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Abstract Materials with large birefringence (Δn, wherenis the refractive index) are sought after for polarization control (e.g., in wave plates, polarizing beam splitters, etc.), nonlinear optics, micromanipulation, and as a platform for unconventional light–matter coupling, such as hyperbolic phonon polaritons. Layered 2D materials can feature some of the largest optical anisotropy; however, their use in most optical systems is limited because their optical axis is out of the plane of the layers and the layers are weakly attached. This work demonstrates that a bulk crystal with subtle periodic modulations in its structure—Sr9/8TiS3—is transparent and positive‐uniaxial, with extraordinary indexne= 4.5 and ordinary indexno= 2.4 in the mid‐ to far‐infrared. The excess Sr, compared to stoichiometric SrTiS3, results in the formation of TiS6trigonal‐prismatic units that break the chains of face‐sharing TiS6octahedra in SrTiS3into periodic blocks of five TiS6octahedral units. The additional electrons introduced by the excess Sr form highly oriented electron clouds, which selectively boost the extraordinary indexneand result in record birefringence (Δn> 2.1 with low loss). The connection between subtle structural modulations and large changes in refractive index suggests new categories of anisotropic materials and also tunable optical materials with large refractive‐index modulation.
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Designing optical anisotropy in low-index nanolattices
This research investigates the optical anisotropy and structure-induced birefringence in low-index nanolattices. By designing the unit-cell geometry using 3-dimentional (3D) colloidal lithography, nanolattices can exhibit different refractive indices along orthogonal directions due to the structure geometry. The out-of-plane and in-plane indices are characterized using spectroscopic ellipsometry and agree well with the anisotropic Cauchy material model. Exhibit positive-uniaxial birefringence, the nanolattices can have up to Δn = 0.003 for nanolattices with low indices that range from 1.04 to 1.12. The birefringence is modeled using the finite-difference-time-domain (FDTD) method, where the reflectance of an anisotropic film is calculated to iteratively solve for the indices. The theoretical model and experimental data indicate that the birefringence can be controlled by the unit-cell geometry based on the relative length scale of the particle diameter to the exposure wavelength. This work demonstrates that it is possible to precisely design optical birefringence in 3D nanolattices, which can find applications in polarizing optics, nanophotonics, and wearable electronics.
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- PAR ID:
- 10579372
- Publisher / Repository:
- Optical Society of America
- Date Published:
- Journal Name:
- Optics Express
- Volume:
- 33
- Issue:
- 7
- ISSN:
- 1094-4087; OPEXFF
- Format(s):
- Medium: X Size: Article No. 15304
- Size(s):
- Article No. 15304
- Sponsoring Org:
- National Science Foundation
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