Optical coherence classical interference

Control of the lattice vibrations in crystals has so far been achieved only through classical interference. Optical control in solids is far more complicated than in atoms and molecules, especially because of the strong interaction between the phononic and electronic subsystems. Nevertheless, we expect that the rapidly developing pulse-shaping techniques will further stimulate pioneering studies on optical control of coherent phonons. 

The phenomenon of optical interference is commonly describable in completely classical terms, in which optical fields are represented by classical waves. Classical and quantum theories of optical interference readily explain the presence of an interference pattern, but there are interference effects that distinguish the quantum (photon) nature of light from the wave nature. In this section, we present elementary concepts and definitions of both the classical and quantum theories of optical interference and illustrate the role of optical coherence. 

Obviously, classical microscopy is the first option. Conventional optical microscopy has a resolution down to about 500 nm, depending on the wavelength of the light used. This is usually not enough to establish sizes and shapes of. for instance, colloidal particles. When coherent light waves are used and the Interference analyzed by computer, normal resolution down to 1 nm, as compared with a horizontal resolution of about 10 pm, is attainable. However, over the past decades a host of other physical techniques have been developed. 

In the phase-coherent, one-color pump/probe scheme (see Section 9.1.9) the wavepacket is detected when the center of the wavepacket returns to its to position, (x)to+nT — (x)to, after an integer number of vibrational periods. The pump pulse creates the wavepacket. The probe pulse creates another identical wavepacket, which may add constructively or destructively to all or part of the original pump-produced wavepacket. If the envelope delay and optical phase of the probe pulse (Albrecht, et al, 1999) are both chosen correctly, near perfect constructive or destructive interference occurs and the total spontaneous fluorescence intensity (detected after the pump and probe pulses have traversed the sample) is either quadrupled (relative to that produced by the pump pulse alone) or nulled. As discussed in Section 9.1.9, the probe pulse is delayed, relative to the pump pulse, in discrete steps of At = x/ojl- 10l is selected by the experimentalist from within the range (ljl) 1/At (At is the temporal FWHM of the pulse) to define the optical phase of the probe pulse relative to that of the pump pulse and the average excitation frequency. However, [(E) — Ev ]/K is selected by the molecule in accord with the classical Franck-Condon principle (Tellinghuisen, 1984), also within the (ojl) 1/At range. When the envelope delay is chosen so that the probe pulse arrives simultaneously with the return of the center of the vibrational wavepacket to its position at to, a relative maximum (optical phase at ojl delayed by 2mr) or minimum (optical phase at u>l delayed by (2n + l)7r) in the fluorescence intensity is observed. 

Second, unlike classical electromagnetism, quantum problems for electrons did challenge physicists. Compare, e.g. quantum interference of electrons in the problem of weak localization and its optical counterpart, i.e. coherent back scattering of EM-waves. 

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