Multipole engineering by displacement resonance: a new degree of freedom of Mie resonance.

The canonical studies on Mie scattering unravel strong electric/magnetic optical responses in nanostructures, laying foundation for emerging meta-photonic applications. Conventionally, the morphology-sensitive resonances hinge on the normalized frequency, i.e. particle size over wavelength, but non-paraxial incidence symmetry is overlooked. Here, through confocal reflection microscopy with a tight focus scanning over silicon nanostructures, the scattering point spread functions unveil distinctive spatial patterns featuring that linear scattering efficiency is maximal when the focus is misaligned. The underlying physical mechanism is the excitation of higher-order multipolar modes, not accessible by plane wave irradiation, via displacement resonance, which showcases a significant reduction of nonlinear response threshold, sign flip in all-optical switching, and spatial resolution enhancement. Our result fundamentally extends the century-old light scattering theory, and suggests new dimensions to tailor Mie resonances.

Discovery of a potent, orally available tricyclic derivative as a novel BRD4 inhibitor for melanoma.

The epigenetic regulation of the protein bromodomain-containing protein 4 (BRD4) has emerged as a compelling target for cancer treatment. In this study, we outline the discovery of a novel BRD4 inhibitor for melanoma therapy. Our initial finding was that benzimidazole derivative 1, sourced from our library, was a powerful BRD4 inhibitor. However, it exhibited a poor pharmacokinetic (PK) profile. To address this, we conducted a scaffold-hopping procedure with derivative 1, which resulted in the creation of benzimidazolinone derivative 5. This new derivative displayed an improved PK profile. To further enhance the BRD4 inhibitory activity, we attempted to introduce hydrogen bond acceptors. This indeed improved the activity, but at the cost of decreased membrane permeability. Our search for a potent inhibitor with desirable permeability led to the development of tricyclic 18. This compound demonstrated powerful inhibitory activity and a favorable PK profile. More significantly, tricyclic 18 showed antitumor efficacy in a mouse melanoma xenograft model, suggesting that it holds potential as a therapeutic agent for melanoma treatment.

Graphene perfect absorber based on degenerate critical coupling of toroidal mode.

Graphene is a two-dimensional material with great potential for photodetection and light modulation applications owing to its high charge mobility. However, the light absorption of graphene in the near-infrared range is only 2.3%, limiting the sensitivity of graphene-based devices. In this study, we propose a graphene perfect absorber based on degenerate critical coupling comprising monolayer graphene and a hollow silicon Mie resonator array. In particular, monolayer graphene achieves perfect absorption by controlling the periods and holes of the Mie resonators. The proposed graphene perfect absorber can significantly improve the sensitivity of graphene-based devices.

Nonlinear heating and scattering in a single crystalline silicon nanostructure.

Silicon nanophotonics has attracted significant attention because of its unique optical properties such as efficient light confinement and low non-radiative loss. For practical applications such as all-optical switch, optical nonlinearity is a prerequisite, but the nonlinearity of silicon is intrinsically weak. Recently, we discovered a giant nonlinearity of scattering from a single silicon nanostructure by combining Mie resonance enhanced photo-thermal and thermo-optic effects. Since scattering and absorption are closely linked in Mie theory, we expect that absorption, as well as heating, of the silicon nanostructure shall exhibit similar nonlinear behaviors. In this work, we experimentally measure the temperature rise of a silicon nanoblock by in situ Raman spectroscopy, explicitly demonstrating the connection between nonlinear scattering and nonlinear heating. The results agree well with finite-element simulation based on the photo-thermo-optic effect, manifesting that the nonlinear effect is the coupled consequence of the red shift between scattering and absorption spectra. Our work not only unravels the nonlinear absorption in a silicon Mie-resonator but also offers a quantitative analytic model to better understand the complete photo-thermo-optic properties of silicon nanostructures, providing a new perspective toward practical silicon photonics applications.

All-dielectric perfect absorber based on quadrupole modes.

In principle, the absorbance of a free-standing ultra-thin film is limited to 50%. To overcome this limitation, an all-dielectric perfect absorber is proposed herein based on the concept of degenerate critical coupling (DCC) of quadrupole modes. We study the absorbance of a dielectric elliptic cylinder and find that perfect absorption can be achieved by spectrally overlapping peaks of electric and magnetic quadrupole modes. This suggests that the DCC method can be extended to the quadrupole modes beyond dipole modes. Such an all-dielectric perfect absorber can be used in photodetectors, optical filters, and optical modulators mediated by the photothermal effect.

Radiative loss control of an embedded silicon perfect absorber in the visible region.

The absorbance of a free-standing ultrathin layer is limited to 50%; we overcome this limitation by numerically investigating a wavelength-selective perfect absorber based on Mie resonance and degenerate critical coupling. We extend the wavelength of close-to-unity absorbance to the entire visible region by controlling the radiative loss and intrinsic loss. Radiative loss can be controlled by embedding the Mie resonator into a thin film with the defined refractive index. Meanwhile, intrinsic loss can be controlled by addition of a dielectric cap with a higher extinction coefficient on the Mie resonator. Such all-dielectric perfect absorbers can be applied to efficient photodetectors, imaging sensor pixels, or all-optical switching devices mediated by the photothermal effect.

Publisher Correction: Giant photothermal nonlinearity in a single silicon nanostructure.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Giant photothermal nonlinearity in a single silicon nanostructure.

Silicon photonics have attracted significant interest because of their potential in integrated photonics components and all-dielectric meta-optics elements. One major challenge is to achieve active control via strong photon-photon interactions, i.e. optical nonlinearity, which is intrinsically weak in silicon. To boost the nonlinear response, practical applications rely on resonant structures such as microring resonators or photonic crystals. Nevertheless, their typical footprints are larger than 10 µm. Here, we show that 100 nm silicon nano-resonators exhibit a giant photothermal nonlinearity, yielding 90% reversible and repeatable modulation from linear scattering response at low excitation intensities. The equivalent nonlinear index is five-orders larger compared with bulk, based on Mie resonance enhanced absorption and high-efficiency heating in thermally isolated nanostructures. Furthermore, the nanoscale thermal relaxation time reaches nanosecond. This large and fast nonlinearity leads to potential applications for GHz all-optical control at the nanoscale and super-resolution imaging of silicon.

Planar hot-electron photodetector utilizing high refractive index MoS2 in Fabry-Pérot perfect absorber.

Hot electron photodetection (HEPD) excited by surface plasmon can circumvent bandgap limitations, opening pathways for additional energy harvesting. However, the costly and time-consuming lithography has long been a barrier for large-area and mass production of HEPD. In this paper, we proposed a planar and electron beam lithography-free hot electron photodetector based on the Fabry-Pérot (F-P) resonance composed of Au/MoS2/Au cavity. The hot electron photodetector has a nanoscale thickness, high spectral tunability, and multicolour photoresponse in the near-infrared region due to the increased round-trip phase shift by using high refractive index MoS2. We predict that the photoresponsivity can achieve up to 23.6 mA W-1 when double cavities are integrated with the F-P cavity. The proposed hot electron photodetector that has a nanoscale thickness and planar stacking is a perfect candidate for large-area and mass production of HEPD.

Plasmonic interpretation of bulk propagating waves in hyperbolic metamaterial optical waveguides.

Hyperbolic metamaterials (HMMs) show great promise in photonics applications because their unconventional open isofrequency surface permits enlargement of wavenumbers without limitation. Although optical behaviors in HMMs can be macroscopically described by theoretical calculations with the effective medium approximation (EMA), neglect of microscopic phenomena in each layer leads to discrepancies from exact numerical results. We clarify the origin of bulk propagating waves in HMMs and we show that they can be classified into two modes: long- and short-range surface-plasmon-based coupled modes (LRSP and SRSP, respectively). Especially, we find that the ratio of the number of LRSP and SRSP couplings dominates the property of each propagation mode. This plasmonic interpretation bridges the gap between the EMA and numerical exact solutions, thereby facilitating studies on HMM applications.

All-Dielectric Dual-Color Pixel with Subwavelength Resolution.

An all-dielectric optical antenna supporting Mie resonances enables light confinement below the free-space diffraction limit. The Mie scattering wavelengths of the antenna depend on the structural geometry, which allows the antennas to be used for colored imprint images. However, there is still room for improving the spatial resolution, and new polarization-dependent color functionalities are highly desirable for realizing a wider color-tuning range. Here, we show all-dielectric color printing by means of dual-color pixels with a subwavelength-scale resolution. The simple nanostructures fabricated with monocrystalline silicon exhibit various brilliant reflection color by tuning the physical dimensions of each antenna. The designed nanostructures possess polarization-dependent properties that make it possible to create overlaid color images. The pixels will generate individual color even if operating as a single element, resulting in the achievement of subwavelength-resolution encoding without color mixing. This printing strategy could be used to further extend the degree of freedom in structural color design.

Ultrabroadband absorber based on single-sized embedded metal-dielectric-metal structures and application of radiative cooling.

The realization of ultrabroadband absorption is a fundamental part of a thermal emitter, especially in the application of radiative cooling. This study involved proposing and systematically analyzing a novel structure termed as an embedded metal-dielectric-metal (EMDM) structure. The results in the case of an individual resonator indicated that the EMDM resonator displayed a broader full width at half maximum (FWHM) that was 1.9 times that of the metal-dielectric-metal (MDM) resonator due to mode matching at the terminated end and enhanced scattering intensity. With respect to the case of periodic resonators, single-sized periodic EMDM resonators are employed to achieve a broader FWHM that is 3.8 times that of the MDM resonators. In addition, a strong coupling effect is confirmed between localized MDM and hybrid modes. An application of lossy-dielectric based periodic three-dimensional EMDM resonators indicated that an average absorptivity of 0.85 in the entire atmospheric window (8-13 µm). The results revealed the potential of EMDM structures for radiative cooling devices and other ultrabroadband absorbers.

Full-Color Subwavelength Printing with Gap-Plasmonic Optical Antennas.

Metallic nanostructures can be designed to effectively reflect different colors at deep-subwavelength scales. Such color manipulation is attractive for applications such as subwavelength color printing; however, challenges remain in creating saturated colors with a general and intuitive design rule. Here, we propose a simple design approach based on all-aluminum gap-plasmonic nanoantennas, which is capable of designing colors using knowledge of the optical properties of the individual antennas. We demonstrate that the individual-antenna properties that feature strong light absorption at two distinct frequencies can be encoded into a single subwavelength-pixel, enabling the creation of saturated colors, as well as a dark color in reflection, at the optical diffraction limit. The suitability of the designed color pixels for subwavelength printing applications is demonstrated by showing microscopic letters in color, the incident polarization and angle insensitivity, and color durability. Coupled with the low cost and long-term stability of aluminum, the proposed design strategy could be useful in creating microscale images for security purposes, high-density optical data storage, and nanoscale optical elements.

Gap Plasmon Resonance in a Suspended Plasmonic Nanowire Coupled to a Metallic Substrate.

We present an experimental demonstration of nanoscale gap plasmon resonators that consist of an individual suspended plasmonic nanowire (NW) over a metallic substrate. Our study demonstrates that the NW supports strong gap plasmon resonances of various gap sizes including single-nanometer-scale gaps. The obtained resonance features agree well with intuitive resonance models for near- and far-field regimes. We also illustrate that our suspended NW geometry is capable of constructing plasmonic coupled systems dominated by quasi-electrostatics.

Modeling and Prediction of Solvent Effect on Human Skin Permeability using Support Vector Regression and Random Forest.

PURPOSE: The solvent effect on skin permeability is important for assessing the effectiveness and toxicological risk of new dermatological formulations in pharmaceuticals and cosmetics development. The solvent effect occurs by diverse mechanisms, which could be elucidated by efficient and reliable prediction models. However, such prediction models have been hampered by the small variety of permeants and mixture components archived in databases and by low predictive performance. Here, we propose a solution to both problems. METHODS: We first compiled a novel large database of 412 samples from 261 structurally diverse permeants and 31 solvents reported in the literature. The data were carefully screened to ensure their collection under consistent experimental conditions. To construct a high-performance predictive model, we then applied support vector regression (SVR) and random forest (RF) with greedy stepwise descriptor selection to our database. The models were internally and externally validated. RESULTS: The SVR achieved higher performance statistics than RF. The (externally validated) determination coefficient, root mean square error, and mean absolute error of SVR were 0.899, 0.351, and 0.268, respectively. Moreover, because all descriptors are fully computational, our method can predict as-yet unsynthesized compounds. CONCLUSION: Our high-performance prediction model offers an attractive alternative to permeability experiments for pharmaceutical and cosmetic candidate screening and optimizing skin-permeable topical formulations.

Identification of Plasmonic Modes in Parabolic Cylinder Geometry by Quasi-Separation of Variables.

This paper describes the plasmonic modes in the parabolic cylinder geometry as a theoretical complement to the previous paper (J Phys A 42:185401) that considered the modes in the circular paraboloidal geometry. In order to identify the plasmonic modes in the parabolic cylinder geometry, analytic solutions for surface plasmon polaritons are examined by solving the wave equation for the magnetic field in parabolic cylindrical coordinates using quasi-separation of variables in combination with perturbation methods. The examination of the zeroth-order perturbation equations showed that solutions cannot exist for the parabolic metal wedge but can be obtained for the parabolic metal groove as standing wave solutions indicated by the even and odd symmetries.

In Silico Predictions of Human Skin Permeability using Nonlinear Quantitative Structure-Property Relationship Models.

PURPOSE: Predicting human skin permeability of chemical compounds accurately and efficiently is useful for developing dermatological medicines and cosmetics. However, previous work have two problems; 1) quality of databases used, and 2) methods for prediction models. In this paper, we attempt to solve these two problems. METHODS: We first compile, by carefully screening from the literature, a novel dataset of chemical compounds with permeability coefficients, measured under consistent experimental conditions. We then apply machine learning techniques such as support vector regression (SVR) and random forest (RF) to our database to develop prediction models. Molecular descriptors are fully computationally obtained, and greedy stepwise selection is employed for descriptor selection. Prediction models are internally and externally validated. RESULTS: We generated an original, new database on human skin permeability of 211 different compounds from aqueous donors. Nonlinear SVR achieved the best performance among linear SVR, nonlinear SVR, and RF. The determination coefficient, root mean square error, and mean absolute error of nonlinear SVR in external validation were 0.910, 0.342, and 0.282, respectively. CONCLUSIONS: We provided one of the largest datasets with purely experimental log kp and developed reliable and accurate prediction models for screening active ingredients and seeking unsynthesized compounds of dermatological medicines and cosmetics.

Multi-spectral plasmon induced transparency via in-plane dipole and dual-quadrupole coupling.

We experimentally demonstrated an approach based on dipole and dual-quadrupole coupling to construct a planar metamaterial supporting multi-spectral plasmon induced transparency. The structure consists of two short silver wires (dipole) and two long silver wires (dual-quadrupole). The in-plane coupling between the dipole and the dual-quadrupole leads to two transmission windows even in the absorbance linewidth of the dipole. This phenomenon is well described and understood by numerical analyses and a classical oscillator model.

Mutual mode control of short- and long-range surface plasmons.

A symmetric metal slab waveguide simultaneously supports two opposite types of propagation mode similar to a metal film: short-range surface plasmon (SRSP) like mode and long-range surface plasmon (LRSP) like mode. The strong field confinement of SRSP-like mode plays a crucial role for nano-optical integrated circuits in spite of short propagation length. In order to avoid the trade-off between field confinement and propagation length, we demonstrate selective mode excitation and mutual mode conversion for nanofocusing mediated by LRSP-like mode.

Colloidal quantum dot-based plasmon emitters with planar integration and long-range guiding.

We present an experimental demonstration of a quantum dot (QD)-based plasmon emitter controllably integrated in designed patterns on a thin metal film. The generation of surface plasmons polaritons (SPPs) from optically excited QDs on a thin metal film is experimentally demonstrated. Long-range, low-dispersion, two-dimensional isotropic guiding, as well as efficient coupling of the SPPs are also shown. The realization of planar, low loss and efficient plasmon emitter-waveguide integration will offer further development of plasmon circuits.