Optical parametric oscillators (OPOs) are attractive tools for research on tissue ablation upon infrared irradiation. Here,
we report on the performance of several mid-infrared nonlinear crystals, namely type I and type II AgGaS2 (AGS) and
type I CdSiP2 (CSP), used in synchronously-pumped OPOs tuned to a wavelength of 6.45 μm, coinciding with the amide
II absorption band of proteins. CSP-based OPOs clearly exhibit better performance in comparison to AGS: First, the
oscillation threshold with CSP is three (five) times lower than type II (type I) AGS. Second, the idler conversion
efficiency is more favourable for CSP and allows reaching 27.5 mW of idler average power, while 13 and 6 mW are
obtained with type II and type I AGS, respectively. Such performance makes CSP suitable for high power 6.45 μm
surgical applications. Preliminary ablation experiments on liver tissues with our CSP-based OPO highlight the promising
future of CSP in medical applications.
The recently developed chalcopyrite CdSiP2 is employed in a picosecond, 90°-phase-matched synchronously pumped
optical parametric oscillator pumped at 1064 nm, to produce quasi-steady-state idler pulses near 6.4 μm with an energy
as high as 2.8 μJ at 100 MHz. The train of 2 μs long macropulses, each consisting of 200 (picosecond) pulses, follows at
a repetition rate of 25 Hz. This corresponds to an average power of 14 mW. The pump depletion (conversion efficiency)
exceeds 40%. Without intracavity etalon, the 12.6 ps long mid-IR micropulses have a spectral width of 240 GHz.
We have developed a Sum-Frequency Generation (SFG) spectrometer allowing sensitive and highly resolved (1.8 cm-1) vibrational spectroscopy in the mid-infrared (2 - 10 micrometers ) using a dedicated picosecond optical parametric oscillator. Combining SFG and infrared absorption-reflection spectroscopy, we pinpoint the occurrence of dynamical charge transfer at the C60/Ag(111) interface. The induced strong infrared activation of the C60 Ag(2) mode (approximately 1445 cm-1) and its softening are quenched upon K-doping. This allows us to infer the coupling strength of the Ag(2) vibration to the t1u orbital of C60.
We have set up an infrared reflection absorption grating spectrometer working under high vacuum conditions. The infrared source is made of a tantalum cavity heated to 2500 K. Its brightness at 3000 cm-1 (emissivity > 0.7) is four times higher than that of a common silicon carbide source at 1500 K. The adsorbate absorption detection technique is based on the use of a photoelastic modulator. All those features permit a high absorbance sensitivity while avoiding the measured infrared spectrum to be affected by the absorption from residual gases in the spectrometer. The measurement of infrared spectra between 2.6 and 5 μm allowed us to specially follow the evolution of the methyl and hydroxyl stretching bands of CH3OH and CD3OD versus exposure at 80 K and versus annealing temperature for a polycrystailine platinum and a Cu(110) substrates. The strong attenuation of the OH band at low coverage indicates that, in the submonolayer coverage range, the methanol molecules are adsorbed with their OH group axes quasi parallel to both metallic surfaces. Because both the symmetric and asymmetric CH3 (or CD3) stretching modes are detected, the methyl group axis must be inclined versus the surface normals. In the multilayer coverage range, the methanol forms a thin amorphous phase which can be crystallized upon annealing. The simultaneous detection of the in-phase and out of phase components of the crystallized layer hydroxyl stretching indicates that this layer is constituted by arbitrarily oriented small crystallites.
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