Distributed Bragg Reflector Layers Research Articles

Linearly polarized GaN micro-LED with adjustable directional emission integrated with a continuous metasurface

Traditional LEDs emit light that exhibits incoherence and displays a Lambertian distribution. To achieve linearly polarized (LP) light and control the emission direction, a variety of optical components are required to be stacked, which is unsuitable for compact applications and results in low deflection efficiency. Here, we propose and and numerically simulate a novel single-chip micro-resonant cavity LED (micro-RCLED) device that generates directional LP light by integrating a continuous metasurface. This device includes a bilayer grating at the GaN layer's bottom, providing high transverse electric (TE) reflectivity above 89.5% and an extinction ratio exceeds 57 dB at 500 nm. The top distributed Bragg reflector (DBR) and the bilayer grating together constitute a TE mode Fabry–Pérot resonant cavity. This not only promotes the emission of the TE wave, but also guarantees its collimation with the appropriate phase, thereby enhancing its spatial coherence. A functional metasurface above the DBR layer precisely controls the TE wave's deflection angle. It maintains a low aspect ratio while enabling efficient, large-angle deflection. The simulation results demonstrate that this device provides an effective solution for generating highly spatially coherent directional LP light, with broad potential applications in fields such as polarized light imaging and advanced 3D micro-LED display systems.

Silica aerogel cavity enhancement of spectral reflection from distributed Bragg reflectors with patterned surfaces.

Distributed Bragg reflectors (DBRs) can experience thermomechanical issues under an intense incident beam. A remedy for this issue is to pattern the DBR structure to facilitate thermal expansion in the structure. However, finite-size patterns reduce the design's electromagnetic performance, including the reflectivity amplitude and the operational bandwidth. Here, we introduce a silica aerogel cavity medium between the DBR layer and the substrate to increase the reflection by stimulating the cavity modes. By altering the cavity layer's thickness and the pattern width, the interference of reflected lights can be controlled and enhance the reflectivity at desired wavelengths. The results show a significant enhancement in the reflection by introducing the cavity in the design. For a comprehensive investigation of the mechanisms, the interaction of the DBR layers with the substrate, the effect of patterns, and the propagation of electric fields, especially in the cavity layer, are examined.

Optimization of strip-based hybrid plasmonic terahertz waveguide with distributed Bragg reflector layers

In view of recent developments in the utilization of terahertz (THz) communications technology for the increasing demands of high speed and data rates for various wireless applications, this research work targets the design and analysis of distributed Bragg reflector (DBR)-based hybrid plasmonic THz waveguide. This THz waveguide operates at a frequency range from 2.5 to 3.5 THz. The proposed DBR-based hybrid plasmonic THz waveguide consists of DBR layers of gallium arsenide (GaAs) and aluminum arsenide, high-density polyethylene (HDPE) as substrate, a GaAs stripe for wave-guidance, and few layers of graphene layers in HDPE for better confinement. To increase the light confinement between DBR and graphene, a GaAs strip is placed into HDPE. By altering the width and height of the GaAs strip, the parameters birefringence, mode field diameter (MFD), confinement loss, effective mode area, beat length, and dispersion have been fully examined. The obtained simulation results shows high birefringence of 0.6276, maximum MFD of 45 mm, low confinement loss of 7.5 × 10 − 9 mm − 1, high effective mode area of 16.5 mm2, and low anomalous dispersion of 0.0023 (ps/THz/cm) in the range of 2.5 to 3.5 THz. This optimized THz waveguide may helps to enable in guided THz applications for photonic integrated circuits.

Coupled Tamm Phonon and Plasmon Polaritons for Designer Planar Multiresonance Absorbers.

Wavelength-selective absorbers (WS-absorbers) are of interest for various applications, including chemical sensing and light sources. Lithography-free fabrication of WS-absorbers can be realized via Tamm plasmon polaritons (TPPs) supported by distributed Bragg reflectors (DBRs) on plasmonic materials. While multifrequency and nearly arbitrary spectra can be realized with TPPs via inverse design algorithms, demanding and thick DBRs are required for high quality-factors (Q-factors) and/or multiband TPP-absorbers, increasing the cost and reducing fabrication error tolerance. Here, high Q-factor multiband absorption with limited DBR layers (3 layers) is experimentally demonstrated by Tamm hybrid polaritons (THPs) formed by coupling TPPs and Tamm phonon polaritons when modal frequencies are overlapped. Compared to the TPP component, the Q-factors of THPs are improved twofold, and the angular broadening is also reduced twofold, facilitating applications where narrow-band and nondispersive WS-absorbers are needed. Moreover, an open-source algorithm is developed to inversely design THP-absorbers consisting of anisotropic media and exemplify that the modal frequencies can be assigned to desirable positions. Furthermore, it is demonstrated that inversely designed THP-absorbers can realize same spectral resonances with fewer DBR layers than a TPP-absorber, thus reducing the fabrication complexity and enabling more cost-effective, lithography-free, wafer-scale WS-absorberss for applications such as free-space communications and gas sensing.

Study of monolithically integrated 940 nm AlGaAs distributed Bragg reflectors on graded GaAsP/bulk Si substrates

We report the fabrication of monolithically integrated 940 nm AlGaAs distributed Bragg reflectors (DBRs) on graded GaAsP/Si substrates. Low-density surface bumps and cross-hatch patterns were observed on the DBR surfaces. Cross-sectional DBR layers are smooth and flat. The reflectance spectra of the GaAsP/Si DBRs have lower intensities than those of the GaAs DBRs and have double peaks. Transfer matrix method calculations, surface scratch and polishing tests were conducted, which suggest that the surface cross-hatch was the cause of the inferior DBR reflectance spectra.

Enhancement and sensing applications of ultra-narrow band circular dichroism of the chiral nanopore films based on Bragg reflector.

Narrow-band circular dichroism (CD) has attracted considerable attention in the high-sensitivity detection of chiral molecules and chiral catalysis. However, achieving dynamic adjustment of narrow-band CD signals is challenging. In this study, we introduce a disruption layer (DL) and molybdenum disulfide (MoS2) into an L-shaped chiral nanohole array based on a distributed Bragg reflector (DBR), forming L-shaped chiral nanoholes (LCNAs/DL-DBR/MoS2), and investigate the mechanism of CD signal generation. Simulation results show that LCNAs/DL-DBR/MoS2 generate three narrow-band CD signals in the visible region. Analysis of the near-field electric field maps reveals that the three CD peaks of LCNAs/DL-DBR/MoS2 are caused by three Tamm resonances in the DBR layer. The producing and adjusting mechanisms of the CD signals are achieved by changing the structural parameters and the number of MoS2 layers. Dynamic adjustment of the CD signals of LCNAs/DL-DBR/MoS2 can be achieved by changing the environmental temperature. Furthermore, by altering the refractive index of the environment and the DBR layer, it is demonstrated that LCNAs/DL-DBR/MoS2 has a high-quality factor. Our theoretical simulations aid in the design of UNB chiral devices, opening up new avenues for environmental monitoring and the detection of chiral molecules with exceptional sensitivity.

Optimization of distributed Bragg’s reflectors in thin film solar cells

Distributed Bragg reflectors (DBRs) or one-dimensional photonic crystals (1DPCs) play key role in advanced opto-electronic and photonic devices. The term photonics is derived from photons in which particle indicates a quantum of light and photon carries energy proportional to the radiation frequency with zero rest mass. In this work, we present the designing 1DPCs by using Si/SiO2 layers using a plane wave expansion method (PWE). For that, various parameters are optimized such as thicknesses (d) of each layer, center wavelength (λc), refractive index (nH/nL) of individual layer and the number of DBR/period to analyze the photonic band gap (PBG). The transverse electric (TE) and magnetic (TM) field distribution at different wavelength is discussed and helpful to understand the light harvesting mechanism within the PBG. Finally, this optimized DBR layers were integrated into a thin film amorphous silicon solar cell as a back-end reflector. Due to the tuned PBG, the proposed and designed amorphous silicon thin film solar cell has yielded an improved of light absorption, which is useful for enhancing the photovoltaic performance.

Enhancement and Sensing Application of Ultra-Narrowband Circular Dichroism in the Chiral Nanostructures Based on Monolayer MoS2 and a Distributed Bragg Reflector.

Narrowband circular dichroism (CD) has aroused wide concerns in high-sensitivity detections of chiral molecular and chiral catalysis. Nevertheless, the dynamical adjustment of ultra-narrowband (UNB) CD signals is hard to achieve. In this work, single-layer molybdenum disulfide (MoS2), vanadium dioxide (VO2), and a distributed Bragg reflector (DBR) are introduced into X-shaped chiral nanostructures (XCNs) for overcoming the above challenge. The simulation results show that XCNs can generate four strong UNB CD signals in the near-infrared band, and XCNs/MoS2 can further enhance the UNB CD signals. The full width at half-minimum of UNB CD signals can reach 0.14 nm. The electric field distributions of XCNs/MoS2 show that the four CD signals originate from the coupling between the guided mode resonances along the x and y axes in the VO2 layer and the Tamm plasmon polaritons along the x and y axes in the DBR layer. Four UNB CD peaks can be actively tuned by varying the structural parameters, the number of MoS2 layers, and the environmental temperature. The FOM of XCNs/MoS2 can reach 1487 by changing the refractive index of the DRB layers. These findings contribute to the design of UNB chiral devices and provide new possibilities for environmental monitoring and ultrasensitive detection of chiral molecules.

(Invited) Achieving High-Power Single-Mode Operation in Vertical-Cavity Surface-Emitting Lasers Via Scalable, Higher-Order Mode Suppression Techniques

Achieving high-power single-mode operation in Vertical-Cavity Surface-Emitting Lasers (VCSELs) has received renewed interest because of applications that span facial recognition and 3D imaging in mobile telephones, Light Detection and Ranging (LiDAR) for autonomous vehicles, and Pulse Amplitude Modulation (PAM-4) for high-speed (>100 Gbps) data transmission within data centers. A key challenge with oxide-confined VCSELs is that the strong refractive index contrast between the oxidized ~AlAs aperture and surrounding AlGaAs layers combined with the oxide aperture size create a photonic cavity that fundamentally supports multiple transverse modes. When the size of the aperture is reduced to create a single-mode cavity, the output power is limited by the electrically pumped gain region within the aperture. There is thus a fundamental trade-off between size and power which manifests itself in a limited capability to simultaneously achieve high power and single mode operation. Various methods have been employed by groups working on VCSELs to overcome this limitation. Neglecting external cavity methods which are fundamentally not scalable, the most promising results have been achieved by groups using etched surface relief where a pattern is etched in the top layer or layers to change the standing wave pattern in the cavity and reduce modal gain by lowering the magnitude of the electric field in regions overlapping with the quantum wells, which provide laser gain. A downside of this approach is that material is removed, impacting the uniformity of current injection across the area of the device. A key question of interest is therefore how to achieve mode suppression in large area devices in a way that does not impact current injection. In this presentation, novel methods for mode control that rely on spatial variation of modal gain across the area of the VCSEL aperture will be discussed, and results on high-power single-mode operations of VCSELs will be presented. Two distinct mode control methods will be reviewed. The first method of mode control uses deposited optical coatings on the top surface of the VCSEL to spatially modify the electric field standing wave in the quantum well heterostructure region. Through patterning the coating in either a pillar or anulus, specific modes can be suppressed by selectively changing the gain available for those modes. These filters have been made using both SiO2/TiO2 stacks and alternatively a single layer of silicon. The capability to pattern these layers in a wafer-scale process makes this method attractive for volume manufacturing. The second method of mode control utilizes impurity-induced layer disordering via patterned diffusion of Zn to selectively intermix Distributed Bragg Reflector (DBR) layers, thereby creating a top laser mirror whose reflectivity varies spatially across the aperture. Designing the diffused region to maximize overlap with high-order modes while minimizing overlap with the fundamental mode provides mode suppression and enhanced single-mode power. An added benefit of this approach is a reduction in resistance through the device top DBR layers brought about by the higher p-doping level in the Zn-diffused regions, minimizing Ohmic losses and heat generation. Through the use of strain control in the diffusion mask, diffusion front shape is also shown to be controllable in a way that can be used to minimize the impact of the diffused region on fundamental mode reflectivity. Tensile and compressive strain of diffusion masks are shown to have differing effects on vertical (into the device) versus lateral (across the area of the device under the mask) diffusion, allowing more precise spatial control of reflectivity. Using these methods, single-mode output powers in excess of 10 mW have been demonstrated, with side-mode suppression ratios in excess of 30 dB. These results will be discussed, as well as further work to enhance VCSEL performance. Figure 1

Regulative control and enhancement of multi-color upconversion luminescence with DBR cavities

Upconversion luminescence (UCL) of lanthanide-doped materials (e.g. NaYF4:Yb3+/Er3+) involves multi-step, multi-channel transitions (corresponding to multi-color emissions) in a multi-level system, and is a low-efficiency nonlinear process. Usually, the UCL is enhanced by local enhancement of the excited field or one of its multi-color emissions is promoted by matching with a resonance cavity mode based on Purcell effect. Here, we propose to regulatively control and enhance the UCL by fostering one color of the UCL emissions with a resonance mode and inhibiting the other with an anti-resonance or nonresonance mode in forbidden band in an optical cavity, so that excited-state ions (e.g. Er3+) transit to the ground state more via the fostered UCL emission channel, rather than the inhibited one. As such, high-contrast single-color UCL emission can be achieved with an enhancement beyond that of Purcell effect. For the purpose, superior mode properties of distributed Bragg reflector (DBR) cavities can be applied, whose forbidden band of the DBR layers and resonance cavity mode in the forbidden band can be independently tuned in positions to match with the UCL emissions for them to be inhibited and fostered respectively. In experimental implementation, multi-color UCL (e.g. red and green) of NaYF4:Yb3+/Er3+ nanoparticles embedded in such DBR cavities are studied. And high-contrast single-color UCL emissions are demonstrated with enhancements factors beyond Purcell factors calculated in numerical simulations. A hypothesis on modifications of intermediate transitions in the UCL processes, as feedbacks to influences of mode characteristics in the DBR cavities for regulative control, is also proposed to explain the phenomena. The work suggests a way to regulatively control multi-channel photon emissions in multi-level systems for enhanced single-channel photon emission.

Excitation of Hybrid Waveguide-Bloch Surface States with Bi2Se3 Plasmonic Material in the Near-Infrared Range.

Bloch surface waves (BSWs) with Bi2Se3 in a composite structure consisting of a coupling prism, distributed Bragg reflector (DBR) and cavity layer have been demonstrated. The design relies on the confinement of surface waves that originates from the coupling between the defective layer of plasmonic material (Bi2Se3) and DBR. The presence of the cavity layer modifies the local effective refractive index, enabling direct manipulation of the BSWs. The transfer matrix method (TMM) is used to evaluate the reflectance and absorptance responses in the spectral domain for various angles of incidence, demonstrating the presence of sharp resonances associated with the BSW. With an optimal thickness of DBR bilayers, the energy of an evanescent wave can be transferred into the periodic stack resulting in the excitation of waveguide modes (WGMs). It is believed that the proposed design possesses the advantage in terms of easy fabrication to develop integrated photonic systems, especially for biological and chemical sensing.

Temperature dependence of refractive indices of Al0.9Ga0.1As and In0.53Al0.1Ga0.37As in the telecommunication spectral range

In this work, we determine the temperature dependence of refractive indices of In0.53Al0.1Ga0.37As and Al0.9Ga0.1As semiconductor alloys at telecommunication wavelengths in the range from room temperature down to 10 K. For that, we measure the temperature-dependent reflectance of two structures: with an Al0.9Ga0.1As/GaAs distributed Bragg reflector (DBR) designed for 1.3 µm and with an In0.53Al0.1Ga0.37As/InP DBR designed for 1.55 µm. The obtained experimental results are compared to DBR reflectivity spectra calculated within the transfer matrix method to determine refractive index values. We further show that changes due to the thermal expansion of the DBR layers are negligible for our method.

Synthesis of SiO2-coated CdSe/ZnS quantum dots using various dispersants in the photoresist for color-conversion micro-LED displays

Quantum dots (QDs) are a promising technology for next-generation display screens owing to their well-defined and tunable emission wavelengths. However, QDs can chemically react with external environment, decreasing their photoluminescence efficiency and reliability. In addition, further work is needed on lithography techniques to make QDs compatible with semiconductor manufacturing processes. In this study, silicon dioxide (SiO2)-coated cadmium selenide/zinc sulfide QDs are synthesized using tetraethoxysilane. The ratio of cadmium oxide to zinc acetate was tuned before coating the SiO2 passivation layer to adjust the emission wavelength of the QDs between 520 and 625 nm. The SiO2 passivation layer not only enhanced the photoluminescence intensity (PLI) by 69% but also effectively decelerated the fluorescence decay rate (<1% PLI decay after 100 h). In addition, the hydrophilicity of the SiO2-coated QDs enabled them to integrate with appropriate dispersants and photoresist for creating a 50-μm × 50-μm pixel on the color-conversion layer of a blue microlight emitting diode (micro-LED) display. Finally, a structure of a distributed Bragg reflector (DBR) was designed using TFCalc software. Subsequently, a DBR layer was deposited on the red–green QDs’ color-conversion layer to fabricate a full-color micro-LED display.

Improved green thermal activated delayed fluorescence OLEDs based on thermally evaporated distributed Bragg reflector (DBR) of MgF2/ZnS

Unlike the traditional fabrication of distributed Bragg reflector (DBR) structure via atomic layer deposition or spin-coating, here the 1–6 pairs of magnesium fluoride (MgF2)/zinc sulfide (ZnS) alternative dielectric layers were grown via thermal evaporation. The absorption, transmission, reflection, and photoluminescence (PL) spectra were evaluated. 5 pair MgF2/ZnS denotes the largest reflectance (88.5% at 535 nm) together with a stopband at 450–650 nm among the 1– 6 pair dielectric layers, exhibiting the potential for using as DBR. Relative to the bare 4,4’-bis(carbazol-9-yl)biphenyl(CBP):(4s,6s)−2,4,5,6-tetra(9H-carbazol-9-yl) isophthalonitrile (4CzIPN) film, the PL intensity of CBP:4CzIPN/5 pair MgF2/ZnS DBR is enhanced and splitted into two peaks. The 5 pair alternative dielectric film presents more uniform aggregation over 4 pair MgF2/ZnS. The cross-sectional scanning electron microscopic image denotes explicit layering for the MgF2 and ZnS. The organic light-emitting diode (OLED) incorporating 5 pair MgF2/ZnS DBR layers illustrates significantly improved electroluminescent (EL) performance due to the photons concentrated in the direction perpendicular to the DBR. The slightly narrowed EL spectrum is originated from the microcavity effect between the two Al electrodes. Here we develop a universal method for the DBR fabrication suitable to most of OLEDs.

Targeted defect analysis in VCSEL oxide windows using 3D slice and view

We report on high resolution analysis of vertical cavity surface emitting lasers (VCSELs) to detect and assess defects in sub-surface layers. We employ a focussed ion beam scanning electron microscope (FIB-SEM) to sputter and image successive cross sections (slice and view technique) in order to produce a 3D reconstruction of the oxide aperture region. High resolution images and measurements of the multilayers and oxide apertures of VCSEL devices were obtained. The process took ∼2.5 h and produced over 270 slice SEM images for a device volume of approximately 13.2 × 16.0 × 13.8 μm3, with a voxel size of 50 nm. On-wafer, single mode VCSEL devices with high and low output powers were analysed to compare their oxide apertures and distributed Bragg reflector (DBR) layer structures. It was found that the low output power VCSEL had DBR layer defects and a 41.8% reduction of effective oxide aperture area, explaining the lower power obtained. The results provide evidence that oxide aperture area and structural defects are major factors that affect the optical output power of VCSEL devices. Outcomes in this work show FIB-SEM slice and view is a valuable method for 3D reconstruction of VCSEL devices, which enables top view, cross-sectional view and angled view of the whole device region as well as designated structures such as oxide aperture or structural defects in various layers. This work demonstrates a promising technique with high resolution (50 nm) 3D imaging for analysis of complex semiconductor devices.

Reflectivity simulation by using transfer matrix method

Efficiency of optoelectronic devices can be enhanced by using Distributed Bragg Reflector (DBR). Distributed Bragg Reflector (DBR) is a multipair layered structure of high and low refractive indices. High reflectivity of DBR is due to constructive interference of reflected light from interfacial layers of DBR. In the present paper, reflectivity of DBR composed of II-VI compound semiconductor is simulated using Transfer Matrix Method (TMM). In this paper, reflectivity of 4, 8, 12, 16 and 20 periods for the incidence angle of 45+ is simulated. Also, the effect of interchange of constituent layers in DBR stack, on reflectivity is simulated.

Colored MAPbI3 perovskite solar cells based on SnO2–SiO2 distributed Bragg reflectors

Organic–inorganic hybrid perovskite solar cells have attracted significant interest as a novel candidate for building-integrated photovoltaics (BIPV) applications. In particular, coloring methods for perovskite solar cells have been actively studied to meet the aesthetic needs for BIPV. To date, studies based on TiO2–SiO2-based distributed Bragg reflectors (DBRs) have been reported owing to their good color selectivity and low power conversion efficiency (PCE) loss. However, the application of TiO2 layers as DBRs for building exterior can result in serious problem due to their high photocatalytic activity. Therefore, in this study, to overcome this problem, the TiO2 layers in both DBR and electron selective layer in perovskite solar cells are replaced by SnO2 films. SnO2–SiO2 DBRs with various SiO2 thicknesses were synthesized by a simple spin-coating process. By combining DBRs and TiO2-free perovskite devices, blue-, purple-, and yellow-colored perovskite solar cells were successfully fabricated and showed PCEs of 9.52%, 5.99% and 6.85%, respectively.

The enhancement in cell performance of CdTe-based solar cell with Si/SiO2 distributed Bragg reflectors

In this study, the very effective design that is Distributed Bragg Reflectors (DBRs) technique and Back Surface Field (BSF) layer are used to fabricate the high-efficiency CdTe-based solar cells. The CdTe-based thin-film solar cells are fabricated as SiO2/Si(DBR)/ZnTe (BSF)/CdTe/CdS/ZnO/FTO. Then, the effects of different number of DBRs layers on CdTe-based solar cell are investigated. 68.77% reflection intensity can be obtained even in 1 period and reflection intensity can reach 99.28% in 3 periods. Significant improvement in cell output parameters are observed with increasing DBRs up to 3 periods. The highest JSC, FF and η values are obtained as 109.82 A/m2, 82.03% and 10.39%, respectively. With the rise in the DBR period, the long wavelength tail of the reflection spectrum is drawn into the center of the reflection band and the long wavelength reflectivity is reduced. This narrowing reduces the rate of reflection of low-energy photons that cannot be absorbed by CdTe layer.

A detailed comparison of measured and simulated optical properties of a short-period GaAs/AlxGa1−xAs distributed Bragg reflector

A 6-period GaAs/Al0.9Ga0.1As distributed Bragg reflector (DBR) has been grown and its optical properties have been both measured and simulated. Incremental improvements were made to the simulation, allowing it to account for internal consistency error, incorrect layer thicknesses, and absorption due to substrate doping to improve simulation accuracy. A compositional depth profile using secondary-ion mass spectrometry (SIMS) has been taken and shows that the Al fraction averages 88.0% ± 0.3%. It is found that the amplitude of the transmission is significantly affected by absorption in the n-doped GaAs substrate, even though the energy of the transmitted light is well below the GaAs band gap. The wavelength of the features in the transmission spectrum are mostly affected by DBR layer thicknesses. On the other hand, the transmission spectrum is found to be relatively tolerant to changes to Al fraction.

Electrically Injected GaN-Based Vertical-Cavity Surface-Emitting Lasers with TiO2 High-Index-Contrast Grating Reflectors

We demonstrate the first electrically injected GaN-based vertical-cavity surface-emitting lasers (VCSELs) with a TiO2 high-index-contrast grating (HCG) as the top mirror. Replacing the top distributed Bragg reflector (DBR) with an HCG offers substantial thickness reduction, polarization-pinning, and setting of the resonance wavelength by the grating parameters. Conventional HCGs are usually suspended in the low refractive index material, such as air, in order to create the largest refractive index contrast. However, the mechanical stability of such structures can be questioned and creating free-hanging GaN-membrane on top of GaN is problematic. We have therefore fabricated TiO2–HCGs resting directly on GaN without an air-gap. No DBR layers are used below the HCG to boost the reflectivity. A VCSEL with an aperture diameter of 10 μm shows a threshold current of 25 mA under pulsed operation at room temperature. The lasing modes locate around 400 nm and are transversely electrically -polarized with a line width of 0.5 nm. The full-width half-maximum beam divergence is 10°. This demonstration of a TiO2–HCG VCSEL offers a new route to achieve polarization pinning and could also allow additional benefits such as postgrowth setting of the resonance wavelength.