Coherent Phonon Signal Research Articles

The influence of surfaces and interfaces on coherent phonons in semiconductors

Experiments have shown that ultrafast optical excitation of semiconductors can produce oscillating changes in the optical properties of the material. The frequency of the oscillations in transmission or reflection usually matches one of the phonon modes, typically theq= 0 optical mode. These oscillations are known as coherent phonons. We discuss the role of surfaces and interfaces on the coherent phonon signal. We show that: (1) the coherent phonon signal can be used as a probe of the surface depletion field and (2) multiple interfaces as in a superlattice, can drastically alter the coherent phonon spectrum: screening of the modes in the superlattices is reduced and acoustic modes can now be excited.

Observation of coherent plasmon–phonon coupled mode in InAs

We have used reflective electro-optic sampling (REOS) technique combined with 20 fs laser pulses to detect coherent phonon signals. We observe the plasmon–phonon coupling mode and its excitation power dependence in InAs. In addition, we interpreted relative intensity of coupled mode L − to LO phonon in terms of the numerical calculation based on phenomenological equations of motion for polarization and lattice displacement.

Real time-space dynamics of zone-folded phonons in [formula omitted] superlattices

Using time delay modulation technique, we have studied the zone-folded longitudinal acoustic phonons in GaAs AlAs superlattices. Up to seven phonon modes are clearly observed and their wave vectors correspond to those expected in forward and backward Raman scattering experiments. This technique leads to the observation of coherent phonon signal of the lowest frequency mode of 1.5 cm −1 and characteristic abrupt changes of its amplitude and phase in the time domain. They are well explained by the back and forth propagation of the single phonon wave packet in the GaAs AlAs superlattices. These experimental findings show quite new aspects of the dynamics of coherent phonons.

Investigation of field, carrier, and coherent phonon dynamics in low-temperature grown GaAs

We compare the dynamics of electric field, transport, and coherent phonons in as-grown and annealed low-temperature (LT) GaAs by an electro-optic technique on a subpicosecond time scale. The buildup and decay of space-charge fields associated with the photo-Dember effect are investigated. The recombination dynamics of trapped carriers is monitored via the ps decay of the electro-optic signal. Differences in annealed and as-grown LT GaAs are related to the different microscopic form of excess arsenic and point defect density. In the coherent phonon signal a large red shift of the LO phonon and an additional local vibration below the LO phonon provides information on structural defects in as-grown LT GaAs.