Development of a Tunable Ultra-Broadband Mid IR Pulsed Source for Nonlinear Spectroscopy

We generate ultra- broadband mid-IR pulses tunable from 2.5 -- 8 $\mu $m by focusing 800 nm/400 nm pulses into various gas media. The input 800 nm light is doubled to 400 nm in a type I BBO crystal. The two orthogonally polarized $\omega $/2$\omega $ pulses encounter a birefringent calcite crystal for time delay compensation and are subsequently focused in various gas media (air, argon, neon and nitrogen) contained within a 1.2 m gas cell using a 1 m focal length silver mirror. The tunability of the broadband mid-IR pulses arises from different gases, pressure of gases and the amount of incident 800 nm/400 nm light focused into the gas cell at a given pressure. We measure IR energies as high as 0.5 $\mu $J/pulse for an input 800 nm energy of 3 mJ/pulse in 900 Torr of Argon. The mid IR pulses exhibit $\sim $2{\%} long term stability. The ultrabroadband IR pulses have a spectral bandwidth of $\sim $2000 cm$^{-1}$ corresponding to a sub-cycle pulse centered at 4.5 $\mu $m. We will present our preliminary efforts on using the ultrabroadband IR pulses in nonlinear experiments. The broad spectral content of this novel source affords the possibility of probing multiple vibrations in a coherent manner.