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 fiber lasers mode-locked by two-dimensional materials:review and prospect

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.

Nanosecond laser-induced controllable periodical surface structures on silicon

In this paper,an effective method is proposed to generate specific periodical surface structures.A 532 nm linearly polarized laser is used to irradiate the silicon with pulse duration of 10 ns and repetition frequency of 10 Hz.Laser-induced periodic surface structures(LIPSSs) are observed when the fluence is 121 mJ/cm;and the number of pulses is 1000.The threshold of fluence for generating LIPSS gradually increases with the decrease of the number of pulses.In addition,the laser incident angle has a notable effect on the period of LIPSS,which varies from 430 nm to 1578 nm,as the incident angle ranges from10° to 60° correspondingly.Besides,the reflectivity is reduced significantly on silicon with LIPSS.

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.

New Semiconducting Polymer Based on Benzo[1,2-b：4,5-bldiselenophene Donor and Diketopyrrolopyrrole/Isoindigo Acceptor Unit： Synthesis, Characterization and Photovoltaics

Two new D-A type polymers PBDSe-DPP and PBDSe-ID were synthesized to explore new ideal semiconduct- ing polymers, by conjugating acceptor unit diketopyrrolopyrrole/isoindigo to a donor unit benzo[l,2-b：4,5-b＇]di- selenophene, which is designed by substituting the sulfur atom with a selenium atom in the benzo[1,2-b：4,5-b＇]- dithiophene. The thermal, optical, electrochemical, photoelectric and photovoltaie properties of the two polymers were studied systematically. Relatively high open circuit voltage （0.7 and 0.75 V） and fill factor （〉65%） were demonstrated for both polymers. Huge increase （by 64% and 120%） of the short circuit current density was achieved for both polymer based devices by using additive compared to the corresponding reference without addi- tive, resulting in decent power conversion efficiency of 3.7% and 2.5% respectively with only simple optimizing consideration. We believe this class of BDSe polymer possesses a good potential to be alternatives of active material for photovoltaic applications.

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.

Optically Controlled Extraordinary Terahertz Transmission of Bi2Se3 Film Modulator

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 AI2O3 substrates, with the pump fluence range from 25(iJ/cm2 to 200 |iJ/cm2 per pulse. After photoexcitation, an ultrafast switching of THz wave with a full recovery time of ?1 Ops is observed. For a longer timescale, a photoinduced increase in the transmitted THz amplitude is found in the 8 and 10 quintuple layers (QL) BizSR, 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 2.2ps and 12.5ps which have a maximum modulation depth of 6.4% and 1.3% under the pump fluence of 200(iJ/cm2, respectively. Furthermore, the absorption of a optical phonon at 1.9 THz shows a time-dependent evolution which is consistent with the cooling of lattice temperature.