Coherence narrowing

Finally, there is another possibility of arranging interaction between light and the analyte. It is called open optical path sensing. Here, no physical waveguide is used. The beam of light is emitted from a coherent, narrow bandwidth source and... 

Fig.16.9. Coherence narrowing and collision broadening o the width of the Hanle effect signals on the state of lead. (After Happer and Saloman (1967).)...

The emitted beam of coherent radiation is narrow and can be focused into a very small area. This means that the density of radiation that can be delivered for any one pulse over a small area is very high, much higher than can be delivered by conventional light sources operating with similar power inputs. 

Because of the unique features of the x-ray radiation available at synchrotrons, many novel experiments ate being conducted at these sources. Some of these unique features are the very high intensity and the brightness (number of photons per unit area per second), the neatly parallel incident beam, the abihty to choose a narrow band of wavelengths from a broad spectmm, the pulsed nature of the radiation (the electrons or positrons travel in bunches), and the coherence of the beam (the x-ray photons in a pulse are in phase with one another). The appHcations are much more diverse than the appHcations described in this article. The reader may wish to read the articles in the Proceedings of the Materials Research Society Hsted in the bibhography. 

Bouche Th., Drier Th., Lange B., Wolfrum J., Franck E. U., Schilling W. Collisional narrowing and spectral shift in coherent anti-Stokes Raman spectra of molecular nitrogen up to 2500 bar and 700 K, Appl. Phys. B50, 527-33 (1990). 

Koszykowski M. L., Farrow R. L., Palmer R. E. Calculation of collisionally narrowed coherent anti-Stokes Raman spectroscopy spectra, Opt. Lett. 10, 478-80 (1985). 

The light produced by a laser has a much more narrow wavelength than the light of an LED or other light sources (Fig. 15.3). In addition, laser light is coherent, i.e., the photons travel in parallel paths from the source. Lasers made of thin-films are similar to bulk lasers (He-Ne, ruby) except that they are more compact and efficient. Due to the short lifetime of the photons, high-frequency modulations are possible. 

In the time-domain detection of the vibrational coherence, the high-wavenumber limit of the spectral range is determined by the time width of the pump and probe pulses. Actually, the highest-wavenumber band identified in the time-domain fourth-order coherent Raman spectrum is the phonon band of Ti02 at 826 cm. Direct observation of a frequency-domain spectrum is free from the high-wavenum-ber limit. On the other hand, the narrow-bandwidth, picosecond light pulse will be less intense than the femtosecond pulse that is used in the time-domain method and may cause a problem in detecting weak fourth-order responses. 

Figure 3.1 (a) Schematic diagram (not to scale) of Young s double-slit experiment. The narrow slits acts as wave sources. Slits S and S2 behave as coherent sources that produce an interference pattern on screen C. (b) The fringe pattern formed on screen C could look like this. (Reproduced with permission from R. A. Serway Physics for Scientists and Engineers with Modern Physics, 3rd ed, 1990, Saunders, Figure 37.1.)... 

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