Propagation of optically generated acoustic phonons in Si

Ja, Shields; Me, Msall; Ms, Carroll; Jp, Wolfe

doi:10.1103/physrevb.47.12510

Cited by 36 publications

(18 citation statements)

References 17 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Following optical excitation, namely, phonons are generated as a result of rapid carrier relaxation. 11,12 Even at the lowest excitation densities used, a helium gas bubble develops near the phonon source, 13 preventing the escape of phonons from the pumped zone directly into the liquid helium.…”

Section: B Phonon Generationmentioning

confidence: 99%

Transport of 29cm−1phonons in hydrogenated amorphous silicon

1996

View full text Add to dashboard Cite

The scattering of 29 cm Ϫ1 phonons in hydrogenated amorphous silicon ͑a-Si:H͒ at 2 K is investigated by means of optically generated heat pulses and the ruby phonon detector. The transport is shown to be governed by elastic spatial diffusion and a diffusion coefficient of ϳ1 cm 2 /s. Resonant scattering by soft oscillators, as well as Rayleigh scattering by microvoids or by the disordered network itself account for the experimental findings. In optically excited a-Si:H, the transmission of 29 cm Ϫ1 phonons is reduced by inelastic scattering, proportional to the optical excitation density in a-Si:H and recovers on a 40-ns time scale. These results are discussed in terms of interaction of 29 cm Ϫ1 phonons with optically generated long-lived very high-frequency phonons in a-Si:H. ͓S0163-1829͑96͒00541-3͔

show abstract

Section: B Phonon Generationmentioning

confidence: 99%

Transport of 29cm−1phonons in hydrogenated amorphous silicon

1996

View full text Add to dashboard Cite

show abstract

“…The crystal must also be very weakly photoexcited. Figure 2 shows the remarkable changes in the heat pulses as we decrease the laser spot size at constant pulse energy, an effect discovered by Shields et al [4,6]. A large increase in the intensity near the ballistic time is observed, together with a decrease in the tail signal.…”

Section: Quasidiffusion and The Localized Phonon Source In Photoexcitmentioning

confidence: 56%

Quasidiffusion and the localized phonon source in photoexcited Si

et al. 1993

Self Cite

View full text Add to dashboard Cite

Previous observations of heat pulses produced by localized photoexcitation of silicon do not support the predictions of phonon "quasidiffusion" via anharmonic decay and elastic scattering. Our experiments, with controlled boundary conditions, verify that quasidiffusive theory is relevant in Si under very weak photoexcitation. Beyond this domain a transition to a localized source of low frequency phonons is attributed to excited carrier interactions. Photoluminescence experiments confirm the presence of electronhole droplets coincident with this localized phonon source. PACS numbers: 66.70.+f, 63.20.Hp, 63.20.LsIn a perfectly harmonic lattice, vibrational waves or phonons of a given frequency are constrained to remain at that frequency. Anharmonicity of the lattice allows a phonon to split into lower frequency phonons, conserving energy and momentum. This down-conversion process (and the corresponding up-conversion) makes the propagation of thermal energy in a solid quite different from that of particles in a gas. If anharmonic decay-a.k.a., inelastic scattering-is combined with a more rapid elastic scattering of phonons from defects in the lattice (such as that due to impurities or isotopic randomness of atoms), then the motion of nonequilibrium phonons from a point source in an otherwise cold crystal is by "quasidiffusive" expansion [l]. In this case, whenever an acoustic phonon undergoes anharmonic decay to a lower frequency (r^ocy 5 ), a large increase in the effective diffusion constant (typically a factor of 2 4 ) occurs due to the rapid frequency dependence of the elastic scattering rate(r-, ocv 4 ) [2].It is predicted [1,3] that quasidiffusion produces a broad distribution in the arrival times of acoustic phonons crossing a crystal. Surprisingly, localized photoexcitation of a Si crystal immersed in a liquid-helium bath produces temporally sharp heat pulses and sharp phonon-focusing caustics [4], characteristic of low frequency (subterahertz) phonons traveling ballistically from the excitation region to the detector. Since the phonons produced by the thermalization of photoexcited carriers should be rich in frequencies above 1 THz, it appears that either the anharmonic decay rate is much faster than calculated or some other process is bypassing this relaxation mechanism.

show abstract

“…In our experiment, the penetration depth of the laser radiation is about 50 pm. In other experiments it can be as short as 1 pm [2]. These distances are much shorter than the mean free path of phonons in the quasidiffusion process, when their frequencies fall below 2 THr.…”

Section: Introductionmentioning

confidence: 80%

The influence of thin helium films on the phonon spectrum of optically excited silicon

1995

View full text Add to dashboard Cite

We have studied the phonon spectra resulting from laser pulse excitation of a silicon surface We used a phonon spectrometer based on pressure tuned boron levels in silicon (Si: B-spectrometer). A new design allowed us to use the spectrometer in vacuum for the first time. We observed phonon frequencies up to 1 THz. The pulse shapes indicated both, quasidiffusion and a long exciton lifetime. On adding helium films to the surface we have found dramatic changes of the spectrum. Full thermalisation was reached already at a thickness of 1.5 monolayers. This indicates a strong anharmonic interaction in the very first monolayers at these high frequencies and, in addition, a strong confinement of the phonons to the surface by quasidiffusion.

show abstract

Propagation of optically generated acoustic phonons in Si

Cited by 36 publications

References 17 publications

Transport of 29cm−1phonons in hydrogenated amorphous silicon

Transport of 29cm−1phonons in hydrogenated amorphous silicon

Quasidiffusion and the localized phonon source in photoexcited Si

The influence of thin helium films on the phonon spectrum of optically excited silicon

Contact Info

Product

Resources

About