Browsing by Subject "Holography"

This dissertation describes three computational sensors. The first sensor is a scanning multi-spectral aperture-coded microscope containing a coded aperture spectrometer that is vertically scanned through a microscope intermediate image plane. The spectrometer aperture-code spatially encodes the object spectral data and nonnegative

least squares inversion combined with a series of reconfigured two-dimensional (2D spatial-spectral) scanned measurements enables three-dimensional (3D) (x, y, λ) object estimation. The second sensor is a coded aperture snapshot spectral imager that employs a compressive optical architecture to record a spectrally filtered projection

of a 3D object data cube onto a 2D detector array. Two nonlinear and adapted TV-minimization schemes are presented for 3D (x,y,λ) object estimation from a 2D compressed snapshot. Both sensors are interfaced to laboratory-grade microscopes and

applied to fluorescence microscopy. The third sensor is a millimeter-wave holographic imaging system that is used to study the impact of 2D compressive measurement on 3D (x,y,z) data estimation. Holography is a natural compressive encoder since a 3D

parabolic slice of the object band volume is recorded onto a 2D planar surface. An adapted nonlinear TV-minimization algorithm is used for 3D tomographic estimation from a 2D and a sparse 2D hologram composite. This strategy aims to reduce scan time costs associated with millimeter-wave image acquisition using a single pixel receiver.

Microwave imaging platforms conventionally take the form of antenna arrays or synthetic apertures. Inspired by methods in the optical regime, computational microwave imaging has recently taken hold as an alternative approach that uses spatially-diverse waveforms to multiplex scene information. In this dissertation, we use dynamic metasurface apertures to demonstrate improved hardware characteristics and capabilities in computational microwave imaging systems. In particular, we demonstrate waveguide-fed and cavity-backed dynamic metasurface apertures. A waveguide-fed dynamic metasurface aperture consists of a waveguide device loaded with numerous independently tunable metamaterial elements, each of which couples energy from the guided mode into a reconfigurable radiation pattern. We explicate design considerations for a waveguide-fed dynamic metasurface aperture, optimize its usage, and utilize it in computational imaging. In addition, we leverage the dynamic aperture's agility to demonstrate through-wall imaging and beamforming for synthetic aperture radar. Significant attention is also devoted to imaging with a single frequency, an approach which can ease the complexity and improve the performance of the required RF components.

Expanding on the waveguide-fed instantiation, we investigate cavity-backed dynamic apertures. These apertures employ disordered cavity modes to feed a multitude of radiating elements. We investigate this approach with two structures: a volumetric cavity, where we tune the boundary condition, and a planar PCB-based cavity, where the radiating elements are tuned. Capable of generating diverse radiation patterns, we use these structures to assess the utility of dynamic tuning in computational imaging systems. Many of the architectures studied in this dissertation chart a path toward a low-cost dynamic aperture with a favorable form factor, a platform which provides immense control over its emitted fields for a variety of microwave applications.

Over the last twenty years, progress on metamaterials (MMs), defined as three-dimensional artificial composites, has sprouted unprecedented phenomena through the manipulation of electromagnetic, acoustic and other waves, making the connection \textit{from structure to function}. By virtue of their spatial and spectral control of wave-matter interactions, MMs have emerged as a powerful building block for practical applications, including imaging, sensing, energy harvesting, beam shaping and steering, and many more. In recent years, the metasurface, as an alternative to volumetric metamaterials, with its reduced 2D profile, has gained increased attention for applications where weight, power and cost are of importance. In this dissertation, I will mainly explore two optical applications where the flexibility in design of a metasurface provides unique capabilities. In one application, waveguide holography, a multifunctional metasurface is used to couple light from a waveguide to free space, forming multicolor or multipolarization holograms. In the second application, a metasurface is used to enhance optical bistability.

First, in this dissertation I will present an investigation of a multicolor computer-generated hologram (CGH) in an all-dielectric metamaterial waveguide system. Light beams from three different color laser sources (red, green and blue) are coupled into the waveguide via a single period grating without any beam-splitters or prisms. A multicolor holographic image can be decoupled in the far field through a binary CGH without any lenses. This technology enables lens-free, ultra-miniature augmented and virtual reality displays. Then, I will continue to illustrate polarization-selective waveguide holography at optical frequencies based on a similar metamaterial multilayer system. I will show that two orthogonally polarized, spatially separated or overlapped holographic images can be incorporated into a single binary CGH, and use these two images to produce composite, stereo vision, 3D effect images observable using linear or circularly polarized lens glasses. Both polarizations are also used to construct radially and azimuthally polarized beams. The fundamental mode and the second mode of TM and TE modes in the waveguide are used to guide the two polarization states. We envision that incorporating polarization selection into waveguide holograms may be used to realize chip-scale displays and beams for optical trapping.

Furthermore, I will introduce another example of the principle \textit{from structure to function}, optical bistablity, in a film-coupled metasurface system, which is a promising platform for low-energy and all-optical switches. The large field enhancements that can be achieved in the dielectric spacer region between a nanopatch optical antenna and a metallic substrate can substantially enhance optical nonlinear processes. Utilizing a dielectric material that exhibits an optical Kerr effect as the spacer layer, we propose a new simulation method to vividly show the optical bistability processes. We expect this new method to be highly accurate compared with other numerical approaches, such as those based on graphical post-processing techniques, since it self-consistently solves for both the spatial field distribution and the intensity-dependent refractive index distribution of the spacer layer. This method offers an alternative approach to finite-difference time-domain (FDTD) modelling. One of the bistability metasurface designs exhibits exceptionally low switching intensities, corresponding to switching energies on the order of tens of attojoules. We propose our method as an effective tool for designing all-optical switches and modulators.