Terahertz imaging with compressive sensing

Most existing terahertz imaging systems are generally limited by slow image acquisition due to mechanical raster scanning. Other systems using focal plane detector arrays can acquire images in real time, but are either too costly or limited by low sensitivity in the terahertz frequency range. To design faster and more cost-effective terahertz imaging systems, the first part of this thesis proposes two new terahertz imaging schemes based on compressive sensing (CS). Both schemes can acquire amplitude and phase-contrast images efficiently with a single-pixel detector, thanks to the powerful CS algorithms which enable the reconstruction of N-by- N pixel images with much fewer than N2 measurements. The first CS Fourier imaging approach successfully reconstructs a 64x64 image of an object with pixel size 1.4 mm using a randomly chosen subset of the 4096 pixels which defines the image in the Fourier plane. Only about 12% of the pixels are required for reassembling the image of a selected object, equivalent to a 2/3 reduction in acquisition time. The second approach is single-pixel CS imaging, which uses a series of random masks for acquisition. Besides speeding up acquisition with a reduced number of measurements, the single-pixel system can further cut down acquisition time by electrical or optical spatial modulation of random patterns. In order to switch between random patterns at high speed in the single-pixel imaging system, the second part of this thesis implements a multi-pixel electrical spatial modulator for terahertz beams using active terahertz metamaterials. The first generation of this device consists of a 4x4 pixel array, where each pixel is an array of sub-wavelength-sized split-ring resonator elements fabricated on a semiconductor substrate, and is independently controlled by applying an external voltage. The spatial modulator has a uniform modulation depth of around 40 percent across all pixels, and negligible crosstalk, at the resonant frequency. The second-generation spatial terahertz modulator, also based on metamaterials with a higher resolution (32x32), is under development. A FPGA-based circuit is designed to control the large number of modulator pixels. Once fully implemented, this second-generation device will enable fast terahertz imaging with both pulsed and continuous-wave terahertz sources

Optical metasurfaces for polarization generation, detection and imaging

Like phase and amplitude, polarization is a fundamental property of light, which can reveal hidden information and has been used in many research fields, including material science, medicine, target detection and biomedical diagnosis. Polarization generation, detection and imaging are of importance for fundamental research and practical applications. Although conventional optics can perform these tasks, it suffers from a complex system, large volume and high cost, which cannot meet the continuing trend of miniaturization and integration. Optical metasurfaces, the two-dimensional counterparts of metamaterials, are planar nanostructured interfaces, which have recently attracted tremendous interest in realizing ultrathin and lightweight planar optical devices. Optical metasurfaces can manipulate light’s amplitude, phase and polarization in a desirable manner, providing a new and compact platform to generate, detect and manipulate light’s polarization. This thesis utilises optical metasurfaces to realise and experimentally demonstrate novel optical devices for polarization generation, detection and imaging. Due to the simplicity of the design and fabrication, this thesis is mainly focused on geometric optical metasurfaces, which are superior to other types of metasurfaces. 2D and 3D polarization structures are generated based on a metalens approach. A ring focal curve, an Archimedean spiral focal curve, and seven-segment-based decimal numbers are experimentally demonstrated in 2D space, while a 3-foil knot, a 4-foil knot, and a 5-foil are realized in 3D space. The geometric metasurfaces are designed based on colour and phase multiplexing and polarization rotation, creating various 3D polarization knots. Various 3D polarization knots in the same observation region can be achieved by controlling the incident wavelengths, providing unprecedented polarization control with colour information in 3D space. Novel polarization detection is experimentally demonstrated through optical holography, light’s orbital angular momentum, and optical ring vortex beams. The measured polarization parameters such as major axis, ellipticity, and handedness are in good agreement with the theoretical prediction. A multifunctional microscope is proposed and developed to image different objects, including biological samples such as cheek cells and beef tendons. For the same sample, five images with different optical properties are obtained on the same imaging plane, which can simultaneously perform edge imaging, polarimetric imaging, and conventional microscope imaging. Benefiting from the ultrathin nature, compactness and multifunctionality of the optical metasurface devices, the integration does not excessively increase the volume of the optical system. With its promising capabilities and potential for expandability, we believe our microscope will herald an exciting new era in biomedical research. The ultrathin nature of optical metasurfaces and their unprecedented capability in light control have provided a compact platform to develop ultrathin optical devices with novel functionalities that are very difficult or impossible to achieve with conventional optics. The metasurface platform for polarization detection and manipulation is very attractive for diverse applications, including polarization sensing and imaging, optical communications, optical tweezers, quantum sciences, display technologies, and biomedical research as well as wearable and portable consumer electronics and optics where miniaturized systems are in high demand