Fig. 2: Third harmonic generation from terahertz-driven superconducting fluctuations.
From: Fano interference between collective modes in cuprate high-Tc superconductors
a Terahertz transmission from an overdoped superconducting La2−xSrxCuO4 thin film as it is pumped by a 0.7 THz driving pulse. The latter drives the 2ω amplitude oscillation of the superconducting order parameter (see Supplementary Materials S5 for a clear experimental visualization of this 2ω oscillation from transient reflectivity measurements). This 2ω collective excitation scatters the driving photon, leading to third harmonic generation (THG) in transmission. The driving field (red) and the THG (blue) waveforms are extracted from the raw transmission data (black) using 1.4 THz Fourier lowpass and highpass filters. b Zoomed-in view of the transmitted driving field and THG from an optimally doped La2−xSrxCuO4 thin film at 30 K and 45 K. The dotted line marks the crest of the driving field, which shifts in time due to the superconducting screening effect. It can be seen that THG does not remain in phase with the driving field at the two temperatures. c THG intensity (I3ω) and phase (Φ3ω, i.e., relative to the linear driving field phase) as a function of temperature from NbN. The peak in I3ω(T) and the continuous evolution of Φ3ω(T) across Tπ indicate the resonance in THG. The error bars on I3ω indicate the noise floors of the FFT power spectra; the error bars on Φ3ω denote the uncertainties in fitting the THG phase and the linear drive phase.
