Dielectric Distributed Bragg Reflectors Research Articles

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.

Distributed Bragg Reflectors for GaN-Based Vertical-Cavity Surface-Emitting Lasers

A distributed Bragg reflector (DBR) is a key building block in the formation of semiconductor microcavities and vertical cavity surface emitting lasers (VCSELs). The success in epitaxial GaAs DBR mirrors paved the way for the ubiquitous deployment of III-V VCSELs in communication and mobile applications. However, a similar development of GaN-based blue VCSELs has been hindered by challenges in preparing DBRs that are mass producible. In this article, we provide a review of the history and current status of forming DBRs for GaN VCSELs. In general, the preparation of DBRs requires an optimization of epitaxy/fabrication processes, together with trading off parameters in optical, electrical, and thermal properties. The effort of epitaxial DBRs commenced in the 1990s and has evolved from using AlGaN, AlN, to using lattice-matched AlInN with GaN for DBRs. In parallel, dielectric DBRs have been studied since 2000 and have gone through a few design variations including epitaxial lateral overgrowth (ELO) and vertical external cavity surface emitting lasers (VECSEL). A recent trend is the use of selective etching to incorporate airgap or nanoporous GaN as low-index media in an epitaxial GaN DBR structure. The nanoporous GaN DBR represents an offshoot from the traditional epitaxial approach and may provide the needed flexibility in forming manufacturable GaN VCSELs. The trade-offs and limitations of each approach are also presented.

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.

GaN-based Vertical-Cavity Surface-Emitting Lasers Incorporating Dielectric Distributed Bragg Reflectors

This paper reviews past research and the current state-of-the-art concerning gallium nitride-based vertical-cavity surface-emitting lasers (GaN-VCSELs) incorporating distributed Bragg reflectors (DBRs). This paper reviews structures developed during the early stages of research into these devices, covering both major categories of GaN-based VCSELs: hybrid-DBR and all-dielectric-DBR. Although both types exhibited satisfactory performance during continuous-wave (CW) operation in conjunction with current injection as early as 2008, GaN-VCSELs have not yet been mass produced for several reasons. These include the difficulty in controlling the thicknesses of nitride semiconductor layers in hybrid-DBR type devices and issues related to the cavity dimensions in all-dielectric-DBR units. Two novel all-dielectric GaN-based VCSEL concepts based on different structures are examined herein. In one, the device incorporates dielectric DBRs at both ends of the cavity, with one DBR embedded in n-type GaN grown using the epitaxial lateral overgrowth technique. The other concept incorporates a curved mirror fabricated on (000-1) GaN. Both designs are intended to mitigate challenges regarding industrial-scale processing that are related to the difficulty in controlling the cavity length, which have thus far prevented practical applications of all-dielectric GaN-based VCSELs.

GaN-on-Si resonant-cavity light-emitting diode incorporating top and bottom dielectric distributed Bragg reflectors

We report here a GaN-based resonant cavity light-emitting diode (RCLED) with top and bottom dielectric TiO2/SiO2 distributed Bragg reflector (DBR) mirrors on a silicon substrate. High data transmission in free space at 200 Mbps with an opening in the eye diagram was achieved. The results show that the combination of GaN-based LED on silicon and double sided dielectric DBR mirror deposition enables a manufacturable process which provides a unique opportunity for commercialization of RCLED in future solid-state lighting and visible light communication applications.

Polariton lasing in InGaN quantum wells at room temperature

In this paper, we report the exciton polaritons in both positive and negative detuning micro cavities based on InGaN multi-quantum wells (MQWs) and the first polariton lasing in InGaN/GaN MQWs at room temperature by utilizing a 4.5λ Fabry-Perot (F-P) cavity with double dielectric distributed Bragg reflectors (DBRs). Double thresholds corresponding respectively to polariton lasing and photonic lasing are observed along with half-width narrowing and peak blue-shifts. The threshold of polariton lasing is about half of the threshold of photonic lasing. Our results paved a substantial way for ultra-low threshold lasers and room temperature Bose-Einstein Condensate (BEC) in nitride semiconductors.

Coherent perfect absorption in wedged organic thin films: a method to determine optical properties

We investigate coherent perfect absorption in layered thin-film structures with organic small molecules as absorbing material. We focus on strongly asymmetric resonator structures, realized with a high-optical-quality dielectric-distributed Bragg reflector and a thermally evaporated wedged organic layer on top. The optical properties of these devices are systematically investigated by selective optical pumping and probing of the structure along the wedge. We show that phases and amplitudes of all waves and their balance relations can be tuned such that coherent perfect absorption is achieved, i.e., almost all incident radiation is absorbed within the thin organic layer. We show that such wedged structures on highly reflective dielectric mirrors can be used as a novel approach to measure optical dispersion relations of absorbing materials in a broad spectral range without requiring any specific a priori knowledge of the absorbing film.

(Invited) the Progress and Outlook of GaN Laser Devices

GaN laser diodes are considered the next frontier for high performance GaN optoelectronics. Currently, GaN edge-emitting laser diodes are finding specific applications in optical information storage, in large-area projection, and in automobile headlights. It is widely anticipated that further advances in GaN laser diodes could enable a much broader range of applications in lighting and display. The purpose of this presentation is to provide a high-level analysis of the state of two types of GaN laser diodes; one is the edge-emitting laser (EEL) and the other is the vertical cavity surface emitting laser (VCSEL). Commercial GaN EEL has been available for more than 15 years but the performance (wall-plug efficiency) is limited to ~35%[1]. For GaN VCSELs, in spite of various laboratory demonstrations in the past 10 years, there has not been any commercial product as a result of complexity involved in the growth and fabrication. For the discussion of GaN EELs we will analyze the factors that define and limit the contemporary GaN EELs. New approaches such as using non-polar/semipolar planes [2], quantum-dot active regions [3], higher power operation [4], and our unique approach of engineering the optical cavity confinement [5] will be discussed. Concerning GaN VCSELs, the technological bottleneck has been the formation of distributed Bragg reflectors (DBRs) for a planar microcavity. We will review the pros and cons of several approaches including epitaxial DBRs (AlGaN [6] or AlInN [7]), dielectric DBRs involving layer separation and transfer [8]–[10], and later overgrowth on dielectric DBRs [11]. Special emphasis will be given to our effort in using a nanoporous medium to form an epitaxial, conductive, and highly reflective DBRs for VCSELs [12], [13]. This work is supported the National Science Foundation (NSF) under Award ECCS-1709149 and by DARPA MTO of the United States. The facilities used were supported by the Yale SEAS cleanroom, YINQE, and NSF MRSEC DMR-1119826.

Thermal properties of GaN-based semiconductor-metal subwavelength grating VCSELs and novel current injection scheme

This paper provides a thermal and electrical analysis of semiconductor-metal subwavelength-grating (SMSG) vertical-cavity surface-emitting lasers (VCSELs). In the proposed design, the top distributed Bragg reflector (DBR) of a GaN-based VCSEL is replaced with a GaN–Ag SMSG, which serves as both an optical reflector and an electrical contact. Using a numerical 3D self-consistent thermo-electric model combined with a 3D optical model, we show that GaN-based SMSG VCSELs with aperture sizes of over 50 µm are capable of laser operation. We also propose spatial tuning of the injected current density to eliminate the current crowding effect in large area VCSELs. The use of GaN–Ag SMSGs in GaN-based VCSELs eliminates the need for a top dielectric DBR and intracavity injection scheme, making the fabrication of monolithic VCSELs possible. It also eliminates the need to use a tunnel junction with optical confinement built into the cavity. Although the GaN–Ag SMSG introduces higher optical losses than a DBR, its advantage is that it permits vertical current injection into the active region, enabling laser emission from broader-area active regions in comparison to conventional VCSELs.

Reduction of Lasing Threshold of GaN-Based Vertical-Cavity Surface-Emitting Lasers by Using Short Cavity Lengths

Ultralow lasing threshold of $413~\mu \text{J}$ /cm2 in GaN-based vertical-cavity surface-emitting lasers (VCSELs) was observed by reducing the cavity lengths between double-side dielectric distributed Bragg reflectors to $6\lambda $ . To the best of our knowledge, the threshold is the lowest value ever reported in nitride optically pumped VCSEL. The spontaneous emission factor ( $\beta $ ) was 0.1, and the degree of polarization was 91%. Effects of short cavity on spontaneous emission factors, gain coefficient enhancement, and absorption reduction were analyzed. In addition, effects of coupled quantum wells on the confinement factor were also discussed. We believe that our results would be meaningful for refining nitride VCSEL structure in the future.

Continuous-wave operation of m-plane GaN-based vertical-cavity surface-emitting lasers with a tunnel junction intracavity contact

We have achieved continuous-wave (CW) operation of an optically polarized m-plane GaN-based vertical-cavity surface-emitting laser (VCSEL) with an ion implanted current aperture, a tunnel junction intracavity contact, and a dual dielectric distributed Bragg reflector design. The reported VCSEL has 2 quantum wells, with a 14 nm quantum well width, 1 nm barriers, a 5 nm electron-blocking layer, and a 23λ total cavity thickness. The thermal performance was improved by increasing the cavity length and using Au-In solid-liquid interdiffusion bonding, which led to lasing under CW operation for over 20 min. Lasing wavelengths under pulsed operation were observed at 406 nm, 412 nm, and 419 nm. Only the latter two modes appeared under CW operation due to the redshifted gain at higher temperatures. The peak output powers for a 6 μm aperture VCSEL under CW and pulsed operation were 140 μW and 700 μW, respectively. The fundamental transverse mode was observed without the presence of filamentary lasing. The thermal impedance was estimated to be ∼1400 °C/W for a 6 μm aperture 23λ VCSEL.

Fabrication and Characterization of GaN-Based Resonant-Cavity Light-Emitting Diodes with Dielectric and Metal Mirrors

We demonstrated GaN-based resonant-cavity light-emitting diodes (RCLEDs) fabricated on a copper substrate by electroplating and laser lift-off techniques. A TiO2/SiO2 dielectric distributed Bragg reflector (DBR) and an aluminum metal reflector were employed as top and bottom mirrors, respectively. As compared to the conventional LED without a top mirror, improved performances were achieved in RCLED devices due to the resonant cavity effect. We also investigated the effects of the top TiO2/SiO2 dielectric DBR on device characteristics. By tuning the reflectivity of the top dielectric DBR, the optimum RCLED device with the top DBR reflectivity of 55% shows a high output power of 62 mW, an external quantum efficiency of 14.8%, a full width at half maximum (FWHM) of 12 nm for emission spectrum, a 50% viewing angle of 122° and a -3 dB modulation bandwidth of 48 MHz. The results show that the RCLED device with the optimum reflectors has considerable potential for future high-performance application.

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.

Mirror design for long-wavelength vertical-cavity surface-emitting lasers

A kind of bottom mirror, fabricated using a nano-scale epitaxy method, is proposed for long-wavelength InP-based vertical-cavity surface-emitting lasers (VCSELs). The design of the bottom mirror combines the conventional dielectric distributed Bragg reflector structure with a fabrication procedure using nano-scale epitaxial lateral overgrowth (ELO). Hence, this ELO mirror structure can provide both a high reflectivity, to form an optical resonance cavity, and a virtual InP substrate for the growth of high quality active regions in the VCSEL. The dependence of the reflectivity on the epitaxial lateral overgrowth mirror structure parameters is analyzed in detail using numerical methods. The results demonstrate that a stopband at more than 99% reflectivity is achieved in the wavelength range from 1.49 to 2.09 µm, and the threshold optical gain can be as low as 202, 257, and 311 cm−1 for the internal loss coefficients of 10, 15 and 20 cm−1.

Suspended light-emitting diode featuring a bottom dielectric distributed Bragg reflector

Here, we propose, fabricate and characterize the light manipulation of a suspended-membrane InGaN/GaN multiple-quantum-well light-emitting diode (MQW-LED) with a dielectric distributed Bragg reflector (DBR) positioned at the bottom, implemented on a GaN-on-silicon platform. Silicon removal is conducted to obtain the suspended MQW-LED architecture, and back wafer thinning of the epitaxial film is performed to improve the device performance. A 6-pair SiO2/Ta2O5 DBR is deposited on the backside to manipulate the emitted light. The experimental results demonstrate that the bottom dielectric DBR exhibits high reflectivity and distinctly changes the light emission, which are consistent with the performed simulation results. This work represents a significant step towards the realization of inexpensive, electrically driven and simply fabricated GaN VCSELs for potential use in number of applications.

Silicon-Integrated Hybrid-Cavity 850-nm VCSELs by Adhesive Bonding: Impact of Bonding Interface Thickness on Laser Performance

The impact of bonding interface thickness on the performance of 850-nm silicon-integrated hybrid-cavity vertical-cavity surface-emitting lasers (HC-VCSELs) is investigated. The HC-VCSEL is constructed by attaching a III–V “half-VCSEL” to a dielectric distributed Bragg reflector on a Si substrate using ultrathin divinylsiloxane-bis-benzocyclobutene (DVS-BCB) adhesive bonding. The thickness of the bonding interface, defined by the DVS-BCB layer together with a thin SiO2 layer on the “half-VCSEL,” can be used to tailor the performance, for e.g., maximum output power or modulation speed at a certain temperature, or temperature-stable performance. Here, we demonstrate an optical output power of 2.3 and 0.9 mW, a modulation bandwidth of 10.0 and 6.4 GHz, and error-free data transmission up to 25 and 10 Gb/s at an ambient temperature of 25 and 85 °C, respectively. The thermal impedance is found to be unaffected by the bonding interface thickness.

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.

Tunable InGaN quantum dot microcavity light emitters with 129 nm tuning range from yellow-green to violet

An electrically pumped wavelength-tunable InGaN quantum dot (QD) based microcavity (MC) lighter emitter with a large tuning range of 129 nm was demonstrated. The multi-mode emission spectrum was tuned by injected current from 564 nm (yellow-green) to 435 nm (violet). The MC light emitter is featured with a double dielectric distributed Bragg reflector structure and a copper substrate fabricated using substrate transfer and laser lift off techniques. By utilizing an InGaN QD active layer with a tunable broad emission spectrum and a Fabry-Pérot cavity which allows multi-longitudinal mode resonating, the emission spectrum could be tuned among several particular cavity modes, which are decided by the gain enhancement factor. In addition, both the enhancement and suppression of MC emission modes caused by the gain enhancement factor were observed in a single MC device. As the first electrically driven III-V nitride semiconductor based tunable MC light emitter with a tuning range of 129 nm, the device is promising for applications such as in wide-gamut compact displays and projectors.

Investigation of single-mode vertical-cavity surface-emitting lasers with graphene-bubble dielectric DBR

An inter-cavty contact single mode 850nm VCSEL was fabricated with a graphene assisted self-assembly curved dielectric bubble Bragg mirror for the first time. Taking the advantage of graphene’s uniform low surface energy, the low cost dielectric bubble DBR (Si3N4/SiO2) was deposited on top of the graphene/half-VCSEL structure via van der Waals Force (vdWF) without using any additional spacing elements and sacrificial layer release-etch process. The continuous-wave operating VCSELs with an aperture diameter of 7μm exhibit single-mode output power of more than 1mW with a slope efficiency of 0.2W/A. The sidemode suppression ratios are >40dB. This novel modification into the lasers can also be applied to a variety of other optoelectronic devices, such as resonance photodetecter and super narrow linewidth VCSEL.

Effect of Cavity Length, Strain, and Mesa Capacitance on 1.5-μm VCSELs Performance

A systematic experimental analysis of the stationary and dynamic performance of 1.5-μm InP-based vertical-cavity surface-emitting lasers with two dielectric distributed Bragg reflectors is presented. The electrical, thermal, and optical characteristics are studied as a function of the cavity length, strain in the active region and mesa capacitance.