Color coded metadevices toward programmed terahertz switching

Terahertz modulators play a critical role in high-speed wireless communication,non-destructive imaging,and so on,which have attracted a large amount of research interest.Nevertheless,all-optical terahertz modulation,an ultrafast dynamical control approach,remains to be limited in terms of encoding and multifunction.Here we experimentally demonstrated an optical-programmed terahertz switching realized by combining optical metasurfaces with the terahertz metasurface,resulting in 2-bit dual-channel terahertz encoding.The terahertz metasurface,made up of semiconductor islands and artificial microstructures,enables effective all-optical programming by providing multiple frequency channels with ultrafast modulation at the nanosecond level.Meanwhile,optical metasurfaces covered in terahertz metasurface alter the spatial light field distribution to obtain color code.According to the time-domain coupled mode theory analysis,the energy dissipation modes in terahertz metasurface can be independently controlled by color excitation,which explains the principle of 2-bit encoding well.This work establishes a platform for all-optical programmed terahertz metadevices and may further advance the application of composite metasurface in terahertz manipulation.

Ultrasensitive polarization-dependent terahertz modulation in hybrid perovskites plasmoninduced transparency devices

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 spincoated 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.

Ultrafast all-optical terahertz modulation based on an inverse-designed metasurface

Metasurface plays a key role in various terahertz metadevices,while the designed terahertz metasurface still lacks flexibility and variety.On the other hand,inverse design has drawn plenty of attention due to its flexibility and robustness in the application of photonics.This provides an excellent opportunity for metasurface design as well as the development of multifunctional,high-performance terahertz devices.In this work,we demonstrate that,for the first time,a terahertz metasurface supported by the electromagnetically induced transparency(EIT)effect can be constructed by inverse design,which combines the particle swarm optimization algorithm with the finite-difference time-domain method.Incorporating germanium(Ge)film with inverse-designed metasurface,an ultrafast EIT modulation on the picosecond scale has been experimentally verified.The experimental results suggest a feasibility to build the terahertz EIT effect in the metasurface through an optimization algorithm of inverse design.Furthermore,this method can be further utilized to design multifunctional and high-performance terahertz devices,which is hard to accomplish in a traditional metamaterial structure.In a word,our method not only provides a novel way to design an ultrafast all-optical terahertz modulator based on artificial metamaterials but also shows the potential applications of inverse design on the terahertz devices.

Polarization-tunable nonlinear absorption patterns from saturated absorption to reverse saturated absorption in anisotropic GeS flake and an application of all-optical switching

Due to the unique anisotropic chemical and physical properties,two-dimensional(2D)layered materials such as IV-VI monochalcogenides with puckered honeycomb structure,have received considerable interest recently.Among the IV-VI layered MX(M=Ge,Sn;X=Se,S)compounds,germanium sulfide(Ge S)stands out for its strongest anisotropic thermal conductivities and figure-of-merit values.Additionally,the layer-independent direct energy bands(Eg^1.6 e V,E1~2.1 e V)of Ge S flake provide excellent insights into further applications as visible photodetectors.Herein,the polarization-tunable nonlinear absorption(NA)patterns of Ge S flake have been systematically investigated.Specifically both the polarization-dependent Raman spectroscopy and the linear absorption(LA)spectroscopy were employed to characterize the lattice orientation and absorption edges of the251-nm Ge S flake.Considering the low damage threshold of Ge S flake,the Ge S/graphene heterostructure was fabricated to increase the threshold without changing the nonlinear properties of Ge S.Our NA results demonstrated that a 600-nm femtosecond laser with different polarizations would excite the saturated-absorption(SA)effect along armchair and reversesaturated-absorption(RSA)effect along zigzag in the Ge S/graphene heterostructure.Moreover,the function of the polarization-based Ge S/graphene heterostructure all-optical switch was experimentally verified.Notably,thanks to the polarization-dependent NA patterns(SA/RSA)of Ge S,the"ON"and"OFF"states of the all-optical switch can be accomplished by high and low transmittance states of continuous-wave laser(532 nm,80 n W),whose state can be controlled by the polarization of femtosecond switching laser(600 nm,35 fs,500 Hz,12 GW cm-2).The ON/OFF ratio can achieve up to 17%by changing polarization,compared with the ratios of 3.0%by increasing the incident power of switching light in our experiment.The polarization-tunable absorption patterns introduced in this work open up real perspectives for the next-generation optoelectronic devices based on Ge S/graphene heterostructure.

Repetition rate locked single-soliton microcomb generation via rapid frequency sweep and sideband thermal compensation

Dissipative Kerr solitons(DKSs)with mode-locked pulse trains in high-Q optical microresonators possess lownoise and broadband parallelized comb lines,having already found plentiful cutting-edge applications.However,thermal bistability and thermal noise caused by the high microresonator power and large temperature exchange between microresonator and the environment would prevent soliton microcomb formation and deteriorate the phase and frequency noise.Here,a novel method that combines rapid frequency sweep with optical sideband thermal compensation is presented,providing a simple and reliable way to get into the single-soliton state.Meanwhile,it is shown that the phase and frequency noises of the generated soliton are greatly reduced.Moreover,by closing the locking loop,an in-loop repetition rate fractional instability of 5.5×10^(-15)at 1 s integration time and a triangular linear repetition rate sweep with 2.5 MHz could be realized.This demonstration provides a means for the generation,locking,and tuning of a soliton microcomb,paving the way for the application of single-soliton microcombs in low-phase-noise microwave generation and laser ranging.

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.

Saturated absorption of different layered Bi_2Se_3 films in the resonance zone

Here, we used the micro P-scan method to investigate the saturated absorption(SA) of different layered Bi_2Se_3 continuous films. Through resonance excitation, first, we studied the influence of the second surface state(SS) on SA. The second SS resonance excitation(～2.07 e V) resulted in a free carrier cross section that was 4 orders of magnitude larger than usual. At the same time, we found that the fast relaxation process of the massless Dirac electrons is much shorter than that of electrons in bulk states. Moreover, the second SS excitation resonance reduced the saturation intensity. Second, we studied the effect of the thickness on the SA properties of materials.The results showed that the saturation intensity was positively correlated to the thickness, the same as the modulation depth, and the thicker the Bi_2Se_3 film was, the less the second SS would influence it. This work demonstrated that by using Bi_2Se_3 as a saturable absorber through changing the thickness or excitation wavelength, a controllable SA could be achieved.

Electron–phonon coupling in topological insulator Bi_2Se_3 thin films with different substrates

Broadband transient reflectivity traces were measured for Bi_2 Se_3 thin films with various substrates via a 400 nm pump–white-light-probe setup. We have verified the existence of a second Dirac surface state in Bi_2 Se_3 and qualitatively located it by properly analyzing the traces acquired at different probe wavelengths. Referring to the band structure of Bi_2 Se_3, the relaxation mechanisms for photo-excited electrons with different energies are also revealed and studied. Our results show a second rise of the transient reflection signal at the time scale of several picoseconds. The types of substrate can also significantly affect the dynamics of the rising signal. This phenomenon is attributed to the effect of lattice heating and coherent phonon processes. The mechanism study in this work will benefit the fabrication of high-performance photonic devices based on topological insulators.

Soliton mode-locked fiber laser with high-quality MBE-grown Bi2Se3 film

In this work, a soliton mode-locked erbium-doped fiber laser(EDFL) with a high-quality molecular beam epitaxy(MBE)-grown topological insulator(TI) Bi2Se3 saturable absorber(SA) is reported.To fabricate the SA device, a 16-layer Bi2Se3 film was grown successfully on a 100 μm thick SiO2 substrate and sandwiched directly between two fiber ferrules.The TI-SA had a saturable absorption of 1.12% and a saturable influence of 160 MW/cm^2.After inserting the TI-SA into the unidirectional ring-cavity EDFL, self-starting mode-locked soliton pulse trains were obtained at a fundamental repetition rate of 19.352 MHz.The output central wavelength, pulse energy,pulse duration, and signal to noise ratio of the radio frequency spectrum were 1530 nm,18.5 p J, 1.08 ps, and 60d Bm, respectively.These results demonstrate that the MBE technique could provide a controllable and repeatable method for the fabrication of identical high-quality TI-SAs, which is critically important for ultra-fast pulse generation.

Temperature insensitive multi-channel light amplification systems on SOI platform

We present a theoretical analysis of a novel multi-channel light amplification photonic system on chip,where the nonlinear Raman amplification phenomenon in the silicon(Si)wire waveguide is considered.Particularly,a compact and temperature insensitive Mach–Zehnder interferometer filter working as demultiplexer is also exploited,allowing for the whole Si photonic system to be free from thermal interference.The propagation of the multi-channel pump and Stokes lights is described by a rigorous theoretical model that incorporates all relevant linear and nonlinear optical effects,including the intrinsic waveguide optical losses,first-and second-order frequency dispersion,self-phase and cross-phase modulation,phase shift and two-photon absorption,free-carriers dynamics,as well as the inter-pulse Raman interaction.Notably,to prevent excessive drift of the transmission window of the demultiplexer caused by ambient temperature variations and high thermo-optical coefficient of Si,an asymmetric waveguide width is adopted in the upper and lower arms of each Mach–Zehnder interferometer lattice cell.A Chebyshev half-band filter is utilized to achieve a flat pass-band transmission,achieving a temperature sensitivity of<1.4 pm=K and over 100 K temperature span.This all-Si amplifier shows a thermally robust behavior,which is desired by future Si-on-insulator(SOI)applications.