Gradient Index Materials Research Articles

Free-space infrared Mach–Zehnder interferometer for relative index of refraction measurement of gradient index optics

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.

Tuning the sound speed in macroporous polymers with a hard or soft matrix.

In this paper, we investigate the factors affecting the sound speed in air-filled macroporous polymer materials at ultrasound frequencies. Due to the presence of large proportion of gas, these porous materials present high compressibility and, as a consequence, low sound speed which may fall down to values as low as 40 m s-1. Using an emulsion-templating method, we synthesize macroporous samples with similar porous structures but with three different matrices, i.e. a hard poly(styrene-divinylbenzene (DVB)) matrix, a soft epoxy-modified polydimethylsiloxane (PDMS) matrix and a very soft polyaddition PDMS matrix. We characterize the matrix mechanical properties by measuring both the bulk modulus K0 and the shear modulus G0. Next, we compare the sound speed measured in porous samples with porosity varying from 0 to 50%. We show that, in agreement with theoretical predictions, the sound speed is mainly controlled by two parameters, the porosity value and the K0/G0 ratio of the polymer matrix. These parameters may be used to control the sound propagation in porous polymers, which opens the way to the realization of gradient-index materials.

Monostatic cloaking using electrically thin inhomogeneous media

We introduce electrically thin cloaking devices that deflect a plane wave based on the principles of phase compensation and wave interference. An electrically large perfectly conducting object is cloaked by materials that control the phase velocity of the refracted wave. The design is based on gradient index materials to reflect incident waves in different directions resulting in a significant reduction in monostatic radar cross section. Unlike earlier cloaking devices that were constrained by specific geometry and involved complex media, the devices introduced here are electrically thin and can be made conformable to the surface of the structure to be cloaked. Numerical simulations are presented to validate the design concept presented here.

Control ultrasound beam by tissues in the head of finless porpoise acting as a tunable gradient index material

Porpoises are well known to emit directional ultrasound beams for detecting and tracking preys; however, how they produce and manipulate directional beams are challenging. Here, we investigated physical mechanism of ultrasound beam formation and control of finless porpoise (N. a. sunameri) by using an integrated scheme of computed tomography, tissue and field measurements, and numerical modeling. The results showed that complex acoustic structures in the porpoise’s forehead contributed to producing directional acoustic field. Furthermore, we demonstrated that the skull, air sacs, connective tissue, muscle, and melon constituted a gradient index (GRIN) structure whose density and sound velocity are positively correlated, and thus regulated the directional beam. The removal or compression deformation of the forehead tissues decentralizes energy and widens sound beam, indicating that the forehead tissues as a tunable natural GRIN material significantly impact beam patterns of the finless porpoise. The results might be valuable for understanding control mechanism of acoustic beam of other toothed whales.

Transparent all-dielectric gradient index waveplates with compact profiles

Purely dielectric waveplates overcome problems typically associated with metals, e.g., corrosion. However, they are often bulky and/or lossy, substantially reducing their applicability. As the operating frequency shifts towards lower frequencies where compactness is a sought after quality, these problems become even more severe. In this letter, we theoretically demonstrate how to combine axially varying gradient index materials with form birefringence to realize nearly transparent and compact dielectric waveplates. The waveplate is subsequently studied considering a discrete distribution of indexes of refraction. Our results indicate that the number of discretization levels required to perform as the continuous gradient index case is small, which would simplify significantly the fabrication process. As an example, a 0.65λ-thick quarterplate with less than 0.1 dB insertion loss at ∼34 GHz is designed and compared with full-wave simulations, proving an excellent agreement.

Surface Wave Cloak from Graded Refractive Index Nanocomposites.

Recently, a great deal of interest has been re-emerged on the possibility to manipulate surface waves, in particular, towards the THz and optical regime. Both concepts of Transformation Optics (TO) and metamaterials have been regarded as one of key enablers for such applications in applied electromagnetics. In this paper, we experimentally demonstrate for the first time a dielectric surface wave cloak from engineered gradient index materials to illustrate the possibility of using nanocomposites to control surface wave propagation through advanced additive manufacturing. The device is designed analytically and validated through numerical simulations and measurements, showing good agreement and performance as an effective surface wave cloak. The underlying design approach has much wider applications, which span from microwave to optics for the control of surface plasmon polaritons (SPPs) and radiation of nanoantennas.

Spherical aberration in electrically thin flat lenses.

We analyze the spherical aberration of a new generation of lenses made of flat electrically thin inhomogeneous media. For such lenses, spherical aberration is analyzed quantitatively and qualitatively, and comparison is made to the classical gradient index rod. Both flat thin and thick lenses are made of gradient index materials, but the physical mechanisms and design equations are different. Using full-wave three-dimensional numerical simulation, we evaluate the spherical aberrations using the Maréchal criterion and show that the thin lens gives significantly better performance than the thick lens (rod). Additionally, based on ray tracing formulation, third-order analysis for longitudinal aberration and optical path difference are presented, showing strong overall performance of thin lenses in comparison to classical rod lenses.

Acoustic beam control in biomimetic projector via velocity gradient

A biomimetic projector (BioP) based on computerized tomography of pygmy sperm whale's biosonar system has been designed using gradient-index (GRIN) material. The directivity of this BioP device was investigated as function of frequency and the velocity gradient of the GRIN material. A strong beam control over a broad bandwidth at the subwavelength scale has been achieved. Compared with a bare subwavelength source, the main lobe pressure of the BioP is about five times as high and the angular resolution is one order of magnitude better. Our results indicate that this BioP has excellent application potential in miniaturized underwater sonars.

Total omnidirectional reflection by sub-wavelength gradient metallic gratings

In this letter, we find that nearly total omnidirectional reflection could be achieved in a metallic grating structure with gradient index materials, regardless of the polarization of the incident wave. By bending the straight structure into a metallic grating ring, we design a metacage that can well shield the electromagnetic wave. All the phenomena are well demonstrated from theoretical analysis and numerical simulations.

Absolute thickness metrology with submicrometer accuracy using a low-coherence distance measuring interferometer.

Absolute physical thickness across the sample aperture is critical in determining the index of a refraction profile from the optical path length profile for gradient index (GRIN) materials, which have a designed inhomogeneous refractive index. Motivated by this application, instrumentation was established to measure the absolute thickness of samples with nominally plane-parallel surfaces up to 50mm thick. The current system is capable of measuring absolute thickness with 120nm (1σ) repeatability and submicrometer expanded measurement uncertainty. Beside GRIN materials, this method is also capable of measuring other inhomogeneous and opaque materials.

Steering light by a sub-wavelength metallic grating from transformation optics.

Transformation optics has shown great ability in designing devices with novel functionalities, such as invisibility cloaking. A recent work shows that it can also be used to design metasurfaces which usually come from the concept of phase discontinuities. However, metasurfaces from transformation optics have very complicated material parameters. Here in this work, we propose a practical design, a sub-wavelength metallic grating with discrete and gradient index materials. Such a design not only inherits some functionalities of metasurfaces from phase discontinuities, but also shows richer physics. Our work will also provide a guidance to recent activities of acoustic metasurfaces, especially for those made of extremely anisotropic metamaterials.

Synthesis and characterization of erbium-doped SiO2-TiO2 thin films prepared by sol-gel and dip-coating techniques onto commercial glass substrates as a route for obtaining active GRadient-INdex materials

Abstract In this work, SiO 2 -TiO 2 films doped with erbium were prepared by dip-coating sol-gel process onto commercial glass substrates. The surface morphology of the films was characterized using atomic force microscopy, while thickness, refractive index, extinction coefficient and porosity of the films were determined by ellipsometric measurements in a wavelength region of 400–1000 nm. Optical constants and porosity were found to vary with erbium concentration. The proof of principle presented in this paper is applicable to systems of different nature by tailoring the sol-gel precursors in such a way that active GRadient-INdex media described by a complex, parabolic-like refractive index distribution for beam shaping purposes is obtained.

A biomimetic projector with high subwavelength directivity based on dolphin biosonar

Based on computed tomography of a Yangtze finless porpoise's biosonar system, a biomimetic structure was designed to include air cavity, gradient-index material, and steel outer-structure mimicking air sacs, melon, and skull, respectively. The mainlobe pressure was about three times higher, the angular resolution was one order of magnitude higher, and the effective source size was orders of magnitude larger than those of the subwavelength source without the biomimetic structure. The superior subwavelength directivity over a broad bandwidth suggests potential applications of this biomimetic projector in underwater sonar, medical ultrasonography, and other related applications.

Базовая сферическая линза с исправленной сферической аберрацией путем применения градиентного материала с осевым линейным распределением показателя преломления

Базовая сферическая линза с исправленной сферической аберрацией путем применения градиентного материала с осевым линейным распределением показателя преломления

Measurement of gradient index materials by beam deflection, displacement, or mode conversion

For media with heterogeneous index of refraction, rays follow curved paths. Methods exploiting knowledge of the relationship between refractive index distribution and ray paths to estimate the refractive index distribution based on laser beam deflection, displacement, and mode conversion on passage through the media are described. The work covers axial, radial (cylindrical), and spherical refractive index distributions.

Color correction in the infrared using gradient-index materials

The ability of gradient index (GRIN) materials to correct color in the infrared is explored. An overview of GRIN materials is provided, includ- ing a review of the most common index profiles (axial, radial, and spheri- cal). Due to the high potential for color correction, a design study is carried out to compare a single ZnS/ZnSe GRIN element's imaging performance to that of homogeneous singlets and doublets. A radial GRIN element is able to correct axial color and provide superior performance over an aspheric homogeneous singlet and offer a reduction in weight when com- pared to a homogeneous doublet. The ZnS/ZnSe GRIN is also shown to be superior to other infrared GRIN materials between 3 and 5 μm. © 2013

Design of radial gradient-index lenses for dual-band visible to short-wave infrared imaging systems

Over 100 doublets were designed using a polychromatic gradient-index (GRIN) design model to analyze the benefits of radial GRIN profiles in broadband visible to short-wave infrared (vis-SWIR) imaging system applications. The polychromatic GRIN design model can be applied to any GRIN material, but for this work, titania silicate glass was investigated. A multielement design study with Petzval lenses was performed to show improved color correction when using GRIN elements. Results from the doublet and Petzval designs illustrate that in broadband vis-SWIR imaging applications, GRIN can either improve system performance or reduce a cemented homogeneous doublet to a GRIN singlet.

Research on a wavefront aberration calculation method for a laser energy gradient attenuator

When a laser energy gradient attenuator is working, there is an inhomogeneous temperature distribution in the whole of the glass because of the non-uniform light energy absorption. This will lead to optical performance reduction. An integrated opto-thermal–mechanical method is proposed to calculate the wavefront aberration for analysis of the thermal effect of the system. Non-sequential optical analysis is used for computing the absorbed energy distribution. The finite element analysis program solves the temperature distribution and the deformations of nodes on the surfaces. An interface routine is created to fit the surface shape and the index field, and extended Zernike polynomials are introduced to get a higher fitting precision. Finally, the parameters are imported to the CodeV optical design program automatically, and the user defined gradient index material is ray traced to obtain the wavefront aberration. The method can also be used in other optical systems for thermal effect analysis.

Three-Dimensional Gradient-Index Materials and Their Applications in Microwave Lens Antennas

In this paper, we introduce a method to realize 3-D inhomogeneous and nearly isotropic gradient-index materials in the microwave regime. The unit cells of such gradient-index materials are drilled-hole dielectric structures, which can be easily fabricated by using the traditional printed circuit boards. To demonstrate the feasibility of the proposed method, we design and realize two functional devices in the microwave frequencies—a half-spherical Luneburg lens and a half Maxwell fisheye lens—based on 3-D gradient-index materials. The measurement results show that both devices have very good performance, which have good agreements to the numerical simulations, illustrating the great application potentials of the 3-D gradient-index materials.

Fabrication Applications of GRIN Materials

In recent years, a series of gradient index (GRIN) materials including Optical glass and polymer have been Research and developed in our laboratory. The Micro-lenses and arrays with the special optical materials also have been fabricated mainly by using Ion-exchange method through refractive index profile, which are applied to micro-optic devices and other applications—— especially in the coupled and connected devices for fiber communications and medical endoscope or they are used primarily for document scanning equipments.