Aluminum-based concurrent photonic and plasmonic energy conversion driven by quasi-localized plasmon resonance.

Plasmon-enhanced sensitive photodetection using plasmonic noble metals has been widely investigated; however, aluminum (Al)-based photoelectric conversion concurrently utilizing photonic and plasmonic approaches is less explored. Here, photodetection driven by quasi-localized plasmon resonance (QLPR) is investigated. Concurrent photonic and plasmonic contributions to strong absorption in the active region require delocalized, slow-propagating resonant electric field to occur around the peripheries of Al nano-structures and depend on the spatial distribution of diffraction efficiencies of all space harmonics. Efficiency limits are shown to be largely determined by the spatial degrees of freedom and the associated traveling distances of hot electrons during carrier transport. With strong absorption and relatively high reaching-emission probabilities structured in the same region, the measured responsivity and the external quantum efficiency of the fabricated device at 638.9 nm are 4.1889 µA/mW and 0.8129% at -0.485 V, respectively. Our results provide physical insights into related problems and may offer a route to more efficient, hot-carrier based photoelectric conversion devices.

Hybrid plasmonic mode converter: theoretical formulation and design with a graphical approach.

The theoretical formulation based on rigorous transmission-line networks is developed for a general mode conversion problem and lays the groundwork for a simpler yet more efficient graphical design approach for hybrid plasmonic mode converters (HPMCs). The concurrence of co- and cross-polarization conversion to and among higher-order photonic and HP modes followed by subsequent power redistributions and losses over the course of the HPMC can lead to performance degradation and largely determines the silicon core thickness. Using gradient ascent of the TM polarization fraction incorporated with modal index contours sets critical perturbation parameters for required transverse structural asymmetry. Polarization reversal estimates are shown to be practically applicable for about 60% of the total device length. The mode conversion efficiency (MCE), insertion loss (IL), and the polarization conversion efficiency of the proposed HPMC (<7×0.4 µm2) at λ0=1550 nm are 90.04%, 0.4691 dB, and 99.96%, respectively. The 85%-bandwidth of the MCE is 135 nm, while the IL stays below 0.5 dB over a 68-nm spectral range.

Suppressing lossy-film-induced angular mismatches between reflectance and transmittance extrema: optimum optical designs of interlayers and AR coating for maximum transmittance into active layers of CIGS solar cells: erratum.

In my publication [Opt. Express, 22, A167-A178 (2014)], the vertical and horizontal axes on Fig. 6 were mislabeled. Here the corrected figure is presented.

On the lasing-like transmission via radiation-mode-enabled TE resonant optical tunneling in asymmetric, passive, layered media with metal.

Transverse-electric (TE) resonant optical tunneling through an asymmetric, single-barrier potential system consisting of all passive materials in two-dimensional (2-D) glass/silver/TiO2/air configuration is quantified at a silver thickness of 35 nm. Resonant tunneling occurs when the incident condition corresponds to the excitation of a radiation mode. Lasing-like transmission occurring at resonance is carefully qualified in terms of power conservation, resonance condition, and identification of the gain medium equivalent. In particular, effective gain (geff) and threshold gain (gth) coefficients, both of which are strong functions of the forward reflection coefficient at the silver-TiO2 interface, are analytically obtained and the angular span over which geff > gth is further verified rigorously electromagnetically. The results show that the present configuration may be treated as a cascade of the gain equivalent (i.e. the silver film) and the TiO2resonator that is of Fabry-Perot type, giving rise to negative gth when resonant tunneling occurs. The transmittance spectrum exhibiting a gain-curve-like envelope is shown to be a direct consequence of the competition of the resonator loss at the silver-TiO2interface and the forward tunneling probability through the silver barrier, all controlled by the effective silver barrier thickness.

Suppressing lossy-film-induced angular mismatches between reflectance and transmittance extrema: optimum optical designs of interlayers and AR coating for maximum transmittance into active layers of CIGS solar cells.

The investigation of optimum optical designs of interlayers and antireflection (AR) coating for achieving maximum average transmittance (T(ave)) into the CuIn(1-x)Ga(x)Se2 (CIGS) absorber of a typical CIGS solar cell through the suppression of lossy-film-induced angular mismatches is described. Simulated-annealing algorithm incorporated with rigorous electromagnetic transmission-line network approach is applied with criteria of minimum average reflectance (R(ave)) from the cell surface or maximum T(ave) into the CIGS absorber. In the presence of one MgF2 coating, difference in R(ave) associated with optimum designs based upon the two distinct criteria is only 0.3% under broadband and nearly omnidirectional incidence; however, their corresponding T(ave) values could be up to 14.34% apart. Significant T(ave) improvements associated with the maximum-T(ave)-based design are found mainly in the mid to longer wavelengths and are attributed to the largest suppression of lossy-film-induced angular mismatches over the entire CIGS absorption spectrum. Maximum-T(ave)-based designs with a MgF2 coating optimized under extreme deficiency of angular information is shown, as opposed to their minimum-R(ave)-based counterparts, to be highly robust to omnidirectional incidence.

Toward maximum transmittance into absorption layers in solar cells: investigation of lossy-film-induced mismatches between reflectance and transmittance extrema.

The mismatch in film thickness and incident angle between reflectance and transmittance extrema due to the presence of lossy film(s) is investigated toward the maximum transmittance design in the active region of solar cells. Using a planar air/lossy film/silicon double-interface geometry illustrates important and quite opposite mismatch behaviors associated with TE and TM waves. In a typical thin-film CIGS solar cell, mismatches contributed by TM waves in general dominate. The angular mismatch is at least 10° in about 37%-53% of the spectrum, depending on the thickness combination of all lossy interlayers. The largest thickness mismatch of a specific interlayer generally increases with the thickness of the layer itself. Antireflection coating designs for solar cells should therefore be optimized in terms of the maximum transmittance into the active region, even if the corresponding reflectance is not at its minimum.

Ultracompact, narrowband three-dimensional plasmonic waveguide Bragg grating in metal/multi-insulator/metal configuration.

Ultracompact three-dimensional (3D) waveguide plasmonic Bragg gratings in a metal/multi-insulator/metal (MMIM) configuration with sinusoidal width modulations are presented. A semi-analytical approach from the eigenvalue problem and finite transmission-line network perspectives is described to facilitate the 3D designs with Bragg wavelength errors being within the range of 0.12%-3.99%. A narrowband design operating in the 1550 nm band with a FWHM bandwidth of 10.8 nm and an extinction ratio of approximately 12 dB is numerically demonstrated within a footprint of <17 µm(2) (10 periods). Unlike other types of plasmonic Bragg gratings, the bandwidth is increased as the MMIM grating length increases. The number of distinct plasmonic z-directed Poynting vector patterns within one period is found to be identical to the corresponding Bragg order. Narrowband characteristics are attributed to delicate, concurrent contra-flow interactions in and between photonic and plasmonic modes occurring simultaneously in multiple places within one period.

Broadband omnidirectional antireflection coatings for metal-backed solar cells optimized using simulated annealing algorithm incorporated with solar spectrum.

Broadband omnidirectional antireflection (AR) coatings for solar cells optimized using simulated annealing (SA) algorithm incorporated with the solar (irradiance) spectrum at Earth's surface (AM1.57 radiation) are described. Material dispersions and reflections from the planar backside metal are considered in the rigorous electromagnetic calculations. Optimized AR coatings for bulk crystalline Si and thin-film CuIn(1-x)GaxSe(2) (CIGS) solar cells as two representative cases are presented and the effect of solar spectrum in the AR coating designs is investigated. In general, the angle-averaged reflectance of a solar-spectrum-incorporated AR design is shown to be smaller and more uniform in the spectral range with relatively stronger solar irradiance. By incorporating the transparent conductive and buffer layers as part of the AR coating in CIGS solar cells (2µm-thick CIGS layer), a single MgF(2) layer could provide an average reflectance of 8.46% for wavelengths ranging from 350 nm to 1200 nm and incident angles from 0° to 80°.

Polarization-insensitive subwavelength sharp bends in asymmetric metal/multi-insulator configuration.

A new silicon-based sharp waveguide bend in asymmetric metal/multi-insulator configuration is described. TE and TM modes are calculated rigorously electromagnetically from which the general design rules are derived. Numerical simulations show that the respective insertion losses of < 0.085 dB and < 0.229 dB for TE and TM modes can be achieved by introducing a low-index layer between the metal and high-index core. The bending length is determined by the TE mode and has much smaller impacts on the TM that exhibits no resonance-like behavior as does the TE. The combined TE modal and radiation power in the air region is shown to couple back to the Si core through an asymmetric output taper, yielding a high transmission efficiency. Structure-enabled successive photonic-plamsonic mode conversions are shown to increase the TM mode confinement in the high-index core while the plasmonic mode carries up to 42.6% of the input power along the bending section.

Design and analysis of metal/multi-insulator/metal waveguide plasmonic Bragg grating.

A metal/multi-insulator/metal waveguide plasmonic Bragg grating with a large dynamic range of index modulation is investigated analytically and numerically. Theoretical formalism of the dispersion relation for the present and general one-dimensional gratings is developed for TMwaves in the vicinity of each stop band.Wide-band and narrow-band designs with their respective FWHM bandwidths of 173.4 nm and < 3.4 nm in the 1550 nm band using a grating length of <16.0 microm are numerically demonstrated. Time-average power vortexes near the silica-silicon interfaces are revealed in the stop band and are attributed to the contra-flow interaction and simultaneous satisfactions of the Bragg condition for the incident and backward-diffracted waves. An enhanced forward-propagating power is thus shown to occur over certain sections within one period due to the power coupling from the backward-diffracted waves.

Birefringence characteristics of nanoscale dielectrics with cubic and tetragonal lattices.

The birefringence in nanometer-scale dielectrics with the largest dimensions ranging from about 3 nm to 20 nm has been quantified by evaluating directly the summation of induced-dipole-electric-field contributions from all individual atoms within the entire dielectric volume. Various configurations in representative cubic and tetragonal systems are investigated by varying the ratio of lattice constants and the number of atoms in various directions to illustrate the chain-like and plane-like behavior regimes. The dielectric properties of the finite cubic crystal lattices change from isotropic to birefringent (uniaxial or biaxial) when the entire dielectric volume is changed from a cube to a rectangular parallelepiped in shape. In finite tetragonal crystals the birefringence increases with the increasing lattice constant ratios. The largest uniaxial birefringence occurs for non-cube dielectric volume with tetragonal lattices.

Induced-dipole-electric-field contribution of atomic chains and atomic planes to the refractive index and birefringence of nanoscale crystalline dielectrics.

The induced-dipole-electric-field contribution to the refractive index at any location within a nanometer-scale dielectric is quantified by summing the electronic dipole contributions due to all the surrounding atoms in the dielectric. Using a tetragonal lattice and varying the ratio of lattice constants illustrates the important limiting chainlike and planelike behaviors. Strong polarizing effects and thus high refractive indices occur for an electric field applied along the length of a chain of atoms or applied in a planar direction to a plane of atoms. In contrast, a strong depolarizing effect and thus low refractive indices occur for an electric field applied normal to a chain of atoms or applied normal to a plane of atoms. Birefringence is increased or decreased by the simultaneous presence or absence of polarizing and depolarizing effects.

Attenuation in waveguides on FR-4 boards due to periodic substrate undulations.

The guided-mode attenuation associated with optical-interconnect-polymer waveguides fabricated on FR-4 printed-circuit boards is quantified. The rigorous transmission-line network approach is used and the FR-4 substrate is treated as a long-period substrate grating. A quantitative metric for an appropriate matrix truncation is presented. The peaks of attenuation are shown to occur near the Bragg conditions that characterize the leaky-wave stop bands. For a typical 400 microm period FR-4 substrate with an 8 microm corrugation depth, a buffer layer thickness of about 40 microm is found to be needed to make the attenuation negligibly small.