Gradient Index Microlenses Research Articles

Three-dimensional gradient index microlens arrays for light-field and holographic imaging and displays.

The geometric, intensity, and chromatic distortions that are a result of the limitations of the material and processes used to fabricate micro-optical lens arrays (MLAs) degrade the performance of light-field systems. To address these limitations, inkjet print additive manufacturing is used to fabricate planar gradient index (GRIN) lenslet arrays, in which volumetric refractive index profiles are used to embed optical functions that would otherwise require multiple homogeneous index MLA surfaces. By tailoring the optical ink feedstock refractive index spectra, independent control over dispersion is achieved, and achromatic performance is made possible. Digital manufacturing is shown to be beneficial for optimizing individual micro-optical channels in arrays wherein the shape, size, aspect ratio, focal length, and optical axis orientation of the lenslets vary as a function of the position within the optical field. Print fabrication also allows opaque inter-lens baffling and aperture stops that reduce inter-channel cross talk, improve resolution, and enhance contrast. These benefits are demonstrated in a light-field display testbed.

Development of gradient index microlenses for the broadband infrared range.

The development of gradient index free-form micro-optic components dedicated to the mid-infrared range is challenging due to the lack of appropriate technology. We propose a method for developing gradient index components for broadband infrared range beyond the transmission window of silicate glass based on nanostructurization using a stack-and-draw fiber drawing technique. A proof-of-concept microlens is developed and verified experimentally in the wavelength range 1.5-4.3 µm. The microlenses are composed of a set of nanorods with a diameter of 940 nm made of a pair of SiO2-PbO-Bi2O3-Ga2O3 based glasses ordered into the preliminary calculated binary pattern. The pattern forms effectively continuous parabolic refractive index distribution for infrared range according to Maxwell-Garnett effective medium model. The development of individual microlenses with a diameter of 118 µm and focal length of 278 µm at the wavelength of 3.75 µm are reported. A large array of 737 microlenses with an individual diameter of 125 µm and focal length of 375 µm is also presented and analyzed.

All-fiber nanostructured gradient optics vortex beam converter

We study optical performance of a vortex beam converter comprising a nanostructured gradient index vortex mask, a compact gradient index microlens and an optical fiber. The converter generates high quality focused optical vortex from the input Gaussian beam without support of any additional external optical elements. Integration of a nanostructured lens allows control of the size and the position of focus of vortex beams, while at the same time maintaining their quality. This confirms that nanostructured gradient index micro-optical components can be used sequentially to form complex optical systems and maintain full compatibility with fiber optics. Our experimental results are supported by numerical simulations.

Perspective of fibre-optical microendoscopy with microlenses.

Fibre-optical microendoscopy is based on fibre-optical confocal scanning microscopy, where optical fibres are introduced for delivery of the source and collection of the signal. Fibre-optical microendoscopy has led to innovations in imaging of freely moving animals, long-term imaging, minimally invasive diagnostics, and microsurgery. The lens system in fibre-optical microendoscopy is significant because of the imaging resolution and miniaturisation possibility. State-of-the-art fibre-optical microendoscopy based on various types of lens systems is introduced and compared. The lens system contains an objective lens, a gradient index microlens, and other novel lens systems fabricated by electric arc discharge or two-photon lithography.

Precise on-Fiber Plasmonic Spectroscopy Using a Gradient-Index Microlens

Plasmonic spectroscopy has emerged as a powerful technique for interrogation of light in the subwavelength regime. Realization of active plasmonic elements on optical fibers can simplify the optical design, allow for performing remote tasks and thus extend the scope of plasmonic utilization. However, the optimization of fiber optics for improving the plasmonic excitation condition has been overlooked. Here we present a plasmonic spectroscopy platform that combines a gradient-index microlens and a single-mode optical fiber. This design ensures large-area and uniform illumination in normal incidence for the excitation of surface plasmon resonance. We demonstrate precise plasmonic spectroscopy with a fiber-optic platform by patterning a nanohole array onto the facet of a gradient-index microlens with our template transfer technique. The experimental spectra are almost identical to the simulated results under ideal conditions, with accurate resonance wavelength (<10 nm deviation), high sensitivity (5 nm/RIU deviation), narrow linewidth (about 5 nm deviation), and diverse shifting patterns. This platform provides a fiber-optic avenue of precise plasmonic spectroscopy for remote interrogation.

Chiral Liquid Crystal Lenses Confined in Microchannels.

It is known that the liquid crystalline smectic-A phase has geometric defects, called focal conic domains, which can be used as gradient-index microlenses. Cholesteric (chiral nematic) phases also have topological defects with a central symmetry and a singularity at their center. We explore a weakly chiral system in which both types of defects can be present in the same material at different temperatures, and with this strategy we create lenses whose focal length is tunable with temperature. We measure the focal length of the tunable lenses, and we investigate the behavior of the defects near the phase transition. We identify the experimental conditions that make the simultaneous presence of the smectic focal conic domains and the circular cholesteric domains possible, such as the concentration of chiral dopant and the rate of heating and cooling. The transformation of focal conic domains into circular cholesteric domains is a new example of memory at the phase transition between smectic-A and nematic liquid crystals.

Development of large diameter nanostructured GRIN microlenses enhanced with temperature-controlled diffusion.

Nanostructured GRIN components are optical elements which can have an arbitrary refractive index profile while retaining flat-parallel entry and exit facets. A method of their fabrication requires assembly of large quantities of glass rods in order to satisfy subwavelength requirement of the effective medium theory. In this paper, we present a development of gradient index microlenses using a combination of methods: nanostructurization of the preform and controlled diffusion process during lens drawing on a fiber drawing tower. Adding a diffusion process allows us to overcome limits of the effective medium theory related to maximum size of nanorods in the lens structure. We show that nanorods are dissolved during the fiber drawing process in high temperature and glass components are locally quasi-uniformly distributed. To demonstrate feasibility of the proposed approach, we have developed and experimentally verified the performance of a nGRIN microlens with a diameter of 115 µm composed of 115 rods on the diagonal, and length of 200 µm devoted to work for the wavelength over 658 nm.

Light field camera based on hexagonal array of flat-surface nanostructured GRIN lenses.

In this paper we present a light field camera system where a flat-surface hexagonal array of nanostructured gradient index lenses was used as a lens matrix. In our approach we use an array of 469 gradient index microlenses with a diameter of 20 µm and 100% fill factor. To develop the single lens and the lenslet array we used a modified stack-and-draw technology. In this technique, variation of refractive index is achieved by using quantized gradient index profiles and rods from different types of glasses. We show experimental results of using this type of lenses for imaging in a system of two kinds of light field cameras. In the first one, the microlens array is located in the focal plane of the main lens. The image is reconstructed, in this case using a Fourier slice photography algorithm. This allowed a partial reconstruction of a 3D scene with spatial and depth resolution of 20 µm and field of view of 500×500×500 µm. In the second configuration, the microlens array is located between a sample and a microscopic objective, thus allowing for superresolution 3D reconstruction of a microscopic image. The scale-invariant feature transform method was used for image reconstruction and obtained a partial 3D reconstruction with a field of view of 150×115×80 µm and a spatial resolution of 2 µm and depth resolution of 10 µm.

Switchable Lensed Linear Micro Axicon in Plasmonic Structures for All Optical Light Processing

We propose a simple route to build up Fresnel bi-mirror microaxicons based on the localized absorption of an astigmatic pump laser beam by a plasmonic metal foil. The corresponding local overheating produces two main effects: the thermal swelling of the glass and the local increase of its refractive index. As a result, a two-mirrors axicon-like element coupled to a gradient index microlens is produced. We modelled the optothermal formation of this microelement by Finite Element Modeling and tested the predicted temperature raise by Scanning Thermal Microscopy (SThM). We calculated and measured the far field distribution of an impinging probe beam that passes through the lensed microaxicon, finding in both cases a noticeable change in the far field pattern when the pumping light is ON evidencing at the same time the good modulation capability of this opto-thermal element. A similar, even if smaller, effect was also found when the probe light impinges from the air side, where no gradient index region is present. This configuration allowed us to measure and confirm the theoretically expected angular deflection and, indirectly, the high value of the vertical thermomechanical deformation induced by the tight astigmatic focusing of the pump beam.

Achromatic nanostructured gradient index microlenses.

We present a development of microlenses achromatically corrected in near-infrared spectral windows. We show that the standard fiber drawing technology can be successfully applied to the development achromatic gradient index microlenses by means of internal nanostructurization. These gradient index microlenses can achieve similar performance to standard aspheric doublets, while utilizing a simpler, singlet element geometry with flat surfaces. A nanostructured lens with a parabolic profile was designed using a combination of the simulated annealing method and the effective medium approximation theory. Measurements on the fabricated lenses show that the microlenses have a nearly wavelength-independent focal plane at a distance of about 35 μm from the lens facet over the wavelength range of 600-1550 nm. The successful design and fabrication of achromatic flat-parallel rod microlenses opens new perspectives for micro-imaging systems and wavelength-independent coupling into optical fibers.

World-smallest fiber-GRIN lens system for optofluidic applications

We have developed a new type of an optical fiber probe which integrates the standard single mode fiber with a gradient index (GRIN) microlens. The system is perfectly suited for optofluidic sensor applications since the diameter of the lens module is exactly the same as the diameter of the optical fiber. Moreover, the performance of the GRIN lens is not degraded by low contrast of the refractive index between of the lens and the fluidic environment. The GRIN lens is made with novel technology of nanostructured optics. Full Text: PDF References M. Woerdemann, C. Alpmann, M. Esseling, C. Denz, Advanced optical trapping by complex beam shaping, Laser Photonics Rev. 7, 839 (2013). CrossRef E. Weber, F. Keplinger, and M. J. Vellekoop, On-Chip Light Modulation Applying Optofluidic Principles, IEEE Sensors Journal 13 4773 (2013). CrossRef F. Hudelist, J. M. Nowosielski, R. Buczynski, A. J. Waddie, M. R. Taghizadeh, Nanostructured elliptical gradient-index microlenses, Optics Letters 35, 130 (2010) CrossRef J. M. Nowosielski, R. Buczynski, F. Hudelist, A. J. Waddie, M. R. Taghizadeh, Nanostructured GRIN microlenses for Gaussian beam focusing, Opt. Comm. 283 1938 (2010). CrossRef A.Sihvola, Electromagnetic Mixing Formulas and Applications (London: The Institution of Electrical Engineers 1999). J. Nowosielski, R. Buczynski, A. J. Waddie, A. Filipkowski, D. Pysz, A. McCarthy, R. Stepien, M. R. Taghizadeh, Large diameter nanostructured gradient index lens, Opt. Ex. 20 11767 (2012). CrossRef R. Stepien, J. Cimek, D. Pysz, I. Kujawa, M. Klimczak, R. Buczynski, Soft glasses for photonic crystal fibers and microstructured optical components, Opt. Eng. 53, 071815 (2014). CrossRef A. Filipkowski, B. Piechal, D. Pysz, R. Stepien, A. Waddie, M. R. Taghizadeh, R. Buczynski, Nanostructured gradient index microaxicons made by a modified stack and draw method, Opt. Lett. 40, 5200-5203 (2015). CrossRef

Femtosecond laser writing of freeform gradient index microlenses in hydrogel-based contact lenses

Femtosecond lasers can be used to write a variety of gradient index refractive devices. Writing devices with an arbitrary optical profile (i.e., freeform) requires knowing the functional dependence of the phase change that the wavefront will experience when passing through a region written under different exposure parameters. We measured this dependence as a function of writing speed and power in hydrogel-based contact lenses. Regions of constant refractive index change were written under different conditions and then the phase change was measured with a Mach-Zehnder interferometer and a phase retrieval algorithm. This functional dependence was tested by writing arbitrary Zernike polynomials with varying magnitudes.

Large diameter nanostructured gradient index lens

In this paper we report on the development and optical properties of nanostructured gradient index microlenses with good chromatic behavior. We introduce a new fabrication concept for the development of large diameter nanostructured gradient index microlenses based on quantized gradient index profiles and the use of nanostructured meta-rods. We show a dependence of the quality of performance on the number of refractive index levels and the lens diameter. Measurements carried out at 633 and 850 nm show good optical properties and similar focal lengths for both wavelengths.

Lateral gradient index microlenses written in ophthalmic hydrogel polymers by femtosecond laser micromachining

Ophthalmic hydrogel polymers are micromachined with near-infrared femtosecond laser pulses. Refractive index changes up to + 0.05 have been obtained, and lateral gradient index refractive structures are written into the flat polymers. By measuring the transmitted wavefront of the micromachined polymer, we find induced astigmatism as high as 0.8 diopters in the micromachined region.

Focusing a Gaussian beam in nanostructured non-periodic GRIN microlenses

Modeling the propagation of a Gaussian beam in a nanostructured gradient index microlens is reported. The nanostructured GRIN microlens is composed of subwavelength discrete dielectric rods. We compare the performance of nanostructured gradient index microlenses against conventional GRIN microlenses for Gaussian beam focusing. The 3D FDTD method is used for numerical analysis of optical systems. Full Text: PDF

Nanostructured GRIN microlenses for Gaussian beam focusing

Gaussian beam propagation through a novel high refractive index contrast nanostructured Gradient-Index (GRIN) microlens with an arbitrary refractive index distribution is investigated. The concept of the nanostructured GRIN microlenses is based on the effective medium theory and we show numerically, using the FDTD method, that nanostructured GRIN microlenses can focus Gaussian beams in an identical manner to conventional GRIN microlenses. Nanostructured GRIN microlenses have a reduced thickness by almost two orders of magnitude compared to conventional GRIN lenses. We also investigate the accuracy of the standard scalar and paraxial GRIN theory in the case of a very high contrast nanostructured GRIN microlens. The paraxial GRIN lens theory is shown only to give an approximate description of Gaussian beam propagation in such a high contrast nanostructured GRIN microlens.

Design and fabrication of nano-structured gradient index microlenses

We present a novel fabrication technology for nano-structured graded index micro-optical components, based on the stack-and-draw method used for photonic crystal fibres. These discrete structures can be described with an effective refractive index distribution. Furthermore we present spherical nano-structured microlenses with a flat facet fabricated with this method and designed using an algorithm based on the Maxwell-Garnett mixing formula. Finally we show theoretical verification by using FDTD simulations for a nano-structured lens as well as experimental data obtained in the microwave regime.

Phase-referenced probe interferometer for biological surface profiling and displacement measurements

We present a probe-based, phase-referenced low coherence interferometer in which the reference field is provided by a fiber end reflection. A gradient-index microlens focuses light onto a sample and collects reflected light. We use the probe interferometer to measure surface profiles of the compound eye of a housefly (Musca domestica) and measure nanometer-scale vibrations in a test sample.

Multiphoton endoscopy.

Despite widespread use of multiphoton fluorescence microscopy, development of endoscopes for nonlinear optical imaging has been stymied by the degradation of ultrashort excitation pulses that occurs within optical fiber as a result of the combined effects of group-velocity dispersion and self-phase modulation. We introduce microendoscopes (350-1000 microm in diameter) based on gradient-index microlenses that effectively eliminate self-phase modulation within the endoscope. Laser-scanning multiphoton fluorescence endoscopy exhibits micrometer-scale resolution. We used multiphoton endoscopes to image fluorescently labeled neurons and dendrites.

Tunable electro-optic microlens array II Cylindrical geometry

Although the concept of an artificial compound eye has been discussed in the literature, its optical arrangement has never been widely adopted for optical design. A design is presented for a tunable gradient-index microlens array, believed to be new, induced electro-optically inside a cylindrical shell. The transparent electrodes on the both sides of the shell are positioned such that the electrodes on the opposite side compensate the phase delay from the electrodes on the front side for a normally incident plane wave, thus suppressing the intrinsic electrode diffraction for the device without applied voltage. The original technique of the electric field calculation was developed to analyze the induced refractive index inside the shell for two types of electro-optic (EO) ceramics: with linear and with quadratic EO effects. For the linear effect it was shown that for given EO coefficients, electric field strength and intrinsic refractive index, the electrode number should exceed a certain amount to make the focal distance less than the cylinder radius. The quadratic effect provides higher sensitivity to the type of the diffracted wave polarization. It was shown how the quadratic coefficient ratio R(12)/R(11) affects the focal-length difference between TE and TM light polarization.