Creating Anti-Chiral Exceptional Points in Non-Hermitian Metasurfaces for Efficient Terahertz Switching.

Non-Hermitian degeneracies, also known as exceptional points (EPs), have presented remarkable singular characteristics such as the degeneracy of eigenvalues and eigenstates and enable limitless opportunities for achieving fascinating phenomena in EP photonic systems. Here, the general theoretical framework and experimental verification of a non-Hermitian metasurface that holds a pair of anti-chiral EPs are proposed as a novel approach for efficient terahertz (THz) switching. First, based on the Pancharatnam-Berry (PB) phase and unitary transformation, it is discovered that the coupling variation of ±1 spin eigenstates will lead to asymmetric modulation in two orthogonal linear polarizations (LP). Through loss-induced merging of a pair of anti-chiral EPs, the decoupling of ±1 spin eigenstates are then successfully realized in a non-Hermitian metasurface. Final, the efficient THz modulation is experimentally demonstrated, which exhibits modulation depth exceeding 70% and Off-On-Off switching cycle less than 9 ps in one LP while remains unaffected in another one. Compared with conventional THz modulation devices, the metadevice shows several figures of merits, such as a single frequency operation, high modulation depth, and ultrafast switching speed. The proposed theory and loss-induced non-Hermitian device are general and can be extended to numerous photonic systems varying from microwave, THz, infrared, to visible light.

Parallel edge extraction operators on chip speed up photonic convolutional neural networks.

We experimentally establish a 3 × 3 cross-shaped micro-ring resonator (MRR) array-based photonic multiplexing architecture relying on silicon photonics to achieve parallel edge extraction operations in images for photonic convolution neural networks. The main mathematical operations involved are convolution. Precisely, a faster convolutional calculation speed of up to four times is achieved by extracting four feature maps simultaneously with the same photonic hardware's structure and power consumption, where a maximum computility of 0.742 TOPS at an energy cost of 48.6 mW and a convolution accuracy of 95.1% is achieved in an MRR array chip. In particular, our experimental results reveal that this system using parallel edge extraction operators instead of universal operators can improve the imaging recognition accuracy for CIFAR-10 dataset by 6.2% within the same computing time, reaching a maximum of 78.7%. This work presents high scalability and efficiency of parallel edge extraction chips, furnishing a novel, to the best of our knowledge, approach to boost photonic computing speed.

Transient Loss-Induced Non-Hermitian Degeneracies for Ultrafast Terahertz Metadevices.

Non-Hermitian degeneracies, also known as exceptional points (EPs), have attracted considerable attention due to their unique physical properties. In particular, metasurfaces related to EPs can open the way to unprecedented devices with functionalities such as unidirectional transmission and ultra-sensitive sensing. Herein, an active non-Hermitian metasurface with a loss-induced parity-time symmetry phase transition for ultrafast terahertz metadevices is demonstrated. Specifically, the eigenvalues of the non-Hermitian transmission matrix undergo a phase transition under optical excitation and are degenerate at EPs in parameter space, which is accompanied by the collapse of chiral transmission. Ultrafast EP modulation on a picosecond time scale can be realized through variations in the transient loss at a non-Hermitian metasurface pumped by pulsed excitation. Furthermore, by exploiting the physical characteristics of chiral transmission EPs, a switchable quarter-wave plate based on the photoactive metasurface is designed and experimentally verified and realized the corresponding function of polarization manipulation. This work opens promising possibilities for designing functional terahertz metadevices and fuses EP physics with active metasurfaces.

Reassessing Fano Resonance for Broadband, High-Efficiency, and Ultrafast Terahertz Wave Switching.

Miniaturized ultrafast switchable optical components with high efficiency and broadband response are in high demand to the development of optical imaging, sensing, and high-speed communication. Sharp Fano-type resonance switched by active materials is one of the key concepts that underpins the control of light in metaoptics with high sensitivity. However, actuating such metasurfaces exhibits a long-standing trade-off between modulation depth and operational bandwidth. Here, the limitations are circumvented by theoretical analysis, numerical simulation, and experimental realization of an achromatic Fano metasurface so that a high contrast of tunability with ultrafast switching rate over a broad range of frequency is achieved. By developing the physics of inter-mode coupling, the Fano metasurface is designed according to a complete phase diagram derived from coupled mode theory. Unlike conventional Fano metasurfaces, the cross-polarized inter-metaatoms coupling is discovered as a superior ability of high-efficiency broadband achromatic polarization conversion. To prove the ultrasensitive nature, a metadevice is constructed by incorporating a thin amorphous Ge layer with a weak photoconductivity perturbation. Transmission modulation over broadband frequency range from 0.6 to 1.1 THz is thus successfully realized, featuring its merits of modulation depth over 90% and On-Off-On switching cycle less than 10 ps.

Phase-Controllable Chemical Vapor Deposition Synthesis of Atomically Thin MoTe2.

Two-dimensional (2D) molybdenum telluride (MoTe2) is attracting increasing attention for its potential applications in electronic, optoelectronic, photonic and catalytic fields, owing to the unique band structures of both stable 2H phase and 1T' phase. However, the direct growth of high-quality atomically thin MoTe2 with the controllable proportion of 2H and 1T' phase seems hard due to easy phase transformation since the potential barrier between the two phases is extremely small. Herein, we report a strategy of the phase-controllable chemical vapor deposition (CVD) synthesis for few-layer (<3 layer) MoTe2. Besides, a new understanding of the phase-controllable growth mechanism is presented based on a combination of experimental results and DFT calculations. The lattice distortion caused by Te vacancies or structural strain might make 1T'-MoTe2 more stable. The conditions for 2H to 1T' phase conversion are determined to be the following: Te monovacancies exceeding 4% or Te divacancies exceeding 8%, or lattice strain beyond 6%. In contrast, sufficient Te supply and appropriate tellurization velocity are essential to obtaining the prevailing 2H-MoTe2. Our work provides a novel perspective on the preparation of 2D transition metal chalcogenides (TMDs) with the controllable proportion of 2H and 1T' phase and paves the way to their subsequent potential application of these hybrid phases.

Ultrafast resonant exciton-plasmon coupling for enhanced emission in lead halide perovskite with metallic Ag nanostructures.

Integrating metal halide perovskites onto plasmonic nanostructures has recently become a trending method of enabling superior emissive performance of perovskite nanophotonic devices. In this work, we present an in-depth study on the spontaneous emission properties of hybrid systems comprising CsPbBr3 nanocrystals and silver nanostructures. Specifically, a 5.7-fold increment of the photoluminescence (PL) intensity and a 1.65-fold enhancement of the PL relaxation rate is attained when the transition energy of CsPbBr3 is spectrally resonant with the oscillational frequency of Ag nanodisks (NDs), which is attributed to the intense exciton-plasmon coupling-induced Purcell effect. Furthermore, a 540-fs ultrafast energy transfer from the CsPbBr3 excitons to Ag plasmons is revealed by femtosecond pump-probe experiments, suggesting the key mechanism responsible for the Purcell-enhanced radiative emission. Our finding offers a unique understanding of the enhanced emissive behavior in the plasmon-coupled perovskite system and paves the way for further applications.

Light-Driven Spintronic Heterostructures for Coded Terahertz Emission.

The extraordinary proliferation of digital coding metasurfaces turns the real-time manipulation of electromagnetic (EM) waves into reality and promotes the programmable operation of multifunctional equipment. However, current studies are mainly involved in the modulation of the transmission process, and little attention has been given to the control of EM wave generation, especially in the terahertz (THz) band. Here, we conceptually propose and experimentally demonstrate coded terahertz emission, which integrates the efficient generation and control of THz waves across a wide frequency band. For validation, two types of stripe-patterned ferromagnetic heterostructures with opposite spin Hall angles were utilized as coding units. The two distinct states in each coding unit (with two polarization or phase states of 0° and 180°) can be characterized as "0" and "1" digits, which can be switched by manipulating the optical field distribution of the pump beam. Such an ability to realize simultaneous terahertz coding and terahertz emission is essential for meeting the increasingly demanding requirements of integration and miniaturization. Our work endows ferromagnetic heterostructures with controllable spatial characteristics and benefits their applications in wireless communications and holographic imaging.

Interacting plexcitons for designed ultrafast optical nonlinearity in a monolayer semiconductor.

Searching for ideal materials with strong effective optical nonlinear responses is a long-term task enabling remarkable breakthroughs in contemporary quantum and nonlinear optics. Polaritons, hybridized light-matter quasiparticles, are an appealing candidate to realize such nonlinearities. Here, we explore a class of peculiar polaritons, named plasmon-exciton polaritons (plexcitons), in a hybrid system composed of silver nanodisk arrays and monolayer tungsten-disulfide (WS2), which shows giant room-temperature nonlinearity due to their deep-subwavelength localized nature. Specifically, comprehensive ultrafast pump-probe measurements reveal that plexciton nonlinearity is dominated by the saturation and higher-order excitation-induced dephasing interactions, rather than the well-known exchange interaction in traditional microcavity polaritons. Furthermore, we demonstrate this giant nonlinearity can be exploited to manipulate the ultrafast nonlinear absorption properties of the solid-state system. Our findings suggest that plexcitons are intrinsically strongly interacting, thereby pioneering new horizons for practical implementations such as energy-efficient ultrafast all-optical switching and information processing.

Visualizing Hot-Carrier Expansion and Cascaded Transport in WS2 by Ultrafast Transient Absorption Microscopy.

The competition between different spatiotemporal carrier relaxation determines the carrier harvesting in optoelectronic semiconductors, which can be greatly optimized by utilizing the ultrafast spatial expansion of highly energetic carriers before their energy dissipation via carrier-phonon interactions. Here, the excited-state dynamics in layered tungsten disulfide (WS2 ) are primarily imaged in the temporal, spatial, and spectral domains by transient absorption microscopy. Ultrafast hot carrier expansion is captured in the first 1.4 ps immediately after photoexcitation, with a mean diffusivity up to 980 cm2 s-1 . This carrier diffusivity then rapidly weakens, reaching a conventional linear spread of 10.5 cm2 s-1 after 2 ps after the hot carriers cool down to the band edge and form bound excitons. The novel carrier diffusion can be well characterized by a cascaded transport model including 3D thermal transport and thermo-optical conversion, in which the carrier temperature gradient and lattice thermal transport govern the initial hot carrier expansion and long-term exciton diffusion rates, respectively. The ultrafast hot carrier expansion breaks the limit of slow exciton diffusion in 2D transition metal dichalcogenides, providing potential guidance for high-performance applications and thermal management of optoelectronic technology.

Ultraefficient Terahertz Emission Mediated by Shift-Current Photovoltaic Effect in Layered Gallium Telluride.

Generating terahertz waves using thin-layered materials holds great potential for the realization of integrated terahertz devices. However, previous studies have been limited by restricted radiation intensity and finite efficiency. Exploiting materials with higher efficiency for terahertz emission has attracted increasing interest worldwide. Herein, with visible-light excitation, a thin-layered GaTe film is demonstrated to be a promising emitter of terahertz radiation induced by the shift-current photovoltaic effect. Through theoretical calculations, a transient charge-transfer process resulting from the asymmetric structure of GaTe is shown to be the origin of an ultrafast shift current. Furthermore, it was found that the amplitude of the resulting terahertz signals can be manipulated by both the fluence of the pump laser and the orientation of the sample. Such high emission efficiency from the shift current indicates that the layered material (GaTe) is an excellent candidate for photovoltaics and terahertz emitters.

Giant Photoluminescence Enhancement and Carrier Dynamics in MoS2 Bilayers with Anomalous Interlayer Coupling.

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.

Enhanced Terahertz Radiation by Efficient Spin-to-Charge Conversion in Rashba-Mediated Dirac Surface States.

The enhancement of terahertz (THz) radiation is of extreme significance for the realization of the THz probe and imaging. However, present THz technologies are far from being enough to realize high-performance and room-temperature THz sources. Fortunately, topological insulators (TIs), with spin-momentum-locked Dirac surface states, are expected to exhibit a high terahertz emission efficiency. In this work, the novel concept of a Rashba-state-enhanced spintronic THz emitter is demonstrated on the basis of ferromagnet/heavy metal/topological insulator (FM/HM/TI) heterostructure. We find that the THz emission intensity changes as a function of HM interlayer thickness, and a 1.98 times higher intensity compared to that of FM/TI can be achieved when a meticulously designed thickness of the HM layer is inserted. The improvement of terahertz radiation is ascribed to the additive effect of Rashba splitting and topological surface states at the HM/TI interface. These results offer new possibilities for realizing spintronic THz emitters in TI-based magnetic heterostructures.

Distinctive Interfacial Charge Behavior and Versatile Photoresponse Performance in Isotropic/Anisotropic WS2/ReS2 Heterojunctions.

Van der Waals (vdWs) heterostructures based on in-plane isotropic/anisotropic 2D-layered semiconducting materials have recently received wide attention because of their unique interlayer coupling properties and hold a bright future as building blocks for advanced photodetectors. However, a fundamental understanding of charge behavior inside this kind of heterostructure in the photoexcited state remains elusive. In this work, we carry out a systematic investigation into the photoinduced interfacial charge behavior in type-II WS2/ReS2 vertical heterostructures via polarization-dependent pump-probe microscopy. Benefiting from the distinctive (ultrafast and anisotropic) charge-transfer mechanisms, the photodetector based on the WS2/ReS2 heterojunction displays more superior optoelectronic properties compared to its constituents with diverse functionalities including moderate photoresponsivity, polarization sensitivity, and fast photoresponse speed. Additionally, this device can function as a self-driven photodetector without the external bias. These results of our work tangibly corroborate the intriguing interlayer interaction in in-plane isotropic/anisotropic heterostructures and are expected to shed light on designing balanced-performance multifunctional optoelectrical devices.

Tunable anisotropic plasmon response of monolayer GeSe nanoribbon arrays.

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.

Inversion Symmetry Breaking in Lithium Intercalated Graphitic Materials.

Intercalation is a unique degree of freedom for tuning the physical and chemical properties of two-dimensional (2D) materials, providing an ideal system to study various electronic states (such as superconductivity, ferromagnetism, and charge density waves). Here, we demonstrate the inversion symmetry breaking in lithium (Li)-intercalated ultrathin graphite (about 20-100 graphene layers) by optical second-harmonic generation (SHG). This inversion symmetry breaking is attributed to nanoscale inhomogeneities (i.e., lattice distortion and dislocations) in lithiated graphite. In addition, the efficiency of the SHG signal in an ultrathin graphite flake is widely tunable by the electrochemical lithiation process, and the efficiency of fully lithiated graphite (LiC6) is comparable to that of other noncentrosymmetric 2D crystals. Our results reveal a novel intercalation-induced inversion symmetry breaking effect and open up possibilities for building 2D intercalated-compounds-based nonlinear optical devices.

Ultrafast nonlinear absorption enhancement of monolayer MoS2 with plasmonic Au nanoantennas.

In this work, we experimentally study the nonlinear absorption enhancement of saturable absorption and two-photon absorption on a hybrid structure comprising a monolayer MoS2 and Au nanoantennas via femtosecond I-scan measurement. Specifically, a 13-fold increment in the linear absorption coefficient is attained at 1.85 eV, along with an 8-fold enhancement of the two-photon absorption coefficient at 1.65 eV, which is attributed to exciton-plasmon coupling resonance and plasmonic hot electron transfer. The exciton-plasmon coupling effect is characterized by stable photoluminescence experiments. Furthermore, the exciton recombination time is extracted from the pump-probe measurement, whose value in the hybrid structure is shortened from 18.5 ps (pure MoS2) to 1.84 ps. Our findings facilitate a new perspective to modulate the nonlinear optical response and to promote the performance of nonlinear photonic devices.

A polarized nonlinear optical response in a topological insulator Bi2Se3-Au nanoantenna hybrid-structure for all-optical switching.

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.

Ultrafast exciton transfer in perovskite CsPbBr3 quantum dots and topological insulator Bi2Se3 film heterostructure.

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.

Visualized charge transfer processes in monolayer composition-graded WS2xSe2(1-x) lateral heterojunctions via ultrafast microscopy mapping.

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.

Ultrafast saturable absorption of MoS2 nanosheets under different pulse-width excitation conditions.

The newly raised two-dimensional material MoS2 is regarded as an ideal candidate for saturated absorbers. Here, the open-aperture Z-scan method is used to study the saturation absorption (SA) response of monolayer and multilayer MoS2, considering laser irradiation with different pulse widths. Specifically, in cases of 10 ns and 10 ps laser pulses, the accumulative nonlinearity [e.g., free carrier absorption (FCA)] coupled with SA is found in both monolayer and multilayer MoS2. However, under a 65 fs pulse laser, the instantaneous nonlinearity [e.g., two-photon absorption (TPA)] and the SA effect turn to play a significant role. Additionally, the saturation of both TPA and FCA is observed in MoS2. Importantly, the modulation depth of MoS2 shows different change trends by adjusting the laser pulse width.