Enhancement of second-harmonic generation in a 64° stacked WSe2/WS2heterobilayer with local strain.

Transition metal dichalcogenides (TMDs) are actively studied in various fields of optics and optoelectronics, including nonlinear optics of second-harmonic generation (SHG). By stacking two different TMD materials to form a heterobilyaer, unique optical properties emerge, with stronger SHG at a twist angle of 0° between TMDs and weaker SHG at a twist angle of 60°. In this work, we demonstrate the enhancement of SHG in a heterobilayer consisting of WSe2and WS2monolayers stacked at a twist angle of 64.1°, using a nanoparticle to induce local strain. The interatomic spacing of the heterobilayer is deformed by the nanoparticle, breaking the inversion symmetry, resulting in a substantial increase in the SHG of the heterobilayer at room temperature. The SHG increases depending on the polarization of the pump laser: 15-fold for linear polarization, 9-fold for right-circular polarization, and up to 100-fold for left-circular polarization. In addition, the SHG enhanced in the heterobilayer with local strain satisfies the same chiral selection rule as in the unstrained TMD region, demonstrating that the chiral selection rule of SHG is insensitive to local strain. Our findings will increase the applicability of TMD heterobilayers in nonlinear optoelectronics and valleytronics.

Cooling Metals via Gap Plasmon Resonance.

We report highly emissive and radiatively cooled metallic surfaces that sustain multiple and high-amplitude gap plasmon cavity modes within the principal thermal radiation spectrum at room temperature (i.e., 8-13 µm). A square-lattice array of Cu/ZnS/Cu gap plasmon cavities with five different widths was designed to avoid the near-field coupling between adjacent cavities and the anticrossing of a cavity mode and the first diffraction mode. The gap plasmon cavities fabricated on a Si substrate exhibited an effective emissivity of >0.62, up to an incidence of 60°. Outdoor solar heating experiments showed that the Cu/ZnS/Cu multicavity array lowered the Si substrate temperature by 4 °C at a maximum solar irradiance of 800 W/m2, which is equivalent to a near-one-sun intensity, relative to a planar Cu/ZnS/Cu multilayer. Such mid-infrared spectrum management of metals enables heat dissipation via radiation, which will be further utilized for designing electrodes that cool optoelectronic devices with the same metal/dielectric/metal configuration.

Tuning of polarized room-temperature thermal radiation based on nanogap plasmon resonance.

When a one-dimensional (1D) metal array is coupled to a planar metal mirror with a dielectric gap, localized plasmon resonance is excited inside the gap at a specific polarization of light in free space. Herein, we report on the completely polarized, mid-infrared thermal radiation that is released from gap plasmon resonators with a nanometer-thick dielectric. We fabricated nanogap plasmon resonators with 1D Au or Ni array of various widths (w) using laser interference lithography. An atomic layer deposition process was used to introduce a 10 nm-thick alumina gap between a 1D metal array and a planar metal mirror. It was observed that only for the Au nanogap plasmon resonators, high-amplitude absorption peaks that were attributed to gap plasmon modes with different orders appeared at discrete wavelengths in a polarization-resolved spectrum. In addition, all the pronounced peaks were gradually redshifted with increasing w. At w = 1.2-1.6 µm, the fundamental gap plasmon mode was tuned to the main wavelengths (8-9 µm) of thermal radiation at room temperature (e.g., ∼300 K), which led to polarization-selective camouflage against standard infrared thermal imaging. The results of electromagnetic simulations quantitatively agreed with the measured absorbance spectra in both peak wavelength and amplitude. We believe that these experimental efforts towards achieving radiation/absorption spectra tailored at mid-infrared wavelengths will be further exploited in thermal-radiation harnessed energy devices, spectroscopic sensors, and radiative coolers.

A Full-Visible-Spectrum Invisibility Cloak for Mesoscopic Metal Wires.

Structured metals can sustain a very large scattering cross-section that is induced by localized surface plasmons, which often has an adverse effect on their use as transparent electrodes in displays, touch screens, and smart windows due to an issue of low clarity. Here, we report a broadband optical cloaking strategy for the network of mesoscopic metal wires with submicrometer to micrometer diameters, which is exploited for manufacturing and application of high-clarity metal-wires-based transparent electrodes. We prepare electrospun Ag wires with 300-1800 nm in diameter and perform a facile surface oxidation process to form Ag/Ag2O core/shell heterogeneous structures. The absorptive Ag2O shell, together with the coating of a dielectric cover, leads to the cancellation of electric multipole moments in Ag wires, thereby drastically suppressing plasmon-mediated scattering over the full visible spectrum and rendering Ag wires to be invisible. Simultaneously with the effect of invisibility, the transmittance of Ag/Ag2O wires is significantly improved compared to bare Ag wires, despite the formation of an absorptive Ag2O shell. As an application example, we demonstrate that these invisible Ag wires serve as a high-clarity, high-transmittance, and high-speed defroster for automotive windshields.

Microstructured void gratings for outcoupling deep-trap guided modes.

Breaking the total internal reflection far above a critical angle (i.e., outcoupling deep-trap guided modes) can dramatically improve existing light-emitting devices. Here, we report a deep-trap guided modes outcoupler using densely arranged microstructured hollow cavities. Measurements of the leaky mode dispersions of hollow-cavity gratings accurately quantify the wavelength-dependent outcoupling strength above a critical angle, which is progressively improved over the full visible spectrum by increasing the packing density. Comparing hollow- and filled-cavity gratings, which have identical morphologies except for their inner materials (void vs. solid sapphire), reveals the effectiveness of using the hollow-cavity grating to outcouple deep-trap guided modes, which results from its enhanced transmittance at near-horizontal incidence. Scattering analysis shows that the outcoupling characteristics of a cavity array are dictated by the forward scattering characteristics of their individual cavities, suggesting the importance of a rationally designed single cavity. We believe that a hollow-cavity array tailored for different structures and spectra will lead to a technological breakthrough in any type of light-emitting device.

Vertical beaming of incoherent quantum emitters via the near-field coupling of Fano resonance.

We report highly collimated radiation from incoherent quantum emitters coupled to photonic dispersion-engineered structures. Two-dimensional free-standing photonic crystal slabs sustained an extremely high density of states for vertically leaky light at discrete frequencies, which results from the constructive interference between directly reflected light and quasi-bound guided modes, referred to as Fano resonance. Electromagnetic simulations showed that an electric dipole that is excited near a photonic crystal slab generates vertically directional radiation at every Fano resonance frequency. The radiation distribution of an electric dipole is strongly correlated with the angular reflectance of a coupled photonic crystal slab. The strategy developed herein will be useful to achieve a vertical beam from quantum emitters such as transition metal dichalcogenide monolayers, facilitating the delivery of light into other external optics.

Polarization-sensitive beam steering from quantum emitters coupled with birefringent metamaterials.

One-dimensional metal/dielectric subwavelength periodic patterns have dielectric or metallic material dispersions depending on the polarization of incident light. This feature enables the development of artificial, ultrathin, birefringent films. In this study, we report polarization-sensitive beam steering from quantum emitters coupled with one-dimensional metal/dielectric metamaterial films. Electromagnetic simulations show that an Al/ITO metamaterial film functioning as a quarter-wave plate leads to vertically directed radiation for one polarization and a saddle-shaped, diverging radiation pattern for the orthogonal polarization. The strategy studied herein is extended to achieve polarized, vertically directed emission from organic light-emitting diodes. A tailored Al/ITO metamaterial mirror yields an approximately 30-fold improvement in polarization ratio, in conjunction with polarization-dependent Purcell factor enhancement.

Microstructured Air Cavities as High-Index Contrast Substrates with Strong Diffraction for Light-Emitting Diodes.

Two-dimensional high-index-contrast dielectric gratings exhibit unconventional transmission and reflection due to their morphologies. For light-emitting devices, these characteristics help guided modes defeat total internal reflections, thereby enhancing the outcoupling efficiency into an ambient medium. However, the outcoupling ability is typically impeded by the limited index contrast given by pattern media. Here, we report strong-diffraction, high-index-contrast cavity engineered substrates (CESs) in which hexagonally arranged hemispherical air cavities are covered with a 80 nm thick crystallized alumina shell. Wavelength-resolved diffraction measurements and Fourier analysis on GaN-grown CESs reveal that the high-index-contrast air/alumina core/shell patterns lead to dramatic excitation of the low-order diffraction modes. Large-area (1075 × 750 µm(2)) blue-emitting InGaN/GaN light-emitting diodes (LEDs) fabricated on a 3 µm pitch CES exhibit ∼39% enhancement in the optical power compared to state-of-the-art, patterned-sapphire-substrate LEDs, while preserving all of the electrical metrics that are relevant to LED devices. Full-vectorial simulations quantitatively demonstrate the enhanced optical power of CES LEDs and show a progressive increase in the extraction efficiency as the air cavity volume is expanded. This trend in light extraction is observed for both lateral- and flip-chip-geometry LEDs. Measurements of far-field profiles indicate a substantial beaming effect for CES LEDs, despite their few-micron-pitch pattern. Near-to-far-field transformation simulations and polarization analysis demonstrate that the improved extraction efficiency of CES LEDs is ascribed to the increase in emissions via the top escape route and to the extraction of transverse-magnetic polarized light.

Three-dimensional grating nanowires for enhanced light trapping.

We propose rationally designed 3D grating nanowires for boosting light-matter interactions. Full-vectorial simulations show that grating nanowires sustain high-amplitude waveguide modes and induce a strong optical antenna effect, which leads to an enhancement in nanowire absorption at specific or broadband wavelengths. Analyses of mode profiles and scattering spectra verify that periodic shells convert a normal plane wave into trapped waveguide modes, thus giving rise to scattering dips. A 200 nm diameter crystalline Si nanowire with designed periodic shells yields an enormously large current density of ∼28 mA/cm2 together with an absorption efficiency exceeding unity at infrared wavelengths. The grating nanowires studied herein will provide an extremely efficient absorption platform for photovoltaic devices and color-sensitive photodetectors.

Design principles for morphologies of antireflection patterns for solar absorbing applications.

Two-dimensional surface texturing is a widespread technology for imparting broadband antireflection, yet its design rules are not completely understood. The dependence of the reflectance spectrum of a periodically patterned glass film on various structural parameters (e.g., pitch, height, shape, and fill factor) has been investigated by means of full-vectorial numerical simulations. An average weighted reflectivity accounting for the AM1.5G solar spectrum (λ=300-1000 nm) was sinusoidally modulated by a rod pattern's height, and was minimized for pitches of 400-600 nm. When a rationally optimized cone pattern was used, the average weighted reflectivity was less than 0.5%, for incident angles of up to 40° off normal. The broadband antireflection of a cone pattern was reproduced well by a graded refractive index film model corresponding to its geometry, with the addition of a diffraction effect resulting from its periodicity. The broadband antireflection ability of optimized cone patterns is not limited to the glass material, but rather is generically applicable to other semiconductor materials, including Si and GaAs. The design rules developed herein represent a key step in the development of light-absorbing devices, such as solar cells.