Interferometry, radiation detection

To see how interferometry can detect the difference between resonant and nonresonant processes, consider the experimental set-up shown in Figure 2 (a) and the shape of the anti-Stokes pulses produced by the long pump and short Stokes pulses previously mentioned. This combination of pulses is illustrated in Figure 2 (b). en the pump and Stokes pulses overlap, the molecule will be excited by SRS. This excitation will remain after the Stokes pulse passes. At the moment of the overlap, nonresonant four-wave-mixing processes can also be excited. However, because there is no persistent state associated with nonresonant processes, the nonresonant emission will end quickly after the Stokes pulse passes. With a Raman-active resonance, however, the pump can produce anti-Stokes radiation via SRS even after the Stokes has passed, because the resonance persists. Therefore, the nonresonant component can be discarded by rejecting any anti-Stokes radiation that occurs coincident with the Stokes pulse. 

What Is Interferometry (1.3) Interferometry deals with the physical phenomena which result from the superposition of electromagnetic (e.m.) waves. Practically, interferometry is used throughout the electromagnetic spectrum astronomers use predominantly the spectral regime from radio to the near UV. Essential to interferometry is that the radiation emerges from a single source and travels along different paths to the point where it is detected. The spatio-temporal coherence characteristics of the radiation is studied with the interferometer to obtain information about the physical nature of the source. 

The fundamental quantity for interferometry is the source s visibility function. The spatial coherence properties of the source is connected with the two-dimensional Fourier transform of the spatial intensity distribution on the ce-setial sphere by virtue of the van Cittert - Zemike theorem. The measured fringe contrast is given by the source s visibility at a spatial frequency B/X, measured in units line pairs per radian. The temporal coherence properties is determined by the spectral distribution of the detected radiation. The measured fringe contrast therefore also depends on the spectral properties of the source and the instrument. 

Transition radiation is considerably weaker than Cerenkov radiation, however since it is a surface phenomenon it avoids problems with radiator thickness and reflections inherent to Cerenkov-generating silica plates. Optical TR can be measured using a streak camera. An optical TR system has been used to time-resolve the energy spread of an electron macropulse in a free-electron laser facility [10]. Interferometry of coherent, far-infrared TR has been used to measure picosecond electron pulse widths and detect satellite pulses at the UCLA Satumus photoinjector, using charges on the order of 100 pC [11],... 

Photoacoustic detection is one of a class of photo-thermal detection techniques that can be used to measure the optical absorption of a sample by monitoring the absorption of modulated ultraviolet (UV)/ visible or infrared radiation and its subsequent conversion to heat by nonradiative processes to produce a periodic thermal signal. The signal is normally measured by the effect that the periodic heat flow has on the absorbing medium, i.e., the sample, or any medium in contact with the sample. For example, measurement of the change in refractive index that occurs on absorption of a modulated laser beam in a transparent or semitransparent medium is the basis of thermal lens detection, photothermal interferometry, and photothermal refraction. The use of these detection techniques and quantitation of absorption at different excitation wavelengths gives rise to photothermal spectrometry. 

The benefit of interferometry is that the time of arrival of the anti-Stokes radiation can be found very precisely by cross-correlating a broad bandwidth reference pulse with the anti-Stokes radiation. Incoherent detection can detect the interference between the resonant and nonresonant components of the anti-Stokes radiation, but does not directly detect the time of arrival of the anti-Stokes light. Figure 2 shows the actual measured cross-correlations between a short reference pulse and the anti-Stokes radiation for a cuvette of acetone with a resonance at 2925 cm as the frequency difference between the pump and Stokes is tuned (i). 

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