Diffractive Waveplates Research Articles

Recent Advances in Photoalignment Liquid Crystal Polarization Gratings and Their Applications

Liquid crystal (LC) circular polarization gratings (PGs), also known as Pancharatnam–Berry (PB) phase deflectors, are diffractive waveplates with linearly changed optical anisotropy axes. Due to the high diffraction efficiency, polarization selectivity character, and simple fabrication process, photoalignment LC PGs have been widely studied and developed especially in polarization management and beam split. In this review paper, we analyze the physical principles, show the exposure methods and fabrication process, and present relevant promising applications in photonics and imaging optics.

Fabrication of Diffractive Waveplates by Scanning Wave Photopolymerization with Digital Light Processor

Fabrication of Diffractive Waveplates by Scanning Wave Photopolymerization with Digital Light Processor

Linearly Polarized Light Transmission from a Pair of Cycloidal Diffractive Waveplates

Linearly Polarized Light Transmission from a Pair of Cycloidal Diffractive Waveplates

Advances in Transparent Planar Optics: Enabling Large Aperture, Ultrathin Lenses

AbstractUnlike electronics, optics do not follow Moore's law. This statement, expressed by Microsoft's Bernard Kress, refers to the hard challenges to solve in augmented reality hardware. While light sources have undergone numerous revolutions from candles to light emitting diodes, the evolution in transparent optics has been much slower. For transparent materials, variation of the shape, bulk refractive index, and/or its distribution leads to control of the transmitted beam in an optical system. An alternative, the control of the optical axis orientation in an anisotropic material in transparent micrometer‐thin films on a variety of substrates, is explored here. In contrast to metamaterials, these diffractive waveplates have a continuous structure allowing multilayer/multifunctional planar optical systems with close to 100% efficiency across broad bands of wavelengths (ultraviolet to infrared) with customizable spectra. The low‐cost and fast fabrication technology of this fourth generation of optics is scalable to very large aperture sizes. In addition to wearable adaptive optics, the technology enables thin and compact non‐mechanical fast beam steering systems for light detection and ultralight space telescopes. This review will first serve as an introduction to these unique transparent, planar optical films, and then recent advances enabled by specific optical designs will be presented.

Cycloidal diffractive waveplates fabricated using a high-power diode-pumped solid-state laser operating at 532 nm

Diffractive waveplates (DWs) are highly efficient optical components realized by means of a polarization holography setup that makes use of UV/blue laser sources. It is more convenient to perform the holographic recording process with green lasers (e.g., continuous wave operating at 532 nm) because they offer compactness, efficiency, and high power. Unfortunately, the photo-alignment materials used for DW fabrication exhibit limited sensitivity at 532 nm. Herein, we report the realization of cycloidal diffractive waveplates (CDWs) by employing a polarization holographic setup using a high-power, diode-pumped solid-state laser operating at 532 nm. The enabling factor is the use of a photo-alignment material with a broad absorption band extending into the visible part of the spectrum. Samples are characterized in terms of morphological and optical properties. They exhibit a well-defined morphology along with very high diffraction efficiency (≈97%). The possibility to realize CDWs with larger apertures is underway, enabling the realization of a new generation of optical components for integration in modern applications.

Single-step creation of polarization gratings by scanning wave photopolymerization with unpolarized light

Liquid crystalline materials with a cycloidal molecular orientation pattern are attractive for fabricating diffractive waveplates, diffracting incident light regardless of its polarization state into left- and/or right-circularly polarized light only in the +1st and/or −1st orders, applicable as next-generation optical devices. However, large-area high-speed processing of such molecular orientation is a challenge, since even state-of-the-art photoalignment methods require a precise spatial modulation of the polarization states of incident light, e.g., polarization holograms. Here, we propose and demonstrate that unpolarized light could easily generate cycloidal molecular orientation patterns over large areas in a single step merely by using our recently developed method of “scanning wave photopolymerization” with a simple optical setup. Importantly, the processing time for fabricating millimeter-scale films was significantly decreased to less than a few minutes. Detailed investigation revealed that the resultant film showed the desired diffraction behavior with a diffraction efficiency of 50%.

Pancharatnam–Berry optical elements for head-up and near-eye displays [Invited

Liquid-crystal-based Pancharatnam–Berry phase optical elements (PBOEs), also known as diffractive wave plates (DWPs), geometric phase optics (GPO), or geometric phase holograms (GPHs), are functional planar structures with patterned orientation of anisotropy axis. Several scientifically interesting yet practically useful electro-optical effects, such as focusing, beam splitting, waveguide coupling, and wavelength filtering, have been realized with PBOEs. Because of the high degree of optical tunability, polarization selectivity, nearly 100% diffraction efficiency, and simple fabrication process, PBOEs have found widespread applications in emerging display systems, particularly virtual/augmented/mixed reality displays and head-up displays. In this review, we will describe the basic operation principles, present device fabrication procedures, discuss numerical modeling methods, and address applications of PBOEs in emerging display systems.

Vector vortex coronagraphy for exoplanet detection with spatially variant diffractive waveplates

Optical vortex coronagraphs have recently been deployed on most of the world’s largest ground-based telescopes to assist in the search for faint exoplanets and dust emission near stars. These instruments typically make use of vector vortex phase masks, and at short near-infrared wavelengths, the phase masks tend to be spatially variant liquid-crystal-polymer-based diffractive waveplates. Optical vortex coronagraphs also hold great promise as a potential means of imaging terrestrial exoplanets in nearby solar systems with space-based telescopes, for which nearly ideal vortex phase masks will be needed. Here, we briefly summarize the optical vortex coronagraph, foreseen performance requirements for terrestrial exoplanet imaging detection, the obstacles to nearly ideal diffractive-waveplate-based vortex phase masks, and recent broadband performance demonstrations.

Doubling the geometric phase of reflective Pancharatnam–Berry diffractive waveplates

Cholesteric liquid crystals, a liquid crystal phase with a helical modulation of the anisotropic optic axis, function as holographic optical elements, whereby Bragg reflected light is phase modulated by the Pancharatnam–Berry phase acquired in proportion to twice the spatial phase of helical structure. One of the limitations of such holographic optical elements is that a π rotation in the helix phase is required to modulate the optical phase by 2π. Here, we propose a double-layered reflective structure comprising uniaxial and helical anisotropic materials that can modulate the reflected optical phase twice as much compared to conventional elements. We present results of numerical simulations and confirm them through the fabrication of two devices: (i) a reflective deflector and (ii) an optical vortex generator, in which a two-fold increase is achieved in diffraction angle and topological charge of the generated optical vortex beam, respectively. The proposed device structure should be useful for use in compact catoptric optical systems.

Beam shaping diffractive wave plates [Invited

We present and discuss opportunities opened up by a new generation of beam shaping optical elements that combine capabilities of digital spatial light polarization converters and diffractive properties of thin liquid crystalline films with patterned orientation of anisotropy axis (diffractive wave plates). Several functions of the new generation beam shapers are demonstrated, among them converting a laser beam of a Gaussian profile into a ring profile in the far field, a flattop profile, and into complex images. We also describe electrically controlled beam shaping optical elements which can be turned off and on within milliseconds by applying a low external voltage. Optical, morphological, and electro-optical properties of the components are characterized.

Ultrafast Photoalignment: Recording a Lens in a Nanosecond

Liquid crystals can be photoaligned with a single nanosecond pulse acting on thin photoanisotropic coatings on the cell substrates. This phenomenon was demonstrated for pulses of 532 nm and 355 nm wavelengths (second and the third harmonics of a Nd:YAG laser). Direct printing of liquid crystal cycloidal diffractive waveplates and diffractive waveplate lenses characterized by high spatial frequencies required only a mJ energy. The liquid crystal alignment dynamics reveal a fast component related to photoalignment of molecules within the photoanisotropic layer and a slower component related to alignment of the liquid crystal within the cell.

High-contrast, low-voltage variable reflector for unpolarized light

GRAPHICAL ABSTRACT

Mid-wave infrared beam steering based on high-efficiency liquid crystal diffractive waveplates.

We demonstrated two liquid crystal diffractive waveplates: one optimized for near-infrared (1.06 µm), and another for mid-wave infrared (MWIR, 3~5 µm). By employing a low loss liquid crystal mixture UCF-M3, whose absorption loss is below 2% in the 4~5 µm spectral region, the grating achieves over 98% diffraction efficiency in a broad MWIR range. To switch the grating, both active and passive driving methods can be considered. In our experiment, we used a polymer-stabilized twisted nematic cell as the polarization rotator for passive driving. The obtained rise time is 0.2 ms and decay time is 10 ms.

Diffractive waveplate arrays [Invited

Diffractive waveplate technology presents an opportunity for designing arrays of all types of optical components. We present here different architectures of arrays of waveplate lenses and vector vortex waveplates. Due to the continuous nature of diffractive waveplate coatings and the high spatial resolution of the technology, the sizes of array elements can span from micrometers to tens of millimeters. Both fixed and electrically switchable arrays are discussed. Arrays of diffractive waveplates present new challenges and opportunities for digital light polarization holography for applications in polarizer-free displays, smart windows, optical communications, beam shaping, and other photonics technologies.

Electrically switchable diffractive waveplates with metasurface aligned liquid crystals.

Diffractive waveplates and equivalent metasurfaces provide a promising path for applications in thin film beam steering, tunable lenses, and polarization filters. However, fixed metasurfaces alone are unable to be tuned electronically. By combining metasurfaces with tunable liquid crystals, we experimentally demonstrate a single layer device capable of electrically switching a diffractive waveplate design at a measured peak diffraction efficiency of 35%, and a minimum switching voltage of 10V. Furthermore, the nano-scale metasurface aligned liquid crystals are largely independent of variations in wavelength and temperature. We also present a computational analysis of the efficiency limits of liquid crystal based diffractive waveplates, and compare this analysis to experimental measurements.

Polarization conversions of diffractive wave plates based on orthogonal circular-polarization holography

A polarization holographic grating, which integrates the functions of a grating and a wave plate and is called a diffractive wave plate, is recorded by two beams (left and right circularly polarized) of a 532 nm laser in an azo polymer with a liquid-crystal structure. The polarization conversion characteristics of the diffractive wave plates are investigated with a detecting light of 650 nm by metering the polarization state of first-order diffracted light. It is confirmed that the diffractive wave plates convert the incident linear polarization into circular polarization for a linearly polarized probe laser and reverse the sense of rotation of the circular polarization when the detecting light is circularly polarized light.

Recording polarization gratings with a standing spiral wave

A scalable and robust methodology for writing cycloidal modulation patterns of optical axis orientation in photosensitive surface alignment layers is demonstrated. Counterpropagating circularly polarized beams, generated by reflection of the input beam from a cholesteric liquid crystal, direct local surface orientation in a photosensitive surface. Purposely introducing a slight angle between the input beam and the photosensitive surface normal introduces a grating period/orientation that is readily controlled and templated. The resulting cycloidal diffractive waveplates offer utility in technologies requiring diffraction over a broad range of angles/wavelengths. This simple methodology of forming polarization gratings offers advantages over conventional fabrication techniques.

All-optical diffractive/transmissive switch based on coupled cycloidal diffractive waveplates

Pairs of cycloidal diffractive waveplates can be used to doubly diffract or collinearly propagate laser radiation of the appropriate wavelength. The use of a dynamic phase retarder placed in between the pair can be utilized to switch between the two optical states. We present results from the implementation of an azo-based retarder whose optical properties can be modulated using light itself. We show fast and efficient switching between the two states for both CW and single nanosecond laser pulses of green radiation. Contrasts greater than 100:1 were achieved. The temporal response as a function of light intensity is presented and the optical switching is shown to be polarization independent.

The Promise of Diffractive Waveplates

Diffractive waveplates exhibit the high diffraction efficiency of Bragg gratings in micron-thick material layers.

Fabrication and measurement of wideband achromatic waveplates for the mid-infrared region using subwavelength features

Subwavelength diffractive features etched into a substrate lead to form birefringence that can produce polarization sensitive elements such as wave plates. Using etched features allows for the development of pixelated devices to be used in conjunction with focal plane arrays in polarimetric imaging systems. Form birefringence exhibits dispersion that can be advantageous to the design of wave plates with an achromatic response. Taking advantage of this dispersion, diffractive wave plates with good achromatic characteristics can be designed for the 2- to 5-µm spectral region. Previous work in this area has produced good results over a subset of this wavelength band, but designing for this extended band is particularly challenging. The fabrication processes for the subwavelength features will be discussed and fabricated devices with a measured average phase retardation of 80.6 deg and rms variation of 9.41 deg will be presented.