No abstract

The optical transitions in undoped, hexagonal GaN layers, grown on Si(111) by molecular beam epitaxy under nitrogen-rich conditions, have been studied by photoluminescence spectroscopy. Several intense excitonic emissions, of free and bound character, are detected as narrow as 1.7 meV at low temperature. The free A, B and C excitons, observed at 3.4786 eV, 3.484 eV and 3.503 eV, respectively, allow the determination of the crystal-field ( cr = 9.9 meV) and spin-orbit ( so = 19.9 meV) splittings. The evolution of their energies with temperature has been analysed with two different fits, the gap shift proportional to T 2 /(T + θ D ) and 1/[exp(θ E /T ) − 1] respectively. Information on the scattering processes is obtained from the peak broadening, which is due to exciton-phonon interactions. Both the free exciton energies and their temperature behaviour agree with those observed in bulk and homoepitaxial GaN, and therefore the studied GaN/Si layers are strain-free. Up to four extrinsic transitions at 3.4755 eV, 3.4714 eV, 3.456 eV and 3.450 eV have also been observed, and their assignment to bound excitons and donor to band transitions is discussed. Finally, a band at 3.41-3.42 eV is attributed to a donor-to-acceptor transition. This interpretation implies the presence of an acceptor lying at 70 meV above the valence band, shallower than those usually employed for p-type doping.

No abstract

A numerical model has been implemented and used for the simulation of the optical frequency combs generated by gain-switching single mode laser diodes, either free-running or subjected to optical injection. The dynamical model consists of three rate-equations including stochastic noise terms. An exhaustive comparison between the simulated and the experimental temporal profiles and optical spectra, generated in a wide range of switching and optical injection conditions has been performed. The range of the explored conditions leads to a variety of scenarios including actual combs and broad incoherent spectra, and frequencies in the range of, and well below, the relaxation oscillation frequencies. The detailed maps generated provide guidelines for selecting suitable operation conditions for specific comb characteristics. The excellent quantitative agreement found in the entire range of the explored operation conditions supports the validity of the model for the analysis of the comb generation mechanisms. The model has been used for ascertaining the role of the frequency chirp in the evolution of the comb spectra and discussing the physics underlying the effects of the optical injection.

We present an experimental study on the generation of high-peak-power short optical pulses from a fully integrated master-oscillator power-amplifier emitting at 1.5 μm. High-peak-power (2.7 W) optical pulses with short duration (100 ps) have been generated by gain switching the master oscillator under optimized driving conditions. The static and dynamic characteristics of the device have been studied as a function of the driving conditions. The ripples appearing in the power-current characteristics under cw conditions have been attributed to mode hopping between the master oscillator resonant mode and the Fabry-Perot modes of the entire device cavity. Although compound cavity effects have been evidenced to affect the static and dynamic performance of the device, we have demonstrated that trains of single-mode short optical pulses at gigahertz frequencies can be conveniently generated in these devices.