Parabolic Lenses Research Articles

Blackbody-cavity ideal absorbers for solar energy harvesting

Spectrally selective solar absorbers (SSAs), which harvest heat from sunlight, are the key to concentrated solar thermal systems. An ideal SSA must have an absorptivity of unity in the solar irradiance wavelength region (0.3–2.5 upmu m), and its infrared thermal emissivity must be zero to depress spontaneous blackbody irradiation (2.5–25 upmu m). Current SSA designs which utilize photonic crystals, metamaterials, or cermets are either cost-inefficient due to the complexity of the required nanofabrication methods, or have limited applicability due to poor thermal stability at high temperatures. We conceptually present blackbody-cavity solar absorber designs with nearly ideal spectrally selective properties, capable of being manufactured at scale. The theoretical analyses show that the unity solar absorptivity of the blackbody cavity and nearly zero infrared emissivity of the SSA’s outer surface allow for a stagnation temperature of 880 ^circ C under 10 suns. The performance surpasses state-of-the-art SSAs manufactured using nanofabrication methods. This design relies only on traditional fabrication methods, such as machining, casting, and polishing. This makes it suitable for large-scale industrial applications, and the “blackbody cavity” feature enables easy integration with existing concentrated solar thermal systems using the parabolic reflector and Fresnel lens as optical concentrators.

A large field-of-view high-resolution hard x-ray microscope using polymer optics.

We present an effective approach using a matched pair of polymer-based condenser-objective lenses to build a compact full-field x-ray microscope with a high spatial resolution. A unique condenser comprising arrays of high-aspect-ratio prisms with equilateral cross section is used for uniformly illuminating samples over a large field of view (FOV) from all angles, which match the acceptance of an objective made of interdigitated orthogonal rows of one-dimensional lenses. State-of-the-art Talbot grating interferometry used to characterize these lenses for the first time revealed excellent focusing properties and minimal wavefront distortions. Using a specific lens pair designed for 20 keV x rays, short-exposure times, and image registration with a cross-correlation technique, we circumvent vibrational instabilities to obtain distortion-free images with a uniform resolution of 240 nm (smallest resolvable line pair) over a large FOV, 80 × 80 µm2 in extent. The results were contrasted with those collected using commercial two-dimensional parabolic lenses with a smaller FOV. This approach implemented on a diffractometer would enable diffraction-contrast or dark-field microscopy for fast observations of "mesoscopic" phenomena in real space complementing reciprocal-space studies using diffraction on the same instrument.

Diffractive tunable lens for remote focusing in high-NA optical systems.

Remote focusing means to translate the focus position of an imaging system along the optical axis without moving the objective lens. The concept gains increasing importance as it allows for quick 3D focus steering in scanning microscopes, leaves the sample region unperturbed and is compatible with conjugated adaptive optics. Here we present a novel remote focusing approach that can be used in conjunction with high numerical aperture optics. Our method is based on a pair of diffractive elements, which jointly act as a tunable auxiliary lens. By changing the mutual rotation angle between the two elements, we demonstrate an axial translation of the focal spot produced by a NA = 0.95 air objective (corresponding to NA = 1.44 for an oil immersion lens) over more than 140 µm with largely maintained focus quality. We experimentally show that for the task of focus shifting, the wavefront produced by the high-NA design is superior to those produced by a parabolic lens design or a regular achromatic lens doublet.

Mimicking an expanding universe by optical interference in a helicoid waveguide.

According to modern cosmology, expansion of the universe is due to the metric changing of spacetime itself. Here, we propose to mimic an expanding universe by utilizing optical interference and helicoid waveguides. The evolution of interference pattern in the helicoid waveguide is investigated theoretically and experimentally. For precise measurements, we design an air helicoid waveguide which allows us to investigate the wave front of laser beams from the waveguide. Redshift of a Gaussian wave packet in the expanding universe is demonstrated with high precision, showing that the helicoid waveguide acts as a parabolic gradient index lens exactly. The proposed waveguide structure can be used as an efficient waveguide adapter.

Beam scanning antenna based on parabolic phase distribution lenses

In this study, the authors present a beam scanning lens antenna for 30 GHz applications, which is composed of a feed horn and a planar lens with parabolic phase distribution. Due to the parabolic phase compensation, the lens antenna can perform the scanning coverage of ±60° with a gain tolerance of 3 dB. The fabricated lens has a size of 10λ0 × λ0 × 0.2λ0 (λ0 is wavelength at the centre frequency of 30 GHz) and the focal length of 24 mm. The beam steering mechanism and the phase error of the lens antenna are analysed in detail. The total system has the advantages of wide scan angle, low profile and lightweight, and it can be readily applied to multi-beam or beam-scanning antenna designs.

Energy and optical analysis of photovoltaic thermal integrated with rotary linear curved Fresnel lens inside a Chinese solar greenhouse

In this paper, a cylindrical Fresnel lens concentrating photovoltaic/thermal (CPV/T) system is established in the non-planting area of the Chinese solar greenhouse. The system changes as the solar elevation angle changes between the summer solstice and the winter solstice, ensuring that the light is incident perpendicular to the lens. A row of Fresnel lens at this location does not block the photosynthetically active radiation of the plants in the planting area, and only reduces part of the direct light from the north wall of the greenhouse. The space utilization rate of non-planting space is 18.2%. Through optical simulation, the acceptance rate has little effect when the axial incident angle is within 10°. In contrast, the concentrating performance of the cylindrical Fresnel lens is better than that of the elliptical and parabolic Fresnel lenses. In actual weather, an experimental study was conducted on a concentrating photovoltaic/thermal system in which a gallium arsenide concentrating cell was used as a receiver. The main parameters such as temperature distribution of the concentrating photovoltaic/photothermal system receiver, output power and thermal power, utilization efficiency of electric energy and thermal energy were tested. The test results under clear sky conditions show that the maximum power generation efficiency is about 18% at noon (11:00–13:00), the maximum thermal efficiency of cooling water is about 45%, and the total efficiency of thermal and electrical power is about 55%. Combined with facility agricultural engineering, it comprehensively solves the problem of land resource utilization and comprehensive utilization efficiency of solar energy system.

High-Efficiency Metasurfaces with 2π Phase Control Based on Aperiodic Dielectric Nanoarrays.

In this study, the high-efficiency phase control Si metasurfaces are investigated based on aperiodic nanoarrays unlike widely-used period structures, the aperiodicity of which providing additional freedom to improve metasurfaces’ performance. Firstly, the phase control mechanism of Huygens nanoblocks is demonstrated, particularly the internal electromagnetic resonances and the manipulation of effective electrical/magnetic polarizabilities. Then, a group of high-transmission Si nanoblocks with 2π phase control is sought by sweeping the geometrical parameters. Finally, several metasurfaces, such as grating and parabolic lens, are numerically realized by the nanostructures with high efficiency. The conversion efficiency of the grating reaches 80%, and the focusing conversion efficiency of the metalens is 99.3%. The results show that the high-efficiency phase control metasurfaces can be realized based on aperiodic nanoarrays, i.e., additional design freedom.

Method and system for the centrifugal fabrication of low cost, polymeric, parabolic lenses.

A method for low-cost, rapid production of aspheric polymeric lenses using fluid rotation is presented. The system utilizes a cylindrical chamber to hold and cure a polymer while spinning. This system is capable of producing lenses with parabolic planar-concave, planar-convex, and meniscus geometries with tunable radii of curvature and focal lengths. Examples are demonstrated for lenses of 25 mm diameter. System models, performance, and components are described in detail, and lens variability is assessed for surface profile, surface roughness, radius of curvature, and single-lens resolution limit. Results show excellent RMS surface roughness for a low-cost lens production technique.

CRL-based ultra-compact transfocator for X-ray focusing and microscopy.

A new ultra-compact transfocator (UCTF) based on X-ray compound refractive lenses (CRLs) is presented. The device can be used to change the number of one- and two-dimensional focusing CRLs by moving the individual parabolic lenses one-by-one independently, thus providing permanent energy and focal-length tunability for scanning and full-field X-ray microscopy applications. The small overall size and light weight of the device allow it to be integrated in any synchrotron beamline, while even simplifying the experimental layout. The UCTF was tested at the Excillium MetalJet microfocus X-ray source and at theP14 EMBL (PETRA-III) beamline, demonstrating high mechanical stability and lens positioning repeatability.

Method to reduce the aberration of a polygonal aperture focus-tunable lens array for high fill factor.

Focus-tunable liquid lens arrays have been used to improve depth in glass-free three-dimensional devices where high fill factor is required. For high fill factors, polygonal apertures should be used, but due to the difficulty of controlling aberrations during focus tuning, polygonal aperture focus-tunable lenses have rarely been studied. This Letter proposes a new structure with walls at the lens boundaries using a square parabolic lens model to control the quadrafoil aberration induced by the square aperture. ANSYS simulation confirmed that the lens surface was closest to the parabolic lens model when the wall thickness was 1.4mm. A fabricated lens array demonstrated the quadrafoil inhibitory effect and confirmed the utility of the new structure.

Embedded parabolic fiber lens for efficient fiber-to-waveguide coupling fabricated by focused ion beam

This work demonstrates an approach for simplifying fiber-to-chip (edge coupling) packaging by virtually eliminating the longitudinal alignment procedure (also increasing compactness and efficiency) through a fiber lens embedded into the structure of the fiber itself. A parabolic lens, fabricated using focused ion beam milling, with a diameter of 15 μm and height of 5 μm, was embedded 6.5 μm (the working distance of the parabolic lens) below the endfacet of the fiber. The lens focuses a 10.4 μm fiber mode into a spot size of 2.6 μm on the surface of an SMF-28e single-mode optical fiber. The properties of the fabricated lens were studied using the three-dimensional finite-difference time-domain numerical method, and the optimal parameters for maximizing the coupling conditions were extracted. The conversion loss of the lens is estimated to be around . The insertion loss and lateral alignment of the proposed parabolic lens is comparable to a commercial lensed fiber, while directly ensuring the longitudinal alignment, easing the angular alignment, and providing additional mechanical and environmental robustness.

Modelling of distribution of circular beams of Airy in parabolic fiber

Recently, the attention of researchers has been turned to various beams possessing the property of autofocusing, among which are circular beams of Airy, beams of Pierce, hypergeometric and other beams. The sharp autofocusing property inherent in the above beams is very useful in optical manipulation, useful for multiphoton polymerization, is used for nonlinear effects and for polarization transformations. A classic focusing element is a lens that has a quadratic dependence on the radius. The diffractive version of the parabolic lens has radial lines, condensing to the edge of the optical element as a linear chirp. This structure can be obtained by “twirling” the Airy function along the circumference with displacement and scaling. The study of the behavior of various types of self-focusing laser beams in parabolic environments extends the spectrum of optical signals used for telecommunications. In particular, a fractional Fourier transform is used to describe fibers with a parabolic refractive index. In this paper we consider circular Airy beams, which have a radial dependence. Modelling of the passage of these beams through an optical fiber with a parabolic change in the refractive index was performed on the basis of the use of a fractional Fourier transform.

Reception Power Pattern of Distributed Absorbers in Focal Plane Arrays: A Fourier Optics Analysis

Passive imaging cameras at millimeter and sub-millimeter wavelengths are currently entering a new era with the development of large format arrays of direct detectors. Several of these arrays are being developed with bare absorbing meshes without any antenna coupling (lens or horn) structures. The design of such arrays is typically done resorting to geometrical considerations or basic broadside plane wave incidence analysis. This paper presents a spectral technique for the analysis of such focal plane arrays in reception using Fourier Optics, which is valid also for moderately skewed incident angles. The analysis constitutes a step improvement with respect to previously used methods by providing an accurate and efficient way to estimate the point-source angular response and the throughput from a distributed incoherent source of an absorbing mesh in the focal plane of a quasi-optical component (e.g., a parabolic reflector or lens). The proposed technique is validated with full-wave simulations. After presenting the analysis, this paper compares the performance of arrays of bare absorber in the focal plane of a quasi-optical component to those of corresponding antenna based arrays. It is found that absorbers lead to a comparable tradeoff, in terms of spill-over and focusing efficiency, only for very tight samplings. For larger samplings, the focusing efficiency of absorbers is significantly lower than the one for antennas.

Scalable, large area compound array refractive lens for hard X-rays

We demonstrate the fabrication of a 2D Compound Array Refractive Lens (CARL) for multi-contrast X-ray imaging. The CARL consists of six stacked polyimide foils with each displaying a 2D array of lenses with a 65 μm pitch aiming for a sensitivity on sub-micrometer structures with a (few-)micrometer resolution in sensing through phase and scattering contrast at multiple keV. The parabolic lenses are formed by indents in the foils by a paraboloid needle. The ability for fast single-exposure multi-contrast imaging is demonstrated by filming the kinetics of pulsed laser ablation in liquid. The three contrast channels, absorption, differential phase, and scattering, are imaged with a time resolution of 25 μs. By changing the sample-detector distance, it is possible to distinguish between nanoparticles and microbubbles.

Birefringence detection of a gradient-index lens based on astigmatic transformation of a Bessel beam

A method is proposed for detecting birefringence of mechanically free parabolic gradient-index lenses based on the astigmatic transformation of the zero-order Bessel beam. Numerical simulation shows a clearly visible distortion of the intensity structure in the propagation of scalable astigmatic Bessel beams. The degree of astigmatism can be determined from the intensity distribution of the distorted beam. Similar patterns are also obtained in an experimental study of the passage of a Bessel beam through a quarter-pitch gradient-index lens, which indicates the presence of birefringence of the lens material. Comparison of experimental data and numerical simulation shows the possibility of measuring the optical path difference of no worse than 0.05λ0 between ordinary and extraordinary beams.

Efficiency and coherence preservation studies of Be refractive lenses for XFELO application.

Performance tests of parabolic beryllium refractive lenses, considered as X-ray focusing elements in the future X-ray free-electron laser oscillator (XFELO), are reported. Single and double refractive lenses were subject to X-ray tests, which included: surface profile, transmissivity measurements, imaging capabilities and wavefront distortion with grating interferometry. Optical metrology revealed that surface profiles were close to the design specification in terms of the figure and roughness. The transmissivity of the lenses is >94% at 8 keV and >98% at 14.4 and 18 keV. These values are close to the theoretical values of ideal lenses. Images of the bending-magnet source obtained with the lenses were close to the expected ones and did not show any significant distortion. Grating interferometry revealed that the possible wavefront distortions produced by surface and bulk lens imperfections were on the level of ∼λ/60 for 8 keV photons. Thus the Be lenses can be succesfully used as focusing and beam collimating elements in the XFELO.

Nanofocusing with aberration-corrected rotationallyparabolic refractive X-ray lenses.

Wavefront errors of rotationally parabolic refractive X-ray lenses made of beryllium (Be CRLs) have been recovered for various lens sets and X-ray beam configurations. Due to manufacturing via an embossing process, aberrations of individual lenses within the investigated ensemble are very similar. By deriving a mean single-lens deformation for the ensemble, aberrations of any arbitrary lens stack can be predicted from the ensemble with \bar{\sigma} = 0.034λ. Using these findings the expected focusing performance of current Be CRLs are modeled for relevant X-ray energies and bandwidths and it is shown that a correction of aberrations can be realised without prior lens characterization but simply based on the derived lens deformation. The performance of aberration-corrected Be CRLs is discussed and the applicability of aberration-correction demonstrated over wide X-ray energy ranges.

Design and analysis of novel two-stage optical receiving antenna for indoor visible light communication technology

White light emitting diode has become the next generation of light source because of its high illuminance efficiency, low power consumption, and long life, and it has also been adopted in the application of indoor visible light communication (VLC) system. The VLC has great development potentials, however, there is a lack of research on optical receiving antenna which is a key component of VLC. Therefore, in this paper we design a novel two-stage optical receiving antenna for indoor VLC system. In the designed antenna, the lens wall of a compound parabolic concentrator with a certain rotation angle and thickness is obtained through rotating the parabolic reflector cross-section reference curve. Furthermore, a novel two-stage optical receiving antenna is designed by taking advantage of lens-wall compound parabolic concentrators and hemispherical lenses. This significantly increases the view angle by nearly 20 in the case of gain retention. The analytical model of the optical antenna in a 5 m5 m3 m open room is established by using a software of TraceProTM. The indoor VLC system is also modelled and implemented by using a software of MatlabTM. The results show that the growth rate of average received power is 757.2%, which is 5.62 times that of the compound parabolic concentrator, and the signal-noise-ratio is increased by 28.07%, on average, which is 1.67 times that of the compound parabolic concentrator. The optical gain of the two-stage optical receiving antenna is 11.49, which is 2.81 times that of the compound parabolic concentrator. The spot radius is only 2.5 mm, which is reduced by nearly 37.5% compared with the spot radius of the compound parabolic concentrator, and the energy concentration is evenly distributed at the same time. This further confirms that the designed two-stage optical receiving antenna is suitable for indoor VLC system. Finally, the performance analysis and experimental verification of the new two-stage optical receiver antenna are also given.

Ultrasmooth metal thin films on curved fused silica by laser polishing

The fabrication of atomically smooth metal films on supporting oxides is a quite demanding task, since most physical vapor deposition methods used on metals do not work properly on oxide substrates. Here, we report an alternative procedure, based on performing laser polishing of a fused silica substrate before depositing the metallic thin film. This reduces the RMS surface roughness of fused silica by ca. 33%, and increases the maximum grain size of the metallic film from 200 nm to 1200 nm. The method has been applied to a fused silica parabolic lens, which has been coated with a graphene-terminated Ru thin film. The reduction of surface roughness caused by laser polishing leads to the formation of ultrasmooth Ru thin films. Crystallinity and subnanometer roughness of the metal coating are demonstrated by the observation of He diffraction from a macroscopically curved surface.

Prototype of Dense-array Concentrator Photovoltaic System Using Non-imaging Dish Concentrators and Cross Compound Parabolic Concentrator

Abstract The paper presents a novel dense-array concentrator photovoltaic prototype system consisted of primary non-imaging dish concentrator and secondary concentrator (an array of crossed compound parabolic lenses). The secondary optics has a geometrical concentration ratio of 5.998 but the average measured concentration ratio is only 4.07 suns. The losses are mainly attributed to both the Fresnel reflection at the interfaces and absorption of the dielectric material. The preliminary measured power conversion efficiency of the system is 17% due to significant optical loss incurred by the secondary concentrator.