Skip to main content
eScholarship
Open Access Publications from the University of California

Scholarly Works (6 results)

  • Thesis
  • Peer Reviewed
UCLA Electronic Theses and Dissertations (2022)

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.

    • Thesis
    • Peer Reviewed
    UCLA Electronic Theses and Dissertations (2018)

    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.

      Cover page: Polarization-Insensitive Plasmonic Photoconductive Terahertz Emitters
      • Article
      • Peer Reviewed
      UCLA Previously Published Works (2023)
      Many exciting terahertz imaging applications, such as non-destructive evaluation, biomedical diagnosis, and security screening, have been historically limited in practical usage due to the raster-scanning requirement of imaging systems, which impose very low imaging speeds. However, recent advancements in terahertz imaging systems have greatly increased the imaging throughput and brought the promising potential of terahertz radiation from research laboratories closer to real-world applications. Here, we review the development of terahertz imaging technologies from both hardware and computational imaging perspectives. We introduce and compare different types of hardware enabling frequency-domain and time-domain imaging using various thermal, photon, and field image sensor arrays. We discuss how different imaging hardware and computational imaging algorithms provide opportunities for capturing time-of-flight, spectroscopic, phase, and intensity image data at high throughputs. Furthermore, the new prospects and challenges for the development of future high-throughput terahertz imaging systems are briefly introduced.
        Cover page: High-throughput terahertz imaging: progress and challenges.
        • Article
        • Peer Reviewed
        UCLA Previously Published Works (2021)
        We demonstrate optical networks composed of diffractive layers trained using deep learning to encode the spatial information of objects into the power spectrum of the diffracted light, which are used to classify objects with a single-pixel spectroscopic detector. Using a plasmonic nanoantenna-based detector, we experimentally validated this single-pixel machine vision framework at terahertz spectrum to optically classify the images of handwritten digits by detecting the spectral power of the diffracted light at ten distinct wavelengths, each representing one class/digit. We also coupled this diffractive network-based spectral encoding with a shallow electronic neural network, which was trained to rapidly reconstruct the images of handwritten digits based on solely the spectral power detected at these ten distinct wavelengths, demonstrating task-specific image decompression. This single-pixel machine vision framework can also be extended to other spectral-domain measurement systems to enable new 3D imaging and sensing modalities integrated with diffractive network-based spectral encoding of information.
          Cover page: Spectrally encoded single-pixel machine vision using diffractive networks
          • Article
          • Peer Reviewed
          UCLA Previously Published Works (2023)
          Terahertz waves offer advantages for nondestructive detection of hidden objects/defects in materials, as they can penetrate most optically-opaque materials. However, existing terahertz inspection systems face throughput and accuracy restrictions due to their limited imaging speed and resolution. Furthermore, machine-vision-based systems using large-pixel-count imaging encounter bottlenecks due to their data storage, transmission and processing requirements. Here, we report a diffractive sensor that rapidly detects hidden defects/objects within a 3D sample using a single-pixel terahertz detector, eliminating sample scanning or image formation/processing. Leveraging deep-learning-optimized diffractive layers, this diffractive sensor can all-optically probe the 3D structural information of samples by outputting a spectrum, directly indicating the presence/absence of hidden structures or defects. We experimentally validated this framework using a single-pixel terahertz time-domain spectroscopy set-up and 3D-printed diffractive layers, successfully detecting unknown hidden defects inside silicon samples. This technique is valuable for applications including security screening, biomedical sensing and industrial quality control.
            Cover page: Rapid sensing of hidden objects and defects using a single-pixel diffractive terahertz sensor.
            • Article
            • Peer Reviewed
            UCLA Previously Published Works (2024)
            Nonlinear optical processing of ambient natural light is highly desired for computational imaging and sensing. Strong optical nonlinear response under weak broadband incoherent light is essential for this purpose. By merging 2D transparent phototransistors (TPTs) with liquid crystal (LC) modulators, we create an optoelectronic neuron array that allows self-amplitude modulation of spatially incoherent light, achieving a large nonlinear contrast over a broad spectrum at orders-of-magnitude lower intensity than achievable in most optical nonlinear materials. We fabricated a 10,000-pixel array of optoelectronic neurons, and experimentally demonstrated an intelligent imaging system that instantly attenuates intense glares while retaining the weaker-intensity objects captured by a cellphone camera. This intelligent glare-reduction is important for various imaging applications, including autonomous driving, machine vision, and security cameras. The rapid nonlinear processing of incoherent broadband light might also find applications in optical computing, where nonlinear activation functions for ambient light conditions are highly sought.
              Cover page: Broadband nonlinear modulation of incoherent light using a transparent optoelectronic neuron array.
              Top