Photon beam coherence

Interaction of an excited-state atom (A ) with a photon stimulates the emission of another photon so that two coherent photons leave the interaction site. Each of these two photons interacts with two other excited-state molecules and stimulates emission of two more photons, giving four photons in ail. A cascade builds, amplifying the first event. Within a few nanoseconds, a laser beam develops. Note that the cascade is unusual in that all of the photons travel coherently in the same direction consequently, very small divergence from parallelism is found in laser beams. 

Coherent Raman spectroscopy Coherent Raman spectroscopy is a term that refers to a series of closely related nonlinear Raman techniques in which the scattered Raman radiation emerges from the sample as a coherent beam -coherent meaning that the photons are all in phase with one another. The coherent techniques include Stimulated Raman Scattering (SRS), Coherent anti-Stokes Raman Spectroscopy (CARS), Coharent Stokes Raman Spectroscopy (CSRS), and Stimulated Raman Gain Spectroscopy (SRGS). Although most of the nonlinear Raman techniques are also coherent techniques, there is one incoherent nonlinear Raman process called Hyper Raman. 

When a photon beam passes through matter, photons can undergo scatter. In the interaction they change direction, with or without loss of energy. In coherent Rayleigh... 

One extension in which there is now renewed interest was performed by the group at Stirling. This was to perform a full polarisation analysis of the coincident photons from which the Stokes parameters may be obtained. The state which was studied was the 3 P state of helium which decays to the 2 S state by emission of a visible photon (501.6nm) in addition to the decay to the ground state with a uv photon (53.7nm). The photons are viewed normal to the scattering plane and the Stokes parameters associated with the linear (ni and ria) circular polarisation (TI2) measurements were obtained. The coherence of a photon beam can be denoted by the coherence parameter defined as... 

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. 

R. Hanbury Brown, R., Twiss, R.Q., 1956, Correlation between photons in two coherent beams of light. Nature 177, 27 Hong, C.K., Ou, Z.Y., Mandel, L., 1987, Measurement of subpicosecond time intervals between two photons by interference, Phys. Rev. Lett. 59, 2044 Kimble, H.J., Dagenais, M., Mandel, L., 1977, Photon Antibunching in Resonance Fluorescence, Phys. Rev. Lett. 39, 691... 

Hanbury Brown, R., Twiss, R. Q., 1956, Correlation between photons in two coherent beams of light, Nature 177, 27 Quirrenbach, A., 2001, Astronomical interferometry, from the visible to sub-mm waves. Europhysics News 32... 

It is highly likely that by the second decade of the new millennium silicon-based computing will have reached fundamental technological or physical limits. Computers will therefore be based on substrates that exhibit superior performance characteristics. One possibility is the photon. Optoelectronic devices, which use substrates such as gallium arsenide, permit the interconversion of electrons and photons. Hybrid computers, which may already be available commercially by 2010, would use silicon for computation and photons for data transfer. The coherent modulation of very-high-frequency light beams enables many high-capacity... 

In addition to the surface/interface selectivity, IR-Visible SFG spectroscopy provides a number of attractive features since it is a coherent process (i) Detection efficiency is very high because the angle of emission of SFG light is strictly determined by the momentum conservation of the two incident beams, together with the fact that SFG can be detected by a photomultiplier (PMT) or CCD, which are the most efficient light detectors, because the SFG beam is in the visible region, (ii) The polarization feature that NLO intrinsically provides enables us to obtain information about a conformational and lateral order of adsorbed molecules on a flat surface, which cannot be obtained by traditional vibrational spectroscopy [29-32]. (iii) A pump and SFG probe measurement can be used for an ultra-fast dynamics study with a time-resolution determined by the incident laser pulses [33-37]. (iv) As a photon-in/photon-out method, SFG is applicable to essentially any system as long as one side of the interface is optically transparent. 

Insertion devices are placed in the electron path of a synchrotron. They increase the photon flux by several orders of magnitude. Similar to the FEL principle they operate by forcing the electrons on a wavy path. At each bend of the path synchrotron light is emitted. In contrast to the FEL device there is no coherence. Instead, the light intensity sums up to form the effective beam. Two kind of insertion devices are used. In wigglers the curvature of the electron path is high. In undulators it is relatively low. 

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