# Theoretical Study of High Harmonic Generation in Monolayer NbSe2

## TL;DR

This paper studies how light interacts with a single layer of NbSe2 to generate high harmonics, revealing how its electronic structure can be probed and controlled.

## Contribution

The paper predicts the high harmonic generation response of monolayer NbSe2 using theoretical simulations, revealing polarization-dependent harmonic control.

## Key findings

- Monolayer NbSe2 can generate even harmonics due to lack of inversion symmetry.
- Even harmonics arise from transverse current response under zigzag polarization.
- Odd harmonics come from longitudinal current response in all polarization directions.

## Abstract

High harmonic generation
(HHG) is a powerful probe of
electron
dynamics on attosecond to femtosecond time scales and has been successfully
used to detect electronic and structural changes in solid-state quantum
materials, including transition-metal dichalcogenides (TMDs). Among
TMDs, bulk NbSe2 exhibits charge density wave (CDW) order
below 33 K and becomes superconducting below 7.3 K. Monolayer NbSe2 also has superconducting and CDW behavior and is therefore
interesting as a material whose different structural and electronic
properties could be probed via HHG. Here, we predict the HHG response
of the pristine 2H and CDW phases of monolayer NbSe2 using
real-time time-dependent density functional theory under the application
of a simulated laser pulse excitation. We find that due to the lack
of inversion symmetry in both monolayer phases, it is possible to
excite even harmonics and that the even harmonics appear as the transverse
components of the current response under excitations polarized along
the zigzag direction of the monolayer, while odd harmonics arise from
the longitudinal current response in all excitation directions. This
suggests that the even and odd harmonic responses can be controlled
via the polarization of the probing field, opening an avenue for potentially
useful applications in optoelectronic devices.