Spilios Riyopoulos

Comparison of optical VCSEL models on the simulation of oxide-confined devices

We compare the results of different optical vertical-cavity surface-emitting laser models on the position-dependent effects of thin oxide apertures. Both scalar and vectorial models as well as hybrid models are considered. Physical quantities that are compared are resonance wavelength, threshold material gain, and modal stability. For large device diameters and low-order modes, the agreement between the different models is quite good. Larger differences occur when considering smaller devices and higher order modes. It is also observed that the spread in the resonance wavelengths is smaller than that for the threshold material gain.

Theory of electron multipactor in crossed fields

A general theory of multipactor in orthogonal electric and magnetic fields is given. The model consists of two parallel plates of known secondary emission properties, across which a time varying voltage is applied, and between which a constant magnetic field is applied. Expressions are derived for the resonant phases at which the RF‐driven cascades occur; these reduce to previously derived expressions in the limit of the vanishing magnetic field. In addition, this work obtains the conditions governing the stability of the motion about those phases, as well as a dynamic constraint from imposing the restriction that each impact on a plate is the first impact that is allowed by the equations of motion. Chaotic effects from the random ejection velocities of the secondaries are addressed for the first time. It is proven that the phase focusing effect from the radio frequency (RF) interaction will overcome the dispersive effect from the random emission, provided that the mean square emission velocity is suffici...

Multipactor saturation due to space-charge-induced debunching

As the multipactor charge builds up, the mutual repulsion overtakes the focusing effect of the rf interaction, and the thin charge ribbon formed during the initial buildup expands into a wide band. Two synergistic effects lead to saturation (a) first, spreading of the impact phases into the region where the vacuum rf retards emission (b) later, and field reversal in the front end of the bunch due to the space-charge field, regardless of the rf phase. The effective secondary yield under opposing field is zero regardless of impact energy. Steady state is achieved when the ensemble-average yield δ drops to unity. The spreads in the impact energy are less important; impacts under reversed field cause saturation even at constant (independent of impact energy) secondary yield. The bunch breakup threshold is identified from bunch stability theory. It is found that near saturation both one-surface and two-surface cascades take place simultaneously; particle orbits alternate between one- and two-surface multipactors until they are finally lost to unfavorable phase/reversed field impacts. The possibility of saturation with a tight bunch, before breakup, is also discussed.

Enhanced internal quantum efficiency and light extraction efficiency from textured GaN∕AlGaN quantum wells grown by molecular beam epitaxy

GaN∕Al0.2Ga0.8N multiple quantum wells (MQWs) were grown by molecular beam epitaxy on randomly textured and atomically smooth (0001) GaN templates. Smooth and textured GaN templates were deposited on (0001) sapphire substrates by varying the III∕V ratio and the substrate temperature during growth by the hydride vapor-phase epitaxy method. We find that the MQWs replicate the texture of the GaN template, which was found to have a Gaussian distribution. The peak photoluminescence intensity from the textured MQWs is always higher than from the smooth MQWs and for GaN (7nm)∕Al0.2Ga0.8N (8nm) MQWs, it is 700 times higher than that from similarly produced MQWs on smooth GaN templates. This result is attributed partly to the enhancement in light extraction efficiency and partly to the enhancement in internal quantum efficiency. The origin of the increase in internal quantum efficiency is partly due to the reduction of the quantum-confined Stark effect, since the polarization vector intersects the quantum well (QW...

Active-cavity vertical-cavity surface-emitting laser eigenmodes with simple analytic representation

Gain-guided eigenmodes in open vertical-cavity surface-emitting laser cavities are constructed by superposition of paraxial (i.e., Gauss–Laguerre) modes employing the unfolded-cavity hard-mirror equivalent to distributed Bragg reflectors. The round-trip matrix is obtained analytically for simple gain profiles, including finite-mirror-size losses, diffraction spreading, and gain-confinement effects. Diagonalization yields the full range of stable, unstable, and steady-state complex eigenmodes and gain eigenvalues, in terms of the cavity parameters. More importantly, it is demonstrated that in cases of interest the lower-order cavity eigenmodes can be approximated by pure Gauss–Laguerre modes with optimum waist size prescribed through a simple variational principle. The Gaussian nature of the cavity modes is confirmed by comparison with experiments. Finally, the new eigenmode properties self-consistently account for wavelength blueshifting and reduction in the mode waist with increasing bias current, without invoking index guiding.

THz instability by streaming carriers in high mobility solid-state plasmas

It is described how the interaction of streaming electron plasma waves with lattice waves may lead to a highly unstable coupled plasmon-phonon excitation in the THz range, in high mobility semiconductor materials. The underlying physical mechanism is related to the ubiquitous beam-plasma instability in electrostatically coupled fluids. Continuous amplification in transit, rather than localized amplification at the system boundary reflections, yields high growth rates and may lead to efficient THz generation.

Collisional multipactor inside ambient gas

The influence of collisions during rf-induced multipactors in the presence of ambient gas is studied in some detail. The effect of the randomization of the electron rf-interaction phases due to collisional kicks is given particular emphasis, since phase locking among successive generations of rf-driven electrons is essential for avalanching. The systematic collisional friction shifts the parameter region for multipactor eruption to higher rf-fields compared to vacuum. Random velocity fluctuations prevent convergence to exact phase locking, causing the impact phase to wander around its nominal resonant value. The rms width of the phase spread is determined by the balance between the rf-induced dynamical phase focusing and random phase diffusion. For low collision frequencies phase wandering in the unfavorable for secondary emission regime reduces the effective secondary yield and may eventually cause multipactor quenching. For high collision frequencies multipactor is completely suppressed due to loss of c...

Elimination of transient vertical-cavity surface-emitting laser oscillations using photoactive feedback

It is demonstrated numerically that photoactive layers placed in a vertical-cavity surface-emitting laser (VCSEL) cavity eliminate the generic transient oscillations caused by the increasing rate of carrier depletion with increasing photon flux ∂(dN/dt)/∂P 0, using appropriately chosen photoactive layers, makes the cavity relaxation frequency purely imaginary; the laser asymptotes to steady state without modulation as an overdamped oscillator. The sign reversal depends mainly on the layer doping parameters, but not on the laser power, thus suppression holds over the laser operation range.

Radiation scattering by apertures in vertical-cavity surface-emitting laser cavities and its effects on mode structure

Microcavities with subwavelength features, such as oxide apertures and etched-mesa edges, suffer wide-angle scattering that cannot be captured by the paraxial propagation limit; hence the scattering cannot be fully accounted for by index guiding or by lenslike phase shifts. We present a systematic treatment by using the Born approximation in the vector Maxwell equations. We then introduce the scattering losses in the cavity round-trip matrix, by using a Gauss–Laguerre representation of the cavity modes. Optimization of the round-trip coefficient including confinement, diffraction, and scattering losses yields the mode waist in laterally open vertical cavity surface emitting laser (VCSEL) cavities. A simple equation relating the current aperture to mode spot size is obtained. The analytic results are applied to etched-mesa and oxide-confined VCSEL designs. Predictions for the mode waist and threshold current are comparable with experimental results in oxide-confined VCSELs.

Guiding center fluid model of the crossed-field amplifier

A closed, nonlinear set of fluid equations, based on the electron guiding center orbits, is developed to model the CFA behavior. In synchronism with the RF signal frame, the streamlines follow the equipotential surfaces of the transformed fields. In steady state, the flow is incompressible. The equations are implemented in a numerical algorithm that is much faster and more efficient than existing particle codes and shows good agreement with experimental results for V-I curves, over a wide parameter regime. Both analysis and simulation show that the hub density controls the RF amplification and saturation mechanisms. Anode current is determined by the difference of the E*B drift at the top of the hub from the RF phase velocity. Secondary production depends on the energy of the impacting electrons, and thus is sensitive to the electrostatic shielding of the cathode. The pass-to-pass fluctuations at the tubes output are connected to the oscillations in the hub density during each pass. >