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In regard to achieve a vertical surface-emitting laser with quantum dots as active medium, we have realized a microcavity consisting of two distributed Bragg reflectors and three layers of self-assembled InAs quantum dots and a similar sample without DBRs for comparison. The electroluminescence of the sample without cavity is rather broad due to size fluctuations of the islands, whereas the electroluminescence of the cavity sample is dominated by a narrow (FWHM 0.38 meV) and intense line at the resonance energy of the cavity. Several transverse modes are observed due to the wave-guiding effect of the etched mesa and identified by a simple model.

We have investigated the influence of the final states of bound-to-continuum transitions within the conduction band of asymmetric quantum well structures on the photocurrent. This influence manifests itself by an energy-dependent oscillation of the current direction. We observe pronounced oscillations at zero bias voltage in a double quantum well structure, induced by an asymmetric excitation into continuum states with positive and negative momentum, i.e. by a photogalvanic effect (PGE). If this effect is superimposed on an asymmetric backrelaxation, similar oscillations are observed in the spectrum when the latter asymmetry is compensated by an external electric field. Theoretically, we find a strong relation between the PGE and a quantum interference effect occurring in the a strong re continuum.

We report the design, fabrication, and characterization of InP-based double-sided-doped (DSD) MODFETs with InAs-layer-inserted channels. Devices based on optimized structures show a significant improvement in the effective saturation velocity, from 2.4/spl times/10/sup 7/ cm/s for lattice-matched MODFETs to 3.1/spl times/10/sup 7/ cm/s for InAs MODFETs. This leads to a maximum extrinsic transconductance of 1.95 S/mm and excellent high-speed performance of f/sub T/=265 GHz for 0.13-/spl mu/m T-gates. A f/sub max/ higher than 300 GHz can be achieved by fabricating a wide lateral recess groove, which simultaneously results in an improved breakdown voltage of 6.7 V. The excellent RF performance is primarily due to the reduction of Coulomb scattering from donor layers, especially under the channel, and to the reduction of scattering caused by the interface roughness. This improvement is achieved by inserting a 4-nm InAs layer, which better confines the two-dimensional electron gas (2DEG) at the center of the channel of MODFET's.

We report about optical experiments on (i) natural quantum dots formed by well width fluctuations in narrow GaAs quantum wells and on (ii) self-assembled InGaAs quantum dots in GaAs. Contributions from a single natural quantum dot are isolated by spatially resolved spectroscopy and resonant charge injection in an electric-field-tunable coupled quantum well structure. In optical experiments the excitonic and bi-excitonic ground states are identified and growth related interface problems are analyzed. Spectral linewidths down to 50 meV allow thereby a complete separation of the individual levels and Zeeman sub-levels of the dot. The natural quantum dots with their fully resolved Zeeman levels are used as local probes to monitor the spin orientation and the creation of spin-polarized excitons in coupled quantum well structures. With cathodoluminescence excited by a scanning tunneling microscope tip and a special sample design we are able to excite selectively the ground state of a single self-assembled InGaAs quantum dot.

We present an electrically pumped and micromechanically tunable InP-based vertical-cavity surface-emitting laser operating in the 1.55-/spl mu/m wavelength range. The current confinement is achieved by a buried tunnel junction. The GaAs-based movable top mirror membrane is fabricated separately, assembled on top of the device, and can be actuated electrothermally. A single mode output power of about 1.7 mW and a tuning range of 28 nm was obtained. By the use of an antireflection coating at the semiconductor-air-interface, we were able to extend the tuning range up to 60 nm as expected from one-dimensional simulations.

Molecular-beam-epitaxy-grown InGaAlAs-InP vertical-cavity surface-emitting lasers with buried tunnel junction for 1.55-/spl mu/m wavelength, passivated with benzocyclobutene (BCB) and coplanar contacts are presented. The devices show 3-dB modulation frequencies above 8 GHz (small signal modulation) and wide open eye diagrams up to 10 Gb/s over more than 4.6-km ITU-T G.653 fiber (data transmission experiment).

CO2 storage in geological structures represents an immediate solution to reduce the greenhouse effect caused by the emission of polluting gases. To this purpose, the best conditions can be found in depleted oil and natural gas fields, methane-bearing coal beds and saline aquifers. In any case it is necessary, during and after the injection, to monitor the effective CO2 distribution in the subsoil, to detect any possible migration outside the reservoir. In this work we simulated cross-well investigations in a realistic saline aquifer model to determine the sensitivity of the joint seismic-electromagnetic (EM) method for monitoring the CO2 plume in the reservoir. To this aim, we performed numerical modelling of seismic and EM wave propagation in the geological formations to simulate a realistic survey, and used traveltime and attenuation tomography as a monitoring tool of the study area. The combined use of these methods can give more reliable results, especially when the interpretation is based on suitable cross-property relations between seismic velocity and conductivity. In particular, the electrical conductivity of reservoir rocks is highly sensitive to changes in brine and CO2 saturation. The sensitivity is lower for P-wave velocities and quality factors, while the S-wave velocity remains nearly constant.

BACKGROUND: Keratoconus is associated with changes in the corneal structure, such as defects of Bowman's layer, a decrease of corneal thickness etc. They result in alterations of some of the biomechanical parameters of the cornea, namely, rigidity and elasticity. The present study was performed to examine how impression tonometry and applanation tonometry for determination of intraocular pressure (IOP) are affected by the changed biomechanical parameters associated with keratoconus. PATIENTS AND METHODS: We examined 20 normal subjects (40 eyes) and 17 keratoconus patients (25 eyes). The corneal thickness was measured by ultrasound pachymetry in the corneal center. In the keratoconus patients, an additional measurement was made at the conus peak. The corneal curvature was determined using the TMS keratoscope. The IOP measurements were made with the Schiotz tonometer (10 g). For comparison, additional IOP measurements in the corneal center and, in the keratoconus group, on the conus peak were made with the applanation tonometer. RESULTS: The normal subjects had a central corneal thickness of 548 +/- 30 microns, compared to 505 +/- 42 microns in the corneal center and 425 +/- 41 microns on the conus peak in keratoconus patients. The average corneal curvature was 43.3 +/- 1.8 D in the normal subjects and 47.8 +/- 4.1 D in keratoconus patients. Applanation tonometry produced results on 11.33 +/- 1.43 mm Hg in the normal group (corneal center) compared to values of 12.00 +/- 2.55 mm Hg (corneal center) and 7.30 +/- 1.95 mm Hg (conus peak) in the keratoconus cohort. The coefficient of rigidity was 0.0236 +/- 0.0026 microliter-1 in the normal subjects, compared to 0.0173 +/- 0.0050 microliter-1 in the keratoconus patients. CONCLUSION: The morphological changes associated with keratoconus may cause tonometry errors.

We demonstrate the improvement of double-sided-doped InAlAs/InGaAs MODFETs by inserting a thin InAs layer in the center of the conventional InGaAs channel. A maximum extrinsic transconductance of 1.4 S/mm is achieved for 0.13-/spl mu/m devices. The current gain cutoff frequency of this device is as high as 265 GHz. Delay time analysis shows a significant improvement in the effective saturated velocity, from 2.4/spl times/10/sup 7/ cm/s for LM devices to 3.1/spl times/10/sup 7/ cm/s for InAs devices. We believe the superior performance of this device is primarily due to the reduction of scattering from donor layers, especially under the channel, and the interface roughness, which is achieved by inserting a 4-nm InAs layer in the channel.

Summary Morphometry is well-established in tumour pathology. To evaluate its potential usefulness for description of developmental processes, histological slides from paraffin-embedded specimens of 67 human fetal lungs were Feulgen-stained, and morphometric characteristics of nuclei of epithelial pulmonary cells were analysed with an automated image analysis system. The measured cytometric features comprised of integrated optical density (IOD), S-phase-related IOD fraction, IOD entropy and nuclear area. Histometric features of the specimens were based upon the minimum spanning tree (MST) and included distances between neighboring epithelial cells, between epithelial cells and neighboring lymphocytes, and assessment of MST entropy. Notably, certain parameters revealed a non-uniform level during prenatal development. S-phase-related IOD fraction increased from 5% to 8% between 14 and 16 weeks of gestation, then declined to 6% until birth. The IOD entropy steadily increased during development, whereas the extent of nuclear area remained constant. In accordance with an increase of the S-phase-related fraction the MST entropy displayed a singular peak between 14 and 16 weeks of gestation, which is probably associated with development of glandular structures in the lung. Correlation of expression of binding sites for markers, presumably involved in functional aspects of development, with such alterations, is shown for binding capacities of biotinylated fucoidan and the S-phase-related fraction. This may be helpful to infer immunoor ligando histochemically defined tissue sites with potential physiological significance in morphometrically distinguished periods of development.

A periodic array of potential pillars is superimposed on a two-dimensional electron gas being ballistic on the length scale of the lattice period. Pronounced maxima occur in the magnetoresistance of such systems. The magnetoconductivity as obtained from the magnetoresistance via a tensor inversion displays maxima or minima depending on the direction of current flow with respect to the lattice orientation. So-called pinned electron orbits as well as runaway trajectories coexist and specifically influence a given element of the conductivity tensor. In contrast to square lattices, both sets of trajectories show up in the experimental data simultaneously. @S0163-1829~97!02119-X#

InAlAs/InGaAs high electron mobility transistors (HEMTs) with an InAs layer inserted in the In 0.53 Ga 0.47 As channel were grown on InP-substrates by solid source molecular beam epitaxy (MBE). By optimizing position, thickness and growth temperature of the InAs layer, the mobility at 300 K has been increased by 47%. HEMTs with a gate length of 0.15 μm showed very high transconductances and cut-off frequencies f T in excess of 1270 mS/mm and 235 GHz, respectively.

Hintergrund: Die PRK ist der heute am haufigsten durchgefuhrte refraktivchirurgische Eingriff. Um die Brechkraft der Hornhaut zu verandern, werden die vorderen Stromaanteile einschlieslich der Bowman-Membran mit einem Laserstrahl verdampft. Diese Operation stellt einen erheblichen Eingriff in die Struktur der Hornhaut dar. Wie verandern sich die biomechanischen Eigenschaften einer so veranderten Hornhaut? Material und Methode: Wir untersuchten 26 Patienten nach photorefraktiver Keratektomie und 25 unoperierte Probanden als Kontrollgruppe mit der von uns entwickelten druckinduzierten Hornhautstabilitatsmessung. Bei dieser Mesmethode werden die Probanden auf einer Kippliege um 180° gedreht, wodurch es zu einem intraokularen Druckanstieg von ca. 16 mmHg kommt. Vor und wahrend des Kippmanovers wird der Hornhautkrummungsradius mit dem Photokeratoskop bestimmt. Die Anderung des Hornhautkrummungsradius unter Druckbelastung ist der Parameter fur die Beurteilung der Hornhautstabilitat. Ergebnisse: Die zentrale Hornhaut flachte bei den PRK-Patienten um 0,038 ± 0,056 dpt (nicht signifikant) und bei den unoperierten Probanden um 0,187 ± 0,045 dpt (p < 0,05) ab. Bei Aufteilung der PRK-Patienten nach der Lange der postoperativen Zeit, zeigten die vor weniger als 1 Jahr operierten Hornhaute zentral eine geringe Versteilerung, wohingegen die vor 1 oder mehr Jahren operierten Hornhaute ein den unoperierten Hornhauten ahnliches Verhalten im Druckversuche zeigten (p < 0,05). Schlusfolgerung: Nach photorefraktiver Keratektomie ist die Stabilitat der Hornhaut im Vergleich zu unoperierten Hornhauten post-operativ verandert. Nach etwa 1 Jahr scheint es im Rahmen von stromalen Umbauvorgangen zu einer Normalisierung der Hornhautstabilitat zu kommen.

Experiments on antidot superlattices with a variety of rectangular geometries are used to test basic symmetry relations. Depending on the direction of current flow with respect to lattice orientation, different classical periodic orbits are probed. The symmetry relations also persist into the quantum-mechanical regime. Our experimental results show that a quantum effect like the Shubnikov‐de Haas oscillations can be modified for the current direction where the resistance is influenced by classical trajectories and periodic orbits. @S0163-1829~96!07447-4#

We present 1.55-/spl mu/m wavelength buried tunnel junction InGaAlAs-InP vertical-cavity surface-emitting lasers with low threshold current and high efficiency. An improved mirror design is accomplished with high-reflective low-loss epitaxial InGaAlAs-InAlAs and hybrid dielectric CaF/sub 2/-ZnS-Au layer stacks, respectively. Lasers with aperture diameters of only around 5 /spl mu/m exhibit continuous-wave single-mode output powers at room temperature well beyond 2 mW. Threshold voltages and series resistances as low as 0.9 V and 30-40 /spl Omega/ have been measured. The spectral behavior shows excellent performance over the relevant current and temperature range.

We present a novel device concept for a low threshold terahertz source. Our device is based on difference frequency generation in a dual-wavelength quantum cascade laser for terahertz emission and utilizes a Cerenkov phase matching scheme. The active region is an injectorless design to reach lower current threshold densities than injectorbased devices.

A mode hop free single mode tuning < 90nm at 40°C and 45nm at 70°C is demonstrated with a MEMS tunable VCSEL for the first time. The device shows a fiber-coupled output power of 2.9mW at 20°C and 0.5mW at 70°C. The side mode suppression ratio is larger than 40 dB over the entire tuning and temperature range of up to 70°C. The presented technology is cost effective and thus capable for mass production. It is applicable for tuneable VCSELs operating in different wavelength regimes, which are limited by the absorption of the DBR materials only.

We present DC transport measurements of the valence to conduction band (Zener) tunneling current in ap–i–ndiode with an ultrathin intrinsic layer containing a (GaAs)5/(AlAs)2multi-quantum well structure. According to recent theoretical predictions, the DC current should show maxima as a function of the reverse bias voltage that reflect the formation of Wannier–Stark resonances. So far, Wannier–Stark resonances have only been observed optically and never in a regime of strong Zener tunneling. Experimentally, we find the second derivative of the current-voltage characteristics to show a weak oscillatory structure indeed, indicating the existence of Wannier–Stark resonances in Zener tunneling.

The effects of microwave radiation on the Coulomb-blockade oscillations in a quantum dot are investigated. The application of microwaves leads to a pronounced splitting of the Coulomb-blockade oscillations, whose magnitude is proportional to the microwave amplitude. From the observed amplitude dependence of the peak splitting, the electron temperature in the quantum dot can be estimated to be 100 mK. Interestingly, this is different from that estimated by the minimum conductance between adjacent Coulomb oscillation peaks.

It has been predicted that resonances between Stark-ladder states and the Zener tunneling current gives rise to oscillatory structure in the derivative of the Zener current. In order to clarify the prediction, we carried out measurements in a p-i-n diode whose intrinsic region consists of a very thin superlattice, and observed oscillatory structure in the second derivative of the current. Electroreflectance (ER) measurements in the same structure were carried out to clarify the relation between the oscillatory structure in the Zener tunneling current and Stark-ladders. The results of the ER measurements are in good agreement with the calculated Zener current.

Scanning tunnelling microscopy studies on (100) surfaces of GaAs/AlGaAs quantum wire structures in dry nitrogen ambient are reported. Even at room temperature distinct local differences between I - V curves taken on wet chemically etched GaAs/AlGaAs inverted heterostructures are found. This enables us to clearly observe quantum wire regions with current imaging tunnelling spectroscopy at 300 K. The spectroscopic difference can be ascribed to the internal potential profile in the subsurface regions of the sample, rather than to a changed dopant concentration.

Resonant inelastic light scattering was used to investigate both charge density excitations and spin density excitations in shallow etched GaAsAl/GaAs quantum wires. A semi-transparent Schottky gate was evaporated on top of the samples which allowed a strong tunability of the electron density and the lateral confinement, even under high illumination conditions used for inelastic light scattering. The range of charge density excitations extended from confined plasmons to a plasmon band structure. Both intra- and intersubband single-particle excitations were observed. The occurrence of a tunable doublet structure in depolarized plane wave spectra is attributed to spin flip intrasubband single-particle excitations.

The time-of-flight method has been used to study the transport of indirect excitons in AlAs/GaAs CQW structures at T≥350 mK and B≤14 T. A large increase of the exciton diffusivity is observed at high magnetic fields and low temperatures, when exciton superfluidity is expected to occur.

We report measurements of electronic excitations in the confinement regime between one and two dimensions. FIR transmission spectroscopy has been successfully used to detect two-dimensional plasmons dispersion, localized plasmons and depolarization shifted one-dimensional intersubband transitions. The far infrared response of arrays of periodic quantum wires has been investigated by cyclotron resonance transmission and photoconductivity (PC) measurements. Due to narrow geometrical dimensions (300 nm), quantum confinement arises and leads to the formation of one-dimensional electronic subbands with a typical energy separation of 1–3 meV in the case of the heterostructures, and up to 9 meV in the case of the quantum wells. The far infrared transmission spectra of the quantum wire structures show one strong resonance, which can be described by a harmonic oscillator model, assuming that the confining potential is of parabolic shape. Depending on the intensity of the bandgap illumination, a well pronounced transition from a one-dimensional electronic system behaviour to a modulated one-dimensional system and finally to a pure two-dimensional system can be achieved for the heterostructure samples. The position of the PC-peak can be assigned to the plasmon-shifted cyclotron resonance. In addition in the samples, which contain a low density (4 × 1010cm−2) and high mobility inversion channel, an extremely sharp line with a linewidth of 0.2 cm−1 can be observed, depending on the temperature and on the illumination of the samples.

Electrically pumped buried tunnel junction InGaAlAs-InP vertical-cavity surface-emitting lasers (VCSELs) with self-adjusted lateral current and optical confinement and record emission wavelengths beyond 2 /spl mu/m are presented. Front and back side mirrors are realized using 31.5 epitaxial layer pairs of alternating InGaAs-InAlAs and a dielectric 2.5 pair CaF/sub 2/-a-Si layer stack. The devices show single-mode continuous-wave operation up to heat sink temperatures over 80/spl deg/C. The maximum output power at 20/spl deg/C reaches 0.43 mW, threshold current and voltage are as low as 0.66 mA and 0.73 V, respectively. To reach the long emission wavelength, we use an optimized active region comprising heavily strained quantum wells. High-resolution X-ray diffraction and photoluminescence measurements reveal excellent material quality without relaxation in the quantum wells.

We report on the behavior of Coulomb-blockade oscillations in a semiconductor quantum dot, when its coupling to the leads is strong, i.e., of the order of the dot's energy-level separation. Under magnetic fields 0.75\ensuremath{\le}B\ensuremath{\le}4 T, we find a periodic amplitude modulation of both the peaks and the valleys of the conductance resonances. Assuming that only the lowest edge channel carries current, we apply a model, describing partly coherent and partly incoherent electron transport, to explain these modulations. Within this model, the period of the amplitude modulation reflects the filling factor inside the dot. The amplitude of the envelope function is determined by the fraction of electrons scattered inelastically in the dot, the electron temperature, and the transmission of the tunnel barriers that couple the dot to the reservoirs. We show that it is only in the strong coupling regime where we can distinguish between thermal broadening and broadening as a consequence of inelastic scattering. We use this model to estimate the electron temperature as well as the phase-coherence length inside the quantum dot. The modulation of the conductance valleys is related to a periodic modulation of cotunneling rates.

Lifetimes of low-lying states have been measured in 132 Nd and 134 Nd using the coincidenceplunger technique. The reaction 32 S+ 105 Pd was used at a bombarding energy of 152 MeV. The measurement has been performed at the NSF Daresbury using the ESSA 30 array. The differential decay-curve method (DDCM) was used to analyze the recoil-distance Doppler-shift (RDDS) data. The experimental B (E2) values in 132 Nd are well described by the predictions of the rotational model and the IBM in the O(6) limit.

Time-differential measurements of the deorientation effect are performed with a plunger technique in 128Ba and 122Xe nuclei produced in heavy-ion induced reactions and recoiling into vacuum with velocities of about 1.2%c. The effect was found to modify the angular distributions of the transitions depopulating the 21+ and 41+ levels in both nuclei. To facilitate the analysis of the experimental attenuation factors Gkexp(t), a new approach for the treatment of the loss of alignment at higher-lying feeding levels is proposed. In the time-range investigated (up to 70–90 ps), the comparison of the data with calculations according to simple phenomenological models favours the formalism of the time-dependent perturbation theory of Abragam and Pound. It is shown, however, that a static interaction with a broad distribution of hyperfine frequencies may also describe the observed attenuation factors. In addition, a procedure is suggested which allows to correct for the deorientation effect recoil-distance Doppler-shift data obtained with only one detector.

Quasi-one-dimensional GaAs structures were fabricated by molecular beam epitaxial regrowth on dry etched (AlGa)As surfaces, making use of the “self-adjusted” facetted growth on structured surfaces. The increased migration lengths and decreased sticking coefficients of the ad-atoms on higher indexed crystal planes lead to the formation of triangular, sharply peaked ridges with perfectly smooth sidewalls, having lateral dimensional fluctuations on the atomic scale over lengths of several 100 μm. The epitaxial deposition of a quantum well, i.e. a thin GaAs layer between (AlGa)As barriers, results in the direct growth of one-dimensional quantum wire structures at the vertex of the peaked ridges. The size of the GaAs wires can be controlled by the amount of Ga deposited between the (AlGa)As barriers. These one-dimensional structures show high yield in cathodoluminescence, without spectral shift over regions of hundreds of microns. Additional peaks in the CL spectra demonstrate the two-dimensional confinement of the quantum wires. Three-dimensionally confined quantum dots, also showing high luminescence yield, were found at the top of pyramidal structures grown on square mesas.

Lattice-matched and pseudomorphic In 0.52 Al 0.48 As / In x Ga 1− xAs high electron mobility transistor (HEMT) structures with In content of 0.53 ≤ x ≤ 0.74 were grown on InP substrates by solid source molecular beam epitaxy (MBE). The extrinsic transconductances of 0.13 μm gate length HEMTs increased from 670 to 1150 mS/mm with increasing In content. High cut-off frequencies f T between 200 and 220 GHz were measured, independent of the In content. The reverse breakdown voltage of the gate diodes of the HEMTs was improved significantly from 2.7 to 6.7 V by the insertion of a 3 nm thick buried AlAs layer between gate and channel.

The mean lifetimes of the lower spin states of the superdeformed band in133Nd have been measured with the coincidence recoil distance method. The reaction used to populate the band was105Pd (32S, 2p2n)133Nd at a beam energy of 152 MeV, and the gamma-rays were detected with the POLYTESSA array. The differential decay curve method was used to analyse the data and transition quadrupole moments,Q 0, were extracted from the measured lifetimes. The results obtained from the three lowest transitions in the superdeformed band are:Q 0=6.3±0.9 eb (21+/2→17+/2),Q 0=6.7±1.1 eb (25+/2→21+/2) andQ 0>5.0 eb (29+/2→25+/2). These results are consistent with previous results for the high spin members of the band, and are compared to theoretical calculations of total routhian surfaces. The calculation of reduced transition probabilities for the transitions that feed out of the band, allows the effect of hindrance due to K-forbiddenness to be investigated.

Time-resolved photoluminescence studies have been performed on shallow etched wires prepared from remote-doped GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum wells. A fast decay is observed for the direct excitonic transitions below the experimental resolution of 10 ns. The indirect luminescence decays nonexponentially with a differential lifetime increasing from 30 to more than 1000 ns. At the same time the transitions shift to lower energies due to an increasing potential modulation. Numerical simulations reveal a good agreement with the measured differential lifetimes.

A photodetector operating with subwavelength spatial resolution is presented. The active region of this detector is defined by the pn-interface region of a highly doped GaAs homojunction. The cleavage plane normal to the interface defines a line detector with high spatial resolution. The actual resolution was tested using a near-field scanning optical microscope. An Al-coated tapered optical fiber of nominal aperture of 60 nm was scanned in the optical near-field vicinity across the active region of the junction. The photocurrent was measured as a function of the position of the fiber tip relative to the cleaved GaAs pn-junction. The width of the observed peak in photocurrent is found to be highly asymmetric with respect to the p or n side of the junction. The reason for this asymmetry is discussed in terms of diffusion length of minority carriers in the n- and p-GaAs close to the suface.

We investigate quantum-dot (QD) lasers in an external cavity using Littrow and Littman configurations. Here, we report on a continuously tunable QD laser with a broad tuning range from 1047 to 1130 nm with high stability and efficient side mode suppression. The full-width at half-maximum of the laser line is 0.85 nm determined mainly by the quality of the external grating. This laser can be operated in a dual-mode modus, where the mode-spacing can be tuned continuously between 1.1 and 34 nm. Simultaneous emission of the two laser modes is shown by sum frequency generation experiments.

We present surface micro-machined tunable vertical-cavity surface-emitting lasers (VCSELs) operating around 1550nm with tuning ranges up to 100nm and side mode suppression ratios beyond 40 dB. The output power reaches 3.5mW at 1555 nm. The electro-thermal and the electro-statical actuation of a micro electro-mechanical system (MEMS) movable distributed Bragg reflector (DBR) membrane increases/decreases the cavity length which shifts the resonant wavelength of the cavity to higher/lower values. The wavelength is modulated with 200 Hz/120 kHz. Both tuning mechanisms can be used simultaneously within the same device. The newly developed surface micro-machining technology uses competitive dielectric materials for the MEMS, deposited with low temperature plasma enhanced chemical vapor deposition (PECVD), which is cost effective and capable for on wafer mass production.

We demonstrate the linewidth characteristics of a micro-electro-mechanical system (MEMS) widely tunable VCSEL based on a newly developed surface micro-machining technology. The minimal linewidth is 98MHz at thermal rollover.

InP-based 1.3μm VCSELs employing two dielectric DBRs are presented. Because of reduced internal losses in the DBRs, high slope-efficiencies above 70%, and output-powers of 4.8mW are reported. The same devices feature modulation bandwidths beyond 12GHz.

OBJECTIVE: To evaluate corneal reinnervation after different techniques of refractive surgery. STUDY DESIGN: Retrospective evaluation of corneal sensivity after keratotomy, epikeratophakia and photorefractive keratectomy. The measurements were performed with the Draeger electromechanical optical-controlled aesthesiometer which is extremely precise and independent of external changes in temperature and humidity. One measurement point was in the corneal centre of patients after radial keratotomy and photorefractive keratectomy and 5 approximately at 1 mm intervals to the limbus. Five measurement points were in the centre of the epikeratophakia lenticle and 4 next to the circumferential trephination on the host cornea. SETTING: Department of Ophthalmology, University of Hamburg, and Lombardi Eye-Clinic, Rome. PATIENTS: 35 patients after radial keratotomy, 7 patients after epikeratophakia and 82 patients after photorefractive keratectomy. RESULTS: Corneal sensitivity after radial keratotomy is not influenced by radial incisions. Even 3 years after epikeratophakia the lenticle centre showed reduced thresholds. The reinnervation of the cornea after photorefractive keratectomy almost parallels the wound healing and corresponds to the ablation depth.

Resonant inelastic light scattering is used to probe electronic excitations in shallow etched GaAs/AlGaAs quantum dots and wires. In both types of structures, intersubband excitations are observed in depolarized scattering geometries. They show significant blueshift with decreasing confinement length. In dot structures these transitions appear as dispersionless, whereas in wires they show strong broadening and an intensity dip at the energetic center of the excitation with increasing wave vector parallel to the wires, as expected for single-particle excitations. Their line shape correlates with the linear wave vector dispersion of additionally observed intrasubband excitations.

Interdigitated gates are employed on heterostructure surfaces to induce strong and tunable lateral superlattice potentials of type II, i.e. of purely field-effect origin. The design of the heterostructures is optimized in a way that avoids statistical potential fluctuations even at very low electron densities where screening of carriers is ineffective. Single-particle electron states confined to one-dimensional electron wires are investigated with capacitance spectroscopy. The precise control of the potential as well as the homogeneity of the potential allow us to investigate 'true' quantum wire arrays, i.e. wires in which only the lowest one-dimensional subband is occupied. Furthermore, we employ interdigitated gates to investigate the optical properties of quantum wells in a type II potential superlattice. The lateral fields are sufficiently high to field-ionize optically excited electron-hole pairs. Absorption spectra of the quantum well under such strong lateral fields show the typical signatures of the Franz-Keldysh effect.

We have studied tunnelling processes between a multiple quantum dot (MQD) system and a two-dimensional electron gas (2DEG) system, which are realized on a GaAs-AlGaAs-GaAs heterostructure. Using a transfer Hamiltonian formalism. It is shown that the tunnelling probability for transitions between a zero-dimensional (0D) and a two-dimensional (2D) state strongly depends on the quantum dot potential profile. In the case of a square well potential, only the resonance of the ground state is pronounced significantly, whereas for a cosine-shaped quantum dot potential profile a multitude of resonance structures is caused by each 0D state. From our experimental results we conclude that the potential of the quantum dots is best described by a cosine-shaped profile. In addition, the subband spacings and the extent of the wavefunctions of the individual subbands are also determined directly.

The magnetic-field-dependent conductivity of a square antidot lattice is calculated from the experimentally determined magnetoresistance and Hall resistance. Close to the commensurability conditions where the magnetoresistance always displays pronounced maxima we find a crossover from minima to maxima in the conductivity as a function of antidot size. We interpret this behavior in terms of different classes of electron trajectories whose relative importance depends on the size of the antidots with respect to the lattice period.

Using a micron-sized photoluminescence (PL) probe enables us to study single islandlike interface defects of a thin GaAs/AlGaAs quantum well. The bound exciton ground state locally emits a distinct sharp line. With increasing excitation of this quantum dot level additional transition lines emerge at lower energy. They are attributed to localized biexciton states. The biexciton correlation energy is about 4 meV. A distinct two-photon resonant absorption peak of the biexciton ground state is observed in PL excitation spectroscopy. Its linewidth is only about 30 \ensuremath{\mu}eV. The spectra and their polarization properties are discussed on the basis of a discrete level scheme and the Pauli exclusion principle.