DBR Structure Research Articles

Tuning strong coupling towards the deep ultraviolet region realized through Tamm-plasmon exciton-polaritons

Light-matter interactions at the nanoscale have attracted considerable attention due to its features of high optical-field confinement and large electromagnetic-field enhancement. Herein, we theoretically demonstrate strong coupling between Tamm plasmon (TP) and exciton polaritons towards the deep ultraviolet (DUV) region in metal Al/distributed Bragg reflector (DBR)-molecular structures. By adjusting the thickness of the spacer layer and the angle of incident light, the obvious anti-crossing phenomena can be observed with a Rabi splitting of 38.4 meV. Moreover, through varying the period number of the DBR structure, the coupling strength can be effectively manipulated from weak to strong. These findings may extend the operating wavelength of strong coupling to the DUV region, and benefit the development of new-style optical filters.

Ultraviolet-C AlGaN Resonant-Cavity Light-Emitting Diodes with Thermal Stability Pipe-AlGaN-Distributed Bragg Reflectors.

Ultraviolet-C AlGaN resonant-cavity light-emitting diodes with top and bottom pipe-AlGaN-distributed Bragg reflectors (DBRs) have been demonstrated. For the top/bottom DBR structures, 20 pairs of n+-AlGaN:Si/n-AlGaN:Si stack structures were transformed into the pipe-AlGaN:Si/n-AlGaN:Si DBRs through a doping-selective electrochemical wet etching process. The reflectivity of the pipe-AlGaN DBR structure was measured as 90% at 276.7 nm with a 20.9 nm flat stopband width. The anisotropic optical properties of the pipe-AlGaN DBR structure had been analyzed through the polarization-dependent reflectance spectra. For temperature-dependent reflectance spectra, the central wavelengths were slightly redshifted from 275 nm (100 K) to 281 nm (600 K) due to thermal expansion, refractive index increase, and partial strain release phenomena in the pipe-DBR structure. High photoluminescence emission intensity and line-width reducing phenomena were observed at 10 K in the UVC-LED with the resonant-cavity structure, which has potential for high-efficiency UV-C light source applications.

Various Applications-Based Optimized DBR Structure With Effect of Different Material Systems

Various Applications-Based Optimized DBR Structure With Effect of Different Material Systems

Optimal design of thermal emitter based on DBR cavity model in thermal photovoltaic technology

To ensure the necessary technical support for the long-term operation of the space vehicle, this paper studies the optimization technology of thermal emitter design in thermophotovoltaic technology. Because of the unsatisfactory experimental results of the traditional DBR cavity physical model, this paper uses the simplified DBR structure model to design multiple multi-layer composite structures named “simplified DBR cavity” and the absorption rate A value exceeds 99.5%, achieving a higher thermoelectric conversion efficiency.

Enhancement of near-infrared response for GaAs-based photocathode with laminated graded-bandgap structure: theory and experiment

To enhance the quantum efficiency of GaAs-based photocathode in the near-infrared range, a laminated graded-bandgap photocathode structure consisting of a distributed Bragg reflection layer and a graded-bandgap emission layer is proposed. The theoretical optical properties and quantum efficiency of this proposed photocathode are simulated based on the finite-different time-domain method and the one-dimensional continuity equations, respectively. The absorptivity at 1064 nm can be significantly increased for the new-type photocathode because of the secondary absorption caused by the distributed Bragg reflection layer. With the improvement of absorptivity at 1064 nm, the quantum efficiency at specific wavelength is enhanced significantly. The contribution of the DBR structure and built-in electric field to the quantum efficiency is theoretical investigated respectively. According to theoretical structure design, the cathode samples grown by epitaxial technique are prepared to verify the theoretical prediction of quantum efficiency enhancement. After the treatment of surface cleaning and Cs/NF3 activation, the experimental results show that the minimum reflectivity at 1064 nm is realized, and the quantum efficiency of the sample with proposed structure is ten times than that of the sample without distributed Bragg reflection layer at 1064 nm. This research provides a guidance for the design of novel photocathode structures with enhanced response at special wavelength.

Investigation of Spectral Properties of DBR-Based Photonic Crystal Structure for Optical Filter Application

In this work, the spectral properties of distributed Bragg reflector-based photonic crystal (DBR-PhC) structures were studied for the near-infrared (NIR) range. Different structural properties were varied to study their effect on the quality of the stopband and the appearance of the resonant dips in the reflection spectra of the DBR-PhC structure. The investigated structural features included the depth of PhC holes, hole radius, and number of PhC elements in the DBR structure. The 11-layered DBR structure was designed with a 2.4/1.4 refractive index contrast of alternating layers. The study aimed to achieve optical filtering properties in the DBR-PhC structure, to simplify the structural complexity of Fabry-Pérot filters by eliminating the FP cavity and upper-DBR mirror. The proposed DBR-PhC device can be used in different optical filtering and sensing applications.

Exploring superlattice DBR effect on a micro-LED as an electron blocking layer.

The role of a superlattice distributed Bragg reflector (SL DBR) as the p-type electron blocking layer (EBL) in a GaN micro-light-emitting diode (micro-LED) is numerically investigated to improve wall-plug efficiency (WPE). The DBR consists of AlGaN/GaN superlattice (high refractive index layer) and GaN (low refractive index layer). It is observed that the reflectivity of the p-region and light extraction efficiency (LEE) increase with the number of DBR pairs. The AlGaN/GaN superlattice EBL is well known to reduce the polarization effect and to promote hole injection. Thus, the superlattice DBR structure shows a balanced carrier injection and results in a higher internal quantum efficiency (IQE). In addition, due to the high refractive-index layer replaced by the superlattice, the conductive DBR results in a lower operation voltage. As a result, WPE is improved by 22.9% compared to the identical device with the incorporation of a conventional p-type EBL.

Tunable Narrowband Silicon-Based Thermal Emitter with Excellent High-Temperature Stability Fabricated by Lithography-Free Methods.

Thermal emitters with properties of wavelength-selective and narrowband have been highly sought after for a variety of potential applications due to their high energy efficiency in the mid-infrared spectral range. In this study, we theoretically and experimentally demonstrate the tunable narrowband thermal emitter based on fully planar Si-W-SiN/SiNO multilayer, which is realized by the excitation of Tamm plasmon polaritons between a W layer and a SiN/SiNO-distributed Bragg reflector. In conjunction with electromagnetic simulations by the FDTD method, the optimum structure design of the emitter is implemented by 2.5 periods of DBR structure, and the corresponding emitter exhibits the nearly perfect narrowband absorption performance at the resonance wavelength and suppressed absorption performance in long wave range. Additionally, the narrowband absorption peak is insensitive to polarization mode and has a considerable angular tolerance of incident light. Furthermore, the actual high-quality Si-W-SiN/SiNO emitters are fabricated through lithography-free methods including magnetron sputtering and PECVD technology. The experimental absorption spectra of optimized emitters are found to be in good agreement with the simulated absorption spectra, showing the tunable narrowband absorption with all peak values of over 95%. Remarkably, the fabricated Si-W-SiN/SiNO emitter presents excellent high-temperature stability for several heating/cooling cycles confirmed up to 1200 K in Ar ambient. This easy-to-fabricate and tunable narrowband refractory emitter paves the way for practical designs in various photonic and thermal applications, such as thermophotovoltaic and IR radiative heaters.

Improvement of Thermal Dissipation of GaN-Based Micro Cavity Light-Emitting Devices

Double dielectric DBR based GaN-based micro cavity light emitting devices with two different structures were fabricated, and their thermal characteristics were investigated. To improve thermal dissipation, an AlN current confinement layer with a much higher thermal conductivity than SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and an electroplated copper heat sink were utilized. The thermal resistance of the device decreased from 923 K/W to 457 K/W, half of that obtained with the typically used SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> current confinement layer and bonded substrate. This is the lowest reported value in GaN-based micro cavity light-emitting devices with double dielectric DBRs. Temperature distribution and heat flux inside of the device was simulated based on a steady state quasi three-dimensional cylindrical model. The results show that heat transport in vertical direction is efficiently bypassing the bottom DBR to the copper plate. This work provides an effective method to improve thermal characteristics of GaN-based micro cavity light-emitting devices with double dielectric DBR structure.

Enhancement of broad-band light absorption in monolayer MoS2 using Ag grating hybrid with distributed Bragg reflector

A hybrid novel structure of monolayer MoS2 with Ag nanograting and DBR on Si substrate has been proposed to obtain broad-band absorption response for two-dimensional (2D) materials. It is effective to reduce light loss and reflect the incident light efficiently for monolayer MoS2 absorption with DBR dielectric layers. Moreover, by combining Ag nanograting with DBR structure, the average absorption achieves as high as 59% within broad wavelength ranging from 420 to 700 nm, which is attributed to the plasmonic resonant effect of metal nanostripes. The absorption would be affected by the duty ratio and period of the Ag nanograting, and shows incident angle dependent characteristics, while an average absorption higher than 60% has been obtained at the incident angle around 40°. These results indicate that 2D MoS2 in combination with DBR and metal nanograting have a promising potential applications for optical nano-devices.

Narrow-linewidth Q-switched DBR laser on Ytterbium-doped glass

In this paper, an integrated passively Q-switched DBR laser made in glass integrated optics technology is designed, fabricated and investigated. The laser, operating at 1030nm wavelength, is formed by an ion-exchanged single mode waveguide realized in an Ytterbium doped phosphate glass. The feedback is provided by a DBR structure made of a Fiber Bragg Grating and a dielectric mirror. The Q-switched behavior is obtained by hybridizing the saturable absorber on the waveguides, so that it interacts with the evanescent part of the guided field. The saturable absorber we use is a bis(4-dimethylaminodithiobenzil)nickel (BDN) dye incorporated in a polymeric film. Using this structure, we were able to demonstrate a Q-switched operation for several BDN concentrations, allowing to vary the pulse duration and the repetition rate of the device. Moreover, we report a very narrow 372MHz wide emission spectrum while in Q-switched regime.

Improvement of light quality by DBR structure in white LED.

This study demonstrates the application of DBR structure into the remote phosphor structure to improve the angular correlated color temperature (CCT) deviation in white light-emitting diodes (WLEDs). In the experiment, the LED device with DBR structure yielded a higher luminous efficiency than a conventional structure. The CCT deviation can be improved from 1758K to 280K in a range of -70 to 70 degree and the luminous flux increases more than 10% due to the enhancement of the light extraction of the blue light. Moreover, the reflectance of the different DBR structures is analyzed with different angles to reveal the reasons of such improvements. As the result, this LED device with DBR structure shows the great potential to use as the next generation lighting source.

Improvement of light quality by DBR structure in white LED

Improvement of light quality by DBR structure in white LED

Air-Hybrid Dbr Structure for the Improvement of Light Output Power in Algainp-Based LED

In this paper, we report the air gap induced hybrid DBR-based LED (AH-DBR LED) structures for the efficient light extraction efficiency. For verifying the influence of the AH-DBR structures in 630 nm emitting LED devices, we compare two LED structures with conventional and air gap induced hybrid DBRs.The conceptual light paths reflected from conventional DBR and AH-DBR structures are depicted in Fig. 1. The conventional DBRs mainly reflect the light within the yellow region propagating vertically. However, the reflectivity becomes negligible when the incident angle is larger than the critical angle (θc) and the only small portion of light can be extracted out of chip through side facet. For supplementing this drawback of the conventional DBR structures, air gap is inserted in the side of the DBR structures. In the AH-DBR structures, the incident light larger than the θc can be effectively reflected by the air gap to enhance the light extraction through the side facet of the chip.To verify the performance of our proposed LED structures, AlGaInP-based LEDs with the conventional DBRs and AH-DBR structures are prepared. Fig. 2 shows the cross-sectional schematics and actual scanning electron microscope (SEM) images of the conventional DBR LED structures and the AH-DBR LED structures. The AlxGa1-xAs DBRs are well etched away to 38 mm in depth and formed relatively well-defined AH-DBR structures as shown in Fig. 2(b). Also, phosphide LED structures are not etched seriously and well maintained.

Use of distributed Bragg reflectors to enhance Fabry–Pérot lasing in vertically aligned ZnO nanowires

An optically pumped ZnO nanowire laser with a 10-period SiO2/SiN x distributed Bragg reflector (DBR) was demonstrated. Stimulated emissions with equally distributed Fabry–Perot lasing modes were observed at pumping powers larger than 121 kW/cm2. This result, when compared to nanowires of the same length and without a DBR structure, shows that a lower threshold of pumping power, higher quality factor, and larger cavity finesse can be achieved due to the high reflectivity of the DBR in the designed wavelength range. A coexistence of stimulated and spontaneous emissions was also observed above threshold and was attributed to partially confined waveguide modes in nanowires with diameters smaller than 100 nm.

Optical magnetic field enhancement through coupling magnetic plasmons to Tamm plasmons

We report on a theoretical investigation of the coupling between magnetic plasmons (MPs) and Tamm plasmons (TPs) in a metal-dielectric Bragg reflector (DBR) containing a gold nanowire pair array embedded in the low refractive index layer closest to the metal film. Strong coupling between MPs and TPs is observed, manifested by large anticrossings in the dispersion diagram. It creates a narrow-band hybridized MP mode with a Rabi-type splitting as large as 290 meV. Upon the excitation of this hybridized MP mode, a 2.5-fold enhancement of the magnetic field in the center of nanowire pairs is achieved as compared with the pure MP of the nanowire pairs embedded in a bare DBR structure (without the metal film). This result holds a promising potential application in magnetic nonlinearity and sensors.

High performance of InGaN light-emitting diodes by air-gap/GaN distributed Bragg reflectors

The effect of air-gap/GaN DBR structure, fabricated by selective lateral wet-etching, on InGaN light-emitting diodes (LEDs) is investigated. The air-gap/GaN DBR structures in LED acts as a light reflector, and thereby improve the light output power due to the redirection of light into escape cones on both front and back sides of the LED. At an injection current of 20 mA, the enhancement in the radiometric power as high as 1.91 times as compared to a conventional LED having no DBR structure and a far-field angle as low as 128.2° are realized with air-gap/GaN DBR structures.

Enhanced Emission from InxGa1-xN-based LED Structures Using III-Nitride based Distributed Bragg Reflector

ABSTRACTInxGa1-xN-based LED structures were grown on digital AlxGa1-xN/GaN DBR substrate to enhance emission extraction. Same LED structure was grown on sapphire substrate as a comparison. LEDs grown on DBR substrate exhibited similar IV characteristics to that grown on sapphire substrate but emission-angle-dependent EL spectra were observed. Also, the resonant vertical cavity modes were observed in EL spectra of LEDs with DBR structure and compared to simulated results. Image processing analysis results show that light extraction of LEDs is enhanced with use of DBR substrate.

Diffraction loss in long-wavelength buried tunnel junction VCSELs analyzed with a hybrid coupled-cavity transfer-matrix model

Intra-cavity diffraction in VCSELs is a loss mechanism that potentially can cause a significant decrease in efficiency and a rise in the threshold current, particularly in cavities with small lateral features with a high index contrast. One such VCSEL type is the 2.3 microm GaSb-based buried tunnel junction (BTJ) VCSEL studied in this work, where the BTJ induced topology of the top layers gives rise to excess loss through diffraction. Diffraction loss is difficult to measure, and also the numerical estimation must be done with care because of the non-axial propagation of the diffracted fields. We present a simulation method with spatially varying dimensionality, such that the field is three-dimensional (3D) in the entire cavity, whereas the material structure of the cavity is modelled in 3D near the BTJ and the layers with a varying topology, but elsewhere is assumed to be 1D like in a regular DBR structure. We find that the diffraction loss displays a non-monotonic behaviour as a function of the BTJ diameter, but as expected it rapidly increases below a certain diameter of the BTJ and may even become the dominant cause of loss in some device designs. We also show that the diffraction loss can be much reduced if the layers above the BTJ can be deposited such that the surface profile becomes smoother with increasing distance from the BTJ.

MOCVD growth and characteristics of high quality AlGaN used in the DBR structure of ultraviolet detector

High quality AlGaN materials used in the DBR structure of ultraviolet detector are grown under different growth conditions. The structure, composition and photographic characteristics are determined by XRD, SEM and AFM. The influence of the growth conditions on the characteristics of the AlGaN materials are discussed.The good performance of the DBR structure of ultraviolet detector is obtained.