Fig. 3: THz-THG signal from FeSe_0.5Te_0.5 in equilibrium.

a Schematic illustration of the terahertz third-harmonic generation (THz-THG) measurement. Narrowband THz probe pulse with the center photon energy of \({\hbar} {\omega}\) = 2 meV (0.5 THz in frequency) is used. A near-infrared pump pulse, used for the photoexcitation, is also shown by red. The results shown in b, c are obtained without the pump pulse. b Time-domain waveform of the transmitted THz pulses at 18 and 4.3 K (red and blue, respectively), along with the power spectra shown in the inset. 1.5 THz bandpass filters are inserted after the sample to reduce the transmitted fundamental component. c Temperature dependence of the squared magnitude of the THG susceptibility, \(\left| {\chi ^{\left( 3 \right)}} \right|^2 = I_{{\mathrm{TH}}}/\left( {I_{\mathrm{F}}} \right)^3\), with intensities of the observed TH and fundamental signals, I_TH and I_F. Error bars represent the standard deviation. The fit curve by assuming the Higgs mode response represented by Eq. (1) in the text is also shown by the solid line.