Jan Greve

Polarization sensitive coherent anti-Stokes Raman scattering spectroscopy of the amide I band of proteins in solutions

Polarization sensitive coherent anti-Stokes Raman scattering (PCARS) spectroscopy is a fruitful technique to study Raman vibrations of diluted molecules under off-electron resonant conditions. We apply PCARS as a direct spectroscopic method to investigate the broad amide I band of proteins in heavy water. In spontaneous Raman spectroscopy, this band is not well resolved. We fit a number of spectra taken of each protein under different polarization conditions, with a single set of parameters. It then appears that some substructure is observed in the amide I band. From this substructure, we determine the percentage of alpha-helix, beta-sheet, and random coil for the proteins lysozyme, albumin, ribonuclease A, and alpha-chymotrypsin.

Coherence effects in laser Doppler blood flowmetry

In laser doppler blood flowmetry (LDBFM), the flow is derived from the first moment of the power spectrum of the photocurrent. This quantity depends on both the spectral width and the modulation depth of the signal. This modulation depth depends on the amount of speckles on the detector surface. The speckle size is determined by the wavelength, the local wave form and the angular distribution of the incoming light.In theoretical treatments of laser speckle, the distance between the light source and the illuminated screen or detector is often taken much larger than the light source dimensions. In direct contact LDBFM where the detector is placed directly on or close to the skin, such assumptions do not hold, making analytic solutions of the problem impossible. The issue is studied by means of a model experiment. A scattering medium is illuminated by tow beams originating from the same laser, one of which is shifted in frequency. With a detector close to the surface of the medium, beats are observed at the difference frequencies. From the modulation depth of these beats, it is possible to derive the speckle size. The main purpose of this paper is to present this conceptually simple experiment. The first experimental results are shown. Although still of limited quality, they show the potential of the method to study the effects of scattering anisotropy, mean free path length, the detector size and its distance to the tissue.

Noise suppression and correction of laser Doppler blood flow spectra after high-pass filtering

Laser Doppler flowmetry (LDF) is a method that can be used for measuring blood flow changes in the microcirculation. We have contributed to the development of a new device for LDF, based on digital signal processing. A method for correcting the disregarding of frequency components was developed, by approximating the noise-free Doppler spectrum with an exponential shape. The frequency components from 40 kHz to 50 kHz can be used to correct for white noise. We introduced variable resistors for the case common mode components from both detectors have different magnitudes. However, after adjustment we found that noise may still be present. We have observed, that cutting off at 150 Hz suppresses many noise contributions and still provides sufficient Doppler information. For the transfer of a moment calculated from 150 Hz - 20 kHz into 0 - infinity Hz, the correction method mentioned above can be applied.

Coherence effects in modeling laser Doppler perfusion flowmetry

When performing direct-contact laser-Doppler flowmetry on experimental flow models, the power spectra of the detector signal can be obtained by homodyne or by heterodyne detection. Especially with uniformly moving probe particles coherence effects are observed, leading to changes in the width of the power spectrum. Due to destructive interference effects at the detector surface, in the measured homodyne spectra the contribution of relatively high Doppler frequencies is suppressed compared with that of lower frequencies. This results in spectra which are narrower than expected theoretically. This effect allows us to investigate the importance of the relative amount of coherence areas at the surface of the detector.