Femtosecond pump-probe spectroscopy of propagating coherent acoustic phonons inInxGa1−xN∕GaNheterostructures

Abstract

We show that large amplitude, coherent acoustic phonon wavepackets can be generated and detected in InxGa1−xN/GaN epilayers and heterostructures in femtosecond pump-probe differential reflectivity experiments. The amplitude of the coherent phonon increases with increasing Indium fraction x and unlike other coherent phonon oscillations, both amplitude and period are strong functions of the laser probe energy. The amplitude of the oscillation is substantially and almost instantaneously reduced when the wavepacket reaches a GaN-sapphire interface below the surface indicating that the phonon wavepackets are useful for imaging below the surface. A theoretical model is proposed which fits the experiments well and helps to deduce the strength of the phonon wavepackets. Our model shows that localized coherent phonon wavepackets are generated by the femtosecond pump laser in the epilayer near the surface. The wavepackets then propagate through a GaN layer changing the local index of refraction, primarily through the Franz-Keldysh effect, and as a result, modulate the reflectivity of the probe beam. Our model correctly predicts the experimental dependence on probe-wavelength as well as epilayer thickness.

Theory of coherent phonons in carbon nanotubes and graphene nanoribbons

“…The development of ultrafast, femtosecond laser sources has enabled researchers to study dynamical properties of molecular systems, semiconductor nanostructures, and carbon nanotubes [20,[60][61][62][63][64][65][66]. Ultrafast femtosecond lasers are ideal for studying electron and hole dynamics since scattering rates typically range from 10 to 100s of femtoseconds in most semiconductors [67,68].…”

Intrinsic coherent acoustic phonons in the indirect band gap semiconductors Si and GaP

“…Such scheme enables the generation of intense acoustic pulses and thereby allows a detailed study * ishioka.kunie@nims.go.jp of acoustic diffraction and dispersion in semiconductors [14][15][16]. One can alternatively use semiconductor heterostructures, such as InGaN/GaN quantum wells and GaN p-n junctions [17][18][19][20][21], as transducers; in these cases intense acoustic phonons are generated via the selective absorption of the pump pulse in the quantum wells and the screening of the built-in piezoelectric field.…”

The influence of charge and magnetic order on polaron and acoustic phonon dynamics in LuFe2O4

“…11)), and c-axis sound velocity, respectively, [27][28][29][30][31] and the measured oscillation period s p $ 48 ps. In our experiments, doubling the probe photon energy caused the oscillation frequency to double, supporting coherent acoustic phonon generation as the origin of the oscillations.…”

“…The observed oscillations are typically attributed to coherent acoustic phonons, which can be manifested as propagating strain waves, generated through the dynamic stress induced by absorption of the femtosecond laser pulses within the penetration depth. 26 These strain waves have often been observed in ultrafast optical experiments on correlated electron materials [27][28][29] and semiconductors, 30,31 with an oscillation period that depends on the probe wavelength, given by…”