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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 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 demonstrate passive optical-to-terahertz conversion through plasmon-coupled surface states. 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 surface-state-induced built-in electric field formed between the nanoantennas and substrate. The nanoantennas are used to couple optically-excited surface waves to the interface region where the built-in electric field is maximized to provide high optical-to-terahertz conversion efficiencies. We have used this scheme to develop a fiber-coupled bias-free terahertz source that provides more than a 110 dB dynamic range over a 5 THz bandwidth.

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 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.

Stability of optical beats in a chaotically oscillating laser is compared to that of a free-running continuous-wave laser using a highly efficient plasmonic photomixer (Anttena). The high stability of optical beats in chaotically oscillating lasers is verified.

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 demonstrate, for the first time, a telecommunication-compatible terahertz frequency-domain spectroscopy system without requiring any short-carrier-lifetime photoconductors. The ultrafast response of the terahertz source and detector is achieved by incorporating plasmonic antenna electrodes and a thin layer of epitaxially-grown In0.53Ga0.47As to confine optical generation very close to the antenna electrodes. As a result, a short carrier transit time for most of the photocarriers is provided, inducing an ultrafast photocurrent for terahertz generation and detection. This design approach enables larger flexibility for the choice of photoconductors and operation wavelengths without being limited by the availability of short-carrier-lifetime photoconductors.

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.

The degenerate mode interaction can possess a clear avoided mode crossing to manipulate the cavity mode spectra by a mode-splitting process under a strong mode-coupling condition in high-Q microresonators. Here, the mode splitting strength can be changed by controlling the pump-resonance effective detuning in a dispersion-managed Si3N4 microresonator through a differential thermo-optic effect. The splitting mode can locally facilitate the frequency matching in normal dispersion microresonators so that the tunable parametric oscillation can be observed by tuning pump-resonance detuning. A broadly tunable THz wave radiation is generated after injecting the tunable parametric oscillation into a bias-free photomixer at room temperature.

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.

Most state-of-the-art terahertz time-domain imaging technologies are based on single-pixel systems, which mechanically scan either the imaging object or the terahertz system, limiting the imaging speed. We present a new terahertz time-domain imaging modality using a terahertz photoconductive focal-plane array. The focal-plane array consists of plasmonic nano-antenna arrays on an LT-GaAs substrate. The dynamic range of a single pixel can reach up to 75 dB with more than a 4 THz bandwidth. We demonstrate clear terahertz images up to 2.5 THz. We also demonstrate that the focal-plane array can operate at video-rate imaging speeds.

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.

Although unique potentials of terahertz waves for chemical identification and material characterization have been recognized for quite a while, the relatively poor performance of current terahertz spectroscopy and spectrometry systems continue to impede their deployment in field settings. This talk describes some of the recent advancements in terahertz spectroscopy systems by using optically-pumped plasmonic photoconductors for terahertz wave generation and detection. Incorporating plasmonic nanostructures inside the active area of photoconductive terahertz sources and detectors offers enhanced quantum efficiencies while maintaining ultrafast operation. This enhancement is due to the unique capability of plasmonic nanostructures to significantly increase the concentration of photo-induced carriers inside the device active area, where they interact with a bias/terahertz electric field to generate/detect terahertz radiation. By the use of this powerful technique, broadband terahertz spectroscopy with record-high signal-to-noise ratio levels exceeding 140 dB and broadband terahertz spectrometry with quantum-level sensitivities are demonstrated. Such high-sensitivity terahertz spectroscopy and spectrometry systems could offer numerous opportunities for e.g., biomedical sensing, atmospheric studies, astrophysics studies, pharmaceutical quality control, and security screening applications.

Graphene is a very attractive material for broadband photodetection in hyperspectral imaging and sensing systems. However, its potential use has been hindered by tradeoffs between the responsivity, bandwidth, and operation speed of existing graphene photodetectors. Here, we present engineered photoconductive nanostructures based on gold-patched graphene nano-stripes, which enable simultaneous broadband and ultrafast photodetection with high responsivity. These nanostructures merge the advantages of broadband optical absorption, ultrafast photocarrier transport, and carrier multiplication in graphene nano-stripes with the ultrafast transport of photocarriers to the gold patches before recombination. Through this approach, high-responsivity operation is achieved without the use of bandwidth- and speed-limiting quantum dots, defect states, or tunneling barriers. We demonstrate high-responsivity photodetection from the visible to the infrared regime (0.6 A/W at 0.8 μm and 11.5 A/W at 20 μm) with operation speeds exceeding 50 GHz. Our results demonstrate an improvement of the response times by more than seven orders of magnitude and an increase in bandwidths of one order of magnitude compared to those of higher-responsivity graphene photodetectors based on quantum dots and tunneling barriers.

Detection of faint fluxes of photons at terahertz frequencies is crucial for various applications including biosensing, medical diagnosis, chemical detection, atmospheric studies, space explorations, high-data-rate communication, and security screening. Heterodyne terahertz spectrometers based on cryogenically cooled superconducting mixers have so far been the only instruments that can provide high spectral resolution and near-quantum-limited sensitivity levels. The operation temperature, bandwidth constraints, and complexity of these terahertz spectrometers have restricted their use to mostly astronomy and atmospheric studies, limiting the overall impact and wide-spread use of terahertz technologies. Here we introduce a spectrometry scheme that uses plasmonic photomixing for frequency downconversion to offer quantum-level sensitivities at room temperature for the first time. Frequency downconversion is achieved by mixing terahertz radiation and a heterodyning optical beam with a terahertz beat frequency in a plasmonics-enhanced semiconductor active region. We demonstrate spectrometer sensitivities down to 3 times the quantum-limit at room temperature. Our presented spectrometry scheme can be applicable to resolve both the high-resolution spectra of gas molecules and mid-resolution spectra of condensed phase samples over a total operable bandwidth of 0.1-5 THz. As an example, we use the presented spectrometer to resolve the spectral information of ammonia, which has a number of narrowband absorption peaks over the 0.1-5 THz frequency range. With a versatile design capable of broadband spectrometry, this plasmonic photomixer has broad applicability to quantum optics, chemical sensing, biological studies, medical diagnosis, high data-rate communication, as well as astronomy and atmospheric studies.

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.

Although unique potentials of terahertz waves for chemical identification and material characterization have been recognized for quite a while, the relatively poor performance of current terahertz spectroscopy and spectrometry systems continue to impede their deployment in field settings. This talk describes some of the recent advancements in terahertz spectroscopy systems by using optically-pumped plasmonic photoconductors for terahertz wave generation and detection. Incorporating plasmonic nanostructures inside the active area of photoconductive terahertz sources and detectors offers enhanced quantum efficiencies while maintaining ultrafast operation. This enhancement is due to the unique capability of plasmonic nanostructures to significantly increase the concentration of photo-induced carriers inside the device active area, where they interact with a bias/terahertz electric field to generate/detect terahertz radiation. By the use of this powerful technique, broadband terahertz spectroscopy with record-high signal-to-noise ratio levels exceeding 110 dB and broadband terahertz spectrometry with quantum-level sensitivities are demonstrated. Such high-sensitivity terahertz spectroscopy and spectrometry systems could offer numerous opportunities for e.g., biomedical sensing, atmospheric studies, space explorations, pharmaceutical quality control, and security screening applications.

Utilizing nano-antennas and plasmonic light concentrators in photoconductive terahertz sources has proven to offer significantly higher terahertz radiation powers by enhancing the photoconductor quantum efficiency while maintaining ultrafast operation. This is because the use of nano-antennas and plasmonic light concentrators in a photoconductive source reduces the average transport path of photocarriers to the terahertz radiating elements, increasing the ultrafast photocurrent that contributes to terahertz radiation generation. In this talk I will present an overview of some of the recent advancements in photoconductive terahertz sources based on plasmonic contact electrodes, enabling significant enhancement in efficiency and output power of photoconductive terahertz sources. I show that the significant performance enhancement offered by plasmonic contact electrodes can be utilized to achieve record-high optical-to-terahertz conversion efficiencies as high as 7.5% and milliwatt terahertz power levels in both continuous-wave and pulsed operation at optical pump wavelengths ranging from 800 nm to 1550 nm.

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.

Terahertz radiation has many unique applications in imaging and sensing currently limited by low efficiency, complexity, and bulky nature of existing terahertz emitters. In this study, we propose an innovative terahertz emitter based on plasmonic nanowire light absorbers that can convert optical beam to terahertz radiation with unprecedented conversion efficiencies. By utilizing nanowire arrays integrated with plasmonic nano-antennae, we confine the majority of the incident optical photons within nanoscale distances from metal contacts. As a result, the majority of the photo-generated carriers quickly drift to the plasmonic nano-antennae in a sub-picosecond time-scale and contribute to efficient terahertz generation. It is predicted that bias-free terahertz emitters based on this novel device architecture can achieve tens of mW terahertz radiation power levels and optical-to-terahertz conversion efficiencies as high as 24%. Additionally, the proposed terahertz emitters operate at telecommunication optical wavelengths and are monolithically integrated on Si or InP substrates, offering a compact and low-cost device platform.

Graphene is a promising two-dimensional material for photo-detection owing to its high mobility, broadband optical absorption, zero band gap nature, and tunable carrier concentration through electrical gating. Despite these unique properties, its 2.3% optical absorption from ultraviolet to infrared wavelengths and short carrier lifetime has limited its usage for practical applications. In this work, we present a broadband, high responsivity, and high speed graphene photodetector. By use of plasmonic nanoantennas, an incident optical field can be strongly concentrated in close proximity to the metallic nanoantennas. This significantly reduces the drift path length of the majority of photo-generated carriers in graphene to the plasmonic nanoantennas that serve as the photodetector contact electrodes. As a result, a large number of the photo-generated carriers can drift to the photodetector contact electrodes despite the short carrier lifetime of graphene, offering high responsivity levels. Moreover, the photodetector is designed to offer high speed operation by minimizing the capacitive parasitics induced by the plasmonic nanoantennas. We demonstrate a broadband photo-detection operation covering the wavelength regime from 800 nm to 1800 nm. We achieve responsivity levels as high as 0.6 A/W at 800 nm, which is close to the theoretical limit of 0.65 A/W. In summary, the combination of the high-responsivity, broad bandwidth, and high-speed performance of the presented plasmonics-enhanced graphene photodetector could find many applications in future optical communication, imaging and sensing systems.

Heterodyne terahertz spectrometers are highly in demand for space explorations and astrophysics studies. A conventional heterodyne terahertz spectrometer consists of a terahertz mixer that mixes a received terahertz signal with a local oscillator signal to generate an intermediate frequency signal in the radio frequency (RF) range, where it can be easily processed and detected by RF electronics. Schottky diode mixers, superconductor-insulator-superconductor (SIS) mixers and hot electron bolometer (HEB) mixers are the most commonly used mixers in conventional heterodyne terahertz spectrometers. While conventional heterodyne terahertz spectrometers offer high spectral resolution and high detection sensitivity levels at cryogenic temperatures, their dynamic range and bandwidth are limited by the low radiation power of existing terahertz local oscillators and narrow bandwidth of existing terahertz mixers. To address these limitations, we present a novel approach for heterodyne terahertz spectrometry based on plasmonic photomixing. The presented design replaces terahertz mixer and local oscillator of conventional heterodyne terahertz spectrometers with a plasmonic photomixer pumped by an optical local oscillator. The optical local oscillator consists of two wavelength-tunable continuous-wave optical sources with a terahertz frequency difference. As a result, the spectrometry bandwidth and dynamic range of the presented heterodyne spectrometer is not limited by radiation frequency and power restrictions of conventional terahertz sources. We demonstrate a proof-of-concept terahertz spectrometer with more than 90 dB dynamic range and 1 THz spectrometry bandwidth.

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.