60,924 research outputs found
Sensitivity of coherent anti-Stokes Raman lineshape to time asymmetry of laser pulses
We show that coherent anti-Stokes Raman lineshapes do not follow known
spectral profiles if the time asymmetry of realistic laser pulses is taken into
account. Examples are given for nanosecond and picosecond laser pulses commonly
employed in frequency-resolved coherent anti-Stokes Raman scattering. More
remarkably, the analysis suggests an effect of line narrowing in comparison to
the customary approach, based primarily on the Voigt lineshape.Comment: 8 pages, 3 short sections, 4 figure
The Interplay of Structure and Dynamics in the Raman Spectrum of Liquid Water over the Full Frequency and Temperature Range
While many vibrational Raman spectroscopy studies of liquid water have
investigated the temperature dependence of the high-frequency O-H stretching
region, few have analyzed the changes in the Raman spectrum as a function of
temperature over the entire spectral range. Here, we obtain the Raman spectra
of water from its melting to boiling point, both experimentally and from
simulations using an ab initio-trained machine learning potential. We use these
to assign the Raman bands and show that the entire spectrum can be well
described as a combination of two temperature-independent spectra. We then
assess which spectral regions exhibit strong dependence on the local
tetrahedral order in the liquid. Further, this work demonstrates that changes
in this structural parameter can be used to elucidate the temperature
dependence of the Raman spectrum of liquid water and provides a guide to the
Raman features that signal water ordering in more complex aqueous systems
Femtosecond Covariance Spectroscopy
The success of non-linear optics relies largely on pulse-to-pulse
consistency. In contrast, covariance based techniques used in photoionization
electron spectroscopy and mass spectrometry have shown that wealth of
information can be extracted from noise that is lost when averaging multiple
measurements. Here, we apply covariance based detection to nonlinear optical
spectroscopy, and show that noise in a femtosecond laser is not necessarily a
liability to be mitigated, but can act as a unique and powerful asset. As a
proof of principle we apply this approach to the process of stimulated Raman
scattering in alpha-quartz. Our results demonstrate how nonlinear processes in
the sample can encode correlations between the spectral components of
ultrashort pulses with uncorrelated stochastic fluctuations. This in turn
provides richer information compared to the standard non-linear optics
techniques that are based on averages over many repetitions with well-behaved
laser pulses. These proof-of-principle results suggest that covariance based
nonlinear spectroscopy will improve the applicability of fs non-linear
spectroscopy in wavelength ranges where stable, transform limited pulses are
not available such as, for example, x-ray free electron lasers which naturally
have spectrally noisy pulses ideally suited for this approach
Nanoscale Vibrational Analysis of Single-Walled Carbon Nanotubes
We use near-field Raman imaging and spectroscopy to study localized vibrational modes along individual, single-walled carbon nanotubes (SWNTs) with a spatial resolution of 10−20 nm. Our approach relies on the enhanced field near a laser-irradiated gold tip which acts as the Raman excitation source. We find that for arc-discharge SWNTs, both the radial breathing mode (RBM) and intermediate frequency mode (IFM) are highly localized. We attribute such localization to local changes in the tube structure (n, m). In comparison, we observe no such localization of the Raman active modes in SWNTs grown by chemical vapor deposition (CVD). The direct comparison between arc-discharge and CVD-grown tubes allows us to rule out any artifacts induced by the supporting substrate
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