# Simulation of Hot-Carrier Dynamics and Terahertz Emission in Laser-Excited Metallic Bilayers

## TL;DR

This paper introduces a multiscale simulation model for understanding and optimizing terahertz emission from laser-excited metallic bilayers, specifically Fe/Pt heterostructures, by modeling spin-current dynamics and electromagnetic wave propagation.

## Contribution

It presents a novel multiscale approach combining microscopic charge carrier transport with electromagnetic modeling to predict THz emission and guide design improvements.

## Key findings

- The model accurately reproduces experimental THz emission characteristics.
- Stacked layer configurations can significantly enhance THz generation efficiency.
- Optimization strategies include using anti-reflection coatings and multilayer stacking.

## Abstract

Metallic bilayer structures have been shown to emit strong terahertz (THz) pulses. We present a predictive multiscale mode that simulates optically induced spin-currents in a Fe/Pt-heterostructure and the emitted electric field. Electronic effects are treated on the nano scale using the Boltzmann transport equation for the dynamics of out-of-equilibrium charge carriers, numerically solved with a particle-in-cell code. The optical effects are simulated with a formalism that bridges the nanometer scale of the structure to the micrometer scale of the emitted waves. The approach helps to understand recent experimental findings on the basis of microscopic scattering effects and transport phenomena. Our theory's versatility allows it to be readily adapted to a wide spectrum of spintronic THz emitter designs. As an example, we show how the THz generation efficiency, defined as output to input power ratio, can be improved and optimized using serially stacked layers in conjunction with THz anti-reflection coatings.