Terahertz metasurfaces have great applications for efficient terahertz modulation, but there are still problems in designing terahertz metadevices in terms of complexity and inefficiency. Herein, we demonstrate an inversely-designed terahertz metasurface with double electromagnetically induced transparency (EIT)-like windows by incorporating a particle swarm optimization (PSO) algorithm with the finite-difference time-domain method. We prepared and tested the metadevices, and the experimental terahertz signals are close to the designed results. By hybridizing amorphous germanium film with the inversely-designed metasurface, two EIT-like windows, including transmission and slow-light effect, exhibit ultrafast modulation behavior in 25 ps excited by a femtosecond laser. The modulation depths of transmission in two transparency windows are 74% and 65%, respectively. The numerical simulations also illustrate the ultrafast dynamic process and modulation mechanism, which match well with the experiment results. Our work thus offers opportunities for designing other objective functions of the terahertz metadevice.

干涉成像光谱技术是利用光的干涉原理获取目标光谱信息的一种成像技术。为研究其在强光下的干扰效果和机理，以大孔径静态成像光谱仪为典型对象，开展了相关仿真实验研究。以实际地物的图像和光谱信息为对象，仿真生成了原始干涉成像图案，并模拟830 nm单波长激光和超连续谱激光两种干扰源，分别研究不同辐照强度下的典型干扰效果，分析时假设光谱角大于30°时原始光谱信息丢失。基于本文的仿真模型，得到的相关结果表明，在830 nm的单波长激光干扰情况下，当干扰与目标成像峰值之比大于0.2∶1时原始光谱信息无法正确复原（光谱角大于30°），但模拟加入830 nm滤光片后，干扰效果被有效滤除。在超连续谱激光干扰情况下，不考虑饱和阈值时光谱角数值最终稳定在21°；考虑探测器饱和阈值为目标成像强度峰值3倍时，干扰与目标成像峰值之比大于2.1∶1时，原始光谱信息便无法分辨。该研究可能为同类型光谱仪的激光辐照效应和损伤机理的后续研究，以及光谱成像系统的激光防护和性能优化提供参考。

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 121mJ/cm2 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 from 10° to 60° correspondingly. Besides, the reflectivity is reduced significantly on silicon with LIPSS.

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.

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.

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.

Broadband transient reflectivity traces were measured for Bi2Se3 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 Bi2Se3 and qualitatively located it by properly analyzing the traces acquired at different probe wavelengths. Referring to the band structure of Bi2Se3, 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.