Heterodyne light scattering

For very accurate line profile measurements, a heterodyne technique has been developed 240) which can be briefly explained as follows the light, scattered into a cone within the angle 0 b9 (50< 1 °). is focused onto the cathode of a photomultiplier. The photocurrent is proportional to the square of the incoming light amplitude but cannot follow the rapid light frequency. Any beat frequencies, however, resulting from interference between the... 

In addition to the intrinsic lack of stationarity, many of the fluctuations in the glass relax so slowly that they appear to be static sources of light scattering on the time scale of the data collection. These static contributions will introduce a heterodyne component into the observed relaxation function. If the fraction of the light which is quasi-static exceeds 90%, then the observed relaxation function can be interpreted as a heterodyne case and an analysis can be carried out. However, it is not clear that this limit is ever reached in practice. Only 60% of the light was slowly relaxing at all in polystyrene. If at least 90% of the slowly relaxing part becomes quasi-static the heterodyne case will still apply to the observed part of the relaxation function. For PMMA and PEMA this is unlikely to be the case at any temperature near Tg. 

We have performed optically heterodyne-detected optical Kerr effect measurement for transparent liquids with ultrashort light pulses. In addition, the depolarized low-frequency light scattering measurement has been performed by means of a double monochromator and a high-resolution Sandercock-type tandem Fabry-Perot interferometer. The frequency response functions obtained from the both data have been directly compared. They agree perfectly for a wide frequency range. This result is the first experimental evidence for the equivalence between the time- and frequency-domain measurements. 

In dynamic light scattering experiments the motion of a particle with constant velocity results in a Doppler shift in the frequency spectrum. In a heterodyne experiment (mixing of a small portion of the unscattered light with the scattered one on the photomultiplier) this shift Aw can be directly determined from the power spectrum [98], providing the electrophoretic mobility by the simple relation... 

Fig. 1.2.2. Schematic illustration of the various techniques used in light-scattering experiments (a) filter methods (b) homodyne (c) heterodyne.

The literature [61,63-82] r rs to QELS by many differoit names, some of which are spediic methods of implemratation. The.se include dynamic light scattering, laser scattering, laser Doppler velocimeby, intensity fluctuation spectroscopy, photon correlation spectroscopy (PCS), light beating spectroscopy and homo- and heterodyne spectroscopy. Most of the techniques discussed here are based on PCS. 

In many technical problems in hydrodynamics or aerodynamics, the velocity profile v(r,t) of a flowing medium in pipes or around solid bodies is of great importance. Doppler anemometry (Sect. 7.12) is a heterodyne laser spectroscopy technique where these velocity profiles are determined from the measured Doppler shifts of the scattered light [1503-1505]. The beam of a HeNe or Ar laser with wave vector A l passes through a volume element dV of the flowing medium. The frequency co of light scattered in the direction ks by particles with velocity v (Fig. 10.34) is Doppler-shifted to... 

Instead of proceeding directly to obtain the autocorrelation using Equation 8.67, in one common application (heterodyne) the scattered field is mixed with incident field/light and the resulting field is E = + Ef. Now the square of... 

Correlation spectroscopy is based on the correlation between the measured frequency spectrum S(co) of the photodetector output and the frequency spectrum I((jo) of the incident light intensity. This light may be the direct radiation of a laser or the light scattered by moving particles, such as molecules, dust particles, or microbes (homodyne spectroscopy). In many cases the direct laser light and the scattered light are superimposed on the photodetector, and the beat spectrum of the coherent superposition is detected (heterodyne spectroscopy) [12.81,12.82]. 

An instmmental alternative to microelectrophoresis is electrophoretic light scattering (ELS). The light scattering at migrating particles leads to phase shift (Doppler effect), which can be detected by a heterodyne DLS set-up (i.e. reference-beating with frequency shift). The method yields an intensity weighted distribution of the zeta-potential. 

Heterodyne detection method for the coherent detection of laser signals, which superposes the optical signal with a reference beam from a coherent light source (local oscillator) in contrast to homodyne detection, the local oscillator is operated at a slightly different frequency than the optical signal heterodyne measurements allow for velocity measurements via Doppler effect and are employed, e.g., in electrophoretic light scattering. 

We also realize that the induced molecular precession requires a deposition of the beam energy in the medium. Since the medium is transparent, this can only happen if part of the beam is downward shifted in frequency. Indeed, the rotation of the polarization ellipse means that the two circularly polarized components of the elliptical polarization have different frequencies (o and (o respectively, with fi) fi) 2ft. Using a heterodyne technique, we were able to measure directly the w component in the output. In this respect, we can also regard the present nonlinear optical effect as a stimulated light-scattering process in which a new frequency component at (o is generated. Details of our experiment and theoretical description will be reported elsewhere. 

Fig. 17 Velocity profiles obtained using heterodyne dynamic light scattering for different mean shear rates along the flow curve 7= Is, 7= 5s, 7= 12s 7= 15s", 7= 22s" , and...

Several well-known experimental techniques are employed in laser light scattering. Here we describe the homodyne and heterodyne methods. Both methods use a laser source for incident radiation. Both use a photomultiplier as a detector followed by an autocorrelator for computer analysis. The autocorrelator is defined as... 

Dynamic light scattering, photon correlation spectroscopy, heterodyne/ homodyne method) QELS Electromagnetic wave interaction 0,005-2... 

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