Snapshot Multispectral Imaging Using a Diffractive Optical Network

Multispectral imaging has been used for numerous applications in e.g., environmental monitoring, aerospace, defense, and biomedicine. Here, we present a diffractive optical network-based multispectral imaging system trained using deep learning to create a virtual spectral filter array at the output image field-of-view. This diffractive multispectral imager performs spatially-coherent imaging over a large spectrum, and at the same time, routes a pre-determined set of spectral channels onto an array of pixels at the output plane, converting a monochrome focal plane array or image sensor into a multispectral imaging device without any spectral filters or image recovery algorithms. Furthermore, the spectral responsivity of this diffractive multispectral imager is not sensitive to input polarization states. Through numerical simulations, we present different diffractive network designs that achieve snapshot multispectral imaging with 4, 9 and 16 unique spectral bands within the visible spectrum, based on passive spatially-structured diffractive surfaces, with a compact design that axially spans ~72 times the mean wavelength of the spectral band of interest. Moreover, we experimentally demonstrate a diffractive multispectral imager based on a 3D-printed diffractive network that creates at its output image plane a spatially-repeating virtual spectral filter array with 2x2=4 unique bands at terahertz spectrum. Due to their compact form factor and computation-free, power-efficient and polarization-insensitive forward operation, diffractive multispectral imagers can be transformative for various imaging and sensing applications and be used at different parts of the electromagnetic spectrum where high-density and wide-area multispectral pixel arrays are not widely available.Comment: 24 Pages, 9 Figure

Video-rate terahertz digital holographic imaging system

Terahertz (THz) imaging has been demonstrated in numerous applications from medical to non-destructive evaluation (NDE), but current systems require expensive components, provide slow frame-rates and low resolutions. THz holography offers a potentially low-cost, high-performance alternative. Here we demonstrate the first full video-rate THz digital holography system at 2.52 THz (118.8 µm) using low-cost optical components. 2D digital reconstructions of samples are performed at frame-rates of 50 Hz - an order of magnitude higher than previous systems, whilst imaging of samples concealed in common packaging types demonstrates suitability for NDE applications. A lateral resolution of 250 µm was determined using a 1951 USAF target

Dissipative Materials Enabled Subwavelength Nanophotonics

Properly structuring materials at subwavelength scale allows for strong light-matter interaction, thereby enhancing near-field effects and engineering far-field scattering through intermodal interference. A majority of such effects are associated with plasmonics where electromagnetic waves created in the vicinity of metallic nanostructures is able to give rise to a variety of novel phenomena and fascinating applications. In the recent years, dielectric nanoparticles with high refractive index based on optically induced electric and magnetic Mie resonances attract a plethora of attention. In this rapidly developing field, dissipative loss in optical materials is considered one of the major challenges. Here, in this dissertation, we show that, counter-intuitively, it contributes positively to sub-wavelength scale light enhancement and confinement, and also improves scattering efficiency in the far field. In the first part of this dissertation, near field enhancement in dissipative dielectric antennas is demonstrated to be orders of magnitude higher than their lossless dielectric counterparts, which is particularly favorable in deep UV applications where metals are plasmonically inactive and transparent dielectrics always have low index. The loss facilitated field enhancement is the result of large material permittivity contrast and electric field discontinuity. These dissipative dielectric nanostructures can be easily achieved with a great variety of dielectrics at their Lorentz oscillation frequencies, thus having the potential to build a completely new material platform boosting light-matter interaction over broader frequency ranges, with advantages such as bio-compatibility, CMOS compatibility and harsh environment endurance. Additionally, manipulation of ultra-violet light through metasurface in the far field utilizing the silicon loss is then presented. We experimentally demonstrate Si metasurfaces working effectively over a broad band down to 290nm, with efficiencies comparable to plasmonic metasurface performance in the infrared regime. And for the first time, we show photolithography enabled by metasurface-generated ultraviolet holograms. We attribute such performance enhancement to the large scattering cross-sections of Si antennas in the ultraviolet range, which is adequately modeled via a circuit model. Our new platform will deepen our understanding of the role of material dissipation and introduce even more material options to broadband metaphotonic applications, including those in integrated photonics and holographic lithography technologies.Dynamically tunable far field with subwavlength nanostructures is always desired for practical applications. In the last section of this dissertation, we introduce a lithography free and field-programmable photonic metacanvas. Previous attempts of realizing such idea used micro-mechanical metamaterials or amorphous-crystalline phase transition materials, which are limited in terms of the functionalities, efficiency, cost, and high working temperature (> 600oC). It is much desired to reconfigure photonic devices in a fast, large-scale, cost-effective, reliable, and free-style way at or near room temperature. Here, we present a completely rewritable meta-canvas on which arbitrary photonic devices can be rapidly written, erased and rewritten. The writing is with a low-power (1 mW) continuous laser and the entire process stays below ~ 90oC. Using these devices we demonstrate dynamical manipulation of optical waves for light propagation, reconstruction and polarization. Such meta-canvas supports physical (re)compilation of photonic operators akin to that of FPGA, opening up possibilities where a single photonic element can be field-programmed to deliver complex, system-level functionalities

Antenna-coupled Infrared Focal Plane Array

In this dissertation a new type of infrared focal plan array (IR FPA) was investigated, consisting of antenna-coupled microbolometer fabricated using electron-beam lithography. Four different antenna designs were experimentally demonstrated at 10-micron wavelength: dipole, bowtie, square-spiral, and log-periodic. The main differences between these antenna types were their bandwidth, collection area, angular reception pattern, and polarization. To provide pixel collection areas commensurate with typical IR FPA requirements, two configurations were investigated: a two-dimensional serpentine interconnection of individual IR antennas, and a Fresnel-zone-plate (FZP) coupled to a single-element antenna. Optimum spacing conditions for the two-dimensional interconnect were developed. Increased sensitivity was demonstrated using a FZP-coupled design. In general, it was found that the configuration of the antenna substrate material was critical for optimization of sensitivity. The best results were obtained using this membranes of silicon nitride to enhance the thermal isolation of the antenna-coupled bolometers. In addition, choice of the bolometer material was also important, with the best results obtained using vanadium oxide. Using optimum choices for all parameters, normalized sensitivity (D*) values in the range of mid 108[√Hz/W] were demonstrated for antenna-coupled IR sensors, and directions for further improvements were identified. Successful integration of antenna-coupled pixels with commercial readout integrated circuits was also demonstrated

A comprehensive survey on antennas on-chip based on metamaterial, metasurface, and substrate integrated waveguide principles for millimeter-waves and terahertz integrated circuits and systems

Antennas on-chip are a particular type of radiating elements valued for their small footprint. They are most commonly integrated in circuit boards to electromagnetically interface free space, which is necessary for wireless communications. Antennas on-chip radiate and receive electromagnetic (EM) energy as any conventional antennas, but what distinguishes them is their miniaturized size. This means they can be integrated inside electronic devices. Although on-chip antennas have a limited range, they are suitable for cell phones, tablet computers, headsets, global positioning system (GPS) devices, and WiFi and WLAN routers. Typically, on-chip antennas are handicapped by narrow bandwidth (less than 10%) and low radiation efficiency. This survey provides an overview of recent techniques and technologies investigated in the literature, to implement high performance on-chip antennas for millimeter-waves (mmWave) and terahertz (THz) integrated-circuit (IC) applications. The technologies discussed here include metamaterial (MTM), metasurface (MTS), and substrate integrated waveguides (SIW). The antenna designs described here are implemented on various substrate layers such as Silicon, Graphene, Polyimide, and GaAs to facilitate integration on ICs. Some of the antennas described here employ innovative excitation mechanisms, for example comprising open-circuited microstrip-line that is electromagnetically coupled to radiating elements through narrow dielectric slots. This excitation mechanism is shown to suppress surface wave propagation and reduce substrate loss. Other techniques described like SIW are shown to significantly attenuate surface waves and minimise loss. Radiation elements based on the MTM and MTS inspired technologies are shown to extend the effective aperture of the antenna without compromising the antenna’s form factor. Moreover, the on-chip antennas designed using the above technologies exhibit significantly improved impedance match, bandwidth, gain and radiation efficiency compared to previously used technologies. These features make such antennas a prime candidate for mmWave and THz on-chip integration. This review provides a thorough reference source for specialist antenna designers.This work was supported in part by the Universidad Carlos III de Madrid and the European Union's Horizon 2020 Research and Innovation Programme under the Marie Sklodowska-Curie Grant 801538, in part by the Icelandic Centre for Research (RANNIS) under Grant 206606, and in part by the National Science Centre of Poland under Grant 2018/31/B/ST7/02369

All-optical switching and strong coupling using tunable whispering-gallery-mode microresonators

We review our recent work on tunable, ultrahigh quality factor whispering-gallery-mode bottle microresonators and highlight their applications in nonlinear optics and in quantum optics experiments. Our resonators combine ultra-high quality factors of up to Q = 3.6 \times 10^8, a small mode volume, and near-lossless fiber coupling, with a simple and customizable mode structure enabling full tunability. We study, theoretically and experimentally, nonlinear all-optical switching via the Kerr effect when the resonator is operated in an add-drop configuration. This allows us to optically route a single-wavelength cw optical signal between two fiber ports with high efficiency. Finally, we report on progress towards strong coupling of single rubidium atoms to an ultra-high Q mode of an actively stabilized bottle microresonator.Comment: 20 pages, 24 figures. Accepted for publication in Applied Physics B. Changes according to referee suggestions: minor corrections to some figures and captions, clarification of some points in the text, added references, added new paragraph with results on atom-resonator interactio