General Modal Properties of Optical Resonances in Subwavelength Nonspherical Dielectric Structures

Subwavelength dielectric structures offer an attractive low-loss alternative to plasmonic materials for the development of resonant optics functionalities such as metamaterials and optical antennas. Nonspherical-like rectangular dielectric structures are of the most interest from the standpoint of device development due to fabrication convenience. However, no intuitive fundamental understanding of the optical resonance in nonspherical dielectric structures is available, which has substantially delayed the development of dielectric resonant optics devices. Here, we elucidate the general fundamentals of the optical resonance in nonspherical subwavelength dielectric structures with different shapes (rectangular or triangular) and dimensionalities (1D nanowires or 0D nanoparticles). We demonstrate that the optical properties of nonspherical dielectric structures are dictated by the eigenvalue of the structure’s leaky modes. Leaky modes are defined as optical modes with propagating waves outside the structure. We also elucidate the dependence of the modal eigenvalue on physical features of the structure. The eigenvalue shows scale invariance with respect to the size of the structure, weak dependence on the refractive index, but linear dependence on the size ratio of different sides of the structure. We propose a modified Fabry–Perot model to account for the linear dependence. The knowledge of leaky modes, including the role in optical responses and the dependence on physical features, can serve as a powerful guide for the rational design of devices with desired optical resonances. It may open up a pathway to design devices with functionality that has not been explored due to the lack of intuitive understanding, for instance, imaging devices able to sense incident angle or superabsorbing photodetectors

Dielectric Core–Shell Optical Antennas for Strong Solar Absorption Enhancement

We demonstrate a new light trapping technique that exploits dielectric core–shell optical antennas to strongly enhance solar absorption. This approach can allow the thickness of active materials in solar cells lowered by almost 1 order of magnitude without scarifying solar absorption capability. For example, it can enable a 70 nm thick hydrogenated amorphous silicon (a-Si:H) thin film to absorb 90% of incident solar radiation above the bandgap, which would otherwise require a thickness of 400 nm in typical antireflective coated thin films. This strong enhancement arises from a controlled optical antenna effect in patterned core–shell nanostructures that consist of absorbing semiconductors and nonabsorbing dielectric materials. This core–shell optical antenna benefits from a multiplication of enhancements contributed by leaky mode resonances (LMRs) in the semiconductor part and antireflection effects in the dielectric part. We investigate the fundamental mechanism for this enhancement multiplication and demonstrate that the size ratio of the semiconductor and the dielectric parts in the core–shell structure is key for optimizing the enhancement. By enabling strong solar absorption enhancement, this approach holds promise for cost reduction and efficiency improvement of solar conversion devices, including solar cells and solar-to-fuel systems. It can generally apply to a wide range of inorganic and organic active materials. This dielectric core–shell antenna can also find applications in other photonic devices such as photodetectors, sensors, and solid-state lighting diodes

Function parameters of OHCs from the 4 genotypes of mice.

<p>Note:</p>**<p>: p<0.01,</p>*<p>: p<0.05, compared to wild-type OHCs, as determined by the Kruskal-Wallis test followed by Student's t test with a Holm correction; NA: not applicable.</p

DPOAE (<i>A</i>) and ABR (<i>B</i>) thresholds of P21–24 mice of the indicated genotypes.

<p>Values are the mean ± SEM; **: <i>P</i><0.01, *: <i>P</i><0.05 by two-way ANOVA followed by Student's t test with a Bonferroni correction.</p

Morphological and immunohistochemical analysis of mutant mice.

<p>(<b><i>A</i></b><b>–</b><b><i>D</i></b>) Representative immunofluorescence staining with prestin (green) and myosin 6 (red) in whole-mount preparations of basal cochlear turns (corresponding to the 60 kHz region of the +/+ cochlea) in the indicated mouse genotypes at P24. 4′, 6-diamidino-2-phenylindole (DAPI) counterstain is shown in blue. Scale bar: 20 µm. <i>Neo/Neo</i> and <i>Neo/-</i> mice show no OHC loss or abnormal distribution of prestin. (<b><i>E</i></b>) Length of OHCs of each genotype at a given location corresponding to 16 kHz region of the +/+ cochlea. Values are the mean ± SEM; **: <i>P</i><0.01, *: <i>P</i><0.05. (<b><i>F</i></b>) Length of OHCs of each genotype at different locations of the cochlea. The shorter OHCs could reduce the mass of the organ of Corti and result in a higher frequency response for a given location, assuming all other material properties remain the same. The x-axis displays a normalized distance from apex (0%) to base (100%). That is, the locations responding to 4, 6, 12, 16, and 22 kHz in wild-type cochleae correspond to 4, 12, 30, 40, and 51% in a normalized distance from the cochlear apex, respectively. Calculated intercepts for each genotype differed significantly by one-way ANOVA, followed by Student's t test with a Bonferroni correction.</p

<i>Prestin</i> knock-in mice.

<p>(<b><i>A</i></b>) Targeted Neo <i>prestin</i> knock-in allele. Solid rectangles represent exons 5 through 9. Diamond indicates the C1 mutation <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0045453#pone.0045453-Gao1" target="_blank">[9]</a>. A neo-selectable marker was inserted into intron 6 of the <i>prestin</i> gene in ES cells by homologous recombination. (<b><i>B</i></b>) Genomic Southern blot analysis of <i>Neo/Neo</i> mice. Genomic DNAs from <i>+/+</i> and <i>Neo/Neo</i> tails were digested with <i>Bam</i>HI and a specific probe indicated in (<b><i>A</i></b>) was used to detect a 12-kb band in the <i>+/+</i> allele and an 8-kb band in the targeted allele. (<b><i>C</i></b>) PCR-based genotyping of <i>+/+</i> and <i>Neo/Neo</i> mice using 3 primers is indicated in (<b><i>A</i></b>). Wild-type mice showed a 242-bp band; the <i>Neo/Neo</i> mice showed a 400-bp band.</p

Membrane capacitance versus membrane potential and electromotility <i>in vitro</i>.

<p>(<b><i>A</i></b>) Voltage-dependent membrane capacitance of OHCs of the indicated genotypes, obtained at a voltage range of −150 mV to 120 mV. (<b><i>B</i></b>) Maximum charge transfer of OHCs of the indicated genotypes, expressed as absolute total charge transfer. Values in <i>Neo/Neo</i> and <i>Neo/-</i> OHCs are between those in wild-type and <i>prestin−/−</i> OHCs. (<b><i>C</i></b>) Linear membrane capacitance in the indicated OHC genotypes. Values in <i>Neo/Neo</i> and <i>Neo/-</i> OHCs fall between those in wild-type and <i>prestin−/−</i> OHCs. (<b><i>D</i></b>) Length of the OHC axial lateral wall in the indicated genotypes. Lengths were measured in the isolated cells in which voltage-dependent membrane capacitance (<b><i>A</i></b>) was obtained. (<b><i>E</i></b>) Estimated charge density in the indicted OHC genotypes. The area of the lateral membrane containing prestin was calculated as <i>A</i>_lat = π<i>DL</i> where <i>D</i> is the diameter and <i>L</i> is the length of the membrane. The charge density of OHCs was calculated by dividing each cell's <i>Q</i>_max×1<i>C</i> (6.24×10¹⁸ elementary charges) by the total prestin-containing surface area. (<b><i>F</i></b>) Linear capacitance vs. the combined surface area of the lateral membrane area, the cuticular plate area, and the basal area in the indicated genotypes. The line represents the membrane capacitance of OHCs per unit of surface area (0.0085 pF/µm²). (<b><i>G</i></b>) Electromotile amplitude in the indicated genotypes. OHC length changes in response to voltage steps (−150 to 120 mV in 30-mV increments) were recorded in whole-cell, voltage-clamp mode. The absolute value of the amplitude was normalized to the OHC length at holding potential to allow comparison between genotypes. Bars show the mean (±SEM) maximum OHC motility expressed as percent length change. **: <i>P</i><0.01, *: <i>P</i><0.05 as determined by the Kruskal-Wallis test followed by Student's t test with a Holm correction.</p

The relationship between prestin activity/feedback efficiency and amplification gain.

<p>The relationship between amplification gain and feedback efficiency proposed by Patuzzi et al. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0045453#pone.0045453-Patuzzi1" target="_blank">[7]</a> is plotted as a black curve. Y-axes represent the input/output (y/x) ratio (left) and the amplification gain [dB SPL = 20log(y/x), right], respectively. The amplification gain (dB SPL) for each genotype is the ABR threshold difference between prestin <i>−/−</i> mice and wild-type, <i>Neo/Neo</i>, or <i>Neo/-</i> mice. Mean values derived from our data are color-coded according to genotype. In this study, averaged ABR threshold changes at 16 kHz were used to derive the amplification gain, although similar results were obtained at frequencies of 4–12 and 22 kHz. We assumed that the prestin amount was linearly correlated with <i>Q</i>_max, charge density or electromotility, which is further linearly correlated with feedback efficiency. Therefore, the normalized prestin activity in <i>Neo/Neo</i> and <i>Neo/-</i> OHCs by wild-type control (100% prestin activity) is expressed as the feedback efficiency (β). Values are the mean ± SEM.</p

Enhancing Multifunctionalities of Transition-Metal Dichalcogenide Monolayers <i>via</i> Cation Intercalation

We have demonstrated that multiple functionalities of transition-metal dichalcogenide (TMDC) monolayers may be substantially improved by the intercalation of small cations (H⁺ or Li⁺) between the monolayers and underlying substrates. The functionalities include photoluminescence (PL) efficiency and catalytic activity. The improvement in PL efficiency may be up to orders of magnitude and can be mainly ascribed to two effects of the intercalated cations: p-doping to the monolayers and reducing the influence of substrates, but more studies are necessary to better understand the mechanism for the improvement in the catalytic functionality. The cation intercalation may be achieved by simply immersing substrate-supported monolayers into the solution of certain acids or salts. It is more difficult to intercalate under the monolayers interacting with substrates stronger, such as as-grown monolayers or the monolayers on 2D material substrates. This result presents a versatile strategy to simultaneously optimize multiple functionalities of TMDC monolayers

Surface-Energy-Assisted Perfect Transfer of Centimeter-Scale Monolayer and Few-Layer MoS₂ Films onto Arbitrary Substrates

The transfer of synthesized 2D MoS₂ films is important for fundamental and applied research. However, it is problematic to translate the well-established transfer processes for graphene to MoS₂ due to different growth mechanisms and surface properties. Here we demonstrate a surface-energy-assisted process that can perfectly transfer centimeter-scale monolayer and few-layer MoS₂ films from original growth substrates onto arbitrary substrates with no observable wrinkles, cracks, and polymer residues. The unique strategies used in this process include leveraging the penetration of water between hydrophobic MoS₂ films and hydrophilic growth substrates to lift off the films and dry transferring the film after the lift off. This is in stark contrast with the previous transfer process for synthesized MoS₂ films, which explores the etching of the growth substrate by hot base solutions to lift off the films. Our transfer process can effectively eliminate the mechanical force caused by bubble generations, the attacks from chemical etchants, and the capillary force induced when transferring the film outside solutions as in the previous transfer process, which consists of the major causes for the previous unsatisfactory transfer. Our transfer process also benefits from using polystyrene (PS), instead of poly(methyl methacrylate) (PMMA) that was widely used previously, as the carrier polymer. PS can form more intimate interaction with MoS₂ films than PMMA and is important for maintaining the integrity of the film during the transfer process. This surface-energy-assisted approach can be generally applied to the transfer of other 2D materials, such as WS₂