Abstract
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.
Highlights
Vibrational sum-frequency generation (VSFG) spectroscopy is a powerful tool to study the structure, orientation, and dynamics at surfaces and buried interfaces between two centrosymmetric or random-phase media at the molecular level.[1]
The advent of tunable femtosecond pulses led to the development of broadband VSFG spectroscopy, where single-shot acquisition of spectra for whole vibrational regions became possible without laboriously scanning the wavelength of a narrowband infrared source.[2]
As in the case in Refs. 11 and 14, the group-delay dispersion (GDD) in the seed/signal arm was chosen to be positive in order that the idler beam generated in the booster optical parametric amplification (OPA) stage exhibits down-chirp, which can be removed by widely available anti-reflection (AR) coated silicon windows in their positive GDD spectral range
Summary
Vibrational sum-frequency generation (VSFG) spectroscopy is a powerful tool to study the structure, orientation, and dynamics at surfaces and buried interfaces between two centrosymmetric or random-phase media at the molecular level.[1]. The narrowband VIS pulses are produced using a sum-frequency generation scheme allowing small, adjustable spectral bandwidths at high conversion efficiency.[17] The two IR beams are obtained from a supercontinuum-seeded dual-beam OPA, where the seed pulses in each OPA stage are centered at ∼1.5 μm, and the MIR pulses are extracted only at the last stage. This design provides more straightforward construction and optimization of the OPA system due to the availability of off-the-shelf optical components and sensors in the pump and seed spectral range in contrast to the MIR. The higher pulse energies at the 1.5-μm output, when the OPA is seeded at 1.5 μm, can be exploited using frequency up-conversion for generating pump pulses with sufficiently high energies for time-resolved experiments
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