Search

Two-dimensional (2D) materials including graphene, transition metal dichalcogenides, black phosphorus, MXenes, and semimetals have attracted extensive and widespread interest over the past years for their many intriguing properties and phenomena, underlying physics, and great potential for applications. The vast library of 2D materials and their heterostructures provides a diverse range of electrical, photonic, mechanical, and chemical properties with boundless opportunities for photonics and plasmonic devices. The infrared (IR) regime, with wavelengths across 0.78 μm to 1000 μm, has particular technological significance in industrial, military, commercial, and medical settings while facing challenges especially in the limit of materials. Here, we present a comprehensive review of the varied approaches taken to leverage the properties of the 2D materials for IR applications in photodetection and sensing, light emission and modulation, surface plasmon and phonon polaritons, non-linear optics, and Smith-Purcell radiation, among others. The strategies examined include the growth and processing of 2D materials, the use of various 2D materials like semiconductors, semimetals, Weyl-semimetals and 2D heterostructures or mixed-dimensional hybrid structures, and the engineering of light-matter interactions through nanophotonics, metasurfaces, and 2D polaritons. Finally, we give an outlook on the challenges in realizing high-performance and ambient-stable devices and the prospects for future research and large-scale commercial applications. Agency for Science, Technology and Research (A*STAR) National Research Foundation (NRF) This work is partially supported by the National Research Foundation, Singapore, under its CRP program (NRF-CRP26- 2021-0004) and A*STAR AME IRG (A20E5c0084). Z.L. acknowledges the support from PHC Merlion Program 2020. L.W. gratefully acknowledges the Start-Up Research Grant from the Singapore University of Technology and Design via Grant No. SRG SMT 2021 169, and National Research Foundation Singapore via Grant Nos. NRF2021-QEP2-02-P03 and NRF2021-QEP2-03-P09. Y.L. acknowledges the support from A*STAR Career Development Fund - Seed Projects (C222812008). M.Z. acknowledges the support from A*STAR Career Development Fund (C210812027).

We measure the ultrafast spectral diffusion, vibronic dynamics, and energy relaxation of a CdSe colloidal quantum wells (CQWs) system at room temperature using two-dimensional electronic spectroscopy (2DES). The energy relaxation of light-hole (LH) excitons and hot carriers to heavy-hole (HH) excitons is resolved with a time scale of ∼210 fs. We observe the equilibration dynamics between the spectroscopically accessible HH excitonic state and a dark state with a time scale of ∼160 fs. We use the center line slope analysis to quantify the spectral diffusion dynamics in HH excitons, which contains an apparent sub-200 fs decay together with oscillatory features resolved at 4 and 25 meV. These observations can be explained by the coupling to various lattice phonon modes. We further perform quantum calculations that can replicate and explain the observed dynamics. The 4 meV mode is observed to be in the near-critically damped regime and may be mediating the transition between the bright and dark HH excitons. These findings show that 2DES can provide a comprehensive and detailed characterization of the ultrafast spectral properties in CQWs and similar nanomaterials. Agency for Science, Technology and Research (A*STAR) Ministry of Education (MOE) Submitted/Accepted version H.V.D. gratefully acknowledges the financial support in part from the Singapore Agency for Science, Technology and Research (A*STAR) SERC under Grant No. M21J9b0085, and the Singapore Ministry of Education Tier 1 grant (MOERG62/20). H.V.D. also gratefully acknowledges the support from TUBA. H.-S.T. gratefully acknowledges the financial support in part from the Singapore Ministry of Education Tier 1 grant (MOE-RG2/19 and MOE-RG14/20). O.V.P. acknowledges the financial support from the United States National Science Foundation under Grant No. CHE-2154367.

The paradigmatic example of a topological phase of matter, the two-dimensional Chern insulator1-5, is characterized by a topological invariant consisting of a single integer, the scalar Chern number. Extending the Chern insulator phase from two to three dimensions requires generalization of the Chern number to a three-vector6,7, similar to the three-dimensional (3D) quantum Hall effect8-13. Such Chern vectors for 3D Chern insulators have never been explored experimentally. Here we use magnetically tunable 3D photonic crystals to achieve the experimental demonstration of Chern vectors and their topological surface states. We demonstrate Chern vector magnitudes of up to six, higher than all scalar Chern numbers previously realized in topological materials. The isofrequency contours formed by the topological surface states in the surface Brillouin zone form torus knots or links, whose characteristic integers are determined by the Chern vectors. We demonstrate a sample with surface states forming a (2, 2) torus link or Hopf link in the surface Brillouin zone, which is topologically distinct from the surface states of other 3D topological phases. These results establish the Chern vector as an intrinsic bulk topological invariant in 3D topological materials, with surface states possessing unique topological characteristics. Ministry of Education (MOE) National Research Foundation (NRF) Submitted/Accepted version We acknowledge funding from the Singapore National Research Foundation Competitive Research Program (grant no. NRF-CRP23-2019-0007) and Singapore Ministry of Education Academic Research Fund Tier 3 (grant no. MOE2016-T3-1-006). P.Z., Z.G. and Y.Y. acknowledge funding from the National Natural Science Foundation of China (grant nos. 52022018, 52021001, 12104211, 6101020101 and 62175215) and Chinese Academy of Engineering (grant no. 2022-XY-127).

For more than a century, abnormal nuclei in tumor cells, presenting subnuclear invaginations and folds on the nuclear envelope, have been known to be associated with high malignancy and poor prognosis. However, current nuclear morphology analysis focuses on the features of the entire nucleus, overlooking the malignancy-related subnuclear features in nanometer scale. The main technical challenge is to probe such tiny and randomly distributed features inside cells. We here employ nanopillar arrays to guide subnuclear features into ordered patterns, enabling their quantification as a strong indicator of cell malignancy. Both breast and liver cancer cells were validated as well as the quantification of nuclear abnormality heterogeneity. The alterations of subnuclear patterns were also explored as effective readouts for drug treatment. We envision that this nanopillar-enabled quantification of subnuclear abnormal features in tumor cells opens a new angle in characterizing malignant cells and studying the unique nuclear biology in cancer. Ministry of Education (MOE) Nanyang Technological University National Research Foundation (NRF) This work is supported by the Singapore Ministry of Education (MOE) (W.Z., RG145/18 and RG112/20), the Singapore National Research Foundation (W.Z., NRF2019-NRF-ISF003- 3292), the Institute for Digital Molecular Analytics and Science (IDMxS) supported by MOE funding under the Research Centres of Excellence scheme (W.Z.), the NTU Start-up Grant (W.Z.), the NTU-NNI Neurotechnology Fellowship (W.Z.), and AIRC IG-24614 and Sapienza AR1181642EE61111 (I.S.).

Organic-inorganic perovskite solar cells demonstrate immense potential for future photovoltaic application due to their remarkable efficiency advancements competitively low-cost. To unlock the full capability of perovskite, developments to the neighbouring layers play immense roles in the ultimate performance of a perovskite solar device. Here, a brief review comprising of the advancements and roles of electron transport layers (ETLs) is discussed. In addition, the effects of ETL on the charge transport, hysteresis, and stability of perovskite solar devices, along with high−performing examples, are also explored. Ministry of Education (MOE) Submitted/Accepted version The research is supported by AcRF Tier2 grant (MOE-T2EP50121-0012) from Singapore Ministry of Education.

The photo-bolometric effect is critically important in optoelectronic structures and devices employing metallic electrodes with nanoscale features due to heating caused by the plasmonic field enhancement. One peculiar case is individual silver nanowires (Ag NWs) and their networks. Ag NW-networks exhibit excellent thermal, electrical, and mechanical properties, providing a simple yet reliable alternative to common flexible transparent electrode materials used in optoelectronic devices. To date, there have been no reports on the photoresponse of Ag NWs. In this study, we show that single Ag NWs and networks of such Ag NWs possess a significant, intrinsic photoresponse, thanks to the photo-bolometric effect, as directly observed and measured using scanning photocurrent microscopy. Surface plasmon polaritons (SPPs) created at the contact metals or plasmons created at the nanowire-metal structures cause heating at the junctions where a plasmonic field enhancement is possible. The local heating of the Ag NWs results in negative photoconductance due to the bolometric effect. Here an open-circuit response due to the plasmon-enhanced Seebeck effect was recorded at the NW-metal contact junctions. The SPP-assisted bolometric effect is found to be further enhanced by decorating the Ag NWs with Ag nanoparticles. These observations are relevant to the use of metallic nanowires in plasmonic applications in particular and in optoelectronics in general. Our findings may pave the path for plasmonics-enabled sensing without spectroscopic detection. T. S. K. acknowledges funding from the Scientific and Technological Research Council of Turkey (TUBITAK) under grant number 118F061.

Colloidal quantum wells (CQWs), also known as nanoplatelets (NPLs), are exciting material systems for numerous photonic applications, including lasers and light-emitting diodes (LEDs). Although many successful type-I NPL-LEDs with high device performance have been demonstrated, type-II NPLs are not fully exploited for LED applications, even with alloyed type-II NPLs with enhanced optical properties. Here, we present the development of CdSe/CdTe/CdSe core/crown/crown (multi-crowned) type-II NPLs and systematic investigation of their optical properties, including their comparison with the traditional core/crown counterparts. Unlike traditional type-II NPLs such as CdSe/CdTe, CdTe/CdSe, and CdSe/CdSexTe1–x core/crown heterostructures, here the proposed advanced heterostructure reaps the benefits of having two type-II transition channels, resulting in a high quantum yield (QY) of 83% and a long fluorescence lifetime of 73.3 ns. These type-II transitions were confirmed experimentally by optical measurements and theoretically using electron and hole wave function modeling. Computational study shows that the multi-crowned NPLs provide a better-distributed hole wave function along the CdTe crown, while the electron wave function is delocalized in the CdSe core and CdSe crown layers. As a proof-of-concept demonstration, NPL-LEDs based on these multi-crowned NPLs were designed and fabricated with a record high external quantum efficiency (EQE) of 7.83% among type-II NPL-LEDs. These findings are expected to induce advanced designs of NPL heterostructures to reach a fascinating level of performance, especially in LEDs and lasers. Agency for Science, Technology and Research (A*STAR) Ministry of Education (MOE) Published version This research is supported by the Singapore Agency for Science, Technology and Research (A*STAR) MTC program, Grant No. M21J9b0085, and the Ministry of Education, Singapore, under its Academic Research Fund Tier 1 (MOERG62/20), and partly from TUBITAK 119N343, 120N076, 121C266, 121N395, and 20AG001. H.V.D. also gratefully acknowledges the support from the TUBA and TUBITAK 2247-A National Leader Researchers Program (121C266). B.L. acknowledges the support from the Science and Technology Program of Guangdong Province under Grant 2021A0505110009 and the National Natural Science Foundation of China under Grant 62104265.

Colloidal quantum wells (CQWs) have emerged as a promising class of gain material in various optical feedback configurations. This is due to their unique excitonic features arising from their 1D quantum confinement. However, existing methods for integrating CQW onto microresonators will cause low laser quality due to uneven CQW coating. To overcome this, the use of liquid-interface kinetically driven self-assembly is proposed to coat ultrathin, close-packed layers of colloidal CdSe/Cd1−xZnxS core/shell CQWs between 7 and 14 nm onto the surface of silica microsphere cavities. The fabricated CQW-whispering-gallery-mode microlasers possess a commendable high quality (Q) factor of 13 000 at room temperature. Stable single-mode lasing output is demonstrated through evanescent field coupling between a CQW-coated microsphere and a thin uncoated microfiber in a 2D-3D microcavity configuration. These promising results highlight the suitability of the liquid-interface kinetically driven self-assembly method for realizing ultrathin CQW-coated microlasers and its high compatibility for integrating colloidal nanocrystals onto complex 3D microstructures for future miniaturized colloidal optoelectronic and photonic applications. Agency for Science, Technology and Research (A*STAR) Economic Development Board (EDB) Ministry of Education (MOE) National Research Foundation (NRF) Submitted/Accepted version This work was supportedby c and AME-IRG- A20E5c0083. The SEM imaging and EDS mapping wereperformed at the Facility for Analysis, Characterization, Testing and Sim-ulation (FACTS) at Nanyang Technological University, Singapore. H.V.D.and E.G.D. gratefully acknowledge the financial support in part from theSingapore Agency for Science, Technology and Research (A*STAR) MTCprogram under Grant No. M21J9b0085, Ministry of Education, Singapore,under its Academic Research Fund Tier 1 (MOE-RG62/20). H.V.D. alsogratefully acknowledges the support from TUBA. W.S.L. and C.X.X.L. ac-knowledge the support of EDB-IPP (REQ0165097).

Various topological laser concepts have recently enabled the demonstration of robust light-emitting devices that are immune to structural deformations and tolerant to fabrication imperfections. Current realizations of photonic cavities with topological boundaries are often limited by outcoupling issues or poor directionality and require complex design and fabrication that hinder operation at small wavelengths. Here we propose a topological cavity design based on interface states between two one-dimensional photonic crystals with distinct Zak phases. Using a few monolayers of solution-processed all-inorganic cesium lead halide perovskite quantum dots as the ultrathin gain medium, we demonstrate a lithography-free, vertical-emitting, low-threshold, and single-mode laser emitting in the green. We show that the topological laser, akin to vertical-cavity surface-emitting lasers (VCSELs), is robust against local perturbations of the multilayer structure. We argue that the design simplicity and reduction of the gain medium thickness enabled by the topological cavity make this architecture suitable for low-cost and efficient quantum dot vertical emitting lasers operating across the visible spectral region. Agency for Science, Technology and Research (A*STAR) Ministry of Education (MOE) National Research Foundation (NRF) Published version Research was supported by the A*STAR-AME programmatic fund on Nanoantenna Spatial Light Modulators for NextGen Display Technologies (Grant A18A7b0058), the Singapore Ministry of Education (MOE2016-T3-1-006), and the Quantum Engineering Programme of the Singapore National Research Foundation (NRF2021- QEP2-01-P01).

Chiral edge states that propagate oppositely at two parallel strip edges are a hallmark feature of Chern insulators which were first proposed in the celebrated two-dimensional (2D) Haldane model. Subsequently, counterintuitive antichiral edge states that propagate in the same direction at two parallel strip edges were discovered in a 2D modified Haldane model. Recently, chiral surface states, the 2D extension of one-dimensional (1D) chiral edge states, have also been observed in a photonic analogue of a 3D Haldane model. However, despite many recent advances in antichiral edge states and chiral surface states, antichiral surface states, the 2D extension of 1D antichiral edge states, have never been realized in any physical system. Here, we report the experimental observation of antichiral surface states by constructing a 3D modified Haldane model in a magnetic Weyl photonic crystal with two pairs of frequency-shifted Weyl points (WPs). The 3D magnetic Weyl photonic crystal consists of gyromagnetic cylinders with opposite magnetization in different triangular sublattices of a 3D honeycomb lattice. Using microwave field-mapping measurements, unique properties of antichiral surface states have been observed directly, including the antichiral robust propagation, tilted surface dispersion, a single open Fermi arc connecting two projected WPs and a single Fermi loop winding around the surface Brillouin zone (BZ). These results extend the scope of antichiral topological states and enrich the family of magnetic Weyl semimetals. Ministry of Education (MOE) Nanyang Technological University National Research Foundation (NRF) Published version Z.G. acknowledges support from the National Natural Science Foundation of China under Grant No. 12104211, Shenzhen Science and Technology Innovation Commission under Grant No. 20220815111105001, and SUSTech under Grant No. Y01236148 and No. Y01236248. Work at Nanyang Technological University was sponsored by Singapore Ministry of Education Academic Research Fund Tier 3 Grant MOE2016-T3-1-006, and the National Research Foundation Competitive Research Program NRF-CRP23-2019-0007. Work at Zhejiang University was sponsored by the National Natural Science Foundation of China under grant number 62175215 and the Fundamental Research Funds for the Central Universities (2021FZZX001-19).

Non-radiative bound states in the continuum (BICs) allow construction of resonant cavities with confined electromagnetic energy and high-quality (Q) factors. However, the sharp decay of the Q factor in the momentum space limits their usefulness for device applications. Here we demonstrate an approach to achieve sustainable ultrahigh Q factors by engineering Brillouin zone folding-induced BICs (BZF-BICs). All the guided modes are folded into the light cone through periodic perturbation that leads to the emergence of BZF-BICs possessing ultrahigh Q factors throughout the large, tunable momentum space. Unlike conventional BICs, BZF-BICs show perturbation-dependent dramatic enhancement of the Q factor in the entire momentum space and are robust against structural disorders. Our work provides a unique design path for BZF-BIC-based silicon metasurface cavities with extreme robustness against disorder while sustaining ultrahigh Q factors, offering potential applications in terahertz devices, nonlinear optics, quantum computing, and photonic integrated circuits. National Research Foundation (NRF) Published version W.W. and Z.W. acknowledge the support from the National Key Research and Development Program of China (No. 2019YFB2203400) and the “111 Project” (Grant No. B20030). W.W., Y.K.S., T.C.W.T. and R.S. acknowledge funding support from Singapore NRF-CRP23-2019-0005 (TERACOMM). W.W. acknowledges the China Scholarship Council for financial support (202006070143).

Cadmium selenide (CdSe) nanoplatelets (NPLs) have attracted significant attention thanks to their favorable optical properties, including narrow emission linewidths, reduced Auger recombination, and a high absorption cross section. However, the photoluminescence (PL) quantum yield (QY) in the near-infrared (NIR) region is poor as compared to that in the visible region. Doping of metal ions is proven to be a successful strategy for inducing Stokes-shifted NIR emission. Here, we report the first account of the successful doping of ytterbium (Yb) into CdSe NPLs by a modified seeded-growth method. The successful incorporation of divalent Yb ions into CdSe NPLs resulted in an additional NIR emission apart from their excitonic emission. By optimizing the dopant concentration, we observed an impressive PL QY of ∼55% for these Yb-doped NPLs. Detailed elemental and optical characterizations were conducted to understand the emerging photophysical properties of these Yb-doped NPLs. These NIR-emitting lanthanide-doped CdSe NPLs might have applications in the next-generation bioimaging, night vision, and photodetection. Agency for Science, Technology and Research (A*STAR) Ministry of Education (MOE) Published version This research was supported by the Ministry of Education, Singapore, under its Academic Research Fund Tier 1 (MOERG62/20), Singapore Agency for Science, Technology and Research (A*STAR) MTC program, grant no. M21J9b0085, and partly from TUBITAK 119N343, 120N076, 121N395, and 20AG001. H.V.D. also acknowledges the support from TUBA.

One of the most exciting breakthroughs in physics is the concept of topology that was recently introduced to photonics, achieving robust functionalities, as manifested in the recently demonstrated topological lasers. However, so far almost all attention was focused on lasing from topological edge states. Bulk bands that reflect the topological bulk-edge correspondence have been largely missed. Here, we demonstrate an electrically pumped topological bulk quantum cascade laser (QCL) operating in the terahertz (THz) frequency range. In addition to the band-inversion induced in-plane reflection due to topological nontrivial cavity surrounded by a trivial domain, we further illustrate the band edges of such topological bulk lasers are recognized as the bound states in the continuum (BICs) due to their nonradiative characteristics and robust topological polarization charges in the momentum space. Therefore, the lasing modes show both in-plane and out-of-plane tight confinements in a compact laser cavity (lateral size ~3λlaser). Experimentally, we realize a miniaturized THz QCL that shows single-mode lasing with a side-mode suppression ratio (SMSR) around 20 dB. We also observe a cylindrical vector beam for the far-field emission, which is evidence for topological bulk BIC lasers. Our demonstration on miniaturization of single-mode beam-engineered THz lasers is promising for many applications including imaging, sensing, and communications. Agency for Science, Technology and Research (A*STAR) Ministry of Education (MOE) National Research Foundation (NRF) Published version This work was supported by the fundings from Singapore Ministry of Education (MOE), A*STAR Programmatic Funds, and the National Research Foundation Competitive Research Program that correspond the grants MOE-T2EP50120-0009, A18A7b0058, and NRF-CRP23-2019-0007, respectively. L.L., A.G.D., and E.H.L. acknowledge founding support from the EPSRC (UK) HyperTerahertz programme (EP/P021859/1), the Royal Society, and the Wolfson Foundation. Y.K. acknowledges a founding support from the Australian Research Council (grant DP200101168).

Perovskite emitters are promising materials as next-generation optical sources due to their low fabrication cost and high quantum yield. In particular, the superradiant emission from a few coherently coupled perovskite emitters can be used to produce a bright entangled photon source. Here, we report the observation of superradiance from mesoscopic (106 emitters). The superradiance is spontaneously generated by off-resonance excitation and detected by time-resolved photoluminescence and second-order photon correlation measurements. We observed a remarkable magnetic tunability of the superradiant photon bunching, indicating a magnetic field-induced decoherence process. The experimental results can be well explained using a theoretical framework based on the microscopic master equation. Our findings shed light on the superradiance mechanism in perovskite emitters and enable low-cost quantum light sources based on perovskite. Ministry of Education (MOE) National Research Foundation (NRF) This work is supported by the Singapore National Research Foundation, Competitive Research Program (CRP Award No. NRF-CRP23-2019-0002) and Quantum Engineering Programme (NRF2021-QEP2-03-P10), ASTAR IRG (M21K2c0116), NRF Investigatorship Programme (Award No. NRF-NRFI05-2019-0003), and the Singapore Ministry of Education (MOE2016-T3-1-006 (S)).

Optical superoscillations are rapid, subwavelength spatial variations of the intensity and phase of light, occurring in complex electromagnetic fields formed by the interference of several coherent waves. The discovery of superoscillations stimulated a revision of the limits of classical electromagnetism — in particular, the studies of phenomena such as unlimitedly small energy hotspots, phase singularities, energy backflow, anomalously high wavevectors and their intriguing similarities to the evanescent plasmonic fields on metals. In recent years, the understanding of superoscillatory light has led to the development of superoscillatory lensing, imaging and metrology technologies. Dielectric, metallic and metamaterial nanostructured superoscillatory lenses have been introduced that are able to create hotspots smaller than allowed by conventional lenses. Far-field, label-free, non-intrusive deeply subwavelength super-resolution imaging and metrology techniques that exploit high light localization and rapid variation of phase in superoscillatory fields have also been developed, including new approaches based on artificial intelligence. We review the fundamental properties of superoscillatory optical fields and examine emerging technological applications. Agency for Science, Technology and Research (A*STAR) Ministry of Education (MOE) National Research Foundation (NRF) Submitted/Accepted version This work was supported by the Engineering and Physical Sciences Research Council UK (grant nos. EP/M009122/1 and EP/T02643X/1), the Singapore National Research Foundation (grant no. NRF-CRP23-2019-0006), the Singapore Ministry of Education (grant no. MOE2016-T3-1-006) and the Agency for Science, Technology and Research (A*STAR) Singapore (grant no. SERC A1685b0005). G.Y. is also supported by the National Innovative Talents Program of China.

A moiré superlattice in transition metal dichalcogenides heterostructure provides an exciting platform for studying strongly correlated electronics and excitonic physics, such as multiple interlayer exciton (IX) energy bands. However, the correlations between these IXs remain elusive. Here, we demonstrate the cascade transitions between IXs in a moiré superlattice by performing energy- and time-resolved photoluminescence measurements in the MoS_{2}/WSe_{2} heterostructure. Furthermore, we show that the lower-energy IX can be excited to higher-energy ones, facilitating IX population inversion. Our finding of cascade transitions between IXs contributes to the fundamental understanding of the IX dynamics in moiré superlattices and may have important applications, such as in exciton condensate, quantum information protocols, and quantum cascade lasers. Ministry of Education (MOE) National Research Foundation (NRF) Published version This work is supported by the Singapore National Research Foundation through its Competitive Research Program (CRP Awards No. NRF-CRP22-2019-0004, No. NRF-CRP23-2019-0002, and Quantum engineering programme) and Singapore Ministry of Education [MOE2016-T3-1-006 (S)].

Colloidal quantum wells (CQWs) provide an appealing platform to achieve emissive many-body correlations for novel optoelectronic devices, given that they act as hosts for strong carrier Coulomb interactions and present suppressed Auger recombination. However, the demonstrated high-order excitonic emission in CQWs requires ultrafast pumping with high excitation levels and can only be spectrally resolved at the single-particle level under cryogenic conditions. Here, through systematic investigation using static power-dependent emission spectroscopy and transient carrier dynamics, we show that Cu-doped CdSe CQWs exhibit continuous-wave-pumped high-order excitonic emission at room temperature with a large binding energy of ∼64 meV. We attribute this unique behavior to dopant excitons in which the ultralong lifetime and the highly localized wavefunction facilitate the formation of many-body correlations. The spectrally resolved high-order excitonic emission generated at power levels compatible with solar irradiation and electrical injection might pave the way for novel solution-processed solid-state devices. Agency for Science, Technology and Research (A*STAR) Ministry of Education (MOE) Published version C.D. is grateful for the financial support from the Ministry of Education, Singapore, under its AcRF Tier 2 grant (MOE-T2EP50121-0012). H.V.D. acknowledges the financial support in part from the Singapore Agency for Science, Technology and Research (A*STAR) MTC program under grant no. M21J9b0085, the Ministry of Education, Singapore, under its Academic Research Fund Tier 1 (MOE-RG62/20), and in part from TUBITAK 119N343, 20AG001, 121N395, and 121C266. H.V.D. also acknowledges support from TUBA and TUBITAK 2247-A National Leader Researchers Program (121C266). M.S. also acknowledges the funding through the Australian Research Council Center of Excellence in Exciton Science (grant no. CE170100026).

The hypothetical Weyl particles in high-energy physics have been discovered in three-dimensional crystals as collective quasiparticle excitations near two-fold degenerate Weyl points. Such momentum-space Weyl particles carry quantized chiral charges, which can be measured by counting the number of Fermi arcs emanating from the corresponding Weyl points. It is known that merging unit-charged Weyl particles can create new ones with more charges. However, only very recently has it been realised that there is an upper limit - the maximal charge number that a two-fold Weyl point can host is four - achievable only in crystals without spin-orbit coupling. Here, we report the experimental realisation of such a maximally charged Weyl point in a three-dimensional photonic crystal. The four charges support quadruple-helicoid Fermi arcs, forming an unprecedented topology of two non-contractible loops in the surface Brillouin zone. The helicoid Fermi arcs also exhibit the long-pursued type-II van Hove singularities that can reside at arbitrary momenta. This discovery reveals a type of maximally charged Weyl particles beyond conventional topological particles in crystals., Comment: 17 pages, 4 figures

Flexural oscillations of free-standing films, nano-membranes and nano-wires are attracting growing attention for their importance to the thermal, electrical and mechanical properties of 2D materials. Here we report on the observation of short-timescale ballistic motion in the flexural mode of a nano-membrane cantilever, driven by thermal fluctuation of flexural phonons, including measurements of ballistic velocities and displacements performed with sub-atomic resolution, using a new free electron edge-scattering technique. Within intervals, Comment: 7 pages, 4 figures

Organic semiconductors have shown great potential as efficient bioelectronic materials. Specifically, photovoltaic polymers such as the workhorse poly(thiophene) derivatives, when stimulated with visible light, can depolarize neurons and generate action potentials, an effect that has been also employed for rescuing vision in blind rats. In this context, however, the coupling of such materials with optically resonant structures to enhance those photodriven biological effects is still in its infancy. Here, we employ the optical coupling between a nanostructured metasurface and poly(3-hexylthiophene) (P3HT) to improve the bioelectronic effects occurring upon photostimulation at the abiotic-biotic interface. In particular, we designed a spectrally tuned aluminum metasurface that can resonate with P3HT, hence augmenting the effective field experienced by the polymer. In turn, this leads to an 8-fold increase in invoked inward current in cells. This enhanced activation strategy could be useful to increase the effectiveness of P3HT-based prosthetic implants for degenerative retinal disorders. Ministry of Education (MOE) Published version This work was supported by the Singapore Ministry of Education Academic Research Fund Tier 1 (2018-T1-002- 040).

Quantum tomography is one of the major challenges of large-scale quantum information research due to the exponential time complexity. In this work, we develop and apply a Bayesian state estimation method to experimentally demonstrate quantum overlapping tomography [Phys. Rev. Lett. \textbf{124}, 100401 (2020)], a scheme intent on characterizing critical information of a many-body quantum system in logarithmic time complexity. By comparing the measurement results of full state tomography and overlapping tomography, we show that overlapping tomography gives accurate information of the system with much fewer state measurements than full state tomography., Accepted in Phys. Rev. Lett., main-text 6 pages, 4 figures, and Supplements 20 pages, 37 figures

This research was supported in part by the National Research Foundation, Prime Minister's Office, Singapore, under its Investigatorship Program (NRF-NRFI2016-08) and the Singapore Agency for Science, Technology and Research (A*STAR) SERC Pharos Program under Grant 152 73 00025. The authors also acknowledge financial support from TUBITAK through 115E679, 115F297, and 117E713 programs. The authors thank Mr. Mustafa Guler and Mr. Ovunc Karakurt for their assistance in TEM imaging, Dr. Gokce Celik for her help on the ellipsometric measurements, Mr. Semih Bozkurt for his support on the AFM characterization, and Mr. Bilge Yagci for his assistance in optical characterization. O.E. acknowledges TUBITAK for financial support through the BIDEB-2211 program. H.V.D. gratefully acknowledges TUBA. Here, the first account of self-resonant fully colloidal mu-lasers made from colloidal quantum well (CQW) solution is reported. A deep patterning technique is developed to fabricate well-defined high aspect-ratio on-chip CQW resonators made of grating waveguides and in-plane reflectors. The fabricated waveguide-coupled laser, enabling tight optical confinement, assures in-plane lasing. CQWs of the patterned layers are closed-packed with sharp edges and residual-free lifted-off surfaces. Additionally, the method is successfully applied to various nanoparticles including colloidal quantum dots and metal nanoparticles. It is observed that the patterning process does not affect the nanocrystals (NCs) immobilized in the attained patterns and the different physical and chemical properties of the NCs remain pristine. Thanks to the deep patterning capability of the proposed method, patterns of NCs with subwavelength lateral feature sizes and micron-scale heights can possibly be fabricated in high aspect ratios. Agency for Science Technology & Research (ASTAR) 152 73 00025 Turkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK) 115E679 115F297 117E713

The realization of electrically tunable plasmonic resonances in the ultraviolet (UV) to visible spectral band is particularly important for active nanophotonic device applications. However, the plasmonic resonances in the UV to visible wavelength range cannot be tuned due to the lack of tunable plasmonic materials. Here, we experimentally demonstrate tunable plasmonic resonances at visible wavelengths using a chalcogenide semiconductor alloy such as antimony telluride (Sb2Te3), by switching the structural phase of Sb2Te3 from amorphous to crystalline. We demonstrate the excitation of a propagating surface plasmon with a high plasmonic figure of merit in both amorphous and crystalline phases of Sb2Te3 thin films. We show polarization-dependent and -independent plasmonic resonances by fabricating one and two-dimensional periodic nanostructures in Sb2Te3 thin films, respectively. Moreover, we demonstrate electrically tunable plasmonic resonances using a microheater integrated with the Sb2Te3/Si device. The developed electrically tunable Sb2Te3-based plasmonic devices could find applications in the development of active color filters. Agency for Science, Technology and Research (A*STAR) Accepted version The authors (K.V.S. and R.S.) acknowledge the funding support from Advanced Manufacturing and Engineering (AME) Programmatic (Grant No. A18A5b0056) by Agency for Science, Technology and Research.

Laser-based generation of random numbers The security of our digital networks is underpinned by the ability to generate streams of random numbers or bits. As networks expand in an ever-connected way, the challenge is to increase the generation rate of the random numbers to keep pace with demand. Kim et al. designed a chip-scale laser diode that generates random bits at an ultrahigh rate (see the Perspective by Fischer and Gauthier). By tailoring the geometry of the cavity, they were able to exploit the spatiotemporal interference of many lasing modes to generate picosecond-scale emission intensity fluctuations in space and time, producing ultrafast random bit streams in parallel. Such a device will find a wide range of applications requiring an ultrafast, compact, robust, and energy-efficient random bit generator. Science , this issue p. 948 ; see also p. 889

We present an efficient 976 nm fiber laser achieved in a step index large-mode-area (LMA) fiber by 915 nm diode pumping. The 976 nm lasing was enabled using spectroscopic engineering in a fiber design that helps suppress the competing 1030 nm gain. Our study shows, alumino-phospho-silicate (Al:P) is advantageous for shaping a gain spectrum in favor of 976 nm compared to other widely used compositions, hence facilitating otherwise the challenging 976 nm lasing. In addition, a germanium (Ge)-cladding was introduced to achieve LMA and low numerical aperture step-index fiber, permitting a wavelength selective bending technique. By applying the bending technique, an efficient 976 nm lasing was achieved with up to 15 W output power (pump power limited) with 28% slope efficiency in the step-index LMA fiber. This work was supported in part by Taihan Fiberoptics Company Ltd.

A CMOS-compatible plasmonic waveguide with a metal or metal-like strip sandwiched in-between dielectrics has been proposed for intra-chip communication in the more-than-Moore era. A sequence of numerical models has been presented to evaluate the plasmonic waveguide performance. For device-level consideration, we demonstrated through simulations that Cu (1450 nm pitch) and PLD-TiN (900 nm pitch) plasmonic waveguides symmetrically sandwiched by SiO₂ with much smaller and hence denser interconnects,are promising candidates for use in global wires for the asynchronous communication. This design of plasmonic waveguide can bridge the CMOS circuitry and high-speed communication at optical frequencies within chip. For a system-level assessment, both of them have the same bandwidth throughput of∼19.8 Gbps. The other performance parameters of Cu and PLD-TiN plasmonic waveguides are respectively, signal latency of ∼0.18 ps and 0.19 ps,energy dissipation per computing bit of ∼2.5×10⁻³ fJ/bit and 3.8×10⁻³ fJ/bit, and 25% crosstalk coupling length of 155μm and 125μm. These findings suggest that plasmonic waveguide for intra-chip communication surpass those of existing electronic interconnects for all the categories of performance parameters. National Research Foundation (NRF) Published version This research was supported by National Research Foundation Singapore project of Integration of Electrically Driven Plasmonic Components in High Speed (NRF2016_CRP001_111).

Hybrid organic–inorganic perovskites (HOIPs) offer long carrier diffusion lengths, high absorption coefficients, tunable bandgaps and long spin lifetimes. The flexible crystal structure and ionic nature of HOIPs makes it possible to allow tune their material properties through rational design, including the incorporation of chiral organic ligands. Recently, chiral HOIPs have emerged as promising materials for chiroptoelectronics, spintronics and ferroelectricity. They exhibit high photoluminescence polarization (17% without an external magnetic field), good device performance (a circularly polarized photodetector had 100 times higher responsivity than one based on chiral metasurface) and high saturated polarization (~2 times higher than that of barium titanate). Here we review the latest advances in chiral HOIPs and investigate the specific benefits of combining chiral organic and inorganic components in perovskites. We discuss demonstrations of chiroptical and ferroelectric applications, and conclude with our perspective on the future opportunities for chiral HOIPs. Agency for Science, Technology and Research (A*STAR) Ministry of Education (MOE) National Research Foundation (NRF) Accepted version W.G., G.K.L. and A.S. acknowledge the support from the Singapore National Research Foundation through 2015 NRF fellowship grant (NRF-NRFF2015-03), Singapore Ministry of Education via AcRF Tier2 grant (No. MOE2016-T2-2-077, No. MOE2017- T2-1-163), and A*Star QTE Programme. R.S and G.L. acknowledge the support from the Australian Research Council Centre of Excellence in Exciton Science (Funding grant number CE170100026). E.H.S. acknowledges support from the U.S. Office of Naval Research (grant award no.: N00014-17-1-2524). We thank Prof. Mingtao Zhang (Nankai University) for helpful discussions.

Recent attempts to synthesize hybrid perovskites with large chirality have been hampered by large size mismatch and weak interaction between their structure and the wavelength of light. Here we adopt a planar nanostructure design to overcome these limitations and realize all-dielectric perovskite metasurfaces with giant superstructural chirality. We identify a direct spectral correspondence between the near- and the far- field chirality, and tune the electric and magnetic multipole moments of the resonant chiral metamolecules to obtain large anisotropy factor of 0.49 and circular dichroism of 6350 mdeg. Simulations show that larger area metasurfaces could yield even higher optical activity, approaching the theoretical limits. Our results clearly demonstrate the advantages of nanostructrure engineering for the implementation of perovskite chiral photonic, optoelectronic, and spintronic devices. Agency for Science, Technology and Research (A*STAR) Ministry of Education (MOE) National Research Foundation (NRF) Published version We acknowledge support from the Singapore National Research Foundation (CRP Award No. NRF-CRP14–2014-03, C.S.), the A*STAR-AME programmatic fund on Nanoantenna Spatial Light Modulators for Next-Gen Display Technologies (Grant A18A7b0058, C.S.), the Singapore Ministry of Education (MOE2016-T3-1-006, C.S.), and from the Fundamental Research Funds for Central Universities, Nankai University (Grant Number: 023-63213067, G.L.) and the NSFC (52103218, G.L.) of China.

We demonstrate a non-contact optical magnetic field sensor that is based on actuation of a metamaterial-microcavity by the magnetic Lorentz force. Magnetic field is transduced to a change of the sensor's reflectivity. The microscale proof-of-concept metamaterial magnetometer can be read from a distance and offers 60 μm spatial, about 10 μs temporal, and sub-microtesla magnetic field resolution. Ministry of Education (MOE) Published version This work was supported by the Office of Naval Research (Grant No. N62909-18-1-2026), the UK’s Engineering and Physical Sciences Research Council (Grant Nos. EP/M009122/1 and EP/T02643X/1), the Singapore Ministry of Education [Grant No. MOE2016-T3-1-006 (S)], and the Basic Scientific Research of Heilongjiang University (Grant No. 2020-KYYWF-0997).

Focusing on the transient regime, we explore atomic two-photon spectroscopy with self-aligned homodyne interferometry in a $\Lambda$-system with large optical depth. The two light sources at the origin of the interference are the single-photon transient transmission of the probe, and the slow light of the electromagnetically induced transparency. By switching off the probe laser abruptly (flash effect), the transient transmission signal is reinforced by cooperativity, showing enhanced sensitivity to the two-photon frequency detuning. If the probe laser is periodically switched on and off, the amplitude of the transmission signal varies and remains large even for high modulation frequency. This technique has potential applications in sensing, such as magnetometry and velocimetry, and in coherent population trapping clocks., Comment: 7 figures and 9 pages including an appendix and references

Rational design of noble metal catalysts with the potential to leverage efficiency is vital for industrial applications. Such an ultimate atom-utilization efficiency can be achieved when all noble metal atoms exclusively contribute to catalysis. Here, we demonstrate the fabrication of a wafer-size amorphous PtSe film on a SiO substate via a low-temperature amorphization strategy, which offers single-atom-layer Pt catalysts with high atom-utilization efficiency (~26 wt%). This amorphous PtSe (1.2 < x < 1.3) behaves as a fully activated surface, accessible to catalytic reactions, and features a nearly 100% current density relative to a pure Pt surface and reliable production of sustained high-flux hydrogen over a 2 inch wafer as a proof-of-concept. Furthermore, an electrolyser is demonstrated to generate a high current density of 1,000 mA cm. Such an amorphization strategy is potentially extendable to other noble metals, including the Pd, Ir, Os, Rh and Ru elements, demonstrating the universality of single-atom-layer catalysts. [Figure not available: see fulltext.], This work was supported by the Singapore National Research Foundation Singapore programme (NRF-CRP21-2018-0007, NRF-CRP22-2019-0060, NRF-CRP18-2017-02 and NRF–CRP19–2017–01) and the Singapore Ministry of Education via AcRF Tier 3 (MOE2018-T3-1-002), AcRF Tier 2 (MOE2017-T2-2-136, MOE2019-T2-2-105 and MOE2018-T2-1-176) and AcRF Tier 1 (RG7/18 and 2019-T1-002-034). It was also supported by the National Key Research and Development Program of China (2019YFA0705400, 2021YFE0194200), the National Natural Science Foundation of China (11772153, 22073048, 21763024, 22175203, 22006023), the Natural Science Foundation of Jiangsu Province (BK20190018), the National Key R&D Program of China (2021YFA1500900), the Fundamental Research Funds for Central Universities (531119200209, NE2018002, NJ2020003) and the High-Performance Computing Center of Nanjing Tech University. Catalan Institute of Nanoscience and Nanotechnology (ICN2) acknowledges funding from Generalitat de Catalunya 2017SGR327 and the project NANOGEN (PID2020-116093RB-C43), funded by MCIN/AEI/10.13039/501100011033/. ICN2 is supported by the Severo Ochoa programme from Spanish MINECO (grant no. SEV-2017-0706) and is funded by the CERCA Programme/Generalitat de Catalunya. This work has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement no. 823717-ESTEEM3.

The second-order (2nd order) nonlinear Hall effect observed in time-reversal symmetric system has not only shown abundant physical contents, but also exhibited potential application prospect. Recently, a third-order (3rd order) nonlinear Hall effect has been observed in MoTe2 and WTe2. However, few-layer MoTe2 and WTe2 are usually unstable in air and the observed 3rd nonlinear Hall effect can be measured only at low temperature, which hinders its further investigations as well as potential applications. Thus, exploring new air-stable material systems with sizable 3rd nonlinear Hall effect at room temperature is an urgent task. Here, in a type-II Weyl semimetal TaIrTe4, we observed pronounced 3rd order nonlinear Hall effect, which can exist at room temperature and maintain stably for months. The 3rd order Hall effect is connected to the Berry-connection polarizability tensor instead of Berry curvature. The possible mechanisms for the observation of the 3rd order nonlinear Hall effect in TaIrTe4 at room temperature has been discussed. Our findings will open an avenue towards exploring room temperature nonlinear devices in new quantum materials. Ministry of Education (MOE) National Research Foundation (NRF) Submitted/Accepted version This work was supported by the Singapore National Research Foundation through its Competitive Research Program (CRP Award No. NRF-CRP21-2018-0007 and NRF-CRP22-2019-0004), Singapore Ministry of Education (MOE2016-T3-1-006 (S), MOE2017-T2-2-108, and MOE2019-T2-1-001), Fundamental Research Funds for the Central Universities (buctrc 202122) and National Natural Science Foundation of China (11947212).

As a dominant oncogenic protein, Ras is well-known to segregate into clusters on the plasma membrane for activating downstream signaling. However, current technologies for direct measurements of Ras clustering are limited to sophisticated high-resolution techniques like electron microscopy and fluorescence lifetime imaging. To further promote fundamental investigations and the related drug development, we hereby introduce a nanobar-based platform which effectively guides Ras clusters into quantifiable patterns in live cells that is resolvable under conventional microscopy. Major Ras isoforms, K-Ras, H-Ras, and N-Ras were differentiated, as well as their highly prevalent oncogenic mutants G12V and G13D. Moreover, the isoform specificity and the sensitivity of a Ras inhibitor were successfully characterized on nanobars. We envision that this nanobar-based platform will serve as an effective tool to read Ras clustering on the plasma membrane, enabling a novel avenue both to decipher Ras regulations and to facilitate anti-Ras drug development. Ministry of Education (MOE) Nanyang Technological University National Research Foundation (NRF) Accepted version This work is funded by Singapore Ministry of Education (MOE) (W. Zhao, RG145/18 and RG112/20), the Singapore National Research Foundation (W. Zhao, NRF2019-NRF-ISF003-3292), the NTU Start-up Grant (W. Zhao), the NTU-NNI Neurotechnology Fellowship (W. Zhao), DFG (K. Rajalingam), and Ageing Research Institute for Society and Education (ARISE) NTU for the research scholarship (H. Mu).

On-chip Floquet photonic topological insulators, which are based on switched-capacitor circulators, could be used to create hybrid electronic–photonic topological integrated circuits for emerging communication technologies.

Negatively charged boron vacancy (V_{B}^{-}) centers in hexagonal boron nitride (h-BN) are promising spin defects in a van der Waals crystal. Understanding the spin properties of the excited state (ES) is critical for realizing dynamic nuclear polarization. Here, we report zero-field splitting in the ES of D_{ES}=2160 MHz and its associated optically detected magnetic resonance (ODMR) contrast of 12% at cryogenic temperature. In contrast to nitrogen vacancy (NV^{-}) centers in diamond, the ODMR contrast of V_{B}^{-} centers is more prominent at cryotemperature than at room temperature. The ES has a g factor similar to the ground state. The ES photodynamics is further elucidated by measuring the level anticrossing of the V_{B}^{-} defects under varying external magnetic fields. Our results provide important information for utilizing the spin defects of h-BN in quantum technology. Ministry of Education (MOE) National Research Foundation (NRF) Published version We acknowledge the Singapore National Research Foundation through QEP Grants No. NRF2021-QEP2-01- P02, No. NRF2021-QEP2-03-P01, No. NRF2021-QEP2-03- P10, No. 2019-0643 (QEP-P2), and No. 2019-1321 (QEP-P3), and Singapore Ministry of Education [No. MOE2016-T3-1- 006 (S)], the Australian Research council (via CE200100010), the Asian Office of Aerospace Research and Development Grant No. FA2386-17-1-4064 and Office of Naval Research Global N62909-22-1-2028.

The interfacial passivation technique is an effective method to improve the stability and photovoltaic performance of perovskite solar cells. Here, we demonstrate the importance of passivating undercoordinated halide ions in minimizing carrier losses at the perovskite/spiro-OMeTAD interface. 4-Chlorobenzene sulfonyl chloride (CBSC) has been utilized as a Lewis acid passivation material. CBSC molecules act as electron acceptors, which bind to the negatively charged undercoordinated halide ions and Pb-I antisite defects (PbI3−). The champion CBSC-passivated perovskite device shows a high power conversion efficiency (PCE) of 20.02%, unlike the pristine device with an efficiency of 18.29%. Significant long term-stability in the CBSC passivated device is also observed, maintaining 93% of the initial PCE after 768 h stored in ambient conditions with 30% relative humidity. Energy Market Authority (EMA) Ministry of Education (MOE) National Research Foundation (NRF) Published version The research is supported by an AcRF Tier2 grant (MOET2EP50121-0012) from Singapore Ministry of Education, and EMA-EP004-EKJGC-0003 grant from the Energy Market Authority (EMA) and National Research Foundation (NRF) Singapore and NRF2018-ITC001-001.

The revolutionary 5G cellular systems represent a breakthrough in the communication network design to provide a single platform for enabling enhanced broadband communications, virtual reality, autonomous driving, and the internet of everything. However, the ongoing massive deployment of 5G networks has unveiled inherent limitations that have stimulated the demand for innovative technologies with a vision toward 6G communications. Terahertz (0.1-10 THz) technology has been identified as a critical enabler for 6G communications with the prospect of massive capacity and connectivity. Nonetheless, existing terahertz on-chip communication devices suffer from crosstalk, scattering losses, limited data speed, and insufficient tunability. Here, we demonstrate a new class of phototunable, on-chip topological terahertz devices consisting of a broadband single-channel 160 Gbit/s communication link and a silicon Valley Photonic Crystal based demultiplexer. The optically controllable demultiplexing of two different carriers modulated signals without crosstalk is enabled by the topological protection and a critically coupled high-quality (Q) cavity. As a proof of concept, we demultiplexed high spectral efficiency 40 Gbit/s signals and demonstrated real-time streaming of uncompressed high-definition (HD) video (1.5 Gbit/s) using the topological photonic chip. Phototunable silicon topological photonics will augment complementary metal oxide semiconductor (CMOS) compatible terahertz technologies, vital for accelerating the development of futuristic 6G and 7G communication era driving the real-time terabits per second wireless connectivity for network sensing, holographic communication, and cognitive internet of everything. National Research Foundation (NRF) Published version A.K., M.G., P.P., N.W., and R.S. acknowledge the research funding support from National Research Foundation (NRF) Singapore, Grant No: NRF-CRP23-2019-0005. We thank the IEMN characterization service (CHOP platform) for the access to various instruments and infrastructure support used during the VNA characterization steps of the different samples. Signal generation and data analysis received support from CPER Photonics for Society and DYDICO research cluster of the I-site ULNE. The 300 GHz datacom setup used for high level modulation formats and associated BER measurements was also partially funded by ANR TERASONIC and SPATIOTERA grants (ANR programs under CE24, 2017 and 2019) as well as ITN TERAOPTICS Marie-Curie Action (H2020- MSCA-ITN-2020, Grant 956857). Last, ASK modulation generations and analysis was carried using a dedicated THz communication setup developed within TERIL-WAVES project from I-site ULNE and Metropole Européenne de Lille (MEL).

Subwavelength confinement of electromagnetic modes in periodic structures is essential to tailor light−matter interaction in space and time. Metamaterials with strong electromagnetic field confinement can be extremely sensitive to surface conditions, thereby enhancing the metadevice response for low-energy electrical, thermal, and optical stimuli. It is important to note that the total field confinement matters, but the contribution of in-plane electric field density is critical to efficiently harness and manipulate light via active metasurfaces. The authors experimentally demonstrate a new planar metal–semiconductor hybrid terahertz (THz) metasurface design that shows highly sensitive active THz amplitude modulation towards optical illumination. In the proposed design, in-plane electric field density has been enhanced by 350% to lower the optical pump fluence requirement for energy-efficient, active modulation of resonances compared to the conventional inductive-capacitive resonant metamaterials. Such metasurfaces with large in-plane electric field density can find many applications in developing ultrasensitive sensors and active THz electrical and optical modulators operational at extremely low energies. Agency for Science, Technology and Research (A*STAR) Submitted/Accepted version The authors acknowledge the financial support from Agency for Science, Technology, and Research (A*STAR), grant number: A18A5b0056.

In modern day photonic integrated circuitries (ICs), near-field Coulomb interaction plays an important role in the device performances. In fact, harvesting Coulomb effect intelligently can immensely help to realize advanced photonic devices in micro and nano length scales. In this context, electrically coupled terahertz metasurfaces are investigated for weak and strong near field coupling regimes under the influence of variable photo excitations. Our study demonstrates active tuning of metasurface resonances at relatively low pump powers (up to 10 mW (pump fluence of 12.7 µJ/cm2) and 20 mW (pump fluence of 25.4 µJ/cm2) for strong and weak near field coupling regimes, respectively). We attribute photo induced modifications of substrate electronic properties along with modifications of near-field interactions to be responsible for the resonance modulations. Further, we have demonstrated Coulomb effect parameter can be dynamically tunable with variable pump power. Our study reveals that change in Coulomb parameter is more pronounced for weak near field coupling regime (more than 6 times enhancement in Coulomb parameter) in comparison to strong near field coupling regime. In addition, the optical switching of the fundamental resonance is validated utilizing numerically simulated patterns of surface current & electric field. We envisage such active modulation of Coulomb effects in near-field coupled metasurfaces can lead to realize dynamically tunable terahertz metaphotonic devices. Submitted/Accepted version D K gratefully acknowledges financial assistance from the Department of Science and Technology (DST) INSPIRE doctoral research fellowship, Ministry of Science and Technology, Government of India. D R C acknowledges partial support from the Department of Science and Technology (DST), Project No. CRG/2019/001656.

The technological revolution of two-dimensional (2D) materials provides vitality for ultrafast optics. Compared with other 2D materials, indium sulfide (In2S3) has become an attractive candidate material in the field of nonlinear optics because of its high carrier mobility, ultrafast photoelectric sensitivity, medium band gap, and high absorption coefficient. However, its nonlinear optical effects have rarely been studied. In this work, we synthesized 2D β-In2S3nanoplates using the hydrothermal method. The saturable absorber based on β-In2S3nanoplates has been investigated. The experimental results demonstrated that the modulation depth and saturable intensity of β-In2S3are measured as 2.32% and 18.4 MW·cm-2, respectively, which indicates that β-In2S3has great potential in ultrafast optics among emerging nonlinear optical materials. Using such unprecedented properties, the mode-locked fiber laser with 247th (1.92 GHz) harmonic mode locking is realized in the C-band for the first time to our knowledge. This paves the way for exploring β-In2S3as an outstanding nonlinear material for high-performance ultrafast optics. Submitted/Accepted version This research was supported by the International Science & Technology Cooperation and Exchanges Project of Shaanxi (no. 2020KW-005); the Funded projects for the Academic Leader and Academic Backbones, Shaanxi Normal University (no. 18QNGG006); the Starting Grants of Shaanxi Normal University (grant nos. 1112010209 and 1110010717); and the Fundamental Research Funds For the Central Universities (nos. GK201802006 and 2020CSLY005).

We propose and investigate an all-solid ytterbium-doped antiresonant fiber (YbARF) design to inherently suppress four-level lasing with >20 dB/m of selective loss and achieve high-efficiency three-level lasing while maintaining near-diffraction-limited operation with an ultra-large mode area of approximately 3630 µm2. The YbARF is designed such that the high-gain wavelengths corresponding to four-level lasing lie in the resonance band characterized by high confinement loss. This enables three-level lasing with high efficiency in a short (0.8-m-long) YbARF, making it a potential candidate for high-peak-power ultrafast lasers at 976 nm. We discuss fiber design considerations and detailed simulation results for three-level lasing performance in the YbARF, which promises >85% lasing efficiency in a single-pass pump configuration. These design concepts can be easily extended to suppress high-gain wavelengths in other rare-earth-doped (e.g., with thulium, erbium, and neodymium) fiber amplifiers or lasers. National Research Foundation (NRF) Submitted/Accepted version National Research Foundation Singapore, Quantum Engineering Programme.

The development of terahertz integrated circuits is vital for realizing sixth-generation (6G) wireless communication, high-speed on-chip interconnects, high-resolution imaging, on-chip biosensors, and fingerprint chemical detection. Nonetheless, the existing terahertz on-chip devices suffer from reflection, and scattering losses at sharp bends or defects. Recently discovered topological phases of light endow the photonics devices with extraordinary properties, such as reflectionless propagation and robustness against impurities or defects, which is vital for terahertz integrated devices. Leveraging the robustness of topological edge states combined with a low-loss silicon platform is poised to offer a remarkable performance of the terahertz devices providing a breakthrough in the field of terahertz integrated circuits and high-speed interconnects. In this Perspective, we present a brief outlook of various terahertz functional devices enabled by a photonic topological insulator that will pave the path for augmentation of complementary metal oxide semiconductor compatible terahertz technologies, essential for accelerating the vision of 6G communication and beyond to enable ubiquitous connectivity and massive digital cloning of physical and biological worlds. National Research Foundation (NRF) Published version A.K., M.G., P.P., N.W., and R.S. would like to acknowledge the research funding support from National Research Foundation (NRF) Singapore (Grant No. NRF-CRP23-2019-0005). M.F. acknowledges the Core Research for Evolutional Science and Technology (CREST) program of the Japan Science and Technology Agency (Grant No. JPMJCR 21C4), KAKENHI, Japan (Grant No. 20H01064), and the commissioned research by National Institute of Information and Communications Technology (NICT), Japan (Grant No. 03001).

Halide perovskite metasurfaces are attracting increasing interest for applications in light-emitting and display technologies. To access the wide range of colors required for these applications, the main mechanism exploited thus far has been chemical engineering of the perovskite compounds – this constitutes a significant limitation for the dynamic switching of optical response desirable in actual devices. Here we demonstrate polarization-dependent, dynamic control of structural color and emission wavelength in an all-dielectric phase-change halide perovskite nanograting metasurface, by temperature tuning. This is underpinned by the significant change in the perovskite optical constants which accompanies its phase-transition around room temperature. The functionalities demonstrated in our work bearing potential for applications in light-emitting devices, displays and spatial-light-modulators. Agency for Science, Technology and Research (A*STAR) Ministry of Education (MOE) Published version This research was supported by the A*STAR-AME programmatic fund on Nanoantenna Spatial Light Modulators for Next-Gen Display Technologies (grant A18A7b0058), and the Singapore Ministry of Education (Tier 3 grant MOE2016-T3-1-006).

Energy-efficient spintronic technology holds tremendous potential for the design of next-generation processors to operate at terahertz frequencies. Femtosecond photoexcitation of spintronic materials generates sub-picosecond spin currents and emission of terahertz radiation with broad bandwidth. However, terahertz spintronic emitters lack an active material platform for electric-field control. Here, we demonstrate a nonlinear electric-field control of terahertz spin current-based emitters using a single crystal piezoelectric Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) that endows artificial magnetoelectric coupling onto a spintronic terahertz emitter and provides 270% modulation of the terahertz field at remnant magnetization. The nonlinear electric-field control of the spins occurs due to the strain-induced change in magnetic energy of the ferromagnet thin-film. Results also reveal a robust and repeatable switching of the phase of the terahertz spin current. Electric-field control of terahertz spintronic emitters with multiferroics and strain engineering offers opportunities for the on-chip realization of tunable energy-efficient spintronic-photonic integrated platforms. National Research Foundation (NRF) Published version R.S. and P.A. would like to acknowledge National Research Foundation, Singapore, for the support through NRF-CRP23-2019-0005.

Nonlinear optical processes are crucial for emerging applications including multiphoton-excited fluorescence microscopy and optical power limiting. Therefore, searching for materials of high multiphoton absorption cross sections is essential for the development of these techniques. We present synthesis of 4.5 monolayer CdSe nanoplatelets (NPLs) doped with silver and copper ions along with the evaluation of their two-photon absorption (TPA) and three-photon absorption (3PA) cross sections. Doping significantly increases the TPA cross section of each NPL sample, which reaches up to 1.33 × 107 GM for the most absorbing copper-doped ones. We also detected 1-2 orders of magnitude-enhanced 3PA cross sections for the doped NPLs in comparison with their undoped counterparts. As TPA and 3PA peaks appear, in the first and the second biological transmission windows, respectively, doped NPLs are promising candidates for multiphoton fluorescence microscopy as bioimaging agents. Moreover, the strong nonlinear response suggests application as active optoelectronic materials in optical sensors. Ministry of Education (MOE) National Research Foundation (NRF) Published version This work has received financial support from the National Science Centre, Poland, under grant no. 2018/29/B/ST4/ 02172, the National Research Foundation of Singapore under program NRF-NRFI2016-08, and the Singapore Ministry of Education under grant MOE-RG62/20.

Lasers based on semiconductor colloidal quantum wells (CQWs) have attracted wide attention, thanks to their facile solution-processability, low threshold and wide range spectral tunability. Colloidal microlasers based on whispering-gallery-mode (WGM) resonators have already been widely demonstrated. However, due to their microscale size typically supporting multiple modes, they suffer from multimode competition and higher threshold. The ability to control the multiplicity of modes oscillating within colloidal laser resonators and achieving single-mode lasers is of fundamental importance in many photonic applications. Here we show that as a unique, simple and versatile architecture of all-colloidal lasers intrinsically enabled by balanced gain/loss segments, the lasing threshold reduction and spectral purification can be readily achieved in a system of a WGM-supported microfiber cavity by harnessing the notions of parity-time symmetry (PT). In particular, we demonstrate a proof-of-concept PT-symmetric microfiber laser employing CQWs as the colloidal gain medium along with a carefully tuned nanocomposite of Ag nanoparticles (Ag NPs) incorporated into a PMMA matrix altogether and conveniently coated around a coreless microfiber as a rigorously tailored colloidal loss medium to balance the gain. The realization of gain/loss segments in our PT-symmetric all-colloidal arrangement is independent of selected pumping, reducing the complexity of the system and making compact device applications feasible, where control over the pumping is not possible. We observed a reduction in the number of modes, resulting in a reduced threshold and enhanced output power of the PT-symmetric laser. The PT-symmetric CQW-WGM microcavity architecture offers new opportunities towards simple implementation of high-performance optical resonators for colloidal lasers. Agency for Science, Technology and Research (A*STAR) Ministry of Education (MOE) Submitted/Accepted version The authors acknowledge support from the Agency for Science, Technology and Research (A*STAR) MTC program, Grant No. M21J9b0085 (Singapore). The research of the project was supported by the Ministry of Education Tier 1 grant MOE-RG62/20 (Singapore). The authors gratefully acknowledge the financial support in part from TUBITAK 115F297, 117E713, 119N343, 121N395 and 20AG001. H. V. D. also acknowledges the support from TUBA.

Low-dimensional α-phase molybdenum trioxide (α-MoO3), a layered van-der-Waals (vdW) semiconductor, has emerged as an attractive natural hyperbolic material supporting mid-infrared hyperbolic phonon polaritons (PhPs), which exhibit strong spatial confinement and low loss. With the advantages of strong in-plane optical anisotropy, many efforts have been devoted to investigating the properties of hyperbolic PhPs in α-MoO3 using scanning near-field optical microscopy. However, the studies of far-field controlling of hyperbolic PhPs in α-MoO3 have been quite limited so far. This work reports the first experimental demonstration of far-field excitation and manipulation of hyperbolic phonon resonances in metamaterial structures consisting of α-MoO3 nanodisks and slabs. The excited phonon resonances show a maximum spatial confinement factor (free space wavelength/period) of 32 and a quality factor of 76. It is shown that the in-plane phonon resonances in α-MoO3 can be selectively excited in the two Reststrahlen bands featuring hyperbolic dispersion relations along its two crystal directions, by simply controlling the incident polarization. In addition, the relative strength of resonances along different in-plane crystal directions and the resultant field distributions can be continuously reconfigured by varying the incident polarization angle. These findings pave the way for future development of novel nanophotonic devices based on hyperbolic PhPs in vdW materials. Agency for Science, Technology and Research (A*STAR) Ministry of Education (MOE) This work was partially supported by Singapore Ministry of Education Academic Research Fund Tier 2 under grant no. MOE2018-T2-1-176 and by A*STAR AME programmatic grant (grant no. A18A7b0058), AME IRG grant (Project No. A20E5c0095), and career development fund (grant no. C210112044).

We proposed and demonstrated mode cleaning in a high-power fiber laser by integrating an anti-resonant hollow-core fiber (AR-HCF) into a multimode laser cavity of an ytterbium (Yb)-doped fiber (YDF). An in-house mode-matched AR-HCF was fusion-spliced to a commercial multimode LMA-YDF, ensuring efficient fundamental mode coupling. The AR-HCF inflicts a high propagation loss selectively on higher-order modes, facilitating fundamental mode operation. Thus, the AR-HCF works as an efficient spatial mode filter embedded in the multimode fiber laser cavity and reinforces preferential amplification of the fundamental mode. Beam quality factor enhancement was achieved from M2 = 2.09 to 1.39 at an output power of 57.7 W (pump-power limited). The beam quality can be further improved by refining the AR-HCF fabrication. The proposed technique has a great potential to be exploited in other multimode fiber laser cavities involving erbium- or thulium-doped fibers and obviates the need for complicated specialty active fiber designs. Compared with the commonly used fiber bending technique, our method can achieve an efficient higher-order mode suppression without inducing mode-field deterioration. National Research Foundation (NRF) Published version National Research Foundation Singapore (QEP-P4).

The Rashba effect, i.e., the splitting of electronic spin-polarized bands in the momentum space of a crystal with broken inversion symmetry, has enabled the realization of spin-orbitronic devices, in which spins are manipulated by spin-orbit coupling. In optics, where the helicity of light polarization represents the spin degree of freedom for spin-momentum coupling, the optical Rashba effect is manifested by the splitting of optical states with opposite chirality in the momentum space. Previous realizations of the optical Rashba effect relied on passive devices determining the surface plasmon or light propagation inside nanostructures, or the directional emission of chiral luminescence when hybridized with light-emitting media. An active device underpinned by the optical Rashba effect is demonstrated here, in which a monolithic halide perovskite metasurface emits highly directional chiral photoluminescence. An all-dielectric metasurface design with broken in-plane inversion symmetry is directly embossed into the high-refractive-index, light-emitting perovskite film, yielding a degree of circular polarization of photoluminescence of 60% at room temperature. Agency for Science, Technology and Research (A*STAR) Ministry of Education (MOE) This research was supported by the A*STAR-AME programmatic fund on Nanoantenna Spatial Light Modulators for Next-Gen Display Technologies (grant A18A7b0058), and the Singapore Ministry of Education (Tier 3 grant MOE2016-T3-1-006).

Nonradiating charge-current configurations have attracted attention in photonics for the efficient localization of the electromagnetic field. Anapole mode is a unique nonradiating state of light induced by the interference of electric and toroidal dipole that possesses rich physics with potential applications in micro-nanophotonics. Active control of an anapole is essential for the design and realization of tunable low-energy photonic devices. Here, an active anapole metasurface device is experimentally demonstrated as a switch for the terahertz waves. The metadevice consists of planar resonators with photoactive inclusions of silicon patches in a hybrid metal–semiconductor configuration. The active element enables dynamic control over the contributions of the multipoles that eventually determine the formation of the exotic anapoles that host extreme nonradiative confinement and its active switching into sub-radiative Fano resonance and highly radiative electric dipoles. Two orders of magnitude change in the near-field intensity of the anapole that leads to 201% extinction modulation is further demonstrated. The anapole metadevice provides a platform to efficiently control both the far-field radiation and near-field enhancement in metaoptics, promoting active micro-nanophotonic devices for potential applications in terahertz modulators, lasers, filters, and dynamic near-field imaging. National Research Foundation (NRF) Submitted/Accepted version W.W. and Z.W. acknowledge the support from the National Key Research and Development Program of China (No. 2019YFB2203400) and the “111 Project” (Grant No. B20030). W.W., Y.K.S., M.G., and R.S. acknowledge funding support from Singapore NRFCRP23-2019-0005 (TERACOMM). W.W. acknowledges the China Scholarship Council for financial support (202006070143)