General Strategy for Nanoscopic Light Source Fabrication

It was about 125 years ago that the light bulb was commercialized by Thomas Edison. No doubt a brilliant invention at the time, today its low power conversion efficiency is one of the reasons why lighting in the western world has such high energy consumption. Thus, the potential for saving energy is enormous in this area. The introduction of halogen, discharge and fluorescent lamps has lead to certain efficiency improvements, however more than half of the energy is still lost as heat. Light-emitting diodes (LEDs) are very promising candidates for high efficiency light sources, with modern devices showing internal quantum efficiencies of virtually 100 %. However, due to the high refractive index of the commonly used semiconductor materials it is very difficult to have a large extraction efficiency; in a standard cubic geometry most of the internally emitted light is trapped inside the device due to total internal reflection. Several methods have been developed in order to circumvent this problem, either by optimizing the device geometry in order to increase the escape cone or by incorporating a resonant structure in order to force the internal emission into the existing escape cone. The latter approach is called microcavity LED (MCLED) or resonant cavity LED (RCLED). In a MCLED the spontaneous internal emission is controlled by placing the emitter inside an optical cavity with a thickness of the order of its emitting wavelength. The resulting interference effects increase the part of the emission that can be extracted. Contrary to the other approaches this is possible without changing the device geometry and thus without additional costly back-end processing steps. The control of the farfield radiation pattern makes these devices particularly interesting for high brightness applications, which demand highly directional emitters, such as for printing, bar code reading, large area displays and optical communication. The extraction efficiency of a MCLED is inversely proportional to the effective cavity length. An ideal cavity, allowing an extraction efficiency close to unity, consists of a low refractive index material and has an optical length of λ/2. In contrast to this, to obtain high internal quantum efficiencies it is necessary to use high index cavities with an optical length of at least λ. It should be noted, that the large penetration depth of the optical field in the semiconductor-based distributed Bragg reflectors (DBRs) leads to a significant increase of the effective cavity length and thus further reduces the achievable extraction efficiencies. In this thesis novel concepts to reduce effective cavity lengths and therefore increase extraction efficiencies are implemented into standard MCLED structures. The phaseshift cavity principle whilst maintaining the electrical properties of a standard A cavity achieves optical properties approaching that of a λ/2 cavity. The use of AlOx instead of AlAs as the, low refractive index component in the DBRs leads to smaller penetration depths and a concomitant reduction of the effective cavity length. A similar effect can be obtained by combining a resonant cavity with a thin-film structure. Thanks to these design improvements, the external quantum efficiency of different types of MCLEDs was increased. Near infrared emitting InGaAs/GaAs MCLEDs including a phase-shift cavity were realized, as both bottom and top emitting structures. The external quantum efficiencies achieved for emission into air were 18 and 19 %, respectively. With the additional incorporation of an oxide based bottom DBR, the efficiency of top emitting near infrared MCLEDs was further increased to 28 %. Red emitting AlGaInP-based structures are not compatible with the phase-shift cavity principle. However the beneficial effect of the implementation of an oxide DBR is greater at their wavelength rather than in the near infrared. Thus with preliminary red emitting GaInP/AlGaInP MCLEDs containing a bottom oxide DBR external quantum efficiencies of 12% could be achieved. Unfortunately, the incorporation of an oxide DBR significantly complicates the device design and the device fabrication. These problems can be avoided by combining the resonant cavity with a thin-film structure instead. Initial non-optimized red emitting thin-film MCLEDs were realized by OSRAM Opto Semiconductors and characterized in this work. They show external quantum efficiencies of 23 % and 18 % with and without encapsulation, respectively. It is assumed that a significant fraction of the high external quantum efficiency is due to a strong photon recycling effect in these devices. Simulations presented in this thesis show that the theoretical limits for the MCLEDs discussed above are slightly higher than the values obtained, encouraging further device optimization. The thin-film MCLEDs seem to hold the biggest potential for high efficiency emission from MCLEDs, independent of the wavelength range of emission.

One of the key mandates of modern optoelectronic research is the development of compact photonic integrated circuits, capable of performing many diverse functions for the generation, manipulation, and detection of light, all on a single chip. A key practical requirement for such circuits is the development of optical devices for the localization and processing of light within extremely small dimensions. In recent years, planar microring and microdisk resonators, in which light is confined by total internal reflection, have emerged as versatile photonic elements for filling this role. The high quality factors and long photon storage times associated with the whispering-gallery modes supported by these microcavities result in several technologically useful characteristics, including narrow-band filter response, and large resonant enhancement of the circulating electric field. These properties have been exploited in numerous passive and active device applications, including optical add/drop multiplexers, all-optical switches, and tunable lasers. This thesis describes the study of several unique ring-based optical microcavity geometries based upon the indium gallium arsenide phosphide/indium phosphide alloy semiconductor material system, undertaken in an effort to explore new optoelectronic architectures for confining and manipulating light. The first portion of this work involves the analysis and demonstration of a new microcavity geometry, in which cylindrical Bragg reflection is used for radial optical confinement, as an alternative to total internal reflection. In this class of structures, collectively known as annular Bragg resonators, light can be guided within a ring or pillar defect layer surrounded by cylindrical Bragg mirrors. Several microcavities based upon this configuration are designed and fabricated using a thin InGaAsP quantum well membrane. Using pulsed optical excitation, the characteristics of these structures as low threshold vertically emitting lasers is explored. Second, a total internal reflection-based coupled waveguide-resonator geometry, having applications to low power optical switching and modulation, is analyzed. This geometry makes use of the hybrid integration of a Mach-Zehnder interferometer with a racetrack resonator. Switching takes place using the Mach-Zehnder to control the coupling parameters in the vicinity of the critical coupling condition. Characterization of the static and dynamic output response of a thermooptically actuated InGaAsP-InP hybrid switch device demonstrates good ON-OFF switching contrast, microsecond response time, and reduced switching power in comparison with a conventional Mach-Zehnder configuration. Finally, this work concludes by examining both the annular Bragg resonator and hybrid switch geometries in application to chemical and biological sensing. Both microcavity devices are shown to possess unique characteristics making them ideal for sensitive monitoring of small changes in the refractive index of a chemical or biological analyte.

Total reflection X-ray photoelectron spectroscopy as a semiconductor lubricant elemental analysis method

Low-index materials are key components in integrated photonics and can enhance index contrast and improve performance. Such materials can be constructed from porous materials, which generally lack mechanical strength and are difficult to integrate. Here we demonstrate enhanced total internal reflection (TIR) induced by integrating robust nanolattice materials with periodic architectures between high-index media. The transmission measurement from the multilayer stack illustrates a cutoff at about a 60° incidence angle, indicating an enhanced light trapping effect through TIR. Light propagation in the nanolattice material is simulated using rigorous coupled-wave analysis and transfer matrix methods, which agrees well with experimental data. The demonstration of the TIR effect in this Letter serves as a first step towards the realization of multilayer devices with nanolattice materials as robust low-index components. These nanolattice materials can find applications in integrated photonics, antireflection coatings, photonic crystals, and low-k dielectric.

Total internal reflection (TIR) is a ubiquitous phenomenon used in photonic devices ranging from waveguides and resonators to lasers and optical sensors. Controlling this phenomenon and light confinement are keys to the future integration of nanoelectronics and nanophotonics on the same silicon platform. We introduced the concept of relaxed total internal reflection in 2014 to control evanescent waves generated during TIR. These unchecked evanescent waves are the fundamental reason photonic devices are inevitably diffraction-limited and cannot be miniaturized. Our key design concept is the engineered anisotropy of the medium into which the evanescent wave extends thus allowing for skin depth engineering without any metallic components. In this article, we give an overview of our approach and compare it to key classes of photonic devices such as plasmonic waveguides, photonic crystal waveguides and slot waveguides. We show how our work can overcome a long standing issue in photonics nanoscale light confinement with fully transparent dielectric media.

Minimizing optical losses in resonant cavities is crucial for improving photonic device performance. This study focuses on the development of a simulation tool to analyze scattering losses in Fabry–Pérot interferometers (FPIs), offering precise modeling of waveguide dynamics and contributing to accurate loss predictions across various platforms. Optical cavities often suffer from scattering losses due to surface roughness and material defects. Our approach integrates theoretical models and simulations to quantify these losses, utilizing the FPI as a model system. We identified upper and lower reflectivity thresholds, beyond which accurate measurement of losses becomes unreliable. For reflectivity below a certain threshold, measurement errors arise, while excessively high reflectivity can reduce fringe visibility and introduce detector sensitivity issues. Simulations were used to validate the model’s ability to predict reflectivity and attenuation in waveguides with varying loss levels. The software’s flexibility to adjust transmission parameters for different cavity configurations enhances its utility for a broad range of photonic systems. Our study offers a novel methodology for optical loss analysis, with practical applications in optimizing photonic devices. By providing a reliable tool for precise loss measurement, this work supports advancements in optical technologies, enabling the design of more efficient, high-performance devices across various applications.

To address the issues of the existing laser and electrochemical machining (LECM) processes in controlling the machining precision, machining depth limit, and taper angle, a novel LECM based on internal total reflection (LECM-ITR) has been proposed. In LECM-ITR, both the electrolyte jet and laser beam were guided and transferred to the machining area, synchronously. Since the tube electrode could feed into the workpiece, the laser could act on the high depth machining area with less laser attenuation. The materials could be removed by using laser-materials interaction and electrochemical dissolution. Small holes with depth of 2 mm have been processed on aluminum alloy and stainless-steel workpiece successfully. Experimental results showed that LECM-ITR could decrease the side gap by 23.6%, reduce the taper angle by 60.8%, and improve materials removal rate by 118.8%. The mechanism of materials removal rate and precision enhancement by LECM-ITR were discussed, including (1) Laser induced temperature rise at the machining area could improve the machining efficiency of ECM with the increased electric current density; (2) The formed anodic oxide layer in larger thickness could enhance the machining precision of ECM with increasing time constant of the electric double layer at the machining area.

Microring resonators are a rapidly-developing area of research in photonic devices, and have a wide range of applications including signal processing filters, sensors, lasers, modulators, switches, memory and slow-light elements, and the list of applications and the performance achievements is growing. Recently microring resonators have become an interesting technology to generate enhanced pulse shapes and accurate wavelengths This review presents a review on generating wavelengths and pulses shapes using microring resonators systems. Although the commercial promise of microring resonators has not yet been realized, this paper continues to flourish in a remarkable variety of applications-areas discussed here.

The motivation of this thesis is to provide quantitative insights into the microscopic interaction mechanisms between atomic hydrogen and hydrogenated amorphous silicon (a-Si:H), particularly the evolution kinetics of the electronic defect states during the interaction. a-Si:H thin films are grown on the total internal reflection surface of a folded miniature optical resonator by thermal decomposition of SiH4 on a hotwire, and are subjected to quantified atomic H fluxes at various substrate temperature. The H-induced defects during the process is monitored in situ by the Evanescent-Wave Cavity Ring-Down Spectroscopy (EW-CRDS), which allows to study the kinetics with a time resolution of 33 ms and sensitivity up to 0.1 ppm in the defect absorption change. The defect evolution process is characterized by a fast increase to a steady state when the H flux is turn on and reversible healing after the H flux is turned off. The effects of the H flux, substrate temperature, film structure and film thickness on the defect evolution process are investigated. The electronic defect state detected by EW-CRDS is recognized as the defect complexes (DC), which are created by insertion of H atoms into strained Si-Si bonds. A comprehensive microscopic kinetic model is proposed to interpret the measured EW-CRDS data, which involves DC creation by H insertion, H-DC recombination, DC self-healing and H diffusion. The kinetic model is studied analytically for the H and DC profiles in the steady state and the DC evolution kinetics in the initial stage. The defect absorption kinetics in the entire time range can be fitted numerically based on the kinetic model, which is consistent with the analytical results. Based on the combined analytical and numerical study, the kinetic parameters of the H/a-Si:H interaction and their temperature dependence can be quantitatively evaluated. The results from the thesis will provide new insights into a number of important H-involved processes in a-Si:H such as the light induced degradation and the H-induced crystallization.

Monitoring thermal barrier coating delamination progression by upconversion luminescence imaging

Two-dimensional photonic crystals drew attentions of researchers because of their manufacturability.. One noticeable advantage of the 2-D photonic crystal is its forgiveness against imperfect fabrication. For example, the each holes of the lattice do not have to be extremely uniform. Even with some irregularities in size and shape, the major properties of the photonic band gap remain almost unchanged except the region near the band edge [1]. The electromagnetic waves interact with a periodic structure over a long range, thereby some kind of natural averaging takes place in the photonic crystal. Experimentally, in 1999, the single defect two-dimensional photonic crystal laser was demonstrated from the triangular lattice air-slab photonic crystal [2]. This demonstration drew attention of the scientists working on photonic crystals and semiconductor lasers because it proved the manufacturability of photonic crystal devices functioning at communication wavelength of 1550 nm. In this example of the 2-D slab triangular photonic crystal, TE-polarized photons are confined by the TE photonic band gap in the plane of the slab and by the total internal reflection in vertical direction.

The switching characteristics of an electroabsorption modulator fabricated on a waveguide-coupled micro-ring-cavity resonator were investigated. The ring cavity resonator consists of ridge waveguide regions, total internal reflector mirrors, and a coupling region. The coupling region is implemented with a multimode interferometer (MMI) waveguide to enhance the coupling coefficient. The total internal reflection mirrors were fabricated by self-aligned RIBE process resulting in minimal insertion losses of around 0.7 dB/mirror. The gain medium was a 1.55 μm InGaAsP MQW ridge waveguide, and the passive waveguide was a re-grown 1.4 μm InGaAsP ridge structure. The sharp variation of the optical transmission through the micro-ring-cavity resonator near the resonance wavelength was utilized to realize efficient semiconductor optical modulators with a high extinction ratio. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical microcavities can trap light within both spatial and temporal dimensions, giving rise to an enhancement of light-matter interaction, which are employed extensively for fundamental studies of optical physics and photonic applications. In particular, because of its ultrahigh- Q factors and small mode volumes, optical whispering-gallery-mode (WGM) microcavities, confining light by total internal reflection, act as the frontier of the state-of-art microcavity studies. As the recent significant development of the micro-/nano-fabrication technology for semiconductor chips and optical materials, on-chip integration becomes one of the most important trends for WGM microcavities. For example, on-chip optical microcavities have been playing a pivotal role in diverse fields, including photonic chip, broadband integrated optical computing, on-chip optical interconnects, microcomb-based precise measurement, etc. In this review, we introduce the research advances of microlasers, nonlinear optics, photonic integrated circuits, and ultrasensitive optical sensing in ultrahigh- Q on-chip microcavities. The specific progresses are organized as follows: (1) Ultralow-threshold lasing in on-chip WGM microcavity can be achieved by either introducing active materials to the resonator or using intrinsic nonlinearities of the resonator material. Moreover, diverse functional microlasers are also realized, such as unidirectional emission, chaotic laser, symmetry-broken laser, orbit angular momentum laser, parity-time reversal symmetry laser, etc. (2) Numerous optical nonlinear effects with high efficiency have been experimentally generated in WGM microcavities, including second harmonic generation, four-wave mixing, stimulated Brillouin scattering, stimulated Raman scattering. Furthermore, high-performance and multifunctional photonic applications have been developed based on microcavity-engineered nonlinear optics. In particular, optical frequency combs based on Kerr nonlinear effects in WGM microcavities, are applied extensively for integrated precision measurement applications like ultrafast LiDAR, optical clocks, and astronomical spectrum calibration. (3) As one of the most important elements in photonic integrated circuit, the optical microcavity has been widely used in the realization of photonic functional devices, such as signal sources, storage, filter, isolator, and modulator. Moreover, it also has been applied to broadband coupling between individual photonic devices. (4) WGM microcavity sensing attracts much attention due to ultra-high sensitivity and label-free nature. The last decade has witnessed the tremendous progress of on-chip microcavity sensing, and the sensing ability has been widely exploited for detecting single nanoparticles, biological molecules, magnetic fields, angular velocity, temperature, etc. Finally, we provide an outlook on the future developments of on-chip ultrahigh- Q microcavity photonics. While numerous progresses have been achieved in ultrahigh- Q on-chip microcavities, a few challenges and obstacles still exist in practical applications. For example, the optical soliton microcombs suffer from the low conversion efficiency and limited wavelength bandwidth; the bandwidth of the optical isolator by the optical microcavities is narrow; the electro-optic modulator based on microcavities cannot combine the broad modulation bandwidth and low power consumption; it is highly challenging for the specific identification in microcavity biosensing. In order to further push forward the development of on-chip microcavity photonics, new principles, new mechanisms, and new materials are required to be explored. The considerable solution may focus on the non-trivial interaction in complex systems, such as hybrid microcavity structures, regulation of optical field by multiple nonlinear optical effects, optical wave chaos in asymmetric microcavities, non-Hermitian physics, and topological photonics.

The potential-induced accumulation of electrolyte ions at the water/1,2-dichloroethane (DCE) electrochemical interface has been examined by total internal reflection second harmonic generation (TIR SHG). TIR SHG experiments provide the first direct optical measurements of the accumulation of the base electrolytes, tetraphenylarsonium (Ph4As+) and tetraphenylborate (Ph4B-), in the interfacial region between two immiscible electrolyte solutions (ITIES). The standard Galvani potential for ion transfer across the water/DCE interface for the tetrabutylammonium (TBA+) is measured from the SHG data. Using the Ph4As+ ion as a reference, a value of 219 ± 5 mV is determined for TBA+. In addition, optically determined potential-dependent concentration profiles for the Ph4As+ and Ph4B- ions are correlated with the modified Verwey−Niessen model for surface excess. Results suggest that less than 10% of the potential drop across the water/DCE interface occurs in the organic phase.

The total internal reflection absorption spectroscopy (TIRAS) method presented in this article uses an inexpensive diode laser to detect solvated electrons produced by a low-temperature plasma in contact with an aqueous solution. Solvated electrons are powerful reducing agents, and it has been postulated that they play an important role in the interfacial chemistry between a gaseous plasma or discharge and a conductive liquid. However, due to the high local concentrations of reactive species at the interface, they have a short average lifetime (~1 µs), which makes them extremely difficult to detect. The TIRAS technique uses a unique total internal reflection geometry combined with amplitude-modulated lock-in amplification to distinguish solvated electrons' absorbance signal from other spurious noise sources. This enables the in situ detection of short-lived intermediates in the interfacial region, as opposed to the bulk measurement of stable products in the solution. This approach is especially attractive for the field of plasma electrochemistry, where much of the important chemistry is driven by short-lived free radicals. This experimental method has been used to analyze the reduction of nitrite (NO2-(aq)), nitrate (NO3-(aq)), hydrogen peroxide (H2O2(aq)), and dissolved carbon dioxide (CO2(aq)) by plasma-solvated electrons and deduce effective rate constants. Limitations of the method may arise in the presence of unintended parallel reactions, such as air contamination in the plasma, and absorbance measurements may also be hindered by the precipitation of reduced electrochemical products. Overall, the TIRAS method can be a powerful tool for studying the plasma-liquid interface, but its effectiveness depends on the particular system and reaction chemistry under study.