Results are presented for measurements of the total and polarized radiances from the ocean surface by a state-of-the-art snapshot hyperspectral imager, which simultaneously acquires spectra with 4nm spectral resolution in the wavelength range of 450-950nm within a 40° field-of-view. The imager does not require any along track movement and allows the continuous collection of hyperspectral imagery from stationary structures or slow moving platforms such as ships or helicopters. In addition, a computer controlled filter wheel is installed in front of the imager allowing for division-oftime Stokes vector images from the ocean surface. Experiments are aimed at the application of the multi-angular polarimetric measurements for the retrieval of water parameters in addition to the ones retrieved from traditional unpolarized methods. Several sets of measurements used in the analysis were acquired from ocean platform in the NYC area, Duck, NC and from shipborne observations in the Gulf of Mexico and along the Florida coast. Measurements made by the imager are compared with simulations using a vector radiative transfer code showing good agreement. Analysis of pixel-to-pixel variability of the total and polarized above water radiance for the viewing angles of 20°-60° in different wind conditions enable the estimation of uncertainties in measurements of these radiances in un-polarized and polarized modes for the whole spectral range, thus setting requirements for the quality of polarized measurements. Impacts of aerosols on spectral variations of both the radiance and the polarized terms of the Stokes vector are studied.
Results of measurements by a novel snapshot hyperspectral polarimetric imager are presented using several data sets acquired from ocean platforms. Based on the unique availability of the pixel-to-pixel total, sky and water leaving radiances at multiple wavelengths, variations of these parameters for wind-roughened surface are assessed and possible errors in measurements of these parameters are estimated. Measurements made by the imager are compared with coincident ones from the green-band SALSA Stokes vector imaging camera, a push-broom hyperspectral polarimetric imager operated by Naval Research Laboratory (NRL), and with simulations using a vector radiative transfer code, all demonstrating excellent agreement.
The behavior of the optical vortices with fractional topological charges in the far-field is assessed through numerical modeling and confirmed by experimental results. The generation of fractional topological charge variations of the phase within a Gaussian beam was achieved by using a liquid crystal spatial light modulator (LCoS SLM). It is shown that a laser beam carrying an optical vortex with a fractional topological charge evolves into a beam with a topological charge of integer value, specifically an integer value closer to the fractional number in the far field. A potential application of this work is for data transmission within optical telecommunication systems.
We propose plasmonic metal-insulator-metal (MIM) metamaterial designs for the sensing of two infrared wavelength bands, the mid-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) band by using a photon sorting technique. The proposed structures can capture light effectively on the metasurfaces based on coupling of free space energy to a subwavelength plasmonic mode. Photon sorting can be performed such that the incident light with a broad spectrum upon the metasurfaces can be "split" according to wavelength, channeling different spectral bands to different physical regions of the array on the surface where it is then absorbed by the insulator. Two different structures described in this work are (1) Square-type structure which consists of MIM resonators being periodically arranged to form a polarization independent sensor and (2) Meander-type structure which consists of MIM resonators being connected to form the meander shaped sensor. Mercury Cadmium Telluride (HgCdTe) posts are used as absorbing material within the MIM structure to generate free carriers and allow for collection of carrier charges. The proposed structures have compact designs and exhibit efficient light splitting and absorption for the IR spectral band. Structural and material properties, the electric field distribution and Poynting vector fields at the resonance frequencies are provided. Applications include thermal imaging, night vision systems, rifle sights, missile detection and discrimination, dual bandwidth optical filters, light trapping, and electromagnetically induced transparency.
A method to generate an optical metasurface is developed. In our experimental setup, we use a pump-probe technique,
where the pump beam is used to project patterns of v-shaped antennas on the surface of a silicon substrate. In the areas
illuminated with the images of v-shaped antennas electron-hole pairs are created. Therefore, the antenna structures on
silicon will have metallic-like properties, we classify this structure as a metasurface. The THz beam probes refraction
and reflection on the metasurface generated on the silicon substrate. The dynamic change of these patterns of
metasurface causes the beam steering effects of THz radiation.
An innovative method of examining properties of metasurfaces is presented. A pump-probe technique is used to create a metasurface composed of conductive shapes on a silicon surface. A wave-front of intense pulse of 82 fs from Ti:Sa laser with wavelength of 800 nm is shaped by a spatial light modulator and then focused into a preprogrammed array of vshaped features on a high purity float zone silicon substrate. The laser pulse generates electron-hole pairs on the silicon substrate, thus a metasurface consisting of an array of metal-like v-shaped antennas is inscribed on the silicon substrate. The lifetime of v-shaped antennas is in millisecond time range. In the meantime, the second, less intense pulse, also of wavelength 800 nm is converted to a pulse of terahertz radiation with a peak-power at wavelength approximately 800 μm and used to probe the metasurface inscribed in the silicon. Tracing the position of the refracted terahertz beam is achieved with a specially designed INO video camera for terahertz radiation.
Two-dimensional compound gratings (2dCGs) are capable of π-radian difference phase resonances (PRs). Circulation and concentration of s- and p-polarized light incident on 2dCG metal structures are studied. In prior work, it has been shown that PRs can occur for s- and p-polarized light in one-dimensional compound gratings (ldCGs). In contrast, the structure studied in this work has two asymmetric holes in the unit cell, each filled with a material of high dielectric permittivity (Epsilon=l0.84) and can support PRs in 2dCGs in the spectral range from 8 to 12 GHz. Due to asymmetry within the system, the two apertures react differently to the incident light and support polarization dependent PRs that are resonantly excited within the apertures. It is shown that PRs occur in 2dCGs with similar characteristics of ldCGs, such as having narrow bandwidths, high Q values, and high concentrations of electromagnetic fields. However, PRs occurring on 2dCGs have a benefit of manipulating in more numerous ways as compared with ldCGs. As the incident light excites waveguide cavity modes, the fields in the corresponding neighboring cavities in 2dCGs are coupled by circulations of counter-propagating modes and the π-radian phase differences produce a concentration and narrowband inversion of the transmissivity/opacity. The dependencies of bandwidth and wavelength of the PRs on structural and material properties, polarization of the incident beam, as well as the Poynting vector fields are described. Applications include narrow bandwidth optical filters, light trapping, antireflection coatings, waveguiding structures, and electromagnetically induced transparency.
Optical metasurfaces demonstrate outstanding capabilities of optical parameters modifications by changes in the structural architecture at the nano-scale level. We demonstrate results of electrophoretic experiments that modify the structure of a metasurface by using diamond nanoparticles with sizes much smaller than the wavelength of light; the nanoparticles are suspended in an aqueous solution and a uniform electric field is applied. The electric field controls the concentration of nanoparticles inside the sub-wavelength apertures and on the top plane of the metasurface. The higher concentration of diamond nanoparticles increases the refractive index of the suspension as well as increasing scattering and absorption. Results of optical material parameter characterization for a wavelength of 512 nm are provided for different concentrations of the diamond nanoparticles dispersions.
Photolithography for patterns with periodicity in the illumination plane (2.5-D lithography) has seen rapid advances over the past decade, with the introduction of holographic lithography and the further development of phase-contrast and grayscale photolithography methods. However, each of these techniques suffers from substantial difficulties preventing further integration into device fabrication: a lack of parallel processing capabilities and dimension limitations. Here, we present a demonstration of controlled layer topography through modulation of both the exposure dose and exposure focal plane yielding reproducible 2.5-D patterns which are applied to the further development of plasmonic gratings. This process is entirely compatible with commercially available i -line photolithography and etch hardware, enabling a path to ready integration.
Optical metamaterials capture the imagination with potential applications such as sub-wave imaging, invisibility
cloaking and solar energy collection. The challenge is to learn how to construct and reconfigure a metamaterial with a
spatially varying refractive index. We describe an approach based on colloidal dispersion of metal nano-rods in a
dielectric fluid placed in a non-uniform electric field. Because of the dielectrophoretic effect, the nano-rods accumulate
in the regions with the maximum field and align along the field lines. High concentration of nano-rods lowers the
effective local refractive index of the dispersion. The nano-rods are much smaller than the wavelength of light. We
illustrate the approach with a dispersion of gold nanorods (length 40-70 nm, diameter 10-20 nm) in toluene, using flat
and cylindrical cells. In the first case, the electric field is created by two mutually perpendicular electrodes, in the second
case, it is created by two coaxial electrodes. When the field is applied, the initially isotropic dispersion of nanorods
transforms into birefringent orientationally ordered structures with the director following the electric field lines. We
describe how the optical properties of the field-induced structures are controlled by dichroism and birefringence of the
dispersion and determine the spatial variation of the field-induced optical phase retardation. In cylindrical capillaries, the
index gradient bends lights around the central electrode, thus reducing its visibility. Our approach can be used as a
starting point for the development of self-assembled and reconfigurable optical metamaterials with optical properties
controlled by the dielectrophoretic effect on submicron scales.
We developed an electrically switchable mirror based on polymer-stabilized, short-pitched
cholesteric liquid crystals using electro-optical cells with planar alignment. The devices enable the switching
of a pre-selected reflective wavelength of the cholesteric to reflect a different wavelength in corresponding to
the magnitude of applied electric field. The principle of the wavelength shift to a shorter wavelength is a
result of field-induced pitch shortening near the boundaries. The spectral wavelength shift of the reflected
wavelength is about 140-nm and the wavelength shift is linearly proportional to the magnitude of applied
voltage. The optical response of the device is also studied.
We describe the application of smectic A (SmA) liquid crystals for beam deflection. SmA materials can be used in digital beam deflectors (DBDs) as fillers for passive birefringent prisms. SmA prisms have high birefringence and can be constructed in a variety of shapes, including single prisms and prismatic blazed gratings of different angles and profiles. We address the challenges of uniform alignment of SmA, such as elimination of focal conic domains. Fast rotation of the incident light polarization in DBDs is achieved by an electrically switched 90-deg twisted nematic (TN) cell.
We developed a broad band achromatic linear polarization switch for visible and near infrared radiation. The achromatic switch is based on a twisted nematic cell filled with a dual-frequency nematic material. The switch is capable of fast switching linearly polarized light between two orthogonal directions with a switching time in the millisecond range.
In order to develop a fine angular beam steering technique (milliradian and less), we propose a nematic liquid crystal cell with a continuous gradient of the refractive index. This continuous gradient is controlled by applying the driving voltage to non-patterned indium-tin oxide electrodes. We employed the dual-frequency nematic liquid crystal in the cell with high pretilt alignment. The experiments with dual-frequency nematic confirmed that non-patterned electrically controlled nematic cell with the continuous gradient of refractive index is capable of angular beam steering in the milliradian range.
We demonstrate a fast optical modulator capable of switching large amount of optical retardation (a few microns) in less than 1 ms. The result is achieved by employing a dual frequency nematic in cells with high pre-tile alignment and by providing a special addressing scheme that features amplitude and frequency modulated voltage. We explore the effect of time delay and dielectric heating in process of the optical retardation switching.
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