Dependence of the stimulated Raman scattering threshold in droplets on the temporal stretching of a nanosecond laser pulse

Abstract

For linear Raman spectroscopy in micro-droplets the excitation laser energy and therefore the signal strength is limited either by the threshold for the excitation of nonlinear processes in the droplets or by the generation of a plasma (gas breakdown) in the vicinity of the droplets. These processes are due to the enhancement of the excitation field by morphology-dependent resonances (MDRs) which are built up in the cavity synthesised by the droplets. As these stimulated signals disturb the detection of the linear signal it is of high interest for different applications to suppress the stimulated Raman scattering (SRS) or to use an excitation intensity below the SRS threshold with a sufficiently large integrated laser energy to obtain an appropriate signal-to-noise ratio for the linear Raman process. In this paper the effect of temporal stretching of the incoming laser pulse with the help of an optical pulse stretcher on the SRS threshold is described and discussed.

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