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.

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.

Helicity-dependent ultrafast spin current generated by circularly polarized photons in topological materials holds the crux to many technological improvements, such as quantum communications, on-chip communication processing and storage. Here, we present the manipulation of helicity-dependent terahertz emission generated in a nodal line semimetal candidate Mg₃Bi₂ by using photon polarization states. The terahertz emission is mainly ascribed to the helicity-dependent photocurrent that is originated from circular photogalvanic effects, and the helicity-independent photocurrent that is attributed to linear photogalvanic effect. Our work will inspire more explorations into novel nodal line semimetals and open up new opportunities for developing ultrafast optoelectronics in the topological system.

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/cm². 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 pJ, 1.08 ps, and 60 dBm, 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.