Organic−inorganic hybrid perovskites hold great potential for various optoelectronic devices with exceptional properties. Although the direct generation of circularly polarized emission from perovskites would enable various compact devices, achieving a large degree of circular polarization (DCP) at room temperature still remains challenging. Herein, we demonstrate that DCP can be strongly enhanced at the narrow mode position of chiral Fano resonances. In our design, a perovskite film is spin-coated on a symmetry-broken structure with a relatively large feature size. A large DCP of more than 0.5 is achieved at room temperature without the direct patterning of the perovskite layer. Reciprocity calculation reveals that chiral field enhancement enables the emission of opposite helicity to couple into counterpropagating slab modes and leads to a large DCP. Our design is very general and scalable. Our work may lead to circularly polarized light sources based on various perovskite materials.

The optical properties of a medium are described by the dielectric permittivity ε and the refractive index N, where ε is a measure how much polarization is induced upon application of an optical electric field, while refractive index N determines how the optical phase develops as optical wave propagates in the optical medium by associating momentum k and energy ω. Optical epsilon-near-zero (ENZ) material possesses the permittivity |ε| → 0 and the phase velocity of optical wave becomes very large while the group velocity is slowing down significantly, owing to the relation between refractive index and permittivity, ε = N .[1] Related to nonlinear optical processes, this simple equation also implies that the optical Kerr nonlinearity is strongly enhanced in the ENZ spectral range. [2] Enhanced Kerr nonlinearities are observed in metamaterials such as conducting oxides and doped inorganic semiconductor thin films showing epsilon-nearzero (ENZ) response in the infrared region. However, to achieve ENZ in the visible, artificial metamaterials with more complex nanostructures have to be specifically designed. [2,4-bis[8-hydroxy-1,1,7,7-tetramethyljulolidin-9-yl] squaraine] organic thin films, ENZ responses between 450 and 620 nm are demonstrated. Both nonlinear refractive index and nonlinear absorption coefficient are enhanced by more than two orders of magnitude in the ENZ spectral region. These optical effects in the visible spectral range come from the strongly dispersive permittivity of molecular aggregates resulting from the coupling of excitonic transition dipoles. These findings open the path toward a next generation of high-performance solution-processable organic nonlinear optical materials with ENZ properties that can be tuned by molecular engineering. Here, using sodium [5,6-dichloro-2-[[5,6-dichloro-1-ethyl-3-(4-sulphobutyl)-benzimidazol-2-ylidene]-propenyl]-1-ethyl-3-(4-sulphobutyl)-benzimidazolium hydroxide] and

Four-dimensional (4D) printing can add active and responsive functions to three-dimensional (3D) printed objects in response to various external stimuli. Light, among others, has a unique advantage of remotely controlling structural changes to obtain predesigned shapes. In this study, we demonstrate multicolor 4D printing of shape-memory polymers (SMPs). Using color-dependent selective light absorption and heating in multicolor SMP composites, we realize remote actuation with light illumination. We experimentally investigate the temperature changes in colored SMPs and observe a clear difference between the different colors. We also present simulations and analytical calculations to theoretically model the structural variations in multicolor composites. Finally, we consider a multicolor hinged structure and demonstrate the multistep actuation by changing the color of light and duration of illumination. 4D printing can allow complex, multicolor geometries with predesigned responses. Moreover, SMPs can be reused multiple times for thermal actuation by simply conducting thermomechanical programming again. Therefore, 4D printing of multicolor SMP composites have unique merits for light-induced structural changes. Our study indicates that multicolor 4D printing of SMPs are promising for various structural changes and remote actuation.

Bound states in the continuum (BICs) or trapped modes can provide an important new avenue for strong light confinement via destructive interference. Dielectric photonic structures have been extensively studied for optical BICs. However, BICs in plasmonic nanostructures have not been explored much yet. Herein, we present a thorough experimental study of plasmonic BICs via Fourier-plane spectroscopy and imaging. Optical mode dispersion in a metal grating covered by a dielectric layer is directly measured in an angle-resolved white light reflection spectrum. Two dielectric layer thicknesses are considered. Both plasmonic and photonics modes are supported in the visible range using a thicker dielectric film; hence, either hybrid or purely plasmonic BICs can be formed. With a thinner dielectric layer, only plasmonic modes are strongly excited and purely plasmonic BICs appear. Our measurements exhibit all features expected for BICs, including a substantial increase in the Q factor. We also demonstrate that the BIC position can be switched from one optical mode branch to the other by tuning a metal grating parameter. Moreover, by mixing luminescent dyes in a dielectric layer, light emission coupling into BICs is investigated. We find that the photoluminescence peak disappears at the BIC condition, which is attributed to the trapping of molecular emission at plasmonic BICs. Therefore, both white light reflection and dye emission measurements in the Fourier plane clearly indicate the formation of trapped modes in plasmonic nanostructures. Our observation implies that plasmonic BICs can enable a highly effective light trapping device despite the simple structure of the device geometry. Plasmonic supercavity design based on the BIC concept may provide many interesting future opportunities for nanolasers, optical sensing, and nonlinear enhancement.

Strong light absorption in ultrathin films has been of great interest for both fundamental studies and device applications. Here we demonstrate and analyze controllable superabsorption in excitonic thin films in the visible region. By adjusting the concentration of J-aggregate dyes, we control the dispersion of excitonic films (from optically metallic to nonmetallic ones) and show that this leads to drastic changes in the optical response of organic thin films. We find that planar excitonic films can have various optical features in the visible region, for example, surface polaritons, epsilon-near-pole, asymmetric Fabry−Perot type resonances, and so on. We leverage these diverse features to study perfect absorption in planar films without additional structural patterning. We also demonstrate that strong light absorption can even occur away from an excitonic absorption peak (i.e., maximum optical loss position) due to cavity-like resonances in the high dielectric constant region. Our work demonstrates that there are unique opportunities for dispersion control in the visible region with easy-to-handle organic molecules, and this can be useful for novel nano-optical studies or energy conversion devices. Collaborative synergy between molecular photonics and nanoscale optics has been demonstrated throughout this work.