Measurement of Photocarrier Mean Free Path via Speckled Laser Pump -- Transient Fourier Microscopy Probe

TL;DR

This paper introduces a novel experimental technique using transient Fourier microscopy to measure the mean free path of photocarriers, revealing hot carriers can have a micron-scale mean free path much larger than previously measured electrically.

Contribution

It presents an innovative method for directly measuring photocarrier mean free paths, especially for hot carriers, using transient grating Fourier transform and lock-in amplification.

Findings

Hot carriers in Silicon and GaAs have micron-scale mean free paths.

The measured mean free path is over an order of magnitude larger than electrical measurements.

The method enables observation of transition from diffusive to ballistic transport.

Abstract

The mean free path of photocarriers is a crucial parameter for material design, device optimization, and new optoelectronics applications. Currently, this parameter remains unknown for many materials, and experimental means available for its measurement are considerably lacking. Meanwhile, it remains an unclear issue whether the mean free path of the photogenerated high-energy hot carriers is significantly different from that of the localequilibrium-state carriers near the Fermi surface or around the band edge. Based on the concept of transient grating Fourier transform and utilizing a virtual lock-in amplification technique, we proposed and demonstrated an efficient experimental technique for measuring the mean free path of photocarriers. This method has facilitated direct observation of the photocarrier transport behavior across the transition between diffusive and ballistic motion, from which we surprisingly find that the mean free path of photogenerated hot carriers in Silicon membrane and GaAs quantum well can reach micron scale, more than an order of magnitude larger compared to the electrically-measured one. This work provides new ideas for characterization of photoelectronic devices under operating status and is expected to greatly enhance the understanding of the photocarrier transport process in opto-electronic or photonic materials.