Utilizing plasmonic nanoantennas capable of concentrating light in the infrared spectrum and serving as terahertz antennas, we have engineered a terahertz focal-plane array (THz-FPA) suitable for integration into terahertz pulsed imaging systems. Comprising 64 pixels, this detector array enables scanning of a 5 cm line width. With its rapid scanning capability and expansive field-of-view, the THz-FPA has the potential to elevate terahertz pulsed imaging systems beyond mere metrology tools, rendering them high-throughput instruments applicable in industrial environments for diverse non-destructive evaluation purposes.
We developed an in-line quality control scanner that can be used to detect defects in lithium-ion battery electrodes during roll-to-roll manufacturing. Lookin’s scanners employ terahertz radiation which is non-ionizing and non-destructive. The patented hardware used in Lookin’s scanners provide scan speeds that allow real-time detection of defects during manufacturing, for the first time. With early detection of these defects, Lookin’s scanners enable manufacturing of lithium-ion batteries with better shelf life, increased safety, lower cost, and decreased production lead-time.
By using plasmonic nanoantennas that can act as light concentrators in infrared band and terahertz antennas in terahertz band, we developed a terahertz focal-plane array (THz-FPA) that can be used in terahertz pulsed imaging systems. The detector array consists of 64 pixels and can scan a line width of 5 cm. By offering high scan speed with large field-of-view, the THz-FPA can transform terahertz pulsed imaging systems from a metrology tool to a high-throughput instrument that can be used in industrial settings for various non-destructive evaluation applications.
We present a diffractive terahertz sensor using a single-pixel detector to rapidly sense hidden defects within a target sample volume. Leveraging multiple spatially-engineered diffractive layers optimized via deep learning, this diffractive sensor can all-optically process the sample scattered waves and generate an output spectrum encoding information for indicating the presence/absence of hidden 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 within silicon samples. By circumventing raster scanning and digital image formation/reconstruction, this framework holds vast potential for various applications requiring high-throughput, non-destructive defect detection.
Terahertz focal-plane arrays (THz-FPAs) can revolutionize the terahertz technology market by addressing all critical needs of practical terahertz pulsed imaging (TPI) systems. Such THz-FPAs would transform TPI systems from a metrology tool with a slow scan speed and limited field-of-view to a high-throughput instrument that can be used in industrial settings for various quality control applications. For this purpose, we developed a 64-pixel one-dimensional THz-FPA that can scan a line width of 5 cm to use it in scenarios where moving, continuous material needs to be inspected. The plasmonic nanoantenna technology realizing THz-FPAs helped achieving broadband and high-sensitivity operation.
Terahertz focal-plane arrays (THz-FPAs) can revolutionize the terahertz technology market by addressing all critical needs of practical terahertz pulsed imaging (TPI) systems. Such THz-FPAs would transform TPI systems from a metrology tool with a slow scan speed and limited field-of-view to a high-throughput instrument that can be used in industrial settings for various quality control applications. We present a high-throughput one-dimensional THz-FPA that can be used for non-destructive quality control applications in field settings.
We report a single-pixel machine vision framework based on deep learning-designed diffractive surfaces to perform a desired machine learning task. The object within the input field-of-view is illuminated with a broadband light source and the subsequent diffractive surfaces are trained to encode the spatial information of the object features onto the power spectrum of the diffracted light that is collected by a single-pixel detector in a single-shot. We experimentally demonstrated the all-optical inference capabilities of this single-pixel machine vision platform by classifying handwritten digits using 3D-printed diffractive layers and a plasmonic nanoantenna-based time-domain spectroscopy setup operating at THz wavelengths.
We utilize diffractive optical networks to design small footprint, passive pulse engineering platforms, where an input terahertz pulse is shaped into a desired output waveform as it diffracts through spatially-engineered transmissive surfaces. Using 3D-printed diffractive networks designed by deep learning, various terahertz pulses with different temporal widths are experimentally synthesized by controlling the amplitude and phase of the input pulse over a wide range of frequencies. Pulse width tunability was also demonstrated by changing the layer-to-layer distance of a 3D-printed diffractive network or by physically replacing 1-2 layers of an existing network with newly trained and fabricated diffractive layers.
We present a photoconductive terahertz detector, which offers high-sensitivity and broadband detection performance for terahertz time-domain spectroscopy at record-low optical pump power levels. The detector employs a plasmonic nanocavity designed to confine the optical pump photons in a thin photoconductive region. By providing an efficient optical absorption in this thin layer, the carrier transport time to the device contact electrodes is maintained in a sub-picosecond range for the majority of the photo-generated carriers. Therefore, ultrafast operation and high quantum efficiency is achieved simultaneously, which significantly increases the detector responsivity and dynamic range even at very low optical pump power levels. We experimentally demonstrate a 110 dB dynamic range over a 0.1-7 THz frequency range at only a 0.1 mW optical pump power level.
We present a telecommunication-compatible terahertz source based on passive transponder nanoantennas. When excited with an optical pump beam, photogenerated carriers in a photo-absorbing substrate are swept to an array of terahertz radiating nanoantennas by a built-in electric field formed between the nanoantennas and substrate. The photoconductive substrate is specifically grown to maximize the strength and overlap of the built-in electric field with the photogenerated carriers to provide high optical-to-terahertz conversion efficiencies. We have used this terahertz generation scheme to develop a fiber-coupled passive transponder that provides more than a 110 dB dynamic range over a 5 THz bandwidth.
Using deep learning-based training of diffractive layers we designed single-pixel machine vision systems to all-optically classify images by maximizing the output power of the wavelength corresponding to the correct data-class. We experimentally validated our diffractive designs using a plasmonic nanoantenna-based time-domain spectroscopy setup and 3D-printed diffractive layers to successfully classify the images of handwritten-digits using a single-pixel and snap-shot illumination. Furthermore, we trained a shallow electronic neural network as a decoder to reconstruct the images of the input objects, solely from the power detected at ten distinct wavelengths, also demonstrating the success of this platform as a task-specific, single-pixel imager.
We report a broadband diffractive optical network that can simultaneously process a continuum of wavelengths. To demonstrate its success, we designed and experimentally validated a series of broadband networks to create single/dual passband spectral filters and a spatially-controlled wavelength de-multiplexer that are composed of deep learning-designed diffractive layers to spatially and spectrally engineer the output light. The resulting designs were 3D-printed and tested using a terahertz time-domain-spectroscopy system to demonstrate the match between their numerical and experimental output. Broadband diffractive networks diverge from intuitive/analytical designs, creating unique optical systems to perform deterministic tasks and statistical inference for machine learning applications.
We present a diffractive network, trained for pulse engineering to shape input pulses into desired optical waveforms. The synthesis of square-pulses with various widths was experimentally demonstrated with 3D-fabricated passive diffractive layers that control both the amplitude and phase profile of the input terahertz pulse across a wide range of frequencies. Pulse-width tunability was also demonstrated by altering the layer-to-layer distances of a diffractive network. Furthermore, the modularity of this framework was demonstrated by replacing part of an already-trained network with newly-trained layers to tune the width of the output terahertz pulse, presenting a Lego-like physical transfer learning approach.
We present a diffractive deep neural network-based framework that can simultaneously process a continuum of illumination wavelengths to perform a specific task that it is trained for. Based on this framework, we designed and 3D printed a series of optical systems including single and double pass-band filters as well as a spatially-controlled wavelength de-multiplexing system using a broadband THz pulse as input, revealing an excellent match between our numerical design and experimental results. The presented optical design framework based on diffractive neural networks can be adapted to other parts of the spectrum and be extended to create task-specific metasurface designs.
We demonstrate a high dynamic range, broadband THz-TDS system that is compatible with 1 μm femtosecond lasers. In order to improve the dynamic range and bandwidth, we designed and fabricated photoconductive terahertz sources and detectors equipped with arrays of plasmonic nano-antennas fabricated on an epitaxially-grown In0.24Ga0.76As substrate. Plasmonic nano-antennas concentrate the photo-generated carriers close to the antenna-photoconductor interface. This ensures a superior performance both in terahertz generation and detection by increasing the induced ultrafast current at the antenna terminals. We demonstrate a THz-TDS system with more than a 100 dB dynamic range and a 4 THz bandwidth.
Utilizing short-carrier-lifetime semiconductors as the photo-absorbing substrate of photoconductive terahertz detectors has been considered a necessity to enable ultrafast operation and to recombine the slow photo-generated carriers that increase the detector noise and reduce the detector responsivity. However, most of the techniques used for growing short-carrier-lifetime semiconductors introduce a high density of defects in the semiconductor lattice, degrading the carrier mobility and drift velocity and, thus, the detector responsivity. To eliminate the need for a short-carrier-lifetime semiconductor, we present a novel photoconductive terahertz detector based on a nanocavity-coupled plasmonic nanoantenna array. The presented photoconductive terahertz detector uses an undoped GaAs layer embedded inside a nanocavity as the photoconductive active region. The nanocavity is specifically designed to confine the optical pump photons very tightly inside the undoped GaAs layer so that all the photo-generated carriers concentrate around an array of plasmonic nanoantennas, which are also designed to operate as broadband terahertz antennas. Therefore, the presented nanocavity-coupled plasmonic nanoantenna array maximizes the photo-generated carrier concentration and the induced terahertz electric field in response to an incident terahertz radiation near the plasmonic nanoantenna contact electrodes. This significantly increases the detector responsivity and offers photo-generated carrier transport times comparable to photoconductive terahertz detectors based on short-carrier-lifetime semiconductors. By using the presented detector in a time-domain terahertz spectroscopy system, we demonstrate resolving terahertz spectra with a large dynamic range over the 0.1-5 THz frequency range.
High-power pulsed terahertz radiation sources are highly in demand for time-domain terahertz imaging and spectroscopy systems. A common way to generate pulsed terahertz radiation is exciting a biased ultrafast photoconductor with a femtosecond optical pulse. The photo-generated carriers drift to a terahertz radiating element under the induced bias electric field and a pulsed terahertz radiation is generated. Developing photoconductive terahertz sources operating at telecommunication wavelengths (~1550 nm) is very attractive because of the availability of high-power, narrow-pulse-width, and compact fiber lasers at these wavelengths. However, photoconductors responsive to telecommunication wavelengths often have low resistivity due to their small bandgap energy, resulting in excessive dark current levels under an applied bias voltage. As a result, telecommunication-compatible photoconductive sources experience a premature thermal breakdown under high bias voltages and cannot offer high terahertz radiation powers. To address this limitation, we introduce a new type of telecommunication-compatible photoconductive terahertz source that does not require an externally applied bias voltage and relies on a built-in electric field formed at the interface between the photoconductor and terahertz antenna contact electrodes. By eliminating the bias voltage, the device operates at a zero dark current, enabling a highly reliable operation. We use an array of plasmonic nanoantennas as the terahertz radiating elements to achieve a broad terahertz radiation bandwidth and high optical-to-terahertz conversion efficiency. We demonstrate pulsed terahertz radiation with powers exceeding 100 μW, enabling time-domain terahertz spectroscopy with a 100 dB dynamic range over a 0.1-3 THz bandwidth.
We present a photoconductive terahertz source that offers broadband pulsed terahertz radiation with enhanced optical-to-terahertz conversion efficiencies compared to photoconductive terahertz sources based on short-carrier-lifetime semiconductors. The performance enhancement is achieved by utilizing a plasmonic nanocavity that tightly confines optical pump photons inside a photoconductive layer near the terahertz radiating elements. The plasmonic nanocavity is implemented by sandwiching the photoconductive layer between a distributed Bragg reflector and plasmonic metallic structures, which are optimized to be resonant at the optical pump wavelength. The plasmonic structures are also designed as a broadband terahertz nanoantenna array. A thin undoped GaAs film is used as the photoconductive layer offering much higher carrier drift velocities compared to short-carrier-lifetime GaAs substrates. The tight confinement of the optical pump photons and the use of a low-defect photoconductive semiconductor layer allow drift of almost all of the photo-generated carriers to the terahertz nanoantennas in a sub-picosecond time scale to efficiently contribute to pulsed terahertz radiation. We experimentally demonstrate that the presented terahertz source offers 60 times higher optical-to-terahertz conversion efficiency compared to a similar terahertz nanoantenna array fabricated on a short-carrier-lifetime semiconductor. We demonstrate pulsed terahertz radiation with powers exceeding 4 mW over 0.1-4 THz frequency range.
One of the main limitations for realizing high-performance time-domain terahertz imaging and spectroscopy systems is the low responsivity and narrow bandwidth of the existing pulsed terahertz detectors. In this work, we present a high-responsivity and broadband large-area terahertz detector that incorporates a two-dimensional array of plasmonic nanoantennas fabricated on a low-temperature-grown GaAs substrate. By using a large-area device architecture, large optical spot sizes can be used, mitigating the carrier screening effect at high optical pump powers. Using a large-area device architecture also makes the device less sensitive to changes in optical and terahertz alignment. The two-dimensional array of plasmonic nanoantennas is designed to offer a broad terahertz detection bandwidth. It is also designed to enhance optical absorption in close proximity to the nanoantennas by exciting surface plasmon waves. This allows drifting a large portion of photo-generated electrons and holes to the nanoantennas in presence of an incident terahertz pulse, offering high responsivity levels. We experimentally demonstrate detection of terahertz pulses with more than 5 THz bandwidth with high responsivity and signal-to-noise ratio levels exceeding that of electro-optic detectors. Such terahertz detectors would play a critical role in realization of the next generation time-domain terahertz imaging and spectroscopy systems.
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