Simplified formulas for the generation of terahertz waves from semiconductor surfaces excited with a femtosecond laser

TL;DR

This paper derives simple formulas to explain terahertz wave generation from semiconductor surfaces excited by femtosecond lasers, aiding understanding and application in semiconductor research.

Contribution

It introduces simplified expressions for THz emission from semiconductors excited by femtosecond lasers, considering photocarrier dynamics and structures like metal-insulator-semiconductor interfaces.

Findings

Emission proportional to electron mobility and laser intensity.

Diffusion-related emission depends on the squared diffusion coefficient.

Formulas applicable to laser THz emission microscopy (LTEM).

Abstract

We derive simple formulas to explain terahertz (THz) emission from semiconductor surfaces excited by a femtosecond (fs) laser. Femtosecond optical pulses with energies larger than the bandgap create photocarriers which travel and generate THz radiation, according to the time derivative of the photocurrent. By assuming that only electrons traveling in an ultrafast time scale, less than a few hundred fs, contribute to THz radiation, one can obtain simple expressions for the emission originating from the photocarrier drift accelerated with a built-in field or from the photocarrier diffusion. The emission amplitude of the former is in proportion with electron mobility, the Schottky-Barrier height, and the laser intensity and one of the latter with the laser intensity and diffusion coefficient squared. We also discuss the formula for emission from metal-insulator-semiconductor structures. The derived expressions are useful in understanding the THz emission properties observed by a laser THz emission microscope (LTEM), bringing LTEM into real applications in the field of semiconductor research and development.