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Recently, emerging two-dimensional (2D) germanium selenide (GeSe) has drawn lots of attention due to its in-plane anisotropic properties and great potential for optoelectronic applications such as in solar cells. However, methods are still sought to enhance its interaction with light to enable practical applications. Herein, we numerically investigate the localized plasmon response of monolayer GeSe nanoribbon arrays systematically, and the results show that localized surface plasmon polaritons in the far-infrared range with anisotropic behavior can be efficiently excited to enhance the light-matter interaction. We further show that the plasmon response of monolayer GeSe nanoribbons could be tuned effectively through the nanoribbon width, local refractive index, substrate thickness and carrier concentration, pointing out the ways for controlling the localized plasmon response. In the case of monolayer GeSe nanoribbons on a substrate of finite thickness, a Fabry-Pérot-like (FP-like) quantitative model has been proposed to explain the overall spectral response originating from overlapped FP and plasmon modes, and it matches well with the simulation results. All in all, we investigate the plasmon response of the novel 2D GeSe nanoribbons thoroughly for the first time, bringing opportunities for potential applications of novel polarization-dependent optoelectronic devices.

Fundamental researches and explorations based on transition metal dichalcogenides (TMDCs) mainly focus on their monolayer counterparts, where optical densities are limited owing to the atomic monolayer thickness. Photoluminescence (PL) yield in bilayer TMDCs is much suppressed owing to indirect-bandgap properties. Here, optical properties are explored in artificially twisted bilayers of molybdenum disulfide (MoS2). Anomalous interlayer coupling and resultant giant PL enhancement are firstly observed in MoS2 bilayers, related to the suspension of the top layer material and independent of twisted angle. Moreover, carrier dynamics in MoS2 bilayers with anomalous interlayer coupling are revealed with pump-probe measurements, and the secondary rising behavior in pump-probe signal of B-exciton resonance, originating from valley depolarization of A-exciton, is firstly reported and discussed in this work. These results lay the groundwork for future advancement and applications beyond TMDCs monolayers.

Nonlinear plasmons are becoming an appealing and intriguing research area due to their remarkable light concentration and manipulation abilities. In this work, the nonlinear absorption (NLA) phenomena of polarized nanoantenna arrays coupled with the low dimensional topological insulator Bi2Se3 are studied at different excitation wavelengths. Our experimental results indicate that a significant enhancement in the linear absorption coefficient is achieved by localized surface plasmon (LSP) resonance, with enhancement factors that are 10- and 8-fold in magnitude for the cases of 800 nm (y polarization) and 970 nm (x polarization), respectively. Moreover, by polarization sensitive studies under 800 nm laser excitation, this new Bi2Se3-Au nanoantenna hybrid-structure exhibits adverse absorption responses of enhancement and suppression compared to pure Bi2Se3 film, providing excellent potential for applications in information converters. In particular, under 800 nm pump light (10 GW cm-2), the transmittance intensity of 450 nm or 1064 nm continuous wave (CW) probe light alters back and forth when the polarization direction changes by 90°. Thus, "ON" and "OFF" modes of this Bi2Se3-Au nanoantenna hybrid structure-based switch are achieved by using 450 nm and 1064 nm light, respectively, with a corresponding modulation depth of 3.4% and 21.9%, which can be applied in versatile photonic devices.

Standing on the potential for high-speed modulation and switching in the terahertz (THz) regime, all-optical approaches whose response speeds mainly depend on the lifetime of nonequilibrium free carriers have attracted a tremendous attention. Here, we establish a novel bi-direction THz modulation experiment controlled by femtosecond laser for new functional devices. Specifically, time-resolved transmission measurements are conducted on a series of thin layers Bi2Se3 films fabricated straightforwardly on Al2O3 substrates, with the pump fluence range from 25 μJ/cm2 to 200 μJ/cm2 per pulse. After photoexcitation, an ultrafast switching of THz wave with a full recovery time of ~10 ps is observed. For a longer timescale, a photoinduced increase in the transmitted THz amplitude is found in the 8 and 10 quintuple layers (QL) Bi2Se3, which shows a thickness-dependent topological phase transition. Additionally, the broadband modulation effect of the 8 QL Bi2Se3 film is presented at the time delays of 2.2 ps and 12.5 ps which have a maximum modulation depth of 6.4% and 1.3% under the pump fluence of 200 μJ/cm2, respectively. Furthermore, the absorption of α optical phonon at 1.9 THz shows a time-dependent evolution which is consistent with the cooling of lattice temperature.

Advances in tunable metamaterials/metasurfaces facilitates their utilization in novel optical components, and lead to many breakthroughs in light tailoring by giving birth to diverse spatiotemporal dynamics. In the ascendant field of terahertz (THz) photonics, the ultrafast modulation is the fundamental process of technological advancements in high-speed wireless communications, sensing, and imaging. However, the current research efforts have been mainly devoted to studies of single functionality under the control of one stimulus, which has plateaued in terms of innovative new features. Here, building on the incident angle-induced C2 symmetry breaking of split ring pairs, we experimentally demonstrate extremely versatile, ultrafast THz switching behaviors at continuously alterable resonant states. The direction-controlled resonance hybridization provides another excellent degree of routing freedom, owing to its robustness, simplicity, and wide tunability. By leveraging such virtues, single LC mode and EIT-like resonance under normal and oblique incidence conditions are both effectively switched-off by means of photon injection. Considering the ultrashort lifetime of free carriers in MoSe2 crystal, the corresponding transient dynamics show an ultrafast recovery time within 700 ps. The strategy proposed here is a viable pathway for multidimensional THz wave manipulation, which gears up a crucial step for diversified functionalities in deployable metaphotonic devices.

Electron-phonon scattering is the key process limiting the efficiency of modern nanoelectronic and optoelectronic devices, in which most of the incident energy is converted to lattice heat and finally dissipates into the environment. Here, we report an acoustic phonon recycling process in graphene-WS2 heterostructures, which couples the heat generated in graphene back into the carrier distribution in WS2. This recycling process is experimentally recorded by spectrally resolved transient absorption microscopy under a wide range of pumping energies from 1.77 to 0.48 eV and is also theoretically described using an interfacial thermal transport model. The acoustic phonon recycling process has a relatively slow characteristic time (>100 ps), which is beneficial for carrier extraction and distinct from the commonly found ultrafast hot carrier transfer (~1 ps) in graphene-WS2 heterostructures. The combination of phonon recycling and carrier transfer makes graphene-based heterostructures highly attractive for broadband high-efficiency electronic and optoelectronic applications. Here, the authors perform transient absorption microscopy on graphene-WS2 heterostructures, and identify a phonon recycling process that couples the heat generated in graphene back into the carrier distribution in WS2 with a picosecond characteristic time.

Incorporating active materials into metamaterials is expected to yield exciting advancements in the unprecedented versatility of dynamically controlling optical properties, which sheds new light on the future optoelectronics. The exploration of emerging semiconductors into terahertz (THz) meta‐atoms potentially allows achieving ultrafast nanodevices driven by various applications, such as biomedical sensing/imaging, ultrawide‐band communications and security scanners. However, ultrafast optical switching of THz radiation is currently limited to a single level of tuning speed, which is a main hurdle to achieve multifunctionalities. Here, a hybrid metadevice which can realize the pump‐wavelength controlled ultrafast switching response by the functionalization of double photoactive layers is demonstrated experimentally. A whole cycle of electromagnetically induced transparency switching with a half‐recovery state changes from 0.78 ns to 8.8 ps as pump wavelength varies from near infrared to near ultraviolet regions. The observed pump‐color selective switching speed changing from nanosecond scale to picosecond scale is ascribed to the wavelength‐dependent penetration length of Ge and the contrasting defect states between noncrystalline Ge and epitaxial Si layers. It is believed that the schemes regarding pump‐color controllable ultrafast switching behavior introduced here can inspire more innovations across the field of ultrafast photonics and can boost the reconfigurable metamaterial applications., All‐optical terahertz modulators with pump‐color selective ultrafast switching speed are realized by combining double layers of semiconductors with electromagnetically induced transparency meta‐atoms. The transmission/group delay of terahertz wave is modulated at contrasting switching speeds from sub‐nanoseconds to picoseconds when pumped by the near‐infrared and near‐ultraviolet beams. This work opens up new avenues for manipulating the dynamics of ultrafast metaphotonic devices.

The energy dissipation issue has become one of the greatest challenges of the modern electronic industry. Incorporating graphene into the electronic devices has been widely accepted as a promising approach to solve this issue, due to its superior carrier mobility and thermal conductivity. Here, using Raman spectroscopy and infrared thermal microscopy, we identify the energy dissipation behavior of graphene device with different thicknesses. Surprisingly, the monolayer graphene device is demonstrated to have a comparable energy dissipation efficiency per unit volume with that of a few-layer graphene device. This has overturned the traditional understanding that the energy dissipation efficiency will reduce with the decrease of functional materials dimensions. Additionally, the energy dissipation speed of the monolayer graphene device is very fast, promising for devices with high operating frequency. Our finding provides a new insight into the energy dissipation issue of two-dimensional materials devices, which will have a global effect on the development of the electronic industry.

Due to the theoretical prediction of large tunneling magnetoresistance (TMR), graphene-based magnetic tunneling junction (MTJ) has become an important branch of high-performance spintronics device. In this paper, the non-collinear spin filtering and transport properties of MTJ with the Ni/tri-layer graphene/Ni structure were studied in detail by utilizing the non-equilibrium Green’s formalism combined with spin polarized density functional theory. The band structure of Ni–C bonding interface shows that Ni–C atomic hybridization facilitates the electronic structure consistency of graphene and nickel, which results in a perfect spin filtering effect for tri-layer graphene-based MTJ. Furthermore, our theoretical results show that the value of tunneling resistance changes with the relative magnetization angle of two ferromagnetic layers, displaying the anisotropic tunneling behavior of graphene-based MTJ. This originates from the resonant conduction states which are strongly adjusted by the relative magnetization angles. In addition, the perfect spin filtering effect is demonstrated by fitting the anisotropic conductance with the Julliere’s model. Our work may serve as guidance for researches and applications of graphene-based spintronics device.

Broadband, ultrafast and scalable topological insulator Sb2Te3/STO heterostructure photovoltaic detectors were fabricated by molecular beam epitaxy.

The emergence of larger-scaled two-dimensional (2D) layered materials has attracted intense research efforts and significant progress in the area of photonics and optoelectronics recently. As a remarkable representative of 2D materials, transition metal dichalcogenides (TMDCs) have demonstrated an astonishing photoconductivity and the ultrafast charge carrier dynamics, which provides excellent potentials for ultrafast optical modulators. Herein, by simply transferring a high-quality CVD-grown WSe2 multilayer on plasmon-induced transparency (PIT) metasurfaces, we demonstrate that the transmission amplitude modulation is as high as 43% and the slow light switching is up to 6 ps in the THz regime. Under photoexcitation, both functionalities are dynamically controlled within ~8 ps owing to a merit of ultrafast free carriers’ relaxation in the WSe2 multilayer. Moreover, a theoretical model consisting of two coupled harmonic oscillators and the near-field distributions are simultaneously employed to verify the strong dependence of the active PIT switching behavior on the suppression of the bright mode of split ring resonators. Our proposed versatile active WSe2-functionalized metasurface with a low cost and simple manufacturing will give researchers new insights into the ultrafast switchable metaphotonic devices.

The year 2019 marks the 10th anniversary of the first report of ultrafast fiber laser mode-locked by graphene. This result has had an important impact on ultrafast laser optics and continues to offer new horizons. Herein, we mainly review the linear and nonlinear photonic properties of two-dimensional (2D) materials, as well as their nonlinear applications in efficient passive mode-locking devices and ultrafast fiber lasers. Initial works and significant progress in this field, as well as new insights and challenges of 2D materials for ultrafast fiber lasers, are reviewed and analyzed.

A new novel method, hyperspectral imaging (HSI), is presented in this work to measure the beam divergence angle and beam profile uniformity of supercontinuum lasers. The obtained results of divergence angles are consistent with theoretically calculated values. The uniformity of different-size projected Gaussian beams was measured through referencing the data sets provided by HSI camera under the wavelength variation. HSI, compared with traditional methods, is much faster and capable of providing critical reference to supercontinuum output parameters measurements and practical application in far-field situation.

Atomically thin transition metal dichalcogenides (TMDCs) have emerged as a new class of two-dimensional (2D) material for novel optoelectronic applications. In particular, 2D TMDCs are viewed as intriguing and appealing materials to construct Q-switching and mode-locked modulators, due to their broadband saturable absorption even of photon energy below their excitonic energies. However, the dynamics and mechanism of saturable absorption inside TMDCs has yet to be investigated. In this paper, the relaxation dynamics of monolayer tungsten disulphide (WS2) was investigated considering different excitonic transitions. WS2 illustrates dramatic changes in optical responses when excited by intense laser pulses, which are characterized by the broadband photo-induced nonresonance absorption and the giant excitonic bands renormalization process. The experimental results show that strong photo-induced restructuring of excitonic bands has picosecond lifetime and full recovery of optical responses takes hundreds of picosecond. Additionally, our observations reveal that heavy renormalization and overlap of excitonic bands are induced by strong many-body Coulomb interactions. Moreover, the broadband absorption feature of WS2 opens up new applications in broadband saturable absorbers and ultrafast photonic devices.

In recent years, CsPbBrsub3/subquantum dots (QDs) have attracted much attention due to their bright prospects in solar cell studies. Dielectric properties are important for the fabrication of optoelectronic devices. Here, the dielectric properties of a CsPbBrsub3/subQD solution are investigated between 0.1 and 2.0 THz by terahertz time-domain spectroscopy. The measured frequency-dependent transmitted ratio is found to decrease from 0.96 to 0.80 in this range. By comparing different concentrations of the QD solution, the frequency-averaged absorption is linearly increased with the increase in QD concentration. After that, the frequency-dependent dielectric constant, including the complex refractive index, complex dielectric constant, and conductivity, is extracted by Fourier transform of the time-domain spectrum. An effective medium approach method is adopted to extract the complex dielectric constant of a CsPbBrsub3/subQD inclusion, and a slight peak around 0.4 THz is found in the imaginary part of the dielectric constant. The result of Drude-Lorentz fitting shows that the phonon plays a dominant role in the dielectric properties of a CsPbBrsub3/subQD solution. Moreover, the THz response of a CsPbBrsub3/subQD is found to be unchanged when the test is conducted under illumination. We attribute this phenomenon to the discrete energy level of excitons in CsPbBrsub3/subQDs due to quantum confinement, and design a comparative experiment to validate it. This study is significant for its deeper insight into the dielectric properties of CsPbBrsub3/subQDs, and thus is helpful through its applications in optoelectronics.

Active control of metamaterial properties with high tunability of both resonant intensity and frequency is essential for advanced terahertz (THz) applications, ranging from spectroscopy and sensing to communications. Among varied metamaterials, plasmon-induced transparency (PIT) has enabled active control with giant sensitivity by embedding semiconducting materials. However, there is still a stringent challenge to achieve dynamic responses in both intensity and frequency modulation. Here, an anisotropic THz active metamaterial device with an ultrasensitive modulation feature is proposed and experimentally studied. A radiative-radiative-coupled PIT system is established, with a frequency shift of 0.26 THz in its sharp transparent windows by polarization rotation. Enabled by high charge-carrier mobility and longer diffusion lengths, we utilize a straightforwardly spin-coated MAPbI3 film acting as a photoactive medium to endow the device with high sensitivity and ultrafast speed. When the device is pumped by an ultralow laser fluence, the PIT transmission windows at 0.86 and 1.12 THz demonstrate a significant reduction for two polarizations, respectively, with a full recovery time of 561 ps. In addition, we numerically prove the validity that the investigated resonator structure is sensitive to the optically induced conductivity. The hybrid system not only achieves resonant intensity and frequency modulations simultaneously, but also preserves the all-optical-induced switching merits with high sensitivity and speed, which enriches multifunctional subwavelength metamaterial devices at THz frequencies.

Recently, topological insulator based heterostructures (HSs) have attracted tremendous research interest, due to their efficient carrier transfer features at the heterointerface induced by metallic surface states. Here, a novel HS comprising 0D perovskite CsPbBr3 quantum dots (QDs) and 2D material topological insulator Bi2Se3 film is proposed and experimentally investigated. Specifically, steady state and time-resolved photoluminescence (PL) measurements are employed, from which a significant quenching behaviour is observed in the HS, with an average quenching factor of 93.2 ± 0.8%. Additionally, time-resolved PL spectroscopy affirms that the carrier transfer efficiency can be up to 92.6 ± 0.2%. Furthermore, the dynamics of carrier transfer within the 0D-2D HS are characterized by utilizing femtosecond broadband transient absorption (TA) spectroscopy, revealing an ultrafast exciton transfer from photoexcited CsPbBr3 QDs to the Bi2Se3 film with a time-scale around 1.1 ± 0.2 ps. An alternative important finding is that the band renormalization is exhibited in CsPbBr3 QDs of the HS, with the dominant factor being the Coulomb screening effect. This work is expected to provide some fundamental understanding of the ultrafast and efficient carrier transfer mechanism underneath HSs based on topological insulators.

Recently lead halide nanocrystals (quantum dots) have been reported with potential for photovoltaic and optoelectronic applications due to their excellent luminescent properties. Herein excitonic photoluminescence (PL) excited by two-photon absorption in perovskite CsPbBr3 quantum dots (QDs) have been studied across a broad temperature range from 80K to 380K. Two-photon absorption has been investigated with absorption coefficient up to 0.085 cm/GW at room temperature. Moreover, the photoluminescence excited by two-photon absorption shows a linear blue-shift (0.25meV/K) below temperature of ~220K and turned steady with fluctuation below 1nm (4.4meV) for higher temperature up to 380K. These phenomena are distinctly different from general red-shift of semiconductor and can be explained by the competition between lattice expansion and electron-phonon couplling.Our results reveal the strong nonlinear absorption and temperature-independent chromaticity in a large temperature range from 220K to 380K in the CsPbX3 QDs, which will offer new opportunities in nonlinear photonics, light-harvesting and light-emitting devices., 4 pages, 5 figures

Two-dimensional transitional metal dichalcogenides (TMDCs) based lateral heterojunctions have emerged as appealing and intriguing materials for applications in the next generation flexible nanoelectronics. The construction of depletion region near the in-plane interface brings rich opto-electrical dynamics, which is essential for future applications. Due to the synchronous requirement of spatial and time resolution, the study of lateral heterojunction dynamics remains a challenging issue. Herein, with a home-built spatiotemporal femtosecond transient absorption (TAS) spectroscopy platform, we have investigated the ultrafast photocarrier dynamics of monolayer spatial composition-graded WS2xSe2(1−x) lateral heterojunctions. At the alloy interface, the charge transfer (CT) processes have been visualized and referred to occur in 1 ps time scale. The mobility difference between electrons and holes results in the space modulation of the interface and a significant broadening of rising edge on the shell region. Moreover, carrier lifetime near the interface is extraordinarily extended by over 3 times from 153 ps to 678 ps. All these results unveil its great potential in designing future low cost logic devices and ultrafast optical applications.

Recent progress in solar cell and light-emitting devices makes halide perovskite a research hot spot in optics. In this Letter, the nonlinear absorption and fluorescence properties of FAPbBr3 nanocrystal, one typical organometallic halide perovskite, have been investigated via Z-scan measurements and a density-dependent photoluminescence (PL) spectrum. The FAPbBr3 nanocrystal exhibits nonlinear absorption under the excitation of 800 nm, whose photon energy is below the bandgap of FAPbBr3. The significant absorption is experimentally confirmed to be induced by two-photon absorption (TPA), and the TPA coefficient is measured to be ∼0.0042 cm/GW. Moreover, the PL induced by TPA in FAPbBr3 nanocrystal shows different temperature-dependent behaviors in the range of 90 to 350 K. The peaks of the PL spectrum remain nearly constant at 100–160 K, with a very shallow trough at around 150 K, while a linear blue shift (0.496 meV/K) of the spectrum is observed when temperature is above 160 K. These temperature-dependent fluorescence behaviors can be ascribed to the structural phase transition at about 150 K and the contribution of thermal expansion. Moreover, the exciton binding energy around 160 meV and the optical phonon energy of 15.3 meV are also extracted from the temperature-dependent PL data.

As a new quantum state of matter, topological insulators offer a new platform for exploring new physics, giving rise to fascinating new phenomena and new devices. Lots of novel physical properties of topological insulators have been studied extensively and are attributed to the unique electron-phonon interactions at the surface. Although electron behavior in topological insulators has been studied in detail, electron-phonon interactions at the surface of topological insulators are less understood. In this work, using optical pump-optical probe technology, we performed transient absorbance measurement on Bi2Te3 thin films to study the dynamics of its hot carrier relaxation process and coherent phonon behavior. The excitation and dynamics of phonon modes are observed with a response dependent on the thickness of the samples. The thickness-dependent characteristic time, amplitude and frequency of the damped oscillating signals are acquired by fitting the signal profiles. The results clearly indicate that the electron-hole recombination process gradually become dominant with the increasing thickness which is consistent with our theoretical calculation. In addition, a frequency modulation phenomenon on the high-frequency oscillation signals induced by coherent optical phonons is observed.

Owing to their unique physical properties, monolayer transition metal dichalcogenides (TMDCs) have been widely used in applications of light-emitting diodes (LEDs). However, monolayers of TMDCs undergo dramatic aging effects, including intense degradation in optical behavior, extensive cracking, and severe quenching of the direct gap photoluminescence (PL), seriously limiting the device performance. In this work, we show that monolayer WS2 stored for three months even in the glovebox exhibits obvious degenerative PL with changed peak position that can be attributed to the presence of a large number of trions induced by the aging effect. PC61BM surface processing was used to modify the surface of the aged monolayer WS2. As expected, higher uniformity in the PL spectrum was obtained. Besides, the PL peak wavelength was modified to be the same as that of the nonaged one and did not change even at higher excitation power. This strategy is shown to successfully suppress the formation of the trion by transferring the excess electrons from WS2 to PC61BM. The results demonstrate the feasibility of applying PC61BM surface modification to improve the performance of the LED based on monolayer WS2.

Laser-induced rear surface breakdown process of sodium chloride (NaCl) optical window was investigated based on the time-resolved shadowgraphy and interferometry. Violent particle ejection behavior lasting from tens of nanoseconds to tens of microseconds after the breakdown was observed. Classified by the particle velocity and propagating direction, the ejection process can be divided into three phases: (1) high-speed ejection of liquid particles during the first 100-ns delay; (2) micron-sized material clusters ejection from ∼ 100 - ns to ∼ 1 - μ s delay; (3) larger and slower solid-state particles ejection from ∼ 1 μ s to tens of microseconds delay. The moving directions of particles in the first and third phases are both perpendicular to the sample surface while particles ejected in the second phase exhibits angular ejection and present a V-like particle pattern. Mechanisms include explosive boiling, impact ejection, and shockwave ejection are discussed to explain this multiple phase ejection behavior. Our results highlight the significance of impact ejection induced by recoil pressure and backward propagating internal shockwave for laser-induced rear surface breakdown events of optical materials with low melting point.