Broadband high resolution sum frequency generation spectrometer for molecular vibrational spectroscopy at interfaces

Abstract

Broadband Sum Frequency Generation (BB-SFG) spectrometer is a complete solution for femtosecond vibrational spectroscopy designed and manufactured by EKSPLA. System produces optically coupled femtosecond broadband midinfrared pulse covering the spectral range of the molecular "fingerprint" region and narrowband visible pulse which are directed to the sample to produce sum frequency signal. Ultrashort pulses of high intensity allow to get better signal to noise ratio using lower pulse energy thus reducing the possibility of sample modification. It is especially important for aqueous and biological samples. Single measurement can cover up to 800 cm -1 bandwidth which shortens the measurement time and lets obtaining spectrum of the same sample state at the beginning and at the end of measurement, which can be different in case of scanning. The design of BB-SFG helps to overcome the main shortcoming of common broadband SFG spectrometersa complex and energy inefficient narrowband visible pulse channel formation. We use a single all-fiber mode-locked oscillator to generate ultrashort pulses for both broadband femtosecond mid-IR and narrowband picosecond visible channels. Up to 10 µJ energy tunable broadband (>300 cm -1 ) pulses in 2.5-10 µm spectral range are mixed at the sample with less than 2 cm -1 bandwidth visible (532 nm) picosecond pulses in order to produce SFG signal at kHz repetition rates. Real-time spectral scanning technique allows even broader simultaneous spectral acquisition.

Vibrational Spectra of Liquid Interfaces with a 100 kHz Sub-1 cm–1 High-Resolution Broadband Sum-Frequency Generation Vibrational Spectrometer (HR-BB-SFG-VS)

“…In summary, a 100 kHz sub-1 cm –1 high-resolution broadband sum-frequency generation vibrational spectrometer (HR-BB-SFG-VS) is successfully built for broad applications in molecular interface studies. This sub-1 cm –1 high-resolution spectrometer is based on the OPA and SHBC unit pumped by a single femtosecond laser source, without complicated and costly systems, such as the synchronization of two laser amplifiers or the system with two amplifiers with different pulse widths using common seed pulses. , Data from the air/DMSO aqueous solution interface and air/D 2 O interfaces show that the heating effect from the 100 kHz laser can be effectively managed through control of the focusing conditions of the visible and IR beams, and the sensitivity of the spectra with high SNR and high-quality line shape can be effectively achieved. This much lower-cost, space saving, and relatively easy to construct high-repetition rate sub-1 cm –1 HR-BB-SFG-VS system can be easily adopted in many academic and research laboratories, and it may gradually phase-out the currently widely used more costly conventional BB-SFG-VS systems, associated with significantly lower spectral resolution and more significant spectral distortion …”

“…Up to now, most conventional BB-SFG-VS employed laser systems with a repetition rate of 1 kHz. − BB-SFG-VS with 5 kHz with about 1.4 cm –1 resolution and 10 kHz with about 5 cm –1 resolution was also reported. , There were also a few reports of BB-SFG-VS spectrometers operating at a repetition rate around 100 kHz on an air/solid interface, with spectral resolution around 3 cm –1 . , Most recently, EKSPLA reported a ready-to-announce commercial fiber laser and dual-amplifier-based high-resolution BB-SFG-VS with ∼3 cm –1 spectral resolution running at 10 kHz with a dual amplifier . Modeling of laser heating effects on a liquid surface showed that when the repetition rate is above 1 kHz, the heating effects essentially resemble the case of the CW laser with the same power .…”