Ultrafast Carrier Relaxation and Second Harmonic Generation in a Higher-Fold Weyl Fermionic System PtAl

TL;DR

This study investigates ultrafast carrier relaxation and strong second harmonic generation in the topologically nontrivial crystal PtAl, revealing potential for ultrafast optoelectronic applications and providing insights into its electronic and nonlinear optical properties.

Contribution

It presents the first detailed analysis of carrier relaxation dynamics and SHG in PtAl, a higher-fold Weyl fermionic system, highlighting its exceptional nonlinear optical response.

Findings

Carrier relaxation involves acoustic and optical phonons at 0.06 and 2.94 THz.

PtAl exhibits a large SHG susceptibility of 468 pm/V.

SHG signal shows a non-perturbative dependence on intensity.

Abstract

In topological materials, shielding of bulk and surface states by crystalline symmetries has provided hitherto unknown access to electronic states in condensed matter physics. Interestingly, photo-excited carriers relax on an ultrafast timescale, demonstrating large transient mobility that could be harnessed for the development of ultrafast optoelectronic devices. In addition, these devices are much more effective than topologically trivial systems because topological states are resilient to the corresponding symmetry-invariant perturbations. By using optical pump probe measurements, we systematically describe the relaxation dynamics of a topologically nontrivial chiral single crystal, PtAl. Based on the experimental data on transient reflectivity and electronic structures, it has been found that the carrier relaxation process involves both acoustic and optical phonons with oscillation frequencies of 0.06 and 2.94 THz, respectively, in picosecond time scale. PtAl with a space group of $P$$2_{1}$3 allows only one non-zero susceptibility element i.e. $d_{14}$, in second harmonic generation (SHG) with a large value of 468(1) pm/V, which is significantly higher than that observed in standard GaAs(111) and ZnTe(110) crystals. The intensity dependence of the SHG signal in PtAl reveals a non-perturbative origin. The present study on PtAl provides deeper insight into topological states which will be useful for ultrafast optoelectronic devices.