The optical fiber bundle depicted in Fig. 2 delivered the Raman scattered light to a spectrograph (HolospecTM VPT System, Kaiser Optical Systems Inc., Ann Arbor, MI), where it was dispersed onto a array back-illuminated deep-depletion CCD camera, CCD1 (Model DU 420-BR-DD, Andor Technology, Belfast, Northern Ireland), achieving a spectral resolution of approximately . A dichroic beam splitter (Chroma Technology Corp., Bellows Falls, Vermont) and a holographic notch filter (Kaiser Optical Systems Inc., Ann Arbor, MI) were used to reject the elastically scattered light and pass only the Stokes-shifted light to the spectrograph. Finally, because of the limited height of the CCD array, only spectra from the 40 central fibers of the fiber bundle were imaged onto CCD1. The position of each sample with respect to the laser illumination spot was confirmed by a white-light image of the sample plane acquired with a second CCD camera, CCD2.