Stimulated Raman scattering with a rapidly tunable non-collinear optical parametric oscillator

Abstract

Summary form only given. The efforts for developing systems which allow for fast and sensitive acquisition of Raman spectra are part of ongoing research. For instance narrowband stimulated Raman scattering (SRS) microscopy with an OPO rapidly tuned by an electro-optical modulator over of a span of 115 cm-1 has been reported just recently [1]. We present the fast acquisition of broadband Raman spectra covering a lager range of 3400-960 cm-1 via SRS with an update rate of 19.6 Hz. For realization this fast acquisition a rapidly tunable ultra-broadband phase-matched non-collinear optical parametric oscillator (NOPO) is employed [2] which provides the required range of excitation frequencies. The NOPO is pumped by a frequency-doubled high power Yb:KLu(WO4)2 thin-disk laser oscillator with a repetition rate of 34 MHz and a pulse duration of 500fs at 1030nm [3].Stimulated Raman scattering is a label-free spectroscopic method technique which provides high sensitivity and therefore enables fast acquisition times. For the generation two beams, pump and Stokes beam, have to coincide on the sample. Only in case the difference frequency matches a particular molecular vibrational frequency stimulated excitation of the vibrational transition occurs. For gathering broad Raman spectra, one of the beams has to be tuned in wavelength over a wide range. Therefore the tunability of the light source has a significant impact on the width of the spectra as well as on the velocity of the acquisition. In our experiment the output of the NOPO is used as pump and a portion of the output of the thin-disk laser as Stokes beam. Due to the non-collinear phase matching geometry of the parametric gain crystal a tuning range from 650 nm to 1200 nm is supported. The instantaneous nature of the parametric gain allows for spectral filtering by dispersive separation of the colors in time and simple resonator length detuning. Hence the system allows for generation of Raman spectra via SRS with video rates over a broad spectral range of 3600-700 cm-1.For our measurements (example see Fig. 1) the Stokes beam is modulated with an AOM at about 385 kHz. Therefore the stimulated Raman loss (SRL) of the pump beam can be detected via lock-in amplification. The minimum integration time of the lock-in amplifier of 100 μs limits the update rates of the current measurements. Aiming for a broad range from 3400-960 cm-1 the resonator length was tuned with 9.8 Hz resulting in an update rate of 19.6 Hz for each spectrum. The update rate can be further enhanced by either choosing smaller ranges or by employment of faster lock-in detection systems. The resolution is not constant over the whole spectral range. At 793 cm-1 it is about 75 cm-1 which is quite low. This can be improved by introduction of more dispersion into the resonator of the NOPO. This is subject of current work.