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.

In this thesis we use three main approaches to enhance the performance of terahertz photoconductive switches (THz PC switches). We first propose two novel materials (GaBiAs and carbon nanotubes) for the substrate. The resulting enhancement in THz emission and reception are significant for GaBiAs. As thoroughly analyzed and addressed in Chapter 2, both the emission bandwidth and the emission amplitude of the device are improved by these materials. A systematic study of CNTs predicts 2 orders of magnitude enhancement in THz emission and one order of magnitude enhancement in THz reception. Experimental results for GaBiAs indicate 0.5 THz increase in bandwidth and 68% increase in the emitted THz wave amplitude. The bandwidth enhancement is in comparison to premium commercial devices. The optical excitation of the PC switch is studied and optimized next as the second enhancement approach (Chapter 3). The study presented in Chapter 3 provides an insight on the subwavelength dynamics of the optical excitation E-field at the edge of the electrodes. The study reveals that majority of the fast photocarriers are collected at the edge of the electrode in a subwavelength scale area. This insight leads to optimization of illumination profile and also the third enhancement approach, namely, the enhancement of electrode structure (Chapter 4). In Chapter 4 we have engineered the electrodes down to nanometer scale. This significantly enhances the optical excitation of the substrate and also overcomes the undesired properties of some substrate materials such as long carrier lifetime. Fabricated devices and fabrication processes are assessed in Chapter 5. Results (Chapter 6) highlight more than two orders of magnitude enhancement for nanostructures on GaAs. / Graduate / 0544

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.

Terahertz technology is a fast growing field with variety of applications in biology and medicine, medical imaging, material spectroscopy and sensing, monitoring and spectroscopy in pharmaceutical industry, security, and high-data-rate short-range communications. Among different terahertz sources, photomixers are potentially compact, low power consuming, coherent, low-cost, and tunable continuous-wave sources. A terahertz photomixer is a heterodyne scheme, in which two laser beams with their frequency difference falling in the terahertz range mix in a nonlinear medium, such as a photoconductor or a superconductor, and generate a signal, whose frequency is equal to the frequency difference of the two lasers. The frequency of the generated terahertz signal can be tuned by tuning the central frequency of one of the lasers. <br ><br /> In this dissertation, the photomixing in superconductors and photoconductors is studied, and comprehensive analytical models for the interaction of two interfering laser beams with these materials are developed. Integrated photomixer-antenna elements as efficient terahertz sources are introduced and arrays of these elements as high power terahertz sources are designed. Also, an array of photoconductive photomixer-antenna elements with integrated excitation scheme is proposed. <br ><br /> In a photo-excited superconductor, the fundamental equations for the motion of the carriers inside the superconductor material are used in connection with the two-temperature model to find an analytic expression for the generated terahertz photocurrent inside the film. In a photo-excited photoconductor, the continuity equations for the electron and hole densities are solved in their general form along with the appropriate boundary conditions to find photocurrent distribution inside the photoconductor film. It is shown that in a continuous-wave (CW) terahertz photomixing scheme, the resulting photocurrent contains a dc component and a terahertz traveling-wave component. The dependency of the amplitude and the phase of the generated photocurrent on the physical parameters of the photomixer, the parameters of the lasers, the applied dc bias, and the configuration of the device is explored in detail for a photoconductive photomixer made of low-temperature grown (LTG) GaAs and for a high-temperature superconductive photomixer made of YBa₂Cu₃O_7-&delta;. <br ><br /> The developed models for the photoconductive and the superconductive terahertz photomixers are used to design new integrated photomixer-antenna devices. In these devices, the photomixing film simultaneously acts as an efficient radiator at the terahertz frequencies. Integrating the photomixing medium with the antenna not only eliminates any source to antenna coupling problem, but also makes the proposed device attractive for array configurations. <br ><br /> To increase the generated terahertz power, arrays of the photoconductive and the superconductive photomixer-antenna elements are proposed as CW terahertz sources. It is shown that a sub-milliwatt terahertz power is achievable from a typical superconductive photomixer-antenna array structure. The beam steering capability of the proposed devices is also investigated. <br ><br /> A photoconductive photomixer-antenna array with integrated excitation scheme is proposed, in which the laser beams are guided inside the substrate and excite the photomixer elements. In this way the laser power is only being consumed by the photomixer elements, and the photomixer-antenna elements can be integrated with other optical components on a single substrate. The whole structure is robust and less sensitive to vibration and other environmental parameters.

Terahertz technology is a fast growing field with variety of applications in biology and medicine, medical imaging, material spectroscopy and sensing, monitoring and spectroscopy in pharmaceutical industry, security, and high-data-rate short-range communications. Among different terahertz sources, photomixers are potentially compact, low power consuming, coherent, low-cost, and tunable continuous-wave sources. A terahertz photomixer is a heterodyne scheme, in which two laser beams with their frequency difference falling in the terahertz range mix in a nonlinear medium, such as a photoconductor or a superconductor, and generate a signal, whose frequency is equal to the frequency difference of the two lasers. The frequency of the generated terahertz signal can be tuned by tuning the central frequency of one of the lasers. <br ><br /> In this dissertation, the photomixing in superconductors and photoconductors is studied, and comprehensive analytical models for the interaction of two interfering laser beams with these materials are developed. Integrated photomixer-antenna elements as efficient terahertz sources are introduced and arrays of these elements as high power terahertz sources are designed. Also, an array of photoconductive photomixer-antenna elements with integrated excitation scheme is proposed. <br ><br /> In a photo-excited superconductor, the fundamental equations for the motion of the carriers inside the superconductor material are used in connection with the two-temperature model to find an analytic expression for the generated terahertz photocurrent inside the film. In a photo-excited photoconductor, the continuity equations for the electron and hole densities are solved in their general form along with the appropriate boundary conditions to find photocurrent distribution inside the photoconductor film. It is shown that in a continuous-wave (CW) terahertz photomixing scheme, the resulting photocurrent contains a dc component and a terahertz traveling-wave component. The dependency of the amplitude and the phase of the generated photocurrent on the physical parameters of the photomixer, the parameters of the lasers, the applied dc bias, and the configuration of the device is explored in detail for a photoconductive photomixer made of low-temperature grown (LTG) GaAs and for a high-temperature superconductive photomixer made of YBa₂Cu₃O_7-&delta;. <br ><br /> The developed models for the photoconductive and the superconductive terahertz photomixers are used to design new integrated photomixer-antenna devices. In these devices, the photomixing film simultaneously acts as an efficient radiator at the terahertz frequencies. Integrating the photomixing medium with the antenna not only eliminates any source to antenna coupling problem, but also makes the proposed device attractive for array configurations. <br ><br /> To increase the generated terahertz power, arrays of the photoconductive and the superconductive photomixer-antenna elements are proposed as CW terahertz sources. It is shown that a sub-milliwatt terahertz power is achievable from a typical superconductive photomixer-antenna array structure. The beam steering capability of the proposed devices is also investigated. <br ><br /> A photoconductive photomixer-antenna array with integrated excitation scheme is proposed, in which the laser beams are guided inside the substrate and excite the photomixer elements. In this way the laser power is only being consumed by the photomixer elements, and the photomixer-antenna elements can be integrated with other optical components on a single substrate. The whole structure is robust and less sensitive to vibration and other environmental parameters.

Recent advances in molecular biology and nanotechnology have enabled scientists to study biological systems at molecular and atomic scales. This level of sophistication demands for new technologies to emerge for providing the necessary sensing tools and equipment. Recent studies have shown that terahertz technology can provide revolutionary sensing techniques for organic and non-organic materials with unprecedented accuracy and sensitivity. This is due to the fact that most of the macromolecules have vibrational and/or rotational resonance signatures in terahertz range. To further increase the sensitivity, terahertz radiation is generated and interacted with the bio-sample on a miniaturized test site or the so-called biochip. From the view point of generation and manipulation of terahertz radiation, the biochip is designed based on the same rules as in high frequency electronic chips or integrated circuits (IC). By increasing the frequency toward terahertz range, the conventional IC design methodologies and analysis tools fail to perform accurately. Therefore, development of new design methodologies and analysis tools is of paramount importance for future terahertz integrated circuits (TIC) in general and terahertz biochips in particular. In this thesis, several advancements are made in design methodology, analysis tool and architecture of terahertz and millimeter-wave integrated circuits when used as a biochip. A global and geometry independent approach for design and analysis of the travelling-wave terahertz photomixer sources, as the core component in a TIC, is discussed in details. Three solvers based on photonic, semiconductor and electromagnetic theories are developed and combined as a unified analysis tool. Using the developed terahertz photomixer source, a resonance-based biochip structure is proposed, and its operation principle, based on resonance perturbation method, is explained. A planar metallic resonator acting as a sample holder and transducer is designed, and its performance in terms of sensitivity and selectivity is studied through simulations. The concept of surface impedance for electromagnetic modeling of DNA self-assembled monolayer on a metal surface is proposed, and its effectiveness is discussed based on the available data in the literature. To overcome the loss challenge, Whispering Gallery Mode (WGM) dielectric resonators with high Q factor are studied as an alternative for metallic resonator. The metallic loss becomes very high at terahertz frequencies, and as a result of that planar metallic resonators do not exhibit high Q factor. Reduced Q factor results in a low sensitivity for any sensor using such resonators. Theoretical models for axially and radially layered dielectric resonators acting on WGM are presented, and the analytical results are compared with the measured data. Excitation of WGM through dielectric waveguide is proposed, and the critical coupling condition is explained through analytical formulation. The possibility of selecting one resonance among many for sensing application is also studied both theoretically and experimentally. A high sensitivity sensor based on WGM resonance in mm-wave and terahertz is proposed, and its sensitivity is studied in details. The performance of the proposed sensor is tested for sensing drug tablets and also liquid droplets through various measurements in mm-wave range. The comprehensive sensitivity analysis shows the ability of the proposed sensor to detect small changes in the order of 10−4 in the sample dielectric constant. The results of various experiments carried out on drug tablets are reported to demonstrate the potential multifunctional capabilities of the sensor in moisture sensing, counterfeit drug detection, and contamination screening. The measurement and simulation results obtained in mm-wave hold promise for WGM to be used for sensing biological solutions in terahertz range with very high sensitivity.

Recent advances in molecular biology and nanotechnology have enabled scientists to study biological systems at molecular and atomic scales. This level of sophistication demands for new technologies to emerge for providing the necessary sensing tools and equipment. Recent studies have shown that terahertz technology can provide revolutionary sensing techniques for organic and non-organic materials with unprecedented accuracy and sensitivity. This is due to the fact that most of the macromolecules have vibrational and/or rotational resonance signatures in terahertz range. To further increase the sensitivity, terahertz radiation is generated and interacted with the bio-sample on a miniaturized test site or the so-called biochip. From the view point of generation and manipulation of terahertz radiation, the biochip is designed based on the same rules as in high frequency electronic chips or integrated circuits (IC). By increasing the frequency toward terahertz range, the conventional IC design methodologies and analysis tools fail to perform accurately. Therefore, development of new design methodologies and analysis tools is of paramount importance for future terahertz integrated circuits (TIC) in general and terahertz biochips in particular. In this thesis, several advancements are made in design methodology, analysis tool and architecture of terahertz and millimeter-wave integrated circuits when used as a biochip. A global and geometry independent approach for design and analysis of the travelling-wave terahertz photomixer sources, as the core component in a TIC, is discussed in details. Three solvers based on photonic, semiconductor and electromagnetic theories are developed and combined as a unified analysis tool. Using the developed terahertz photomixer source, a resonance-based biochip structure is proposed, and its operation principle, based on resonance perturbation method, is explained. A planar metallic resonator acting as a sample holder and transducer is designed, and its performance in terms of sensitivity and selectivity is studied through simulations. The concept of surface impedance for electromagnetic modeling of DNA self-assembled monolayer on a metal surface is proposed, and its effectiveness is discussed based on the available data in the literature. To overcome the loss challenge, Whispering Gallery Mode (WGM) dielectric resonators with high Q factor are studied as an alternative for metallic resonator. The metallic loss becomes very high at terahertz frequencies, and as a result of that planar metallic resonators do not exhibit high Q factor. Reduced Q factor results in a low sensitivity for any sensor using such resonators. Theoretical models for axially and radially layered dielectric resonators acting on WGM are presented, and the analytical results are compared with the measured data. Excitation of WGM through dielectric waveguide is proposed, and the critical coupling condition is explained through analytical formulation. The possibility of selecting one resonance among many for sensing application is also studied both theoretically and experimentally. A high sensitivity sensor based on WGM resonance in mm-wave and terahertz is proposed, and its sensitivity is studied in details. The performance of the proposed sensor is tested for sensing drug tablets and also liquid droplets through various measurements in mm-wave range. The comprehensive sensitivity analysis shows the ability of the proposed sensor to detect small changes in the order of 10−4 in the sample dielectric constant. The results of various experiments carried out on drug tablets are reported to demonstrate the potential multifunctional capabilities of the sensor in moisture sensing, counterfeit drug detection, and contamination screening. The measurement and simulation results obtained in mm-wave hold promise for WGM to be used for sensing biological solutions in terahertz range with very high sensitivity.