Autocorrelation study of terahertz and second-harmonic pulsed generations in reflection at the surface of a nonlinear dielectric crystal

Second-order nonlinear generation at the surface of a nonlinear dielectric crystal by two successive laser pulses separated by a variable delay is studied in reflection. For the very first time, the waveforms of the terahertz radiation generated via optical rectification and the second-harmonic generation are measured simultaneously versus the time delay between two pump laser pulses using detectors sensitive to the beam powers. The resulting autocorrelation signals are first recorded using a very high temporal resolution over a short time window, which allows us to acquire the terahertz part of the related spectra as well as the spectral content at the fundamental and second-harmonic frequencies for both the terahertz and the second-harmonic waveforms. Next, the envelopes of the autocorrelation signals are recorded over a longer window using a coarse temporal resolution. In contrast, this permits us to obtain the terahertz part of the spectra of both the terahertz and the second-harmonic waveforms with high spectral precision but at the cost of the fundamental and second-harmonic parts at higher frequencies. To conclude, this technique facilitates a new route for surface spectroscopy at terahertz frequencies, since nonlinear generation in reflection occurs within the surface region of the crystal, that could be useful for studies of surface energy states, oxidation, nanometer-thick films deposited at the surface, adsorption of molecules, etc.