Imaging systems operating in the terahertz part of the electromagnetic spectrum are in great demand because of the distinct characteristics of terahertz waves in penetrating many optically-opaque materials and providing unique spectral signatures of various chemicals. However, the use of terahertz imagers in real-world applications has been limited by the slow speed, large size, high cost, and complexity of the existing imaging systems. These limitations are mainly imposed due to the lack of terahertz focal-plane arrays (THz-FPAs) that can directly provide the frequency-resolved and/or time-resolved spatial information of the imaged objects. During my doctoral studies, I designed, fabricated, and characterized the first THz-FPA that can directly provide the spatial amplitude and phase distributions, along with the ultrafast temporal and spectral information of an imaged object. It consists of a two-dimensional array of ~0.3 million plasmonic photoconductive nanoantennas optimized to rapidly detect broadband terahertz radiation with a high SNR. Terahertz time-domain imaging on amplitude and phase objects is demonstrated in both terahertz lens-free and lens-based imaging systems. By eliminating the need for raster scanning and spatial terahertz modulation, our THz-FPA offers more than a 1000-fold increase in imaging speed compared to the state-of-the-art. The high imaging speed enables the first terahertz video taken in a terahertz time-domain imaging system with a frame rate of 16 fps. The unprecedented rich information in the terahertz time-domain imaging data can facilitate many new applications. As the first proof-of-concept, we utilized the multispectral nature of the amplitude and phase data captured by the plasmonic photoconductive THz-FPA to realize pixel super-resolution imaging of objects. We successfully imaged and super-resolved etched patterns in a silicon substrate and reconstructed both the shape and depth of these structures with an effective number of pixels that exceeds 1 kilo pixels. We also super-resolved the terahertz video at 16 fps of a water flow in three adjacent plastic pipes using a holographic reconstruction algorithm. Beyond these proof-of-concept super-resolution demonstrations, the unique capabilities enabled by our plasmonic photoconductive THz-FPA offer transformative advances in a broad range of applications that use hyperspectral and three-dimensional terahertz images of objects for e.g., industrial inspection, security screening, and medical diagnosis, among others.

Terahertz optoelectronics have shown significant promise in the development and enhancement of technologies in chemical identification, biological sensing, and medical imaging. The practical advancement of such devices, however, has been hindered by the characteristics of the frequency range, 0.3 to 3 THz in the electromagnetic radiation spectrum. Presented here is a demonstration of the significant advancements possible in creating efficient and ultra-fast plasmonic THz sources by incorporating an optical cavity using a distributed Bragg reflector (DBR). Plasmonic electrodes enable increased transmission of photons into a GaAs photo-absorbing substrate, and the DBR enhances the quantum efficiency of the device by creating an optical cavity which allows for nearly 100\% of the incoming light getting absorbed in the active GaAs layer.

Photoconductive antennas are extensively used for the generation of terahertz radiation. They generally consist of an antenna fabricated on a photo-absorbing semiconductor substrate. When illuminated by an optical pump beam with terahertz frequency components, photocarriers are generated in the substrate. The photogenerated carriers drift to the antenna under an external bias voltage to induce a terahertz photocurrent that drives the antenna to generate terahertz radiation.

One of the major limitations of existing photoconductive antennas, especially those operating at telecommunication optical wavelengths (~1550 nm), is their large dark current level, which degrades device reliability due to excessive Joule heating. To address this limitation, a new class of bias-free terahertz emitters was recently introduced, which utilizes the naturally induced built-in electric field at the surface of semiconductors to drift the photo-generated carriers. By eliminating the bias voltage and dark current, a highly reliable optical-to-terahertz conversion is achieved.

The first generation of these bias-free terahertz emitters utilized an array of plasmonic nanoantennas in the form of gratings fabricated on epitaxially-grown undoped and p+ doped InAs layers on a semi-insulating GaAs substrate. Using this bias-free terahertz emitter, record-high optical-to-terahertz conversion efficiencies were achieved compared to other bias-free terahertz emitters based on nonlinear optical process, photo-Dember effect, and spintronics. Despite its superior efficiency compared to the state-of-the art, the optical-to-terahertz conversion efficiency of this emitter is limited by leakage of the terahertz current from the terahertz radiating elements to the highly doped substrate and destructive interference of the photocurrent components injected to the terahertz radiating elements. This research is focused on the design and optimization of a new generation of plasmonic nanoantenna arrays to address these efficiency limitations and further enhance the optical-terahertz conversion efficiency of bias-free terahertz emitters.

Terahertz waves can be used for many potential applications including security screening, chemical sensing and medical diagnosis. However, the practical applications are limited by the size, cost and complexity of terahertz systems. This is due to the lack of monolithically integrated terahertz optoelectronic devices. During my doctoral studies, I have extensively explored the possibility of using quantum well structures as a platform for monolithic integration of terahertz optoelectronics. Based on a GaAs/AlGaAs quantum well structure operating at ~800 nm wavelength, high performance SOA-integrated terahertz photomixers are demonstrated and characterized as both terahertz transmitter and receiver. An in-depth theoretical model is given to predict the performance of the fabricated terahertz transmitter and receiver prototypes, accounting for the optical absorption, RC parasitics and ultrafast carrier dynamics. Terahertz transmitters with radiation power levels of –10, –20, –26, and -35 dBm are demonstrated at 140, 310, 400, and 500 GHz and show record-high optical-to-terahertz conversion efficiencies of 2% at 140 and 230 GHz. With the same device structure, terahertz receivers with noise equivalent power levels of 2.3, 8.1, 13.7, 15.1, and 93 pW/Hz0.5 are demonstrated at 140, 250, 310, 400 and 500 GHz, which are equivalent or better than the state-of-the-art room temperature terahertz receivers. The wafer structure can be modified to further improve the device performance. The presented scheme is compatible with photonic integrated system foundry processes, e.g., InP based processes, which could enable fabrication of compact, low-cost, scalable terahertz systems with high volumes.

Surface states generally degrade semiconductor device performance by raising the charge injection barrier height, introducing localized trap states, inducing surface leakage current, and altering the electric potential. Therefore, there has been an endless effort to use various surface passivation treatments to suppress the undesirable impacts of the surface states. During my doctoral studies, I have investigated the unique electrochemical characteristics of semiconductor surface states and showed that the giant built-in electric field created by the surface states can be harnessed to enable passive wavelength conversion with unprecedented efficiencies without utilizing any nonlinear optical phenomena. Photo-excited surface plasmons are coupled to the surface states to generate an electron gas, which is routed to a nanoantenna array through the giant electric field created by the surface states. The induced current on the nanoantennas, which contains mixing product of different optical frequency components, generates radiation at the beat frequencies of the incident photons. We utilize the unprecedented functionalities of plasmon-coupled surface states to demonstrate passive wavelength conversion of nano-joule optical pulses at a 1550 nm center wavelength to terahertz regime with record-high efficiencies that exceed nonlinear optical methods by 4-orders of magnitude. The presented scheme can be used for optical wavelength conversion to different parts of the electromagnetic spectrum ranging from microwave to far-infrared regimes by using appropriate optical beat frequencies.

Terahertz waves have great potentials for security screening, medical imaging, and chemical identification. The use of plasmonic contact electrodes in photoconductive terahertz emitters has been very effective in enhancing terahertz radiation power and signal-to-noise ratio levels of terahertz imaging and sensing systems. However, previously demonstrated plasmonic photoconductive terahertz emitters have all utilized grating-based plasmonic contact electrodes, which are sensitive to the polarization state of their optical pump beam. This polarization sensitivity can degrade the performance of plasmonic photoconductive terahertz emitters in practical application settings. In this thesis, we develop a polarization-insensitive plasmonic photoconductive terahertz emitter, which utilizes a periodic array of subwavelength cross-shaped apertures as the plasmonic contact electrodes. By using cross-shaped aperture arrays, surface plasmon waves can be excited near the metal contact-substrate interface, bringing more photo-generated carriers close to this interface, which reduces the carrier transport path length to the contact electrodes. The two-dimensional symmetry of the cross-shaped apertures leads to a polarization-insensitive interaction between the plasmonic contact electrodes and optical pump beam. The geometry of the cross-shaped aperture arrays is selected to achieve maximum optical pump absorption in the photo-absorbing substrate at the metal-contact interface. Prototypes of this plasmonic photoconductive emitter are fabricated. The fabricated emitter prototypes show an excellent polarization insensitivity to the optical pump. The terahertz radiation power and efficiency of this emitter are comparable to those of a grating-based plasmonic photoconductive emitter with the same terahertz radiating elements.

Time-domain terahertz imaging and spectroscopy systems have unique functionalities for chemical identification, material characterization, biological sensing, medical imaging, and security screening. Photoconductive sources and detectors are commonly used for the generation and detection of terahertz pulses in compact time-domain terahertz systems operating at room temperature. However, the low radiation power of conventional photoconductive sources and the low responsivity of conventional photoconductive detectors have limited the scope of the potential uses of these systems. To improve the performance of time-domain terahertz systems, photoconductive sources and detectors consisting of large-area plasmonic nanoantenna arrays are designed, fabricated and experimentally characterized.

To increase the radiation power of photoconductive sources, the plasmonic nanoantenna arrays are designed to enhance the absorption of optical pump photons near them. The enhanced optical absorption leads to stronger ultrafast photocurrent levels coupled to the nanoantenna arrays. The nanoantenna arrays are designed to serve as broadband terahertz radiating elements. Hence, high-power and broadband pulsed terahertz radiation is generated. Higher terahertz radiation powers and device efficiencies are achieved by a tighter optical pump confinement enabled by plasmonic nanocavities formed near the plasmonic nanoantenna arrays. Record-high terahertz radiation powers as high as 4 mW are achieved over a 0.1 – 5 THz frequency range.

To increase the responsivity of photoconductive detectors, the plasmonic nanoantenna arrays are designed to maximize the interaction between the optical pump photons and the incident terahertz radiation at the nanoscale. The impact of the nanoantenna geometry on terahertz detection responsivity and bandwidth is analyzed theoretically and experimentally. By optimizing the geometry of the nanoantenna arrays, broadband time-domain terahertz spectroscopy with a record-high dynamic range of 107 dB is demonstrated.

Photoconductive terahertz devices require ultrafast (< 1 ps) carrier dynamics to generate and detect terahertz radiation. Conventionally, they have been heavily reliant on the use of defect-introduced short-carrier-lifetime photoconductors. While quite a few excellent results have been achieved by using short-carrier-lifetime photoconductors, the high concentration of defects intentionally incorporated in the active photo-absorbing material leads to lower carrier mobility and substantial degradation of photoconductive gain due to photocarrier scattering, trapping and recombination. Additionally, realization of many short-carrier-lifetime photoconductors is only possible at limited facilities due to the need for non-standard processes, as well as rare elements as defect-introducing dopants. Therefore, alternative approaches to realize terahertz sources and detectors have attracted increasing attention. During my doctoral studies, I have established two different approaches to develop photoconductive terahertz sources and detectors without using short-carrier-lifetime photoconductors. These approaches are based on carrier transit time reduction and photoconductor band engineering to introduce ultrafast carrier dynamics. With the strong optical field enhancement offered by plasmonic nanostructures, photocarrier generation in plasmonic photoconductors can be highly confined near the terahertz radiating elements, leading to significantly reduced transport distances for the photocarriers that facilitates broadband terahertz operation. As a result, by utilizing the plasmonics-enabled carrier transit time reduction, high-efficiency terahertz sources and detectors are realized for both pulsed and continuous-wave operation. This scheme provides a generic and reliable approach for designing photoconductive terahertz devices using various semiconductors and optical wavelengths. Moreover, by engineering the photoconductor band structure, highly reliable bias-free terahertz sources are realized with multiple orders of magnitude improvement in optical-to-terahertz conversion efficiency compared to other passive terahertz generation techniques. In particular, a record-high-power pulsed terahertz source with a radiation power of 860 �W is demonstrated using a novel graded-composition InGaAs structure. Furthermore, to minimize terahertz propagation loss in the substrate, bias-free sources are successfully implemented on a silicon substrate, which not only leads to an increased radiation bandwidth, but also enables integrability with silicon photonic platforms, largely extending the scope and potential uses of terahertz sources to a diversity of practical applications.