Conventional Distributed Bragg Reflector Research Articles

Improvement of Near-Infrared Light-Emitting Diodes' Optical Efficiency Using a Broadband Distributed Bragg Reflector with an AlAs Buffer.

This study developed an advanced 850 nm centered distributed Bragg reflector (DBR) (broadband DBR) composed of nanomaterial-based multiple structures to improve the optical efficiency of an 850 nm near-infrared light-emitting diode (NIR-LED). A combined 850 nm centered broadband DBR was fabricated by growing an 800 nm centered ten-pair DBR on a 900 nm centered ten-pair DBR (denoted as a combined DBR). The combined DBR exhibited a slightly wider peak band than conventional DBRs. Furthermore, the peak band width of the combined DBR significantly increased upon using a reflective AlAs buffer layer that reduced the overlapped reflection. The output power (20.5 mW) of NIR-LED chips using the combined DBR with an AlAs buffer layer exceeded that of a conventional 850 nm centered DBR (14.5 mW) by more than 40%. Results indicated that combining the optical conditions of wavelengths and the AlAs buffer layer effectively strengthened the broadband effect of the DBR and increased the optical efficiency of the 850 nm NIR-LED.

Flexible multi-layered coloring transparent electrode composed of AZO–based materials

Color implementation in flexible photovoltaics (PVs) is aesthetically important in modern vehicles and buildings. To implement color in flexible PVs using conventional distributed Bragg reflector (DBR) methods, it is necessary to reduce its brittleness and minimize the light loss due to refractive index differences (Δn). Here, color-implemented transparent front electrodes for flexible and transparent solar cells (FTSCs) have been proposed using an aluminum-doped zinc oxide (AZO)–based multilayer with periodic hydrogenation (AZO:H). The hydrogenation of AZO causes passivation in vacancies of oxygen and zinc and strengthening of the O–H bonds, resulting in improved of electrical and optical properties and a very low Δn of ∼5%. Using the AZO:H DBR–diffuser electrodes, we demonstrate three primary colors and colored FTSCs made of silicon thin film. The FTSCs show little change in power conversion efficiencies for the different colors owing to low return loss by the very low Δn. They are lightweight for their high specific power and exhibit excellent flexibility that enables them to endure the bending stress of 600 cycles at a 6 mm bending radius. The developed device can be used for vehicle and building-integrated PV applications, and the methodology can be used for other thin-film solar cells that require color and flexibility.

Rational Distributed Bragg Reflector Design for Improving Performance of Flip-Chip Micro-LEDs

The distributed Bragg reflector (DBR) has been widely used in flip-chip micro light-emitting diodes (micro-LEDs) because of its high reflectivity. However, the conventional double-stack DBR has a strong angular dependence and a narrow reflective bandwidth. Here, we propose a wide reflected angle Ti3O5/SiO2 DBR (WRA-DBR) for AlGaInP-based red and GaN-based green/blue flip-chip micro-LEDs (RGB flip-chip micro-LEDs) to overcome the drawbacks of the double-stack DBR. The WRA-DBR consisting of six sub-DBRs has high reflectivity within the visible light wavelength region at an incident angle of light ranging from 0° to 60°. Furthermore, the influence of the WRA-DBR and double-stack DBR on performances of RGB flip-chip micro-LEDs is numerically investigated based on the finite-difference time-domain method. Owing to higher reflectivity and less angular dependence of the WRA-DBR, the RGB flip-chip micro-LEDs with the WRA-DBR have a stronger electric field intensity in the top side in comparison with RGB flip-chip micro-LEDs with the double-stack DBR, which indicates that more photons can be extracted from micro-LEDs with the WRA-DBR.

Influence of top mirror on performance of GaN-based resonant cavity light-emitting diode

In this paper, two kinds of distributed Bragg reflectors (DBRs) with high-reflective-film structure and filter structure are designed and evaporated on the top of GaN-based resonant cavity light emitting diode (RCLED), respectively. Firstly, the reflectivity spectra of the two kinds of DBRs are simulated. Then, the differences in performance including optical longitudinal modes, spectral linewidth, and output light intensity between the two kinds of RCLED devices with different top mirrors, are compared and analyzed. Finally, the influence of the top mirror reflection characteristics on the output spectrum of the RCLED is studied in detail. The results show that the top mirror is an important part of RCLED, and its reflection characteristics determine the optical performance of the device. For the conventional DBR with high-reflective-film structure, its reflectivity spectrum has a wide high-reflection band. Accordingly, the spectral linewidth of the RCLED can be effectively narrowed by using the conventional DBR as the top mirror. However, the spectrum still consists of multi-longitudinal modes. For the DBR with filter structure, its reflectivity spectrum has a narrow high-transmittance band at the central wavelength. Depending on the modulation effect of the high-transmittance band to the output light, single longitudinal mode light emission is realized for the RCLED with the specially designed DBR as the top mirror, which shows a broad application prospect in optical communication and optical fiber sensing. Moreover, the spectral characteristics of the RCLED can be further optimized to meet its application requirements in much more fields, by designing the top mirror structure and changing its reflectivity spectrum characteristics.

An electrically pumped surface-emitting semiconductor green laser.

Surface-emitting semiconductor lasers have been widely used in data communications, sensing, and recently in Face ID and augmented reality glasses. Here, we report the first achievement of an all-epitaxial, distributed Bragg reflector (DBR)-free electrically injected surface-emitting green laser by exploiting the photonic band edge modes formed in dislocation-free gallium nitride nanocrystal arrays, instead of using conventional DBRs. The device operates at ~523 nm and exhibits a threshold current of ~400 A/cm2, which is over one order of magnitude lower compared to previously reported blue laser diodes. Our studies open a new paradigm for developing low-threshold surface-emitting laser diodes from the ultraviolet to the deep visible (~200 to 600 nm), wherein the device performance is no longer limited by the lack of high-quality DBRs, large lattice mismatch, and substrate availability.

Monolithic High Contrast Grating Nitride-Based VECSEL

This paper proposes a new design for a nitride-based vertical-external-cavity surface-emitting laser (VECSEL) emitting at 394 nm. The structure contains a monolithic high contrast grating (MHCG) instead of a conventional distributed Bragg reflector (DBR), makes the vertical design of the laser more compact. In a numerical analysis, we consider several designs for MHCG VECSELs and show that replacing the DBR with an MHCG improves laser performance by 50%, in terms of output energy and slope efficiency. Further optimization of the MHCG VECSEL design achieves an almost threefold improvement in both values.

2-μm band active distributed Bragg reflector laser for CO2 gas sensing

We have developed a 2-μm-band active distributed Bragg reflector (DBR) laser for CO2 gas sensing employing single quantum well (SQW) gain in the tuning sections. The laser covers the double wavelength band with a <3-dB power penalty during tuning compared with our conventional DBR laser having no active SQW. The absorption-line detection of CO2 gas using the laser is also demonstrated. Furthermore, we show that the ∼60% tuning efficiency for the tuning current of the active DBR laser compared with our conventional one is explained with an assumed rate equation including the carrier consumption for the active SQW gain.

Thermal transport of nanoporous gallium nitride for photonic applications

Recently, nanoporous (NP) GaN has emerged as a promising photonic material in the III-N family. Due to its attractive properties, such as its large refractive index contrast and perfect lattice matching with GaN, as well as its good electrical conductivity, photonic components and devices involving NP GaN have been successfully demonstrated. However, further development of high-performance NP GaN based electrically injected devices, such as vertical-cavity surface-emitting lasers (VCSELs) and edge emitting lasers, requires efficient heat dissipation. Therefore, in this paper, we study thermal conductivity (TC) of NP GaN, especially when incorporated into a practical distributed Bragg reflector (DBR) in a VCSEL device. Through an effective medium model, we study the theoretical effect of NP GaN morphological properties over its TC. We then experimentally measure the TC of NP GaN, with different porosities and pore wall thicknesses, which shows a high agreement with the theoretical model. We also fabricate actual NP GaN DBRs and study the large tunability and interdependence among their TC (1–24 W/m K), refractive index (0.1–1.0), and electrical conductivity (100–2000 S/m) compared to other conventional DBRs. Finally, we perform a finite-element simulation of the heat dissipation within NP GaN-VCSELs, revealing their superior thermal dissipation compared to dielectric DBR based VCSELs. In this regard, this study lays the foundation for nanoscale thermal engineering of NP GaN optoelectronic and photonic devices and paves the way for their successful commercialization.

High-Performance Green Flip-Chip LEDs with Double-Layer Electrode and Hybrid Reflector

High-performance green flip-chip light-emitting diode (FC-LED) was realized by combining double-layer electrode and highly reflective hybrid reflector. The introduction of double-layer electrode improved uniformity of current spreading over the entire active region. The highly reflective hybrid reflector, composed of SiO2 total internal reflection (TIR) layer and Nb2O5/SiO2 distributed Bragg reflector (DBR), offer higher reflectance and wider bandwidth compared to conventional DBR. Numerical simulation and experimental results indicated that the green FC-LED has advantages of uniform current spreading and high light extraction efficiency (LEE) compared to green top-emitting LED (TE-LED). Due to uniform current spreading, the forward voltage of the green FC-LED is 0.28 V lower than that of the TE-LED at 20 mA. Compared to TE-LED, the light emission intensity of the green FC-LED at 20 mA is enhanced by 18% due to the higher reflectance of hybrid reflector and increased utilization ratio of active region area caused by double-layer electrode structure.

What is the best planar cavity for maximizing coherent exciton-photon coupling

We compare alternative planar cavity structures for strong exciton-photon coupling with the conventional distributed Bragg reflector (DBR) and three unconventional types of cavity mirrors—air/GaAs DBR, Tamm-plasmon mirror, and sub-wavelength grating mirror. We design and optimize the planar cavities built with each type of mirror at one side or both sides for the maximum vacuum field strength. We discuss the trade-off between performance and fabrication difficulty for each cavity structure. We show that cavities with sub-wavelength grating mirrors allow simultaneously the strongest field and high cavity quality. The optimization principles and techniques developed in this work will guide the cavity design for research and applications of matter-light coupled semiconductors, especially new material systems that require greater flexibility in the choice of cavity materials and cavity fabrication procedures.

FDTD and transfer matrix methods for evaluating the performance of photonic crystal based microcavities for exciton-polaritons

We investigate alternative microcavity structures for exciton-polaritons consisting of photonic crystals instead of distributed Bragg reflectors. Finite-difference time-domain simulations and scattering transfer matrix methods are used to evaluate the cavity performance. The results are compared with conventional distributed Bragg reflectors. We find that in terms of the photon lifetime, the photonic crystal based microcavities are competitive, with typical lifetimes in the region of ∼20 ps being achieved. The photonic crystal microcavities have the advantage that they are compact and are frequency adjustable, showing that they are viable to investigate exciton-polariton condensation physics.

Diode lasers with front surface high-order distributed Bragg reflector

Control of spectral and spatial parameters of semiconductor lasers is a key element of their development and application. Surface integrated high-order distributed Bragg reflector (DBR) is the way to narrow diode laser spectrum preserving the conventional postgrowth process of laser diodes. On the other hand, high-order DBR has specific optical losses, which reduce optical output power. In this work we analyzed these specific optical losses and present design of laser resonator containing short front DBR, that allow to significantly reduce losses. With this approach, slope efficiency was greatly increased in contrast with conventional rear DBR laser diodes and 4 W optical output power with spectral width of less than 3 A was attained.

High quality-factor Si/SiO(2)-InP hybrid micropillar cavities with submicrometer diameter for 1.55-μm telecommunication band.

We theoretically demonstrate high quality(Q)-factor micropillar cavities at 1.55-μm wavelength based on Si/SiO(2)-InP hybrid structure. An adiabatic design in distributed Bragg reflectors (DBRs) improves Q-factor for upto 3 orders of magnitude, while reducing the diameter to sub-micrometer. A moderate Q-factor of ~3000 and a Purcell factor of ~200 are realized by only 2 taper segments and fewer conventional DBR pairs, enabling single photon generation at GHz rate. As the taper segment number is increased, Q-factor can be boosted to ~10(5)-10(6), enabling coherent exchange between the emitter and the optical mode at 1.55 μm, which is applicable in quantum information networks.

Optical Properties of GaAs/AlO&lt;formula formulatype="inline"&gt;&lt;tex Notation="TeX"&gt;$_{x}$&lt;/tex&gt;&lt;/formula&gt; and Si/SiO &lt;formula formulatype="inline"&gt;&lt;tex Notation="TeX"&gt;$_{x}$&lt;/tex&gt;&lt;/formula&gt; High Contrast Gratings Designed for 980-nm VCSELs

A 3-D, fully vectorial optical numerical model is used to design a high contrast grating for use as the top mirror of a 980-nm arsenide-based vertical-cavity surface-emitting laser. We study the parameters of two high contrast gratings made from GaAs/AlO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> and Si/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> , respectively. Parameters providing high degrees of reflectivity, high levels of polarization discrimination, and fine tuning of the reflectivity of the high contrast grating are determined. The results demonstrate that both configurations if carefully designed by advanced numerical models can offer better performance in comparison to conventional distributed Bragg reflectors.

High-Power Single-Frequency Thulium-Doped Fiber DBR Laser at 1943 nm

We report on a high-power single-frequency mode-hop-free fiber distributed Bragg reflector (DBR) laser fabricated from Tm-doped photosensitive alumino-silicate fiber and in-band pumped by an Er/Yb fiber laser at 1565 nm. The fiber DBR laser yielded up to 580 mW of single-frequency single-ended output at 1943 nm for ~ 5 W of absorbed pump power. Further scaling of the DBR laser output power was achieved with the aid of a simple Tm-doped fiber amplifier stage spliced directly to the DBR fiber without the need of an optical isolator. The maximum output power from the DBR laser and fiber amplifier was 2.9 W for a combined absorbed pump power of 9 W. The merits of this DBR laser architecture compared to conventional DBR and distributed-feedback laser configurations are discussed.

AlGaN membrane grating reflector

We report on AlGaN grating reflectors for short-wavelength applications, an alternative to conventional distributed Bragg reflectors that are difficult to make in a GaN-based system. An array of air-bridge AlGaN grating reflectors, with a period and filling factor of 430 nm and 0.55, was fabricated through holographic lithography and photoelectrochemical etching. Polarization-dependent microreflectance spectra were measured to confirm their consistency with simulation results. The reflectance measured at 532 nm for transverse-electric polarization was as high as ∼90%. We also found that the spectral range for high reflection correlates with the photonic Bloch band-edge modes.

Semiconductor Double-Ring-Resonator-Coupled Tunable Laser for Wavelength Routing

A monolithic widely tunable semiconductor laser based on a double-ring resonator is developed for use in a wavelength-routing switch. By using the double-ring resonator as a wavelength-selective filter, operation over a wide wavelength tuning range is achieved with a low tuning current. This low-tuning-current operation makes the laser very promising as a high-speed tunable light source for a wavelength-routing-based switch by effectively suppressing the thermal wavelength drift induced by current injection. In addition, the laser fabrication process is simpler compared to conventional distributed Bragg reflector tunable lasers. A tuning range of 50.0 nm, covering the entire C-band, is successfully demonstrated with an injection current of less than 5.2 mA. The wavelength drift caused by the thermal transients is less than 5 GHz.

High-Index-Contrast Grating (HCG) and Its Applications in Optoelectronic Devices

We review recent advances in subwavelength high-index-contrast gratings (HCGs) and a variety of applications in optoelectronic devices, including vertical-cavity surface-emitting lasers (VCSELs), tunable VCSELs, high- <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> optical resonators, and low-loss hollow-core waveguides (HWs). HCGs can serve as broadband (Delta lambda/lambda ~ 35%), high-reflectivity (>99%) mirrors for surface-normal incident light, which is useful to replace conventional distributed Bragg reflectors in optical devices. HCGs can also be designed as high- <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> resonators with output coupling in the surface-normal direction. Finally, we discuss a novel design of HCG as shallow angle reflectors and HWs.

Angle-tuned, evanescently-decoupled reflector for high-efficiency red light-emitting diode

We propose and demonstrate evanescently-decoupled, solid-angle-optimized distributed Bragg reflectors (DBRs) for AlGaInP light-emitting diodes (LEDs). The thickness of each DBR layer is tuned to the wavelength slightly longer than the emission peak of the active medium in order to maximize the radiated power integrated over the top surface. In addition, to increase the horizontal radiation through the side facets, the glancing-angle reflectivity at the AlInP/AlAs interface is improved by employing an AlAs layer thicker than the attenuation length of the evanescent field. With the improved DBR, the integrated output power of AlGaInP LEDs is enhanced by a factor of 1.9 in comparison to those of LEDs with conventional DBRs. Additional 1.25-fold enhancement is observed by incorporating an square-lattice hole array (a=1200nm) into the top GaP surface by a conventional photolithography.

Enhancement of Pumping Efficiency in a Vertical-External-Cavity Surface-Emitting Laser

We report an improved optical pumping efficiency in a vertical-external-cavity surface-emitting laser (VECSEL) by employing the double-band mirror (DBM), which acts as a pump beam reflector as well as a cavity mirror. The optical pumping efficiency of a VECSEL with the DBM was increased by 23%, as compared to one with the conventional distributed Bragg reflector. It was found that the use of the DBM was more effective in the VECSEL with the shorter absorption layer thickness.