Ultrasensitive Terahertz Nano-Probing for Semiconductors Using Nanogap Structures

Abstract

There has been a growing interest in exploring novel opto-electrical behaviors and response of carriers under nonequilibrium conditions in semiconductor nanoscale materials for applications of photocurrent and operation of photodetectors. Terahertz waves have shown substantial promise for these applications, with the merit of direct observation of transient carrier dynamics and conductivity changes in semiconductor materials. Especially, to investigate surface carrier dynamics of such nanoscale semiconductor materials under photoexcitation, one can use nanosized materials such as nanofilms, nanowires, and nanoparticles, which have a very large surface-to-volume ratio. In this article, we introduce ultrafast phenomena in semiconductors investigated by terahertz probes. In particular, recent studies on how to observe carrier dynamics in a semiconductor with a spatial resolution of nanoscale levels below the diffraction limit of terahertz wave, using well-tailored metallic nanogap structures are mainly introduced. Metallic nanostructures having the ability to confine electromagnetic waves in a volume much smaller than wavelength play an important role in many application areas such as subwavelength waveguides, metamaterials, biochemical sensing, and photovoltaic technology. Strong local electromagnetic field confinement and enhancement accompanied by metallic nanostructures including nanogaps and nanotips have been widely used to manipulate interaction between electromagnetic waves and target materials. In addition to their diverse applications, their fundamental importance has received a great deal of attention in nano-optics and nano-photonics. The tunability of resonances by the geometrical effect of metal nanostructures can be exploited to increase the interaction efficiency between the nanostructures and electromagnetic waves at specific wavelength regions of interest.