Traditional polarization gratings (PGs) have been studied with increasing intensity since 2005, in part because they can manifest 100% single-order diffraction efficiency and strong sensitivity to input polarization, in both theory and practice. They can be made using patterned anisotropic materials (e.g., liquid crystals) or nanostructures (e.g., metasurfaces). Nearly every prior work on traditional PGs has implemented a linear spatial phase-shift that is either continuous or which samples the 2π phase period with multiple (≥ 4) discrete phase levels. As far as we know, only two prior works (Bhandari et al, Phys. Rep. 281 (1997); and Wang et al, Appl. Phys. Lett. 108 (2016)) have considered the circumstance when the phase is sampled with exactly two phase levels, with π radians between them. We call this a Binary PG (Bin-PG). In this work, we apply Jones calculus and the small angle (i.e., paraxial) approximation to derive the fundamental optical behavior of Bin-PGs: far-field efficiencies, input polarization sensitivity, and output polarizations. We show that Bin-PGs manifest properties that are a compelling and unique mixture of both traditional (non-binary) PGs and standard diffraction gratings (e.g., surface-relief-gratings (SRGs)). Like non-binary PGs, their output polarization is often different than the input and diffraction efficiencies are dependent on the effective retardation of the film or surface. However, like SRGs, they show a maximum of 80% total first-order efficiency and are insensitive to input polarization.
Planar, anisotropic liquid crystal (LC) optics, along with metasurfaces, have shown to be the predominant meth- ods of producing a geometric (or Pancharatnam-Berry) phase hologram (GPH). One of the simplest GPHs, the traditional continuous polarization grating (PG), implements a continuous linear phase ramp. This PG has received significant attention due to its polarization-selective nature and 100% diffraction efficiency. However, when this linear phase is sampled with a 0-π alternating phase profile, theoretical reasoning predicts polarization- independent qualities. In order to distinguish this grating from continuous PGs, we call this a binary polarization grating (bin-PG). Traditional PGs, with a continuously varying nematic director profile, are simple to manufacture with many holographic methods. However, no bin-PG fabricated with patterned LCs have yet been reported. In this work, we experimentally study bin-PGs formed using a photo-aligned LC polymer network. Particular attention is brought to the problematic rotational ambiguity of LC at the phase step. To prevent disclination lines, a rotation biasing pixel of varying size is implemented at the phase transition boundary. We measure the diffraction efficiencies, the input polarization response, and the impact of the non-zero transition region. At the smallest transition pixel size (0.625μm) an average +1-order efficiency of 36% was measured with an input-polarization sensitivity of only ±1.7%.
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).
The direct-write technology for liquid-crystal patterns allows for manufacturing of extreme geometric phase patterned coronagraphs that are inherently broadband, e.g. the vector Apodizing Phase Plate (vAPP). We present on-sky data of a double-grating vAPP operating from 2-5 μm with a 360-degree dark hole and a decreased leakage term of ∼ 10−4 . We report a new liquid-crystal design used in a grating-vAPP for SCExAO that operates from 1-2.5μm. Furthermore, we present wavelength-selective vAPPs that work at specific wavelength ranges and transmit light unapodized at other wavelengths. Lastly, we present geometric phase patterns for advanced implementations of WFS (e.g. Zernike-type) that are enabled only by this liquid-crystal technology.