Phonon generation

Systematic TRSHG studies on alkali-atom adsorbed metal surfaces by Matsumoto and coworkers provided a deep insight on how coherent motions are created under very different electronic configurations [15, 77, 78]. The results showed that the coherent phonon generation critically depends on the surface and bulk electronic structure of the substrate. 

The earliest control experiments were performed in double- (or multiple-) pump and probe scheme on optical phonons generated via ISRS in transparent materials by Nelson and coworkers [24,25], Shortly later, similar experiments were carried out on coherent phonons generated in semiconductors via TDFS by Dekorsy and coworkers [26], and on those generated in semimetals via DECP by Hase and coworkers [27] (Fig. 2.1 in the previous chapter). These experiments demonstrated that the amplitude of the coherent oscillation can be controlled by varying the temporal separation At between the two pump pulses. At = nT leads to the maximum enhancement of the amplitude with an integer n and the phonon period T, while At = (n + 1/2)T results in complete cancelation. 

In order to image the interaction of phonons with internal structure in a specimen, scattering by thermal phonons can be used to attenuate the acoustic beam in a cryogenic microscope (Foster 1984). Experiments to test this confirmed that the cryogenic microscope was indeed able to detect on one surface of a sapphire disc the arrival of thermal phonons generated by a... 

Here N(sQ is the electron density of states on the Fermi surface for one direction of spin, is the effective volume of phonon generation, is the point contact form factor, averaged over the Fermi surface. It should be noted that point contacts of sizes d > l, d l can work also in diffusive or thermal current regimes [5] and are used for the study of EPI, phase transitions, superconductivity and other interesting physical phenomena. 

Doublets of folded longitudinal acoustic (LA) phonons due to the superlattice periodicity [143] can also be seen in the Raman spectra of the SLs (indicated by arrows in Fig. 21.2). The positions of the doubled peaks agree well with the first doublet frequencies calculated within the elastic continuum model [144]. The observation of the LA phonon folding suggests that these superlattices possess the requisite structural quality for acoustic Bragg mirrors and cavities to be used for potential coherent phonon generation applications [145-147]. 

As indicated above, for solids it is possible to transfer energy between nuclear spins and the lattice via phonons generated within the sample. Kastler [4] and Altershulter [5-7] proposed that it should also be possible to utilize this path in reverse, with applied phonons (ultrasound) causing a nuclear spin system to experience a net absorption of energy, and result in detectable acoustic nuclear magnetic resonance (ANMR) spectra. 

In ceramics, the heat flow is primarily due to phonon generation, and the thermal conductivity is generally lower than that of metals. Crystalline structures, such as alumina and beryllia, are more efficient heat conductors than amorphous structures such as glass. Organic materials used to fabricate printed circuit boards or epoxy attachment materials are highly amorphous electrical insulators, but tend to be very poor thermal conductors. 

K Krai, B Hejda. Long-wavelength LO-phonon generation during hot-electron cooling in polar semiconductors. Phys Rev B 48 11,461-11,464, 1993. 

Figuie 6.2 Lower part Spectral profile of a photochemical hole with and without phonon generation during the detection process. Upper part Directly measured difference between these two profiles as obtained by modulating the IR light at a frequency of 150 Hz. 

Finally, a phononic band gap stmcture could be incorporated to improve photovoltaic efficiency. Due to zero phonon population within a phononic band gap frequency range, materials with a phononic band gap stmcture may reduce phonon generation, i.e., thermalization, which is a major energy loss mechanism in... 

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