Pump-induced thermo-optic manifestation lead adiabatic ultrashort-pulse optical parametric generation in long LiNbO₃ crystals

Optical parametric processes offer a powerful tool to generate frequency-tunable broadband optical pulses across any spectral band. They have been widely employed for a variety of applications in the field of spectroscopy and medical science. The conventional route to generate broadband mid-infrared (MIR) pulses through optical parametric processes rely on 'gain-bandwidth' trade-off relationship and therefore, thin ${\chi }^{\left(2\right)}$-based nonlinear optical crystals are used for this purpose. Adiabatic following in optical parametric processes offer to partially relax the stringent requirement imposed by the phase-matching condition and consequently, this allows deployment of long length crystals. Therefore, adiabatic following provides a plausible route for efficient broadband frequency conversion to MIR spectrum using widely-available near-infrared (NIR) laser sources. We present a simple route to generate broadband (ultrashort) pulses through a single-pass optical parametric generation (OPG) process through satisfying the adiabatic constraints. The experimental configuration is based on realizing an optimum longitudinal temperature gradient using an ultrashort pump pulse laser ($\Delta \tau \approx 370$ fs). This enables creation of a suitable conditions for obeying the adiabatic constraints for broadband MIR generation and the OPG process delivers Fourier-transform limited ultrashort signal pulses tunable across $150$nm spectral band in the NIR spectrum as well as compressed ultrashort idler pulses tunable across $400$nm-band in the MIR spectrum. The adiabatic OPG process result in conversion efficiencies ≈70% across the entire frequency tunable range. (paper)