Optical Properties of Quasiperiodically Arranged Semiconductor Nanostructures

This work consists of two parts which are entitled "One-Dimensional Resonant Fibonacci Quasicrystals" and "Resonant Tunneling of Light in Silicon Nanostructures". A microscopic theory has been applied to investigate the optical properties of the respective semiconductor nanostructures. The studied one-dimensional resonant Fibonacci quasicrystals consist of GaAs quantum wells (QW) that are separated by either a large spacer L or a small one S. These spacers are arranged according to the Fibonacci sequence LSLLSLSL... The average spacing satisfies a generalized Bragg condition with respect to the 1s-exciton resonance of the QWs. A theory, that makes use of the transfer-matrix method and that allows for the microscopic description of many-body effects such as excitation-induced dephasing caused by the Coulomb scattering of carriers, has been applied to compute the optical spectra of such structures. Based on an appropriate single set of fixed sample parameters, the theory provides reflectance spectra that are in excellent agreement with the corresponding measured linear and nonlinear spectra. A pronounced sharp reflectivity minimum is found in the vicinity of the heavy-hole resonance both in the measured as well as in the calculated linear 54-QW spectra. Such sharp spectral features are suitable for application as optical switches or for slow-light effects. Hence, their properties have been studied in detail. Specifically, the influence of the carrier density, of the QW arrangement, of a detuning away from the exact Bragg condition, of the average spacing as well as of the ratio of the optical path lengths of the large and small spacers L and S, respectively, and of the QW number on the optical properties of the samples have been studied. The features of measured spectra could have been attributed to different sample properties related to the sample setup. Additionally, self-similarity among reflection spectra corresponding to different QW numbers that exceed a Fibonacci number by one is observed, which identifies certain spectral features as true fingerprints of the Fibonacci spacing. In the second part, resonant tunneling of light in stacked structures consisting of alternating parallel layers of silicon and air have been studied theoretically. While usually total internal reflection is expected for light shined on a silicon-air interface under an angle larger than the critical angle, light may tunnel through the air barrier due to the existence of evanescent waves inside the air layers if the neighboring silicon layer is close enough. This tunneling of light is in analogy to the well-known tunneling of a quantum particle through a potential barrier. In particular, the wave equation and the stationary Schrödinger equation are of the same form. Hence, the resonant tunneling of light can be understood in analogy to the resonant tunneling of e.g. electrons as well. The characteristic feature of resonant tunneling is a complete transmission through the barrier at certain resonance energies. The transmission, reflection, and propagation properties of the samples have been determined numerically using a transfer-matrix method. Analytical expressions for the energetic resonance positions have been derived and are in excellent agreement with the numerical simulations. Special attention has been drawn to the lowest resonance out of a series of resonant-tunneling resonances. There, light has been observed to be concentrated within silicon layers the extension of which is smaller than the corresponding wavelength of the light. Specifically, the quality factor is large at the resonance energies, so that the resonant light leaves the sample delayed, which allows for the study of slow light. A detailed investigation of how the sample geometry influences the optical properties of the sample has been presented. In particular, it has been outlined how to design a sample to obtain certain desired optical properties. The optical properties that are related to the resonant tunneling strongly rely on the (mirror-)symmetry of the samples. If asymmetries - especially of the silicon wells inside the air barrier - are present in the sample setup, the resonant-tunneling efficiency is diminished. Such asymmetries are unavoidable in the production of the samples. Therefore, a parameter range has been identified in which reasonable transmission above a transmission probability of 50% can be expected taking typical fluctuations caused by the production process into account. Silicon-based resonant-tunneling structures of a setup proposed by the presented theory have already been fabricated and first experiments are under way. This will allow for theory-experiment comparisons

Photodetectors

In this book some recent advances in development of photodetectors and photodetection systems for specific applications are included. In the first section of the book nine different types of photodetectors and their characteristics are presented. Next, some theoretical aspects and simulations are discussed. The last eight chapters are devoted to the development of photodetection systems for imaging, particle size analysis, transfers of time, measurement of vibrations, magnetic field, polarization of light, and particle energy. The book is addressed to students, engineers, and researchers working in the field of photonics and advanced technologies

The Third International Symposium on Space Terahertz Technology: Symposium proceedings

Papers from the symposium are presented that are relevant to the generation, detection, and use of the terahertz spectral region for space astronomy and remote sensing of the Earth's upper atmosphere. The program included thirteen sessions covering a wide variety of topics including solid-state oscillators, power-combining techniques, mixers, harmonic multipliers, antennas and antenna arrays, submillimeter receivers, and measurement techniques

Integral Optics: Lecture Notes

An introduction is given to the principles of integrated optics and optical guided-wave devices. The characteristics of dielectric waveguides are summarized and methods for their fabrication are described. An illustration is given of recent work on devices including directional couplers, filters, modulators, light deflectors, and lasers. The textbook reflects the latest achievements in the field of integrated optics, which have had a significant impact on the development of communication technology and methods for transmitting and processing information

Resonant tunnelling diodes for THz communications

Resonant tunnelling diodes realised in the InGaAs/AlAs compound semiconductor system lattice-matched to InP substrates represent one of the fastest electronic solid-state devices, with demonstrated oscillation capability in excess of 2 THz. Current state-of-the-art offers a poor DC-to-RF conversion efficiency. This thesis discusses the structural issues limiting the device performance and offers structural design optimums based on quantum transport modelling. These structures are viewed in the context of epitaxial growth limitations and their extrinsic oscillator performance. An advanced non-destructive characterisation scheme based on low-temperature photoluminescence spectroscopy and high-resolution TEM is proposed to verify the epitaxial perfection of the proposed structure, followed by recommendations to improve the statistical process control, and eventually yield of these very high-current density mesoscopic devices. This work concludes with an outward look towards other compound semiconductor systems, advanced layer structures, and antenna designs

Doctor of Philosophy

dissertationMetamaterials have gained significant attention over the last decade because they can exhibit electromagnetic properties that are not readily available in naturally occurring materials. This dissertation describes our work on design, fabrication and characterization of liquid metal-based metamaterials with focus on their applications in the terahertz (THz) frequency range. In contrast to the more conventional approaches to fabricating these structures, which rely on vacuum deposited solid metal films, we used metals that are liquid at room temperature. This family of materials is especially attractive for such applications, since it enables large-scale reconfigurability in the overall geometry of the device. We demonstrate a number of unique plasmonic and metamaterial devices. Within the topic of plasmonics, we demonstrate a device that allows for mechanical stretching that is reversibly deformable. In an analogous structure, we can change the geometry dramatically by injecting or withdrawing liquid metals from specific area of the pattern. We also developed a liquid metal-based reconfigurable THz metamaterial device that is not only pressure driven, but also exhibits pressure memory. As an alternate approach to demonstrate reconfigurability, we developed a technique for creating dramatic configuration changes in a device via selective erasure and refilling of metamaterial unit cells that utilizes hydrochloric acid. While the approach is successful in changing the geometry, it does not allow for fine spatial control of the pattern. Thus, we have refined the approach by developing an electrolytic process to change the geometry of a liquid metal-based structured device in a more localized and controlled manner. Since liquid metals can be solidified under certain conditions, we have demonstrated a novel technique for fabrication of free-standing two-dimensional and three-dimensional terahertz metamaterial devices using injection molding of gallium. Finally, we developed a technique of printing three-dimensional solid metal structures by pulling liquid gallium out of a reservoir via solid/liquid interface. Based on these results, we are currently extending our work towards development of metamaterials that can be used in real-world applications. Based on the significant progress made the THz field over the last two decades, the likelihood of THz systems level applications is much brighter

Terahertz quantum-cascade lasers for spectroscopic applications

Terahertz (THz) quantum-cascade lasers (QCLs) are unipolar semiconductor heterostructure lasers that emit in the far-infrared spectral range. They are very attractive radiation sources for spectroscopy, since they are very compact and exhibit typical output powers of severalmWas well as linewidths in the MHz to kHz range. This thesis presents the development of methods to tailor the emission characteristics of THz QCLs and employ them for spectroscopy with highest resolution and sensitivity. In many cases, these spectroscopic applications require that the far-field distribution of the THz QCLs exhibits only a single lobe. However, multiple lobes in the far-field distribution of THz QCLs were experimentally observed, which were unambiguously attributed to the typically employed mounting geometry and to the cryogenic operation environment such as the optical window. Based on these results, a method to obtain a single-lobed far-field distribution is demonstrated. A critical requirement to employ a THz QCL for high-resolution spectroscopy of a single absorption or emission line is the precise control of its emission frequency. This long-standing problem is solved by a newly developed technique relying on the mechanical polishing of the front facet. A QCL fabricated in this manner allows for spectroscopy at a maximal resolution in the MHz to kHz range, but its accessible bandwidth is usually limited to a few GHz. In contrast, a newly developed method to utilize QCLs as sources for THz Fourier transform spectrometers enables highly sensitive spectroscopy over a significantly larger bandwidth of at least 72 GHz with a maximal resolution of typically 100 MHz. The application of QCLs as sources for THz Fourier transform spectroscopy leads to a signal-to-noise ratio and dynamic range that is substantially increased by a factor of 10 to 100 as compared to conventional sources. The results presented in this thesis pave the way to routinely employ THz QCLs for spectroscopic applications in the near future

Space Communications: Theory and Applications. Volume 3: Information Processing and Advanced Techniques. A Bibliography, 1958 - 1963

Annotated bibliography on information processing and advanced communication techniques - theory and applications of space communication

Monitoring Charge Carrier Dynamics at Atomic Length Scales by STM-induced Luminescence

The goal of this thesis is the combination of the high spatial resolution of scanning tunneling microscopy (STM) with the high temporal resolution of optical spectroscopy, by monitoring the light emitted from the tunnel junction. Two complementary techniques are presented, which allow (a) access to the local photon statistics and (b) following the luminescence response to nanosecond voltage pulses at atomic length scales. The versatility of these methods is demonstrated by using two different model systems: pure C60 films on Ag(111) and Au(111) substrates, as well as single fac-tris(2-phenylpyridine)iridium(III) (Ir(ppy)3) molecules adsorbed on them. These two systems reveal the possibility to investigate both the local charge carrier dynamics of mesoscopic systems as well as individually and selectively addressable quantum systems. Furthermore, these methods are not limited to pure emission processes such as the recombination of electron hole pairs; rather, it is also possible to study the dynamics of non-emitting processes via the excitation and radiation of surface plasmon polaritons (SPPs) from the tunnel junction. To achieve high time resolutions in the luminescence response of an investigated system, a further technique is developed that permits a quantitative mapping of the voltage at the tunnel junction with millivolt and nanosecond accuracy. Knowing the precise shape of the voltage pulses arriving at the tunnel junction offers compensation for ubiquitous pulse distortions arising from reflections and attenuations of the pulses on their way to the tunnel junction, thus enabling pulse rise and falling times of a few nanoseconds. Investigations of the electronic structure of pure C60 films show that the electronic states of C60 multilayers experience a band bending in STM due to the electric field between the STM tip and the metal substrate. As the band bending is sufficiently strong to align the lowest unoccupied molecular orbitals with the Fermi energy of the substrate, electrons can be injected by the substrate. At structural defects, these electrons can recombine with holes in the highest occupied molecular orbitals injected by the STM tip. At such defects, local deviations from the highly ordered structure of the C60 film result in localized electronic states within the band gap. These states act as traps for the injected electrons and holes and increase their lifetime such that they can recombine with each other. The underlying injection dynamics are accessible by the luminescence time response of these defects to nanosecond voltage pulses. Measurements of the second-order intensity correlation function prove that these defects act as single photon sources with exciton lifetimes of <0.7 ns. To clarify whether C60 films can be used as electronic decoupling layers, the electronic structure of individual Ir(ppy)3 molecules is investigated on various C60 film thicknesses. While the single molecules on a C60 monolayer exhibit a vacuum level alignment, their electronic structure on C60 bi- and trilayers suggests a charge transfer to the C60 film. Hence, C60 films are less suitable as inert decoupling layers. Nevertheless, single Ir(ppy)3 molecules adsorbed on various C60 film thicknesses prove to be an ideal model system to study the influence of molecules on the SPP excitation in tunnel junctions. Surprisingly, the level of control systematically increases from one to three C60 layers due to the changing electronic structure