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

Abstract

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.

Interfacial photoconductivity effect of type-I and type-II Sb2Se3/Si heterojunctions for THz wave modulation

“…2(c), for 173 nm Sb 2 Se 3 /Si, the electrons and holes would both transport to the Si due to the band alignment of the type-I heterojunction. [1] On the one hand, the simultaneous transfer of electrons and holes to Si will enhance the recombination of photocarriers and reduce the concentration of photogenerated carriers. [33] This property presents promising potential in light-emitting devices.…”

“…[33] This property presents promising potential in light-emitting devices. [1] On the other hand, the photoconductivity of Si is described as [13,34] σ = e (n e µ e + n h µ h ), where the n h (n e ) and µ h (µ e ) represent the carrier density and mobility of holes (electrons), respectively. Since Si is an n-type semiconductor, its photoconductivity is mainly determined by the electrons.…”

“…Nobel Prize laureate H. Kroemer once said 'the interface is the device', since the interface controls the charge transfer, the distribution of quasi-particles and so forth. [1,2] Currently, most studies of interfacial effects and applications usually concentrate on electrical conductivity at interfaces. [3] However, photovoltaic devices and optoelectronic devices are generally utilized under light irradiation, which can be described by the photoconductivity.…”

“…[4] Hence, a deep comprehensive understanding of the photoconductivity at the interface is necessary to improve the performance of optoelectronic devices. Semiconductor interfaces include three types of heterojunctions according to semiconductor band alignment: [1] type-I, straddling gap; type-II, staggered gap; type-III, broken gap. Transfer and separation of electron-hole pairs between materials are allowed to occur in type-I and type-II heterojunctions, while these processes are hindered in type-III heterojunctions due to the absence of overlapping band gaps.…”

Disentangling complex current pathways in a metallic Ru/Co bilayer nanostructure using THz spectroscopy

“…THz time-domain spectroscopy (THz-TDS) is a contact-free, all-optical spectroscopy method, which is ideally suited for studies of electrical transport on a nanoscale (see e.g. [11][12][13] ). Here, we applied THz-TDS to studies of conduction in technologically-relevant metallic bilayer Ru/Co nanostructures of different thicknesses [14][15][16] , and of their individual constituent metals Ru and Co.…”