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We study the band structure and scattering of in-plane coupled longitudinal and shear stress waves in linear layered media and observe that exceptional points (EP) appear for elastic (lossless) media, when parameterized with real-valued frequency and tangential wave vector component. The occurrence of these EP pairs is not limited to the original stop bands. They could also appear in all mode pass bands, leading to the formation of new stop bands. The scattered energy near these locations is studied along with the associated polarization patterns. The broken phase symmetry is observed inside the frequency bands book-ended by these EP pairs. This is especially manifested by the chirality of the trajectory of the particle velocity, which gets selected by a ‘‘direction’’ of the wave, e.g. the imaginary part of normal component of the wave vector, or the energy flux direction just outside the band. Additionally, EP pairs also appear in the spectrum of the (modified) scattering matrix when mechanical gain is theoretically included to balance the loss in a parity-time symmetric finite structure. These EP pairs lead to amplification of transmission to above 1 and single-sided reflectivity, both phenomena associated with broken phase symmetry, with intriguing potential applications.
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Compensation of differential dispersion: application to multiband stellar interferometry.
Abstract With the aim of pushing the limiting magnitude of interferometric instruments, the need for wide-band detection channels and for a coordinated operation of different instruments has considerably grown in the field of long-baseline interferometry. For this reason, the Center for High Angular Resolution Astronomy (CHARA), an array of six telescopes, requires a new configuration of longitudinal dispersion compensators to keep the fringe contrast above 95% simultaneously in all spectral bands, while preserving the transmission above 85 %. In this paper, we propose a new method for defining the longitudinal dispersion compensators (LDC) suited for multi-band observations. A literal approximation of the contrast loss resulting from the dispersion residues enables us to define a general criterion for fringe contrast maximisation on several bands simultaneously. The optimization of this criterion leads to a simple solution with only two LDC stages per arm and existing differential delay lines, to the glass choice and a simple linear formula for thickness control of all these media. A refined criterion can also take into account glass transmission. After presenting this criterion, we give the optimal solution (medium, configuration) and its expected performance for the planned observing modes on CHARA.
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- PAR ID:
- 10288778
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- ISSN:
- 0035-8711
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Polarization aberrations are found in most optical components due to a materials differing response to s- and p-polarizations. This differing response can manifest either as diattenuation, retardance, or both. Correction of polarization aberrations, such as these, are critical in many applications such as interferometry, polarimetry, display, and high contrast imaging, including astronomy. In this work, compensators based on liquid crystal polymer and anti-reflection thin-films are presented to correct polarization aberrations in both transmission and reflection configurations. Our method is versatile, allowing for good correction in transmission and reflection due to optical components possessing differing diattenuation and retardance dispersions. Through simulation and experimental validation we show two designs, one correcting the polarization aberrations of a dichroic spectral filter over a 170nm wavelength band, and the other correcting the polarization aberration of an aluminum-coated mirror over a 400nm wavelength band and a 55-degree cone of angles. The measured performance of the polarization aberration compensators shows good agreement with theory.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1093/mnras/stab1922
