In optical thin-films and surfaces, geometric phase is utilized to control the phase beyond that possible through optical path differences. Geometric-phase lenses, which are significantly thinner than refractive lenses for the same numerical aperture (NA), most commonly use a spherical phase profile. This is especially effective for normally incident light, but like other thin lenses, the performance degrades noticeably for off-axis incidence and wider fields-of-view. In this study, we investigate whether or not various aspheric designs provide better off-axis performance. We simulate aspheric singlet and doublet liquid crystal geometric-phase lenses (24.5 mm diameter, 40 mm back focal length at 633 nm), aiming to optimize spot size performance at 0, 3, and 7 degrees field angles, using Zemax OpticStudio 16.5. By using Zernike fringe phase expansions, we find conditions which provide improved off-axis performance. We demonstrate improved performance of a compact lens system utilizing these polarization-dependent optics.
A prior simulation-only study of aspherical phase profiles [Hornburg et al, Proc SPIE 10743, 10743-4 (2018)] in geometric-phase lenses (GPLs) indicated that aspherical doublet lens systems should provide substantially improved off-axis performance than those using spherical phase profiles. In this work, we fabricate a liquid crystal GPL doublet (24.5 mm diameter, 40 mm back focal length at 633 nm) and compare it to with a reference spherical GPL singlet. We characterized the liquid crystal alignment quality, efficiencies, and spot performance. With these compact GP lens systems, we realize improved performance for wider fields of view, while maintaining low loss.
All diffractive lenses manifest chromatic aberration/dispersion. If the focal length f0 at a given wavelength λ0 is known, then the focal length f(λ) = f0 λ0 / λ at other wavelengths λ. This can be considerable, even for lenses of a few diopters. Geometric-phase lenses (GPLs), are no exception, which manipulate incident light’s wavefront by the Pancharatnam-Berry phase effect. Several years ago, we developed achromatic coatings based on photo-aligned chiral liquid crystals that achieve nearly 100% efficiency into the primary and conjugate waves, and more recently we demonstrated fast, defect-free GPLs down to F/1.5 for red light. Until now, no one has reported how to generally reduce chromatic aberration and ensure that two or more wavelengths can have the same focal length. Here, we report on a new approach to correct for chromatic aberration using a stack of GPLs and retarders to arrange red, green, and blue wavelengths to have precisely the same focal length. A simple arrangement of these elements results in a thin, monolithic, and flat GPL, which can either converge or diverge three wavelength sources (R/G/B) with the same focal length, positive or negative, depending on the handedness of the circular input polarization. Here, we describe the concept and characterize our first prototypes by evaluating focal lengths, efficiency, and polarization contrast. We also discuss the realistic opportunities and limitations for this approach.
Certain wavelengths bands, especially Y, J, H, and K, have become the main measurement pathway for many of the world’s largest telescopes. Additionally, the study of stellar light within of near-infrared (NIR) bands has become the staple in the field of direct imaging. Because of this, there is a growing necessity for customized broadband optics in the near infrared to meet the needs of the astronomers and allow for more precise measurements. We report on complex birefringent films developed for NIR operation, useful to implement wave-plates, vector apodizing phase plates, and polarization gratings. The combination of multi-twist retarders (MTRs) with both direct-write laser scanning or holographic lithography, and allows us to fabricate arbitrary phase patterns via a geometric phase effect and achromatic, super-achromatic, and highly chromatic (dual-band) spectra from 0.5 to 5 microns. MTRs are complex birefringent films with an optic axis variation along 1D/2D/3D. They consist of two or more chiral liquid crystal (LC) layers on a single substrate and with a single photo-alignment layer. Importantly, subsequent LC layers are aligned directly by prior layers, allowing simple fabrication, achieving automatic layer registration, and resulting in a monolithic film with a continuously varying optic axis. MTRs can be used for a wide range of remote optical sensing, both earth- and space-based. Here, we will review our current and prior MTR films being used for NIR astronomical observation, and discuss the realistic opportunities and limitations ahead for improved precision and design-complexity for retardation and wavefront(phase).
We report on the properties of a fast F/1.5 geometric-phase lens with a focal length of 37 mm at 633 nm and a 24.5 mm diameter. This lens employs photo-aligned liquid crystal layers to implement the spatially varying Pancharatnam-Berry phase, leading to the expected polarization- and wavelength-dependent focusing. An achromatic spectrum is achieved using (chiral nematic) multi-twist retarder coatings, with high first-order (≥98%) and low zero-order (≤1%) transmittance across 450-700 nm. We measure traditional optical metrics of the GP lens including focused spot profile and modulation transfer function through knife edge testing and NBS 1963a resolution charts. This work includes a comparison to similar F/# conventional thick and thin lenses.
We show how highly chromatic Multi-Twist Retarder (MTR) films can be used to create a single-film color filter wherein the color may be selected only by the MTR orientation angle. By this approach, we can create multi- color images with just an MTR between polarizers. We study the design method and limits of the available color gamut possibilities in this approach, and experimentally demonstrate several designs of continuous and discrete patterns. This technique may be useful in art, displays, microscopy, and remote sensing.
We introduce and demonstrate an approach to create highly chromatic retardation spectra across various wave lengths. The design approach is based on Multi-Twist Retarder (MTR) principle where multiple liquid crystal polymer layers are coated on top of each other on a single substrate. Previous MTRs have been applied to develop broadband achromatic retarders, but here we show that MTRs are quite flexible, and their retardation spectrum can be tuned to create arbitrary profiles. As a representative example, we show this tailorability by creating a retarder which produces approximately zero retardation in visible (500-900 nm) and half-wave retardation in near- infrared (1-2.7 μm) wavelength region. This would provide enhancement in remote sensing, telecom, and spectroscopy systems where it is advantageous to have an optical element which affects only one band, but is largely transparent otherwise.
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