Enhanced laser-induced single-cycle terahertz generation in a spintronic emitter with a gradient interface

TL;DR

This paper demonstrates that a gradient interface in a spintronic THz emitter doubles the efficiency of laser-to-THz conversion by increasing laser-driven spin current transmittance, with performance affected by laser fluence.

Contribution

The study introduces a gradient interface in a spintronic emitter, significantly improving THz pulse generation efficiency over conventional abrupt interfaces.

Findings

Gradient interface doubles THz emission efficiency.

Enhanced transmittance of spin-polarized current through gradient interface.

Efficiency decreases at higher laser fluences due to spin accumulation.

Abstract

The development of spintronic emitters of broadband THz pulses relies on designing heterostructures where processes of laser-driven spin current generation and subsequent spin-to-charge current conversion are the most efficient. An interface between ferromagnetic and nonmagnetic layers in the emitter is one of the critical elements. Here, we study experimentally single-cycle THz pulse generation from a laser-pulse excited Pt/Co emitter with a composition gradient interface between Pt and Co and compare it with the emission from a conventional Pt/Co structure with an abrupt interface. We find that the gradient interface enhances the efficiency of optics-to-THz conversion by a factor of two in a wide range of optical fluences up to 3 mJ cm$^{-2}$. We reveal that this enhancement is caused by a pronounced increase in transmittance of the laser-driven spin-polarized current through the gradient interface compared to the abrupt one. Furthermore, we find that such a transmission deteriorates with laser fluence due to the spin accumulation effect.