Harry A. Atwater

Barium titanate (BTO), one of the earliest reported ferroelectric materials, exhibits strong second-order nonlinearity and an ultrafast Pockels effect, making it highly attractive for photonics. However, its optical constants remain uncertain largely due to the ubiquitous ferroelectric–ferroelastic multidomain microstructure, which depolarizes and averages out the anisotropic optical response, thereby complicating conventional ellipsometric approaches. Here, we report a Kramers–Kronig-consistent complex refractive-index tensor of tetragonal BTO from the ultraviolet (190 nm) to the far-infrared (40 μm) derived from multimodal Mueller-matrix spectroscopic ellipsometry. We further quantify optical loss by incorporating transmission intensity measured over the 2–10 μm transparency window, resolving the extinction coefficient down to ∼10–5. To verify the underlying domain configuration, we performed independent structural analyses, including electron backscatter diffraction, confocal Raman microscopy, and Kelvin probe force microscopy. We then reconstruct an effective optical model for our multidomain BTO sample, formulated as a mixture of three tetragonal variants with the determined intrinsic refractive indices, to link microscopic domains to the macroscopic optical response. Our work presents a unified, experimentally cross-validated data set, enabling accurate modeling for electro-optic modulators and high-Q nanophotonic resonators without ad hoc approximations, and provides a general strategy for optical metrology of complex ferroic crystals.

We report herein phototropic growth toward, and skototropic growth away from, an incoherent, unstructured, low-intensity light stimulus in the deposition of nonliving, inorganic material. Fundamental light–matter interactions yield growth of inorganic deposits with ordered mesostructures that are a function of the characteristics of the light stimulus. Illumination at normal incidence, as well as at substantial inclination, resulted in phototropic growth toward the direction of incidence. However, illumination incident at shallow inclinations resulted in growth away from the stimulus, a skototropic response. We show that the skototropic behavior of these materials is an emergent phenomenon produced primarily by nearest-neighbor optical scattering interactions during growth. We moreover observe similar scaling relations between the morphologies of the inorganic deposits and the growth habits of trees and forest stands, despite the differences in growth mechanisms and differences of >10 ⁸ in scale length.

Light in the mid-infrared (mid-IR) spans wavelengths from 3 to 8 micrometers and is important to many applications such as gas sensing and thermal imaging. Due to materials challenges, there is now a lack of mid-IR reconfigurable optical elements. Here, we present an electrically addressable metasurface for modulation of coherent and incoherent mid-IR radiation in two spatial dimensions. Our device achieves optical modulation due to the field-effect free-carrier depletion in a lightly doped (10 ¹⁹ per cubic centimeter) film of indium tin oxide coupled to a gap plasmon resonator. By addressing 32 individual elements across the metasurface, we first demonstrate tunable diffraction of coherently reflected mid-IR light. Next, we introduce a scalable perimeter-addressed driving scheme for tunable diffraction in two dimensions. Last, we demonstrate modulated emissivity with spatially reconfigurable two-dimensional patterns at elevated temperatures. This work advances the development of solid-state reflective beam-steering devices in the mid-IR and manipulation of thermally emitted incoherent radiation.

Achieving spatiotemporal control of light at subwavelength and subcycle scales is an important milestone in the development of new photonic materials for signal processing, pulse shaping and ultrafast imaging. Spatiotemporal light modulation currently relies on electronic interband and intraband transitions that yield pronounced refractive index changes but typically suffer from slow, picosecond response times due to carrier relaxation. Here we show that by leveraging resonant light-matter interactions in a high-quality factor metasurface it is possible to use the optical Kerr effect, a weaker but subfemtosecond optoelectronic polarization effect, to achieve ultrafast, reconfigurable light modulation. By the subwavelength all-optical tuning of the refractive index of the dielectric metasurface unit cells with a spatially structured pump beam, we experimentally demonstrate pulse-limited beam steering with a 74-fs response time at angles up to ±13° in the near-infrared, where the deflection angles are programmable by the pump pattern. The steering originates from the Kerr effect, with a background contribution arising from a slower two-photon-excited free carrier absorption. Additionally, we observe pump self-modulation and self-diffraction, linear frequency conversion, and demonstrate arbitrary subpicosecond spatial light modulation in two dimensions.

Achieving laser-driven relativistic light sails would represent a tremendous breakthrough for humankind. Numerous sail designs have been proposed, but none satisfy all the stringent optical, mechanical, and mass constraints. Here we demonstrate a class of nanolaminate sails with strong and flexible hexagonally-corrugated microstructures. Our prototypes, fabricated from alumina and molybdenum disulfide using scalable semiconductor processing techniques, feature areal densities of < 1 g ⋅ m ⁻² , achieve experimentally-measured broadband reflectivities of > 50%, and feature broadband absorptivities of < 4% with a measurement uncertainty that overlaps with zero - indicative of our sail class’s potential for fast acceleration and ultra-low photon absorption. Moreover, we propose a sail’s maximum achievable relative velocity as a performance benchmark, and analyze optical, mechanical, and mass constraints for our design and others in the literature to highlight the strong potential of our class of sails. Our approach represents a promising step toward plausible relativistic interstellar propulsion.

Accessing diverse polarization states across the Poincaré sphere via electrical control is highly desirable in optical communications, bio-imaging and quantum information processing, where fast continuous switching, compactness, and ease of integration into photonikic circuits are essential. Layered anisotropic 2D materials enable polarization modulation in a fast and compact manner, thereby overcoming the speed and size limitations of liquid-crystal based spatial light modulators and electro-optic Pockels cell modulators. Here, we report a Fabry-Perot cavity that integrates two cross-aligned black phosphorus layers, with each layer being independently gated to access the inherently two-dimensional range of polarization states across the Poincaré sphere surface. Our heterostructure design predicts electronic access to 86% of the Poincaré sphere at its S-band operating wavelength. We experimentally validate the design by fabricating and testing such a device in reflection, where we demonstrate independent, two-parameter electronic control of the output polarization.