Tracking ultrafast change of multiterahertz broadband response functions in a photoexcited Dirac semimetal Cd$_3$As$_2$ thin film

TL;DR

This study investigates ultrafast broadband response functions in photoexcited Dirac semimetal Cd$_3$As$_2$ thin films, revealing electron dynamics and a significant refractive index change useful for ultrafast optoelectronic switches.

Contribution

It provides the first detailed measurement of multiterahertz response functions in a Dirac semimetal during ultrafast excitation, highlighting electron thermalization and large refractive index modulation.

Findings

Electron nonthermal to thermal transition observed

80% reduction in refractive index detected

Ultrafast broadband response characterized in 10-45 THz range

Abstract

The electromagnetic response of Dirac semimetals in the infrared and terahertz frequency ranges is attracting growing interest for potential applications in optoelectronics and nonlinear optics. The interplay between the free-carrier response and interband transitions in the gapless, linear dispersion relation plays a key role in enabling novel functionalities. Here we investigate ultrafast dynamics in thin films of a photoexcited Dirac semimetal Cd$_3$As$_2$ by probing the broadband response functions as complex quantities in the multiterahertz region (10-45 THz, 40-180 meV, or 7-30 $\mu$m), which covers the crossover between the inter and intraband response. We resolve dynamics of the photoexcited nonthermal electrons which merge with originally existing carriers to form a single thermalized electron gas and how it is facilitated by high-density excitation. We also demonstrate that a large reduction of the refractive index by 80% dominates the nonequilibrium infrared response, which can be utilized for designing ultrafast switches in active optoelectronics.