Characterization of microdot apodizers for imaging exoplanets with next-generation space telescopes

Abstract

A major science goal of future, large-aperture, optical space telescopes is to directly image and spectroscopically analyze reflected light from potentially habitable exoplanets. To accomplish this, the optical system must suppress diffracted light from the star to reveal point sources approximately ten orders of magnitude fainter than the host star at small angular separation. Coronagraphs with microdot apodizers achieve the theoretical performance needed to image Earth-like planets with a range of possible telescope designs, including those with obscured and segmented pupils. A test microdot apodizer with various bulk patterns (step functions, gradients, and sinusoids) and 4 different dot sizes (3 µm, 5 µm, 7 µm, and 10 µm) made of small chrome squares on anti-reflective glass was characterized with microscopy, optical laser interferometry, as well as transmission and reflectance measurements at wavelengths λ=600 nm and λ=800 nm. Microscopy revealed the microdots were fabricated to high precision. Results from laser interferometry showed that the phase shifts observed in reflection vary with the local microdot fill factor. This effect is not explained purely by interference between reflected fields from the chrome and glass portions. Transmission measurements showed that microdot fill factor and transmission were linearly related for dot sizes ≥5 µm. However, anomalously high transmittance was measured when the dot size is <5× the wavelength (i.e. ∼3 µm) and the fill factor is approximately 50%, where the microdot pattern becomes periodic. The transmission excess is not as prominent in the case of larger dot sizes suggesting that it is likely to be caused by the interaction between the incident field and electronic resonances in the surface of the metallic microdots. We used our empirical models of the microdot apodizers to optimize a second generation of reflective apodizer designs, specifically for demonstrating end-to-end instrumentation for planet characterization at Caltech's High Contrast Spectroscopy Testbed for Segmented Telescopes (HCST), and confirmed that the amplitude and phase of the reflected beam closely matches the ideal wavefront.

Enhancing Direct Exoplanet Spectroscopy with Apodizing and Beam Shaping Optics

“…The MDA for KPIC uses a semi-random pattern of 25 μm squares in a 200 nm thick chrome layer on an AR-coated CaF 2 substrate (seen in Figure 2). The size of the microdots was chosen to maintain a ∼10:1 ratio between the mask feature size and the wavelength of light to avoid anomalous diffraction effects (Martinez et al 2009b;Zhang et al 2018). This design is lossy because the chrome dots block light, creating a grayscale beam intensity distribution.…”

“…We repeated the optimization for various values of N and opted to use N = 5 since adding more terms did not significantly improve performance. We converted the continuous apodization function into a binary microdot pattern using the Floyd-Steinberg error diffusion algorithm (Floyd & Steinberg 1976;Dorrer & Zuegel 2007;Zhang et al 2018) taking into account the relative transmission of the 200 nm thick chrome layer and the exposed substrate. Figure 2 shows the designed pattern and a photo of the fabricated MDA, which has approximately 500 microdots across the expected 12.3 mm beam size.…”

“…In addition, it is also important to minimize the stellar leakage into the planet fiber to reduce the photon noise contribution from the star. Improved AO correction can also help here, but specific focal plane wave front control techniques such as speckle nulling (Bottom et al 2016;Sayson et al 2019) or the use of a pupil plane apodizing mask (Zhang et al 2018), are better suited to reducing speckle noise. Although these technologies achieve different goals, they are used to reduce total exposure time, which is critical when characterizing faint exoplanets with high spectral resolution on large telescopes.…”

High-contrast Demonstration of an Apodized Vortex Coronagraph

“…4. Defects in the microdot matrix (Zhang et al 2018), and/or subtle nonlinear vector diffraction effects due to the subwavelength feature size of microdot edges (Sivaramakrishnan et al 2013).…”