Optimized Spintronic Terahertz Emitters Based on Epitaxial Grown Fe/Pt Layer Structures

TL;DR

This paper demonstrates optimized Fe/Pt bilayer spintronic emitters for broadband terahertz radiation, achieving up to 8 THz bandwidth with low pump power, and models the spin current generation process.

Contribution

It introduces an optimized design of spintronic terahertz emitters based on epitaxial Fe/Pt layers, with a comprehensive model of spin current generation and experimental validation.

Findings

Achieved up to 8 THz bandwidth in THz emission.

Optimized layer thicknesses match spin current simulations.

Low pump power of 25 mW suffices for effective THz generation.

Abstract

We report on generation of pulsed broadband terahertz radiation utilizing the inverse spin Hall effect in Fe/Pt bilayers on MgO and sapphire substrates. The emitter was optimized with respect to layer thickness, growth parameters, substrates and geometrical arrangement. The experimentally determined optimum layer thicknesses were in qualitative agreement with simulations of the spin current induced in the ferromagnetic layer. Our model takes into account generation of spin polarization, spin diffusion and accumulation in Fe and Pt and electrical as well as optical properties of the bilayer samples. Using the device in a counterintuitive orientation a Si lens was attached to increase the collection efficiency of the emitter. The optimized emitter provided a bandwidth of up to 8 THz which was mainly limited by the low-temperature-grown GaAs (LT-GaAS) photoconductive antenna used as detector and the pulse length of the pump laser. The THz pulse length was as short as 220 fs for a sub 100 fs pulse length of the 800 nm pump laser. Average pump powers as low as 25 mW (at a repetition rate of 75 MHz) have been used for terahertz generation. This and the general performance make the spintronic terahertz emitter compatible with established emitters based on nonlinear generation methods.