New generation of 1-μm pumped mid-IR light sources for interface-specific nonlinear spectroscopy

A wide range of fundamental chemical and biological processes is governed by molecular interactions at interfaces, where structural and dynamic properties of biomacromolecules can differ significantly from their behavior in the bulk. Gaining molecular-level insight into these systems is crucial for understanding interfacial functions and for the rational design of advanced materials. Vibrational sum frequency generation (VSFG) spectroscopy provides powerful surface specificity and chemical selectivity, enabling in situ determination of interfacial molecular structure and orientation. However, conventional broadband VSFG systems face limitations in both spectral resolution and acquisition speed, constraining their applicability for real-time studies of dynamic biomolecular interfaces.

To address these challenges, recent progress in diode-pumped solid state Yb laser technology, combined with the development of novel wide-bandgap non-oxide nonlinear optical crystals, has created new opportunities for nonlinear vibrational spectroscopy in the molecular fingerprint region. In this context, we demonstrate high-efficiency, single-stage optical parametric amplifiers (OPA) based on novel materials such as LiGaS₂, BaGa₄S₇, and Cd_xHg_1-xGa₂S₄. These systems facilitate direct frequency down-conversion of femtosecond Yb-laser pulses (1.03 μm, 100 kHz) into broadly tunable mid-infrared pulses spanning the 4.5‑13.5 μm spectral range. As an example, starting from 4 W of average pump power at 1.03 μm, the OPAs deliver near transform-limited, few-cycle pulses with average powers exceeding 30 mW and durations as short as 101 fs at 10.6 μm (2.9 optical cycles). This approach demonstrates significantly higher conversion efficiencies of novel, non-oxide-based nonlinear crystals compared to conventional cascaded schemes.

In this talk, I will present our recent results in mid-infrared light source development and demonstrate how the integration of such OPAs with VSFG spectroscopy substantially enhances both temporal and spectral performance, enabling real-time, high-resolution probing of macromolecular interactions at interfaces.