J.-P. R. Wells

Femtosecond studies of electron capture times in InGaN'GaN multiple quantum wells

Subpicosecond time-resolved differential transmission spectroscopy has been used to investigate the carrier density and temperature dependence of the quantum well electron capture time of blue-emitting InGaN/GaN multiple quantum well structures. It is found that the capture time varies significantly with both temperature and carrier density, the latter effect being consistent with carrier-induced band bending or increased carrier–carrier scattering. At room temperature, the electron capture time is in the range 0.4–0.8 ps for carrier densities ⩽5×1018u2002cm−3.

Infrared modulated interlevel spectroscopy of 1.3μm self-assembled quantum dot lasers using a free electron laser

A modulated far-IR technique is applied to determine the separations of the confined electron and hole states in a 1.3μm emitting self-assembled quantum-dot laser. By utilizing the unusual temperature behavior of the threshold current density, which exhibits a minimum at ∼200K, it is possible to eliminate heating effects associated with the far-IR pulses. In addition, the intense monochromatic laser output results in a relatively strong modulated signal strength. Electron and hole quantization energies of 46.7±0.2 and 13±1meV, respectively, are determined.

Precise measurement of the fraction of charged dots in self-assembled quantum dot ensembles using ultrafast pump-probe techniques

We demonstrate that polarization-resolved ultrafast pump-probe spectroscopy can be used to measure accurately (<10%) the fraction of charged dots in quantum dot (QD) ensembles. The method is applied to study the dependence of charging levels in thermally annealed InGaAs QDs, grown both as nominally undoped and n-type modulation-doped structures. We also show that the method can be used to study the spectral distribution of charges (or charging profiles) in quantum dot ensembles.

Control of the nonlinear response time of AlGaAs photonic crystal waveguides by sample design

We demonstrate the picosecond switching in 1D AlGaAs photonic crystal waveguides. We show the decay times vary with the structure parameters demonstrating excellent potential for ultrafast nonlinear switches with a controllable recovery time.

Long-lived spin coherence and temperature-induced dephasing in inas quantum dots measured via quantum beats

Long-lived spin coherence up to 80 K is deduced from quantum beats measured on InAs dots using a highly sensitive pump-probe technique. The beat pattern can in addition be manipulated by thermal annealing of dots

Ultrafast resonant nonlinear response of AlGaAs photonic crystals

This study reports experimental measurements of the ultrafast nonlinear response of AlGaAs photonic crystals with a band gap at around 900 nm. An ultrafast nonlinear shift of the band gap wavelength was observed when using femtosecond pulsed excitation at 800 nm. The magnitude of the shift is very large (> 6 nm), and the 1/e response time is around 30 ps. These results confirm the excellent potential of semiconductor photonic crystals for applications in nonlinear optical devices.