High Sensitivity Terahertz Receivers Based on Plasmonic Photoconductors.

Terahertz radiation has unique properties that enable new functionalities for various imaging and sensing applications, such as security screening, bio sensing, medical imaging, and astronomical studies, etc. Despite great benefits that terahertz radiation can offer to these applications, high-power terahertz transmitters and sensitive terahertz receivers are still in demand to realize practical terahertz systems. This PhD research focuses on high sensitivity terahertz receivers based on plasmonic photoconductors. Two types of terahertz receivers have been studied to achieve high terahertz detection sensitivity levels. The first type is photoconductive terahertz receivers, which are widely used for detecting terahertz pulses in time-domain terahertz spectroscopy systems. By utilizing plasmonic contact electrodes in photoconductive terahertz receivers, significantly higher detection sensitivities can be achieved compared to conventional photoconductive terahertz receivers that do not use plasmonic contact electrodes. The second type of terahertz receivers that have been studied is plasmonic heterodyne terahertz receivers, which can be used to detect continuous wave (CW) terahertz radiation and provide accurate intensity and frequency information simultaneously. A novel scheme for heterodyne terahertz receivers based on plasmonic photomixers is presented, which replaces the terahertz local oscillator of conventional heterodyne receivers with two wavelength tunable lasers to provide large dynamic range and broadband operation at room temperature.PhDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/133342/1/niwa_1.pd

Radio frequency and terahertz signals generated by passively mode-locked semiconductor lasers

There are several different approaches to generating periodic signals using semiconductor lasers, for example: Q-switching, gain switching or mode-locking schemes. In general the active or passive mode-locking techniques require the use of a modulator or a saturable absorber in order to achieve the phase synchronisation. The laser diodes studied in this thesis, are demonstrated to operate in the mode-locked regime, while not requiring any direct or external modulation, nor the saturable absorbtion element in order to achieve the phase synchronisation. It has been demonstrated previously, that in a multimode semiconductor laser, the third order nonlinearities of a gain medium resulting in the four-wave-mixing effects, are responsible for the phase synchronisation and lead to phase locking. The repetition rate of the generated signal is fixed by the free-spectral range of the longitudinal spectrum. Therefore, with a passively mode-locked laser (PMLL) it is possible to cover a wide range of frequencies from the Radio-Frequency (RF) to the TeraHertz (THz) domain. Radio frequency signals generated by semiconductor lasers have many applications in optical communications, such as radio-over-fibre, or all-optical clock extraction. Terahertz signals are the focus of many research bodies nowadays, due to their interaction with matter. They have potential applications in areas like: industry, pharmacy, security (military), telecommunication and medicine. With continuous improvement of materials processing and technology, new ways of generation and detection of such types of signals have appeared. The key advantage of the optical RF or THz generation is that this type of device is direct current biased and operates at room temperature. In this thesis, a comprehensive study of various PMLLs, from distributed Bragg reflector bulk laser to quantum dashed Fabry-Perot lasers is given, demonstrating the origin of the phase synchronisation in these structures and some applications for these lasers such as all-optical clock recovery or THz signal generation

Superconducting nanowire single-photon detectors: physics and applications

Single-photon detectors based on superconducting nanowires (SSPDs or SNSPDs) have rapidly emerged as a highly promising photon-counting technology for infrared wavelengths. These devices offer high efficiency, low dark counts and excellent timing resolution. In this review, we consider the basic SNSPD operating principle and models of device behaviour. We give an overview of the evolution of SNSPD device design and the improvements in performance which have been achieved. We also evaluate device limitations and noise mechanisms. We survey practical refrigeration technologies and optical coupling schemes for SNSPDs. Finally we summarize promising application areas, ranging from quantum cryptography to remote sensing. Our goal is to capture a detailed snapshot of an emerging superconducting detector technology on the threshold of maturity.Comment: 27 pages, 5 figures, Review article preprint versio

Optoelectronic Devices Exploiting the Gunn Effect

Semiconductor integrated Electro-Optic modulators which exploit the Gunn effect, have been produced in gallium arsenide. Both vertical and planar diode structures incorporating a form of optical waveguide were investigated. Optical, electrical, and Electro-Optical, design calculations were carried out for each of the proposed device designs. The vertical Gunn diode structures incorporated either a slab or a rib waveguide, whereas the planar Gunn diode structure incorporated a photoelastic waveguide. Various methods, such as the Effective Index Method, the Variational Analysis technique, and the Finite Difference method, were used to assess the propagation characteristics of the rib guide structures. The results of this assessment provided a comparison between the different techniques available, and revealed the advantages and disadvantages of each method. The propagation characteristics of the photoelastic waveguides were analysed by the Finite Difference method. A computer program was developed to analyse the electrical characteristics of the integrated devices. The program solves Poisson's equation and the Current Continuity equation under the conditions for a stable propagating dipole Gunn domain, for: an approximate velocity-field characteristic, v(E), and field independent diffusion coefficient, D; an analytical v(E) characteristic and field independent D; or an analytical v(E) and a field dependent D. The program supplies an estimate of the maximum electric field within the domain, and also of the domain length. The results for the three possible analysis situations were compared. The perturbations induced in the optical guiding characteristics, at above bandgap optical wavelengths, due to the presence of a domain, have been studied. A knowledge of the characteristics of the domain, as it propagates through the device, leads to an estimate of the magnitude of the induced optical changes, eg. in the refractive index, or in the optical absorption. A measure of the Electro-Optic interaction between the propagating domain and the guided optical wave was therefore obtained. In order to test the electrical characteristics of the vertical devices, spot contact diodes, varying in diameter from 20mum to 90mum. were fabricated. Also, for the purpose of testing both the optical and Electro-Optical properties of the vertical devices, rib guides, varying in width from 4mum to 15mum, were fabricated by optical photolithography and dry etching techniques. Unique devices incorporating a vertical Gunn diode and a rib waveguide were consequently designed and fabricated, and assessed for their modulation depth. Optical modulation, at a frequency of 24GHz, was observed in the vertical diode structure. While at a wavelength of 1.15mum modulation was solely due to the Linear Electro-Optic effect, at a wavelength of 905nm modulation was seen to be due to both the Linear Electro-Optic effect, and the Electro-Absorption effect. Estimated values of the observed Electro-Optic coefficient, r41, at wavelengths of 1.15mum and 905nm were obtained, along with an estimate of the observed Electro-Absorption coefficient, alpha, at 905nm

Specialty Fibers for Terahertz Generation and Transmission: A Review

Terahertz (THz) frequency range, lying between the optical and microwave frequency ranges covers a significant portion of the electro-magnetic spectrum. Though its initial usage started in the 1960s, active research in the THz field started only in the 1990s by researchers from both optics and microwaves disciplines. The use of optical fibers for THz application has attracted considerable attention in recent years. In this paper, we review the progress and current status of optical fiber-based techniques for THz generation and transmission. The first part of this review focuses on THz sources. After a review on various types of THz sources, we discuss how specialty optical fibers can be used for THz generation. The second part of this review focuses on the guided wave propagation of THz waves for their transmission. After discussing various wave guiding schemes, we consider new fiber designs for THz transmission

Monolithic Colliding Pulse Mode-Locking of AlGaAs/GaAs and InGaAs/InGaAsP Quantum Well Lasers

The fabrication of T-section colliding pulse mode-locked (T-CPM) GaAs/AlGaAs quantum well lasers is described. These devices have MOVPE grown ridge waveguide laser structures, with monolithically integrated side-injection waveguides, orthogonal to the laser waveguide, passing through the saturable absorber from the edge of the laser chip. This orthogonal waveguide is included to enable synchronisation applications which involve the injection of external optical pulse streams into mode-locked diode lasers. Intensity autocorrelations are performed on these devices, the first time for monolithic CPM lasers in this material system, showing a 400 mum long device mode-locked at a repetition rate of 175 GHz producing near transform-limited 1 ps pulses at wavelengths around 0.87 mum. A new technique for measuring the gain and absorption spectra in semiconductor material is presented, which utilises the multi-section laser fabrication technology developed for the CPM lasers. Using this new technique the TE and TM absorption spectra of the MOVPE grown T-CPM laser saturable absorber is measured in situ under the influence of increasing reverse bias, showing large exciton absorption features, which shift and broaden in accordance with the QCSE. With the measurement of the forward bias gain spectra, the sub-linear form of the gain - current density relationship was directly measured for the mode-locked laser wafer. The crucial role of quantum well doping in the achievement of high repetition rate monolithic mode-locking has been demonstrated by the realisation of short cavity CPM and multiple colliding pulse (MCPM) lasers in MBE grown GaAs/AlGaAs quantum well material only through background doping of the active region to simulate the effects of the residual carbon impurities incorporated in MOVPE grown AlGaAs. By inspection of the absorption spectra, it is shown that a distinct broadening of the exciton absorption peaks occur with the presence of background doping in MBE material. This indicates that the corresponding decease in the exciton lifetime has a beneficial effect on saturable absorber recovery, allowing high repetition mode-locking to take place. Low damage dry etching and wet etching techniques have been developed to enable the successful fabrication of ridge waveguide CPM lasers in InP/InGaAs/InGaAsP laser material. Spectral measurements show the mode-locked operation of CPM and T-CPM lasers in strained and lattice-matched material at wavelengths around 1.5 mum. Also, spectral results are presented showing harmonic CPM operation at repetition rates of 150 GHz and 300 GHz (two and four pulse CPM action), from a 600 mum MCPM laser (emitting at a wavelength of 1.53 mum). This is the first reported demonstration of an MCPM laser diode in the long wavelength region around 1.55 mum

THz: Research Frontiers for New Sources, Imaging and Other Advanced Technologies

The THz region of the electromagnetic spectrum is a frontier research area involving application of many disciplines, from outdoor to indoor communications, security, drug detection, biometrics, food quality control, agriculture, medicine, semiconductors, and air pollution. THz research is highly demanding in term of sources with high power and time resolution, detectors, and new spectrometer systems. Many open questions still exist regarding working at THz frequencies; many materials exhibit unusual or exotic properties in the THz domain, and researchers need new methodologies to exploit these opportunities. This book contains original papers dealing with emerging applications, new devices, sources and detectors, and materials with advanced properties for applications in biomedicine, cultural heritage, technology, and space

Development and Applications of Terahertz Near-Field Microscopes for Surface Plasmon Imaging

The confined nature of surface plasmons (SPs) often imposes challenges on their experimental detection and makes specific near-field probes necessary. While various SP detection methods have been developed in the optical domain, only a few examples of SP imaging have been reported in the terahertz range. In this thesis, specific problems of current terahertz near-field detection systems have been addressed which has led to the development of two new SP imaging methods. In the first method, SP imaging is demonstrated using the integrated subwavelength aperture near-field probe. The photoconductive antenna inside the probe is sensitive to the SP electric-field despite the orthogonal spatial orientation between the antenna and the SP polarisation. This enables SP imaging directly on a metallic surface employing a photoconductive antenna. This unexpected sensitivity has been applied to SP imaging in two examples: first, the SP propagation has been imaged on a resonant THz bow-tie antenna and second, the SP excitation by a strongly focused terahertz beam directly on the metallic probe surface has been investigated. The second method presents an electro-optic micro-resonator for SP imaging. A micro-resonator structure has the potential to provide a better sensitivity and spatial resolution, as well as a lower level of invasiveness compared to bulk crystals, which are commonly used in terahertz near-field systems. The micro-resonator design is explained in detail and the impact of the micro-resonator geometry on the probe performance is discussed. This micro-resonator has then been fabricated and embedded into an electro-optic detection system. This detection system has been fully characterised with the focus on two functional units which are essential for its performance: a tapered parallel plate waveguide for broadband terahertz transmission and the balanced detector for noise reduction. The overall performance of the detection system has been evaluated for its use as a terahertz near-field microscope