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Low-cost spectroscopy has received a great deal of attention in recent years in applications such as food inspection, disease detection, and manufacturing. Current spectroscopic systems rely on multiple optical components, making them mechanically fragile systems. In our previous work, we demonstrated the use of Fourier filtering using thin dielectric films. The sampling effect from the cavity resonances can be used to decompose a signal into its Fourier components. Although the thin films were deposited directly on the face of the detectors, filters of varying thicknesses were needed, which required multiple lithographic processes. To overcome this challenge, in this work, we use a continuously variable filters deposited by a single-step electron-beam evaporation technique. We demonstrate a novel, to our knowledge, method that utilizes the glancing angle deposition technique with a continuously varying angle in order to produce tens of variable Fourier filters in a single deposition run. To prove this technique, we deposit this variable filter on a 38-channel linear detector and show the results from this device.

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Two-dimensional (2D) transition metal dichalcogenides (TMDCs) exhibit compelling dimension-dependent exciton-dominated optical behavior influenced by thickness and lateral quantum confinement effects. Thickness quantum confinement effects have been observed; however, experimental optical property assessment of nanoscale lateral dimension monolayer TMDCs is lacking. Here, we employ ex situ spectroscopic ellipsometry to evaluate laterally coalescing monolayer metalorganic chemical vapor deposited MoS2. A multisample analysis is used to constrain Bruggeman and Maxwell-Garnett effective medium approximations and the effective dielectric functions are derived for laterally coalesced and uncoalesced MoS2 films (∼10-94% surface coverage for ∼10-140 nm lateral grain sizes). This analysis demonstrates the ability to probe MoS2 optical exciton behavior at growth-relevant grain sizes in relation to chemical vapor nucleation density, ripening, and lateral growth conditions. Our analysis is pertinent toward expected in situ epitaxial 2D TMDC film growth metrology to enable the facile development of monolayer films with targeted process-dependent optical properties.

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We demonstrate the design, fabrication, and measurement of a switchable distributed Bragg reflector (DBR) that can be thermally switched from a close-to-zero reflective OFF state to a more than 70% reflection in its ON state. This is accomplished using a multilayer thin film stack using germanium (Ge) and the phase change material (PCM) Ge2Sb2Te5 (GST). The refractive indexes of Ge and GST in the amorphous state are closely matched, resulting in a nearly zero interface reflection. With appropriate antireflection coatings at the cavity ends, the overall reflection can be designed to be close to zero. When the GST is switched to the crystalline state, the refractive index contrast between the Ge and GST layers will increase dramatically contributing to the DBR reflection. Using this unique feature, we were able to design and experimentally demonstrate more than 70% reflection in the ON state and close to zero reflection in the OFF state at a wavelength of 2 µm.

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In this paper we present a tunable filter using Ge2Sb2Se4Te1 (GSST) phase change material. The design principle of the filter is based on a metal-insulator-metal (MIM) cavity operating in the reflection mode. This is intended for night vision applications that utilize 850nm as the illumination source. The filter allows us to selectively reject the 850nm band in one state. This is illustrated through several daytime and nighttime imaging applications.

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Interference Lithography (IL) is a powerful and inexpensive tool for large area precision nanoscale patterning of periodic structures. In this work we extend IL's capability to create features in arbitrary shapes and locations through the use of binary contact masks with wavefront division deep-UV interference lithography. Grating couplers for use in a streak measurement system and a focal plane division polarimeter are created to demonstrate the viability and versatility of the technique. Simultaneous fabrication of 90nm and 20µm features proves the potential of this process to simplify and streamline common fabrication processes in research and in industrial applications.

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We report the design, fabrication and characterization of a 1-by-5 geometric-phase polarization fan-out grating for coherent beam combining at 1550 nm. The phase profile of the grating is accurately controlled by the local orientation of the binary subwavelength structure instead of the etching depth and profile empowering the grating to be more tolerant to fabrication errors. Deep-UV interference lithography on silicon offers an inexpensive, highly efficient and high damage threshold solution to fabricating large-area fan-out gratings than electron beam lithography (EBL) and photoalignment liquid crystals. The theoretical and experimental diffraction efficiency of the grating is 87% and 85.7% respectively. Such a fan-out grating may find application to high-power beam combining in the infrared regime.

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Extrinsic or pseudo-chiral (meta)surfaces have an achiral structure, yet they can give rise to circular dichroism when the experiment itself becomes chiral. Although these surfaces are known to yield differences in reflected and transmitted circularly polarized light, the exact mechanism of the interaction has never been directly demonstrated. Here we present a comprehensive linear and nonlinear optical investigation of a metasurface composed of tilted gold nanowires. In the linear regime, we directly demonstrate the selective absorption of circularly polarised light depending on the orientation of the metasurface. In the nonlinear regime, we demonstrate for the first time how second harmonic generation circular dichroism in such extrinsic/pseudo-chiral materials can be understood in terms of effective nonlinear susceptibility tensor elements that switch sign depending on the orientation of the metasurface. By providing fundamental understanding of the chiroptical interactions in achiral metasurfaces, our work opens up new perspectives for the optimisation of their properties.

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We describe a new fabrication method for making wire-grid polarizers for the visible and near-IR based on deep-UV interference lithography, nanoimprint, and glancing angle deposition. We fabricated aluminum wire grids with periods ranging from 375 to 230 nm with heights from 145 to 110 nm, respectively. The measured extinction ratio was as high as 220:1 at 1064 nm. The performance of the polarizer is limited by the roughness and porosity of the Al film and the underlying SU-8 structure. This method allows patterning of wire grids on any substrate material, which makes this an attractive method for fabricating wire-grid micropolarizers in a timely and cost-effective manner.

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Current multispectral imaging systems use narrowband filters to capture the spectral content of a scene, which necessitates different filters to be designed for each application. In this paper, we demonstrate the concept of Fourier multispectral imaging which uses filters with sinusoidally varying transmittance. We designed and built these filters employing a single-cavity resonance, and made spectral measurements with a multispectral LED array. The measurements show that spectral features such as transmission and absorption peaks are preserved with this technique, which makes it a versatile technique than narrowband filters for a wide range of multispectral imaging applications.

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Here we present measurements on a metasurface composed by tilted gold nanowires. The metasurface can induce an optical chiral response of the whole sample when the light impinges on the sample out of the normal incidence angle. In order to investigate the symmetry breaking induced by the geometry, we measured the second harmonic generation (SHG) signal generated by circular polarized pulsed light. This technique has demonstrated to be a powerful method in order to investigate the chiral morphology of nanostructures.

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Fluorescence enhancement using photonic crystals can produce a significant improvement in the signal-to-noise ratio for single molecule and low molecule-concentration fluorescence imaging in biological and biochemical studies. In this Letter, a pixelated one-dimensional photonic band gap structure was designed to enhance both transverse electric and transverse magnetic polarizations through a spatially multiplexed photonic crystal resonance. The average enhancement of 15.6 and 17.9 fold were experimentally verified for the transverse and longitudinal fields on the same substrate. This device may be used as an optical platform for molecular orientation determination.

Asunto(s)

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We report the optical characterization of a metal wiregrid micropolarizer array for IR imaging polarimetry. The micropolarizers are designed for operation in the 1.5-5.0 microm band with a specially designed thin SiO(2) layer between the silicon substrate and the wiregrids to improve the performance at the shorter wavelengths. Deep-UV projection lithography is used to fabricate 140-nm-deep wiregrids with a 400 nm period. The extinction ratio and the transmission coefficient are measured with a tunable IR laser. A TM transmission coefficient greater than 70% with an extinction ratio greater than 10(4) is achieved for the midwave-IR region while maintaining an extinction ratio better than 10(2) for the near-IR region above 1.5 microm.

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We derived a temperature-dependent Sellmeier equation for Al(x)Ga(1-x)As material by measuring the refractive index of GaAs and AlAs with temperature dependence in a resonant cavity enhanced structure. The equation is applicable in the range of 1460-1580 nm and 26-86 degrees C and can be extrapolated to the other wavelengths and temperature ranges as well.

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We report two emission bands corresponding to the spectral line narrowing (SLN) of the conjugated polymer [2-methoxy-5-(2'-ethylhexyloxy)-1, 4-phenylenevinylene] (MEH-PPV) in films. The SLN emission coming from the polymer chains closer to the glass substrate are at a different spectral position compared to the chains that lay further away from the glass substrate. We explain this phenomenon as a direct consequence of the "gas-to-crystal" effect. In solution form, as concentration was increased, and thus the proportion of aggregates, a decrease in the SLN bandwidth and a red shift of the emission peak was observed.

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The dipole selection rule limits the maximum achievable efficiency in corrugated quantum well infrared photodetectors (C-QWIPs) to 50%. We consider what is believed to be a novel design that utilizes a resonant cavity enhancement technique to increase the efficiency beyond 50% by rotating the photon polarization at each pass around the cavity. Simulation results show that the quantum efficiency of this device can be enhanced up to 38% compared to that of the standard C-QWIP device.

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An experimental study of the lasing characteristics of photonic crystal lasers based on the conjugated polymer 2-methoxy-5-(2'- ethylhexyloxy)-1,4-phenylenevinylene (MEH-PPV) is reported in this letter. One and two dimensional (1D, 2D) photonic crystal structures were patterned on a glass substrate through interferometric lithography on photoresist layers. A 1.5 microm layer of polymethylglutarimide (PMGI) was deposited to prevent photoxidation of the polymer. Lasing action was observed under optically pumped conditions. Instabilities associated with pumping geometries were demonstrated in the case of 2D photonic crystal laser. As a result, the laser spectrum and threshold gain were found to be strongly dependent on the excitation geometry. The broad spectrum of the amplified spontaneous emission (ASE) allows laser tunability by engineering the effective refractive index of the devices or by controlling the periodicity of the photonic crystal.

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The resonant cavity enhanced (RCE) photodetectors is analyzed using the finite difference time domain (FDTD) method. Unlike the analytical models, FDTD includes all of the essential considerations such as the cavity build-up time, standing wave effect and the refractive index profiles across every layer. The fully numerical implementation allows it to be used as a verification of the analytical models. The simulation is demonstrated in terms of time and space enabling one to visualize how the field inside the cavity builds up. The results are compared with the analytical models to point out the subtle differences and assumptions made in the analytical models.