Moritsugu Sakamoto

Taking the vector vortex coronagraph to the next level for ground- and space-based exoplanet imaging instruments: review of technology developments in the USA, Japan, and Europe

The Vector Vortex Coronagraph (VVC) is one of the most attractive new-generation coronagraphs for ground- and space-based exoplanet imaging/characterization instruments, as recently demonstrated on sky at Palomar and in the laboratory at JPL, and Hokkaido University. Manufacturing technologies for devices covering wavelength ranges from the optical to the mid-infrared, have been maturing quickly. We will review the current status of technology developments supported by NASA in the USA (Jet Propulsion Laboratory-California Institute of Technology, University of Arizona, JDSU and BEAMCo), Europe (University of Li`ege, Observatoire de Paris- Meudon, University of Uppsala) and Japan (Hokkaido University, and Photonics Lattice Inc.), using liquid crystal polymers, subwavelength gratings, and photonics crystals, respectively. We will then browse concrete perspectives for the use of the VVC on upcoming ground-based facilities with or without (extreme) adaptive optics, extremely large ground-based telescopes, and space-based internal coronagraphs.

Stable and flexible ring-shaped optical-lattice generation by use of axially symmetric polarization elements

To overcome a trade-off issue between stability and flexibility in the generation of ring-shaped optical lattices, we proposed and demonstrated a novel generation method by using axially symmetric polarization elements. While two optical vortices were coaxially generated, electrically controlled phase difference between them by an electro-optic modulator enabled a precise rotation of the lattice. Our method has the capability to fulfill the high stability and rapid rotation.

Design and laboratory demonstration of an achromatic vector vortex coronagraph

A vector vortex coronagraph (VVC) is one of promising means for imaging extremely faint objects around bright stars such as exoplanets. We present a design of an achromatic VVC, in which an axially-symmetric half-wave plate (AHP) is placed between crossed polarization filters (circular polarizer and analyzer). The circular polarizer and the analyzer are both composed of a polarizer and a quarter-wave plate (QWP). We demonstrate, via Jones calculus and Fourier analysis, that the achromatic stellar elimination can theoretically be realized by optimal polarization filters, even when chromatic AHP and QWPs are used. We carried out laboratory demonstrations of the designed VVC using a photonic-crystal AHP. As a result, we observed achromatic coronagraphic performance, a light suppression level of 7 × 10(-5), over a wavelength from 543 nm to 633 nm.

Coronagraph Focal-Plane Phase Masks Based on Photonic Crystal Technology: Recent Progress and Observational Strategy

Photonic crystal, an artificial periodic nanostructure of refractive indices, is one of the attractive technologies for coronagraph focal-plane masks aiming at direct imaging and characterization of terrestrial extrasolar planets. We manufactured the eight-octant phase mask (8OPM) and the vector vortex coronagraph (VVC) mask very precisely using the photonic crystal technology. Fully achromatic phase-mask coronagraphs can be realized by applying appropriate polarization filters to the masks. We carried out laboratory experiments of the polarization-filtered 8OPM coronagraph using the High-Contrast Imaging Testbed (HCIT), a state-of-the-art coronagraph simulator at the Jet Propulsion Laboratory (JPL). We report the experimental results of 10-8-level contrast across several wavelengths over 10% bandwidth around 800nm. In addition, we present future prospects and observational strategy for the photonic-crystal mask coronagraphs combined with differential imaging techniques to reach higher contrast. We proposed to apply the polarization-differential imaging (PDI) technique to the VVC, in which we built a two-channel coronagraph using polarizing beam splitters to avoid a loss of intensity due to the polarization filters. We also proposed to apply the angular-differential imaging (ADI) technique to the 8OPM coronagraph. The 8OPM/ADI mode mitigates an intensity loss due to a phase transition of the mask and provides a full field of view around central stars. We present results of preliminary laboratory demonstrations of the PDI and ADI observational modes with the phase-mask coronagraphs.

Three-dimensionally modulated anisotropic structure for diffractive optical elements created by one-step three-beam polarization holographic photoalignment

A diffractive optical element with a three-dimensional liquid crystal (LC) alignment structure for advanced control of polarized beams was fabricated by a highly efficient one-step photoalignment method. This study is of great significance because different two-dimensional continuous and complex alignment patterns can be produced on two alignment films by simultaneously irradiating an empty glass cell composed of two unaligned photocrosslinkable polymer LC films with three-beam polarized interference beam. The polarization azimuth, ellipticity, and rotation direction of the diffracted beams from the resultant LC grating widely varied depending on the two-dimensional diffracted position and the polarization states of the incident beams. These polarization diffraction properties are well explained by theoretical analysis based on Jones calculus.

Holographic binary grating liquid crystal cells fabricated by one-step exposure of photocrosslinkable polymer liquid crystalline alignment substrates to a polarization interference ultraviolet beam

Holographic binary grating liquid crystal (LC) cells, in which the optical anisotropy was rectangularly modulated even as the grating was fabricated using holographic exposure, were fabricated by one-step polarization holographic exposure of an empty glass cell, the interior of which was coated with a photocrosslinkable polymer LC (PCLC). The present study is of great significance in that three types of holographic binary grating LC cells containing twisted alignments can be fabricated by simultaneous exposure of two PCLC substrates to the UV interference beams, which are sinusoidally modulated. The polarization conversion properties of the diffracted beams are explained well by theoretical analysis based on Jones calculus.

Diffraction properties of a vector grating liquid crystal cell fabricated using a one-step exposure of a nonorthogonal elliptically polarized interference beam

Vector grating liquid crystal (LC) cells, with periodically intermixed 0° planar and 90° twisted nematic alignments and director distributions rotated in the grating vector, were fabricated using a one-step polarization holographic exposure. A nonorthogonal elliptically polarized interference UV beam was irradiated to the empty glass cell, which had the inner walls coated with a photocrosslinkable polymer liquid crystal film. The dependence of the diffraction properties on the birefringence of the nematic LC (E7) in the resultant vector grating LC cell was determined through experimental and theoretical analysis. The polarization azimuth and ellipticity of the diffracted beams were varied using the temperature of the nematic LC.

Three-dimensional vector recording in polarization sensitive liquid crystal composites by using axisymmetrically polarized beam.

Three-dimensional anisotropic structures were fabricated by a recording axisymmetrically polarized beam in azobenzene (azo)-dye doped liquid crystal polymer composites. Polarization and wavefront modulation properties of fabricated anisotropic structures are investigated by experimentally and theoretically analyzing the diffraction properties. Photo-induced anisotropic structures would be utilized to generate singular light waves, such as optical and polarization vortices.

Polarization grating fabricated by recording a vector hologram between two orthogonally polarized vector vortex beams

Polarization gratings (PGs) were fabricated by the recording of vector holograms between two orthogonally polarized vector vortex beams (VVBs). The polarization and diffraction properties of the resulting PGs were analyzed theoretically. The feasibility of the process was demonstrated experimentally using a photo-crosslinkable liquid crystal polymer film as the polarization-sensitive material. The fabricated PGs can convert homogeneously polarized laser beams into VVBs, vector beams, optical vortices, and ring-shaped optical lattices by controlling the incident beam’s state of polarization. The PGs that are presented will be applicable to optical communications and optical manipulation processes as vortex generators and converters.

Tunable dichroic polarization beam splitter created by one-step holographic photoalignment using four-beam polarization interferometry

A tunable dichroic polarization beam splitter (tunable DPBS) simultaneously performs the follow functions: 1. Separation of a polarized incident beam into multiple pairs of orthogonally polarized beams; 2. Separation of the propagation direction of two wavelength incident beams after passing through the tunable DPBS; and 3. Control of both advanced polarization and wavelength separation capabilities by varying the temperature of the tunable DPBS. This novel complex optical property is realized by diffraction phenomena using a designed three-dimensional periodic structure of aligned liquid crystals in the tunable DPBS, which was fabricated quickly with precision in a one-step photoalignment using four-beam polarization interferometry. In experiments, we demonstrated that these diffraction properties are obtained by entering polarized beams of wavelengths 532 nm and 633 nm onto the tunable DPBS. These diffraction properties are described using the Jones calculus in a polarization propagation analysis. Of si...