RESUMEN
The temporal Talbot effect is a passive phenomenon that occurs when a periodic signal propagates through a dispersive medium with a quadratic phase response that modulates the output pulse repetition rate based on the input period. As previously proposed, this effect enables innovative applications such as passive amplification. However, its observation in the microwave regime has been impractical due to the requirement for controlled propagation through a highly dispersive waveguide. To overcome this challenge, we employed an ultra-wide band linearly chirped Bragg grating within a standard microwave X-Band waveguide. By utilizing backwards Talbot array illuminators aided by particle swarm optimization, we achieved passive amplification with a gain of 3.45 dB and 4.03 dB for gaussian and raised cosine pulses, respectively. Furthermore, we numerically verified that with higher quality substrates this gain can be theoretically increased to over 8 dB. Our work paves the way for numerous applications of the Talbot effect in the microwave regime, such as temporal cloaking, sub-noise microwave signal detection, microwave pulse shaping, and microwave noise reduction.
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Visual arts and entertainment related industries are continuously looking at promising innovative technologies to improve users' experience with state-of-the-art visualization platforms. This requires further developments on pixel resolution and device miniaturization which can be achieved, for instance, with high contrast materials, such as crystalline silicon (c-Si). Here, a new broadband stereoscopic hologram metasurface is introduced, where independent phase control is achieved for two orthogonal polarizations in the visible spectrum. The holograms are fabricated with a birefringent metasurface consisting of elliptical c-Si nanoposts on Sapphire substrate. Two holograms are combined on the same metasurface (one for each polarization) where each is encoded with four phase levels. The theoretical bandwidth is 110 nm with a signal to noise ratio (SNR) >15 dB. The stereoscopic view is obtained with a pair of cross-polarized filters in front of the observers' eyes. The measured transmission and diffraction efficiencies are about 70% and 15%, respectively, at 532 nm (the design wavelength). The metasurfaces are also investigated at 444.9 nm and 635 nm to experimentally assess their bandwidth performance. The stereoscopic effect is surprisingly good at 444.9 nm (and less so at 635 nm) with transmission and diffraction efficiencies around 70% and 18%, respectively.
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This paper reports on the first hologram in transmission mode based on a c-Si metasurface in the visible range. The hologram shows high fidelity and high efficiency, with measured transmission and diffraction efficiencies of ~65% and ~40%, respectively. Although originally designed to achieve full phase control in the range [0-2π] at 532 nm, these holograms have also performed well at 444.9 nm and 635 nm. The high tolerance to both fabrication and wavelength variations demonstrate that holograms based on c-Si metasurfaces are quite attractive for diffractive optics applications, and particularly for full-color holograms.
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In this paper, we propose a new hybrid network solution based on asynchronous optical code-division multiple-access (OCDMA) and free-space optical (FSO) technologies for last-mile access networks, where fiber deployment is impractical. The architecture of the proposed hybrid OCDMA-FSO network is thoroughly described. The users access the network in a fully asynchronous manner by means of assigned fast frequency hopping (FFH)-based codes. In the FSO receiver, an equal gain-combining technique is employed along with intensity modulation and direct detection. New analytical formalisms for evaluating the average bit error rate (ABER) performance are also proposed. These formalisms, based on the spatially correlated gamma-gamma statistical model, are derived considering three distinct scenarios, namely, uncorrelated, totally correlated, and partially correlated channels. Numerical results show that users can successfully achieve error-free ABER levels for the three scenarios considered as long as forward error correction (FEC) algorithms are employed. Therefore, OCDMA-FSO networks can be a prospective alternative to deliver high-speed communication services to access networks with deficient fiber infrastructure.
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The coupling of light between free space and thin film semiconductors is an essential requirement of modern optoelectronic technology. For monochromatic and single mode devices, high performance grating couplers have been developed that are well understood. For broadband and multimode devices, however, more complex structures, here referred to as "coupling surfaces", are required, which are often difficult to realise technologically. We identify general design rules based on the Fourier properties of the coupling surface and show how they can be used to determine the spatial resolution required for the coupler's fabrication. To our knowledge, this question has not been previously addressed, but it is important for the understanding of diffractive nanostructures and their technological realisation. We exemplify our insights with solar cells and UV photodetectors, where high-performance nanostructures that can be realised cost-effectively are essential.
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We design and fabricate efficient, narrow-band, transmission color filters whose operating principle resides in a narrow-band guided-mode resonance associated with a surface-plasmon resonance. The fundamental device consists of an aluminum grating over a 200-nm-thick aluminum oxide film on a glass substrate. Numerical simulations show a sharp resonance-derived spectral profile that is additionally shaped by a neighboring Rayleigh anomaly. Besides the Rayleigh effect, we show numerically that the narrow bandwidth is predominantly due to the low refractive-index contrast between the waveguide film and the substrate. Red, green, and blue filters are fabricated using ultraviolet holographic lithography followed by a lift-off process. The experimental spectral efficiency in transmission exceeds 80% with full-width-at-half-maximum linewidths near 20 nm. We provide color images of the zero-order transmitted spectra, and illustrate the pure colors associated with the modal resonance extracted as side-coupled output light.
RESUMEN
Silver and gold films with thicknesses in the range of 120-450 nm were evaporated onto glass substrates. A sequence of slits with widths varying between 70 and 270 nm was milled in the films using a focused gallium ion beam. We have undertaken high-resolution measurements of the optical transmission through the single slits with 488.0 nm (for Ag) and 632.8 nm (for Au) laser sources aligned to the optical axis of a microscope. Based on the present experimental results, it was possible to observe that (1) the slit transmission is notably affected by the film thickness, which presents a damped oscillatory behavior as the thickness is augmented, and (2) the transmission increases linearly with increasing slit width for a fixed film thickness.
RESUMEN
This paper carries out a rigorous analysis of supercontinuum generation in an improved highly asymmetric microstructured fiber (MF) design. This geometry, defined simply as D-MF, has the advantage of being produced with a regular stacking and drawing technology. We have obtained birefringence values on the order of 4.87x10(-3) at the adopted pump wavelength and a significantly smaller effective area when compared to a whole MF, which makes this fiber quite attractive for SCG. Therefore, this D-MF design is a promising alternative for SCG since it provides new degrees of freedom to control field confinement, birefringence, and dispersion characteristics of MFs.