Terahertz wave generation from semiconductor thin films and its applications

Abstract

Pulsed terahertz (THz) wave technology that utilizes ultrafast laser system for THz waves generation and detection has been studied extensively for several decades. It has seen great success and interest in spectroscopy and more applications are being developed such as security scanning, bio-chemical identification, defect characterization, and so on. However, the major technical obstacles for most THz imaging and applications are absorption loss by water vapor, the difficulty in miniaturizing the system size, and the rather poorer imaging resolution than that of the conventional spectral region. Also, to overcome low signal to noise ratio, several types of semiconductors for the purpose of powerful THz wave generation and its application have been investigated. Narrow band gap semiconductor such as InAs has attracted more and more interest due to their powerful THz wave generation via photo-Dember effect. With a comparatively longer wavelength, Rayleigh’s criterion limits its use for imaging of single cells and other microstructures. Typical human cell sizes range from a few to hundreds of microns. A method for achieving sub-wavelength resolution is to adopt near-field imaging techniques by the usage of a small aperture, near-filed tip, focused optical laser beam. In this manner, we have developed sub-wavelength THz emission microscope by using MBE-grown thin InAs film and small optical-fiber THz emission tip.

In this dissertation, we have studied and demonstrated a new method of displaying temporal phase information of THz pulses for sub-wavelength imaging, whereby a semiconductor film with diffraction apertures is illuminated by ultrafast laser pulses to produce THz waves. Temporal phase advancement of THz pulses occurs when THz pulses are generated from sub-wavelength InAs emission apertures. This temporal phase shift is explained by transverse spatial confinement of sub-wavelength THz radiation from thin InAs apertures. The geometrical phase shift of rad...