H. De Neve

Impact of planar microcavity effects on light extraction-Part II: selected exact simulations and role of photon recycling

In this paper we use an exact calculation of dipole emission modifications in an arbitrary multilayer structure to obtain the extraction efficiency from realistic planar microcavities, additional insights gained through this exact approach compared to the simplified one of Part I of this paper [see ibid., p. 1612, 1998] are first discussed in the case of a dielectric slab. We next optimize for the extraction purpose asymmetric cavities bounded by metal on one side and dielectric mirrors on the output side for any pair of material indices in a broad range (n=1.4-4). The decrease of extraction when taking into account relative linewidths of the source of a few percent is shown to be moderate, allowing the large enhancements of monochromatic light to be maintained in many useful cases. The fractions of power emitted into guided modes, leaky modes, etc., are detailed. The beneficial role of possible photon recycling (reabsorption of emitted photons by the active layer) on extraction efficiency is evaluated using the fractions of power in guided and leaky modes. Extraction efficiencies in the 50% range are predicted for optimized, hybrid, planar metal-semiconductor structures for a wide range of active materials and wavelengths. We show that exact calculations justify the simple model used in Part I evaluating the extraction efficiency of a microcavity-based light-emitting diode as 1/m/sub c/ where m/sub c/ is the effective cavity order.

Angular spectroscopic analysis: An optical characterization technique for laterally oxidized AlGaAs layers

We present an optical characterization technique to determine both the refractive index and the shrinkage of laterally oxidized AlAs and AlGaAs layers. The technique consists of measuring the angular dependence of the Fabry-Perot dip wavelength in a simple cavity structure. Over standard ellipsometry, it has the advantage of measuring more realistic layer structures. Over transmission electron microscopy cross sections to determine the final aluminum-oxide layer thickness, it has the benefit of performing the measurement without elaborate sample preparation. We find that AlAs shrinks by approximately 3% during oxidation, and that the refractive index of oxidized AlAs is 1.52.

Microcavity LEDs with an overall efficiency of 4% into a numerical aperture of 0.5

Summary form only given. Microcavity LEDs were optimised for optical interconnect requirements. Overall quantum efficiency of up to 4.3% into a numerical aperture of 0.5 and a FWHM beam divergence angle of 105 degrees at a drive current of 1 mA was achieved. Microcavity LEDs with one gold and one GaAs-AlAs DBR-mirror have been optimized for efficiency into a limited NA of 0.5. Simulations indicate that an efficiency of 8% can be achieved. Experimental devices give a best value of 3.7%.

Gigahertz modulation of tunneling-based GaAs light emitters

We demonstrate GaAs surface-emitting light-emitting diodes (LEDs) containing a resonant tunneling structure that combine ultrahigh modulation bandwidths in excess of 2 GHz with high-external efficiency. In contrast to previously demonstrated resonant tunneling LEDs that were used for quantum-well (QW) emission at cryogenic temperatures, these devices operate at room temperature (RT). These devices are less affected by nonradiative recombination compared to the conventional heavy-doping approach, in which a high-modulation bandwidth was obtained only at the expense of a more than proportional reduction in quantum efficiency.

Constrained shortest path first algorithm for lambda-switched mesh optical networks with logical overlay OCh/SP rings

We describe a constrained shortest path first (CSPF) routing algorithm for lambda-switched optical networks with logical overlay OCh/SP rings. Speed is definitely a requirement if the algorithm is to be used for lambda-switching, but several features of the network architecture complicate the routing problem. The algorithm we present in this paper meets the requirement of being fast. Moreover, the routing of a lightpath is based on a limited amount of link state information, which can be disseminated by a lightweight routing protocol. Our numerical results show that the performance of the algorithm does not improve significantly if the routing decisions are based on more detailed information about the configuration of the network.

Study of scalability properties of two LP-based algorithms for traffic engineering in a flat differentiated services network

Two mixed-integer linear programming (MILP) formulations of a centralized, off-line traffic engineering tool are compared with respect to their scalability properties. Such an algorithm can be used to optimize the routing configuration in a DiffServ network. It is shown that a considerable reduction of the number of variables and constraints can be obtained by pairing primary and protection path candidates prior to the routing optimization. Simulations of networks with different sizes evidence this result,.

Gigahertz microcavity light emitters using resonant tunneling diodes

By designing a planar microcavity around a resonant tunneling light-emitting diode (RTLED), the high modulation speed of resonant tunneling devices is combined with the high external quantum efficiency of microcavity devices. Two designs are proposed and realized. The first design is a quantum well resonant tunneling light emitter and the second a bulk tunneling-based light emitter. Whereas the first only operates well at cryogenic temperatures, the bulk light emitter operates efficiently at room temperature. Design, realization and the electrical and optical steady-state and transient characteristics of the two kinds of tunneling-based microcavity light emitters are presented.