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Discusses developments in the field of gradient index optical lenses for resolving problems of lens spherical abberration. (SL)

The annular folded lens (AFL) is a design form offering large aperture, high-resolution imaging in a very axially compact package. The folded optic can be made monolithic for easier fabrication and alignment, yet the introduction of refractive surfaces with a dispersive optical material gives way to chromatic aberrations. AFL designs using homogeneous media are generally limited to the monochromatic regime with polychromatic performance greatly reduced. By introducing freeform gradient-index (F-GRIN) media, monolithic AFL designs can achieve higher monochromatic performance as well as provide color correction for diffraction-limited polychromatic imaging. Monochromatic and polychromatic design methodologies are surveyed where the F-GRIN is constrained to remain feasible for fabrication.

A raytrace method is developed for evaluation of progressive addition lens (PAL) designs with freeform surfaces and gradient index (GRIN) profiles. The raytrace analysis is adapted from a Hartmann test method. More common surface-geometric evaluations for freeform surface PAL designs are presented for comparison. Sur- face geometric and real raytrace methods are compared for evaluation of homogeneous freeform surface PALs. Raytrace evaluation is compared with evaluation results from commercial lens design software for single vision lenses. A GRIN PAL base design is evaluated using the real raytrace method, and characteristic PAL behavior is demonstrated. This raytrace evaluation method enables new directions in using GRIN for ophthalmic lenses.

Annular folded lenses (AFLs) offer high resolution monochromatic imaging in a low telephoto ratio package. Monolithic designs offer advantages but are inflicted with chromatic aberrations. Applying freeform gradient- index media enables more advanced, diffraction-limited monochromatic AFL designs. By optimizing the GRIN profile along with its dispersion, chromatic aberrations can also be corrected, granting high performing poly- chromatic designs.

In an era of new developments in nanomaterials analysis enabled by the unprecedented spatial and energy resolutions of electron energy-loss spectroscopy in the scanning transmission electron microscope (STEM-EELS), it remains that the vast majority of works concern collective or single-particle excitations that are well described by the electrostatic approximation, which neglects retardation and magnetic field effects. Here we demonstrate a simple case in which that approximation is fundamentally inadequate. When the beam energy is above the Cherenkov threshold and the geometric dimensions of the nanomaterial sample are on the order of the wavelength of light in the material, spatial variations in low-loss (less than or similar to 5 eV) spectral maps from guided light modes may be observed. We demonstrate such observations for amorphous silicon disks and offer an interpretation of the results based on the waveguide modes of a cylinder. We also demonstrate explicitly that spatial variations from waveguide modes are manifest in analytic models for the especially simple geometry of a STEM beam penetrating a dielectric ribbon. We discuss how these modes relate to those that have been observed more generally in dielectric nanomaterials.

This guest editorial introduces the Special Section on Advances in Gradient Index Optics Technology.

Conventional optical designs with gradient index (GRIN) use rotationally-invariant GRIN profiles described by polynomials with no orthogonality. These GRIN profiles have limited effectiveness at correcting aberrations from tilted/decentered or freeform systems. In this paper, a three-dimensional orthogonal polynomial basis set (the FGRIN basis) is proposed, which enables the design of GRIN profiles with both rotational and axial variations. The FGRIN basis is then demonstrated via the design of a 3D GRIN corrector plate targeted to correct the rotationally-variant aberrations induced from a tilted spherical mirror. A sample corrector is manufactured and tested, showing significant correction of astigmatism. The FGRIN basis opens a new design space of 3D rotational variant GRIN profiles, which has the potential of replacing multiple freeform surfaces and simplifying complex systems.

Raytrace evaluation capable of evaluating progressive addition lens (PAL) designs with freeform surface and gradient index (GRIN) contributions is presented. The method is validated on an analytically generated PAL start design and on optimized surface designs. Surface raytrace evaluations are compared with the surface-geometric evaluation commonly presented for freeform surface PAL designs. The evaluation is also tested on analytically generated freeform GRIN PAL designs with spherical and plano surfaces. The raytrace method agrees with the analytic performance and surface-geometric performance near the center of the lens and deviates at the edge of the lens, due to ray obliquity with the surfaces and aggregate contributions of surfaces and/or GRIN. These deviations are expected, as the raytrace model accounts for more physical contributions to optical performance, including pupil diameter and eye position. This raytrace method enables the evaluation of lens performance contributions other than from polished surfaces on homogeneous materials, enabling further exploration of GRIN in PAL designs.

Freeform optics enable irregular system geometries and high optical performance by leveraging rotational variance. To this point, for both imaging and illumination, freeform optics has largely been synonymous with freeform surfaces. Here a new frontier in freeform optics is surveyed in the form of freeform gradient-index (F-GRIN) media. F-GRIN leverages arbitrary three-dimensional refractive index distributions to impart unique optical influence. When transversely variant, F-GRIN behaves similarly to freeform surfaces. By introducing a longitudinal refractive index variation as well, F-GRIN optical behavior deviates from that of freeform surfaces due to the effect of volume propagation. F-GRIN is a useful design tool that offers vast degrees of freedom and serves as an important complement to freeform surfaces in the design of advanced optical systems for both imaging and illumination.

The ability to obtain three-dimensional (3-D) information about morphologies of nanostructures elucidates many interesting properties of materials in both physical and biological sciences. Here we demonstrate a novel method in scanning transmission electron microscopy (STEM) that gives a fast and reliable assessment of the 3-D configuration of curvilinear nanostructures, all without needing to tilt the sample through an arc. Using one-dimensional crystalline defects known as dislocations as a prototypical example of a complex curvilinear object, we demonstrate their 3-D reconstruction two orders of magnitude faster than by standard tilt-arc TEM tomographic techniques, from data recorded by selecting different ray paths of the convergent STEM probe. Due to its speed and immunity to problems associated with a tilt arc, the tilt-less 3-D imaging offers important advantages for investigations of radiation-sensitive, polycrystalline, or magnetic materials. Further, by using a segmented detector, the total electron dose is reduced to a single STEM raster scan acquisition; our tilt-less approach will therefore open new avenues for real-time 3-D electron imaging of dynamic processes., Scientific Reports, 7, ISSN:2045-2322

The dn/dT of the IR Chalcogenide glass is measured in LWIR by using a Twyman-Green Interferometer. The measured sample is a round disk with parallel optically flat surfaces. The results are provided from -53°C ~32°C.

Scanning confocal Raman microscopy is proposed to measure a gradient index (GRIN) profile at an optical surface. The Raman microscope is calibrated to index of refraction for a binary copolymer GRIN material, and then the index of refraction is mapped on the plano surface of a GRIN polymer lens. The measurement deduces axial shift of 680 μm and identifies lateral tilt or decenter with respect to the nominal position of the GRIN profile. Results suggest that the mapping method is a nondestructive way to measure the GRIN profile of a GRIN lens and its positioning within the lens geometry, to within the sampling precision of the Raman microscope.

A typical light field virtual reality head-mounted display (VR HMD) is comprised of a lenslet array and a display for each eye. An array of tiled subobjects shown on the display reconstructs the light field through the lenslet array, and the light field is synthesized into one image on the retina. In this paper, we present a novel compact design of binocular spatially multiplexed light field display system for VR HMD. Contrary to the flat lenslet array and flat display used in current light field displays, the proposed design explores the viability of combining a concentric curved lenslet array and curved display with optimized lenslet shape, size and spacing. The design of placing lenslet array on a spherical surface is investigated and the specification tradeoffs are shown. The system displays highest resolution at the direction wherever the eye gazes. The design form is thin and lightweight compared to most other VR optical technologies. Furthermore, the use of a curved display reduces the complexity of optical design and wastes fewer pixels between subobjects. The design simultaneously achieves a wide field of view, high spatial resolution, large eyebox and relatively compact form factor.

A smart glass augmented reality (AR) display system is designed with a streamlined form factor featuring an off-axis mirror design. The main component of the combiner optics is in the shape of a regular pair of eyeglasses or sunglasses, with no diffractive gratings, waveguides (lightguides), prisms or Fresnel surfaces involved. High quality see-through performance is achieved with a low-cost combiner that consists of only highly manufacturable reflective surfaces. The 20-degree full field of view of the AR display is centered at about 30 degrees with respect to the center of the ocular vision. Such a design allows the user to have a clear unobscured central field of view. At the same time, the projected image is accessible by moving the eyeball off the central vision. The system is designed with a circular eye box with more than 10 mm in diameter.

In this paper, two freeform prism combiner designs with different geometries were studied. The first design, whose geometry is driven by the need for total-internal-refraction, achieves optical performance suitable for use in AR/VR applications, but involves highly complex surfaces and highly non-uniform performance. The second design, which removes the total-internal-refraction requirement, adopts a modified geometry which enables significantly improved aberration correction potential. The nodal-aberration-theory based design process is shown for both prism designs, and the optical performance of each design was analyzed. Performance exceeds 10% MTF at 50lp/mm over centered and decentered 3mm effective subpupils, evaluated at nine different positions within an 8mm diameter eyebox.

Freeform prism systems are commonly used for head mounted display systems for augmented, virtual, and mixed reality. They have a wide variety of applications from scientific uses for medical visualization to defense for flight helmet information. The advantage of the freeform prism design over other designs is their ability to have a large field of view and low f-number while maintaining a small and light weight form factor. Current designs typically employ a homogeneous material such as polymethyl methacrylate (PMMA). Using a GRIN material gives the designer extra degrees-of-freedom by allowing a variable material refractive index within the prism. The addition of the GRIN material allows for light to bend within the material instead of only reflecting off the surfaces. This work looks at implementing a freeform gradient-index (GRIN) into a freeform prism design to improve performance, increase field of view (FOV), and decrease form factor by the use of 3D printable polymers. A prism design with freeform GRIN is designed with a FOV of 45°, eye relief of 18.25 mm, eyebox of 8 mm, and performance greater than 10% at 50 lp/mm.

An extensive design study was conducted to find the optimal power distribution and stop location for a 7.5x afocal zoom that controls the pupil walk through zoom such that the lens can be coupled to a high-resolution microscope objective and tube lens with minimal vignetting and performance loss.

Optical nanoantennas have a great potential for enhancing light-matter interactions at the nanometer scale, yet fabrication accuracy and lack of scalability currently limit ultimate antenna performance and applications. In most designs, the region of maximum field localization and enhancement (i.e., hotspot) is not readily accessible to the sample since it is buried into the nanostructure. Moreover, current large-scale fabrication techniques lack reproducible geometrical control below 20 nm. Here, we describe a new nanofabrication technique that applies planarization, etch back and template stripping to expose the excitation hotspot at the surface, providing a major improvement over conventional electron beam lithography methods. We present large flat surface arrays of in-plane nanoantennas, featuring gaps as small as 10 nm with sharp edges, excellent reproducibility and full surface accessibility of the hotspot confined region. The novel fabrication approach drastically improves the optical performance of plasmonic nanoantennas to yield giant fluorescence enhancement factors up to 104-105 times, together with nanoscale detection volumes in the 20 zeptoliter range. The method is fully scalable and adaptable to a wide range of antenna designs. We foresee broad applications by the use of these in-plane antenna geometries ranging from large-scale ultra-sensitive sensor chips, to microfluidics and live cell membrane investigations.

An interferometer was developed to measure the absolute thickness map of plane-parallel optical samples up to 50 mm diameter, with 30 nm repeatability, and

A system combining an interferometer with an environmental chamber for measuring both coefficient of thermal expansion (CTE) and temperature-dependent refractive index (dn/dT) simultaneously is presented. The operation and measurement results of this instrument are discussed.

A high resolution, long working distance zooming microscope of two design forms is compared: 1. a zoomed afocal system is paired with a fixed focal length objective and a fixed focal length tube lens and 2. an objective is zoomed and paired with a fixed focal length tube lens.

A light guide alternative design to Fresnel lenses for building integrated photovoltaic systems is discussed. The manufacturing limitations and their effects on the lens design and system performance are reviewed.

This paper gives new insights into the role of both the microstructure and the interfaces in microcrystalline silicon (μc- Si) single-junction solar cells. A 3-D tomographic reconstruction of a μc-Si solar cell reveals the 2-D nature of the porous zones, which can be present within the absorber layer. Tomography thus appears as a valuable technique to provide insights into the μc- Si microstructure. Variable illumination measurements enable to study the negative impact of such porous zones on solar cells performance. The influence of such defectivematerial can bemitigated by suitable cell design, as discussed here. Finally, a hydrogen plasma cell post-deposition treatment is demonstrated to improve solar cells performance, especially on rough superstrates, enabling us to reach an outstanding 10.9% efficiency microcrystalline singlejunction solar cell.

Characterizing the thermal properties of optical materials is necessary for understanding how to design an optical system for changing environmental conditions. A method is presented for simultaneously measuring both the linear coefficient of thermal expansion and the temperature-dependent refractive index coefficient of a sample interferometrically in air. Both the design and fabrication of the interferometer is presented as well as a discussion of the results of measuring both a steel and a CaFsub2/subsample.

Electron energy-loss spectroscopy (EELS) has become an experimental method of choice for the investigation of localized surface plasmon resonances, allowing the simultaneous mapping of the associated field distributions and their resonant energies with a nanoscale spatial resolution. The experimental observations have been well-supported by numerical models based on the computation of the Lorentz force acting on the impinging electrons by the scattered field. However, in this framework, the influence of the intrinsic properties of the plasmonic nanostructures studied with the electron energy-loss (EEL) measurements is somehow hidden in the global response. To overcome this limitation, we propose to go beyond this standard, and well-established, electron perspective and instead to interpret the EELS data using directly the intrinsic properties of the nanostructures, without regard to the force acting on the electron. The proposed method is particularly well-suited for the description of coupled plasmonic systems, because the role played by each individual nanoparticle in the observed EEL spectrum can be clearly disentangled, enabling a more subtle understanding of the underlying physical processes. As examples, we consider different plasmonic geometries in order to emphasize the benefits of this new conceptual approach for interpreting experimental EELS data. In particular, we use it to describe results from samples made by traditional thin film patterning and by arranging colloidal nanostructures.

Surmet continuously strives to develop novel, advanced optical ceramics products for current and future defense and commercial systems. Using conventional powder processing techniques, Surmet has made substantial progress in its ability to manufacture large ALON® sensor windows, lenses, domes and transparent armor. In addition to transparency, Surmet has demonstrated the ability to incorporate other capabilities into its optical ceramic components, including: EMI shielding, heating, internal antennas and cooling channels. Working closely with the University of Rochester, Surmet has developed gradient index (GRIN) optics in ALON for use in the visible through the MWIR applications. Surmet has demonstrated the ability to tailor the refractive index of ALON® Optical Ceramic by either varying its composition or through the addition of dopants. Smooth axial and radial gradient profiles with ~0.055 change in refractive index, over depths of 1-8 mm (axial) and over 20 mm radius (radial) have been demonstrated. Initial design studies have shown that such elements provide unique capabilities. Radial gradients in particular, with their optical power contribution, provide additional degrees of freedom for color correction in broadband imaging systems. Surmet continues to mature ALON® GRIN technology along with the associated metrology. Surmet is committed to the development of its ALON® GRIN capability as well as finding insertion opportunities in novel imaging solutions for military and other commercial systems.

We have designed, fabricated, and tested a small, integrated photovoltaic module comprised of two separately-contacted, high efficiency, multijunction solar cells and non-imaging optics that both concentrate and spectrally split the incoming light. This hybrid design allows us to individually optimize the tandem cells and optical elements. The system has a measured module efficiency, including optical and packaging losses but not power combination losses, of 38.5 ± 1.9% under the AM1.5 direct terrestrial spectrum. The internal optics concentrate the light by a factor of approximately 20. We find excellent agreement between the modeled and measured performance. This is the highest confirmed conversion efficiency demonstrated for a photovoltaic module. Copyright © 2010 John Wiley & Sons, Ltd.

With the advancement in sensors, hyperspectral imaging in short wave infrared (SWIR 0.9 μm to 1.7 μm) now has wide applications, including night vision, haze-penetrating imaging, etc. Most conventional optical glasses can be material candidates for designing in the SWIR as they transmit up to 2.2 μm. However, since SWIR is in the middle of the glasses’ major absorption wavebands in UV and IR, the flint glasses in SWIR are less dispersive than in the visible spectrum. As a result, the glass map in the SWIR is highly compressed, with crowns and flints all clustering together. Thus correcting for chromatic aberration is more challenging in the SWIR, since the Abbe number ratio of the same glass combination is reduced. Conventionally, fluorides, such as CaF 2 and BaF 2 , are widely used in designing SWIR system due to their unique dispersion properties, even though they are notorious for poor manufacturability or even high toxicity. For lens elements in a zoom system, the ray bundle samples different sections of the each lens aperture as the lens zooms. This creates extra uncertainty in correcting chromatic aberrations. This paper focuses on using only commercially available optical glasses to color-correct a 3X dual-band zoom lens system in the VIS-SWIR. The design tools and techniques are detailed in terms of material selections to minimize the chromatic aberrations in such a large spectrum band and all zoom positions. Examples are discussed for designs with different aperture stop locations, which considerably affect the material choices.

A design study is compiled for a VIS-SWIR dual band 3X zoom lens. The initial first order design study investigated zoom motion, power in each lens group, and aperture stop location. All designs were constrained to have both the first and last lens groups fixed, with two middle moving groups. The first order solutions were filtered based on zoom motion, performance, and size constraints, and were then modified to thick lens solutions for the SWIR spectrum. Successful solutions in the SWIR were next extended to the VIS-SWIR. The resulting nine solutions are all nearly diffraction limited using either PNNP or PNPZ (“Z” indicating the fourth group has a near-zero power) design forms with two moving groups. Solutions were found with the aperture stop in each of the four lens groups. Fixed f-number solutions exist when the aperture stop is located at the first and last lens groups, while varying f-number solutions occur when it is placed at either of the middle moving groups. Design exploration included trade-offs between parameters such as diameter, overall length, back focal length, number of elements, materials, and performance.

A method is presented for the quantitative investigation of microstructure and texture evolution in polycrystalline thin films based on in-plane automated crystal orientation mapping in transmission electron microscopy, from the substrate up. To demonstrate the method we apply it to the example of low pressure metal-organic chemical vapor deposited ZnO layers. First, orientation mapping is applied to standard cross-section and plan-view transmission electron microscopy samples of films, illustrating how plan-view samples both reduce the occurrence of grain overlap that is detrimental to reliable orientation mapping and also improve sampling statistics compared to cross-sections. Motivated by this, orientation mapping has been combined with a double-wedge method for specimen preparation developed by Spiecker et al. (2007) [11, which creates a large area plan-view sample that traverses the film thickness. By measuring > 10,000 grains in the film, the resulting data give access to grain size, orientation and misorientation distributions in function of height above the substrate within the film, which are, in turn, the inputs necessary for quantitative assessment of growth models and simulations. The orientation data are directly related to microstructural images, allowing correlation of orientations with in-plane and out-of-plane grain sizes and shapes. The spatial correlation of the entire data set gives insights into previously unnoticed growth mechanisms such as the presence of renucleation or preferred misorientations. Finally, the data set can be used to guide targeted, local studies by other transmission electron microscopy techniques. This is demonstrated by the site-specific application of nano-beam diffraction to validate the presence of coherent [2 11 0]/(0 11 3) twin boundaries first suggested by the orientation mapping. (C) 2015 Elsevier B.V. All rights reserved.

The availability of more affordable, high resolution, infrared (IR) detector arrays has opened up the need for a larger set of IR materials. This has created a renewed interest in chalcogenide glasses as their transmission spans from 1 to 14 μm. The Naval Research Laboratory has shown that the chalcogenide glasses can be diffused to make gradient index (GRIN) lenses. GRIN materials are interesting because they can have unique dispersion properties that do not exist in the currently discovered homogeneous materials, a key parameter for optical designers. When fabricating a GRIN material, the ability to test and characterize its properties is essential. This has prompted the need and development of an IR Mach–Zehnder interferometer for relative index of refraction measurements of GRIN materials. These same, more affordable IR detector arrays have also allowed for what was commonly done in the visible to be developed in the IR. Now, two-dimensional change in refractive index can be measured without scanning, and the fringe density becomes less of an issue in the IR, allowing for larger changes in refractive index over a small area to be measured. An axial chalcogenide GRIN with Δn of 0.13 was measured at a wavelength of 3.39 μm.

An all-plastic high-performance eyepiece design utilizing a polymer spherical gradient-index optical element is presented. The use of a gradient-index lens in the eyepiece offers better off-axis and chromatic aberration correction, as well as overall performance improvement compared to a similar eyepiece with all homogeneous lenses.

Gradient-index (GRIN) zoom lenses are shown to offer superior imaging performance to homogenous designs over the visible spectrum. For a given element count, copolymer GRIN designs are better corrected for axial and lateral color than homogeneous aspheric designs. A macro was developed in CODE V® to calculate the surface contributions to both axial and lateral color for a radial GRIN lens. This macro confirms the improved color correction of the GRIN systems over the homogeneous ones.

A design study is conducted in the 1-5μm wavelength band for an F/3, 15 degree full field of view, 38mm focal length imaging system. A survey of preferred materials shows the chromatic properties of homogeneous materials in different regions of this spectrum. A survey of GRIN materials, including zinc selenide zinc sulfide GRIN, aluminum oxynitride GRIN, and chalcogenide GRIN, expands the available chromatic properties in this spectral band. Baseline homogeneous triplet designs are explored and compared to previous studies in the literature. The inclusion of a GRIN material in the three element design improves the chromatic correction and results in a system that is nearly diffraction-limited. The three element design is reduced to two elements, where both elements are GRIN, while maintaining comparable performance to the homogeneous triplet.

A diffusion model for the fabrication of gradient-index aluminum oxynitride is developed. The deviation of the diffused profile from the design profile has a similar effect on imaging systems as mid-spatial frequency surface errors.

What do ocean, optical, seismic, and acoustic waves have in common? In most materials the composition varies as a function of x, y, and z. This talk will tie together these four phenomena and describe one of the difficulties of locating Malaysia Air Flight 370. Article not available.

Gradient-index (GRIN) zoom lenses are shown to offer superior imaging performance to homogenous designs over the visible spectrum. For a given element count, copolymer GRIN designs are better corrected for axial and lateral color than homogeneous aspheric designs.