Professor Robert Taylor

Abstract:

We present femtosecond pump-probe reflectance measurements of excitons in GaN for various pump intensities. Saturation of the excitonic absorption with increasing carrier density has been measured in the case of resonant and non-resonant excitations during the rising times, and the exciton bleaching densities for various excitation energies have also been measured. We found that the resonant excitons are bleached at higher densities (∼10 times) than the non-resonant excitons due to the absence of free carriers. The stimulated emission mechanism is investigated by measuring simultaneously the photoluminescence and the time-resolved reflectance near the band edge, over a range of excitation densities. The onset of the stimulated emission coincides with the bleaching density of the non-resonant excitons as well as a theoretical gain threshold density. These results suggest that the stimulated emission in GaN is due to the electron-hole plasma.

Abstract:

We present femtosecond pump-probe reflectance measurements of excitons in GaN for various pump intensities. Saturation of the excitonic absorption with increasing carrier density has been measured in the case of resonant and non-resonant excitations during the rising times, and the exciton bleaching densities for various excitation energies have also been measured. We found that the resonant excitons are bleached at higher densities (similar to10 times) than the non-resonant excitons due to the absence of free carriers. The stimulated emission mechanism is investigated by measuring simultaneously the photoluminescence and the time-resolved reflectance near the band edge, over a range of excitation densities. The onset of the stimulated emission coincides with the bleaching density of the non-resonant excitons as well as a theoretical gain threshold density. These results suggest that the stimulated emission in GaN is due to the electron-hole plasma.

Abstract:

InxGa1-xN quantum dots have been fabricated by the selective growth of GaN micropyramid arrays topped with InGaN/GaN quantum wells. The spatially, spectrally, and time-resolved emission properties of these structures were measured using cathodoluminescence hyperspectral imaging and low-temperature microphotoluminescence spectroscopy. The presence of InGaN quantum dots was confirmed directly by the observation of sharp peaks in the emission spectrum at the pyramid apices. These luminescence peaks exhibit decay lifetimes of approximately 0.5 ns, with linewidths down to 650 μeV (limited by the spectrometer resolution). © 2004 American Institute of Physics.

Abstract:

Transient gain spectra were measured for an In0.02Ga 0.98N / In0.16Ga0.84N multiple quantum well using the variable stripe length method (VSLM) in combination with the ultrafast optical Kerr gate (OKG) technique. Gain dynamics were measured for a range of excitation lengths from short (50 μm) to long (350 μm) stripes with the sample under femtosecond photoexcitation. Analysis of the temporal behaviour of gain and chemical potential suggests that stimulated emission originates from a photoexcited electron-hole plasma at early times; at later times localized states dominate as the electron-hole plasma becomes exhausted. Gain reduction at early times is attributable to coupling of the electron-hole plasma with photons along the stripe, whilst localized states are less susceptible to gain saturation. © 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Abstract:

This paper reports on time-integrated and time-resolved microphotoluminescence (μ-PL) measurements of single InGaN quantum dots (QDs). The linewidths of the μ-PL peaks originating from single metal-organic vapor phase expitaxy-grown III/V InGaN QDs are measured, implying dephasing times of at least 5 ps. Temporal fluctuations of the QD emission energy are observed, and these are explained in terms of randomly generated local electric fields inducing a Stark shift in the optical emission of the InGaN QDs. Time-resolved measurements demonstrate that decay dynamics from single InGaN QDs are exponential in nature. Measurements of the effect of temperature upon the recombination times in individual InGaN QDs have been performed from 4 to 60 K.