Fig. 2: Terahertz four-wave mixing in a HgTe Dirac semimetal.
From: Highly efficient broadband THz mixing and upconversion with Dirac materials
a Time-domain dynamics of the sub-THz pulses from the TELBE facility (blue-green line) and a photoconductive antenna (PCA, red line) passed through the HgTe layer and detected with electro-optic sampling (EOS). The gray shaded area indicates the time window used to calculate the fast Fourier transform (FFT) spectrum from TELBE. b The echo is attributed to the internal reflection of the THz pulse, where the temporal signal shift directly corresponds to the sample thickness. c The power FFT of the time-domain sub-THz pulses from TELBE (blue-green) and PCA (red). d Time-domain dynamics of the resulting FWM signal. The black line shows a negligible FWM signal from the bare CdTe substrate. The THz fields from TELBE and PCA below 1 THz as well as the third harmonic from TELBE above 2 THz are suppressed in the detection path using bandpass filters (BPFs), as schematically shown in Fig. 1a. e The shaded area shows the power FFT of the THz FWM, revealing two pronounced maxima at the frequency sum fhigh = 2fT + fa and difference flow = 2fT–fa. The red solid line shows the calculated spectrum as described in the text. The vertical axes in (a), (c), (d) and (e) are normalized such that the area under the power FFT from the PCA signal is equal 1.
