EXPRES. III. Revealing the Stellar Activity Radial Velocity Signature of ϵ Eridani with Photometry and Interferometry

The distortions of absorption line profiles caused by photospheric brightness variations on the surfaces of cool, main-sequence stars can mimic or overwhelm radial velocity (RV) shifts due to the presence of exoplanets. The latest generation of precision RV spectrographs aims to detect velocity amplitudes ≲ 10 cm s−1, but requires mitigation of stellar signals. Statistical techniques are being developed to differentiate between Keplerian and activity-related velocity perturbations. Two important challenges, however, are the interpretability of the stellar activity component as RV models become more sophisticated, and ensuring the lowest-amplitude Keplerian signatures are not inadvertently accounted for in flexible models of stellar activity. For the K2V exoplanet host ϵ Eridani, we separately used ground-based photometry to constrain Gaussian processes for modeling RVs and TESS photometry with a light-curve inversion algorithm to reconstruct the stellar surface. From the reconstructions of TESS photometry, we produced an activity model that reduced the rms scatter in RVs obtained with EXPRES from 4.72 to 1.98 m s−1. We present a pilot study using the CHARA Array and MIRC-X beam combiner to directly image the starspots seen in the TESS photometry. With the limited phase coverage, our spot detections are marginal with current data but a future dedicated observing campaign should allow for imaging, as well as allow the stellar inclination and orientation with respect to the debris disk to be definitively determined. This work shows that stellar surface maps obtained with high-cadence, time-series photometric and interferometric data can provide the constraints needed to accurately reduce RV scatter.

A Metasurface Beam Combiner Based on the Control of Angular Respons

Beam combiners are widely used in various optical applications including optical communication and smart detection, which spatially overlap multiple input beams and integrate a output beam with higher intensity, multiple wavelengths, coherent phase, etc. Since conventional beam combiners consist of various optical components with different working principles depending on the properties of incident light, they are usually bulky and have certain restrictions on the incident light. In recent years, metasurfaces have received much attention and become a rapidly developing research field. Their novel mechanisms and flexible structural design provide a promising way to realize miniaturized and integrated components in optical systems. In this paper, we start from studying the ability of metasurfaces to manipulate the incident wavefront, and then propose a metasurface beam combiner in theory that generates an extraordinary refracted beam based on the principle of phase gradient metasurface. This metasurface combines two monochromatic light incidents at different angles with identical polarization but arbitrary amplitudes and initial phases. The combining efficiency, which is defined as the ratio of the power in the combining direction to the total incident power, is 42.4% at the working wavelength of 980 nm. The simulated results indicate that this proposed method is able to simplify the design of optical combiners, making them miniaturized and integrated for smart optical systems.

See-through reflective optical elements with embedded wavelike films.

The recently proposed optical elements containing semitransparent wavelike films embedded into the transparent material are investigated. Such optical elements do not distort a wave transmitted through them. Novel optical elements can be fabricated using well-known embossing methods. Their possible applications are discussed. The optical elements containing holographic wavelike films can be used for color holography, for augmented reality wearable glasses, as a beam combiner for head-up displays. The optical elements containing diffuse wavelike films can be used as an alternative for tinted car windows, instead of mirror-glass windows in high-rise buildings, for shop-windows, for one-way privacy windows, for printing images on eyeglasses. The scheme of a waveguide display with a single-layer holographic wavelike films is analyzed. The feasibility of the proposed applications is verified by estimation of the optical elements’ parameters and computer simulation, and measurements conducted on an experimental specimen.

Design and Realization of a Compact Efficient Beam Combiner, Based on Liquid Crystal Pancharatnam–Berry Phase Gratings

We demonstrate a laser beam combiner based on four photo-patterned Pancharatnam–Berry (PB) phase gratings, which is compact and has high diffraction efficiency for incident circularly polarized light. The nematic liquid crystal mixture E7 is used as anisotropic material, and the thickness of the layer is controlled by spacers. The beam combiner can bring two parallel laser beams closer to each other while remaining parallel. This work shows the potential to realize components based on flat optical LC devices.