Top Distributed Bragg Reflector 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.

Room-Temperature Continuous-Wave Microcavity Lasers from Solution-Processed Smooth Quasi-2D Perovskite Films with Low Thresholds

Continuous-wave (CW) lasing in quasi-two-dimensional (2D) perovskite-based distributed feedback cavities has been achieved at room temperature; however, CW microcavity lasers comprising distributed Bragg reflectors (DBRs) have rarely been prepared using solution-processed quasi-2D perovskite films because the roughness of perovskite films significantly increases intersurface scattering loss in the microcavity. Herein, high-quality spin-coated quasi-2D perovskite gain films were prepared using an antisolvent to reduce roughness. The highly reflective top DBR mirrors were deposited via room-temperature e-beam evaporation to protect the perovskite gain layer. Lasing emission of the prepared quasi-2D perovskite microcavity lasers under CW optical pumping was clearly observed at room temperature, featuring a low threshold of ∼1.4 W cm-2 and beam divergence of ∼3.5°. It was concluded that these lasers originated from weakly coupled excitons. These results elucidate the importance of controlling the roughness of quasi-2D films to achieve CW lasing, thus facilitating the design of electrically pumped perovskite microcavity lasers.

(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

(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 due to performance needs driven by facial recognition and 3D imaging in mobile telephones. Two distinct mode control methods that rely on exploiting the spatial field distribution of optical transverse modes to achieve high-power single-mode operation in VCSELs will be discussed. The first method uses a surface-deposited optical coating of multilayer SiO2/TiO2 or single-layer silicon to achieve single-mode emission. The capability to pattern these layers in a wafer-scale process makes this method attractive for high-volume manufacturing. The second method utilizes impurity-induced layer disordering (IID) to selectively intermix the top distributed Bragg reflector (DBR) in a VCSEL, thereby creating a mirror whose reflectivity spatially varies across the aperture. Using these techniques, single-mode output power in excess of 10 mW has been demonstrated with side-mode suppression ratios in excess of 35 db.

Design, Modeling, and Characterization of Hot Electron Light Emission and Lasing in Semiconductor Heterostructure-VCSOA with Optical Gain up to 36 dB

Vertical-cavity semiconductor optical amplifiers (VCSOAs) are interesting devices for optical communication applications. In this work, we have studied the characteristics of gain spectra and amplifier bandwidth in reflection mode at 1300 nm GaInNAs/GaAs hot electron light emission and lasing in semiconductor heterostructure-VCSOA structure using MATLAB program. The device contains 16 Ga0.7In0.3N0.038As0.962 multiple quantum wells (QWs) in its intrinsic region; the active region is bounded between eight pairs of GaAs/AlAs top distributed Bragg reflectors (DBRs) mirror and 25 pairs of AlAs/GaAs bottom DBRs mirror. Simulation results suggest that the resonance cavity of QW of HILLISH-VCSOA is varied with temperature and number of DBRs periods. We compare the relation between the wavelength and gain at a different single-pass gain in both reflection and transmission modes. The highest gain at around 36 dB is observed in reflection mode. Moreover, we calculated the amplifier bandwidth at different periods of top mirror’s reflectivity, in which when the peak reflection gains increases, the amplifier bandwidth decreases.

Low Divergence Coherently Coupled Vertical Cavity Surface Emitting Laser Ring Arrays

We report the theory and experimental verification of a method to reduce beam divergence from coherently-coupled ring arrays of photonic crystal vertical cavity surface emitting lasers (VCSELs). The photonic crystal etched into the top distributed Bragg reflector defines the optically coupled array elements, each of which has independent current injection and thus independent gain. Using a 2-dimensional complex waveguide model we simulate the supermodes of a 6-element ring array of VCSELs. The influence of the photonic crystal, free carrier loss, laser gain, and carrier-induced index suppression arising from the spatially defined gain regions are included in our analysis. Our simulations show the latter enables preferred anti-guided supermodes with reduced beam divergence. Optical characterization of the spectral and beam characteristics of 6-ring photonic crystal implanted VCSEL arrays shows that when the arrays are current-tuned to quasi-single mode operation, we find the beam divergence angle approaches 2° consistent with our simulations. This method of reduced beam divergence may have advantages for high-brightness applications.

Large-area narrowband Fabry–Pérot interferometers for long-wavelength infrared spectral sensing

This paper presents a proof-of-concept for microelectromechanical system (MEMS)-based fixed cavity Fabry–Pérot interferometers (FPIs) operating in the long-wavelength infrared (LWIR, 8 to 12 μm) region. This work reports for the first time on the use of low-index BaF2 thin films in combination with Ge high-index thin films for such applications. Extremely flat and stress-free ∼3-μm-thick free-standing distributed Bragg reflectors (DBRs) are also presented in this article, which were realized using thick lift-off of a trilayer structure fabricated using Ge and BaF2 optical layers. A peak-to-peak flatness was achieved for free-standing surface micromachined structures within the range of 10 to 20 nm across large spatial dimensions of several hundred micrometers. Finally, the optical characteristics of narrowband LWIR fixed cavity FPIs are also presented with a view toward the future realization of tunable wavelength MEMS-based spectrometers for spectral sensing. The measured optical characteristics of released FPIs agree with the modeled optical response after taking into consideration the fabrication-induced imperfections in the free-standing top DBR such as an average tilt of 15 nm and surface roughness of 25 nm. The fabricated FPIs are shown to have a linewidth of ∼110 nm and a suitable peak transmittance value of ∼50 % , which meets the requirements for their utilization in tunable MEMS-based LWIR spectroscopic sensing and imaging applications requiring spectral discrimination with narrow linewidth.

A Comparison of Top Distributed Bragg Reflector for 1300 nm Vertical Cavity Semiconductor Optical Amplifiers Based on III–V Compound

In this work, the design of GaAs/AlGaAs distributed Bragg reflector (DBR) has been implemented for 1300 nm vertical cavity semiconductor optical amplifiers (VCSOAs) for optical fiber communication applications. The top DBR period and Al concentration are varied, the peak reflectivity of the DBR is increasing from 50% to 97.5% for 13 periods with increasing Al concentration, whereas the reflectivity bandwidth is increased to almost 190 nm. The relation between wavelength and incidence angle variation on DBR reflectivity is increasing with the incident angle (0°, 20°, 30°, and 50°), the resonant wavelength and bandwidth of the measured reflectance spectra shifts to shorter wavelength and wider bandwidth, respectively. In addition, a comparison between the linear, the graded, and the parabolic DBRs has been achieved with transfer matrix method using MATLAB software to show the influence of layer in DBRs and its effect on lasing wavelength. It is shown that using grading DBR mirror is much more beneficial compared to abrupt DBR, whereas it has lower reflectivity of almost 10% due to VCSOAs device which needs less number of top layers until prevent reaching lasing threshold.

Circularly polarized light emission from a GaN micro-LED integrated with functional metasurfaces for 3D display.

This Letter proposes a circularly polarized (CP) light GaN micro-LED which is integrated with functional metasurfaces. The one-dimensional metallic nanograting can achieve a high transverse electric (TE) reflectivity (${{\rm{R}}_{\rm{TE}}}$) and extinction ratio (ER) of TE and transverse magnetic (TM) waves, which is highly polarized output for micro-LEDs. Besides, the nanograting, which is integrated on the bottom of the GaN layer, can also support a resonant cavity, together with the top distributed Bragg reflector, which can shape the radiation pattern. By optimizing the structure parameters of nanograting, the ${{\rm{R}}_{\rm{TE}}}$ achieves over 80%, and the ER reaches higher than 38 dB at 450 nm for the GaN micro-LED. Additionally, the metasurface, which acts as a quarter-wave plate, was investigated to control the phase delay between the polarization state of the electric wave in two orthogonal components. Finally, the circular shape of the transmitted pattern denotes the high performance of the metasurface which is integrated in the micro-LED for CP light emission. The work reported in this Letter might provide potential application in a 3D polarized light display.

Design optimization for bright electrically-driven quantum dot single-photon sources emitting in telecom O-band.

A combination of advanced light engineering concepts enables a substantial improvement in photon extraction efficiency of micro-cavity-based single-photon sources in the telecom O-band at ∼1.3 µm. We employ a broadband bottom distributed Bragg reflector (DBR) and a top DBR formed in a dielectric micropillar with an additional circular Bragg grating in the lateral plane. This device design includes a doped layer in pin-configuration to allow for electric carrier injection. It provides broadband (∼8-10 nm) emission enhancement with an overall photon-extraction efficiency of ∼83% into the upper hemisphere and photon-extraction efficiency of ∼79% within numerical aperture NA=0.7. The efficiency of photon coupling to a single-mode fiber reaches 11% for SMF28 fiber (with NA=0.12), exceeds 22% for 980HP fiber (with NA=0.2) and reaches ∼40% for HNA fiber (with NA=0.42) as demonstrated by 3D finite-difference time-domain modeling.

Design and Modeling of High-Performance DBR-Based Resonant-Cavity-Enhanced GeSn Photodetector for Fiber-Optic Telecommunication Networks

In this work, we present a novel distributed Bragg reflector (DBR)-based resonant-cavity-enhanced (RCE) GeSn photodetector on Si substrates to achieve high-performance photodetection in terms of responsivity and 3-dB bandwidth (BW) for the short-wave infrared (SWIR) high-speed applications. The proposed structure consists of Si/SiO2 distributed Bragg reflectors (DBRs) to enhance the performance of the device. The top and bottom DBRs create a high-quality ( ${Q}$ ) optical cavity to enable multiple pass reflection schemes to increase the responsivity and also high wavelength selectivity with a sharp response. In addition, with an increase in Sn concentration in the active ( i- GeSn) layer, the photodetection range extends to longer wavelengths due to the shrinkage of bandgap energy. The calculated result shows an enhanced 3-dB BW and responsivity as compared to the existing p-i-n PDs. Therefore, the proposed DBR-based RCE GeSn PD can be a promising device for high-speed SWIR photodetection applications.

InGaN Resonant-Cavity Light-Emitting Diodes with Porous and Dielectric Reflectors

InGaN based resonant-cavity light-emitting diode (RC-LED) structures with an embedded porous-GaN/n-GaN distributed Bragg reflector (DBR) and a top dielectric Ta2O5/SiO2 DBR were demonstrated. GaN:Si epitaxial layers with high Si-doping concentration (n+-GaN:Si) in the 20-period n+-GaN/n-GaN stacked structure were transformed into a porous-GaN/n-GaN DBR structure through the doping-selective electrochemical wet etching process. The central wavelength and reflectivity were measured to be 434.3 nm and 98.5% for the porous DBR and to be 421.3 nm and 98.1% for the dielectric DBR. The effective 1λ cavity length at 432nm in the InGaN resonant-cavity consisted of a 30 nm-thick Ta2O5 spacer and a 148 nm-thick InGaN active layer that was analyzed from the angle-resolved photoluminescence (PL) spectra. In the optical pumping PL spectra, non-linear emission intensity and linewidths reducing effect, from 6.5 nm to 0.7 nm, were observed by varying the laser pumping power. Directional emission pattern and narrow linewidth were observed in the InGaN active layer with bottom porous DBR, top dielectric DBR, and the optimum spacer layer to match the short cavity structure.

Design of electrically driven single photon source based on dielectric passive cavity structure at 1.3 μm

A combination of advanced concepts is applied for designing micro-cavity structures aimed for single-photon sources with high photon-extraction efficiency in the telecom O-band at ~1.3 μm. The device design consists of a broad stop-band bottom distributed Bragg reflector (DBR), a top DBR formed in a dielectric micropillar with additional circular Bragg grating and a central dielectric passive cavity. This combination of photonic elements is compatible with electric carrier injection and provides overall photon-extraction efficiency of ~83% as shown by 3D finite-difference time-domain simulations.

Comprehensive design and simulation of a composite reflector for mode control and thermal management of a high-power VCSEL

We propose a composite top reflector composed of a distributed Bragg reflector (DBR) and a subwavelength high-contrast grating (HCG) for a high-power 808-nm vertical-cavity surface-emitting laser (VCSEL). The DBR and HCG in the reflector are connected by an indium tin oxide (ITO) surrounding layer, which makes it possible for the reflector to improve current injection uniformity and reduce heat generation while providing high reflectivity. The angle-dependent reflectivity of the composite reflector is optimized to suppress the high-order transverse modes of VCSEL while ensuring sufficient fundamental mode feedback. The number of top DBR periods and the thickness of the ITO surrounding layer are optimized to reduce the loss and provide high out-coupling efficiency. The double resonator coupled by top DBR is designed to provide optimal resonant wavelength stability, longitudinal optical confinement factor, and thermoelectric characteristics. Optical simulation results demonstrate that the well-designed configuration can provide a highest fundamental mode reflectivity of 99.7%, an out-coupling efficiency of 65%, a wavelength stability rate of 0.011 with the thickness of the ITO layer, and a confinement factor of 0.05. The transverse modes with order greater than 2 are effectively suppressed. The result of the thermoelectric model shows that the composite reflector-based VCSEL has low operating temperature and uniform current injection; thermal resistance of 0.87 K/mW is realized. In this context, devices with high emission efficiency and beam quality can be expected.

High performance planar microcavity organic semiconductor lasers based on thermally evaporated top distributed Bragg reflector

High performance organic semiconductor lasers (OSLs), especially those under current injection, have been sought for decades due to their potentially great applications in fields such as spectroscopy, displays, medical devices, and optical interconnection. The design and fabrication of high-quality resonators is a prerequisite for high performance OSLs. In the case of planar microcavities, the fabrication process of top distributed Bragg reflectors (DBRs) usually requires electron beam evaporation or manual lamination on top of organic thin-film layers, which can lead to issues including degradation of the organic materials, large-scale non-uniformity, and difficulties for current injection. Here, we report a non-destructive way of fabricating a top DBR by thermal evaporation. The top DBR based on thermally evaporated alternative TeOx/LiF stacks shows low morphological roughness, high process tolerance, and high reflectivity. Moreover, the deposition process causes negligible damage to the organic thin-film layers underneath. With the combination of a conventional e-beam evaporated bottom DBR, a high performance planar microcavity OSL with a low threshold of 1.7 μJ cm−2, an emission linewidth of 0.24 nm, and an angular divergence of <3° has been achieved under nitrogen laser pumping. Similar performance, with a high Gaussian beam quality comparable with that of an ideal diffraction-limited beam, was also obtained under diode pumping, showing the potential of this technique for building compact and cost-effective organic lasers with good beam quality. Our result will open a promising route for future high performance microcavity optoelectronic devices, especially for laser devices under current injection.

The fabrication technology of VCSELs emitting in the 1.55 μm waveband

The paper presents the results on fabrication technique of a 1,55 μm wafer-fused vertical-cavity surface-emitting lasers (VCSELs) based on the InAlGaAsP/InP optical cavity and AlGaAs/GaAs distributed Bragg reflectors (DBRs) grown by solid-source molecular beam epitaxy. The optical cavity InAlGaAsP/InP containes an active region based on multiple InGaAs quantum wells, n++/p++-In (Al)GaAs buried tunnel junction (BTJ), InGaAsP intracavity contact layers and n-InP spreading layers. The top and bottom AlGaAs/GaAs DBRs were grown on GaAs substrate, while the optical cavity InAlGaAsP/InP was grown on InP substrate. The main fabrication proceseses of 1,55 μm VCSELs such as: BTJ fabrication, dry etching of first mesa on top DBR, wet etching of second VCSEL mesa, forming of the ohmic contacts and passivation of the VCSEL structure were described in details.

Cavity designs for nitride VCSELs with dielectric DBRs operating efficiently at different temperatures

This study set out to analyze the performance of modern nitride GaN-based tunnel-junction vertical-cavity surface-emitting lasers (VCSELs) at various ambient and elevated temperatures, with the aid of an advanced self-consistent optical-electrical-recombination-thermal model. Particular attention was given to the impact that both the length of the laser cavities and the mirror reflectivities have on laser operation across the temperature range. The optimal number of pairs in the top Distributed Bragg Reflector (DBR) was determined for stable operation at high temperatures and relatively high output powers. The effect on laser performance of expanding the metallization around the bottom DBR was also investigated.

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.

Design and Analysis of Static Characteristics of VCSEL at 1160 nm for Optical Interconnects

Abstract VCSEL can be used as the transmitter for optical interconnect applications especially for short reach communication. In this research paper, we proposed a VCSEL structure design which can generate the wavelength of 1160 nm. The VCSEL structure is proposed with the aim of small aperture diameter because for optical interconnects applications the size should reduce. Two distributed Bragg reflectors (DBR), i. e. top DBR and bottom DBR are used to confine the active cavity area and InGaAsP/InP material is used for the formation of active cavity region. Then, the performance of the proposed VCSEL is evaluated at various oxide aperture diameters in the range 2–7 µm for anode voltage, anode current and total power emitted is also calculated to achieve optimized performance.

Analysis of pulse amplification characteristics in vertical cavity semiconductor optical amplifiers

Based on the transfer matrix method (TMM), a comprehensive dynamic numerical model describing the optical pulse amplification in vertical cavity semiconductor optical amplifiers (VCSOAs) is established, which treats the whole VCSOA as one multilayer dielectric structure, including the two Distributed Bragg Reflectors (DBRs) and the middle active region. In this model, the spatial dependence of the carriers and optical field in the longitudinal direction and the discontinuity of refractive index distribution in the resonant cavity are accounted for. Moreover, in this study the influence of the two photon absorption (TWP) is also taken into consideration. By employing this model, we investigate the signal distortion and the pulse energy gain characteristic in single pulse amplification of VCSOAs operated in reflection mode in detail. The numerical results show that the pulse distortion can be mitigated in a satisfactory manner via reducing the period of top DBR. It further shows that the energy gain during the pulse amplification can be increased by enhancing the pump power; larger saturation input pulse energy (SIPE) could be obtained by either lowering the period of top DBR or reducing the pump power; for the input pulse with duration ranging from several to tens of picoseconds, the energy gain characteristics of VCSOA is pulsewidth independent.