Low Optical Absorption Research Articles

Precise measurements of the low optical absorption coefficients of massive lithium triborate crystal boules with untreated facets formed during the growth process.

A spatial diagnostic of optical quality of the massive lithium triborate crystal boule with untreated facets was performed by measuring optical absorption coefficients at 1070 and 1908nm wavelengths using the piezoelectric resonance laser calorimetry technique.

Role of metallic nanoparticles in the optoelectronic performance enhancement of InP ultrathin film solar cell

III-V material-based ultrathin Solar Cells (SCs) garnered a great deal of interest due to their benefits such as efficient charge transport, recycling of photons, adaptability, and material usage but suffer from low optical absorption due to thicker active material. In order to increase the optical absorption in the ultra-thin SC, metal nanoparticles (MNPs) are employed that scatter the incident light in the absorber layer resulting in higher optical absorption with the reduced requirement of active material. The role of MNPs in the efficiency improvement of InP SCs has not yet been explored. Therefore, in this paper, we have investigated the impact of plasmonic MNPs of different materials such as gold (Au), silver (Ag) and aluminium (Al) placed on the top of ITO coated 280 nm ultra-thin InP SC with FDTD simulations, which employ the surface plasmon resonance (SPR) to increase the optical absorption by tuning the dimensions of the MNPs. The thickness of InP thin-film and an anti-reflective coating of ITO are optimized to achieve maximum absorption. In addition, the diameter and period of Au, Ag, and Al MNPs are also optimized to achieve higher optical Jsc, and results show that Au nanoparticle with diameter of 50 nm and period of 100 nm shows higher Jsc when compared to Ag and Al nanoparticles of optimized diameter. The device analysis shows the Au nanoparticles placed on the top surface of ITO coated InP ultrathin SC with a power conversion efficiency (PCE) of 22.3% outperform the Ag NPs and Al NPs ITO coated InP ultrathin SC and ITO/InP ultrathin SC without MNPs.

ФОТОДИНАМИЧЕСКИ-АКТИВНЫЕ ПРЕПАРАТЫ В ЛЕЧЕНИИ РАЗЛИЧНЫХ ЗАБОЛЕВАНИЙ

A multifactorial effect on damaged tissues of the body in the process of a photosensitive reaction in photodynamic therapy is carried out. It includes the killing of pathologically altered cells with intensive production of reactive oxygen species, the effect on blood vessels and the activation of the body's immune system. The effectiveness of this method depends on the properties of photosensitizing drugs, namely, their selectivity, the ability to photoproduce reactive particles and ability to absorb in the near infrared spectrum. This review discusses three generations of photosensitizers. The first is the photodynamically active compound hematoporphyrin «Photofrin I». Taking into account the shortcomings of the first generation of drugs, such as: the complexity of the chemical composition, moderate photodynamic activity, a high degree of photosensitization of normal skin areas, insufficient selectivity and low optical absorption, second-generation photosensitizers were developed. These include Levulan, Foscan, Talaporfin and others. However, clinical trials of some of them have not been completed due to a number of reasons, to a greater extent, the low effectiveness of drugs or the financial costs of research. The third generation preparations were obtained using the methods of bioconjugation and encapsulation of photosensitizers of previous generations. To date, special attention is paid to the development of new systems for targeted delivery of photosensitizers, such as nanoparticles and monoclonal antibodies, which can significantly reduce the time required for targeted accumulation in tumor tissue.

Photo-electric properties of preferred orientation of tin oxide studied by first-principles

Tin oxide (SnO2) is a material owning competitive properties of optical transparency and electrical conductivity. Different preferred orientation films have been prepared by experiments. However, it is not clear that the effect of preferred orientation on photo-electric properties. In this work, structure and photo-electric properties of four preferred orientations were calculated by first-principles. The results showed that surface energy of (110) and (200) surfaces was lower than that of (211) and (301) surfaces. Furthermore, (200) surface had charge densities of valence band maximum (VBM) and conduction band minimum (CBM) distributed in the whole system and suitable VBM and CBM positions, which could enhance carrier concentration and conductivity. (211) and (301) surfaces are easy to achieve p-type doping, but (110) and (200) surfaces are opposite. Meanwhile, (200) and (301) preferred orientations have low optical absorption in visible spectrum. Our results provide a useful guidance for preparation of high performance SnO2 films.

Enhancing light emission near low-refractive-index contrast nanostructures via mirror-symmetry breaking

A platform of recent interest and large application potential is one in which the light emitters are directly integrated with an optical metasurface. Plasmonic metals and high-refractive-index, dielectric Mie metasurfaces have been explored in this connection but have their challenges. We propose low-refractive-index, contrast nanostructured thin films for studying metasurface-emitter systems, specifically focusing on two configurations of practical importance: LED white light generation using color converting polymers and fluorescence-based sensing in aqueous media. To achieve light emission enhancement in a low-index contrast, low optical absorption setting, we exploit the excitation of quasi-bound states in the continuum modes in a mirror-symmetry broken grating. Our numerical study predicts widely tuneable sharp-linewidth emission enhancements, near-zero quenching, and large and controllable active volume in the grating vicinity, which are significant improvements in comparison with both plasmonic and high-index contrast, all-dielectric platforms. When compared with simple gratings, mirror-symmetry broken gratings give four times larger radiative enhancement. Our results are of interest in furthering experimental activity and realizing applications such as light converter for efficient white LEDs and smart detection electronics-integrated substrates for sensing.

Quasi-monocrystalline silicon for low-noise end mirrors in cryogenic gravitational-wave detectors

Mirrors made of silicon have been proposed for use in future cryogenic gravitational-wave detectors, which will be significantly more sensitive than current room-temperature detectors. These mirrors are planned to have diameters of $\ensuremath{\approx}50$ cm and a mass of $\ensuremath{\approx}200$ kg. While single-crystalline float-zone silicon meets the requirements of low optical absorption and low mechanical loss, the production of this type of material is restricted to sizes much smaller than required. Here we present studies of silicon produced by directional solidification. This material can be grown as quasi-monocrystalline ingots in sizes larger than currently required. We present measurements of a low room-temperature and cryogenic mechanical loss comparable with float-zone silicon. While the optical absorption of our test sample is significantly higher than required, the low mechanical loss motivates research into further absorption reduction in the future. While it is unclear if material pure enough for the transmissive detector input mirrors can be achieved, an absorption level suitable for the highly reflective coated end mirrors seems realistic. Together with the potential to produce samples much larger than $\ensuremath{\approx}50$ cm, this material may be of great benefit for realizing silicon-based gravitational-wave detectors.

Broadband and perfect absorption in monolayer MoS2-based multilayered structure arranged in Kolakoski sequence

Monolayer MoS2 has excellent optical properties, but its low optical absorption hinders its application in optical absorbers. To improve its absorption performance, we design an aperiodic multilayer structure with monolayer MoS2, the dielectric materials Si and SiO2 arranged by Kolakoski sequence, and achieve the perfect absorption in the visible wavelength range because of its omnidirectional reflectivity. By optimizing the thicknesses of SiO2 (db), Si (dc) and the repetition number (N) of the unit cell, the absorption of the aperiodic multilayer structure can be greatly improved at normal incidence. When N is 3, this aperiodic multilayer structure achieves high absorption of up to 98% in the wavelength range from 350 to 450 nm. As N increases to 5 or 7, the average absorption of this aperiodic multilayer structure exceeds 90% in the 350–600 nm wavelength range. In addition, we verify that the broadband and high absorption obtained in this aperiodic multilayer structure is robust against the angle of incidence and polarization of the incident wave. Our results suggest an alternative configuration with monolayer MoS2 to realize the broadband and perfect absorption, which is useful for future applications in solar cell or photodetectors.

Low frequency photoacoustic optical absorption measurement of aerosols

Atmospheric optical absorption, including the contribution from aerosols, is important in modeling optical propagation and for climate research. An open-celled photoacoustic aerosol sensing system was developed to operate at 150 Hz for measurement of absorption aerosols up to 3 microns in diameter with the design goal to achieve a sensitivity adequate to measure an absorption corresponding to a 1/e optical absorption length of 1 Mm. The advantage of a photoacoustic system is that it measures optical absorption only and is not sensitive to optical extinction due to scattering at low absorption levels. The system was calibrated against an existing black carbon absorption measurement unit made by Droplet Measurement Technologies, which operates at a shorter optical wavelength and higher acoustic frequency. With previous work focusing on measuring optical absorption by black carbon aerosols [J.A. Case and R.W. Smith,182nd meeting of the Acoustical Society of America in Denver, CO (5aPA)], this work covers measuring absorption by wet salt solutions of similar salt concentration to that oceanic aerosols. Experiments with the current system showed an anomalous photoacoustic signal appearing in the absence of any aerosols. Several solutions to eliminating the unwanted signal from the desired measurement are proposed and tested.

Efficient interfacial charge transfer of BiOCl-In2O3 step-scheme heterojunction for boosted photocatalytic degradation of ciprofloxacin

• Step-scheme In 2 O 3 -BiOCl heterojunctions were synthesized by molten salt method. • Molten-salt method without organic solvent for the BiOCl-In 2 O 3 composites could endow clean surface, which is beneficial to the adsorption of reactants in the photocatalytic degradation process. • In 2 O 3 -BiOCl heterojunctions exhibited excellent photocatalytic degradation of ciprofloxacin. • Boosting photocatalytic performance is due to the formation of step-scheme heterojunction. • • O 2− and H + played critical roles in photocatalytic degradation process of ciprofloxacin. BiOCl as a representative layered bismuth-based photocatalyst with Sillén-structure has aroused wide public concern on photocatalytic degradation. However, the photocatalytic efficiency of pristine BiOCl is currently restricted by its low optical absorption and charge separation efficiency. Herein, step-scheme (S-scheme) heterojunctions of In 2 O 3 nanoparticle and BiOCl micron-sheet were constructed by a convenient molten salt method by using a LiNO 3 -KNO 3 system. The In 2 O 3 -BiOCl heterojunctions exhibit higher optical absorption performance from 380 nm to 700 nm than the pristine BiOCl and enhanced photocatalytic property toward ciprofloxacin (CIP) degradation under Xenon lamp illumination. The sample 20%In 2 O 3 -BiOCl showed the highest photodegradation efficiency, attaining 91% removal of CIP within 35 min, which was 39.6 times and 3.2 times higher than that of pristine In 2 O 3 and BiOCl, respectively. The improved photodegradation property mainly resulted from the novel S-scheme mechanism, which boosted highly efficient separation of the photo-induced carriers. The photoluminescence spectrometric test and transient photocurrent response results demonstrated that In 2 O 3 -BiOCl composite exhibited efficient separation of photo-generated charge carriers. This work would provide new insights into the design of novel S-scheme photocatalytic systems with applicability in photocatalytic water treatment.

A Transparent Electrode Based on Solution-Processed ZnO for Organic Optoelectronic Devices

Achieving high-efficiency indium tin oxide (ITO)-free organic optoelectronic devices requires the development of high-conductivity and high-transparency materials for being used as the front electrode. Herein, sol-gel-grown zinc oxide (ZnO) films with high conductivity (460 S cm−1) and low optical absorption losses in both visible and near-infrared (NIR) spectral regions are realized utilizing the persistent photoinduced doping effect. The origin of the increased conductivity after photo-doping is ascribed to selective trapping of photogenerated holes by oxygen vacancies at the surface of the ZnO film. Then, the conductivity of the sol-gel-grown ZnO is further increased by stacking the ZnO using a newly developed sequential deposition strategy. Finally, the stacked ZnO is used as the cathode to construct ITO-free organic solar cells, photodetectors, and light emitting diodes: The devices based on ZnO outperform those based on ITO, owing to the reduced surface recombination losses at the cathode/active layer interface, and the reduced parasitic absorption losses in the electrodes of the ZnO based devices.

First-principles investigations of Na2CuMCl6 (M = Bi, Sb) double perovskite semiconductors: Materials for green technology

Investigations of stable lead-free perovskites have ignited an increasing interest in overcoming lead-based perovskites' instability and toxicity problems. This study thoroughly investigated the transport, nature of electronic, stability, and optical properties of inorganic halide double perovskites', namely Na2CuMCl6 (M = Bi, Sb), to better understand their possible applications. The theory of density function was applied to determine the physical characteristics of these materials. This cubic material's stability was validated by optimizing the mechanical stability test, tolerance factor, and structure. Small bandgap semiconductors with outstanding optoelectronic performance caused by low reflectivity, high conductivity, and optical absorption, as well as a high potential for optoelectronic application, were used. Due to the small bandgap, we also identified multiple transport parameters with chemical potential (μ). Based on this study, our findings revealed that the figure of merit (ZT) was near to unity due to the semiconducting nature of the materials, implying that it will be effective in thermoelectric technology.

Effects of Hydrogen Peroxide Concentration on Properties of Black Silicon Fabricated by Two-Step Silver-Assisted Wet Chemical Etching for Photovoltaics

Crystalline silicon (c-Si) has low optical absorption due to its high surface reflection of incident light. Nanotexturing of c-Si which produces black silicon (b-Si) offers a promising solution. In this work, effect of H2O2 concentrations towards surface morphological and optical properties of b-Si fabricated by two-step silver-assisted wet chemical etching (Ag-based two-step MACE) for potential photovoltaic (PV) applications is presented. The method involves a 30 s deposition of silver nanoparticles (Ag NPs) in an aqueous solution of AgNO3:HF (5:6) and an optimized etching in HF:H2O2:DI H2O solution under 0.62 M, 1.85 M, 2.47 M, and 3.7 M concentrations of H2O2 at 5 M HF. On the b-Si, nanowires with 250-950 nm heights and an average diameter of 150-280 nm are obtained. Low concentrations of H2O2 result in denser nanowires with an average length of 900-950 nm and diameters of about 150-190 nm. The b-Si exhibit outstanding broadband antireflection due to the refractive index grading effect represented as WAR within the 300-1100 nm wavelength region. B-Si obtained after etching in a solution with 0.62 M concentration of H2O2, demonstrate WAR of 7.5%. WAR of 7.5% results in an absorption of up to 95.5 % at a wavelength of 600 nm. The enhanced broadband light absorption yields maximum potential short-circuit current density (Jsc(max)) of up to 38.2 mA/cm2, or 45.2% enhancement compared to the planar c-Si reference.

Superiorly low half-wave voltage electro-optic polymer modulator for visible photonics.

Chip-scale optical devices operated at wavelengths shorter than communication wavelengths, such as LiDAR for autonomous driving, bio-sensing, and quantum computation, have been developed in the field of photonics. In data processing involving optical devices, modulators are indispensable for the conversion of electronic signals into optical signals. However, existing modulators have a high half-wave voltage-length product (VπL) which is not sufficient at wavelengths below 1000 nm. Herein, we developed a significantly efficient optical modulator which has low VπL of 0.52 V·cm at λ = 640 nm using an electro-optic (EO) polymer, with a high glass transition temperature (Tg = 164 °C) and low optical absorption loss (2.6 dB/cm) at λ = 640 nm. This modulator is not only more efficient than any EO-polymer modulator reported thus far, but can also enable ultra-high-speed data communication and light manipulation for optical platforms operating in the ranges of visible and below 1000 nm infrared.

Transparent conducting oxides: from all-dielectric plasmonics to a new paradigm in integrated photonics

During the past few years, the optics and photonics communities have renewed their attention toward transparent conducting oxides (TCOs), which for over two decades have been broadly employed for the fabrication of transparent electrodes in photovoltaic and communication technologies. This reinvigorated research curiosity is twofold: on the one hand, TCOs, with their metal-like properties, low optical absorption, and fabrication flexibility, represent an appealing alternative to noble metals for designing ultra-compact plasmonic devices. On the other hand, this class of hybrid compounds has been proved to possess exceptionally high optical nonlinearities when operating on a frequency window centered around their crossover point, the wavelength point at which the real part of the dielectric permittivity switches sign. Because TCOs are wide-bandgap materials with the Fermi level located in the conduction band, they are hybrid in nature, thus presenting both interband and intraband nonlinearities. This is the cause of a very rich nonlinear physics that is yet to be fully understood and explored. In addition to this, TCOs are epsilon-near-zero (ENZ) materials within a broad near-infrared spectral range, including the entire telecom bandwidth. In this operational window a myriad of novel electromagnetic phenomena have been demonstrated experimentally such as supercoupling, wavefront freezing, and photon doping. Furthermore, TCOs stand out among all other ENZ systems due to one fundamental characteristic, which is hardly attainable even by using structured materials. In fact, around their ENZ wavelength and for a quite generous operational range, these materials can be engineered to have an extremely small real index. This peculiarity leads to a slow-light effect that is ultimately responsible for a significant enhancement of the material nonlinear properties and is the cornerstone of the emerging field of near-zero-index photonics. In this regard, the recent history of nonlinear optics in conductive oxides is growing extremely fast due to a great number of experiments reporting unprecedentedly remarkable effects, including unitary index change, bandwidth-large frequency shift, efficient ultra-low-power frequency conversion, and many others. This review is meant to guide the reader through the exciting journey of TCOs, starting as an industrial material for transparent electrodes, then becoming a new alternative for low-loss plasmonics, and recently opening up new frontiers in integrated nonlinear optics. The present review is mainly focused on experimental observations.

Extreme learning machine computational method of modeling energy gap of doped zinc selenide nano-material semiconductor

Zinc selenide (ZnSe) semiconductor belongs to a class of wide energy gap material with low visible region optical absorption, high optical coefficient (nonlinear) and fascinating optoelectronic as well as photocatalytic features which foster and strengthen its applications in resisting thermal shock, photoelectric fields (which include dielectric mirrors, shot wavelength lasers and light emitting diode), buffer layer of solar cell and many photocatalytic environmental cleanliness applications. Nanostructured ZnSe semiconductors occupy a special place in application domain due to the characteristic features attached to the crystallite size of the material which include high fluorescence quantum yields, solvent dispersibility and adjustable emission color among others. Effective utilization of nanostructured ZnSe semiconductor involves tuning of its energy gap through doping mechanisms which is accompanied with distortion in the parent lattice structure. In this contribution, extreme learning machine (ELM) computational intelligent method is proposed for predicting the band gap energy of doped ZnSe nanostructured semiconductor using the material crystallite size and lattice parameter as descriptors to the models. The developed ELM based model using sine (Sine), sigmoid (Sig) and transig (Trang) activation functions were compared with the existing SVR-GA and SPR models in the literature using various performance metrics. The developed Sine-ELM model shows superior performance over Sig-ELM, Trang-ELM, SVR-GA (2021) and SPR (2021) models with performance improvement of 12.74%, 64.89%, 68.87% and 75.62% using mean squared error (MSE) metric for testing data samples. The superiority of the model developed was further justified through validation with external set of data. The precision and superiority of the ELM based models developed would eventually ease quick characterization and tuning of energy gap of ZnSe nanostructured semiconductors for various applications.

Near‐Infrared and Visible‐Range Optoelectronics in 2D Hybrid Perovskite/Transition Metal Dichalcogenide Heterostructures

AbstractThe application of ultrathin 2D perovskites in near‐infrared and visible‐range optoelectronics is limited owing to their inherent wide bandgaps, large excitonic binding energies, and low optical absorption at higher wavelengths. Here, it is shown that by tailoring interfacial band alignments via conjugation with low‐dimensional materials like monolayer transition metal dichalcogenides (TMD), the functionalities of 2D perovskites can be extended to diverse, visible‐range photophysical applications. Based on the choice of individual constituents in the 2D perovskite/TMD heterostructures, first principles calculations demonstrate widely tunable type‐II bandgaps, carrier effective masses, and band offsets to enable an effective separation of photogenerated excitons for enhanced photodetection and photovoltaic applications. In addition, the possibilities of achieving a type‐I band alignment for recombination‐based light emitters as well as a type‐III configuration for tunneling devices are shown. Further, the effect of strain on the electronic properties of the heterostructures are evaluated to show a significant strain tolerance, making them prospective candidates in flexible photosensors.

Using Ligand Engineering to Produce Efficient and Stable Pb-Sn Perovskite Solar Cells with Antioxidative 2D Capping Layers.

Pb-Sn binary halide perovskites are a promising photovoltaic material due to their low toxicity and optical absorption spectrum well matched to the solar spectrum. However, the ready oxidation of Sn2+ to Sn4+ makes the material system currently too unstable to commercialize. Herein, ligand engineering based on antioxidative tyramine (hydrochloride, TACl) is presented for the first time to increase the stability of this material system. Using this strategy, we generate a two-dimensional (2D) capping layer on top of a standard three-dimensional Pb-Sn film. After capping, the surface defects can be passivated and the TACl-based 2D perovskite effectively protected Sn2+ from oxidation, which stabilized the Sn-Pb perovskite composition, avoiding the Pb-based perovskite formation. It is further found that the TACl treatment suppressed the halide segregation and improved the perovskite film photostability. Cell efficiency increases from 16.25 to 18.28% and device lifetime (T80) increases from less than 100 to over 1000 h. Our finding suggests that tuning ligand form/function represents a potentially highly productive direction to explore when trying to produce stable tin-based perovskite devices.

Measurements of small optical absorption and scattering coefficients of high-power polarised laser radiation in lithium triborate crystals

We present the results of measurements of low optical absorption and scattering coefficients in lithium triborate crystals using the method of piezoelectric resonance laser calorimetry under the action of polarised single-mode laser radiation at wavelengths of 1070 and 532 nm in the power ranges of 50 – 300 and 5 – 50 W, respectively. It is shown that when radiation propagates along the crystal axis a, these coefficients remain constant in the specified power ranges and do not depend on the direction of laser radiation polarisation. The measured optical scattering coefficients are compared with the results obtained by the method of integrating spheres.

Photodetection Tuning with High Absorptivity Using Stacked 2D Heterostructure Films.

Graphene-based photodetection (PD) devices have been broadly studied for their broadband absorption, high carrier mobility, and mechanical flexibility. Owing to graphene’s low optical absorption, the research on graphene-based PD devices so far has relied on hybrid heterostructure devices to enhance photo-absorption. Designing a new generation of PD devices supported by silicon (Si) film is considered as an innovative technique for PD devices; Si film-based devices are typically utilized in optical communication and image sensing owing to the remarkable features of Si, e.g., high absorption, high carrier mobility, outstanding CMOS integration. Here, we integrate (i) Si film via a splitting/printing transfer with (ii) graphite film grown by a pyrolysis method. Consequently, p-type Si film/graphite film/n-type Si-stacked PD devices exhibited a broadband detection of 0.4–4 μm (in computation) and obtained good experimental results such as the responsivity of 100 mA/W, specific detectivity of 3.44 × 106 Jones, noise-equivalent power of 14.53 × 10−10 W/(Hz)1/2, external quantum efficiency of 0.2, and rise/fall time of 38 μs/1 μs under 532 nm laser illumination. Additionally, our computational results also confirmed an enhanced light absorption of the above stacked 2D heterostructure film-based PD device compatible with the experimental results.

Designs for III-nitride edge-emitting laser diodes with tunnel junction contacts for low internal optical absorption loss

The lasing mode and internal optical absorption loss profiles of III-nitride edge-emitting laser diode (LD) designs operating at 440 nm were modeled using the transfer matrix method. These models indicate absorption losses are minimized in LDs utilizing tunnel junction (TJ) contacts that (1) optimize the TJ to be as close to fully depleted as possible; (2) spatially separate the metal contact from the lasing mode using lightly doped n-GaN located above the TJ; (3) in designs with optimized TJs, minimize the total thickness of p-type material. These features reduce absorption losses in the model, relative to comparable LD models with an unoptimized TJ contact or with a transparent conducting oxide p-contact, by up to 15.3% and 12.5%, respectively, while at the same time eliminating most or nearly all p-type material.