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Conventional CMOS image sensors with a linear transfer characteristic only have a limited dynamic range (DR) of about 60–70 dB. To extend the dynamic range considerably, the already successfully demonstrated concept of a linear self-reset pixel was employed in this work. With the self-reset concept the limit of the maximum analyzable photo generated charge (Qmax⁡) during the exposure time is extended to a multiple of the saturation charge of the photo diode (Qsat) by asynchronous self-resets of the photo diode. Additionally, the remaining charge at the end of the exposure time is evaluated to increase the resolution of the opto-electronic conversion. Thus we achieved pixels with a DR of more than 120 dB combined with an improved low light sensitivity using a pinned photodiode. This paper focuses on two topics: One is the realization and opto-electronic characterization of further self-reset pixel cells for an experimental optimization of the functionality with respect to linearity and high signal-to-noise ratio. The second one is the assembly and digital readout of a cluster structure composed of 16 × 16 pixel matrix on a CMOS test chip. One constraint for later usage of the pixel cells in a high resolution (> VGA) image sensor is the required layout area of the individual circuit blocks. For the cluster structure a size of 20 × 20 µm2 for the analog part of the pixel containing the photo diode and the other analog circuit blocks, the comparator and the signal shaping, was desired. The circuit design and layout work included several variants of the pinned photo diode with floating diffusion (FD) readout node, which is also used for analog voltage storage, and different control transistors. Further for the comparator a telescopic differential amplifier with high gain was implemented as well as peripheral 10 bit counter/shift register as static and dynamic versions. Test chips have been fabricated in an advanced 0.18 µm CMOS technology for optical sensors with low leakage currents. The sensor chips have been evaluated with a specifically developed test setup which gives the flexibility to arbitrarily generate the digital and analog control signals in terms of timing and voltage levels. Based on this, the number of asynchronous self-resets could be read out from the counters of the pixel cells as coarse values. The remaining charge at the end of the integration time was digitized using a ramp analog to digital conversion and could be read out as fine values. An opto-electronic characterization with adjustable illumination from 0 lx to 13 klx was done to measure and analyze the opto-electronic conversion function (OECF) and the noise of six different self-reset pixel cells having the high-gain differential amplifier as comparator. Finally the coarse values of two implemented 16 × 16 pixel clusters could be read out as a mini camera using a lens for optical image formation.

This paper investigates transfer in the use of English intensifiers by Croatian students of English. Since transfer is an important factor in second language acquisition and is very common in different areas of language use, it has been the subject of various studies. In this paper, we focus on lexical transfer investigating its effects on the use of intensifiers among first-year undergraduate university students of English at Zadar University, Croatia. The aim of this cross-sectional study is to examine whether there is a connection between the use of intensifiers in L2 and L1, i.e. whether the knowledge of intensifiers in L1 Croatian influences learners' use or knowledge of intensifiers in L2 English. Intensity is a basic human cognitive category and has an important function in communication. Specifically, the speaker expresses emotions or an attitude towards a topic using intensifiers. Furthermore, intensifiers are used in different collocations modifying adjectives, adverbs, and verbs. However, this study focuses only on intensifiers that appear in adverb-adjective collocations, and their use is observed in a relatively large group of language learners at a single point in time. Relevant data are collected using questionnaires as well as cloze-tests and translation tasks which focus on semantics and collocation.

Terahertz (THz) quantum-cascade lasers (QCLs) are narrow band, high-power, and frequency-agile sources. These properties make them attractive for applications in high-resolution molecular and atomic spectroscopy. In the past few years, various techniques and methods regarding high-resolution spectroscopy with THz QCLs have been developed, namely, direct absorption spectroscopy, detection schemes such as wavelength and frequency modulation, differential spectroscopy, photoacoustic spectroscopy, and heterodyne spectroscopy. We briefly review the most relevant technological and methodological advances in this field, and we discuss perspectives and future directions.

We report a novel cavity feedback mechanism for stabilizing single-longitudinal-mode (SLM) operation of diamond Raman lasers. Polarization-dependent Raman gain and in-grown stress birefringence in diamond were investigated as sources for Hansch-Couillaud-type locking signals. The power range of SLM operation increased from 2.1 W to 7.2 W, compared to the free-running laser, for a simple standing-wave laser cavity without frequency-selective elements. Methods for further increasing power range and frequency stability are discussed.

The non-destructive collection of ultrathin sections on silicon wafers for post-embedding staining and volumetric correlative light and electron microscopy traditionally requires exquisite manual skills and is tedious and unreliable. In MagC introduced here, sample blocks are augmented with a magnetic resin enabling the remote actuation and collection of hundreds of sections on wafer. MagC allowed the correlative visualization of neuroanatomical tracers within their ultrastructural volumetric electron microscopy context., eLife, 8, ISSN:2050-084X

Tetragonal tungstate CsLa(WO4)2:Nd3+ was found to be an attractive multifunctional simultaneously Nd3+-laser and stimulated Raman scattering (SRS)-active crystal. A multitude of Stokes and anti-Stokes components is generated in the visible and near-infrared spectral region producing frequency combs with widths of ~8600 cm−1 and ~9560 cm−1 under pumping at 0.532 07 µm and 1.064 15 µm wavelengths, respectively. All registered nonlinear emission lines are identified and attributed to a single SRS-promoting vibration ν s(WO4) mode with energy of ω SRS ~956 cm−1. Moreover, a passively Q-switched LD-pumped nanosecond self-Raman CsLa(WO4)2:Nd3+ laser is reported. An overview of Ln3+-doped tungstate self-Raman lasers is given as well.

With quantum information processing in mind, homodyne optical amplification tremendously enhances the sensitivity of spin noise spectroscopy applied to delicate few- and single-spin systems. In realizing the allied experimental techniques, the authors are able to boost the low-frequency spin noise signal of a few remaining impurities in an isotopically enriched Rb vapor above the electronic noise level dictated by the photodetector system. This proof-of-principle experiment advances semiconductor spin research and facilitates higher-order spin noise measurements on semiconductor qubits, such as single (In,Ga)As quantum dots.

We report on real-time gas sensing with a terahertz quantum-cascade laser (QCL). The method is solely based on the modulation of the external cavity length, exploiting the intermediate optical feedback regime. While the QCL is operated in continuous-wave mode, optical feedback results in a change of the QCL frequency as well as its terminal voltage. The first effect is exploited to tune the lasing frequency across a molecular absorption line. The second effect is used for the detection of the self-mixing signal. This allows for fast measurement times on the order of 10 ms per spectrum and for real-time measurements of gas concentrations with a rate of 100 Hz. This technique is demonstrated with a mixture of D2O and CH3OD in an absorption cell.

This article presents a novel microfluidic impedance cytometer enabling dielectric characterization of single cells at frequencies up to 500 MHz. The dielectric properties of cells at lower frequencies contain information about their size and membrane capacitance. The increased frequency range of the presented cytometer potentially allows for characterization of intracellular components, such as vacuoles or the cell nuclei. We demonstrate the overall capabilities of the cytometer through discrimination of polystyrene beads from Chinese hamster ovary (CHO) cells. The discrimination is based on the difference in dielectric properties at frequencies up to 500 MHz.

Dieser Kurs stellt eine Einführung in das Gebiet der Optoelektronik dar. In den Einleitungskapiteln werden die grundlegenden Begriffe vorgestellt, die die Lichttransmission betreffen. Die physikalischen Mechanismen, die einen Bezug zu den optoelektronischen Bauelementen besitzen, sind im vierten Kapitel beschrieben. Die wichtigsten optoelektronischen Bauelemente werden im fünften Kapitel behandelt. Kapitel 6. und 7. beschreiben die Prinzipien der optischen Kommunikation und die grundlegenden Anwendungen der Optoelektronik.

We present a side-respectively end-pumped Nd:YAG laser system at a laser wavelength of 1116 nm. The third harmonic of this wavelength at 372 nm can be used for LIDAR detection of molecular iron in the middle atmosphere at the height of about 70 to 120 km. Such measurements give information about temperature profiles and wind speeds in this atmospheric region and thus help in validating and calibrating global circulation models [1, 2].

Unravelling the nature of the interactions between photogenerated charge carriers in solar energy conversion devices is key to enhance performance. In this perspective, we discuss electroabsorption spectroscopy (EAS), as the spectral bandshape of the electroabsorption (EA) signal directly depends on the strength of the charge carrier interactions. For instance, the electroabsorption response in molecular or confined excitonic systems can be modelled perturbatively yielding the Stark effect. In contrast, most solids exhibit weaker interactions, and a perturbative approach cannot be taken in general. For solids with negligible charge carrier interactions, one resorts to the Franz-Keldysh theory of a continuum in a field, that, in the low-field limit, simplifies to the low-field FKA effect. Alternatively, when the continuum approximation breaks down, the problem of a Wannier exciton in a field has to be solved, and numerical methods emerged as the best solution. We illustrate our discussion with two examples involving lead-halide perovskites, a new, high-stake solar cell material. In the first example, we discuss the lineshape of the electroabsorption response for thin-films of lead-iodide perovskite, that sustains the photogeneration of free carriers. In the second example, we address a confined excitonic case with lead-bromide perovskite nanoparticles, and demonstrate the presence of so-called charge-transfer excitons.

Colloidal semiconductor nanocrystals display remarkably bright, strongly size-dependent photoluminescence properties. Following photoexcitation of these materials, temporary charge carrier separation can occur where one or both charge carriers are trapped. Charge detrapping can reform the emissive state on long time scales up to seconds, causing delayed luminescence. This delayed luminescence has not yet been thoroughly explored, and appears to be closely associated with a phenomenon observed at the single particle level, i.e. photoluminescence intermittency (blinking). Here, some of our recent work on the delayed luminescence properties of nanocrystals of different chemical composition is reviewed. These results provide insight into the mechanism of carrier detrapping, and are discussed in the context of photoluminescence blinking.

Collective low-frequency molecular motions have large impact on chemical reactions and structural relaxation in liquids. So far, these modes have mostly been accessed indirectly by off-resonant optical pulses. Here, we provide evidence that intense terahertz (THz) pulses can resonantly excite reorientational-librational modes of aprotic and strongly polar liquids through coupling to the permanent molecular dipole moments. We observe a significantly enhanced response because the transient optical birefringence is up to an order of magnitude higher than obtained with optical excitation. Frequency-dependent measurements and a simple analytical model indicate that the enhancement arises from resonantly driven librations and their coupling to reorientational motion, assisted by the pump field and/or a cage translational mode. Our results open up the path to applications such as efficient molecular alignment, enhanced transient Kerr signals and systematic resonant nonlinear THz spectroscopy of the coupling between intermolecular modes in liquids.

Zusammenfassung Die Anforderungen an die Sicherheit von Lithium-Ionen-Zellen rücken durch deren Einsatz in Elektrofahrzeugen zunehmend in den Vordergrund. Als Basis für die Zellsicherheit müssen typische Produktionsfehler in Batterieseparatoren zuverlässig erkannt werden. Die Bildverarbeitung stellt hierfür eine geeignete Methode dar. Nachfolgend wird ein System zur Qualitätskontrolle beschrieben, das eine Detektion dieser Fehler ermöglicht. Die Besonderheit ist dabei die Unterscheidung auftretender Defektarten untereinander und die Abgrenzung von nicht qualitätsrelevanten optischen Effekten mittels eines expertenbasierten Ansatzes zur Fehlerklassifikation.

We describe a new procedure of solving the electrostatic potentials in the silicon film of an undoped DG SOI MOSFET structure. Starting from a model previously described in the literature by Malobabic et al. (2004), we propose the bisection method for the solution of transcendental equation giving the surface electrostatic potential of the silicon channel, as a function of the gate to source voltage and the voltage along the channel. The above calculated results are used for obtaining the charges and corresponding drain current in the DG MOSFET transistor. The entire model is implemented in Verilog A and can be used inside Cadence for the determination of the static regime of electrical circuits based on undoped symmetric DG SOI MOSFET. As a case study, a simple common-source amplifier built with such a novel device is analyzed, showing the currents and voltages present in the circuit.

This paper presents a CMOS bandgap reference that employs a curvature correction technique for compensating the nonlinear voltage temperature dependence of a diode connected BJT. The proposed circuit cancels the first and the second order terms in the VBE(T ) expansion by using the current of an autopolarizedWidlar source and a small correction current generated by a MOSFET biased in weak inversion. The voltage reference has been fabricated in a 0.35µm 3Metal/2Poly CMOS technology and the chip area is approximately 70µm × 110µm. The measured temperature coefficient is about 10.5 ppm/K over a temperature range of 10– 90°C while the power consumption is less than 1.4mW.

Photovoltaics have the potential to become a major contributor to future sustainable energy generation. To this end, device efficiency needs to be brought beyond present physical limits. Solar cells cannot absorb light with less energy than the bandgap (absorption threshold of the photovoltaic absorber material), and consequently this energy source is usually uncaptured. Upconversion (UC) devices harvest those unused subthreshold photons behind the solar cell, create one higher-energy photon out of (at least) two transmitted photons, and radiate upconverted light back toward the solar cell, thus expanding the usable solar spectrum (see Figure 1). Key requirements for UC units are broad absorption and high UC quantum yield under even the low-intensity incoherent illumination that is available to solar energy conversion devices. Upconversion was proposed as a means to increase solar cell efficiency about a decade ago,1 and research has since then been primarily focused on UC processes occurring in lanthanide-doped glasses or nanoparticles.2 Although transitions between the rare earth 4f orbitals lie in an energy range promising for combination with common crystalline silicon solar cells, UC systems based on erbium (Er) or ytterbium (Yb) suffer from weak absorption by Laporte-forbidden transitions, a narrow absorption bandwidth, and the resulting low efficiency. Recently, triplet-triplet annihilation (TTA) in organic molecules has been demonstrated to be an alternative UC mechanism highly promising for solar energy application.3 It proceeds efficiently even for low photon flux and displays highly tunable absorption characteristics. The key to high quantum yield from TTA-UC lies in the combination of two different classes of organic molecules dissolved together in organic solvents or in a polymeric host matrix.4 A socalled sensitizer species, which in many cases is a metalated porphyrin, absorbs a low-energy photon (process 1 in Figure 2) and undergoes fast intersystem crossing (ISC, process 2), whereby Figure 1. Principle of a solar cell assisted by upconversion (UC). The creation of usable electron-hole pairs in the photovoltaic absorber material (left) is augmented by light, usually transmitted unused, which is converted to higher photon energies in the UC unit (right).

A terahertz imaging system based on a quantum-cascade laser (QCL), a fast scanning mirror, and a sensitive Ge:Ga detector is demonstrated. Transmission images are obtained by scanning the beam of the QCL across an object. Images with a diameter of approximately 40 mm and a signal-to-noise ratio of up to 28 dB were obtained within 1.1 s. The system was also used to obtain three-dimensional images of objects in an ellipsoidal volume with axes of approximately 40 mm by computed tomography within 87 s.

The plasmonic resonances of nanostructured silver films produce exceptional surface enhancement, enabling reproducible single-molecule Raman scattering measurements. Supporting a broad range of plasmonic resonances, these disordered systems are difficult to investigate with conventional far-field spectroscopy. Here, we use nonlinear excitation spectroscopy and polarization anisotropy of single optical hot spots of supercontinuum generation to track the transformation of these plasmon modes as the mesoscopic structure is tuned from a film of discrete nanoparticles to a semicontinuous layer of aggregated particles. We demonstrate how hot spot formation from diffractively-coupled nanoparticles with broad spectral resonances transitions to that from spatially delocalized surface plasmon excitations, exhibiting multiple excitation resonances as narrow as 13 meV. Photon-localization microscopy reveals that the delocalized plasmons are capable of focusing multiple narrow radiation bands over a broadband range to the same spatial region within 6 nm, underscoring the existence of novel plasmonic nanoresonators embedded in highly disordered systems.

We present thorough measurements of the intrinsic detection efficiency in the wavelength range from 350 to 2500 nm for meander-type TaN and NbN superconducting nanowire single-photon detectors with different widths of the nanowire. The width varied from 70 nm to 130 nm. The open-beam configuration allowed us to accurately normalize measured spectra and to extract the intrinsic detection efficiency. For detectors from both materials the intrinsic detection efficiency at short wavelengths amounts at 100% and gradually decreases at wavelengths larger than the specific cut-off wavelengths, which decreases with the width of the nanowire. Furthermore, we show that applying weak magnetic fields perpendicular to the meander plane decreases the smallest detectable photon flux.

This paper describes the synthesis and characterization of core-shell structures, based on SnO2 and TiO2, for use in dye-sensitized solar cells (DSC). Atomic layer deposition is employed to control and vary the thickness of the TiO2 shell. Increasing the TiO2 shell thickness to 2 nm improved the device performance of liquid electrolyte-based DSC from 0.7% to 3.5%. The increase in efficiency originates from a higher open-circuit potential and a higher short-circuit current, as well as from an improvement in the electron lifetime. SnO2-TiO2 core-shell DSC devices retain their photovoltage in darkness for longer than 500 seconds, demonstrating that the electrons are contained in the core material. Finally core-shell structures were used for solid-state DSC applications using the hole transporting material 2,2',7,7',-tetrakis(N, N-di-p-methoxyphenyl-amine)-9,9',-spirofluorene. Similar improvements in device performance were obtained for solid-state DSC devices.

To unleash the full potential of hybrid solar cells, it is imperative to get significant photocurrent contribution from both the sensitizing dye and the polymeric hole transporter. Here we report on the interfacial modifier 4-mercaptopyridine (4-MP), which induces controlled orientation of poly(3-hexylthiophene) (P3HT), the most widely used hole transporting polymer for hybrid solar cells, at the interface. 4-MP optimizes the charge separating interface between P3HT and a squaraine dye-decorated TiO2, inducing enhanced contribution to photocurrent generation by the polymer. In combination with 4-tert-butylpyridine, which enhances the open circuit potential in dye-sensitized and hybrid solar cells but reduces the photocurrent, a synergistic effect is observed and it is possible to enhance both open circuit voltage and photocurrent simultaneously. Similar effects on device performance are also found for two other commonly used dye molecules, a fullerene derivative and a common indoline dye.

Wir stellen eine Kopplungsmethode für resonatorgestützte Absorptionsmessungen vor, bei der Licht durch einen im Resonator platzierten akustooptischen Modulator aktiv ein- und ausgekoppelt wird. Dies ermöglicht es, Cavity-Ring-Down-Spektroskopie (CRDS) mit breitbandigen und zeitlich inkohärenten Lichtquellen niedriger spektraler Leistungsdichte durchzuführen. Das Verfahren wird zum ersten Mal mit einer breitbandigen Superkontinuum-Quelle demonstriert. A new coupling scheme for cavity enhanced absorption spectroscopy makes use of an intracavity acousto-optical modulator to actively switch light into (and out of) a resonator. This allows cavity ringdown spectroscopy (CRDS) to be implemented with broadband temporally incoherent light sources with low spectral power densities. The method is demonstrated for the first time using a broadband supercontinuum source.

Dye-sensitized solar cells (DSCs) are one of the most promising environmental friendly and low material costs photovoltaic devices. DSCs accomplish the separation of the optical absorption and charge separation processes by the association of a sensitizer as light absorbing material with a wide band gap semiconductor. The mesoscopic morphology of the semiconductor produces an interface with a huge area endowing these systems with intriguing optoelectronic properties. In recent years the DSC has made excellent progress. Conversion efficiencies over 12% and excellent stability have been reached rendering it a credible alternative to conventional p-n junction photovoltaic devices. Commercial exploitation of DSCs started in 2009. In addition to the DSC, Laboratory of Photonics and Interfaces performs research in the field of photoelectrochemical water splitting and materials design for organic light emitting diodes.

In preparation for picosecond pump-probe experiments at the SwissFEL X-ray laser facility, the feasibility of collectively initiating surface chemical reactions using energetic pulses of terahertz radiation is being tested. © Schweizerische Chemische Gesellschaft.

Powerful THz radiation confined in one field period or less is an adequate tool for triggering nonlinear actions. We show results towards the realization of a tunable high-power THz source based on a laser-driven frequency conversion scheme in plasma and nonlinear crystals. A powerful THz source in combination with the future X-ray Free Electron Laser facility in Switzerland (SwissFEL) holds promise for exciting experiments in a variety of different research areas.

InGaN/GaN devices are currently used for many applications, for example, full color display, white (RGB) illumination systems and for the realization of shorter wavelength emitters for optical data storage. We previously reported a blinking phenomenon in the photo-luminescence of InGaN device ready single quantum well materials. In this study we observe in high resolution this optical instability with a near-field nano-probe. The phenomenon appears only in local confined domains and does not seem to behave as a bistable state process like reported on quantum dots generated photo-luminescence. We investigated by a modified scanning near-field optical microscope (SNOM) and studied the time/intensity profile of the optical signal with a resolution in the range of 100nm. The dynamics of the blinking was time-resolved and its behaviour studied with Fourier analysis. Despite the intensity oscillations were found to have chaotic component (autocorrelation coefficient is about 0.63), the optical oscillations appear to include regular characteristics. Fourier analysis of the light intensity from confined domains exhibit peaks in the range of 4–5 s. The emergence of these intriguingly slow and partially regular dynamics should shed light on the inner mechanism that are involved in the fundamental processes of optical emission in these devices.

Junctions based on mesoscopic Hg electrodes are used to characterize the electrical properties of the organic molecules organized in self-assembled monolayers (SAMs). The junctions M-SAM//SAM-Hg are formed by one electrode based on metals (M) such as Hg, Ag, Au, covered by a SAM, and by a second electrode always formed by a Hg drop carrying also a SAM. The electrodes, brought together by using a micromanipulator, sandwich SAMs of different nature at the contact area (?0.7 ?m2). The high versatility of the system allows a series of both electrical and electrochemical junctions to be assembled and characterized: i) The compliant nature of the Hg electrodes allows incorporation into the junction and measurement of the electrical behavior of a large number of molecular systems and correlation of their electronic structure to the electrical behavior; ii) by functionalizing both electrodes with SAMs exposing different functional groups, X and Y, it is possible to compare the rate of electron transfer through different X···Y molecular interactions; iii) when the junction incorporates one of the electrode formed by a semitransparent film of Au, it allows electrical measurements under irradiation of the sandwiched SAMs. In this case the junction behaves as a photoswitch; iv) incorporation of redox centres with low lying, easily reachable energy levels, provides electron stations as indicated by the hopping mechanism dominating the current flow; v) electrochemical junctions incorporating redox centres by both covalent and electrostatic interactions permit control of the potential of the electrodes with respect to that of the redox state by means of an external reference electrode. Both these junctions show an electrical behavior similar to that of conventional diodes, even though the mechanism generating the current flow is different. These systems, demonstrating high mechanical stability and reproducibility, easy assembly, and a wide variety of produced results, are convenient test-beds for molecular electronics and represent a useful complement to physics-based experimental methods.

The mechanically controllable break-junction technique (MCBJ) enables us to investigate charge transport through an individually contacted and addressed molecule in ultra-high vacuum (UHV) environment at variable temperature ranging from room temperature down to 4 K. Using a statistical measurement and analysis approach, we acquire current–voltage (I-V) characteristics during the repeated formation, manipulation, and breaking of a molecular junction. At low temperatures, voltages accessing the first molecular orbitals in resonance can be applied, providing spectroscopic information about the junction's energy landscape, in particular about the molecular level alignment in respect to the Fermi energy of the electrodes. Thereby, we can investigate the non-linear transport properties of various types of functional molecules and explore their potential use as functional building blocks for future nano-electronics. An example will be given by the reversible and controllable switching between two distinct conductive states of a single molecule. As a proof-of-principle for functional molecular devices, a single-molecule memory element will be demonstrated.

High-powered light emitting diodes (LED) are investigated for possible uses as light sources in flow diagnostics, in particular, as an alternative to laser-based illumination in particle imaging flow velocimetry in side-scatter imaging arrangements. Recent developments in solid state illumination resulted in mass-produced LEDs that provide average radiant power in excess of 10 W. By operating these LEDs with short duration, pulsed currents that are considerably beyond their continuous current damage threshold, light pulses can be generated that are sufficient to illuminate and image micron-sized particles in flow velocimetry. Time-resolved PIV measurements in water at a framing rate of 2 kHz are presented. The feasibility of LED-based PIV measurements in air is also demonstrated.

We report on a compact, tunable ultraviolet laser system that consists of an optical parametric oscillator (OPO) and a longitudinally diode-pumped Nd:YAG master oscillator-power amplifier (MOPA). The pump energy for the whole laser system is supplied via a single delivery fiber. Nanosecond pulses are produced by an oscillator that is passively Q-switched by a Cr(4+):YAG crystal. The OPO is pumped by the second harmonic of the Nd:YAG MOPA. Continuously tunable radiation is generated by an intracavity sum-frequency mixing process within the OPO in the range of 245-260 nm with high beam quality. Maximum pulse energies of 1.2 mJ were achieved, which correspond to an optical efficiency of 3.75%, relating to the pulse energy of the MOPA at 1064 nm.

From the detailed balance between absorbed and emitted radiation follows the efficiency limit of Shockley and Queisser (SQ-limit). In the framework of the SQ-theory, the exact working principle and the transport mechanism in the solar cell are irrelevant. The efficiency is only determined by the absorptance and emissivity of the material, which are equal according to Kirchhoff’s law. This abstraction from all internal properties of the solar cell makes the SQ-theory general, however, it also implies a fundamental difference between the SQ-theory and typical device simulators, which base on the solution of the continuity equations for charge carrier transport. The present dissertation presents a model, which is compatible with the SQ-theory and which includes the internal details of the device like e.g. charge carrier transport. The simplest version of the model solves the continuity equations for electrons and holes as well as the Poisson equation and allows thereby to simulate inorganic pn and pin-junction solar cells. This model is used for instance to calculate the mobility dependent efficiency limit of SiO2/Si superlattice absorbers. To allow simulations of organic solar cells, the model has to include the continuity equation for excitons, which are relevant in organic materials due to the higher exciton binding energy. The special geometry of organic solar cells, which usually consist of a bulk heterojunction, is also considered. The model for organic polymer-fullerene solar cells allows both the reproduction of experimental data on current/voltage and quantum efficiency measurements and the prediction of the device behaviour as a function of mobility, band offsets and the geometry of the phase separation. The final version of the model also includes the effect of multiple exciton generation, meaning the creation of multiple electron/hole pairs or excitons from a single high energy photon. The model for multiple exciton generating absorber layers allows the identification of possible bottlenecks as well as the calculation of efficiency limits for future solar cell concepts. The experimental part of this thesis uses a reciprocity theorem between electroluminescence (EL) emission and the photovoltaic quantum efficiency, which is a direct consequence of the principle of detailed balance. EL measurements of crystalline silicon, Cu(In,Ga)Se2 and GaInP/GaInAs/Ge triple cells were carried out and interpreted with the help of the reciprocity theorem. In case of crystalline silicon, the focus was on the investigation of recombination and light trapping with spectrally and spatially resolved EL. First the relation between EL and quantum efficiency is verified experimentally before the application for the extraction of diffusion lengths and back side reflectances is presented. Temperature dependent measurements of Cu(In,Ga)Se2 solar cells characterize the devices with respect to their stoichiometric inhomogeneity, which may lead to a detrimental lateral variation of the band gap. The EL measurements show that there is no significant inhomogeneity in high efficiency samples. In case of multijunction solar cells, EL measurements give access to the internal voltages of the individual subcells in the stack. By varying the injection current and detecting the spectral EL emission, this method allows determining the amount of non-radiative recombination and the diode quality factors of each individual cell.

The doping profile in different n-doped GaAs homoepitaxial structures grown by molecular beam epitaxy is investigated in the time domain by employing a laser based picosecond ultrasound technique in a contactless and noninvasive way. Experiments based on asynchronous optical sampling employ two femtosecond lasers, which allow us to detect changes in the optical reflectivity over a 1 ns time delay with a signal-to-noise ratio of 10(exp 7) and 100 fs time resolution in

Optische Schichten dienen zum Einstellen der optischen Eigenschaften einer Oberflache. Ihre Funktion beruht auf dem Interferenzeffekt und/oder den intrinsischen Eigenschaften der Schicht- und Substratmaterialien. Im Folgenden werden alte Faustregeln wiederholt, die grundlegend fur das Design optischer Schichten sind. Old rules useful to the designer of optical coatings Optical coatings are usually composed of multilayer film structures which are used to obtain a desired transmittance, reflectance and absorptance from a surface. The spectral characteristic may be due to the intrinsic property of the material (e.g. metal reflectors) or due to interference effects arising from the multilayer stack. The following is a set of guidelines (“old rules”) useful to the designer of optical systems of which coated surfaces are an integral part.

We report on epitaxial growth of InAs nanowires and the steps necessary to create devices for electrnic transport experiments. Growth conditions were found by the use of metal organic vapor phase epitaxy (MOVPE) resulting in nanowires with designable length and diameter. Electrical properties indicate diffusive electron transport with an elastic mean free path of around hundred nanometers. Coherent quantum mechanical effects and single electron tunneling can be observed at low temperatures in quantum dots created along the nanowire. We demonstrate the realization of highly tunable quantum dots with metallic top-gates. Beyond that, alternative techniques to introduce potential barriers based on local constrictions are investigated., Chimia, 60 (11), ISSN:0009-4293

Some of the recent research in optical sensor and switch molecules at Queen's University of Belfast is out-lined. The ion-induced absorption spectral changes of molecules with ICT (Internal Charge Transfer) excited states are used to demonstrate superposed logic where multiple logic configurations are simultaneously observable. Similar spectral effects in multi-receptor structures give rise to XOR logic. The ion-induced luminescent spectral changes of molecules with ICT excited states can be combined with the phenomenon of Photoinduced Electron Transfer (PET) to exemplify tristable systems. Insights can also be gained into the luminescence effects initiated in designed molecules by transition metal ions in non-aqueous media. The ion-induced luminescent spectral changes of PET systems with one or two receptors display YES and AND logic respectively. The latter yield coincidence sensors for the direct detection of ion pairs. Overall, interesting optical sensors and switches can be produced from rather small molecules when the latter are fitted with ion-receptors.

Duisburg, Essen, Univ., Diss., 2004 This work explains a method for optoelectronic integration of an heterojunction bipolar transistor (HBT) and a waveguide electroabsorption modulator (EAM). For this the epitactical layers of the individual devices based on III/V-semiconductors are merged into each other. The method has the advantage to reuse layers and results in a layer stack, which is easier to process technologically. Additionally to the manufacturing of the individual devices HBT and EAM, this integration enables a multifunctional device, which works in the optical and electronic regime simultaneously (HBT-EAM). This corresponds to an EAM with integrated amplifier. Presented are epitaxy, technological processing and measurement results.

DAST (4-N,N-dimethylamino-4'-N'-methyl stilbazolium tosylate) is an organic salt with one of the highest nonlinear optical susceptibilities and electro-optical coefficients. DAST crystals are therefore seen as good candidates for many photonic applications such as microring resonators or for THz-devices. The quality of such organic crystal is crucial for any application. We discuss here two different growth methods of single crystalline DAST from solution. Bulk crystals, 1 cm3 in size, and thin crystals, 1 mm2 for a thickness of about 100 ?m are obtained. The use of a DAST crystal for a microring resonator is also presented

We report on a room temperature detector which allows to determine and monitor the state of polarization of terahertz radiation with picosecond temporal resolution. The detector is based on the circular photogalvanic effect recently observed in GaAs/AlGaAs quantum wells. The circular photogalvanic effect yields in response to elliptically polarized radiation a current signal proportional to the degree of circular polarization. The peak current signal occurs in unbiased samples for circular polarization, vanishes at linear polarization and changes sign by switching the helicity from right-handed to left-handed. The detector consists of a (113)A MBE grown p-GaAs/AlGaAs multiple quantum well structure. The response has been measured in the wavelength range between 76 μm and 280 μm at normal incidence of the radiation on the sample.

Atomic resolution by noncontact atomic force microscopy with a self-sensing piezoelectric force sensor is presented. The sensor has a stiffness of 1800 N/m and is operated with sub-nanometer amplitudes, allowing atomic resolution with relatively bluntly etched tungsten tips. Sensitivity and noise are discussed.

We study the emission of photons from porous Si after high-level vibrational excitation in the Si-O absorption band using a pulsed CO2 laser. Narrow resonances in the efficiency of light generation are discovered at 1030 and 1084 cm-1. Time-resolved experiments on the luminescence and the ir-induced changes in optical transparency show that the two excitation modes are characterized by different temporal responses. Additionally, their spectral distributions differ substantially. For excitation at 1030 cm-1 the familiar photoluminescence band of porous Si appears. We suggest microscopic processes responsible for the light emissions.