Terahertz third-harmonic generation of lightwave driven Weyl fermions far from equilibrium

Abstract

We report on time-resolved ultrafast terahertz third-harmonic generation spectroscopy of nonequilibrium dynamics of Weyl fermions in a nanometer thin film of the Weyl semimetal TaP. Terahertz third-harmonic generation is observed at room temperature under the drive of a multicycle narrowband terahertz pulse with a peak field strength of down to tens of kV/cm. The observed terahertz third-harmonic generation exhibits a perturbative cubic power-law dependence on the terahertz drive. By varying the polarization of the drive pulse from linear to elliptical, we realize a sensitive tuning of the third harmonic yield. By carrying out theoretical analysis based on the Boltzmann transport theory, we can properly describe the experimental results and ascribe the observed THz nonlinearity to field-driven kinetics of the Weyl fermions.

Figures (3)

• Figure 1: (a) Crystal structure of TaP, with two mirror planes indicated by the grey planes. The THz electric field is parallel to the surface of a (001)-oriented thin film sample plane. (b) Sketch of the time-resolved THz third-harmonic generation spectroscopy. Two band pass filters (BPF) with central frequency of $1f=0.5$ THz and two with $3f=1.5$ THz are used for preparing a narrowband driving pulse and detection of THG, respectively. (c) Time-domain traces of the driving pulse and the third-harmonic emission from the sample, with the frequency-domain Fourier spectra of the time-domain signals in (d).
• Figure 2: (a) Sketch of tuning THz drive field strength by using two wire-grid polarizers (WGPs). The field strength varies with $E_0\cos^2\alpha$, where $\alpha$ represents the polarizer angle, without changing the field polarization. (b) Emitted THz field is recorded through a $3f$ bandpass filter in the time domain, for various peak field strengths of an $f=0.5$ THz drive. (c) Frequency-domain spectra derived through Fourier transformation, exhibiting an evident driving field dependence of the THG intensity. (d) Fluence dependece of the integrated THG intensity follows a cubic power-law dependence i.e. $I_{3f} \propto I_f^3$ (solid line).
• Figure 3: (a) Time-domain signal of the emitted third harmonic field $E_{3f}$ for $f=0.5$ THz at various angles $\beta$ of the THz quarter wave plate (QWP). $\beta=0^\circ$ corresponds to linear polarization. For elliptical polarization the dominant parallel component $E_\parallel$ is recorded. (b) The corresponding frequency-domain spectra derived through Fourier transformation. (c) Integrated THG intensity $I_{3f,\parallel}$ versus $\beta$. (d) Schematic illustration of the driving pulse ellipticity tunable from an incoming linearly polarized THz wave by a QWP.