<p> Vertical-cavity surface-emitting lasers (VCSELs) have a long history of development in GaAs-based and InP-based systems, however III-nitride VCSELs research is still in its infancy. Yet, over the past several years we have made dramatic improvements in the lasing characteristics of these highly complex devices. Specifically, we have reduced the threshold current density from &sim;100 kA/cm² to &sim;3 kA/cm², while simultaneously increasing the output power from &sim;10 &mu;W to &sim;550 &mu;W. These developments have primarily come about by focusing on the aperture design and intracavity contact design for flip-chip dual dielectric DBR III-nitride VCSELs. We have carried out a number of studies developing an Al ion implanted aperture (IIA) and photoelectrochemically etched aperture (PECA), while simultaneously improving the quality of tin-doped indium oxide (ITO) intracavity contacts, and demonstrating the first III-nitride VCSEL with an n-GaN tunnel junction intracavity contact. Beyond these most notable research fronts, we have analyzed numerous other parameters, including epitaxial growth, flip-chip bonding, substrate removal, and more, bringing further improvement to III-nitride VCSEL performance and yield. This thesis aims to give a comprehensive discussion of the relevant underlying concepts for nonpolar VCSELs, while detailing our specific experimental advances. In Section 1, we give an overview of the applications of VCSELs generally, before describing some of the potential applications for III-nitride VCSELs. This is followed by a summary of the different material systems used to fabricate VCSELs, before going into detail on the basic design principles for developing III-nitride VCSELs. In Section 2, we outline the basic process and geometry for fabricating flip-chip nonpolar VCSELs with different aperture and intracavity contact designs. Finally, in Section 3 and 4, we delve into the experimental results achieved in the last several years, beginning with a discussion on the epitaxial growth developments. In Section 4, we discuss the most noteworthy accomplishments related to the nonpolar VCSELs structural design, such as different aperture and intracavity contact developments. Overall, this thesis is focused on the nonpolar VCSEL, however our hope is that many of the underlying insights will be of great use for the III-nitride VCSELs community as a whole. Throughout this report, we have taken great effort to highlight the future research fronts that would advance the field of III-nitride VCSELs generally, with the goal of illuminating the path forward for achieving efficient CW operating III-nitride VCSELs.</p>

<p> This dissertation concerns the fabrication, analysis, and simulation of computer generated geometric phase holograms (CGHs). The current knowledge of CGHs is advanced to enable the creation of new sophisticated optical elements with unique characteristics. These elements enable new technologies related to displays, astronomy, sensing, beam-steering, beam-shaping, and more.</p><p> First, a novel direct-write system for CGH creation is presented. A mathematical description of the system is developed which allows the result of a given scan pattern to be predicted. The accuracy of the model is validated with various scan patterns, then a high-quality direct-write polarization grating and q-plate are fabricated for the first time.</p><p> With a system capable of creating CGHs, the most common and useful CGHs are explored in depth: the polarization grating, the geometric phase lens, and the Fourier geometric phase hologram. For each element, the possible scan patterns and parameters and their effect on the resulting element's quality are studied. Ultimately, the optimal scan patterns and parameters are found, then best-quality elements of each type are created and characterized. </p><p> Finally, a new tool for simulating periodic CGHs is developed. This begins with the derivation of the algorithm, which is based on the finite-difference time-domain (FDTD) method. Next tool's capabilities are verified by simulating many test structures and comparing the results to known solutions. The tool is used to simulate, for the first time, a CGH multiple beam splitter and a GPL array.</p>

<p> Analog RF-photonic links hold the potential to increase the precision of time and frequency synchronization in commercial applications by orders of magnitude. However, current RF-photonic links that are used for synchronization must suppress optical-fiber-induced noise by using bi-directional active feedback schemes, in which light must travel through the optical fiber in both directions. These schemes are incompatible with most existing fiber-optic networks. Unless this noise can be suppressed using different methods, RF-photonic time and frequency synchronization will remain accessible only to the research community. As a first step towards identifying alternate means of suppressing the optical-fiber-induced noise, this dissertation presents an extensive experimental characterization and limited theoretical discussion of the dominant optical-intensity and RF-phase noise source in a laboratory setting, where environmental fluctuations are small. The experimental results indicate that the optical-fiber-induced RF-phase noise and optical-intensity noise are caused by the same physical mechanism. The experimental results demonstrate that this mechanism is related to the laser-phase noise but not the laser intensity noise. The bandwidth of the optical-fiber-induced noise depends on the optical-fiber length for lasers with low phase noise, while for lasers with high phase noise, the bandwidth is constant. I demonstrate that the optical-intensity and RF-phase noise can be mitigated by dithering the laser frequency. Based on these results, I hypothesize that interference from Rayleigh scattering is the underlying mechanism of the optical-intensity and RF-phase noise. Prior theoretical work, carried out with high phase noise lasers, predicts that the noise induced by this process will have a bandwidth that is proportional to the laser linewidth and that is constant with respect to the optical-fiber length, for lasers with high-phase noise, which is consistent with the experimental results. I derive a simple model that is valid for low-phase-noise lasers. I compare this model with the experimental results and find that it matches the optical-fiber-length-dependent bandwidth that is measured for low-phase-noise lasers.</p>

Thesis (Ph. D.)--Syracuse University, 2005. / "Publication number AAT 3193856."

<p> Space division multiplexing (SDM) has become a promising approach in the telecom industry to reduce the cost-per-bit of optical fiber transmission and to resolve the approaching bandwidth crunch. Meanwhile, intermodal nonlinear effects in few-mode fibers (FMF) potentially provide some novel applications along with sophisticated optical signal processing functionality. Recently, such spatial channels and modes have been applied in optical sensing applications with the returned echo analyzed for the collection of essential environmental information. The key advantages of implementing SDM techniques in optical measurement systems include the multi-parameter discriminative capability and accuracy improvement. In this dissertation, we conduct theoretical and experimental study on the SDM systems using FMFs for both optical transmission and sensing applications. </p><p> We first investigate a fast-convergence single-stage adaptive frequency-domain recursive-least-square algorithm for simultaneously compensating chromatic dispersion and differential mode group delay in a 224 Gbit/s six-mode polarization-division multiplexed 16 quadrature amplitude modulation FMF transmission system, which increases convergence speed by 53.7% over conventional frequency-domain least-mean square method, with 11% hardware complexity reduction over two-stage recursive-least square approach. </p><p> We then present an ultrafast all-optical simultaneous wavelength and mode conversion scheme based on intermodal four-wave mixing, with the capability of switching polarization and mode degeneracy orientation in FMFs. The relation among the conversion efficiency, pump power and phase matching conditions is investigated in theory analysis and simulation. The cross-polarization modulation and cross-mode modulation can be achieved, by in the best case up to 50% conversion efficiency. </p><p> Finally, a single-end FMF-based distributed sensing system that supports simultaneous temperature and strain monitoring is demonstrated via Brillouin optical time-domain reflectometry and heterodyne detection. Theoretical analysis and experimental assessment of multi-parameter discriminative measurement applied to the distributed sensors are presented, which endows with good sensitivity characteristics and can prevent catastrophic failure in many applications.</p><p>

<p>Laboratory results on some of the optical properties of Deuteporfin, a relatively new photosensitizing drug that has been in clinical trials in China since around 2009, is discussed. The drug was characterized on the basis of one photon absorption and fluorescence emission for providing data for proper drug applications and dosimetry. In addition, the effects of photobleaching were investigated to characterize decay kinetics. The results of this research on this photosensitizer were also compared against HMME Hematoporphyrin monomethyl ether (HMME) (Hemoporfin^&reg;) key characterization data which includes Q-band absorption to compare peak wavelengths and fluorescence intensity to show that Deuteporfin has similar absorption profile to HMME while it has superior fluorescence characteristics. The findings help to support the manufacturer&rsquo;s claim that Deuteporfin can be an effective photosensitizer for tumor treatment. </p>

<p> Even though image sensor performance has improved tremendously over the years, there are two key areas where sensor performance leaves room for improvement. Firstly, small pixel performance is limited by low full well, low dynamic range and high crosstalk, which greatly impact the sensor performance. Also, low light color image sensors, which use color filter arrays, have low sensitivity due to the selective light rejection by the color filters. The quanta image sensor (QIS) concept was proposed to mitigate the full well and dynamic range issues in small pixel image sensors. In this concept, spatial and temporal oversampling is used to address the full well and dynamic range issues. The QIS concept however does not address the issue of crosstalk. In this dissertation, the high spatial and temporal oversampling of the QIS concept is leveraged to enhance small pixel performance in two ways. Firstly, the oversampling allows polarization sensitive QIS jots to be incorporated to obtain polarization information. Secondly, the oversampling in the QIS concept allows the design of alternative color filter array patterns for mitigating the impact of crosstalk on color reproduction in small pixels. Finally, the problem of performing color imaging in low light conditions is tackled with a proposed stacked pixel concept. This concept which enables color sampling without the use of absorption color filters, improves low light sensitivity. Simulations are performed to demonstrate the advantage of this proposed pixel structure over sensors employing color filter arrays such as the Bayer pattern. A color correction algorithm for improvement of color reproduction in low light is also developed and demonstrates improved performance.</p>

<p> This dissertation explores three related embodiments of light&ndash;matter interactions at the micro&ndash; and nano&ndash;scales, and is focused towards tangible device applications. The first topic provides a fundamentally different transistor or electronic switch mechanism, which is termed a light&ndash;effect transistor (LET). The LET, unlike exotic techniques, provides a practical and viable approach using existing fabrication processes. Electronic devices at the nanoscale operate within the ballistic regime, where the dominate source of energy loss comes from impurity scattering. As a LET does not require extrinsic doping, it circumvents this issue. Electron&ndash;phonon coupling, however, is the second largest source, and it is a pertinent and important parameter affecting electronic conductivity and energy efficiency, such as in LETs. The third topic is laser writing, or the use of a laser to perform post&ndash;growth modifications, to achieve specific optical and electrical characteristics. </p><p> A LET offers electronic&ndash;optical hybridization at the component level, which can continue Moore&rsquo;s law to the quantum region without requiring a FET&rsquo;s fabrication complexity, e.g., physical gate and doping, by employing optical gating and photoconductivity. Multiple independent gates are therefore readily utilized to achieve unique functionalities without increasing chip space. LET device characteristics and novel digital and analog applications, such as optical logic gates and optical amplification, are explored. Prototype cadmium selenide (CdSe) nanowire&ndash;based LETs show output and transfer characteristics resembling advanced FETs, e.g., on/off ratios up to ~1.0x10⁶ with a source-drain voltage of ~1.43 V, gate-power of ~260 nW, and a subthreshold swing of ~0.3 nW/decade (excluding losses). The LET platform offers new electronic&ndash;optical integration strategies and high speed and low energy electronic and optical computing approaches.</p><p> Electron&ndash;phonon coupling is typically studied as an intrinsic property, which is particularly important for electronic transport properties at the nanoscale, where controversy and even contradictory experimental and theoretical findings still persist. Zinc telluride (ZnTe) has important uses in optical or laser refrigeration, and the existing studies do not consider extrinsic effects, such as laser&ndash;forming tellurium&ndash;based species. Nanostructures, with their large surface&ndash;to&ndash;volume ratios, are more susceptible to extrinsic perturbations that ultimately effect coupling. In this dissertation, ZnTe is studied in bulk, thin film, and nanowire forms with primary focus on the latter. Raman spectroscopy under near resonant excitation is used to extract electron&ndash;phonon coupling strengths, which is obtained through the ratio of the first and second order Raman peaks, <i>R</i> = <p style="font-variant: small-caps"> I2LO/I1LO</p> (and is proportional to the Huang&ndash;Rhys factor). Laser&ndash;formation of tellurium&ndash;based species on ZnTe nanowires dynamically altered the ratio R from ~6-7 to 2.4 after laser processing, while tuning the (532 nm) laser power from a few microwatts to 150 microwatts (with constant optical exposure time) did not significantly impact the EPC strength. Other explored effects include size dependence, chemical effects (methanol exposure), and interface effects (e.g., at a gold&ndash;nanowire junction). The findings suggest that the previously reported size dependence in ZnTe was extrinsic in nature. Tunable coupling strengths also suggest the possibility of novel electronic and optoelectronic devices.</p><p> The electrical characteristic of CdSe nanowire M-S-M devices are shown to be tunable with laser illumination. As with any semiconductor material, sufficiently low optical powers produce stable and reproducible electrical properties, while higher optical powers and exposure times can induce laser modifications of the material. Drastic modification of electrical characteristics were observed, such as from converting an ohmic response (linear slope change) to rectified characteristics, and modification of both forward and reverse currents. Results suggest the potential to laser write wavelength&ndash;specific electronic functions that could be used in applications requiring wavelength discrimination, such as with night vision products. Using a combination of laser modification and device fabrication processes provides the ability to offer a menu of electrical behaviors using the same materials and fabrication processes.</p>

<p> In the past few decades, imaging technology has made great strides. From high resolution sensors for photography to 3D scanners in autonomous driving, imaging has become one of the key drivers of the modern society. However, there are still many scenarios where the traditional methods of imaging are woefully inadequate. Examples include high-resolution non-visible light imaging, 3D scanning in the presence of strong ambient light, and imaging through scattering media. In these scenarios, the two classical solutions of single-shot imaging using 2D sensors and point scanning using photodiodes have severe shortcomings in terms of cost, measurement rate and robustness to non-idealities in the imaging process. </p><p> The goal of this dissertation is the design of computational imagers that work under traditionally difficult conditions by providing the robustness and economy of point scanning systems along with the speed and resolution of conventional cameras. In order to achieve this goal, we use line sensors or 1D sensors and make three contributions in this dissertation. The first contribution is the design of a line sensor based compressive camera (LiSens) which uses a line sensor and a spatial light modulator for 2D imaging. It can provide a measurement rate that is equal to that of a 2D sensor but with only a fraction of the number of pixels. The second contribution is the design of a dual structured light (DualSL) system which uses a 1D sensor and a 2D projector to achieve 3D scanning with same resolution and performance as traditional structured light system. The third contribution is the design of programmable triangulation light curtains (TriLC) for proximity detection by rotating a 1D sensor and a 1D light source in synchrony. This device detects the presence of objects that intersect a programmable virtual shell around itself. The shape of this virtual shell can be changed during operation and the device can perform under strong sunlight as well as in foggy and smoky environments. We believe that the camera architectures proposed in this dissertation can be used in a wide range of applications, such as autonomous driving cars, field robotics, and underwater exploration.</p><p>

<p> The search for robust, low-noise modelocked comb sources has attracted significant attention during the last two decades. Passively modelocked fiber lasers are among the most attractive comb sources. The most important design problems for a passively modelocked laser include: (1) finding a region in the laser&rsquo;s adjustable parameter space where it operates stably, (2) optimizing the pulse profile within that region, and (3) lowering the noise level. Adjustable parameters will typically include the cavity length, the pump power, and the amplifier gain, which may be a function of the pump power, the pump wavelength, and both the material and geometry of the gain medium. </p><p> There are two basic computational approaches for modeling passively modelocked laser systems: the evolutionary approach and the dynamical approach. In the evolutionary approach, which replicates the physical behavior of the laser, one launches light into the simulated laser and follows it for many round trips in the laser. If one obtains a stationary or periodically-stationary modelocked pulse, the laser is deemed stable and, if no such pulse is found, the laser is deemed unstable. The effect of noise can be studied by using a random number generator to add computational noise. In the dynamical approach, one first obtains a single modelocked pulse solution either analytically or by using the evolutionary approach. Next, one finds the pulse parameters as the laser parameters vary by solving a root-finding algorithm. One then linearizes the evolution equations about the steady-state solution and determines the eigenvalues of the linearized equation, which we refer to as the equation&rsquo;s dynamical spectrum. If any eigenvalue has a positive real part, then the modelocked pulse is unstable. The effect of noise can be determined by calculating the noise that enters each of the modes in the dynamical spectrum, whose amplitudes are described by either a Langevin process or a random walk process. </p><p> The evolutionary approach is intuitive and straightforward to program, and it is widely used. However, it is computationally time-consuming to determine the stable operating regions and can give ambiguous results near a stability boundary. When evaluating the noise levels, Monte Carlo simulations, which are based upon the evolutionary approach, are often prohibitively expensive computationally. By comparison, the dynamical approach is more difficult to program, but it is computationally rapid, yields unambiguous results for the stability, and avoids computationally expensive Monte Carlo simulations. The two approaches are complementary to each other. However, the dynamical approach can be a powerful tool for system design and optimization and has historically been undertilized. </p><p> In this dissertation, we discuss the dynamical approach that we have developed for design and optimization of passively modelocked laser systems. This approach provides deep insights into the instability mechanisms of the laser that impact or limit modelocking, and makes it possible to rapidly and unambiguously map out the regions of stable operation in a large parameter space. For a given system setup, we can calculate the noise level in the laser cavity within minutes on a desktop computer. </p><p> Compared to Monte Carlo simulations, we will show that the dynamical approach improves the computational efficiency by more than three orders of magnitude. We will apply the dynamical approach to a laser with a fast saturable absorber and to a laser with a slow saturable absorber. We apply our model of a laser with a slow saturable absorber to a fiber comb laser with a semiconductor absorbing mirror (SESAM) that was developed at National Institute of Standards and Technology (NIST), Boulder, CO. We optimize its parameters and show that it is possible to increase its output power and bandwidth while lowering the pump power that is needed.</p><p>