Terahertz Hyper-Raman Time-Domain Spectroscopy

Abstract

A new spectroscopic method has been demonstrated on the benchmark crystal α-SiO2. The new technique makes use of femtosecond optical pulses and intense, sub-ps, broadband terahertz (THz) pulses to generate a THz-optical four wave mixing in the investigated material. The spectrum of the generated signal is resolved in wavelength and displays two pronounced frequency sidebands close to the optical second harmonic central frequency 2ωL, where ωL is the optical central frequency of the fundamental beam. The two sidebands develop around the central frequency at the (anti-) Stokes side of ωs;a = 2ωL ∓ ωT, where ωT is the THz central frequency, thus resembling the spectrum of standard hyper-Raman scattering, and hence, we named this effect "THz Hyper-Raman"—THYR. Due to the large laser and THz bandwidths, it is not possible to resolve the THYR signal in the frequency domain. Nonetheless, by taking advantage of the same principle at work in THz time-domain spectroscopy, it is possible to follow the evolution of the THYR signal in time and access the frequency domain again by Fourier Transform. In this way we were able to observe pronounced oscillations in time of the THYR signal whose frequencies correspond to a large variety of material excitations including Γ-point phonons, polaritons, and phonons out of the Γ-point, which are usually observed only by neutron scattering techniques. To complement the richness of these observations, we will show that the selection rules of the THYR process allow the simultaneous observation of both IR- and Raman-active material modes, thus highlighting the potential of this innovative experimental method.