 Femtosecond stimulated Raman spectroscopy

Femtosecond stimulated Raman spectroscopy, or FSRS, measures a Raman spectrum through gain induced in a short broadband probe by a narrowband Raman pump. At a molecular vibration frequency matching the pump–probe frequency difference, Stokes Raman scattering transfers energy from the pump into the probe. The narrow pump and spectrograph supply spectral selectivity while the short probe samples the system in time; the probe duration alone is not the spectral linewidth.^[ref]

The2003 apparatus of David W. McCamant, Philipp Kukura and Richard A. Mathies filtered a Ti:sapphire amplifier output to make a narrowband pump near $793\mathrm{nm}$ and generated a broadband probe in sapphire. Overlapped beams traversed the sample; a spectrograph dispersed probe and reference spectra onto a dual-diode array. The paper demonstrates gain bands in cyclohexane.^[ref]

After dark correction, let $S$ and $R$ denote probe and reference counts. The Raman pump-on pump-off gain ratio isFor an original chosen spectral channel, take $(S,R{)}_{\mathrm{off}}=(10000,10000)$ and $(S,R{)}_{\mathrm{on}}=(11440,11000)$. The normalized ratio is $1.04$, a four-percent Raman gain; the unnormalized probe ratio $1.144$ would mistake a source fluctuation for additional gain. This assumes a linear detector and a reference that tracks that fluctuation.

A second original calculation uses an $800\mathrm{nm}$ pump and a $1000\mathrm{c}{\mathrm{m}}^{-1}$ vibration. The Stokes probe channel is at wavenumber $12500-1000=11500\mathrm{c}{\mathrm{m}}^{-1}$, or about $869.6\mathrm{nm}$. A broadband probe can sample many such differences simultaneously. Spatial and temporal overlap, wavelength calibration, probe chirp and non-Raman nonlinear backgrounds still require characterization. A short pulse label does not by itself establish the time resolution of a changing molecular response.