12 research outputs found
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><sub>lat</sub> = 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><sub>max</sub>×1<i>C</i> (6.24×10<sup>18</sup> 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/µm<sup>2</sup>). (<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><sub>max</sub>, 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<sup>+</sup> or Li<sup>+</sup>) 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 MoS<sub>2</sub> Films onto Arbitrary Substrates
The transfer of synthesized 2D MoS<sub>2</sub> films is important for fundamental and applied research. However, it is problematic to translate the well-established transfer processes for graphene to MoS<sub>2</sub> 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 MoS<sub>2</sub> 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 MoS<sub>2</sub> 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 MoS<sub>2</sub> 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 MoS<sub>2</sub> 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 WS<sub>2</sub>