Origin of the side peak in terahertz third-harmonic generation

Determine whether the side peak near 1.97 THz in the 2.1 THz terahertz third-harmonic-generation spectrum of 1T-TaS2 results from quadratic coupling between the 0.7 THz terahertz field and the 1.43 THz Raman-active phonon, and identify the other degree or degrees of freedom responsible for the resulting periodic modulation of the phonon amplitude and terahertz third-harmonic signal.

Background

The authors observe a side peak around 1.97 THz when 1T-TaS2 is driven by a 2.1 THz third-harmonic-generation configuration using a 0.7 THz multicycle terahertz field. Related optical pump–probe measurements reveal periodic amplitude modulation of a 1.43 THz Raman-active phonon, as well as of the 2.05 and 2.37 THz modes, suggesting coherent energy transfer between the phonon and additional degrees of freedom.

The proposed explanation is not established: the authors speculate that the 0.7 THz field couples quadratically to the 1.43 THz phonon, while an unresolved coherent coupling to other degrees of freedom produces the observed modulation and side peak. Establishing the relevant coupling mechanism and participating degrees of freedom would clarify the microscopic origin of the terahertz nonlinear response.