G. H. Döhler

Above band gap absorption spectra of the arsenic antisite defect in low temperature grown GaAs and AlGaAs

Room temperature absorption spectra of low temperature molecular beam epitaxy grown GaAs (LT‐GaAs) and AlGaAs (LT‐AlGaAs) are reported. We performed measurements in an extended spectral range from 0.8 eV to photon energies of 2.8 eV far above the band gap. For as‐grown LT‐materials, the absorption coefficients at the band gap are twice as high as for high temperature grown materials. By annealing the samples, we obtained a drastic reduced absorption coefficient below as well as above the band gap. We observed absorption changes up to 17 000 cm−1 for LT‐GaAs and 9000 cm−1 for LT‐AlGaAs taking place in a two phase process.

Polarization threshold switches based on ordered GaInP

We report on optoelectronic switching devices with high photoconductive gain which are sensitive to the polarization direction of the optical input. Due to the large polarization anisotropy of the absorption coefficient of ordered GaInP, an electrical output can be switched “on” and “off” by rotating the linear polarization of the input light. Depending on the external adjustment of the working point, the structure can operate as a polarization threshold switch or a polarization detector, respectively. First results exhibit a switching contrast of 25 dB and a contrast of the detector signal of 7 dB between two perpendicularly polarized incident light beams, respectively.

Polarization effect in light emitting diodes with ordered GaInP active layers

We report on polarization effects in surface emitting light emitting diodes based on p‐i‐n AlGaInP/GaInP double heterostructures grown by metal organic vapor phase epitaxy. Devices with an ordered GaInP active layer show polarized light output with a ratio of 4:3 for polarization along the [011] and [011] crystal direction, respectively. This polarization is nearly independent of diode current and mesa geometry, but is not observed if disordered GaInP active layers are used. The effect is considered to be an important means for polarization control in vertical cavity surface emitting lasers.

Guiding light in fluids

In contrast to conventional waveguides with high-refractive-index core, we describe a concept for guiding light within a low-refractive-index medium surrounded by a medium with higher refractive index. This concept is especially compatible with fluidic sensors because the channel itself can be used as the core of the waveguide and thereby enabling a strong light-analyte interaction over an extended distance. The concept is based on antiresonant modes, and experiments have shown that these modes can be excited by collimating excitation light under an appropriate angle onto the waveguide structure.

Polarization anisotropy in the electroabsorption of ordered GaInP

The polarization dependence of the Franz–Keldysh effect (FKE) in metalorganic vapor phase epitaxially grown AlGaInP/GaInP/AlGaInP p-i-n double heterostructures was investigated for samples with different ordering parameters. We determine an ordering induced shift of the FK spectra of up to 13 meV between light polarized along the [011] and [01-1] crystal directions. Due to the FKE the transmitted light intensity ratio between the respective polarizations was changed from 0.2 to 2.8 dB by applying an electric field of 335 kV/cm.

Novel shadow mask molecular beam epitaxial regrowth technique for selective doping

We present a novel molecular beam epitaxial regrowth technique which provides a simple and convenient way for the in situ lateral structuring of the doping profiles and growth rates on a μm scale. We achieve excellent selective contacts to the respective doping layers for device dimensions varying from several 100 μm down to several μm. Keldysh based n‐i‐p‐i modulator structures, fabricated with our new method, exhibit an on/off ratio of 6:1 for a voltage swing of 7 V without enhancement by additional Bragg mirrors.

Polarization dependence of the electroabsorption in low‐temperature grown GaAs for above band‐gap energies

We have measured the electroabsorption in low‐temperature grown GaAs by performing room‐temperature transmission experiments in the spectral range from 1.3 to 1.9 eV for different electric fields induced by a voltage applied to a metal‐semiconductor‐metal structure. The devices were separated from the substrate by using an epitaxial liftoff technique. Therefore, we have been able to observe the electro‐optic effect at the fundamental band gap as well as at the split‐off band edge. The absorption is clearly polarization dependent at the fundamental band gap but only weakly at the split‐off band gap, in agreement with the theory of the Franz–Keldysh effect.

Polarization-insensitive high-contrast GaAs/AlGaAs waveguide modulator based on the Franz-Keldysh effect

A GaAs/AlGaAs p-i-n double heterostructure waveguide modulator based on the Franz-Keldysh effect is reported. On/off ratios up to 40 dB are obtained over a broad range of wavelengths (>or=32 dB between 905 nm and 960 nm), while absorption loss in the on-state is very low (<1 cm/sup -1/). The polarization dependence of the Franz-Keldysh effect is relatively weak and only causes a shift of the transmission-voltage characteristics of TE-polarized light towards higher reverse bias compared to the corresponding TM curves. Therefore, very high contrast ratios are achievable over the whole range of wavelengths even for arbitrary polarization of the light.<<ETX>>

Low power (bistable) opto‐electrical threshold switches with high gain based on n‐i‐p‐i doping superlattices

We report on the realization of an electro‐optical switch on n‐i‐p‐i doping superlattices for both bistable and step‐like threshold operation at very low input power levels (≂200 pW) and with high opto‐electrical gain (6.8×106). The configuration presented in this letter consists of a (two terminal) photodiode with negative current‐voltage characteristic and a novel three‐terminal photoconductive detector structure. The minimum optical switching power is determined by the reverse dark currents of the devices that are below pA. The switches are suitable for monolithical integration into arrays.

High contrast electro‐optic n‐i‐p‐i doping superlattice modulator

Transmission changes in a n‐i‐p‐i doping superlattice have been measured. The structures were grown by a newly developed epitaxial shadow mask technique allowing to apply selective contacts to the n and p layers. For a voltage swing between only Upn=−4 and 0.8 V relative transmission changes of 65% below the band gap have been measured in a 2.66‐μm‐thick structure. The changes are larger than 50% in a wavelength range of more than 13 nm. The pure absorption changes are enhanced by a carrier‐ and field‐induced‐Bragg effect within the n‐i‐p‐i crystal. The experimental results are in very good quantitative agreement with calculations.