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With the emerging trend of big data and internet-of-things, sensors with compact size, low cost and robust performance are highly desirable. Spectral imaging and spectral LIDAR systems enable measurement of spectral and 3D information of the ambient environment. These systems have been widely applied in different areas including environmental monitoring, autonomous driving, biomedical imaging, biometric identification, archaeology and art conservation. In this review, modern applications of state-of-the-art spectral imaging and spectral LIDAR systems in the past decade have been summarized and presented. Furthermore, the progress in the development of compact spectral imaging and LIDAR sensing systems has also been reviewed. These systems are based on the nanophotonics technology. The most updated research works on subwavelength scale nanostructure-based functional devices for spectral imaging and optical frequency comb-based LIDAR sensing works have been reviewed. These compact systems will drive the translation of spectral imaging and LIDAR sensing from table-top toward portable solutions for consumer electronics applications. In addition, the future perspectives on nanophotonics-based spectral imaging and LIDAR sensing are also presented.

Rapid scaling of semiconductor devices has led to an increase in the number of processor cores and integrated functionalities onto a single chip to support the growing demands of high-speed and large-volume consumer electronics. To meet this burgeoning demand, an improved interconnect capacity in terms of bandwidth density and active tunability is required for enhanced throughput and energy efficiency. Low-loss terahertz silicon interconnects with larger bandwidth offer a solution for the existing inter-/intrachip bandwidth density and energy-efficiency bottleneck. Here, a low-loss terahertz topological interconnect-cavity system is presented that can actively route signals through sharp bends, by critically coupling to a topological cavity with an ultrahigh-quality (Q) factor of 0.2 × 10

The computing speeds in modern multi-core processors and big data servers are no longer limited by the on-chip transistor density that doubles every two years following the Moores law, but are limited by the on-chip data communication between memories and microprocessor cores. Realization of integrated, low-cost, and efficient solutions for high speed, on-chip data communications require terahertz (THz) interconnect waveguides with tremendous significance in future THz technology including THz-wave integrated circuits and THz communication. However, conventional approaches to THz waveguiding suffer from sensitivity to defects and considerable bending losses at sharp bends. Here, building on the recently-discovered topological phase of light, we experimentally demonstrate robust THz topological valley transport on low-loss, all-silicon chips. We show that the valley polarized topological kink states exhibit unity transmission over a bulk band gap even after propagating through ten sharp corners. Such states are excellent information carriers due to their robustness, single-mode propagation, and linear dispersion-key properties for next generation THz communications. By leveraging the unique properties of kink states, we demonstrate error-free communication through a highly-twisted domain wall at an unprecedented data rate (10 Gbit/s) and uncompressed 4K high-definition video transmission. Our work provides the first experimental demonstration of the topological phases of THz wave, which could certainly inspire a plethora of research on different types of topological phases in two and three dimensions., Comment: 12 pages, 4 figures

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).

We investigate an optical phase shifter based on Ge2Sb2Te5 (GST) integrated with a Si waveguide at mid-infrared (MIR) wavelengths. Since the optical absorption of both amorphous and crystalline GST can be reduced at a longer wavelength, we demonstrate that the optical loss of the phase shifter can be reduced at MIR wavelengths. The measured optical loss per π phase shift of a phase-change material (PCM) phase shifter at 2.32 µm wavelength is 2.6 dB/π, which is more than 80 times smaller than that at 1.55 µm wavelength (21.7 dB/π) and more than 5 times smaller than that at 1.92 µm wavelength (9.7 dB/π). Moreover, resonance wavelength tuning of an add-drop micro-ring resonator using a PCM phase shifter at 2.32 µm wavelength is demonstrated owing to the low-loss optical phase shift. These findings reinforce the applicability of the approach toward a low-loss optical phase shifter based on PCMs operating at MIR wavelengths on a Si photonic platform for quantum computing, sensing, and optical communication.

An ultrasensitive photonic sensor is vital for sensing matter with absolute specificity. High specificity terahertz photonic sensors are essential in many fields, including medical research, clinical diagnosis, security inspection, and probing molecular vibrations in all forms of matter. Widespread photonic sensing technology detects small frequency shifts due to the targeted specimen, thus requiring ultra-high quality (Q) factor resonance. However, the existing terahertz waveguide resonating structures are prone to defects, possess limited Q-factor, and lack the feature of chip-scale CMOS integration. Here, inspired by the topologically protected edge state of light, we demonstrate a silicon valley photonic crystal based ultrasensitive, robust on-chip terahertz topological insulator sensor that consists of a topological waveguide critically coupled to a topological cavity with an ultra-high quality (Q) factor of 𝑄=0.14×106. Topologically protected cavity resonance exhibits strong resilience against disorder and multiple sharp bends. Leveraging on the extremely narrow linewidth (2.3 MHz) of topological cavity resonance, the terahertz sensor shows a record-high figure of merit of 4000 RIU mm−1. In addition to the spectral shift, the intensity modulation of cavity resonance offers an additional sensor metric through active tuning of critical coupling in the waveguide-cavity system. We envision that the ultra-high Q photonic terahertz topological sensor could have chip-scale biomedical applications such as differentiation between normal and cancerous tissues by monitoring the water content. National Research Foundation (NRF) Published version All the authors acknowledge the research funding support from the National Research Foundation (NRF) Singapore (Grant No. NRF-CRP-2019-0005).

Phase change materials provide unique reconfigurable properties for photonic applications that mainly arise from their exotic characteristic to reversibly switch between the amorphous and crystalline nonvolatile phases. Optical pulse based reversible switching of nonvolatile phases is exploited in various nanophotonic devices. However, large area reversible switching is extremely challenging and has hindered its translation into a technologically significant terahertz spectral domain. Here, this limitation is circumvented by exploiting the semiconducting nature of germanium antimony telluride (GST) to achieve dynamic terahertz control at picosecond timescales. It is also shown that the ultrafast response can be actively altered by changing the crystallographic phase of GST. The ease of fabrication of phase change materials allows for the realization of a variable ultrafast terahertz modulator on a flexible platform. The rich properties of phase change materials combined with the diverse functionalities of metamaterials and all-optical ultrafast control enables an ideal platform for design of efficient terahertz communication devices, terahertz neuromorphic photonics, and smart sensor systems. Agency for Science, Technology and Research (A*STAR) Ministry of Education (MOE) National Research Foundation (NRF) The authors acknowledge research funding support from the Ministry of Education, Singapore (AcRF Tier 1, Grant RG191/17, MOE2017-T2-1-110, MOE2019-T2-1-058, and MOE2016-T3-1-006(S)), Advanced Manufacturing and Engineering (AME) Programmatic grant (A18A5b0056) from Agency for Science, Technology and Research (A*STAR) and NRF CRP on Oxide Electronics on silicon Beyond Moore (NRF-CRP15-2015-01).

We demonstrated a low-loss, non-volatile optical phase shifter based on phase-change material, Ge 2 Sb 2 Te 5 operating at a $2\ \mu m$ wavelength. Using a $10-\mu m$ -long phase-change phase shifter, we obtained a $0.7\pi$ phase shift at $2\ \mu m$ with approximately 10 times smaller loss per phase shift than at $1.55\ \mu m$ .

Spatiotemporal manipulation of electromagnetic waves has recently enabled a plethora of exotic optical functionalities, such as non-reciprocity, dynamic wavefront control, unidirectional transmission, linear frequency conversion, and electromagnetic Doppler cloak. Here, an additional dimension is introduced for advanced manipulation of terahertz waves in the space-time, and frequency domains through integration of spatially reconfigurable microelectromechanical systems and photoresponsive material into metamaterials. A large and continuous frequency agility is achieved through movable microcantilevers. The ultrafast resonance modulation occurs upon photoexcitation of ion-irradiated silicon substrate that hosts the microcantilever metamaterial. The fabricated metamaterial switches in 400 ps and provides large spectral tunability of 250 GHz with 100% resonance modulation at each frequency. The integration of perfectly complementing technologies of microelectromechanical systems, femtosecond optical control and ion-irradiated silicon provides unprecedented concurrent control over space, time, and frequency response of metamaterial for designing frequency-agile spatiotemporal modulators, active beamforming, and low-power frequency converters for the next generation terahertz wireless communications. National Research Foundation (NRF) The authors acknowledge the research funding support from National Research Foundation (NRF) Singapore and Agence Nationale de la Recherche (ANR), France- NRF2016- ANR004 (M4197003), NRF CRP on Oxide Electronics on silicon Beyond Moore (NRF-CRP15-2015-01) and Advanced Manufacturing and Engineering (AME) Programmatic grant (A18A5b0056) from Agency for Science, Technology and Research (A*STAR).

Tunable transmission filters are essential for various key terahertz applications including miniaturized spectrometers, hyperspectral imagers and channel selectors in high-speed wireless communication systems. However, tunable terahertz transmission filters have remained elusive, so far. Here, we report an electromechanically reconfigurable microcantilever integrated complementary metamaterial to function as a frequency-agile bandpass transmission filter at terahertz spectral region. A large tunable range of 70% at central frequency of 0.455 THz is designed and the experimental results show switching of terahertz transmission frequency range of 54% at central frequency of 0.5 THz. The proposed electromechanically reconfigurable metadevice provides large tunable range, uses electrical control, and is fabricated using CMOS compatible materials and process, thus making it highly attractive for the realization of plethora of miniaturized high-performance terahertz components and devices.

Terahertz (THz) part of the electromagnetic spectrum (0.1–10 THz) holds the key for next-generation high-speed wireless communication, non-destructive biosensing, fingerprint chemical detection and imaging for astronomy and security surveillance. The limited THz response of naturally occurring materials had left a technological gap in the THz region of the electromagnetic spectrum. Artificially engineered materials termed as ‘metamaterials’, have shown great potential in THz wave interaction and its active counterpart termed as ‘metadevices’ have been widely reported for on-demand manipulation of THz waves. One of the most efficient means of realizing metadevices is to reconfigure the shape of unit cells and hence the corresponding THz response. The 50+ years of development in microelectromechanical systems (MEMS) and the wide array of microactuator designs provide a perfect platform to achieve structural reconfiguration of microscale metamaterial unit cells in both in-plane and out-of-plane directions. In this review, we present a comprehensive overview of various MEMS approaches adopted for the demonstration of THz metadevices, their advantages and limitations. The future research directions of THz MEMS metadevices are also discussed. The seamless integration of matured MEMS technology with incipient THz metamaterials provides significant advantages in terms of enhanced performances, advanced functionalities and large scale manufacturability, that is critical for the development of future THz technologies.

We demonstrate a new CMOS compatible metal–dielectric–metal (Mo–AlN–Mo) platform of metamaterial absorber for refractory and narrowband applications at mid-IR. Comparison with the recently reported CMOS compatible plasmonic TiN shows superior reflectivity of Mo thin film at mid-IR wavelengths (3–8 μm), while AlN provides large thermal stability and thermal conductivity, mid- to far-IR transparency and both second and third order nonlinear effect and satisfies the matching condition of thermal expansion coefficient with Mo toward minimizing the thermal stress. We demonstrate the proof-of-concept of reducing the thermal stress up to 400° by considering a high stress, CMOS platform of SiO2. We further report temporal measurement of the resonance intensity and wavelength-shift of the absorber structures and confirm the robust performance of the platform over prolonged heating. Finally, we propose a method to perform surface enhanced infrared absorption (SEIRA) spectroscopy of biological samples demanding bioc...

Metamaterials are well-known for their tunable and unnatural properties that are not usually accessible in natural materials. Recently, there is a large growing interest in the actively tunable metamaterials, where their structural/optical can be actively tuned using an external means, such as optical pulse, thermal and electrical controls. Among them the microelectromechanical systems (MEMS) based metamaterials have given useful features of multiple controls in engineering their structural geometry in all the three-spatial directions of the sample at the THz frequencies. This allows to probe and engineer unique and intriguing near-field coupling phenomena in metamaterials, thereby obtaining the electro-optical properties on demand.

The strikingly contrasting optical properties of various phases of chalcogenide phase change materials (PCM) has recently led to the development of novel photonic devices such as all-optical non-von Neumann memory, nanopixel displays, color rendering, and reconfigurable nanoplasmonics. However, the exploration of chalcogenide photonics is currently limited to optical and infrared frequencies. Here, a phase change material integrated terahertz metamaterial for multilevel nonvolatile resonance switching with spatial and temporal selectivity is demonstrated. By controlling the crystalline proportion of the PCM film, multilevel, non-volatile, terahertz resonance switching states with long retention time at zero hold power are realized. Spatially selective reconfiguration at sub-metamaterial scale is shown by delivering electrical stimulus locally through designer interconnect architecture. The PCM metamaterial also features ultrafast optical modulation of terahertz resonances with tunable switching speed based on the crystalline order of the PCM film. The multilevel nonvolatile, spatially selective, and temporally tunable PCM metamaterial will provide a pathway toward development of novel and disruptive terahertz technologies including spatio-temporal terahertz modulators for high speed wireless communication, neuromorphic photonics, and machine-learning metamaterials.

We experimentally show the excitation of sharp Fano resonances in a MEMS reconfigurable metasurface exhibiting multiple-input-output states in its electro-optical properties. Further, a set of composite logic gates such as exclusive-OR and XNOR are readout using THz pulse.

Excitation of the non-radiative eigenmodes in photonics and metamaterials above their light cone, that is, the bound states in the continuum (BICs), has drawn fundamental and technological interest. However, the current studies on photonic BICs are mainly restricted to the characterization of the eigenmodes with fixed point group. Here, a fourfold rotationally symmetric (C4v) array of silicon resonators in a square lattice is investigated that reveals the traditional symmetry-protected (Γ-state) and accidental (off-Γ) BICs in the momentum space. Upon symmetry-breaking, BICs at Γ-state transform into quasi-states with decaying quality factors that obey the inverse-square law α−2 as a function of the structural asymmetry parameter α. Surprisingly, at larger asymmetries, multiple new states of extended BICs are observed beyond typical inverse-square law behavior. The structural asymmetry of a resonator provides an important degree of freedom for exploring the Friedrich–Wintgen and new accidental BICs that are topologically protected by robust polarization vortices. The extended family of sharp BIC resonances in all-dielectric silicon metasurfaces is extremely important for scalable applications in terahertz sensing, higher harmonic generation, and ultralow-power switching. Ministry of Education (MOE) The authors acknowledge the financial support from the Singapore Ministry of Education (MOE) Academic Research Fund (AcRF) Tier 1 grant RG96/19, Tier 2 grant MOE2017-T2-1-110, and Tier 3 grant MOE2016-T3-1-006.

1 wileyonlinelibrary.com C o m m u n iC a io n and their possible applications. The ultimate form of active manipulation of EIT phenomenon will be when all three primary parameters are controlled independently. The independent control of individual resonators demands for the controllability at unit cell level, and conventional approaches such as optical pumping of photoconductive elements or thermally controlled superconductor are restricted to provide only global control. Recently, microelectromechanical systems (MEMS) based tunable metamaterials have been reported to achieve controllability at unit cell level, along with the added advantage of being electrically controlled, miniaturized size and enhanced electrooptic performance. The versatility of MEMS design has enabled active manipulation of numerous THz properties such as magnetic resonance,[19–22] electrical resonance,[23–25] anisotropy,[26] broadband response,[27] isotropic resonance switching[28] multiresonance switching,[29–31] and coupling strength between resonators.[32] The enhanced controllability and direct integration of MEMS actuators into metamaterial unit cell geometry is an ideal fit for the realization of selective control of coupled mode resonators. In this Communication, reconfigurable metamaterial with independently controlled bright and dark mode resonators is proposed for advanced manipulation of the classical analog of EIT and slow light effects in THz spectral region. The active control of bright mode resonator enables modulation of EIT intensity, while the tuning of dark mode resonance causes the EIT peak to tune in frequency. Furthermore, simultaneous switching of bright and dark mode resonators results in dynamic switching of the system between coupled and uncoupled states. The proposed approach of selective reconfiguration can be scaled for multiresonator systems, which can be coupled either through inductive, capacitive, or conductive means. The metamaterial consists of 80 × 80 periodic array of cut wire resonator (CWR) with closely placed split ring resonators (SRRs), as shown in Figure 1 and Figure 2. The periodicity of unit cell is 100 μm along both axial directions. The CWR has length, lC = 60 μm and width, wC = 5 μm, respectively. The SRRs have a base length, bS = 30 μm, side length, lS = 20 μm, and split gap, gS = 4 μm. The SRRs are placed at a distance of S = 2 μm from the CWR. When the polarization of the excitation field is along the CWR arm, the dipole mode resonance of the CWR will be the bright mode and the inductive-capacitive (LC) mode of SRR resonance acts as the dark mode. Thus for the incident THz polarization, the direct excitation of the bright mode induces image charges on the nearby SRRs through nearfield inductive coupling, thereby exciting the LC resonance of the SRRs. These bright-dark resonances have contrasting line widths with identical resonance frequencies and under a strong coupling regime they experience an EIT-type of interference that gives rise to a sharp transmission peak. Thus, through Active Control of Electromagnetically Induced Transparency Analog in Terahertz MEMS Metamaterial

Tracking human gestures and movement is crucial for development of advanced input technologies and human computer interfaces. In this paper we demonstrate a novel flexible, low cost and wearable self-powered motion sensor to detect human finger motion for static positions and dynamic motion. As the finger moves to different positions, the capacitance between finger skin and device electrode changes. The device uses the change in capacitive coupling with human skin (epidermis) to measure the static and dynamic positions of finger. It is proposed that the device can utilize low frequency electric fields generated by the household power lines and equipment to passively sense the human finger movement. The device also acts as a triboelectric nanogenerator using outer epidermis as an active triboelectric layer for surface charge generation. It is shown to generate a maximum voltage of 70 V and a current area density of 2.7 μA/cm2 at a load resistance of 5 MΩ. As a demonstration of a new approach for detection of static finger gestures and dynamic motion which is self-powered and intuitive, this work contributes towards development of self-powered sensors for human computer interfacing and osteoarthritis rehabilitation applications.

A microelectromechanically reconfigurable digital metamaterial for dynamic switching of terahertz anisotropy is experimentally demonstrated. Out-of-plane reconfigurable microcantilevers placed in orthogonal direction with isolated control allows active making and breaking of unit cell symmetry. The electrical control, miniaturized size, and ease of process integration make the reported microelectromechanical system metamaterial ideal for the realization of THz polarimetric devices. ASTAR (Agency for Sci., Tech. and Research, S’pore) MOE (Min. of Education, S’pore) Accepted version

A broad range of dynamic metasurfaces has been developed for manipulating the intensity, phase and wavefront of electromagnetic radiation from microwaves to optical frequencies. However, most of these metasurfaces operate in single-input–output state. Here, we experimentally demonstrate a reconfigurable MEMS Fano resonant metasurface possessing multiple-input–output (MIO) states that performs logic operations with two independently controlled electrical inputs and an optical readout at terahertz frequencies. The far-field behaviour of Fano resonance exhibits XOR and XNOR operations, while the near-field resonant confinement enables the NAND operation. The MIO configuration resembling hysteresis-type closed-loop behaviour is realized through inducing electromechanically tuneable out-of-plane anisotropy in the near-field coupling of constituent resonator structures. The XOR metamaterial gate possesses potential applications in cryptographically secured terahertz wireless communication networks. Furthermore, the MIO features could lay the foundation for the realization of programmable and randomly accessible metamaterials with enhanced electro-optical performance across terahertz, infrared and optical frequencies., Here, the authors demonstrate excitation and active tuning of sharp Fano resonances in a MEMS reconfigurable metasurface possessing multiple-input-output states. They realize XOR, XNOR, NOT and NAND logic gate operations by using two independently controllable electrical inputs and an optical readout using terahertz beam.

The mechanism of Cooper pair formation and its underlying physics has long occupied the investigation into high temperature (high-Tc ) cuprate superconductors. One of the ways to unravel this is to observe the ultrafast response present in the charge carrier dynamics of a photoexcited specimen. This results in an interesting approach to exploit the dissipation-less dynamic features of superconductors to be utilized for designing high-performance active subwavelength photonic devices with extremely low-loss operation. Here, dual-channel, ultrafast, all-optical switching and modulation between the resistive and the superconducting quantum mechanical phase is experimentally demonstrated. The ultrafast phase switching is demonstrated via modulation of sharp Fano resonance of a high-Tc yttrium barium copper oxide (YBCO) superconducting metamaterial device. Upon photoexcitation by femtosecond light pulses, the ultrasensitive cuprate superconductor undergoes dual dissociation-relaxation dynamics, with restoration of superconductivity within a cycle, and thereby establishes the existence of dual switching windows within a timescale of 80 ps. Pathways are explored to engineer the secondary dissociation channel which provides unprecedented control over the switching speed. Most importantly, the results envision new ways to accomplish low-loss, ultrafast, and ultrasensitive dual-channel switching applications that are inaccessible through conventional metallic and dielectric based metamaterials. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore) Accepted version

Advances in plasmonic metamaterials have been rapidly evolving with innovations aimed at developing metadevices for real-world applications. In reality, energy losses in plasmonic systems are prevalent and it is of paramount importance to come up with solutions that could overcome the limitations that impede further advancements toward the miniaturization of optoelectronic metadevices. High-Q Fano resonance as a scattering phenomenon can be easily triggered by introducing asymmetry into plasmonic systems, and thus it offers a simple approach for reducing radiative losses through lineshape engineering. High-Q Fano resonance possesses narrow linewidth and intensely confined electromagnetic fields, which makes it viable for widespread applications. The purpose of this review is to consolidate the current advances and contributions that high-Q Fano resonance has made in the metamaterial community. Two general modes of energy loss including radiative and nonradiative losses are introduced and possible ways to overcome these challenges are examined. Furthermore, applications based on high-Q Fano resonance including sensors, lasing spasers, and optical switches are discussed, embracing the future of Fano resonance based high performance photonic technologies. MOE (Min. of Education, S’pore) Accepted version

The terahertz spectral region has received tremendous attention for label free chemical and biological sensing, due to the presence of molecular fingerprints, low energy characteristics, and remote sensing capabilities. However, a major hindrance for the realization of a high performance terahertz bio-chemical sensor comes from the large absorption of terahertz waves by aqueous solution. Here, we overcome this limitation by confining the analyte-aqueous solution in a nanovolumetric fluidic chamber, integrated on metamaterial resonant cavities. The metamaterial resonators confine electromagnetic fields in extremely subwavelength space and hence allow for the enhanced interaction between the nanovolumetric analyte-aqueous solution and terahertz waves, while minimizing the absorption loss. We compare the sensing performance of split ring resonator and Fano resonator systems as metamaterial resonators. As a demonstration of chemical sensing, three alcoholic solutions with different concentrations were measured. Selective adenosine triphosphate (ATP) sensing capability was examined through ATP aptamer functionalization on gold metamaterials, where a decrease in the transmittance value was observed as the ATP concentration increased. The proposed sensing approach has the potential to be an effective tool for molecular analysis through exploiting the advantages offered by low energy terahertz, subwavelength metamaterial resonators and nanofluidic technologies. MOE (Min. of Education, S’pore) NMRC (Natl Medical Research Council, S’pore) Published version

We experimentally demonstrate a simple approach for surface current engineering in a cross-coupled bow-tie nanoantenna by inserting a plasmonic cavity that simultaneously offers (i) improved Fano-like dipolar resonance contrast, (ii) electrically induced magnetic resonance, and (iii) enhanced sensitivity. By introducing a small geometric perturbation, we propose two physical parameters, offset (f) and split gap (s), for strong modulation of resonance location and intensity. We report at least 3.75-fold better dipolar resonance compared with the conventional design and demonstrate a unique mechanism for exciting magnetic plasmonic resonance. Finally, we obtain a large wavelength shift of 777.5 and 904 nm per refractive index unit (RIU) with a thin PMMA coating (110 nm) for the dipolar resonance and magnetic resonance, respectively. Numerical study indicates the potential of the proposed bow-tie nanoantenna array structure with a self-similar plasmonic cavity (SSP BNA) for sensitive recording of binding eve...

Controlling the phase of local radiation by using exotic metasurfaces has enabled promising applications in a diversified set of electromagnetic wave manipulation such as anomalous wavefront deflection, flat lenses, and holograms. Here, we theoretically and experimentally demonstrate an active phase transition in a micro-electromechanical system-based metadevice where both the phase response and the dispersion of the metamaterial cavity are dynamically tailored. The phase transition is determined by the radiative and the absorptive losses in a metal-insulator-metal cavity that obeys the coupled-mode theory. The complete understanding of the phase diagram in a reconfigurable configuration would open up avenues for designing multifunctional metadevices that can be actively switched between different phases leading to a plethora of applications in polarization control, beam deflectors, and holographic metamaterials.

We experimentally report the bidirectional reconfiguration of an out-of-plane deformable microcantilever based metamaterial for advanced and dynamic manipulation of terahertz waves. The microcantilever is made of a bimaterial stack with a large difference in the coefficient of thermal expansion of the constituent materials. This allows for the continuous deformation of microcantilevers in upward or downward direction in response to positive or negative temperature gradient, respectively. The fundamental resonance frequency of the fabricated microcantilever metamaterial is measured at 0.4 THz at room temperature of 293 K. With decreasing temperature, the resonance frequency continuously blue shifts by 30 GHz at 77 K. On the other hand, with increasing temperature, the resonance frequency gradually red shifts by 80 GHz and saturates at 0.32 THz for 400 K. Furthermore, as the temperature is increased above room temperature, which results in the downward actuation of the microcantilever, a significant resonance line-narrowing with an enhanced quality factor is observed due to tight field confinement in the metamaterial structure. The thermal control of the microcantilever possesses numerous inherent advantages such as enhanced tunable range (∼37.5% in this work compared to previously reported microcantilever metamaterials), continuous tunability, and repeatable operations. The microcantilever metamaterial also shows high robustness to operate at cryogenic conditions and hence opens up the possibility of using meta-devices in harsh environments such as space, polar, and deep sea applications. Published version

Free-standing polycrystalline silicon (Si) based photonic crystal (PhC) membranes with etched circular and square holes are developed to display high reflectivity in the mid-infrared (MIR) region. Greater than 90% reflection was measured in the MIR wavelengths around 3.58 μm. By using square air holes in the PhC membrane, the mechanical strength of the polycrystalline Si membrane can be enhanced as square air holes have a lower filling factor of 36% of air holes, compared to 49% in circular air holes while keeping the reflectance around 3.45 μm more than 90%. Such Si PhC membranes offer opportunities for specific applications like filters. To illustrate the feasibility of such devices, simulation works are done by configuring two Si PhC membranes to create a Fabry-Perot interferometer operating in MIR region. The filtered peak shows a full width half maximum of 0.08 nm which corresponds to a quality factor of around 43800, thus demonstrating the possibility of high-resolution applications such as gas sensing and hyperspectral imaging.

Coupling of diffracted waves in gratings with the waveguide modes gives rise to the guided mode resonances (GMRs). The GMRs provide designer linewidth and resonance intensity amidst a broad background, and thus have been widely used for numerous applications in visible and infrared spectral regions. Here, terahertz GMRs are demonstrated in low‐loss, all‐dielectric metasurfaces, which are periodic square lattices of silicon cuboids on quartz substrates. The silicon cuboid lattice simultaneously acts as a diffraction grating and an in‐plane slab waveguide, thereby resulting in the formation of terahertz GMRs. At oblique incidence, two distinct frequency detuned GMRs are observed. The frequency difference between these two GMRs increases at larger angle of incidence. However, extremely small angle of incidence causes destructive interference between these counter‐propagating GMRs that leads to a nonradiative symmetry‐protected bound state in the continuum. GMRs in all‐dielectric silicon metasurfaces can have potential applications in the realization of efficient terahertz devices such as high‐Q transmission filters with angular spectral selectivity, ultrafast modulators, and free‐space couplers. MOE (Min. of Education, S’pore) Accepted version

We experimentally demonstrate the active manipulation of terahertz (THz) waves using a periodic array of electrostatically actuated subwavelength microelectromechanical system cantilevers, which effectively behave like a metamaterial. The design methodology for achieving desired ON- and OFF-state resonance frequencies through electromechanical optimization is presented. The microcantilever metamaterial has a switching range of 0.29 THz and a modulation depth of 60% at 0.59 THz. Utilizing metal layer thickness to optimize the devices, an improvement of 40% is achieved in switching range. The microcantilever metamaterials are highly miniaturized, extremely scalable, and electrically controlled with attractive electro-optic performance. Multiple cantilevers can be placed in a desired fashion to form complex unit cell geometry to realize advanced THz manipulation, such as polarization switching, bandwidth tunable filters, multicolor imagers, and so on. [2015-0090]

This work presents the polarization control of interchangeable multispectral absorption based on the dual-band metamaterial absorber in split mode. Large modulation depth of absorption is obtained during multi-band transition through polarization control.

Out-of-plane reconfigurable microcantilevers integrated into metamaterial unit cells is explored as a technological platform for the realization of advanced polarization control of THz waves. By varying the positioning of the microcantilever structures and isolating the control signals to these cantilevers, various designs are reported for active polarization-dependent resonance switching, polarization-independent resonance switching and polarization response switching characteristics. These metamaterials will greatly aid in the development of numerous polarimetric devices operating in terahertz spectral region.

Metamaterials operating in terahertz (THz) region is an attractive platform for the detection of micro-sized particles. However, only certain location in the metamaterial pattern offers maximum sensitivity. In this work, microfluidic structure was integrated with THz metamaterial in order to precisely handle the position of particles. This enables sensitive and effective detection of microorganism detection in the THz spectral range.

Classical analogue of electromagnetically induced transparency mediated by near field coupling in meta-atom resonators form the basis for design of slow light metamaterial devices. Here, we experimentally demonstrate the active control of individual resonators in the near-field coupled system. This allows for active modulation and spectral tuning of the transparency peak and hence the slow light behavior in terahertz spectral range.

This work presents a graphene plasmonic resonator whose electrically tunable capability at mid-infrared spectral range is experimentally demonstrated and characterized. We believe this device can pave a way for a variety of sensing applications.

We report the first thermal study of a triple band plasmonic nanoantenna strongly coupled to a molecular mode at mid IR wavelength (MW IR). The hybrid plasmonic structure supports three spatially and spectrally variant resonances of which two are magnetic and one is dipolar in nature. A hybridized mode is excited by coupling the structure’s plasmonic mode with the vibrational mode of PMMA at 5.79 μm. Qualitative agreement between the spectral changes in simulation and experiment clearly indicates that resistive heating is the dominant mechanisms behind the intensity changes of the dipolar and magnetic peaks. The study also unveils the thermal insensitivity of the coupled mode intensity as the temperature is increased. We propose a mechanism to reduce the relative intensity change of the coupled mode at elevated temperature by mode detuning and surface current engineering and demonstrate less than 9% intensity variation. Later, we perform a temperature cycling test and investigate into the degradation of the Au-PMMA composite device. The failure condition is identified to be primarily associated with the surface chemistry of the material interface rather than the deformation of the nanopatterns. The study reveals the robustness of the strongly coupled hybridized mode even under multiple cycling.

We report experimental results of the active switching of electromagnetically induced transparency (EIT) analogue by controlling the dark mode excitation pathways in a microelectromechanical system based tri-atomic metamolecule, operating in the terahertz spectral region. The tri-atomic metamolecule consists of two bright cut wire resonators (CWRs) on either side of the dark split ring resonators (SRRs). Each of the CWRs can independently excite the dark inductive-capacitive resonance mode of the SRRs through inductive coupling, and this allows for the dual pathways of dark mode excitation. The CWRs are made movable along the out-of-plane direction and electrically isolated to achieve selective reconfiguration. Hence, by controlling the physical position of these CWRs, the excitation pathways can be actively reconfigured. This enables the strong excitation of EIT analogue at 0.65 THz, only when one of the pathways is made accessible. Moreover, the transparency peak is completely modulated when both pathways are made either inaccessible or equally accessible. The proposed approach of realizing independent control of constituent resonators in a multi-resonator coupled system, enables the realization of efficient slow light devices and tunable high-Q resonators in terahertz spectral region. NRF (Natl Research Foundation, S’pore) Published version

Microelectromechnical system based metamaterial with independent control of TE and TM mode response is experimentally reported. At 0.8 THz, the metamaterial can be reconfigured to be either TE responsive, TM responsive, polarization insensitive or non-responsive for both polarization of incident light by controlling the states of the microcantilevers forming the unit cell. These metamaterials are highly attractive for the development of THz tunable wave plates, filters and polarimetric devices.

We demonstrate an approach to enhance dipolar resonance contrast and magnetic resonance in highly dense cross coupled bow-tie nanostructure. We further observe strong hybridization of the Fano effect with the vibrational mode of the PMMA organic substance.