Bottom Distributed Bragg Reflector Research Articles

Influence of Various Bottom DBR Designs on the Thermal Properties of Blue Semiconductor-Metal Subwavelength-Grating VCSELs.

In this paper, we consider several designs for nitride-based vertical-cavity surface-emitting lasers (VCSELs) with a top semiconductor-metal subwavelength grating (SMSG) as the facet mirror. The constructions of the bottom distributed Bragg reflectors (DBRs) used in the VCSEL designs were inspired by devices demonstrated recently by several research groups. A multiparameter numerical analysis was performed, based on self-consistent thermal and electrical simulations. The results show that, in the case of small aperture VCSEL designs, dielectric-based DBRs with metallic or GaN channels enable equally efficient heat dissipation to designs with monolithically integrated DBRs. In the case of broad aperture designs enabled by SMSGs, monolithically integrated DBRs provide much more efficient heat dissipation in comparison to all other considered designs.

High-Efficiency Grating Coupler in 400 nm and 500 nm PECVD Silicon Nitride With Bottom Reflector

We design and experimentally demonstrate highly efficient Silicon Nitride based grating couplers with bottom distributed Bragg reflectors. All the layers were deposited using plasma enhanced chemical vapor deposition processing. We present gratings on two Silicon Nitride thickness (400 nm and 500 nm) platforms. On a 500 nm thick Silicon Nitride, we show a peak coupling efficiency of -2.29 dB/coupler at a wavelength of 1573 nm with a 1 dB bandwidth of 49 nm. On a 400 nm thick platform, we demonstrate a coupling efficiency of -2.58 dB/coupler at 1576 nm with a 1 dB bandwidth of 52 nm. The demonstrated coupling efficiency is the best reported as yet, for both 400 nm and 500 nm thick, plasma deposited Silicon Nitride platforms.

Demonstration of polarization control GaN-based micro-cavity lasers using a rigid high-contrast grating reflector

We reported on GaN microcavity (MC) lasers combined with one rigid TiO2 high-contrast grating (HCG) structure as the output mirror. The HCG structure was directly fabricated on the GaN structure without an airgap. The entire MC structure comprised a bottom dielectric distributed Bragg reflector; a GaN cavity; and a top HCG reflector, which was designed to yield high reflectance for transverse magnetic (TM)- or transverse electric (TE)-polarized light. The MC device revealed an operation threshold of approximately 0.79 MW/cm2 when pulsed optical pumping was conducted using the HCG structure at room temperature. The laser emission was TM polarized with a degree of polarization of 99.2% and had a small divergence angle of 14° (full width at half maximum). This laser operation demonstration for the GaN-based MC structure employing an HCG exhibited the advantages of HCGs in semiconductor lasers at wavelengths from green to ultraviolet.

(Invited) Development of Ultraviolet Vertical-Cavity Surface-Emitting Light Sources

Nitride-based vertical-cavity surface-emitting lasers (VCSELs) at wavelengths below 400 nm have drawn tremendous interest as they are potentially utilized in various applications as highly-efficient and temperature-insensitive optical sources. One example is for a compact, energy efficient, portable ytterbium-ion-based atomic clock that requires a high-stability CW optical pump at near ultraviolet (UV) wavelengths. In this work, we have used metalorganic chemical vapor deposition (MOCVD) to grow optically pumped UV VCSEL epitaxial structures and UV resonant-cavity light-emitting diodes (RCLEDs) to demonstrate several design concepts targeting the ~360-370nm UVA spectral range. The UV VCSELs are designed using a proprietary III-N device simulation tool developed at Georgia Institute of Technology. Previously, we have demonstrated a variety of optically pumped III-N VCSELs lasing at ~375 nm that showed the feasibility of building the near-UV coherent light source on GaN materials platform. The test structures consisted of a five-pair InGaN/Al0.15Ga0.85N multiple-quantum-well (MQW) active region embedded in Al0.1Ga0.9N spacers with the total optical cavity thickness of 3λ. The lowest threshold incident power density for optically pumped VCSELs was estimated to be ~270 kW/cm2 under pulsed conditions at the room temperature. In further work, we implemented a hybrid distributed Bragg reflectors (DBR) combination to demonstrate vertical resonant cavity light emitting diodes (RCLEDs) targeting at 370-nm peak emission wavelengths. The hybrid mirrors use a dielectric DBR with twelve pairs of ¼-wavelength HfO2/SiO2 layers as the topside mirror, and a semiconductor-based mirror composed of five periods of ¾λ air-gap/Al0.05Ga0.95N DBR at the bottom side of the vertical cavity. The RCLED structure comprises three distinct regions: (1) bottom DBR mirror of five-pair 3λ/4-air-gap/ Al0.045Ga0.95fN, (2) a 6λ-cavity LED active region of n-Al0.08Ga0.92N (n~8x1018cm-3) spacer/InGaN-AlGaN MQW/p-AlGaN electron blocking/p-Al0.08Ga0.92N (p~3x1017cm-3) spacer/graded p +-AlGaN, and (3) a 12-pair λ/4 HfO2/SiO2 dielectric DBR as the top-side mirror. The device fabrication employed a nitrogen ion-implantation isolation to form a current confinement aperture. The mesa-type devices have the anode and the cathode electrodes on the topside of the wafer. The air-gap DBRs were formed using optical lithography and subsequent inductively coupled plasma dry etching to create a deep trench, followed by laterally removing sacrificial layers using a conductivity-selective electrochemical etching process. Once formed, this airgap DBR provides a high reflectivity of ~99.9% covering the spectral range from 360 to 400 nm while the dielectric DBR has a reflectivity of >99% covering the range from 350 to 405 nm. The fabricated RCLEDs can be operated at a current density >50 kA/cm2 in the pulsed current mode with a peak emission wavelength of ~375 nm at the room temperature. Details of MOCVD growth, device fabrication, and characterization will be presented.

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.

GaN-based vertical-cavity surface-emitting lasers using n-type conductive AlInN/GaN bottom distributed Bragg reflectors with graded interfaces

A 4.0λ-cavity GaN-based vertical-cavity surface-emitting laser (VCSEL) using an n-type conductive AlInN/GaN bottom distributed Bragg reflector (DBR) showed a light output power of 1.8 mW with a low differential resistance of 90 Ω. In order to obtain low resistive AlInN/GaN DBRs, 5 nm AlGaInN graded interfaces were introduced. A 40-pair AlInN/GaN DBR with the graded interfaces showed a peak reflectivity over 99.8% and a series resistance of 17 Ω. At the same time, the maximum light output power of a 1.5λ-cavity GaN-based VCSEL with the conductive DBR was only 0.03 mW due to a high thermal resistance of 2700 K W−1 caused by a high thermal resistivity of AlInN. We also conclude that a short cavity typically used in GaAs-based VCSELs is not appropriate in GaN-based VCSELs.

Polarization-pinned emission of a continuous-wave optically pumped nonpolar GaN-based VCSEL using nanoporous distributed Bragg reflectors.

A nonpolar GaN-based vertical-cavity surface-emitting laser (VCSEL) using nanoporous bottom epitaxial distributed Bragg reflector (DBR) is demonstrated at room temperature (RT) under continuous-wave (CW) optical pumping. The porous layers enable the epitaxial growth of lattice-matched high-reflectance DBRs without sacrificing the conductive properties needed for high-performance VCSELs. The 2-λ cavity VCSEL reported here employs a hybrid design with top dielectric DBR and bottom nanoporous DBR. Single longitudinal mode lasing is observed at 462 nm with a threshold power density of ~5 kW/cm2 and a FWHM of ~0.12 nm. The emission polarization was pinned in the a-direction at all measured locations.

A study on the voltage-dependent response of a GaInNAs-based pin photodetector with a quasi-cavity

We present a characterisation of a GaInNAs/GaNAs quantum well-based photodetector with a bottom distributed Bragg reflector (quasi-cavity). The detector is designed to be used at the 1.3 μm optical fibre communication window. The quantum efficiency of the photodetector is measured as 24% at 1286 nm under −2 V applied reverse bias. As the reverse bias voltage is increased, a carrier multiplication-related increase and oscillations are observed in the voltage-dependent responsivity curve. The observed carrier multiplication is explained by the high electrical field-induced impact ionisation mechanism in the pin junction region, while the observed voltage-dependent oscillations are explained by the Franz–Keldysh effect (FKE). At the wavelength of 1286 nm, which is close to the absorption wavelength of the active region of the photodetector, FKE-related oscillations (FKOs), start at very low reverse bias values and the responsivity of the photodetector is dominated by FKOs. On the other hand, FKOs quench at higher wavelengths and an impact ionisation-related increase at the voltage-dependent responsivity curve dominates. At λ = 1310 nm, only impact ionisation mechanisms have an effect over the R (V) curve. The multiplication factor for 1310 nm is calculated as M = 12 at room temperature. The applied electric field and excitation wavelength dependence of the absorption coefficient is calculated and a good match with the experimental results at the applied voltages is achieved.

Quantum-Dot Photonic-Crystal Surface-Emitting Lasers with Bottom Distributed Bragg Reflector

Quantum-dot (QD) photonic-crystal (PC) surface-emitting laser (SEL) devices with bottom distributed Bragg reflector (DBR) were fabricated based on vertical-cavity SEL structure with top DBR completely removed. Two-dimensional (2D) PCs were deeply etched through QD multilayers to yield strong diffraction coupling. Room-temperature optically pumped lasing emissions at 1194 nm and 1296 nm were demonstrated for two lattice periods of 360 nm and 395 nm, respectively. Two lasing wavelengths separated over 100 nm; however, there were less than two times difference in threshold power densities while slope efficiencies were comparable. The unique spectral gain characteristics of QDs were considered in interpretation of gain-cavity detuning. Moreover, simulation revealed the sub-cavity should be designed so that its resonant wavelength is in phase with lasing wavelength.

Enhanced photoresponse of monolayer molybdenum disulfide (MoS2) based on microcavity structure

There is an increasing interest in using monolayer molybdenum disulfide (MoS2) for optoelectronic devices because of its inherent direct band gap characteristics. However, the weak absorption of monolayer MoS2 restricts its applications, novel concepts need to be developed to address the weakness. In this work, monolayer MoS2 monolithically integrates with plane microcavity structure, which is formed by the top and bottom chirped distributed Bragg reflector (DBR), is demonstrated to improve the absorption of MoS2. The optical absorption is 17-fold enhanced, reaching values over 70% at work wavelength. Moreover, the monolayer MoS2-based photodetector device with microcavity presents a significantly increased photoresponse, demonstrating its promising prospects in MoS2-based optoelectronic devices.

A comparative study of thermal characteristics of GaN-based VCSELs with three different typical structures

Thermal characteristics of GaN-based vertical cavity surface emitting lasers (VCSELs) with three typical structures were investigated both theoretically and experimentally. The simulation results based on a steady state quasi three-dimensional cylindrical model show that the thermal resistance (Rth) is affected by cavity length, mesa size, as well as the bottom distributed Bragg reflector (DBR) size, and the detail further depends on different structures. Among different devices, GaN VCSEL with a hybrid cavity formed by one nitride bottom DBR and another dielectric top DBR is featured with lower Rth, which is meanwhile affected strongly by the materials of the epitaxial bottom DBR. The main issues affecting the thermal dissipation in VCSELs with double dielectric DBRs are the bottom dielectric DBR and the dielectric current-confinement layer. To validate the simulation results, GaN-based VCSEL bonded on a copper plate was fabricated. Rth of this device was measured and the results agreed well with the simulation. This work provides a better understanding of the thermal characteristics of GaN-based VCSELs and is useful in optimizing the structure design and improving the device performance.

High-temperature operation of GaN-based vertical-cavity surface-emitting lasers

We report GaN-based vertical-cavity surface-emitting lasers (VCSELs) capable of high-temperature operation. The GaN-based VCSELs include double dielectric distributed Bragg reflectors and epitaxially grown p–i–n InGaN multiple-quantum-well active layers initially deposited on c-plane sapphire substrates that are bonded to a silicon substrate with a p-side-down and patterned mirror configuration, allowing effective heat dissipation. GaN-based VCSELs with an emission aperture 10 µm in diameter were fabricated, and their temperature-dependent lasing characteristics revealed that the VCSELs can endure 350 K, as measured under quasicontinuous-wave operation conditions. The temperature-dependent lasing wavelength shift occurs at a rate of dλFP/dT ≈ 0.012 nm/K. The high-temperature operation of GaN-based VCSELs was attributed to the well-matched gain-mode offset, the p-side-down configuration, and the reduced lateral size of the bottom distributed Bragg reflector with recessed metal.

Heat dissipation effect on modulation bandwidth of high-speed 850-nm VCSELs

Vertical-cavity surface-emitting lasers (VCSELs) have been widely used in short-reach optical communication links and interconnects because of their high modulation speed and low energy consumption. In this paper, the effect of heat dissipation on the modulation bandwidth of high-speed 850-nm VCSELs has been investigated. With increasing bottom distributed Bragg reflector (DBR) diameter from 34 μm to 46 μm, optical output power increased as well as the −3 dB modulation bandwidth. The maximum small-signal modulation bandwidth of 16.2 GHz was achieved with an aperture of 5 μm. Thermal analysis showed that the heat dissipation of the bottom DBR diameter, which determined the differential gain of the quantum wells, has a strong effect on the modulation bandwidth.

Efficiency enhancement for resonant-cavity-enhanced InGaN/GaN multiple quantum well solar cells

We proposed a new approach of enhancing the quantum efficiency of InGaN/GaN multiple quantum well (MQW) solar cells using resonant cavity in this study. The resonant-cavity-enhanced (RCE) structure is designed with a top mirror and bottom distributed Bragg reflector (DBR). The origin of the enhancement in quantum efficiency was the resonance-induced increase of the optical field, which caused more photons to be absorbed in the InGaN absorption layers. We simulated the photovoltaic performance of InGaN/GaN MQW solar cells by comparing RCE-type and conventional solar cells. The best structural parameters are estimated to be 600 nm for cavity length, 10% for the reflectivity of top mirror and as high as possible for the reflectivity of bottom DBR (in simulation we used 99%). As a result, the RCE-type InGaN/GaN MQW solar cell shows an enhancement in short-circuit current density by 2.12 times and conversion efficiency by 2.13 times, as compared to those of a conventional InGaN/GaN MQW solar cell.

Design Considerations for Single-Mode Vertical-Cavity Surface-Emitting Lasers With Impurity-Induced Intermixing

We report on the modeling of 850-nm (Al,Ga)As/GaAs vertical cavity surface-emitting lasers (VCSELs), where the electrical and optical confinement is realized by impurity-induced intermixing. The 3-D simulations of electromagnetic fields are performed by the finite-element method using full vector Maxwell’s equations. An example of a transverse junction VCSEL with Zn-induced intermixing of the areas surrounding the quantum well region is compared with the case of a VCSEL, where the optical modes are confined by selective oxidation of the Al-rich GaAlAs aperture layers. We show that Zn-induced intermixing results in a high efficiency of the leakage of the VCSEL aperture-confined modes into the surrounding intermixed area. Furthermore, there is a strong mode selectivity through the leakage loss between the fundamental and excited transverse modes of the optical cavity. This enables potential realization of a single transverse mode VCSEL at the aperture diameters of 10 $\mu \text{m}$ or above and consequently the realization of high-power single-mode VCSELs. Optimum mode selectivity is realized once the impurity intermixing profile proceeds down to the depth of about 2.6 $\mu \text{m}$ into the bottom distributed Bragg reflector below the quantum well region. Leakage of the optical modes into the intermixed area is accompanied by the evolution of narrow tilted lobes in the far-field emission spectra (at $\sim 60^{\circ }$ tilt angles for the design considered). This effect is similar to the one previously observed in oxide-confined leaky VCSELs and originates from the laterally propagating optical modes outside the non-intermixed aperture region. Where necessary, by applying such phenomenon, a phase-coupled array can be configured. We believe that such single-mode VCSELs may become extremely important in 3-D sensing, printing, and laser illumination. Furthermore, LIDARs with an option of beam steering without moving parts can be realized in 2-D phase-coupled arrays.

New type of porous silicon-based optical microcavity biosensor using a thin metal film

This paper proposes a new type of a porous silicon-based optical microcavity biosensor using a thin metal layer. The detection system was composed of a bottom distributed Bragg reflector (DBR) of porous silicon as an overlayer. After changing the refractive index of the defect layer through interactions of biomaterial, a thin metal layer was deposited onto the defect layer as a top mirror to form a complete microcavity. The optical sensitivity of the new microcavity biosensor with a single DBR and metal mirror can still be high only with the level f modification of a single porous layer, which is comparable to that of the conventional microcavity biosensor with two DBRs, where the modification of multiple porous layers is required. The results of the numerical simulation for the cavity quality and sensitivity of the proposed microcavity biosensor were provided.

Vertical cavity lasing from melt-grown crystals of cyano-substituted thiophene/phenylene co-oligomer

Vertical-cavity organic lasers are fabricated with melt-grown crystals of a cyano-substituted thiophene-phenylene co-oligomer. Due to lying molecular orientation, surface-emitting lasing is achieved even in the half-cavity crystal grown on a distributed Bragg reflector (DBR) under optical pumping at room temperature. Anticrossing splits in angle-resolved photoluminescence spectra suggest the formation of exciton-polaritons between the cavity photons and the confined Frenkel excitons. By constructing the full-cavity structure sandwiched between the top and bottom DBRs, the lasing threshold is reduced to one order, which is as low as that of the half cavity. Around the threshold, the time profile of the full-cavity emission is collapsed to a pulsed shape accompanied by a finite turn-on delay. We discuss these observed characteristics in terms of a polariton contribution to the conventional photon lasing.

Ultra efficient silicon nitride grating coupler with bottom grating reflector.

We theoretically propose a silicon nitride (Si(3)N(4)) grating coupler (GC) with both ultrahigh efficiency and simplified fabrication processes. Instead of using a bottom distributed Bragg reflector (DBR) or metal reflector, a bottom Si grating reflector (GR) with comparable reflectivity is utilized to improve the coupling efficiency. The fully etched Si GR is designed based on an industrially standard silicon-on-insulator (SOI) wafer with 220 nm top Si layer. By properly adjusting the trench width and period length of the Si GR, a high reflectivity over 90% is obtained. The Si(3)N(4) GC is optimized based on a common 400 nm Si(3)N(4) layer sitting on the Si GR with a SiO(2) separation layer. With an appropriate distance between the Si(3)N(4) GC and bottom Si GR, a low coupling loss of -1.47 dB is theoretically obtained using uniform GC structure. A further record ultralow loss of -0.88 dB is predicted by apodizing the Si(3)N(4) GC. The specific fabrication processes and tolerance are also investigated. Compared with DBR, the bottom Si GR can be easily fabricated by single step of patterning and etching, simplifying the fabrication processes.

MEMS-based air-gap cavity for entangled photon generation via polariton-polariton scattering

A structure for entangled photon generation via polariton-polariton scattering is proposed. We have designed an air-gap planar cavity with embedded quantum wells, in which the photonic modes are in strong coupling regime with excitons of quantum wells. An electrical tuning mechanism is designed by introducing an air-gap layer and an integrated electrical actuator to the distributed Bragg reflector (DBR) mirror structure. Our structure consists of 40 (17) bottom (top) DBRs with 12 GaAs/AlAs (7/3 nm) quantum wells at the antinodes of the electromagnetic field working at 767-nm wavelength. Initial air-gap thickness is 1500 nm. The Q factor and Rabi splitting of the structure are 1742 and 14 meV, respectively. Reflectivity of cavity/air interface is 0.32, and suspended DBR reflectivity is 0.97. Average indices of suspended mirror and cavity are 3.2062 and 3.1901, respectively. According to our simulation results, by changing air-gap thickness from 1000 to 1350 nm, the wavelength tuning range is extended to 30 nm via applying a maximum voltage of 12 V.

High-power green and blue electron-beam pumped surface-emitting lasers using dielectric and epitaxial distributed Bragg reflectors

ZnSe-based electron-beam pumped vertical-cavity surface-emitting lasers for the green (λ = 530 nm) and blue (λ = 462 nm) spectral region have been realized. Structures with and without epitaxial bottom distributed Bragg reflector have been fabricated and characterized. The samples consist of an active region containing 20 quantum wells with a cavity length varying between an optical thickness of 10 λ to 20 λ. The active material is ZnCdSSe in case of the green devices and ZnSe for the blue ones. Room temperature single mode lasing for structures with and without epitaxial bottom mirror with a maximum output power up to 5.9 W (green) and 3.3 W (blue) is achieved, respectively.