Heterodyne correlation spectroscopy

For heterodyne correlation spectroscopy the scattered light that is to be analyzed is superimposed on the photocathode by part of the direct laser beam (Fig. 7.34). Assume the scattered light has the amplitude Es at the detector and a frequency distribution around cos, whereas the direct laser radiation l = 0 exp(—ict oO acts as monochromatic local oscillator with constant amplitude Eq. The total amplitude 

Inserting (7.69) into (7.70a), one obtains 16 terms, where three are time-independent. We note, however, that for Eg Eq the terms with E can be neglected. Furthermore, the time average E Eg) is zero. Therefore (7.70a) reduces to 

The power spectrum of the photocurrent i is then obtained from (7.56) as 

The first term represents the shot noise, the second is the dc term, and the third term gives the heterodyne beat spectrum with the difference (co — coq) and sum (o) -h coo) frequencies. The detector is not fast enough to detect the sum frequency. The output signal of the third term therefore contains only the difference frequency (co — ct)o)- If this difference frequency lies in an inconvenient frequency range, the local oscillator can be shifted with an acousto-optic modulator to the frequency COL Act) in order to bring the difference frequency 0)+ Acd — cog into an easily accessible range [947]. 

The heterodyne spectrum is a Lorentzian like the homodyne spectrum (7.67), but its maximum is shifted from wl to w = ( Ws — l)- 

For heterodyne correlation spectroscopy the scattered light that is to be analysed is superimposed on the photocathode by part of the direct laser beam. Assume the scattered light has the amplitude Eg at the detector and a 

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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. 

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]. 

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

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