Gottfried H. Doehler

Optically pumped semiconductor thin-disk laser with external cavity operating at 660 nm

The optically pumped semiconductor thin-disk laser with external-cavity (OPS-TDL) is a new type of semiconductor laser structure with the capability of achieving high output power while retaining good beam quality. We demonstrate the first AlGaInP-based red light emitting OPS-TDL structure. The device has been pumped optically with an argon-laser at 514~nm. The device has an epitaxial backside mirror and a multiple quantum well active region, consisting of strained InGaP quantum wells arranged in several groups as a periodic gain structure. A peak single-mode output power of more than 200mW at 660nm has been obtained in pulsed operation. Various designs for the active layer have been investigated.

Electromodulation and photomodulation spectroscopy of long-period doping superlattices

We performed room temperature photo- and electro-modulation measurements on MBE-grown GaAs pnp and pin structures with large layer thicknesses. In these crystals the optical properties are expected to be dominated by the local field-induced changes of the dielectric function rather than by subband-transitions. The dominating effect in the pnp-structure turns out to be the spatial dependence of the varying refractive index, resulting in a characteristic interference pattern. For the case of the pin structure we clearly observe Franz-Keldysh oscillations changing in amplitude and width with the internal field. Theoretical calculations using effective mass theory are in very good quantitative agreement with the experimental results. This agreement can be achieved only by the inclusion of excitonic effects in the calculation of the field-dependent absorption.

Light extraction mechanisms in surface-textured light-emitting diodes

Light-emitting diodes (LEDs) with high efficiencies can be fabricated by a combination of surface texturing and the application of a rear reflector. We demonstrate an external quantum efficiency of 43% for unencapsulated surface-textured thin-film LEDs, which increases to 54% after encapsulation. At low temperatures, the efficiency of unencapsulated devices increases up to 68%. We investigate the light extraction mechanism from such LEDs employing a Monte Carlo simulation of the light propagation inside the LED structure. One essential input parameter for the simulation are the light scattering properties of the textured surface, which have been investigated experimentally. For light incidence below the critical angle of total internal reflection, the transmission through a textured surface is reduced compared to a flat surface. However, due to surface texturing, transmission becomes possible for incident angles above the critical angle. As a result, the internal scattering during internal reflection at the textured surface is not necessary for an efficient extraction of the light generated inside the LED structure. In addition, the Monte Carlo simulation also explains the strong increase of the LED efficiency at low temperatures quantitatively by photon recycling effects. Photon recycling is also demonstrated to be partially responsible for the shift of the emission wavelength in thin- film LEDs, as compared to conventional LEDs.

Size dependence of record-efficiency non-resonant cavity light-emitting diodes

Non-resonant cavity light-emitting diodes (NRC-LEDs) are based on the combination of surface texturing and the application of a back mirror. With this concept, the extraction efficiency of LEDs can be enhanced considerably. We fabricated NRC-LEDs with a more sophisticated design employing an oxidized current aperture, which is similar to the commonly used for vertical-cavity surface-emitting lasers. In our NRC-LEDs, it confines the injection current to the center of the device in order to reduce light generation below the top contact. We analyze the impact of the aperture size on the device performance, and we show that both the maximum efficiency and the injection current where it is reached are strongly dependent on the device size. Its correlation with the temperature in the active region and the current density is discussed. In addition, we demonstrate that a considerably fraction of the light can be extracted from lateral guided modes in the LED structure by extending the surface texturing beyond the device mesa. Devices fabricated by applying all of the above techniques result in record external quantum efficiencies of 31%.

Nonresonant-cavity light-emitting diodes

The external efficiency of conventional light-emitting diodes (LEDs) is limited by total internal reflection at the semiconductor-air interface. For conventional GaAs-based LEDs, this results in an extraction efficiency of 2%. In non- resonant cavity (NRC) LEDs, this problem is overcome by a combination of internal scattering at a textured top surface and reflection on a back mirror, which increases the probability of escape. Using this approach, we demonstrate external quantum efficiencies of up to 40% without encapsulation of the LED. To gain a more detailed understanding of the out-coupling mechanisms in NRC-LEDs, the scattering properties of the textured surface are investigated experimentally. The optimum surface texture is found to randomize the direction of the internally reflected light almost perfectly. In addition, NRC-LEDs also enables the enhancement of the external quantum efficiency for small and fast LEDs. With efficiencies of about 15%, we demonstrate bitrates of more than 1.3 GBit/s. In order to modify the lambertian output characteristics, we have successfully applied microlenses, allowing 50% coupling efficiency into optical fibers with NA equals 0.5.

Frequency limits of high-efficiency non-resonant cavity light-emitting diodes

In this paper, we present measurements of the switch-on times and of the switch-off times of non-resonant cavity light-emitting diodes, compared to those of conventional reference diodes. From this comparison, we infer that the high quantum efficiency of NRC-LEDs is not achieved by photon recycling, but purely by efficient extraction of generated photons. This is further corroborated by the good matching that is achieved between the measured switch-on times and theoretical predictions of the switch-on times. The latter are calculated with a model that includes only the electrical charging of the active layer and assumes that photon recycling does not occur. It is furthermore shown that the switch-on can be made faster by switching the diode between a non-zero low-state and the required high state. Doing so, an open eye diagram is achieved at 622 Mbit/s for a NRC-LED having an external quantum efficiency of 17%.

Novel VCSEL-based smart pixel for high-contrast, high-gain, and high-speed optical switching

We present a novel smart pixel composed of an optoelectronic threshold switch with gain and a vertical cavity surface-emitting laser (VCSEL). In this smart pixel two surface-normal input optical beams control an output optical beam emitted by the VCSEL. In its present hybrid version the VCSEL-based smart pixel is capable of opto-optical switching with an output contrast ratio in excess of 30 dB at an optical power of about 1.5 mW. For quasi-stationary operation we achieve an optical gain of up to 3 X 105. We also report drastic improvements on the switching dynamics. Operating the receiver with an optical input power of 130 (mu) W we achieve bitrates of up to 160 Mbit/s and an optical gain of 11, while optical inputs of 410 (mu) W result in a maximum bitrate of 400 Mbit/s and an optical gain of 3.6. The minimum input optical energy required for switching is 765 fJ, the AC output contrast ratio is 9 dB. Optically performed NAND and NOR logic operations are demonstrated at 40 Mbit/s with a fan-out of 7.6. We further show that the functionality of this smart pixel can easily be extended to electric read-out of input optical data and to direct electric control of the VCSEL within the smart pixel configuration. In particular, we demonstrate conversion of electric input to optical output data at 1 Gbit/s.

Optical bistability of p-i-n and n-i-p-i structures at very low optical power

We report on opto-electrical and opto-optical bistability with high electrical/optical gain occurring at very low optical power. In p-i-n and n-i-p-i structures large field induced changes of the absorption coefficient up to 5000 cm-1 can be observed due to the Franz- Keldysh effect. Slightly above the bandgap the absorption decreases with increasing internal field. This leads to n-shape current voltage characteristics which can be used for bistable opto- electrical switching. At an optical power smaller than 200 pW we achieve an on/off ratio of more than 107 for switching the n-layer current corresponding to an opto-electrical gain of 6.8 (DOT) 106. A smart pixel consisting of an electro-optical n-i-p-i modulator controlled by the bistable switch allows the observation of opto-optical bistability. With our smart pixel concept we have achieved an opto-optical gain of up to 3 (DOT) 105. This bistable switching is observed at extremely low switching powers smaller than 500 pW and exhibits a very broad hysteresis width of 200 pW. The minimum switching power is only limited by the dark current of the switch and can be reduced further.

Novel monolithic waveguide-based smart pixel for high-contrast, high-gain, and high-speed optical switching

We present results on optical switching of a novel monolithic waveguide-based smart pixel. In this smart pixel two surface-normal optical input beams control an optical output beam propagating in-plane through a waveguide modulator. For the operation only DC biases are required. The optical front end of our waveguide-based smart pixel consists of a specially designed n-i-p photoconductive detector in series with a reference n-i-p photodiode. Together, both devices are forming a digital opto-electronic switch, which is directly controlling the waveguide modulator. All components are based on the Franz-Keldysh effect. For a first demonstration of our monolithic waveguide-based smart pixel all components have been processed from the same GaAs/AlGaAs double hetero n-i-p structure grown by MBE on a semi-insulating GaAs substrate. With a non-optimized sample design we obtain an output contrast ratio of 17 dB and an optical gain in excess of 320. The optical input energy is estimated to be 2.6 pJ (detector area 20 X 20 micrometers 2). In this case the wavelength of the input beams was 783 nm, while the waveguide modulator was operated at 910 nm. An evaluation of the switching dynamics indicates that high-speed operation in excess of 400 Mbit/s can be achieved. This waveguide-based smart pixel could for example be used in optical routing networks.

In-situ structured MBE-grown crystals for applications in optoelectronics

We present the shadow mask molecular beam epitaxial (MBE) growth technique which allows an in situ lateral structuring of the doping profile and the growth rate on a micrometers scale. The electrical dc characteristics show that excellent quality selective contacts have been achieved on devices with lateral dimensions down to the micrometers range. The leakage currents are, even for highly doped structures, in the nA range. High reflectivity Bragg mirrors and pronounced exciton peaks observed on MQW structures confirm the high quality of the regrown material. The influence of the aspect ratio on the growth rates is very small. We have applied this novel technique to fabricate various selectively contacted optoelectronic devices based on n-i-p-i doping superlattices. For GaAs Franz Keldysh n-i-p-i modulators with selective contacts an on/off ratio of 6:1 has been achieved. High frequency results obtained on medium size devices indicate that 3 dB frequencies in the GHz range should be possible for n-i-p-i devices with dimensions < 4 micrometers fabricated with this technique. By selectively contacting the QWs in a modulation doped hetero n-i-p-i structure constructive superposition of field and carrier induced absorption changes have been achieved.