Electrooptic and electroabsorptive modulation properties in interdiffusion-modified AlGaAs-GaAs quantum wells

The electric field induced refractive index change and absorption coefficient change in TE polarization are analyzed at room temperature for several interdiffusion modified Al0.3Ga0.7As-GaAs quantum-well structures. The results show that for the small and medium interdiffusion lengths with fields of 100 and 50 kV/cm, respectively, improved chirping and electroabsorption can be obtained. Further, in a selected set of interdiffusion lengths and fields, the material can be used for an electroabsorption modulator with reduced chirping in a wide range of operation wavelengths (758-874 nm).published_or_final_versio

Genetic algorithm for design of reflective filters: Application to AlxGa1-xN based Bragg reflectors

A genetic algorithm (GA) with adaptive mutations has been employed for the design of Bragg reflectors. The algorithm enables three different design types a) composition and thickness of two layers are chosen and the pair is repeated b) two compositions are chosen for the two alternating materials, and thickness of each layer is optimized c) composition and thickness of each layer are optimized. In all cases, the wavelength and composition dependence of the index of refraction is taken into account. Also, it is possible to impose constraints on the composition difference of the neighbouring layers, either with a penalty function or with narrowing the boundaries for possible compositions. This feature is important because the large lattice mismatch between GaN and AlN can cause poor surface morphology, so measured reflectivity would be lower than the calculated one due to the surface roughness. The algorithm enables finding the optimal design for two chosen incident and final media, and it is capable of taking into account the existence of a finite, optically thick substrate. We have investigated two systems: air/sapphire/AlxGa1-xN reflector/GaN and GaN/AlxGa1-xN/air.published_or_final_versio

Polarization insensitivity of AlGaAs/GaAs interdiffused quantum wells

Modeling is used to show that interdiffusion can generate a polarization independent parabolic-like quantum well. Criteria to achieve the parabolic-like quantum wells by interdiffusion are discussed. The results indicated that interdiffused quantum wells can produce equal eigen-state spacing, polarization insensitive Stark shift and modulation characteristics similar to an ideal parabolic quantum well. The design process to obtain polarization insensitive ON- and OFF- states in the parabolic-like interdiffused quantum wells is discussed. The predicted modulation depth is comparable to those measured using parabolic quantum wells. The diffused quantum wells have the advantage of using an as-grown rectangular quantum well with post-growth annealing to tailor its confinement profile. These features suggest that the interdiffused quantum well structure can be use to produce polarization insensitive electro-absorptive modulation.published_or_final_versio

Interdiffusion contributions in optimized surface-acoustic-wave quantum-well modulators

The electro-absorption of AlGaAs/GaAs diffused quantum well (DFQW) modulator using surface acoustic wave (SAW) QW interactions was studied theoretically. The modulator was also optimized through the use of both the steeper potential at the device surface for modulation and a long λSAW and a low power SAW. The non-uniform feature of the SAW induced potential was also taken into account in optimizing the device structure. Interdiffusion took place into the W stack to modify modulation performance so that λop and the optical properties of the optimized structure can be further modified. The modulation efficiency of an incident optical field depended on the guiding properties in the QW active region.published_or_final_versio

Dielectric function of GaN: Model calculations

In this work we have modeled the optical functions of hexagonal GaN (corresponding to E⊥c) in the range from 1 eV to 10 eV using a modified critical points model. The difference between the model employed and the standard critical points model is that the exponent m is an adjustable parameter, and does not have fixed value depending on the type of critical point. Excellent agreement with the experimental data has been achieved over the entire investigated spectral range. Obtained relative rms errors equal 0.6% for the real part, and 2.0% for the imaginary part of the index of refraction.link_to_subscribed_fulltex

Optical properties of interdiffused quantum well modulators

In this paper, the waveguide type and vertical type electro-absorptive diffused-quantum-well modulators will be reviewed. The contributions of interdiffusion of the quantum wells, which act as the active region of the modulator, to the performance of the modulator in terms of the operating wavelength, optical properties and the modulation strength. For waveguide type optical devices, the polarization independent optical properties including absorption coefficient and optical gain using interdiffusion will be discussed. As a consequence, with appropriate interdiffusion, the operating wavelength can be adjusted while the quality and optical properties of the quantum well can be improved for use of modulation devices.link_to_subscribed_fulltex

Polarization-insensitive electroabsorption by use of quantum well interdiffusion

The use of interdiffusion in different as-grown square QW structures to produce a paraboliclike potential profile to achieve polarization insensitivity of electroabsorptive modulation was studied. These QW's were optimized to achieve three equivalent parabolic AlxGa1-xAs/GaAs QW's of various x from 0.3 to 0.4. Two criteria were proposed for the successful achievement of paraboliclike DFQWs: the first is to start with a narrower as-grown rectangular well width than that of the targeted PQW whereas the second is to use a less extensive interdiffusion. The chronological procedures to obtain polarization-insensitive on-state and off-state electroabsorptive modulation were also proposed.link_to_subscribed_fulltex

Modeling the optical constants of AlN and 6H-SiC

Optical constants of hexagonal AlN in the range 6-20 eV and 6H-SiC in the range 1-30 eV for the component perpendicular to the c axis are modeled using modified Adachi's model of the optical properties of semiconductors. Model parameters are determined by acceptance-probability-controlled simulated annealing. Main distinguishing feature of the model employed here is the use of variable broadening instead of the conventional Lorentzian one. In such a manner, broadening function can vary over a range of functions with similar kernels but different wings. Therefore, excessive absorption inherent to Lorentzian broadening can be reduced so that better agreement with experimental data can be achieved. Relative rms errors for the real and imaginary parts of the index of refraction, respectively, equal 3.5% and 5.2% for 6H-SiC and 1.5% and 1.9% for AlN.link_to_subscribed_fulltex

Polarization insensitive electro-absorption and gain-switching quantum-well modulators using interdiffusion

A theoretical study of the polarization independent quantum-well gain using interdiffusion is presented here. Group V sublattice interdiffusion in InGaAs/InP quantum wells is used to produce polarization independent optical gain. The reverse bias and carrier effects on the subband structures, transition energy and optical gain of the interdiffused quantum well are discussed. The interdiffused quantum well structures are optimized in terms of their subband structure, carrier density, structural parameters and properties of optical gain spectra. The results show that an optimized interdiffused quantum well structure can produce polarization independent optical gain over a range of operation wavelengths around 1.5 μm, although the differential gain and linewidth enhancement factor are slightly degraded. The required tensile strain for the polarization independence of a lattice-matched quantum well structure is generated here using interdiffusion. These results suggest that polarization independent optical devices can be fabricated using interdiffusion using a lattice-matched InGaAsP quantum well structure.link_to_subscribed_fulltex

Double quantum-well phase modulators using surface acoustic waves

Electro-optic modulators using the interaction of surface acoustic waves (SAWs) with III-V semiconductor multiple quantum well structures have gained interests. A SAW induces potential field which provides the phase modulation. In order to improve the phase modulation, an AlGaAs/GaAs asymmetric double quantum well (DQW) optical phase modulator using SAWs is investigated theoretically. The optimization steps of the DQW structure are discussed. The optimized phase modulator structure is found to contain a five-period DQW active region. Analysis of the modulation characteristics show that by using the asymmetric DQW, the large change of the induced potential at the surface and thus large modification of the QW structure can be utilized. The modification of each QW structure is consistent, although this consistency is not always preserved in typical SAW devices. Consequently, the change of refractive index in each of the five DQWs is almost identical. Besides, the change of effective refractive index is 10 times larger here in comparison to a modulator with a five periods single QW as the active region and thus produces a larger phase modulation. In addition, a long wavelength and a low SAW power required here increase the size of the SAW transducer and simplify its fabrication.link_to_subscribed_fulltex