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Conference on Optoelectronic and Microelectronic Materials and Devices (COMMAD)

Femtosecond three-pulse photon echo and transient grating studies of the yellow band of GaN

Lap Van Dao, Martin Lowe, Peter Hannaford

Published Aug 25, 2005• 0 citations
Publisher

Abstract

Using femtosecond three-pulse two-colour photon echo measurements we study the optical nonlinearity induced by pump and probe in the short time scale (< 500 fs) where the free induction decay and photon echo are the main contribution to observed signal and in a long time scale (up to 20 ps) in the trace of the population grating. The variation of the pump and probe wavelength and use of spectrally resolved measurements allows us to study the origin of the yellow band and the phase and energy relaxation properties of this band. The results suggest that the light emission in the yellow band is the recombination between shallow donors and deep acceptors and the carriers trapping time is very short.

DOI
10.1109/commad.2002.1237248

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Laser Physics Letters•Oct 7, 2004•12 citations
Using femtosecond three-pulse two-colour photon echo measurements we study the optical nonlinearity induced by pump and probe in the short time scale (< 500 fs) where the free induction decay and photon echo are the main contribution to observed signal and in a long time scale (up to 20 ps) in the trace of the population grating. The variation of the pump and probe wavelength and use of spectrally resolved measurements allows us to study the origin of the yellow band and the phase and energy relaxation properties of this band. The results suggest that the light emission in the yellow band is the recombination between shallow donors and deep acceptors and the carriers trapping time is very short.
The nonlinear properties of an undoped GaN epitaxial layer in the spectral region of the yellow band have been investigated using two- and three-pulse two-colour photon echo measurements. The optical nonlinearity induced by three sequential femtosecond pulses is studied on a short timescale (<500 fs) where free induction decay and photon echoes are the main contributions to the observed signal and on a long timescale (up to 20 ps) in the case of a population grating. Variation of the wavelength of the femtosecond pulses and the use of spectrally resolved measurements allows us to study the origin of the yellow band and the phase and energy relaxation properties of this band. The results suggest that the emission of light in the yellow band arises from recombination between shallow donors and deep acceptors and that the trapping time of carriers into defect levels is very short (<1 ps).
GaN/AlN quantum dots are semiconductor nanostructures where the huge conduction band offset of 1.75 eV leads to intraband transitions at exceptionnaly short wavelengths compared to III-As semiconductor materials. For specific quantum dot dimensions, it was shown that the absorption spectrum covers the optical communication spectral range. The ultrafast relaxation dynamics observed for these materials motivates the development of opto-electronic devices, and especially ultrafast switches. However, a detailed insight into the microscopic processes governing the relaxation dynamics is still missing although of fundamental importance for the understanding and the optimization of the devices operation. In particular, there is no study of the homogeneous broadening which drives the saturation properties of GaN/AlN quantum dots-devices. In this paper, we present homogeneous linewidth measurements by means of the nonlinear optical technique of spectral hole-burning. Spectrally-resolved pump-probe experiments with cw-lasers allow us to characterize the coherence relaxation dynamics down to 5K by resolving the homogeneous linewidth in an ensemble of GaN/AlN quantum dots. The differential transmission measured at 5K for a TM-polarized pump shows a pronounced bleaching with an amplitude of 2x10-4 on a background signal of 5x10-5. According to the selection rules of the s-pz intraband transition in GaN/AlN QDs, we attribute the photoinduced increase of the probe transmission to the partial saturation of the quantum dots by the pump laser, and we deduce a homogeneous linewidth of 15 meV at 5K. We find that the differential transmission signal scales like the square-root of the incident pump power, thus revealing the importance of electron-electron scattering in the population relaxation dynamics. As a matter of fact, we find that the population relaxation from the pz excited state is predominantly governed by Auger-type processes whereas optical phonon emission only plays a minor role, even if it may contribute to secondary processes during the carrier cascade to the fundamental s state via intermediate confined states.
The first observation is reported of the primary and stimulated femtosecond photon echoes at liquid nitrogen and room temperatures in a dye-doped polymer film at a wavelength of 780 nm. Echo signals were generated by pulses with the duration of 130 fs. The primary echo signal decay at a liquid nitrogen temperature can not be described by a simple exponential function. There is a fast decay region below 250-300 fs and a slow decay region at longer time intervals. The decay of primary echo signal at room temperature demonstrates complex behavior caused by the presence of the phonon-side band in the optical band of impurity centers. There were also detected signals of the self-diffraction of exciting pulses on a nonequilibrium population difference grating, induced in resonant medium by these pulses.
The first observation is reported of the primary femtosecond photon echo at a liquid nitrogen temperature in a dye-doped polymer film at a wavelength of 780 nm. Echo signals were generated by pulses with the duration of 130 fs. The echo signal decay can not be described by a simple exponential function. There is a fast decay region below 250-300 fs and a slow decay region at longer time intervals. There were also detected signals of the self-diffraction of exciting pulses on a nonequilibrium population difference grating, induced in resonant medium by these pulses.