Light Trapping Applications Research Articles

Ultrafast Q-boosting in semiconductor metasurfaces

Abstract All-optical tunability of semiconductor metasurfaces offers unique opportunities for novel time-varying effects, including frequency conversion and light trapping. However, the all-optical processes often induce optical absorption that fundamentally limits the possible dynamic increase of their quality factor (Q-boosting). Here, we propose and numerically demonstrate the concept of large Q-boosting in a single-material metasurface by dynamically reducing its structural anisotropy on a femtosecond timescale. This balance is achieved by excitation with a structured pump and takes advantage of the band-filling effect in a GaAs direct-bandgap semiconductor to eliminate the free-carrier-induced loss. We show that this approach allows a dynamic boosting of the resonance quality factor over orders of magnitude, only limited by the free-carrier relaxation processes. The proposed approach offers complete dynamic control over the resonance bandwidth and opens applications in frequency conversion and light trapping.

Smart strategy of butterfly wing scales to control the light diffusion and absorption

The sparkling colors on the wing scales of butterflies are one of the most fascinating light–matter interactions in nature and have been an intense area of research in recent times. Controlling the light diffusion and absorption due to multiple scattering induced by the inherent disorder in the wing scales is required for applications in imaging, light trapping, and localization using such bio-inspired photonic structures. Here, we study the selective anisotropic light diffusion and absorption in the nanoarchitectures of Pieris rapae and Graphium sarpedon butterfly wing scales in the visible range. We have measured broadband spatially independent low values of specular reflectivity and ballistic transmission from the ventral and the dorsal sides of the wing scales, which are supported with finite-difference time-domain simulations. The specular reflectivity value as small as 3% indicates strong diffuse scattering within the scales, probed using total transmission and reflection measurements. We have found finite wavelength-dependent absorption < 100 % in these wing scales. Further, we have obtained negative scattering anisotropy ( g ) values from the dorsal side, whereas it is positive from the ventral side for both butterflies. The negative g value originating due to the inherent structural correlation is differentiated from that arising due to pigment absorption. The results are useful for generating the optimized light scattering in bio-inspired photonic devices.

Photonic non-Bloch quadrupole topological insulators in coupled ring resonators

We investigate the second-order topological phases in a two-dimensional ring resonator array with each plaquette occupied by \ensuremath{\pi} gauge flux and imaginary gauge field. The real and imaginary gauge fields are induced by shifting the displacement and integrating gain or loss into the two half perimeters of the auxiliary rings. The system supports topological corner modes with their emergence being determined by the non-Bloch topological invariant due to skin effects. The bulk modes, exhibiting second-order skin effects in both trivial and nontrivial phases, are accumulated at opposite corners depending on whether clockwise or counterclockwise modes are excited. By introducing an interface with different imaginary gauge fields, we show the bulk modes exist at the interface while the topological corner modes are localized at the physical corners. Furthermore, the skin effects are also presented in the passive ring resonators. The study may find applications in lasers and broadband light trapping.

Improved thermal stability of antireflective moth-eye topography imprinted on PMMA/TiO2 surface nanocomposites

The thermal stability of antireflective moth-eye topographical features fabricated by nanoimprint lithography on poly (methyl methacrylate) (PMMA) incorporating TiO2 nanoparticles is explored. The effect of nanoparticle load on the relaxation dynamics of the moth-eye nanostructure is evaluated via grazing incidence small angle x-ray scattering measurements by in situ monitoring the structural decay of the nanopatterns upon thermal annealing. It is demonstrated that the incorporation of TiO2 nanoparticles to the imprinted surface nanocomposite films delays greatly the pattern relaxation which, in turn, enhances the stability of the patterned topography even at temperatures well above the polymer glass transition (Tg). The improved thermal behavior of the antireflective films will significantly enhance their functionality and performance in light-trapping applications where temperatures typically rise, such as solar devices or solar glass panels.

Square-root non-Bloch topological insulators in non-Hermitian ring resonators.

We investigate the topological skin effect in a ring resonator array which can be mapped into the square root of a Su-Schrieffer-Heeger (SSH) model with non-Hermitian asymmetric coupling. The asymmetric coupling is realized by integrating the same amount of gain and loss into the two half perimeters of linking rings that effectively couple two adjacent site rings. Such a square-root topological insulator inherits the properties from its parent Hamiltonian, which has the same phase transition points and exhibits non-Bloch features as well. We show the band closing points for open chain are different from that of periodic chain as a result of the skin effect. Moreover, the square-root insulator supports multiple topological edge modes as the number of band gaps is doubled compared to the original Hamiltonian. The full-wave simulations agree well with the theoretical analyses based on a tight-binding model. The study provides a promising approach to investigate the skin effect by utilizing ring resonators and may find potential applications in light trapping, lasers, and filters.

Design and parametric analysis of a wide-angle polarization-insensitive metamaterial absorber with a star shape resonator for optical wavelength applications

Optical wavelengths considered as the key source of electromagnetic waves from the sun, and metamaterial absorber (MMA) enables various applications for this region like real invisible cloaks, color imaging, magnetic resonance imaging, light trapping, plasmonic sensor, light detector, and thermal imaging applications. Contemplated those applications, a new wide-angle, polarization-insensitive MMA is presented in this study. The absorber was formatted with three layers that consisted of a sandwiched metal-dielectric-metal structure. This formation of metamaterial absorber showed a good impedance match with plasmonic resonance characteristics. The structure was simulated using the FIT and validated with the FEM. A variety of parametric studies were performed with the design to gain best physical dimension. The mechanism of absorption also explained immensely by various significant analysis. The design had average 96.77% absorption from wavelengths of 389.34 nm to 697.19 nm and a near-perfect absorption of 99.99% at a wavelength of 545.73 nm for TEM mode. For an ultra-wide bandwidth of 102 nm, the design exhibited above 99% absorbance. The proposed is wide-angle independent up to 60° for both TE and TM mode, which is useful for solar energy harvesting, solar cell, and solar thermophotovoltaics (STPV). This MMA can be used for an optical sensor or as a light detector. Moreover, this proposed design can be employed in some applications mentioned above.

Selective light trapping of plasmonic stack metamaterials by circuit design.

Plasmonic metamaterials have wide applications in light trapping and manipulation. However, most of their design typically rely on solving Maxwell's equations via computational electromagnetics, which is time-consuming and limits design flexibility. Here, we combined the transmission line circuit theory with full wave simulation to design plasmonic stack metamaterials in the near-infrared range. By virtue of the simplicity and high efficiency of circuit theory, we designed various light trapping functions by using plasmonic stack metamaterials, including comb filtering, short pass, long pass, band pass and band stop. Our study reveals the field-circuit relationship for the light-matter interaction of nanostructure stacks and provides a powerful method for the quick design of functional plasmonic metamaterials.

Ag@SiO<sub>2</sub> coupled structure’s design and regulation and control of response to thin film solar cells

The coupled nano-structure Ag@SiO<sub>2</sub> has both plasmon excitation like metallic nanoparticles and diffraction scattering like a dielectric nanosphere, which effectively controls the propagation path and the energy distribution of incident light and shows great potential applications in light trapping for thin film solar cells. In this work, we construct a three-dimensional electromagnetic model based on the finite-difference time-domain (FDTD) and rigorous coupled-wave analysis (RCWA) method to investigate the regulation mechanism of Ag@SiO<sub>2</sub> coupling structure to the spectral response of amorphous silicon cells. By being optimally designed, a high-efficiency cell device is achieved. The results show that the transmitted light into the active layer reaches a maximum value when Ag and SiO<sub>2</sub> have their feature sizes of 18 and 150 nm, respectively. The absorption spectrum of the corresponding cell device also arrives at its maximum value. The photoelectric conversion efficiency is enhanced from 7.19% to 7.80%, with an increment of 8.48% compared with the flat solar cell with an equivalent thickness of absorbing layer.

Rapid prototyping of highly ordered subwavelength silicon nanostructures with enhanced light trapping

High index nanostructures lead to unique light-matter interactions and strong optical resonances for potential light trapping applications and improved optical performances. In this work, Gaussian shaped subwavelength 3D nanostructures are introduced and rapid prototyping of these nanostructures is carried out on silicon (Si) in a single-step using focused ion beam (FIB) milling. The tapered geometry of Gaussian pillar structures provided reflection suppression from the smaller tip in contrast to its extended base, enabling trapping of light, confirmed through light reflection calculations and hot spots observed through absorption density distributions. The nature of accelerated ions in a FIB system is directly used and highly ordered Gaussian pillar like subwavelength nanostructures are fabricated in a single-step on Si. Simulations and experiments demonstrate nanostructure evolution from periodically spaced subwavelength nanoholes to Gaussian pillar structures with tuning of beam processing parameters, providing fabrication flexibility. The proposed light management structures, investigated through Finite Difference Time Domain (FDTD) calculations, exhibit 10–20% reflection for a broadband visible wavelength range against planar Si and previously reported Si structures (such as nano-cones, nano-wires, nano-pyramids etc.). Experimental results support theoretical predictions and provide evidence for antireflection properties from the fabricated nanostructures. Considering the rapid fabrication approach and tunable nature of Gaussian shaped nanostructures, exhibiting unique optical properties; this work provides realization of nanoscale light management 3D structures for localized light trapping and antireflection applications.

Intracavity cylindrical vector beam generation from all-PM Er-doped mode-locked fiber laser.

We demonstrate a practical method that is used to generate on-demand first- and higher-order cylindrical vector beams, in the 1550 nm band, directly from an all polarization maintaining mode-locked Er-fiber laser. On demand typical 1st order CVBs, including the radially and azimuthally polarized beams, can be easily achieved by properly adjusting the angle of a half-wave plate with respect to the fast axis of the vortex wave plate. The spatial beam mode can be flexibly switched with no disturbance on the time domain mode-locking output. The laser outputs the desired vector beams at 1571 nm with a spectral bandwidth at full-width at half-maximum of 32 nm. The mode-locked laser pulses have a repetition rate of 74.9 MHz. Moreover, the proposed method can be easily extended to create higher-order CVBs. Our research provides a convenient way to generate ultrafast pulses in highly flexible-controlled structured modes, which is essential for optical fabrication and light trapping applications.

Large-area flexible colloidal photonic crystal film stickers for light trapping applications

We report on a way to make and transfer large-area (100 cm2) free-standing 2D or 3D colloidal photonic crystal (CPC) films deposited using a roll-to-roll Langmuir-Blodgett technique from silica or polymer particles. Using this approach, a ‘sticker’ composed of a monolayer array of silica particles with 120 nm diameter in polyvinyl alcohol (PVA) glue was transferred onto a commercial flexible organic photovoltaic cell improving its conversion efficiency by some 5.2-8.3% at off-normal 0°-45° angles. The results presented here open up a range of new options for application of CPC films in the areas of photovoltaics and light-emitting devices.

Effectively infinite optical path-length created using a simple cubic photonic crystal for extreme light trapping

A 900 nm thick TiO2 simple cubic photonic crystal with lattice constant 450 nm was fabricated and used to experimentally validate a newly-discovered mechanism for extreme light-bending. Absorption enhancement was observed extending 1–2 orders of magnitude over that of a reference TiO2 film. Several enhancement peaks in the region from 600–950 nm were identified, which far exceed both the ergodic fundamental limit and the limit based on surface-gratings, with some peaks exceeding 100 times enhancement. These results are attributed to radically sharp refraction where the optical path length approaches infinity due to the Poynting vector lying nearly parallel to the photonic crystal interface. The observed phenomena follow directly from the simple cubic symmetry of the photonic crystal, and can be achieved by integrating the light-trapping architecture into the absorbing volume. These results are not dependent on the material used, and can be applied to any future light trapping applications such as phosphor-converted white light generation, water-splitting, or thin-film solar cells, where increased response in areas of weak absorption is desired.

Transparent Long-Pass Filter with Short-Wavelength Scattering Based on Morpho Butterfly Nanostructures

We combine the principles of moth-eye antireflection, Bragg scattering, and thin-film interference to design and fabricate a short-wavelength scattering/long-pass filter with sharp cutoff, high transmission of infrared light, and strong reflection of visible light into high angles. Based on the lamellae-edge features on Morpho didius butterfly wings, nanostructures are self-assembled via sequential one-chamber chemical vapor deposition, metal nanoparticle formation, and wet-chemical etching. Finite-element modeling demonstrates strong (>45%) reflection into the first diffracted order for short wavelengths, while retaining >80% transmission for longer wavelengths. Fabricated nanostructures couple more than 50% of reflected light into angles of >10° while enabling broadband long-pass transmission. Such structures have potential applications in light trapping for tandem solar cells, stealth, and signals processing.

Fabrication and characterization of TiO2/SiO2 based Bragg reflectors for light trapping applications

Distributed Bragg reflectors (DBRs) have received an intensive attention due to their increasing demand in optoelectronic and photonic devices. Such reflectors are capable to prohibit the light propagation within the specified wavelength range of interest. In this paper, we present the fabrication of TiO2/SiO2 stacks based Bragg reflectors by using a simple and in-expensive sol-gel spin coating technique. The prepared single-layer thin films of TiO2 and SiO2 onto glass substrates were characterized for their optical constants. By tuning the process parameters, one-seven DBR stacks of TiO2/SiO2 were prepared. The corresponding shift of the Bragg reflection peak was observed with the increased number of DBR stacks and as much as about 90% reflectance is observed from the 7DBR stacks. The experimentally measured reflectance was compared with the simulated one, which showed good in agreement. FESEM measurement has confirmed the formation of bright and dark strips of TiO2 and SiO2 films with their thicknesses 80 and 115nm respectively. The simulation study was explored to a design of thin film silicon solar cell using 7DBR stacks. An enhancement in light absorption in the visible wavelength range is observed which coincides with the experimental result of the reflectance. The use of DBR at the bottom of the solar cell could felicitate the better light harvesting with the occurrence of Fabry-Perot resonances in the absorbing layer.

Surface Plasmon Enhanced Light Trapping in Metal/Silicon Nanobowl Arrays for Thin Film Photovoltaics

Enhancing the light absorption in thin film silicon solar cells with nanophotonic and plasmonic structures is important for the realization of high efficiency solar cells with significant cost reduction. In this work, we investigate periodic arrays of conformal metal/silicon nanobowl arrays (MSNBs) for light trapping applications in silicon solar cells. They exhibited excellent light-harvesting ability across a wide range of wavelengths up to infrared regimes. The optimized structure (MSNBsH) covered by SiO2 passivation layer and hemisphere Ag back reflection layer has a maximal short-circuit density (Jsc) 25.5 mA/cm2, which is about 88.8% higher than flat structure counterpart, and the light-conversion efficiency (η) is increased two times from 6.3% to 12.6%. The double-side textures offer a promising approach to high efficiency ultrathin silicon solar cells.

Design and Optimization of Novel ZnS–Metal Core-Shell Random Structures for Light Harvesting—a Computational Study

We present a systematic computational study on the optical properties of a new composite structure based on ZnS–metal core-shell particles, which can find applications in light trapping. Three metals Au, Ag and Cu whose surface plasmon resonance (SPR) fall in visible region are used as shell materials. Effect of shell material, thickness and core radius on the SPR values is studied using Mie theory. Optimization of aspect ratios for each composite is done for maximum extinction in visible region. Uniform random distribution of selected particles with optimized aspect ratios is modelled using Lumerical FDTD software. Transmission of wavelengths 400, 450, 500, 550, 600 and 650 nm through random media is investigated using finite difference time domain simulations. Shell material is found to have a large influence on light trapping ability of the composite. Depending on the shell material, thickness and core radius, wavelengths at different regions of visible spectrum can be trapped, thus offering scope for designing novel light harvesting systems tunable over specific wavelength regions.

Numerical simulation of light absorption enhancement in microcrystalline silicon solar cells with Al nanoparticle arrays

Metal nanoparticles with low cost and high performance have good potential applications in light-trapping of solar cells. In this paper, a three-dimensional model is proposed to simulate the light absorption of microcrystalline silicon (μc-Si:H) thin film solar cells. The effects of spherical and hemispherical Al nanoparticle arrays located on the front surfaces of solar cells are investigated, and the particle radius and array period are optimized by the finite element method. The results show that the optimal Al nanoparticle arrays can enhance broadband absorption in thin film solar cells. For spherical particle arrays, the key parameter that influences light absorption in solar cells is period/radius ratio (P/R) or particle surface coverage. When P/R=4-5, the optimum integrated absorption enhancement (Eabs) is over 20% under AM1.5 illumination compared with the solar cell without nanoparticles. The value of Eabs is small and decreases with the increase of P/R when P/R>5, and Eabs is less than zero when P/RP=500 nm and R=120 nm, the spectral absorption rate as a function of wavelength shows broadband absorption including four distinct peaks, which are attributed to quadrupole plasmon resonance mode, dipole resonance mode and waveguide mode respectively according to the electric field distribution in the solar cell. For hemispherical particle arrays, the maximum value of Eabs is 24.5%, which is higher than that of the solar cell with optimized spherical particle arrays. This is due to the high coupling efficiencies of the particles, so that most of the scattered light is directly coupled into the substrate. However, the value of Eabs is very sensitive to the hemispherical particle radius. As the radius decreases, the scattering cross-section and scattering efficiency of the particle decrease dramatically. As the radius increases, the dipole plasmon resonance wavelength rapidly shifts towards longer wavelength (red shift). Both of these are detrimental to absorption enhancement of solar cells. Thus we conclude that spherical Al particle arrays are more preferable in actually fabricating the light-trapping of solar cells.

Repurposing Blu-ray movie discs as quasi-random nanoimprinting templates for photon management.

Quasi-random nanostructures have attracted significant interests for photon management purposes. To optimize such patterns, typically very expensive fabrication processes are needed to create the pre-designed, subwavelength nanostructures. While quasi-random photonic nanostructures are abundant in nature (for example, in structural coloration), interestingly, they also exist in Blu-ray movie discs, an already mass-produced consumer product. Here we uncover that Blu-ray disc patterns are surprisingly well suited for light-trapping applications. While the algorithms in the Blu-ray industrial standard were developed with the intention of optimizing data compression and error tolerance, they have also created quasi-random arrangement of islands and pits on the final media discs that are nearly optimized for photon management over the solar spectrum, regardless of the information stored on the discs. As a proof-of-concept, imprinting polymer solar cells with the Blu-ray patterns indeed increases their efficiencies. Simulation suggests that Blu-ray patterns could be broadly applied for solar cells made of other materials.

Fabrication of silicon nanostructures with large taper angle by reactive ion etching

Micro- and nanostructures with a tapered sidewall profile are important for antireflection and light trapping applications in solar cell, light emitting diode, and photodetector/imager. Here, the authors will show two etching processes that offer a large taper angle. The first process involved a mask-less etching of pre-etched silicon structures having a vertical profile, using a recipe that would give a vertical profile when masked. The authors obtained a moderate taper angle of 14° using CF4/O2 etching gas. The second process involved a one-step etching step with Cr as mask using a recipe that was drastically modified from a nonswitching pseudo-Bosch process that gives a vertical profile. The gas flow ratio of C4F8/SF6 was greatly increased from 38/22 to 59/1 to result in a taper angle of 22°. Further reduction of the RF bias power led to an unprecedented large taper angle of 39° (at the cost of greatly reduced etching rate), which is even higher than the angle obtained by anisotropic wet etching of silicon.

Tuning the peak position of subwavelength silica nanosphere broadband antireflection coatings.

Subwavelength nanostructures are considered as promising building blocks for antireflection and light trapping applications. In this study, we demonstrate excellent broadband antireflection effect from thin films of monolayer silica nanospheres with a diameter of 100 nm prepared by Langmuir-Blodgett method on glass substrates. With a single layer of compact silica nanosphere thin film coated on both sides of a glass, we achieved maximum transmittance of 99% at 560 nm. Furthermore, the optical transmission peak of the nanosphere thin films can be tuned over the UV-visible range by changing processing parameters during Langmuir-Blodgett deposition. The tunable optical transmission peaks of the Langmuir-Blodgett films were correlated with deposition parameters such as surface pressure, surfactant concentration, ageing of suspensions and annealing effect. Such peak-tunable broadband antireflection coating has wide applications in diversified industries such as solar cells, windows, displays and lenses.