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Compact, high-repetition-rate source for broadband sum-frequency generation spectroscopy
We present a high-efficiency optical parametric source for broadband vibrational sum-frequency generation (BB-VSFG) for the chemically important mid-infrared spectral range at 2800-3600 cm-1 to study hydrogen bonding interactions affecting the structural organization of biomolecules at water interfaces. The source consists of a supercontinuum-seeded, dual-beam optical parametric amplifier with two broadband infrared output beams and a chirped sum-frequency mixing stage providing narrowband visible pulses with adjustable bandwidth. Utilizing a pulse energy of only 60 μJ from a turn-key, 1.03-μm pump laser operating at a repetition rate of 100 kHz, the source delivers 6-cycle infrared pulses at 1.5 and 3.2 μm with pulse energies of 4.6 and 1.8 μJ, respectively, and narrowband pulses at 0.515 μm with a pulse energy of 5.0 μJ. The 3.2-μm pulses are passively carrier envelope phase stabilized with fluctuations at the 180-mrad level over a 10-s time period. The 1.5-μm beamline can be exploited to deliver pump pulses for time-resolved studies after suitable frequency up-conversion. The high efficiency, stability, and two orders of magnitude higher repetition rate of the source compared to typically employed systems offer great potential for providing a boost in sensitivity in BB-VSFG experiments at a reduced cost.
Few-cycle 65-µJ pulses at 11.4 µm for ultrafast nonlinear longwave-infrared spectroscopy
Low-energy excitations can provide insight into the basic ultrafast nonequilibrium dynamics of condensed matter. High-energy femtosecond pulses in the long-wavelength infrared are required to induce such processes, and can be generated in an optical parametric chirped pulse amplification (OPCPA) system comprising three GaSe stages. A femtosecond Cr:ZnS laser serves as the front-end, providing the seed for the 2.0-µm pump and the 2.4-µm signal pulses without nonlinear conversion processes. The OPCPA system is pumped at 2.05 µm by a picosecond Ho:YLF regenerative amplifier at a 1-kHz repetition rate. The recompressed idler pulses at 11.4 µm have a duration of 185 fs and an unprecedented energy of 65 µJ, corresponding to a pump-to-idler conversion efficiency of 1.2%. Nonlinear transmission experiments in the range of the L2 infrared band of liquid water demonstrate the potential of the pulses for nonlinear vibrational spectroscopy of liquids and solids.