Young Chul Jun

Combining semiconductor quantum dots (QDs) with dielectric metasurfaces offers a promising platform for enhancing and controlling light emission properties. Highly enhanced directional and polarized emission is experimentally demonstrated from dielectric metasurfaces integrated with colloidal QDs. Specifically, emission enhancements from InP/ZnS QD‐ and CdSe/ZnS QD‐integrated metasurfaces are directly compared. It is found that low absorption losses in the emitter layer can drastically increase the maximum achievable enhancement when different QDs are coated on the same metasurface. In particular, the lower intrinsic losses of the InP/ZnS QD layer enable dramatically enhanced vertical emission with narrow angular divergence, achieving a 140‐fold enhancement in experiment. Our detailed experimental and theoretical investigations indicate the critical role of the emitter layer in the design of the emitter‐integrated metasurface. Strong agreement between simulation and experiment confirms the critical‐coupling behavior in maximized emission enhancement. Our study provides general guidelines for the development of directional light sources. Compact light sources with high directionality and controlled polarization hold strong potential for applications in displays, optical communications, sensing, and imaging.

Chiral perfect absorbers that enable the handedness-dependent near-unity absorption of circularly polarized light are highly desirable for enhancing various chiral interactions. However, the experimental realization of ultranarrowband absorbers with extreme chiral responses remains challenging. Here, we realize such chiral absorbers by incorporating Brillouin zone folding and mirror symmetry breaking in the design of planar dielectric metasurface. When the metasurface is backed by a reflection mirror, strong chiral absorption with ultranarrow line width can be achieved in the normal direction. We experimentally achieve a substantial differential absorptance of 0.75 with an ultranarrow line width of 1.4 nm in the visible region. Furthermore, by leveraging handedness-dependent strong field enhancement, high levels of chiral emission with a luminescence line width of 1.75 nm are demonstrated from quantum dots coated on the absorber surface. Our findings may unlock interesting opportunities in chiral photodetectors, optical filters, nonlinear optics, light sources, and quantum photonic devices.

Critical coupling can induce maximized field enhancement in resonant optical modes. Therefore, it is important for various photonic technologies. Here, it is shown that directional light sources with highly enhanced emission intensities can be realized via critical coupling. A clear experimental demonstration of maximized emission enhancement is presented in quantum dot (QD)‐coated Brillouin zone folding (BZF) metasurfaces. BZF dielectric metasurfaces support guided‐mode resonances, where the radiative quality factor can be gradually tuned by structural parameters, allowing critical coupling to occur at the QD emission wavelength. Maximized enhancements of polarized and unpolarized emissions are demonstrated in the normal direction, resulting in highly enhanced, directional, and narrow‐angled emissions. The investigations indicate that light emission from quantum emitters can be optimized via critical coupling and that BZF metasurfaces can provide a highly tunable platform for both polarization‐sensitive and polarization‐insensitive critical coupling. Maximized field enhancement and highly enhanced light–matter interactions in BZF metasurfaces are important for a wide range of photonic technologies such as light sources, photodetectors, sensors, nonlinear enhancement, and quantum photonic devices.

Chiral emission exhibiting a large degree of circular polarization (DCP) is important in diverse applications ranging from displays and optical storage to optical communication, bioimaging, and medical diagnostics. Although chiral luminescent materials can generate chiral emissions directly, they frequently suffer from either low DCP or low quantum efficiencies. Achieving high DCP and quantum efficiencies simultaneously remains extremely challenging. This review introduces an alternative approach to chiral emission. Chiral emission with large DCP can be readily achieved by combining conventional achiral emitters with chiral metasurfaces. Particularly, this article focuses on recent experimental and theoretical studies on perovskite metasurfaces and metacavities that employ achiral perovskite materials. First, chiral photoluminescence from extrinsic and intrinsic perovskite metasurfaces is explained together with theoretical discussions on metasurface design based on reciprocity and critical coupling. Chiral photoluminescence from other achiral materials is also explained. Subsequently, chiral electroluminescence from perovskite metacavities and other achiral materials is discussed. Finally, it is concluded with future perspectives. This review provides physical insights into how ideal chiral emission can be realized by optimizing the design of metasurfaces and metacavities. Compact chiral light sources with both near‐unity DCP and strong emission intensities can have far‐reaching consequences in a wide range of future applications.

Exciton polaritons are hybrid light‐matter quasi‐particles that hold exceptional opportunities for future optoelectronic devices. Taking the synergic advantages of room‐temperature perovskite excitons and topological photonic structures, topological exciton‐polaritons are experimentally demonstrated in organic–inorganic hybrid perovskite thin films. Topological junction structures based on perovskite gratings are realized using a momentum‐space analog of the 1D Dirac system. Desired enhancement phenomena are observed including narrow‐beam polariton emission from a tightly localized junction region, polaritonic nonlinearity boost, and enhanced luminescence. These remarkable features are obtained from highly compact devices with footprint widths on the order of a few micrometers and are efficiently tailorable with simple unit‐cell geometry control. Therefore, the proposed approach can be a powerful platform for room‐temperature topological exciton‐polaritons and concomitant device applications.

Three-dimensional (3D) printing allows the fabrication of complex shapes with high resolutions. However, the printed structures typically have fixed shapes and functions. Four-dimensional printing allows the shapes of 3D-printed structures to be transformed in response to external stimuli. Among the external stimuli, light has unique advantages for remote thermal actuation. However, light absorption in opaque structures occurs only near the sample surface; thus, actuation can be slow. Here, we propose and experimentally demonstrate the rapid and selective actuation of 3D-printed shape-memory polymer (SMP) composites using microwave heating. The SMP composite filaments are prepared using different amounts of graphite flakes. Microwave radiation can penetrate the entire printed structures and induce rapid heating. With sufficient graphite contents, the printed SMP composites are heated above their glass transition temperature within a few seconds. This leads to rapid thermal actuation of the 3D-printed SMP structures. Finally, dual-material 3D printing is demonstrated to induce selective microwave heating and control actuation motion. Our experiments and simulations indicate that microwave heating of SMP composites can be an effective method for the rapid and selective actuation of complex structures.

Chiral light sources realized in ultracompact device platforms are highly desirable for various applications. Among active media used for thin-film emission devices, lead-halide perovskites have been extensively studied for photoluminescence due to their exceptional properties. However, up to date, there have been no demonstrations of chiral electroluminescence with a substantial degree of circular polarization (DCP) based on perovskite materials, being critical for the development of practical devices. Here, we propose a concept of chiral light sources based on a thin-film perovskite metacavity and experimentally demonstrate chiral electroluminescence with a peak DCP approaching 0.38. We design a metacavity created by a metal and a dielectric metasurface supporting photonic eigenstates with a close-to-maximum chiral response. Chiral cavity modes facilitate asymmetric electroluminescence of pairs of left and right circularly polarized waves propagating in the opposite oblique directions. The proposed ultracompact light sources are especially advantageous for many applications requiring chiral light beams of both helicities.

Exceptional points (EPs) are degenerate singularities in a non‐Hermitian system that can be induced by controlling the interaction between resonant photonic modes. EPs can enable unusual optical phenomena and significantly enhance the optical sensitivity under small perturbations. However, most studies thus far have been limited to static photonic structures. In this study, we propose and experimentally demonstrate electrically addressable EP in a plasmonic structure. Inspired by optical microcavity studies, we employ a localized spoof plasmon structure that supports circulating plasmonic modes in compact single‐resonator geometry. The plasmonic modes are perturbed by an angled metal line, and the interaction between the plasmonic modes is electrically controlled using a varactor. Continuous electrical tuning of the varactor capacitance facilitates simultaneous coalescence of the real and imaginary parts of the eigenfrequency, allowing the direct addressing of EPs. We first investigate the eigenmodes and their coupling in localized plasmonic structures using numerical simulations. We then present experimentally measured spectra that manifest the coalescence of the two resonant modes in both the resonance frequency and linewidth. Electrically addressable EPs in compact plasmonic structures may provide exciting opportunities for highly functional and tunable elements in integrated device platforms.

Exceptional points (EPs) are degenerate singularities in a non-Hermitian system that can be induced by controlling the interaction between resonant photonic modes. EPs can enable unusual optical phenomena and significantly enhance the optical sensitivity under small perturbations. However, most studies thus far have been limited to static photonic structures. In this study, we propose and experimentally demonstrate electrically addressable EP in a plasmonic structure. Inspired by optical microcavity studies, we employ a localized spoof plasmon structure that supports circulating plasmonic modes in a compact single-resonator geometry. The plasmonic modes are perturbed by an angled metal line, and the interaction between the plasmonic modes is electrically controlled using a varactor. Continuous electrical tuning of the varactor capacitance facilitates simultaneous coalescence of the real and imaginary parts of the eigenfrequency, allowing the direct addressing of EPs. We first investigate the eigenmodes and their coupling in localized plasmonic structures using numerical simulations. We then present experimentally measured spectra that manifest the coalescence of the two resonant modes in both the resonance frequency and linewidth. Electrically addressable EPs in compact plasmonic structures may provide exciting opportunities for highly functional and tunable elements in integrated device platforms.

Chiral light sources realized in ultracompact device platforms are highly desirable for various applications. Among active media employed for thin-film emission devices, lead-halide perovskites have been extensively studied for photoluminescence due to their exceptional properties. However, up to date, there have been no demonstrations of chiral electroluminescence with a substantial degree of circular polarization (DCP), being critical for the development of practical devices. Here, we propose a new concept of chiral light sources based on a thin-film perovskite metacavity and experimentally demonstrate chiral electroluminescence with DCP approaching 0.38. We design a metacavity created by a metal and a dielectric metasurface supporting photonic eigenstates with close-to-maximum chiral response. Chiral cavity modes facilitate asymmetric electroluminescence of pairs of left and right circularly polarized waves propagating in the opposite oblique directions. The proposed ultracompact light sources are especially advantageous for many applications requiring chiral light beams of both helicities.