Near-field spectroscopy of colloidal quantum dots and vertical cavity surface emitting lasers.

A scanning near-field optical microscope (SNOM) has been employed for developments and measurements that allow spectroscopic characterisations on the nanoscale. To obtain spectral properties on spatial resolutions beyond the diffraction limit a spectrometer has been integrated into the SNOM system, together with various avalanche photodiode detectors, a cooled charge-coupled device and various filters, suitable to the experiments conducted. The system was optimised such that it allowed subsequent probe-positioning to perform point spectroscopy, using localised stimulation and collection techniques in the near-field. This spectral detection scheme has been applied to two areas of study, laser devices and quantum dot systems. The simultaneous topographical and optical study of semiconductor lasers, specifically vertical cavity surface emitting lasers (VCSEL), was carried out to spatially and spectrally map individual transverse mode emissions at the aperture surfaces in the nearfield. These measurements showed the clear presence of modulation of the intensity of the transverse modes in the form of concentric rings. The effect was attributed to a subsurface defect within the aperture of the device, clipping the Gaussian emission profile of the fundamental transverse mode. Higher order transverse modes were also found to be affected, revealing a more complex modulation structure due to their non-Gaussian emission. Structural defects, in or at the surface of such devices, have been shown to have significant effects on far-field characteristics. It is therefore important to spatially map the spectral source of such effects to gain insight into their origin. Spectral SNOM studies on quantum dots were conducted on mixed colour cadmium selenide/zinc sulphide (CdSe/ZnS), and cadmium selenide/hexadecylamine CdSe(HDA) quantum dots. Low concentrations were immobilised within a 2-3 nm thick layer of a PMMA polymer matrix, spin coated onto cleaved mica. Optical stimulation in the nearfield revealed simultaneous topographic and optical detection of single and small clusters of quantum dots. Spectroscopic measurements of single and small clusters of quantum dots in the near-field, showed a minimum spectral full width half maximum (FWHM) of ~14-16 nm. Repeated imaging of single quantum dots also showed fluorescence intermittency events on a range of time scales from below the time resolution of the set-up, to over an hour, in addition to fluorescence brightening. Conclusions as to the potential for quantum dots in biological imaging are discussed

Material Properties of MBE Grown ZnTe, GaSb and Their Heterostructures for Optoelectronic Device Applications

abstract: Recently a new materials platform consisting of semiconductors grown on GaSb and InAs substrates with lattice constants close to 6.1 A was proposed by our group for various electronic and optoelectronic applications. This materials platform consists of both II-VI (MgZnCdHg)(SeTe) and III-V (InGaAl)(AsSb) compound semiconductors, which have direct bandgaps spanning the entire energy spectrum from far-IR (~0 eV) up to UV (~3.4 eV). The broad range of bandgaps and material properties make it very attractive for a wide range of applications in optoelectronics, such as solar cells, laser diodes, light emitting diodes, and photodetectors. Moreover, this novel materials system potentially offers unlimited degrees of freedom for integration of electronic and optoelectronic devices onto a single substrate while keeping the best possible materials quality with very low densities of misfit dislocations. This capability is not achievable with any other known lattice-matched semiconductors on any available substrate. In the 6.1-A materials system, the semiconductors ZnTe and GaSb are almost perfectly lattice-matched with a lattice mismatch of only 0.13%. Correspondingly, it is expected that high quality ZnTe/GaSb and GaSb/ZnTe heterostructures can be achieved with very few dislocations generated during growth. To fulfill the task, their MBE growth and material properties are carefully investigated. High quality ZnTe layers grown on various III-V substrates and GaSb grown on ZnTe are successfully achieved using MBE. It is also noticed that ZnTe and GaSb have a type-I band-edge alignment with large band offsets (delta_Ec=0.934 eV, delta_Ev=0.6 eV), which provides strong confinement for both electrons and holes. Furthermore, a large difference in refractive index is found between ZnTe and GaSb (2.7 and 3.9, respectively, at 0.7 eV), leading to excellent optical confinement of the guided optical modes in planar semiconductor lasers or distributed Bragg reflectors (DBR) for vertical-cavity surface-emitting lasers. Therefore, GaSb/ZnTe double-heterostructure and ZnTe/GaSb DBR structure are suitable for use in light emitting devices. In this thesis work, experimental demonstration of these structures with excellent structural and optical properties is reported. During the exploration on the properties of various ZnTe heterostructures, it is found that residual tensile strains exist in the thick ZnTe epilayers when they are grown on GaAs, InP, InAs and GaSb substrates. The presence of tensile strains is due to the difference in thermal expansion coefficients between the epilayers and the substrates. The defect densities in these ZnTe layers become lower as the ZnTe layer thickness increases. Growth of high quality GaSb on ZnTe can be achieved using a temperature ramp during growth. The influence of temperature ramps with different ramping rates in the optical properties of GaSb layer is studied, and the samples grown with a temperature ramp from 360 to 470 C at a rate of 33 C/min show the narrowest bound exciton emission peak with a full width at half maximum of 15 meV. ZnTe/GaSb DBR structures show excellent reflectivity properties in the mid-infrared range. A peak reflectance of 99% with a wide stopband of 480 nm centered at 2.5 um is measured from a ZnTe/GaSb DBR sample of only 7 quarter-wavelength pairs.Dissertation/ThesisPh.D. Physics 201

Compound Semiconductor-Based Thin-Film and Flexible Optoelectronics.

Compound semiconductors are the basis of modern optoelectronics due to their intrinsically superior optical and electronic properties compared with elemental semiconductors. However, their applications remain limited due to a prohibitive substrate cost. This limitation has driven the development of epitaxial lift-off (ELO) technology that separates the thin-film epitaxial layer from the substrate by selectively removing a sacrificial layer between them. However, ELO has its own limitations including a long process time, complicated transfer to a secondary, low cost host substrate, and wafer surface degradation which prevents wafer recycling. In this thesis, we address all of these limitations by developing a new, non-destructive ELO (ND-ELO) process. When combined with adhesive-free cold-weld bonding of the wafer directly to a plastic substrate, ND-ELO provides an approximately 100 times reduction in process time, and a considerably simplified transfer process compared with conventional ELO. Furthermore, it allows indefinite wafer reuse by employing the epitaxial protection layers, eliminating surface degradation of the parent wafer encountered in conventional ELO. We demonstrate the feasibility and generality of this process by applying it to optoelectronic devices including photovoltaic cells, LEDs, MESFETs and photodetectors on two compound semiconductor systems, InP and GaAs. Furthermore, we present an approach that can achieve an estimated cost of only 3% that of conventional GaAs solar cells using an accelerated ND-ELO wafer recycling process, and integrated with lightweight, thermoformed plastic, truncated mini-compound parabolic concentrators (CPC) that avoid the need for active solar tracking. Using solar cell/CPC assemblies, without daily solar tracking, the annual energy harvesting is increased by 2.8 times compared with planar solar cells. This represents a drastic cost reduction in both the module and balance of systems costs compared with heavy, rigid conventional modules and trackers that are subject to wind loading damage and high installation costs. The demonstration of cost-efficient and high performance compound semiconductor-based flexible thin-film optoelectronics is a critical step toward allowing their widespread deployment in mainstream state-of-the-art applications including wearable, flexible and conformal devices.PhDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/111479/1/kyusang_1.pd

1996 Laboratory directed research and development annual report

This report summarizes progress from the Laboratory Directed Research and Development (LDRD) program during fiscal year 1996. In addition to a programmatic and financial overview, the report includes progress reports from 259 individual R&D projects in seventeen categories. The general areas of research include: engineered processes and materials; computational and information sciences; microelectronics and photonics; engineering sciences; pulsed power; advanced manufacturing technologies; biomedical engineering; energy and environmental science and technology; advanced information technologies; counterproliferation; advanced transportation; national security technology; electronics technologies; idea exploration and exploitation; production; and science at the interfaces - engineering with atoms

Development of a Full-Field Time-of-Flight Range Imaging System

A full-field, time-of-flight, image ranging system or 3D camera has been developed from a proof-of-principle to a working prototype stage, capable of determining the intensity and range for every pixel in a scene. The system can be adapted to the requirements of various applications, producing high precision range measurements with sub-millimetre resolution, or high speed measurements at video frame rates. Parallel data acquisition at each pixel provides high spatial resolution independent of the operating speed. The range imaging system uses a heterodyne technique to indirectly measure time of flight. Laser diodes with highly diverging beams are intensity modulated at radio frequencies and used to illuminate the scene. Reflected light is focused on to an image intensifier used as a high speed optical shutter, which is modulated at a slightly different frequency to that of the laser source. The output from the shutter is a low frequency beat signal, which is sampled by a digital video camera. Optical propagation delay is encoded into the phase of the beat signal, hence from a captured time variant intensity sequence, the beat signal phase can be measured to determine range for every pixel in the scene. A direct digital synthesiser (DDS) is designed and constructed, capable of generating up to three outputs at frequencies beyond 100 MHz with the relative frequency stability in excess of nine orders of magnitude required to control the laser and shutter modulation. Driver circuits were also designed to modulate the image intensifier photocathode at 50 Vpp, and four laser diodes with a combined power output of 320 mW, both over a frequency range of 10-100 MHz. The DDS, laser, and image intensifier response are characterised. A unique method of measuring the image intensifier optical modulation response is developed, requiring the construction of a pico-second pulsed laser source. This characterisation revealed deficiencies in the measured responses, which were mitigated through hardware modifications where possible. The effects of remaining imperfections, such as modulation waveform harmonics and image intensifier irising, can be calibrated and removed from the range measurements during software processing using the characterisation data. Finally, a digital method of generating the high frequency modulation signals using a FPGA to replace the analogue DDS is developed, providing a highly integrated solution, reducing the complexity, and enhancing flexibility. In addition, a novel modulation coding technique is developed to remove the undesirable influence of waveform harmonics from the range measurement without extending the acquisition time. When combined with a proposed modification to the laser illumination source, the digital system can enhance range measurement precision and linearity. From this work, a flexible full-field image ranging system is successfully realised. The system is demonstrated operating in a high precision mode with sub-millimetre depth resolution, and also in a high speed mode operating at video update rates (25 fps), in both cases providing high (512 512) spatial resolution over distances of several metres

Development of electrically pumped vertical external cavity surface emitting lasers (EP-VECSELs).

In this thesis design, development and realisation of a substrate emission electrically pumped vertical external cavity surface emitting lasers (EP-VECSELs) emitting in the 980 nm wavelength range is discussed. Chapter 1 provides a literature review of the relevant VCSEL and (OP-VECSEL) technology required for the design of an EP-VECSEL. In chapter 2, different areas of the device design are highlighted, including electrical and optical performance of the distributed Bragg reflectors (DBRs), active region design, device detuning and antirefiective coating design. Chapter 3 provides a description of the method used to fabricate EP-VECSEL devices and focuses on optimisation of different process steps, namely the trench etch profile and depth, as well as the contact metalisation. A method for characterising the detuning of a wafer is also presented. In chapter 4 measurements of fabricated EP-VECSEL are presented, with a method for the characterisation of the EP-VECSEL material by modulating the output coupler mirror reflectivity demonstrated. This method is then used to examine the affect of different substrate dopings on device performance. Data is also presented on beam quality, power scaling and thermal properties. Chapter 5 investigates methods for improving electrical aspects of device operation, with improved nand p DBR designs proposed. In addition, analysis of SIMS data for an EP-VECSEL and n-DBR are presented, along with an investigation of the top contact geometry. In chapter 6 a discussion of the QW active region is provided, first by analysing the epitaxial material used in chapter 4 and then proposing improvements to the growth process. A comparison of a 3, 6 and 9 QW active region is then presented, where the trade offs in the optimum number of QWs are examined. Finally, this thesis is summarised and a new device design is proposed from the findings

High speed energy efficient incoherent optical wireless communications

The growing demand for wireless communication capacity and the overutilisation of the conventional radio frequency (RF) spectrum have inspired research into using alternative spectrum regions for communication. Using optical wireless communications (OWC), for example, offers significant advantages over RF communication in terms of higher bandwidth, lower implementation costs and energy savings. In OWC systems, the information signal has to be real and non-negative. Therefore, modifications to the conventional communication algorithms are required. Multicarrier modulation schemes like orthogonal frequency division multiplexing (OFDM) promise to deliver a more efficient use of the communication capacity through adaptive bit and energy loading techniques. Three OFDM-based schemes – direct-current-biased OFDM (DCO-OFDM), asymmetrically clipped optical OFDM(ACO-OFDM), and pulse-amplitude modulated discrete multitone (PAM-DMT) – have been introduced in the literature. The current work investigates the recently introduced scheme subcarrier-index modulation OFDM as a potential energy-efficient modulation technique with reduced peak-to-average power ratio (PAPR) suitable for applications in OWC. A theoretical model for the analysis of SIM-OFDMin a linear additive white Gaussian noise (AWGN) channel is provided. A closed-form solution for the PAPR in SIM-OFDM is also proposed. Following the work on SIM-OFDM, a novel inherently unipolar modulation scheme, unipolar orthogonal frequency division multiplexing (U-OFDM), is proposed as an alternative to the existing similar schemes: ACO-OFDMand PAM-DMT. Furthermore, an enhanced U-OFDMsignal generation algorithm is introduced which allows the spectral efficiency gap between the inherently unipolar modulation schemes – U-OFDM, ACO-OFDM, PAM-DMT – and the conventionally used DCO-OFDM to be closed. This results in an OFDM-based modulation approach which is electrically and optically more efficient than any other OFDM-based technique proposed so far for intensity modulation and direct detection (IM/DD) communication systems. Non-linear distortion in the optical front-end elements is one of the major limitations for high-speed communication in OWC. This work presents a generalised approach for analysing nonlinear distortion in OFDM-based modulation schemes. The presented technique leads to a closed-form analytical solution for an arbitrary memoryless distortion of the information signal and has been proven to work for the majority of the known unipolar OFDM-based modulation techniques - DCO-OFDM, ACO-OFDM, PAM-DMT and U-OFDM. The high-speed communication capabilities of novel Gallium Nitride based μm-sized light emitting diodes (μLEDs) are investigated, and a record-setting result of 3.5Gb/s using a single 50-μm device is demonstrated. The capabilities of using such devices at practical transmission distances are also investigated, and a 1 Gb/s link using a single device is demonstrated at a distance of up to 10m. Furthermore, a proof-of-concept experiment is realised where a 50-μm LED is successfully modulated using U-OFDM and enhanced U-OFDM to achieve notable energy savings in comparison to DCO-OFDM

Parallel reconfigurable single photon avalanche diode array for optical communications

There is a pressing need to develop alternative communications links due to a number of physical phenomena, limiting the bandwidth and energy efficiency of wire-based systems or economic factors such as cost, material-supply reliability and environmental costs. Networks have moved to optical connections to reduce costs, energy use and to supply high data rates. A primary concern is that current optical-detection devices require high optical power to achieve fast data rates with high signal quality. The energy required therefore, quickly becomes a problem. In this thesis, advances in single-photon avalanche diodes (SPADs) are utilised to reduce the amount of light needed and to reduce the overall energy budget. Current high performance receivers often use exotic materials, many of which have severe environmental impact and have cost, supply and political restrictions. These present a problem when it comes to integration; hence silicon technology is used, allowing small, mass-producible, low power receivers. A reconfigurable SPAD-based integrating receiver in standard 130nm imaging CMOS is presented for links with a readout bandwidth of 100MHz. A maximum count rate of 58G photon/s is observed, with a dynamic range of ≈ 79dB, a sensitivity of ≈ −31.7dBm at 100MHz and a BER of ≈ 1x10−9. We investigate the properties of the receiver for optical communications in the visible spectrum, using its added functionality and reconfigurability to experimentally explore non-ideal influences. The all-digital 32x32 SPAD array, achieves a minimum dead time of 5.9ns, and a median dark count rate (DCR) of 2.5kHz per SPAD. High noise devices can be weighted or removed to optimise the SNR. The power requirements, transient response and received data are explored and limiting factors similar to those of photodiode receivers are observed. The thesis concludes that data can be captured well with such a device but more electrical energy is needed at the receiver due to its fundamental operation. Overall, optical power can be reduced, allowing significant savings in either transmitter power or the transmission length, along with the advantages of an integrated digital chip