The optical response of the intersubband excitation of multiple two-dimensional electron gases within a semiconductor microcavity has been studied through angle-dependent reflectance measurements. Using a resonator based on total internal reflection, a clear splitting of about 14 meV of the coupled intersubband cavity modes is observed from 10 K to room temperature, with resulting polaritonlike dispersion. The experimental findings are in good agreement with theoretical calculations performed in a transfer-matrix formalism.

Mobile laser projection is of great commercial interest. Today, a key parameter in embedded mobile applications is the optical output power and the wall plug efficiency of blue and green lasers. We report on improvements of the performance of true blue riedge waveguide InGaN lasers at 452nm with cw-output power up to 800mW in overstress and mono mode operation up to 500mW in a temperatures range of 20°C to 80°C. We succeeded in high and almost temperature independent wall plug efficiencies >20% at stable output power levels from 200 to 500mW in cw-operation. Due to several improvements of our blue laser diodes we now estimate life times is in the order of 40khrs for 80mW output power in cw-operation at 40°C. Additional overstress degradation tests at power levels up to 200mW show a strong dependency of lifetime with output power. Furthermore, we present pioneering results on true green InGaN laser diodes on c-plane GaN-substrates. The technological challenge is to achieve In-rich InGaN-quantum wells with sufficiently high material quality for lasing. We investigated the competing recombination processes below laser threshold like nonradiative defect recombination by electro-optical measurements, such confirming that low defect densities are essential for stimulated emission. A model for alloy fluctuations in In-rich InGaN-MQWs based on spectral and time resolved photoluminescence measurements yields potential fluctuations in the order of E0=57meV for our blue laser diodes. To get a closer insight into the physics of direct green InGaN-Laser we investigated the inhomogeneous broadening of experimentally measured gain curves via Hakki-Paoli-measurements in comparison to calculated gain spectra based on microscopic theory showing the importance of strong LO-phonon coupling in this material system. Investigations of current dependent gain measurements and calculations yield a factor of 2 higher inhomogeneous broadening for our green lasers than for our blue laser diodes on c-plane GaN. Based on the improvements of the material quality and design we demonstrate true green InGaN-Laser in cw-operation at 522nm with more than 80mW output power on c-plane GaN. The combination of low laser threshold ~60-80mA, high slope efficiency ~0.65W/A and low operating voltage 6.9-6.4V of our green monomode RWG-Laser results in a high wall plug efficiency of 5-6% in a temperature range of 20-60°C

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Surface acoustic waves ͑SAWs͒ were used to study the spin transition of the fractional quantum Hall effect at a filling factor of 2 / 3. Although very distinct and hysteretic anomalies were observed at the transition in the transport measurements, no indication was visible in the SAW data. This holds true not only for the small current regime but also for the large current regime where dynamic nuclear-spin polarization enhances the effect. We suggest that this result can be accounted for by highly conducting domain walls with thicknesses of at most 1% of the typical domain size.

We investigated the short term stability of the optical output power of 450nm InGaN test lasers. The short term degradation strongly depended on ridge width. It was mainly caused by an increase in threshold current. From measurements of subthreshold wave-length blueshift, carrier lifetime, and output power, we found a decrease in carrier density after 15h of aging. We show a direct correlation of the short term aging with current spreading effects.

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With a small signal frequency-modulation of the driving current, the resonance frequency and the damping factor of the optical output power response of blue nitride-based ridge lasers grown on [0001]-plane gallium-nitride substrates were investigated with a network analyzer setup. From the linear dependence of the squared resonance frequency on the driving current, the gain coefficients of the logarithmic gain model could be extracted being 7680 cm−1 for blue nitride-based lasers. For this purpose, additional parameters such as the carrier density and the confinement factor were assigned by carrier lifetime and quantum efficiency measurements and one dimensional transfer matrix simulations, respectively.

IntroductionThe spin degree of freedom plays an important role in the fractional quantum Hall effect (FQHE) [1]. It is responsible for a variety of phenomena such as phase transitions between different ground states, elementary spin excitations and interactions of the electronic system with the nuclei. This was initially not obvious since the first approach to solve this strongly correlated many-body problem considered all quantum Hall states at filling factors less than ν = 1 to be spin polarized [2]. Even though this indeed applies to fractional filling factors 1 m ν = / , where m is an odd integer, it was soon pointed out that for other fractional filling factors at sufficiently low densities, QH ground states with partial or zero spin polarization do exist [3].These states are energetically favorable whenever the exchange part of the Coulomb interaction becomes of the order of the Zeeman energy. Furthermore, at the transition, a domain pattern of differently polarized phases is believed to form [4][5][6][7]. In this review, we report on spectroscopic evidence for the existence of these domains, based on NMR and SAW (surface-acoustic wave) measurements. We focus primarily on the unpolarized-polarized phase transition at filling factor ν = 2/3, al-