Giant enhancement of terahertz high-harmonic generation by cavity engineering of Dirac semimetal

Abstract

Engineered micro- or nano-structures based on nonlinear optical materials offer versatile opportunities for optoelectronic applications. While extensive efforts have been devoted to design tailored microcavities to promote and increase the optical nonlinearities of graphene, the potential of engineering its three-dimensional counterparts -- three-dimensional Dirac semimetals -- remains largely unexplored. Here we report on exceptionally strong terahertz nonlinearities in a cavity-engineered Dirac semimetal microstructure, and demonstrate a giant enhancement of terahertz third- and fifth-order harmonic yields by more than three orders of magnitude. By fabricating a designed structure of metallic metasurface microcavities on a nanometer thin film of the threedimensional Dirac semimetal Cd3As2, we significantly enhance the near-field intensity of a picosecond terahertz excitation pulse in resonance with the microcavity eigenmode. This transiently modifies the nonlinearities of the thin film and drives the nonlinear responses of the Dirac fermions from a weakly to a deeply nonperturbative regime where the observed high-harmonic generation essentially saturates.

Figures (3)

• Figure 1: Terahertz high-harmonic generation experiment of a cavity Dirac semimetal. (A) Image of regularly arranged gold split-ring resonators microcavities on top of Cd$_3$As$_2$ nanometer thin film. (B) Illustration of THz high-harmonic generation spectroscopic experiment of a cavity Dirac semimetal. The color codes indicate the simulated electric field strength in one cavity unit. (C) Emitted electric field of bare Cd$_3$As$_2$ thin film sample (grey curve) and of the cavity sample (red curve) under the drive of an $f=0.35$ THz multicycle pulse with a peak driving THz field of $E_f=18$ kV/cm measured in far field. The grey curve is shifted vertically for clarity. (D) The corresponding frequency-domain spectra of the harmonic emission, exhibiting peaks at both the fundamental $f$ and the third-harmonic frequencies $3f=1.05$ THz. The data were recorded after a $3f$ bandpass filter.
• Figure 2: Tuning of the third-harmonic yield by varying the driving field strength. (A), Time-resolved and (B), frequency-domain spectra of third-harmonic electric fields emitted from the cavity Cd$_3$As$_2$ sample for various peak driving electric field strengths $E_f$ measured in far field. (C), Time-resolved and (D), frequency-domain spectra of third-harmonic electric fields emitted from the bare Cd$_3$As$_2$ sample for various peak driving electric field strengths $E_f$ measured in far field. The data are recorded after a $3f$-bandpass filter. The curves are shifted vertically for clarity. (E), The intensity of the third-harmonic yield versus the driving pulse intensity for the cavity-engineered Cd$_3$As$_2$ sample and the bare Cd$_3$As$_2$ sample. The bare Cd$_3$As$_2$ sample exhibits a nonperturbative dependence $I_{3f} \propto I^{2.3}_f$ (dashed line), in clear contrast to the perturbative behavior $I_{3f} \propto I^3_f$ (solid line). The dotted lines indicate power-law fits to the experimental data. In the cavity-engineered sample, the third-harmonic yield is enhanced by more than three orders of magnitude at the lowest driving field, and approaches nearly saturation towards higher fields.
• Figure 3: Simultaneous observation of terahertz third- and fifth-order harmonic generation. (A), Time-resolved emitted electric field and (B), the corresponding frequency-domain spectra from a THz field driven bare Cd$_3$As$_2$ sample and its microcavity heterostructure for a driving peak field of 72 kV/cm (measured in far field) at the frequency $f=0.35$ THz. The emitted electric fields are recorded after a $5f$-bandpass filter. (B), The corresponding spectra in the frequency domain exhibit clear fifth- and third-harmonic generation in addition to the signal of the fundamental frequency. The frequency-domain spectra of the emitted electric fields (C), from the cavity and (D), from the bare Cd$_3$As$_2$ thin-film samples for various driving electric fields from to 29 to 72 kV/cm. (E), The intensity of the fifth-harmonic yield versus the driving pulse intensity for the bare and the cavity-engineered Cd$_3$As$_2$ samples. For the bare Cd$_3$As$_2$, the fifth-harmonic generation at lower drive intensities can hardly be resolved within the experimental uncertainties. In contrast, the fifth-harmonic generation in the heterostructure is enhanced by more than two orders of magnitude, whose fluence dependence follows $I_{5f} \propto I^{1.9}_f$ towards the highest field (dotted line). This deviates far from $I_{5f} \propto I^{5}_f$, hence the system is deep in the nonperturbative nonlinear regime.