Generation of THz radiation via polariton parametric scattering in a rectangular LiNbO3 waveguide
Résumé
Polariton parametric scattering in polar crystals such as Lithium Niobate (LN) is a promising method to generate coherent THz radiations [1]. This process has been mostly investigated to deliver tuneable narrow-band THz radiation [2] when a long pulse pumps the material. A significant challenge hindering this technique lies in the generation of a THz spectrum centred at frequencies exceeding 2 THz, primarily due to the gain spectrum of LN, and the important absorption coefficient at higher frequencies [3][4]. In this manuscript, we highlight the possibility to overcome this limitation by pumping the LN in a transient regime. An ytterbium based ultra-fast laser, delivering ultra-short pulse at a repetition rate of 85 kHz has been used to pump a 500 × 500 μm rectangular LiNbO₃ waveguide at room temperature. The length of the waveguide is 15 mm. Electro-optic detection together with the measured spectrum at the output of the waveguide have provided a comprehensive characterization of the temporal and spectral profiles of the generated THz wave, along with associated Stokes fields. Figure 1 shows an example of a temporal trace recorded when the average power is set at 600 mW. The fast Fourier spectrum of the EO sampling measurement (Figure 2) shows a broadband peak with a width of ~ 4 THz (at full-width at half maximum) centred at 3 THz. The optical spectra at the output of the waveguide is shown in Figure 3. This latter shows multiple stokes fields emitted from the spontaneous polariton parametric scattering process. Our results agree with the theoretical phonon polariton dispersion curve and the diffraction modified Schwarz-Maier plane wave model [5]. In this case, the mode area mismatch between the optical and the THz fields together with the unique waveguide structure which minimize the diffraction-induced absorption lead to a modification in the gain spectrum, centred at 3 THz. Given the compatibility of waveguides with on-chip fabrication and their compact footprint, this research is a good step forward to develop a good platform for realizing on-chip THz radiation generation through nonlinear frequency conversion processes.