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

Modelling and Design of Efficient Photomixer Based Terahertz Antennas

The lack of unoccupied and unregulated bandwidth for wireless communication vanished at lower frequency spectrum and the increasing demand of high data transmission rate leads to an intensive interest in the research of THz technologies at 0.3THz to 30THz spectrum. However, the limitation of the low output power and low efficiency of current THz devices obstacles the utilization of THz technologies. Also, compared with microwave antenna, the signal generation and excitation of THz antenna require new simulation approach. Therefore, the motivation of this thesis is theoretically analyse the reason that cause the inefficiency of THz antenna, from which, the performance of such antennas is improved from the aspects of THz source with low efficiency, THz antenna with low match efficiency and THz antenna with low gain. These investigations are necessary for the development of the THz photomixer antenna in various applications . First of all, an new equation of the generated THz power from photomixer is developed from the equivalent circuit of photomixer fed antenna. Through this equation, various factors that affect the behaviour of photomixer is examined. Furthermore, a computational simulation process that solving both optoelectronic and electromagnetic problem in a full wave electromagnetic solver. This is a prerequisite for the analysis of improving the optical to THz conversion efficiency of photomixer. After that, the optical to THz conversion efficiency of the photomixer has been gradually improved through three different aspects, by optimizing photomixer electrodes, by utilizing reflectors underneath photomixer and by implementing superstrate. As a result, the highest enhancement factor of optical to THz conversion efficiency achieved is 494. Moreover, instead of exciting planar antenna with photomixer, the concept of truncating the photoconductive substrate of photomixer to form a dielectric resonator antenna is proposed. Such design eliminated the substrate effect to improve the radiation efficiency and to avoid using bulky lens. In addition, choke filter network and dielectric superstrate are used to improve the matching and radiation of these DRAs. The proposed DRA improved the matching efficiency and antenna gain by 10 times and 3dBi, respectively. Finally, a realization design that provide physically support to the dielectric superstrate and replace central feeding slot with coplanar waveguide is presented

Development of terahertz systems using quantum cascade lasers and photomixers

The terahertz (THz) region of the electromagnetic spectrum lies between the more established bands of microwave and infrared radiation. In the past few decades, this region has seen huge growth in the development of both THz sources and detectors for a growing number of potential applications including security, wireless communications, medical diagnostics and astronomy. This thesis makes use of three different methods of generation of THz radiation, these being, THz quantum cascade lasers (QCLs), THz time-domain spectroscopy (TDS) and terahertz photomixing. In the first set of experiments, diffuse reflectance imaging of a range of powered samples has been demonstrated using a THz QCL. Imaging was done at four discrete frequencies in the range of 3–3.35 THz by electrically tuning the emission wavelength of the laser. Absorption coefficients of the samples was inferred using Kubelka–Munk model and was found to be in good agreement with the Beer–Lambert absorption coefficient obtained from broadband (0.3–6 THz) THz-TDS measurements. In the second part of the work, photomixers were designed and fabricated on low-temperature-grown (LTG) GaAs substrates. Ex-situ annealing temperature of LTG GaAs was optimised for maximum bandwidth of the photomixers and the impact on recombination lifetime and resistivity of LTG GaAs was also studied. The final set of experiments examined locking a THz QCL to an external stable source. This would allow access to both amplitude and phase information of the laser emission, which in turn would significantly improve the quality of the data obtained from QCL based imaging techniques, making them useful in many different applications. After investigates of various techniques to achieve this, photomixers driven at telecommunications wavelengths (~1550 nm) were successfully used to obtain injection locking a THz QCL

Development of terahertz photomixer technology at telecommunications wavelength

Terahertz (THz) region is one of the least developed regions of the electromagnetic spectrum. Lack of compact and high power sources and detectors in this wavelength range has limited its use for various key applications. In this thesis, three different approaches adopted for the generation of THz radiation are discussed, quantum cascade lasers (QCLs), photoconductive emitters and photomixers and emphasis is given to photomixing. Photomixers generate continuous wave THz radiation by beating two independent laser beams on a semiconductor material. Beat frequency between the laser beams determines the emission frequency. In this work, two different materials, iron (Fe)– doped indium gallium arsenide (Fe:InGaAs) and Fe–doped indium gallium arsenide phosphide (Fe:InGaAsP) is used for THz photomixing at telecommunications wavelength. Characterizing the materials gave an idea about its intrinsic properties. With a standard antenna design, exemplar performance in terms of bandwidth (>2.4 THz) and output power was obtained from these materials. In order to improve the THz power from photomixers, two different electrode designs with nanometre dimensions were attempted on Fe:InGaAsP wafer. The spectral bandwidth and power from the emitters were studied at different bias orientations and polarizations. Mapping the emitters gave an insight into the geometrical dependence of the emission mechanism. The design was tested in a THz time domain system to confirm the results. Using photomixers, a 2.0 THz QCL was injection locked to a heterodyne source. The emission frequency of the QCL was locked over ~20 MHz. QCL voltage modulation was monitored for different emitter modulation frequencies. Locking experiment was performed at different injected signal strengths and QCL biases. QCL emission frequency was monitored at the injection locked frequency and Fabry-Perot modes

Terahertz quantum cascade laser application in local oscillator development

Terahertz (THz) quantum cascade lasers (QCLs) are high-quality THz sources in terms of power (>1W) and compact size, and the application of THz QCLs has been widely investigated. Spectroscopy is one of the most widespread application for THz QCLs. The project presented in this thesis is an application of THz quantum cascade laser. The project is focused on development of a local oscillator (LO system) aimed to be used in Earth observation. A single mode THz QCL which produces continuous-wave signals is designed, fabricated and characterized. In order to improve the performance of the LO (THz QCL), it is integrated within a waveguide block, which is more mechanically robust than a normal packed QCL and a significant improvement in beam profile was obtained by the integration with little change in the electrical and thermal performance. Next, the detector (Schottky diode detector) use in the application was investigated in terms of heterodyne detection and detection calibration. The heterodyne signal from the Schottky diode detector was used to study a Fabry–Perot (FP) QCL, whose neighbouring FP modes are coupled into the Schottky diode detector. The investigation gives a QCL emission linewidth and thermal equilibrium speed of the THz QCL. QCLs are used to calibrate a new Schottky diode detector designed by Rutherford Appleton Laboratory (RAL), which gives a 3.67 THz room temperature cut-off frequency. Lastly, spectroscopy with the THz QCL is carried out. A single mode CW lasing QCL and a photomixer are used in this application. The system gives a clear demonstration of methanol spectroscopy. Different partial pressure and absolute pressure of methanol are investigated. The measured result is backed by the simulation result from a JPL database. The measured result also proves the possibility of obtaining absorption linewidth broadening by increase methanol pressure. The noise level of the system is also investigated, which gives a detection limitation of 6.0×〖10〗^18 molecules in the 73 cm long gas cell that can be detected for the system

A Novel THz Photoconductive Source and Waveguide Based on One-dimensional Nano-grating

A terahertz photoconductive source structure with nano-grating electrodes is proposed. The resonance modes of the one-dimensional nano-grating and their affect the optical power absorption are studied. In addition, an approach for optimal design of the grating to maximize the photocurrent for different proposed DC biases, is presented. The dependence of the photocurrent on physical parameters of photomixer are analyzed. A fast analysis method for a new terahertz waveguide for photo-mixing is proposed. The wave-guiding mixer structure is a modified parallel plate waveguide (PPWG) in which the top plate is replaced by a periodic array of sub-wavelength nano-slits. The substrate of the PPWG is made of a fast photoconductive material in which laser photomixing/absorption occurs. The characteristic equation of the modified PPWG when used as a THz waveguide is derived analytically, and its guided modes are studied in details over THz range of frequencies. The accuracy of the analytical results are verified by comparison with full-wave numerical simulations. The criteria for choosing the suitable mode for photomixing application are also discussed. Finally, based on dyadic Green’s function representation, a systematic approach is provided for calculating the amplitude of the guided modes that are excited by an arbitrary photocurrent

Cellular effects of terahertz waves

Significance: An increasing interest in the area of biological effects at exposure of tissues and cells to the terahertz (THz) radiation is driven by a rapid progress in THz biophotonics, observed during the past decades. Despite the attractiveness of THz technology for medical diagnosis and therapy, there is still quite limited knowledge about safe limits of THz exposure. Different modes of THz exposure of tissues and cells, including continuous-wave versus pulsed radiation, various powers, and number and duration of exposure cycles, ought to be systematically studied. Aim: We provide an overview of recent research results in the area of biological effects at exposure of tissues and cells to THz waves. Approach: We start with a brief overview of general features of the THz-wave–tissue interactions, as well as modern THz emitters, with an emphasis on those that are reliable for studying the biological effects of THz waves. Then, we consider three levels of biological system organization, at which the exposure effects are considered: (i) solutions of biological molecules;(ii) cultures of cells, individual cells, and cell structures; and (iii) entire organs or organisms; special attention is devoted to the cellular level. We distinguish thermal and nonthermal mechanisms of THz-wave–cell interactions and discuss a problem of adequate estimation of the THz biological effects’ specificity. The problem of experimental data reproducibility, caused by rareness of the THz experimental setups and an absence of unitary protocols, is also considered. Results: The summarized data demonstrate the current stage of the research activity and knowledge about the THz exposure on living objects. Conclusions: This review helps the biomedical optics community to summarize up-to-date knowledge in the area of cell exposure to THz radiation, and paves the ways for the development of THz safety standards and THz therapeutic applications

New photonic architectures and devices for generation and detection of sub-THz and THz waves

The development of high-quality and reliable devices in the THz frequency region to fill the existing technological gap has become a major concern. This is chiefly motivated by the need of a widespread exploitation of the extensive variety of identified applications in this frequency region by a wide range of users, including the non-scientific community. The photonic approaches used for these purposes offer important and exclusive advantages over other existing alternatives, which have as a main representative the all-electronic technology, especially in terms of frequency range coverage, possibility of photonic distribution using optical fibers, weight and Electromagnetic Interference (EMI) immunity. Nevertheless, the optical techniques have traditionally provided with worse performance in terms of phase noise, tunability and dynamic range (in generation), and conversion ratio (in detection) when compared to state-of-theart all-electronic THz technology. The work accomplished in this thesis focuses on the design, development and validation of new photonic architectures and devices for both generation and detection of sub-THz and THz waves which overcome the drawbacks of optical techniques at this frequency region while maintaining all their advantages. In this thesis, several photonic sub-THz and THz generation systems have been developed using Difference Frequency Generation (DFG) architectures in which the DFG source is provided by an Optical Frequency Comb Generator (OFCG) and optical mode selection. Different devices and techniques are investigated for each part of the system before arriving to the final high performance synthesizer. Passively Mode-Locked Laser Diodes (PMMLDs) are firstly evaluated as integrated OFCG. An improved design of the OFCG is achieved with a scheme based on a Discrete Mode (DM) laser under Gain- Switching (GS) regime and optical span expansion by the use of a single Electro- Optical (EO) phase modulator. As optical mode selection, both high selective optical filtering and Optical Injection Locking (OIL) are used and evaluated. A commercial 50 GHz photodiode (PD) and an n-i-pn-i-p superlattice THz photomixer are employed as photodetector for Optical to THz conversion. The final reported system consists on an OFCG based on GS, OIL as mode selection strategy and an n-i-pn-i-p superlattice photomixer. This synthesizer offers a wide frequency range (60-140 GHz), readily scalable to a range between 10 GHz and values well above 1 THz. Quasi-continuous tunability is offered in the whole frequency range, with a frequency resolution of 0.1 Hz at 100 GHz that can be straightforwardly improved to 0.01 Hz at 100 GHz and 0.1 Hz at 1 THz. The measured FWHM at 120 GHz is <10 Hz, only limited by the measurement instrumentation. The system offers excellent frequency and power stability with frequency and power deviations over 1 hour of 5 Hz and 1.5 dB, respectively. These values are also limited by both the accuracy and uncertainty of the measurement setup. The performance achieved by this photonic sub-THz and THz synthesizer for most figures of merit matches or even surpasses those of commercial stateof- the-art all-electronic systems, and overcomes some of their characteristics in more than one million times when compared to commercial state-of-the-art photonic solutions. The detection part of this thesis explores the use of photonic architectures based on EO heterodyne receivers and the key devices that encompass these architectures: photonic Local Oscillators (LOs) and EO mixers. First results are developed at microwave frequencies (<15 GHz) using an Ultra-Nonlinear Semiconductor Amplifier (XN-SOA) as EO mixer and a GS based photonic LO. It is demonstrated how this LO device based on GS provides with a significant improvement in the performance of the overall EO receiver when compared to a traditional linearly modulated LO. Furthermore, this detection architecture is validated in an actual application (photonic imaging array), featuring scalability, flexibility and reasonable conversion ratios. After this, an EO heterodyne receiver is demonstrated up to frequencies of 110 GHz. The photonic LO employed is the abovementioned photonic sub- THz synthesizer developed in this thesis, while the EO mixer is an np-i-pn quasi ballistic THz detector. The first fabricated sample of this novel device is used, which is optimized for homodyne/heterodyne detection. The resulting sub-THz EO heterodyne receiver has conversion ratios around -75 dB. It works under zero-bias conditions, which together with the photonic distribution of the LO offers a high potential for remote detection of sub-THz and THz waves. In summary, new photonic architectures and devices are able to provide with state-of-the-art performance for generation of sub-THz and THz waves. In the case of EO heterodyne detection at sub-THz and THz frequency regions, photonic techniques are improving their performance and are closer to offer an alternative to all-electronic detectors. ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------El desarrollo de dispositivos fiables y de alta calidad en el rango frecuencial de Terahercios (THz) con el fin de cubrir el actual vacío tecnológico se ha convertido en una importante inquietud científica. Esto está principalmente motivado por la necesidad de explotar el gran número de aplicaciones identificadas en esta región frecuencial por un gran número de usuarios, incluyendo a usuarios no científicos. El enfoque fotónico empleado para estos propósitos ofrece importantes y exclusivas ventajas sobre otras alternativas existentes, que tienen como principal representante a la tecnología electrónica, especialmente en términos de rango de frecuencia de funcionamiento, posibilidad de distribución fotónica con fibras ópticas, peso, e inmunidad electromagnética. No obstante, las técnicas fotónicas tradicionalmente han ofrecido peores prestaciones en términos de ruido de fase, sintonía y rango dinámico (en generación) y ratio de conversión (en detección) con respecto a la tecnología electrónica de THz en el estado del arte. El trabajo realizado en esta tesis se centra en el diseño, desarrollo y validación de nuevas arquitecturas y componentes fotónicos tanto para generación como detección de ondas de sub-THz y THz que permitan solucionar las desventajas de las técnicas ópticas manteniendo todas sus ventajas. En esta tesis, varios sistemas de generación de sub-THz y THz han sido desarrollados utilizando arquitecturas Difference Frequency Generation (DFG) en las que la fuente DFG es proveída por un Optical Frequency Comb Generator (OFCG) y selección de modos ópticos. Diferentes dispositivos y técnicas son investigados para cada parte del sistema hasta conseguir un sintetizador de altas prestaciones. Passively Mode-Locked Laser Diodes (PMMLDs) son inicialmente evaluados como OFCG integrados. Un diseño mejorado del OFCG es conseguido mediante el uso de un esquema basado en un láser Discrete Mode (DM) bajo régimen Gain Switching (GS) y expansión del ancho de banda óptico mediante el uso de un modulador de fase Electro-Óptico (EO). Como estrategia de selección de modos ópticos, tanto filtrado óptico altamente selectivo como Optical Injection Locking (OIL) son usados y evaluados. Un fotodiodo comercial de ancho de banda 50 GHz y un fotomezclador de THz de superred n-i-pn-i-p son empleados. El sistema de generación final que se presenta en esta tesis consiste en un OFCG basado en GS, OIL como técnica de selección de modos ópticos y un fotomezclador de THz de superred n-i-pn-i-p. Este sintetizador ofrece un rango de funcionamiento de 60 a 140 GHz, directamente escalable a un rango entre 10 GHz y valores más allá de un THz. Sintonía cuasi-continua es ofrecida en todo el rango de frecuencia de operación, con una resolución en frecuencia de 0.1 Hz a 100 GHz que puede ser directamente escalable a 0.01 Hz a 100 GHz y 0.1 Hz a 1 THz. El ancho de línea a 3-dB de la señal a 120 GHz es menor de 10 Hz, solo limitada por la instrumentación de medida. El sistema ofrece una excelente estabilidad en potencia y frecuencia, con desviaciones sobre una hora de operación de 1.5 dB y 5 Hz, respectivamente. Estos valores también están limitados por la precisión e incertidumbre de la instrumentación de medida. Las prestaciones conseguidas por este sintetizador fotónico de sub-THz y THz para la mayoría de figuras de mérito, igualan o superan aquellas de las mejores soluciones comerciales electrónicas en el estado del arte, y supera algunas de estas características en más de un millón de veces en el caso de soluciones fotónicas comerciales en el estado del arte. La parte de detección de esta tesis explora el uso de arquitecturas fotónicas basadas en receptores EO heterodinos y los componentes clave que forman estas arquitecturas: Oscilador Local (OL) fotónico y mezcladores EO. Los primeros resultados son desarrollados en el entorno de microondas (<15 GHz) usando un amplificador de semiconductor óptico ultra no lineal (XN-SOA) como mezclador EO y un OL fotónico basado en GS. Se demuestra como este OL basado en GS ofrece una mejora significativa de las prestaciones del receptor con respecto al uso de OL fotónicos tradicionales basados en modulación lineal. Además, esta arquitectura de detección es validada en una aplicación real (imaging array fotónico), ofreciendo escalabilidad, flexibilidad y ratios de conversión razonables. Tras esto, un receptor EO heterodino es demostrado hasta frecuencias de 110 GHz. El OL fotónico empleado es el sintetizador de altas prestaciones presentado en esta tesis, mientras que el mezclador EO es un nuevo detector de THz: el np-i-pn cuasi-balístico. La primera muestra fabricada de estos nuevos dispositivos, especialmente diseñados y optimizados para detección homodina y heterodina, es empleada. El receptor EO heterodino resultante ofrece ratios de conversión de -75 dB. Este dispositivo es capaz de trabajar sin alimentación, lo que unido a la distribución fotónica del OL, ofrece un gran potencial para detección remota de ondas de sub-THz y THz. En resumen, las nuevas arquitecturas y dispositivos fotónicos presentados en esta tesis son capaces de ofrecer prestaciones en el estado del arte para generación de ondas de sub-THz y THz. En el caso de detectores EO heterodinos en frecuencias de sub-THz y THz, las técnicas fotónicas están mejorando sus prestaciones significativamente y están cada vez más cerca de ofrecer una alternativa a detectores electrónicos en el estado del arte

Advanced Graphene Microelectronic Devices

The outstanding electrical and material properties of Graphene have made it a promising material for several fields of analog applications, though its zero bandgap precludes its application in digital and logic devices. With its remarkably high electron mobility at room temperature, Graphene also has strong potential for terahertz (THz) plasmonic devices. However there still are challenges to be solved to realize Graphene’s full potential for practical applications. In this dissertation, we investigate solutions for some of these challenges. First, to reduce the access resistances which significantly reduces the radio frequency (RF) performance of Graphene field effect transistors (GFETs), a novel device structure consisting of two additional contacts at the access region has been successfully modeled, designed, microfabicated/integrated, and characterized. The additional contacts of the proposed device are capacitively coupled to the device channel and independently biased, that induce more carriers and effectively reduce access resistance. In addition to that, in this dissertation, bandgap has been experimentally introduced to semi-metallic Graphene, by decorating with randomly distributed gold nano-particles and zinc oxide (ZnO) nano-seeds, where their interaction breaks its sublattice symmetry and opens up bandgap. The engineered bandgap was extracted from its temperature dependent conductivity characteristics and compared with reported theoretical estimation. The proposed method of device engineering combined with material bandgap engineering, on a single device, introduces a gateway towards high speed Graphene logic devices. Finally, THz plasmon generation and propagation in Graphene grating gate field effect transistors and Graphene plasmonic ring resonators have been investigated analytically and numerically to explore their potential use for compact, solid state tunable THz detectors