Vibrational Coherence Modulated THG Spectroscopy

We examine a pump-probe method measuring surface vibrational coherences. We excite vibrations through non-resonant impulsive stimulated Raman scattering in a variety of solid, thin crystal samples and probe through coherence-modulated third-harmonic generation (THG).

Figure 1 shows THG signals collected as a function of pump-probe delay for three different solid crystal materials: Bi₃Ge₄O₁₂; BaF₂, and LiTaO₃. A Fourier transformation yields Raman spectra that are in good agreement with published vibrational mode frequencies.

THG is sensitive to material interface and discontinuities in a tight focusing geometry. Under the experimental conditions, three contributions are evident:

• Direct perturbation of the third harmonic generation tensor via coherent second-hyper Raman scattering. The perturbation has the form and dominates when the focus is just inside the sample.
  
  
• Cascaded amplitude effects from the perturbation of the linear susceptibility, which will modulate both the fundamental of the probe as well as the already-generated third harmonic. The perturbation is proportional to .
  
  
• Modulation of Fresnel coefficients through the alteration of the linear susceptibility at the interface. This dominates when the focus is just outside the sample interface. The Fresnel coefficient modulation alters the signal in the form of -.