Frequency-selective subwavelength-size nanophotonic devices for snapshot spectral imaging
Keywords: Electrical Engineering.

From imaging to spectroscopy, in many areas of science and technology, it is necessary to use frequency selective devices to detect spectral components of light. To achieve spectral detection, most imaging systems in snapshot imaging, where the detection of spectral and spatial information happens simultaneously, either use techniques that are not photon efficient or structures that are larger in size than their operating wavelength. Towards the goal of obtaining smaller snapshot imaging systems, it is desired to achieve subwavelength size, photon efficient, spectrally selective devices. In this dissertation, we show different frequency selective subwavelength size nanophotonic approaches that are photon efficient. Firstly, we show a novel device, spectral light separator, which simultaneously separates different spectral components of light into different locations in a subwavelength size device without being dependent on periodicity in its operation. For its operation, the device utilizes a sequence of deep subwavelength slits in metallic films. Each slit supports a resonance, and at the resonance wavelength, it has an electromagnetic cross section that is much larger than its physical size. Moreover, the resonant frequency of each slit is controlled by slit height through a Fabry Pérot like resonance mechanism. Secondly, we demonstrate a planar spectral light separator, which is an ultrathin device where the thickness of this device is deep subwavelength in size. For its operation, this device uses an assemble of subwavelength size rectangular apertures in a planar metallic film. Each aperture in resonance has an electromagnetic cross section that is much larger than its physical size and the frequency of the light collected in each aperture is controlled by aperture length through a Fabry Pérot like resonance mechanism. We show that this device achieves spectral separation both in an isolated (non periodic) configuration as well as in an array configuration. The device operation is largely independent of the incidence angle of light. Additionally, while the device achieves spectral separation without being dependent on the polarization, it can also simultaneously separate different linear polarization components of light. Lastly, we show an integrated multilayer color separator, which is a snapshot photon absorbing device for color imaging. This device is based on silicon and operates in the visible wavelength range. In this device, three vertically stacked, nanotextured silicon layers are separated from each other by an insulator. Each silicon layer effectively absorbs a different color band; hence, this subwavelength size, photon efficient, integrated device does not need to use a separate color filter for its operation. The gratings in the device are used to suppress reflections or to perform light trapping in order to enhance photon efficiency and spectral selectivity. This device concept offers an engineerable spectra with many available design parameters from thickness and material selection for each layer to the choices of gratings for each surface.