Broadband Absorption Enhancement Research Articles

In‐Plane and Out‐of‐Plane Optical Properties of Monolayer, Few‐Layer, and Thin‐Film MoS2 from 190 to 1700 nm and Their Application in Photonic Device Design

Monolayer, few‐layer, and thin‐film MoS2 is synthesized using chemical vapor deposition (CVD) and thermal vapor sulfurization (TVS) methods. The complex refractive index of these samples is assessed using variable angle spectroscopic ellipsometry (VASE) measurements over a broad spectral range between 190 and 1700 nm. The ellipsometry data are sensitive to birefringence effects in the thickest thin‐film sample. These birefringence effects are investigated, and an analysis method is developed to extract the in‐plane and out‐of‐plane optical properties. The complex refractive index is then used to calculate reflectance, transmittance, and absorption of the MoS2 films using the transfer‐matrix method (TMM) and is matched with experimentally measured transmittance of the same samples. The modeled results show that the monolayer, few‐layer, and thin‐film MoS2 absorbs 7.4%, 12.6%, and 32.4% of the incident light, respectively, between 300 and 700 nm. When normalized to per unit‐thickness absorption, they absorb 12.1%, 5.9%, and 1.1% nm−1, respectively, clearly showing superior light–matter interaction in the monolayer and few‐layer films. These new complex refractive index data are further used to design optical coatings for these films to either confine absorption in a narrow bandwidth for photodetector applications or enhance broadband absorption for photovoltaic applications.

Broadband Absorption in Thin Films Motivated by Strong Light Bending

Photon management toward the absorption over a wide spectral range of the solar light is important to various absorption‐based photonic devices. The key for high absorption in thin films is the ability to “bend the light” from the normal direction into the in‐plane direction. Broadband absorption enhancement is reported in thin films decorated with top surface light funnel arrays (LF arrays) due to strong bending of the light by the arrays into the films, in contrast with excitation of thin films by top surface nanopillar arrays (NP arrays) that is governed by forward scattering. The broadband absorption in the film below the LF array (LF film) is 40% higher than that in a film below an optimized NP array (NP film). Also, the strong light bending by the LF arrays into the underlying film induces strong absorptivity peaks substantially exceeding the Yablonovitch limit for both the LF film and the total array–film complex. Finally, the superior LF film broadband absorption is due to strong light bending from the normal direction into the in‐plane direction which is manifested in efficient generation of mode hybridizations.

Thin silicon interference solar cells for targeted or broadband wavelength absorption enhancement.

We present the concept of interference solar cells reliant on spectrum filtering or splitting to enhance absorption in thin (<13 µm) silicon absorber layers, both for targeted wavelengths and broadband absorption. Absorption enhancement in the long wavelength regime is achieved by fine-tuning of device layer thicknesses to provide destructive interference between reflected and escaped waves. We suggest this concept is also suitable for broadband absorption enhancement when combined with spectrum splitting optics through gradual thickness changes laterally across the device. Using the example of silicon heterojunction solar cells, we have computationally demonstrated a short circuit current density enhancement of 19% (from 25.8 mA/cm2 to 30.7 mA/cm2) compared to a silicon heterojunction cell of the same absorber layer thickness.

Broadband absorption enhancement with ultrathin MoS2 film in the visible regime* *Project supported by the National Natural Science Foundation of China (Grant No. 61405217), the Zhejiang Provincial Natural Science Foundation, China (Grant No. LY20F050001), the Anhui Polytechnic University Research Startup Foundation, China (Grant No. 2020YQQ042), and the Pre-research Project of Natural Science Foundation of Anhui Polytechnic University, China (Grant No.

The broadband absorption enhancement effect in ultrathin molybdenum disulfide (MoS2) films is investigated. It is achieved by inserting the MoS2 film between a dielectric film and a one-dimensional silver grating backed with a silver mirror. The broadband absorption enhancement in the visible region is achieved, which exhibits large integrated absorption and short-circuit current density for solar energy under normal incidence. The optical properties of the proposed absorber are found to be superior to those of a reference planar structure, which makes the proposed structure advantageous for practical photovoltaic application. Moreover, the integrated absorption and short-circuit current density can be maintained high for a wide range of incident angles. A qualitative understanding of such broadband absorption enhancement effect is examined by illustrating the electromagnetic field distribution at some selected wavelengths. The results pave the way for developing high-performance optoelectronic devices, such as solar cells, photodetectors, and modulators.

Optimization of etching time for enhanced broadband absorption in flexible black silicon on stainless steel foil

This paper presents optimization of etching time in metal-assisted chemical etching (MACE) process for enhanced broadband absorption in flexible black silicon (b-Si) on stainless steel (SS) foil. Flexible monocrystalline silicon (mono c-Si) wafers with 65 μm thickness are used in this work. For the optimization of etching time, one-step MACE process is utilized with 10−25 min etching to fabricate the b-Si absorber. The SS foil beneath the b-Si also acts as a back reflector (BR) which ensures zero transmission loss in the long wavelength region (above 800 nm). After 20 min etching, b-Si with average nanowires (NWs) length of 982 nm is formed. This condition exhibits the lowest average weighted reflection (AWR) and the highest broadband light absorption in the b-Si within 300−1100 nm wavelength region, owing to refractive index grading effect at the air/b-Si interface. This corresponds to the maximum potential short-circuit density (Jsc(max)) of 39.3 mA/cm2, or 62.4 % enhancement when compared to the planar c-Si reference. The findings demonstrate the potential of flexible b-Si on SS foil to deliver high photocurrent in the future solar cells.

Reflectance spectroscopy-guided broadband spectral derivative approach to detect glauconite-rich zones in fossiliferous limestone, Kachchh region, Gujarat, India

In this study, we processed visible near-infrared (VNIR) and shortwave infrared (SWIR) bands of Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) data to delineate glauconite-bearing zones in fossiliferous limestones, Quaternarysediments and Paleogene shales in the Lakhpat.Region of the Kachchh Province in India. Glauconitic limestone has subtle absorption features at 2.3 µm when compared to the absorption features of pure limestones that are dominated by the strong absorption feature of calcite at 2.33 µm. Additionally, the spectrum of glauconite-rich limestone also has an absorption feature at about 0.9 µm due to the crystal field effect of Fe2+ present in the glauconite structure and a feeble spectral feature due to the presence of Al-OH bonds at about 2.2 µm. ASTER resampled reflectance spectra of these rocks have been used as the reference to construct different image-enhanced products. We derived index images and Eigen matrix analysis-based principal components (PCs), using ASTER bands, to delineate different rocks in the study area. These index images are derived to enhance broadband absorption features observed in shales, glauconite-bearing and pure limestones, respectively. Principal component images are prepared using the spectral bands defining the broadband absorption features of these rocks. We found that the PC image composite, derived using PC 8, PC 2 and PC 7, is suitable to discriminate basalts, limestones and shales from each other, while an index image colour composite, prepared using three independent index images, is suitable for the identification of lithological contacts between Quaternary sediments, shales and fossiliferous limestones, respectively. In addition, the ASTER resampled rock spectrum of glauconite has a relatively lower spectral gradient in comparison to the gradient observed in the spectrum of fossiliferous limestone within the spectral range of bands 6 and 8. The spectral derivative images are derived for discriminating glauconite-rich zones within the limestone using the contrast in the spectral gradient of limestone and glauconite-rich limestone. The image composite of three spectral derivatives was compiled from the derivative image of bands 4 to 3, bands 5 to 7, and bands 6 to 8 to discriminate glauconite-rich zones from pure fossiliferous limestone. Our study proposes the potential use of broadband reflectance spectroscopy in the mineral mapping of sedimentary provinces.

Self‐Assembly of Mechanoplasmonic Bacterial Cellulose–Metal Nanoparticle Composites

AbstractNanocomposites of metal nanoparticles (NPs) and bacterial nanocellulose (BC) enable fabrication of soft and biocompatible materials for optical, catalytic, electronic, and biomedical applications. Current BC–NP nanocomposites are typically prepared by in situ synthesis of the NPs or electrostatic adsorption of surface functionalized NPs, which limits possibilities to control and tune NP size, shape, concentration, and surface chemistry and influences the properties and performance of the materials. Here a self‐assembly strategy is described for fabrication of complex and well‐defined BC–NP composites using colloidal gold and silver NPs of different sizes, shapes, and concentrations. The self‐assembly process results in nanocomposites with distinct biophysical and optical properties. In addition to antibacterial materials and materials with excellent senor performance, materials with unique mechanoplasmonic properties are developed. The homogenous incorporation of plasmonic gold NPs in the BC enables extensive modulation of the optical properties by mechanical stimuli. Compression gives rise to near‐field coupling between adsorbed NPs, resulting in tunable spectral variations and enhanced broadband absorption that amplify both nonlinear optical and thermoplasmonic effects and enables novel biosensing strategies.

Theoretical investigation of broadband absorption enhancement in a-Si thin-film solar cell with nanoparticles

Thin-film solar cells have attracted increasing attention due to its low material cost and large flexibility, but they also face the challenge of low solar absorption due to reduced active layer thickness. Through exciting surface plasmon resonance, plasmonic metal nanoparticles are usually placed on the cell front surface to enhance solar absorption. However, if eliminating the unuseful intrinsic absorption in nanoparticles, we find that dielectric ones are better choices to enhance a-Si thin-film solar cell absorption efficiency. Moreover, a composite light trapping structure with dielectric nanoparticles on the front surface and metal hemispheres on the rear surface is proposed to achieve broadband absorption enhancement in both short and long wavelengths, with the aim to get a higher conversion efficiency. The finite-difference-time-domain simulation results show that, compared with bare 100-nm-thick amorphous silicon solar cell, the short-circuit current density and photoelectric conversion efficiency could be respectively improved by 21% and 18% with addition of optimized composite light trapping structure. The general method proposed in this study could provide valuable guidance to light trapping structure design for various kinds of thin-film solar cells.

Review of Polarization Optical Devices Based on Graphene Materials.

Graphene has received extensive scholarly attention for its extraordinary optical, electrical, and physicochemical properties, as well as its compatibility with silicon-based semiconductor processes. As a unique two-dimensional atomic crystal material, graphene has excellent mechanical properties, ultra-high carrier mobility, ultra-wide optical response spectrum, and strong polarization dependence effect, which make it have great potential in new optical and polarization devices. A series of new optical devices that are based on graphene have been developed, showing excellent performance and broad application prospects. In this paper, the recent research progress of polarizers, sensors, modulators, and detectors that are based on the polarization characteristics of graphene is reviewed. In particular, the polarization dependence effect and broadband absorption enhancement of graphene under total reflection structure are emphasized, which enhance the interaction between graphene and light and then provide a new direction for research of graphene polarization devices.

Design and fabrication of vertically aligned single-crystalline Si nanotube arrays and their enhanced broadband absorption properties

We propose and demonstrate a new and room-temperature approach for the fabrication of well-ordered arrays of vertically aligned, diameter-, length-, and interspacing-controllable Si nanotubes on (0 0 1)Si substrates. In this approach, a unique, hexagonally-ordered Au disk/hole dual nanostructure pattern with uniform size and spacing was first produced on the surface of (0 0 1)Si substrate and was then used as the catalyst to etch vertically downward into the Si substrate by Au-catalyzed chemical etching. All the produced vertical Si nanotubes were identified to be single crystalline with the same axial orientation of [0 0 1] and their lengths could be readily tuned by adjusting the etching time. The produced long vertical Si nanotubes with catalytic Au disk/hole dual nanostructures show greatly enhanced hydrophobicity (water contact angle: ~146°) and broadband absorption properties (average visible-light absorptance: ~96%; average near-IR absorptance: >70%) compared to the corresponding bare (0 0 1)Si substrate and Si nanorod samples. The obtained results present the exciting prospects that the new approach proposed here promises to open opportunities for the design and construction of various 1-D hollow semiconductor nanodevices with multi-function.

Dual-microcavity resonance and plasmon-cavity polaritons enhanced the broadband absorption of polymer-based organic solar cells by employing micro-nano composite gratings

Abstract A broadband-absorption enhancement in two kinds of polymer-based organic solar cells (OSCs) is demonstrated in this work by employing micro-nano composite gratings (MNCGs). The structure of MNCGs is nano-grating covering on the surface of micro-grating. By embedding the MNCGs at the interface of the active layer/Ag cathode, the multi-absorption-enhancement mechanisms, such as dual-microcavity resonance, plasmon-cavity polaritons and light-scattering effects, are excited and result in a broadband-absorption enhancement. The light absorption of active layer in the MNCG-based OSC is numerically calculated. As a result, comparing to the planar OSCs, the absorption intensity is increased by 50.41% and 20.43% in these two kinds of polymer-based OSCs, respectively. Meanwhile, the MNCGs can also be universally used in all kinds of polymer-based OSCs to increase the light absorption of the active layer.

Broadband absorption enhancement of graphene in the ultraviolet range based on metal-dielectric-metal configuration

Light absorption of suspended monolayer graphene in the ultraviolet range is relatively low, which impedes the potential applications for graphene-based optoelectronic devices. To enhance the light-matter interaction in a wide spectral range, we propose two plasmonic structures based on a metal-dielectric-metal configuration. After rigorous optimization of geometric parameters, broadband and enhanced ultraviolet light absorption of graphene can be achieved. The bandwidth above 30% absorption rate can reach up to 245 nm (from 240 nm to 485 nm), while the relative bandwidth can reach up to 68%. The optimal absorption peak is as high as 54%. Besides, the absorption spectrum can be tuned by adjusting the geometric parameters. Physical insight is revealed with electric and magnetic field distributions. Furthermore, the tolerance for the incident angle is demonstrated, and the dependence of the polarization angle is discussed. Our approach shows promising potential for novel graphene-based applications in the ultraviolet range.

Performance improvement of thin-film silicon solar cells using transversal and longitudinal titanium nitride plasmonic nanogratings

Taking advantage of plasmonic nanostructures for light trapping in thin-film silicon solar cells has attracted ample attention among researchers. Titanium nitride (TiN) has recently been introduced as a promising material exhibiting plasmonic properties similar to gold while taking advantage of low cost, low loss, and CMOS compatibility. Moreover, utilization of TiN offers a good tunability since the optical properties of TiN depend on many fabrication parameters. In this work, transversal and longitudinal TiN nanograting arrays have been employed to act like perpendicular polarizers trapping the incident light in the active layer of the cell. Using optical and electrical simulations, it has been shown that the design provides a significant enhancement in the performance of thin-film silicon solar cells owing to the excitation of surface plasmon resonances and their resultant light trapping. Thanks to the employment of TiN nanogratings, the device offers a broadband absorption enhancement with a considerable improvement at the near infrared wavelengths in which the absorption of bare silicon layer is weak. The proposed thin-film cell results in short-circuit current and power conversion efficiency of about 26.46 mA/cm2 and 12.27%, respectively proving the proficiency of the design for performance improvement of thin-film photovoltaic systems.

Ultrabroadband absorber based on a funnel-shaped anisotropic metamaterial

A design of a funnel-shaped anisotropic metamaterial absorber is reported with a periodic array of nickel–germanium (Ni/Ge). The optical properties of the suggested anisotropic metamaterial absorber are numerically calculated using the 3D finite-difference time-domain technique. Composed of both cylindrical and conical units, the reported anisotropic metamaterial attains high absorption over a wavelength range from 200 nm to 9000 nm with an average absorption of 96%. A wide wavelength absorption band is achieved from the ultraviolet to near infrared including the visible range. The cylindrical unit could harvest low-order modes, while the longer wavelengths are trapped through the conical part. The enhanced broadband absorption is due to the excitation of multiple orders of slow-light modes. In addition, the effect of the incident angle on the absorption of transverse magnetic and transverse electric-polarized waves is also investigated. Further, the structural geometrical parameters are studied in order to maximize the absorption through the proposed design.

Impact of micro-texturization on hybrid micro/nano-textured surface for enhanced broadband light absorption in crystalline silicon for application in photovoltaics

This paper reports investigation on hybrid micro/nanotextures for enhanced broadband absorption in crystalline silicon(c-Si) for application in photovoltaics. Microscale pyramids are fabricated using sodium hydroxide (NaOH) solution, etched at different durations. Nanowires on pyramids are formed using metal-assisted chemical etching (MACE). With 30 min of NaOH etching, pyramids with 3–5 μm heights (3–7 μm widths) are formed. The pyramids reduce broadband reflection of the c-Si through enhanced scattering. After MACE, nanowires with 450–600 nm heights (30–40 nm widths) are obtained. With the nanowires, weighted average reflection (WAR) for all samples reduces significantly, owing to enhanced light coupling by the nanowires and increased light scattering by the pyramids. The sample with 30 min NaOH etching and MACE demonstrates WAR of 7.5%, which is the lowest broadband reflection achieved in this work. This corresponds to potential Jsc(max) of 38.7 mA/cm2, or 13.5% enhancement compared to the Jsc(max) of the pyramids alone.

Graphene oxide-ferrite hybrid framework as enhanced broadband absorption in gigahertz frequencies

The present investigation is focused on the in-situ synthesis of Graphene oxide (GO)-ferrite nanoparticle hybrid framework by gel-combustion method followed by fabrication of homogeneous, structurally stable thin (~100–120 μm) hybrid-polyurethane coating on a metallic aluminum substrate and its application on the properties of broadband absorption over the microwave frequency region. Microstructure studies of hybrid materials illustrated that small sized ferrite nanoparticles (~17 nm) are grafted on and through the graphene layers, which forms a homogeneous coating thereby. The hybrid-nanocomposite coating demonstrated superior broadband absorption properties with absorptivity higher than 90% throughout a bandwidth of ~6 GHz, and moreover, it was found that with increased loading of GO in the nanocomposite, the bandwidth range of absorption frequency increases with enhanced absorptivity. The real part and imaginary part of the surface impedance values of the coating was obtained as 377 Ω and 0 Ω, respectively, which imply that the free-space impedance of the hybrid-nanocomposite coating is matching correctly. The nanocomposite coating showed ultra-high absorptivity over the frequency band of 8–12 GHz, which has numerous practical applications as radar absorbing materials (RAM), stealth technology, electromagnetic shielding, and radiated electromagnetic interference (EMI) management in onboard spacecraft and many more.

Broadband absorption enhancement of monolayer graphene by prism coupling in the visible range

We experimentally demonstrated the broadband absorption enhancement of monolayer graphene embedded in the center of a dielectric cavity of the prism coupling configuration in the visible spectral range. The absorption peak is enhanced up to 86.1% at λ = 650 nm with a 3 dB bandwidth of 314 nm (from 542 nm to 856 nm). The absorption performance is the best experimental result for monolayer graphene in the visible range, to the best of knowledge. The improved absorption is attributed to enhanced light-graphene interaction via the cavity mode formed in the dielectric layer with the properly chosen graphene position. The proposed structure is suitable to a high-efficiency broadband photodetector based on monolayer graphene due to the simple fabrication process and the high fabrication tolerance.

Large-scale, broadband absorber based on three-dimensional aluminum nanospike arrays substrate for surface plasmon induced hot electrons photodetection

Performance of plamson induced hot electrons-based photodetectors largely relies on the photon absorption capability. To improve the optical absorption, many perfect absorbers based on the periodic metallic nanostructures have been designed and fabricated through low-throughput, costly and time-consuming lithographic processes, which seriously limit the future potential applications of plasmonic hot electrons optoelectronics devices. Here, a large-scale, broadband absorber consisting of ITO film, ZnO layer, Au film and Al nanospike array substrate was designed and fabricated for hot electrons-based photodetection. The new designed absorber’s absorptivity can be up to 70% in the broad wavelength range from 400 nm to 800 nm (even up to 90% in the wavelength range from 400–550 nm) and most of the absorption comes from the Au film, which is effective for the generation of hot electrons. The enhanced broadband absorption is ascribed to the surface plasmon polariton mode and localized surface plasmon resonance mode supported by the nanospike arrays. The influence of geometry and material parameters on the optical absorption properties is also specifically investigated through numerical simulation. The efficient and broadband absorption of a nanospikes device results in a much larger photocurrent compared with that of a planar reference device. Our approach, which is compatible with large-scale manufacturing, paves the way for the practical implementation of hot electrons-based optoelectronic devices.

Combined Effects of pH and Borohydride Reduction on Optical Properties of Humic Substances (HS): A Comparison of Optical Models.

The combined effects of pH and borohydride reduction on the optical properties of a series of humic substances and a lignin model were examined to probe the molecular moieties and interactions that give rise to the observed optical properties of these materials. Increasing the pH from 2 to 12 produced significantly enhanced absorption across the spectra of all samples, with distinct spectral responses observed over pH ranges attributable to the deprotonation of carboxylic acids and phenols. Borohydride reduction substantially attenuated the broadband absorption enhancements with pH, clearly indicating that the loss of absorption due to ketone/aldehyde reduction is coupled with the pH-dependent increase in absorption due to deprotonation of carboxylic acids and phenols. These results cannot be easily explained by a superposition of the spectra of independently absorbing chromophores (superposition model) but are readily interpretable within a charge transfer (CT) model. Changes of fluorescence emission with pH for both untreated and borohydride reduced samples suggest that a pH-dependent structural reorganization of the HS may also be influencing the fluorescence emission. Independent of optical model, these results demonstrate that chemical tests targeted to specific moieties can identify distinct structural differences among HS sources as well as provide insight into the molecular moieties and interactions that produce the observed optical and photochemical properties.

Controllable Synthesis of Monodispersed Fe1- xS2 Nanocrystals for High-Performance Optoelectronic Devices.

The optical properties of stoichiometric iron pyrite (FeS2) nanocrystals (NCs) are characterized by strong UV-Visible (UV-Vis) absorption within the cutoff while negligible absorption beyond the cutoff in near-infrared and longer wavelengths. Herein, we show this bandgap limitation can be broken through controllable synthesis of nonstoichiometric Fe1- xS2 NCs ( x = 0.01-0.107) to induce localized surface plasmonic resonance (LSPR) absorption beyond the cutoff to short-wave infrared spectrum (SWIR, 1-3 μm) with remarkably enhanced broadband absorption across UV-Vis-SWIR spectra. To illustrate the benefit of the broadband absorption, colloidal LSPR Fe1- xS2 NCs were printed on graphene to form LSPR Fe1- xS2 NCs/graphene heterostructure photodetectors. Extraordinary photoresponsivity in exceeding 4.32 × 106 A/W and figure-of-merit detectivity D* > 7.50 × 1012 Jones have been demonstrated in the broadband of UV-Vis-SWIR at room temperature. These Fe1- xS2 NCs/graphene heterostructures are printable and flexible and therefore promising for practical optical and optoelectronic applications.