Quantum Optoelectronics

Abstract:

Transient gain spectra were measured for an In0.02Ga0.98N / In0.16Ga0.84N multiple quantum well by 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 mu m) to long (350 mu m) 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.

Abstract:

We present measurements of microphotoluminescence decay dynamics for single InGaN quantum dots. The recombination is shown to be characterized by a single exponential decay, in contrast to the non-exponential recombination dynamics seen in the two-dimensional wetting layer. The lifetimes of single dots in the temperature range 4 K to 60 K decrease with increasing temperature. Microphotoluminescence measurements of exciton complexes in single MOVPE-grown InGaN quantum dots are also reported. We find the exciton-biexciton and exciton-charged exciton splitting energies to be 25 meV and 10 meV to the higher-energy side of the exciton ground state, respectively. Assignments of the ground state exciton, biexciton and charged exciton are supported by theoretical calculations. These measurements have been extended to investigate the time-resolved dynamics of biexciton transitions in the quantum dots. The measurements yield a radiative recombination lifetime of 1.0 ns for the exciton and 1.4 ns for the biexciton. The data can be fitted to a coupled differential equation rate equation model, confirming that the exciton state is refilled as biexcitons undergo radiative decay.

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 hyper-spectral 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). © 2005 American Institute of Physics.

Abstract:

Micro-photoluminescence measurements on single InGaN quantum dots at 4K exhibit a rich structure of lines to the high-energy side of the free exciton (X⁰), as a function of excitation density. Lines possibly attributable to biexcitons (2X⁰) and charged excitons (X⁺ and X^-) have been measured. We model the dots as truncated cone shapes, using a self-consistent finite difference method in the Hartree approximation to compute the ground-state wavefunctions of the four exciton complexes. We calculate the exciton recombination energy (X⁰) and the magnitude of the blue-shifts of the biexciton and charged excitons as a function of dot size and composition. The transition energy depends strongly on both dot composition and dot height, while the magnitude of the blue shift is determined primarily by the piezoelectric field-induced separation of electrons and holes. We use these two separate effects to place constraints on allowed models for the QDs. The experimentally measured energies are in agreement with these theoretical calculations. © 2005 American Institute of Physics.

Abstract:

The effect of an externally applied lateral electric field upon an exciton confined in a single InGaN quantum dot is studied using microphotoluminescence spectroscopy. The quantum-confined Stark effect causes a shift in the exciton energy of more than 5 meV, accompanied by a reduction in the exciton oscillator strength. The shift has both linear and quadratic terms as a function of the applied field. © 2005 American Institute of Physics.