Dielectric Reflectivity Research Articles

Dielectric Bragg Reflector as Back Electrode for Semi‐Transparent Organic Solar Cells with an Average Visible Transparency of 52%

A crucial challenge in the development of semi‐transparent solar cells is to maintain a reasonable power conversion efficiency (PCE) while reaching a high average visible transparency (AVT). Typically, organic semiconductors are favorable for this application since they can selectively absorb infrared light while transmitting visible light. This ability stems from limited electronic states at high(er) energies in contrast to inorganic semiconductors with their typical rise of the absorption coefficient toward higher photon energies. To increase PCE at high AVTs, a series of infrared dielectric Bragg reflectors is developed for semi‐transparent organic solar cells. Using the multi‐layered back electrode (TiO2|SiN|TiO2|AZO|Ag|AZO) with PV‐X Plus as photoactive layer and a metal‐free PEDOT:PSS top electrode, a light utilization efficiency (LUE = AVT × PCE) of up to 4.32% is achieved, together with an AVT of 47.9%. Although the short circuit current and AVT agree well with optical simulations, a low fill factor (FF) and partial shunting limit the overall device performance. Using ZnO and PFN‐Br as additional electron transport layers and modifying the back electrode stack (TiO2|SiO2|TiO2|AZO|Ag|AZO) accordingly leads to an LUE of up to 4.6% with a remarkable AVT of 51.9% and a maximum PCE of 8.79%.

Low-threshold green lasing in heterogeneously integrated InGaN-based micro-rings covered by distributed Bragg reflectors on Si (100)

In this work, combining a series of wafer bonding, laser lift-off and chemical mechanical polishing processes, submicron-thick wafer-scale GaN-based thin-film epilayers are successfully transferred on Si (100), which provides a heterogeneous platform for fabricating microcavities for nitride-based integrated photonics. Low-threshold lasing via optical pumping from these transferred dry-etched green micro-ring cavities on Si is demonstrated by covering the whole micro-rings with dielectric distributed Bragg reflectors (DBRs), which greatly reduces the lasing threshold upon a better optical confinement at the ring rim. A high quality-factor of ∼3800 can be observed from the micro-rings beyond the lasing threshold under pulsed excitation conditions. Furthermore, room-temperature continuous-wave (CW) lasing at a wavelength of 521.7 nm with an ultralow threshold of 0.35 kW/cm2 is achieved. Our results suggest the use of a burying DBR layer notably improves the WGM microcavity confinement, providing insights for the design of low-threshold micro-lasers and low-loss waveguides for potential integrated photonic applications in the visible light range on the Si platform.

Suppressing the radiation loss by hybrid Tamm-surface plasmon BIC modes

Tamm plasmon polaritons (TPPs), localized near the boundary of a dielectric Bragg reflector (DBR) and a thin metal film, have attracted much attention for the lower ohm loss and flexible excitation. However, the radiation loss resulting from the direct coupling to the surroundings hinders their applications. Here, we propose and experimentally demonstrate a new type of hybrid plasmonic quasi-bound state in the continuum (BIC) in a Tamm-surface plasmon polariton system to suppress the radiation loss. Leveraging the scattering of the periodic metal array, the TPP interacts with the surface plasmon polariton (SPP) mode and form a Friedrich-Wintgen type quasi-BIC state that originated from the interference of two surface waves with different natures. Through angle resolved reflectance spectrum measurement, the hybrid plasmonic quasi-BIC was observed in the experiment. Our work proposes a new method to design a high Q mode in plasmonic systems, and thus holds promise for applications in the field of light matter interactions.

Optical response and temperature shielding coatings using tri-layer structure composed of titania/silica/titania

The fabrication of metamaterials with inspiration from nature paved the door for the creation of revolutionary passive and active devices. Butterfly wings are one of them, and their multilayered structure motivated nanotechnologists, physicists, and other optoelectronic and photonic engineers to develop photonic crystals, dielectric reflectors, and Bragg reflectors for use in visible light communication, solar cells, and other photonic and optoelectronic applications. We report the sol–gel spin coating of a dielectric reflector on the glass substrate, an optical passive component consisting of titania and silica thin films for infrared radiation reflection. Individual thin films of titania and silica on glass substrates were studied using an x-ray diffractometer, which indicated anatase of titania and the amorphous nature of silica at a Bragg angle of 25° with a sharp and wide peak, respectively. The multilayer structure of titania/silica/titania was further investigated using Fourier transform infrared spectroscopy, which revealed the presence of Ti–O–Ti and Si–O–Si vibrational bonds at wavenumbers 546 and 973 cm−1, respectively, as well as the presence of Ti–O–Si vibrational bond at 1100 cm−1. The thickness of the multilayer titania/silica/titania was measured using a cross-sectional field emission scanning electron microscope (FESEM) and found to be 160/240/160 nm, respectively. Finally, reflection investigation on the multilayer structure using ultraviolet–visible–near-infrared spectroscopy validated the reflection of the infrared spectrum area by around 70% and showed to be beneficial for temperature shielding applications on glass furnishings.

Revisiting the Luce diode in the context of recent research on multi-vircators with dielectric reflectors

Recently, in a series of articles Mumtaz, Choi, and others [i.e., S. Mumtaz and E. Choi, IEEE Electronic Device Lett. 43, 1756 (2022)] studied a relativistic gridded vircator with one or more dielectric anodes, in which considerable increase in the efficiency of high-power microwave (HPM) generation compared to ordinary vircators was observed. This is very important since the main disadvantage of vircators is their low efficiency. It was suggested that the repelling of electrons by the initial impacting electron charge collected on the dielectric reflectors forms a multi-vircator, which increases HPM production. The present article reports the results of experiments and numerical simulations, which show that this mechanism does not result in high multi-vircator efficiency. Similar dielectric anodes were investigated 50 years ago in a configuration known as the Luce diode, in which collective acceleration of ions was studied. It was also known that the operation of the Luce diode is accompanied by a HPM burst, which was never measured. Results of experiments and simulations in various configurations based on a gridded vircator and dielectric reflectors confirm the existence of high-energy ion acceleration. Unfortunately, none of these configurations produce HPM of higher efficiency than a regular gridded vircator.

High-β lasing in photonic-defect semiconductor-dielectric hybrid microresonators with embedded InGaAs quantum dots

We report an easy-to-fabricate microcavity design to produce optically pumped high-β quantum dot microlasers. Our cavity concept is based on a buried photonic-defect for tight lateral mode confinement in a quasi-planar microcavity system, which includes an upper dielectric distributed Bragg reflector (DBR) as a promising alternative to conventional III–V semiconductor DBRs. The cavities show distinct emission features with a characteristic photonic-defect size-dependent mode separation and Q-factors up to 17 000. Comprehensive investigations further reveal lasing operation with a systematic increase (decrease) of the β-factor (threshold pump power) with the number of mirror pairs in the upper dielectric DBR. Notably, due to the quasi-planar device geometry, the microlasers show high temperature stability, evidenced by the absence of temperature-induced redshift of emission energy and linewidth broadening typically observed for nano- and microlasers at high excitation powers. The device exhibits remarkable lasing performance, maintaining efficacy even under elevated temperatures of up to 260 K.

A novel methodology in fabricating dielectric reflectors for the desired wavelength spectrum using sol-gel spin coating technique

The use of passive photonic devices such as dielectric reflectors in electronics to replace semiconductor technology is being extensively researched. Dielectric reflectors with the required stop band are in high demand as smart windows, visual light communication boosters, and UV radiation defenders. The determination of a single approach for the envisioned wavelength spectrum of a dielectric reflector is unique and original. We offer a unique approach for synthesizing and developing dielectric reflectors in the required wavelength domain. The constructed reflectors were examined using XRD, which indicated the anatase phase of TiO2 in the multilayer structure. Cross-sectional FE-SEM examination revealed periodic bright (TiO2) and dark (SiO2) bands, and thickness analysis demonstrated that increasing the spin rate can lower the film thickness of the produced films. In addition, reflectance analysis on the structures using a UV-VIS-NIR spectrophotometer demonstrated that increasing the film thickness of the multilayer structure can move the reflectance band towards higher wavelengths.

Controlled Cavity Length and Wide-Spectrum Lasing in FAMACsPb(BrI)3 Ternary Perovskite Vertical-Cavity Surface-Emitting Lasers with an All-Dielectric Dielectric Bragg Reflector

In this study, we utilized a dielectric Bragg reflector (DBR) as a mirror and positioned a wide-spectrum FAMACsPb(BrI)3 halide perovskite film between two DBRs to construct a vertical-cavity surface-emitting laser (VCSEL) structure. The top and bottom DBRs were connected using optical adhesive, allowing us to control the cavity length by applying external force. Through this approach, we achieved operation at the desired wavelength. Due to the exceptional optical gain provided by FAMACsPb(BrI)3, we successfully observed multimode and lasing phenomena at room temperature under continuous-wave (CW) laser excitation. The outcomes of this study provide valuable insights for the application of novel VCSEL structures and highlight the potential of using FAMACsPb(BrI)3 halide perovskites in optical gain. This work holds significant implications for the fields of optical communication and laser technology.

Metamaterial-based smart and flexible Optical Solar Reflectors

In the frame of projects funded by the European Commission and the ESA, we developed a new type of Optical Solar Reflector (OSR) that combines the flexibility and easy handling of Second Surface Mirrors with the temperature-variable emittance necessary to ensure both effective radiative cooling in the hot phase and reduced heat losses in the cold phase. The new smart OSR consists of a metamaterial coating deposited on Kapton film. The coating is made of two functional blocks, namely a variable emitter topped by a dielectric solar reflector. The variable emitter is a Metamaterial Perfect Absorber designed for strong and broadband plasmonic resonance absorption in the thermal IR. It consists of a metal back-reflector, a dielectric spacer, and an array of doped VO2 thermochromic micro-antennas that are switched-off when the temperature drops below the metal-to-insulator transition point. The solar reflector is a wideband dielectric filter made of materials that are transparent across the entire spectrum from the VIS to the thermal IR. All the layers of the two blocks are deposited by standard vacuum techniques, while the array is patterned by Nanoimprint Lithography, a technique that is often performed at the wafer level but allows for up-scaling via roll-to-roll or roll-to-plate production setups. The paper reports on the characterization and testing of samples of size up to 100 mm x 100 mm, at the Beginning of Life and after thermal, humidity, irradiation and handling tests.

Low-threshold lasing of optically pumped micropillar lasers with Al0.2Ga0.8As/Al0.9Ga0.1As distributed Bragg reflectors

We report on the design, realization, and characterization of optically pumped micropillar lasers with low-absorbing Al0.2Ga0.8As/Al0.9Ga0.1As dielectric Bragg reflectors (DBRs) instead of commonly used GaAs/AlGaAs DBRs. A layer of (In, Ga)As quantum dots is embedded in the GaAs λ-cavity of as an active medium. We experimentally study the lasing characteristics of the fabricated micropillars by means of low-temperature photoluminescence with varying pump laser wavelength between 532 and 899 nm. The incorporation of 20% Al content in the DBRs opens an optical pumping window from 700 to 820 nm, where the excitation laser light can effectively reach the GaAs cavity above its bandgap while remaining transparent to the DBRs. This results in a substantially improved pump efficiency, a low lasing threshold, and a high thermal stability. Pump laser wavelengths outside of the engineered spectral window lead to low pump efficiency due to strong absorption by the top DBR or inefficient excitation of pump-level excitons. The superiority of the absorption-free modified DBRs is demonstrated by simply switching the pump laser wavelength from 671 to 708 nm, which crosses the DBRs absorption edge and drastically reduces the lasing threshold by more than an order of magnitude from (363.5 ± 18.5) to (12.8 ± 0.3) μW.

Polarization-sensitive narrowband infrared photodetection triggered by optical Tamm state engineering.

Polarization-sensitive narrowband photodetection at near-infrared (NIR) has attracted significant interest in optical communication, environmental monitoring, and intelligent recognition system. However, the current narrowband spectroscopy heavily relies on the extra filter or bulk spectrometer, which deviates from the miniaturization of on-chip integration. Recently, topological phenomena, such as the optical Tamm state (OTS), provided a new solution for developing functional photodetection, and we experimentally realized the device based on 2D material (graphene) for the first time to the best of our knowledge. Here, we demonstrate polarization-sensitive narrowband infrared photodetection in OTS coupled graphene devices, which are designed with the aid of the finite-difference time-domain (FDTD) method. The devices show narrowband response at NIR wavelengths empowered by the tunable Tamm state. The full width at half maximum (FWHM) of the response peak reaches ∼100 nm, and it can potentially be improved to ultra-narrow of about 10 nm by increasing the periods of dielectric distributed Bragg reflector (DBR). The responsivity and response time of the device reaches 187 mA/W and ∼290 µs at 1550 nm, respectively. Furthermore, the prominent anisotropic features and high dichroic ratios of ∼4.6 at 1300 nm and ∼2.5 at 1500 nm are achieved by integrating gold metasurfaces.

Reflectance investigation on dielectric reflectors using ethanol, methanol and isopropanol as solvents in the synthesis process

Dielectric reflectors are made up of a range of solvents in the wet chemical sol–gel based synthesis technique. The unexpected scarcity of solvents such as ethanol complicates matters, influencing the optical characteristics of the created multilayer structure. To eliminate the need for solvents, a new method for generating dielectric reflectors employing a variety of solvents must be created. We provide here an experimental inquiry on the synthesis and fabrication procedure of dielectric reflectors employing different solvents such as ethanol, methanol, and isopropanol that has not been reported previously to the best of our knowledge. The fabricated reflectors are characterized with AFM for roughness analysis and found 26, 5, and 1.2 nm for the solvent ethanol, methanol, and isopropanol respectively. Cross-sectional FE-SEM analysis supported the fabrication of titania and silica thin films with bright and dark colours on the dielectric reflectors. When ethanol was employed as the solvent, reflectance analysis using UV–vis spectroscopy approach showed 100 % reflectivity in the near-infrared region, and when methanol and isopropanol were used, improved reflectance as well as shift in the stop band were observed respectively.

Design and optimization of a blue fluorescent microcavity-organic light-emitting diode (MOLED) on AZO anode for an algae excitation light source application

In this work, we present simultaneous organic light-emitting diode (OLED) stack optimization and optical modelling for a blue microcavity-OLED (MOLED) based on AZO anode to be used as algae excitation light in optical biosensor. Fluorescent materials (MADN and DPAVBi) were chosen as host:guest for the doped emissive layer due their known stability compared to phosphorescent and thermally activated delayed fluorescence (TADF) materials. The MOLED modelling was performed by targeting 470 nm as the maximal excitation wavelength and suppressing the emission in the algae fluorescence bandwidth (600–800 nm) in order to fulfil the sensor requirements. By using a bilayer hole transport layer/electron blocking layer (HTL/EBL) instead of a single HTL, the total thickness was adjusted to meet the resonance wavelength condition without loss of efficiency, while at the same time preserving a maximum electric field intensity in the emissive layer (antinode position). MOLED devices were fabricated by organic semiconductor evaporation on three dielectric distributed Bragg reflectors (DBRs) with 3 different numbers of TiO2/SiO2 (high-index/low-index) pairs and aluminum-doped zinc oxide (AZO) transparent electrode. Devices with 1.5 pairs as DBR showed not only an improved external quantum efficiency (+33%) compared to a standard OLED but also an increase of about 3 times the intensity of the peak at 470 nm combined to a lower emission in the 600–800 nm bandwidth, as aimed. This increase in the peak intensity should lead to a longer device lifetime, as the current density necessary to excite algae at 470 nm is significantly lower owing to the microcavity effect.

Economical construction of spin coater for the fabrication of dielectric reflectors

Spin-coating is a versatile technique that utilizes only a few drops of the synthesized gel as an empirical method to fabricate the appropriate thin films. However, the economical design of a spin coater is always ground-breaking research. This paper reports the development of a spin coater using low-cost components such as SMPS, a brushless D.C. motor, and a proximity sensor. All of these components were contained in a stainless-steel unit and the entire equipment cost only 63 dollars, making it significantly less expensive than a standard one. The constructed spin coater was tested by fabricating three-and-five multilayer structures of TiO2/SiO2 with their respective sol–gel synthesized gels, and an X-ray diffraction (XRD) studies on the structures revealed a broad peak at 25°, confirming the existence of anatase TiO2 (titania) phase and non-crystalline amorphous phase of SiO2 (silica). Fourier transform infrared (FT-IR) spectroscopy endorsed the presence of titania and silica functional linkages by revealing vibrational bonds at 736 and 612 cm−1, respectively. Field emission scanning electron microscopy (FESEM) in cross-sectional mode on the three- and five-multilayer titania and silica structures evidenced the presence of titania and silica layers with intense (bright) and gloomy (dark) nature respectively. Finally, the reflectance study of the multilayer structures utilising ultraviolet–visible and near-infrared (UV–vis-NIR) spectroscopic showed 68 % and 100 % reflectance in the near-infrared region respectively, and demonstrated to be a dielectric reflector.

An Efficient Vircator With High Output Power and Less Drifting Electron Loss by Forming Multivirtual Cathodes

Multi virtual cathodes (MVCs) were created in an axial vircator utilizing two dielectric reflectors (DRs). The wall charge of drifting electrons on the surface of DRs led to the creation of VC2 and VC3 in the downstream area of drift tube, according to phase space simulations. The microwave generated by VC1 interacted with the charge of VC2 and VC3 to raise the power amplitude, resulting in an increase in output power from 284 MW to 1028 MW. The use of two dielectric reflectors was proven to have a remarkable impact in reducing electron loss, increasing output power, and improving efficiency. The suggested vircator’s efficiency with MVCs improved from 7.9 % to 28.6 %. The findings indicate important signs of progress in vircator research to decrease electron losses and boost beam–wave interaction, which might lead to the advancement of high-efficiency vircators.

Gallium nitride-based resonant cavity light-emitting diode with single-longitudinal-mode emission.

A novel, to the best of our knowledge, gallium nitride (GaN)-based resonant cavity light-emitting diode (RCLED) with single-longitudinal-mode light emission was demonstrated. A Ta2O5/SiO2 dielectric distributed Bragg reflector (DBR) with a filter-film structure was adopted as the top mirror. In contrast to the flat-topped reflectivity spectrum of the conventional high-reflective-structure DBR, for this filter-film-structure DBR, there is a light-transmitting concave band on the reflectivity spectrum. Owing to the modulation effect of this band on the output light, a single-longitudinal-mode light emission with a full width at half maximum as low as 0.63 nm was realized. Furthermore, the novel RCLED exhibited better wavelength stability. With an increase in the injection current from 50 to 500 mA, the redshift of the emission peak was only 0.2 nm. This novel RCLED with ultra-narrowband emission has a high potential for application in optical communication systems and optical fiber sensing applications.

Sol-gel spin coating of dielectric reflector for energy harvesting and heat shielding applications

Sol-gel spin coating of dielectric reflector for energy harvesting and heat shielding applications

Photonic Materials Cloud: An Online Interactive Open Tool for Creating, Comparing, and Testing Photonic Materials.

Recent advances in nanoscale fabrication and characterization further accelerated research on photonics and plasmonics, which has already attracted long-standing interest. Alongside morphological constraints, phenomena in both fields highly depend on the materials’ optical properties, dimensions, and surroundings. Building up the required knowledge and experience to design next-generation photonic devices can be a complex task for novice and experienced researchers who intend to evaluate the impact of subtle material and morphology variations while setting up experiments or getting a general overview. Here, we introduce the Photonic Materials Cloud (PMCloud), a web-based, interactive open tool for designing and analyzing photonic materials. PMCloud allows identification of the subtle differences between optical material models generated from a database, experimental data input, and inline-generated materials from various analytical models. Furthermore, it provides a fully interactive interface to evaluate their performance in important fundamental (numerical) optical experiments. We demonstrate PMCloud’s applicability to state-of-the-art research questions, namely the comparison of the novel plasmonic materials aluminium-doped zinc oxide and zirconium nitride and the design of an optical, dielectric thin-film Bragg reflector. PMCloud opens a rapid, freely accessible path towards prototyping optical materials and simple fundamental devices and may serve as an educational platform for photonic materials research.

Assembly of Guanine Crystals as a Low-Polarizing Broadband Multilayer Reflector in a Spider, Phoroncidia rubroargentea.

The diminishing of the polarization effect is important in the applications of dielectric multilayer reflectors in many optical systems, such as low-loss broadband waveguides, optical fibers, and LEDs. Low-polarizing broadband reflections were identified from birefringent-guanine-crystal-based multilayer reflectors in the skins of some fish. Previous models for these intriguing natural optical phenomena suggested the combined action of two populations of guanine crystals with an orthogonal low-refractive-index optic axis. Here we report a novel realization of polarization-insensitive broadband reflectivity in a spider, Phoroncidia rubroargentea, based solely on the type of guanine crystals with the low-refractive-index optic axis normal to the crystal plates. We examined the three-dimensional structure of the guanine assembly in the spider and performed finite-difference time-domain (FDTD) optical modeling of the guanine-based multilayer reflector. Comparative modeling studies reveal that the biological selection of the guanine crystal type and specific spatial arrangement work synergistically to optimize the polarization-insensitive broadband reflection. This study demonstrates the importance of both crystallographic characteristics and 3D arrangement of guanine crystals in understanding relevant natural optical effects and also provides new insights into similar broadband, low-polarizing reflections in biological optical systems. Learning from relevant biofunctional assembly of guanine crystals could promote the bioinspired design of nonpolarizing dielectric multilayer reflectors.

InGaN-Based Orange-Red Resonant Cavity Light-Emitting Diodes

InGaN-based orange-red resonant cavity light-emitting diodes (RCLEDs), were fabricated by using an AlN current-confinement aperture and double dielectric distributed Bragg reflectors (DBRs). For realizing the structure of device, a substrate transfer technique was employed in process of fabrication. The device exhibited optical resonant effect, a high Q factor (∼3010), and a narrow emission linewidth (FMHW ∼0.2 nm), indicating low optical loss in the resonant cavity. Additionally, due to the three-dimensional (3D) optical confinement effect, the discrete modes of red emission were clearly observed in the far field via an angular resolved measurement system. And then, the energies of the photon states of a red RCLED consists generally with the simulation results based on a circular waveguide model. The saturated emission intensity of the orange-red RCLED was proportional to the area of current injection. This work demonstrated the feasibility of InGaN-based electrically injected orange-red RCLEDs that are useful for the development of displays and communication systems.