Terahertz interface physics: from terahertz wave propagation to terahertz wave generation

Terahertz (THz) interface physics as a new interdiscipline between THz technique and condensed matter physics has undergone rapid developments in recent years. Especially, the developments of advanced materials, such as graphene, transitional metal dichalcogenides, topological insulators, ferromagnetic metals, and metamaterials, have revolutionized the interface field and further promotes the development of THz functional devices based on interface physics. Moreover, playing at the interface with these advanced materials could unveil a wealth of fascinating physical effects such as charge transfer, proximity effect, inverse spin-Hall effect, and Rashba effect with THz technology by engineering the charge, spin, orbit, valley, and lattice degrees of freedom. In this review, we start from the discussion of the basic theory of THz interface physics, including interface formation with advanced materials, THz wave reflection and transmission at the interface, and band alignment and charge dynamics at the interface. Then we move to recent progresses in advanced materials from THz wave propagation to THz wave generation at the interface. In the THz wave propagation, we focus on the THz wave impedance-matching, Goos–Hänchen and Imbert–Fedorov shifts in THz region, interfacial modulation and interfacial sensing based on THz wave. In the THz wave generation, we summarize the ongoing coherent THz wave generation from van der Waals interfaces, multiferroic interfaces, and magnetic interfaces. The fascinating THz interface physics in advanced materials is promising and promoting novel THz functional devices for manipulating the propagation and generation of THz wave at the interfaces.

Unusual terahertz-wave absorptions in δ/α-mixed-phase FAPbI3 single crystals: interfacial phonon vibration modes

The terahertz (THz)-wave absorption properties in organic-inorganic hybrid perovskite (OHP) materials are investigated with the in-depth development of OHP-based THz applications. In the THz range from 0.5 to 3 THz, OHPs typically show several interesting phonon modes such as transverse, longitudinal, and halogen self-vibrations. To modulate these frequencies, the density changes in defect-incorporated structures and element mixtures were tested and confirmed. In the literature, the origin of phonon modes in OHP materials have been mostly explained. However, we found new phonon vibration modes in formamidinium (FA)-based hybrid perovskite structures. FAPbI3 single crystals, organic–inorganic hybrid perovskites, of the δ-, δ/α-mixed-, and α-phases were prepared. We intriguingly found that the δ/α-mixed-phase exhibited significant THz-wave absorption peaks at 2.0 and 2.2 THz that were not related to any phonon modes from either the δ- or α-phases, although the δ/α-mixed-phase sample was confirmed to be formed by a physical combination of the δ- and α-phases without the creation of any new chemical states. Our theoretical study performed with ab initio calculations provides an explanation for these unusual THz-wave absorption behaviors; they originate from the novel vibration modes excited at the seamless interfaces in the mixed phase of FAPbI3.

Enhanced terahertz emission from mushroom-shaped InAs nanowire network induced by linear and nonlinear optical effects

The development of powerful terahertz (THz) emitters is the cornerstone for future THz applications, such as communication, medical biology, non-destructive inspection, and scientific research. Here, we report the THz emission properties and mechanisms of mushroom-shaped InAs nanowire (NW) network using linearly polarized laser excitation. By investigating the dependence of THz signal to the incidence pump light properties (e.g., incident angle, direction, fluence, and polarization angle), we conclude that the THz wave emission from the InAs NW network is induced by the combination of linear and nonlinear optical effects. The former is a transient photocurrent accelerated by the photo-Dember field, while the latter is related to the resonant optical rectification effect. Moreover, the p-polarized THz wave emission component is governed by the linear optical effect with a proportion of ~85% and the nonlinear optical effect of ~15%. In comparison, the s-polarized THz wave emission component is mainly decided by the nonlinear optical effect. The THz emission is speculated to be enhanced by the localized surface plasmon resonance absorption of the In droplets on top of the NWs. This work verifies the nonlinear optical mechanism in the THz generation of semiconductor NWs and provides an enlightening reference for the structural design of powerful and flexible THz surface and interface emitters in transmission geometry.

Circularly-polarized THz wave emission from a micro-thin water flow

Intense THz wave sources are highly expected for further progresses in nonlinear THz science and practical implementation of non-ionizing radiation in sensing and communications. Solid-based sources have inherent limits of material breakdown, while intense laser irradiation of liquids is a promising emerging technique for THz wave and hard X-ray emission. Water-based THz emission shows intensity enhancements up to 10 times when laser-pulse pairs with nanosecond delay are used. Here we show circularly-polarized THz wave emission from thin water flow irradiated by two time-separated and linearly-polarized femtosecond laser pulses. THz time-domain spectroscopy reveals the circularly-polarized THz emission dominates 4.7 ns after the first pulse irradiation. THz wave detection delay in the spectroscopy and time-resolved micrography indicate that the THz wave emission originates from the rarefied volume in front of the flow. Radial relaxation of charges in the focal volume where ponderomotive charge depletion occurred on the optical axis is the origin for the circular polarization (due to spiraling currents). Tight focusing of fs-laser pulses localized THz wave emission to the sub-wavelength (tens-of-micrometers) region.