High Reflectivity Distributed Bragg Reflectors Research Articles

Structural analysis of distributed Bragg reflector mirrors

Quantum cascade (QC) lasers and vertical-cavity surface-emitting lasers (VCSELs) are of great interest due to their potential importance for a variety of device applications. Both kinds of lasers call for very highly reflective mirrors. Usually distributed Bragg reflector (DBR) mirrors, which consist of periodic quarter wavelength stacks of high and low refractive index compound semiconductors are used. These stacks are superlattices containing more than 40 individual layers. To obtain very high reflectivity DBRs alternating GaAs and AlAs layers are used for both the high and low index mirrors. GaAs/AlAs DBR structures containing 15 periods were characterized by the complementary use of RBS/channeling, TEM and HRXRD. Since the total thickness of a DBR exceeds 2 μm the RBS analysis was performed at two He-ion energies: 1.7 MeV and 3.82 MeV. After some stopping power corrections TEM and RBS provided similar results. Discrepancies with HRXRD data were attributed to the lateral inhomogeneity of produced superlattice. Virtues and pitfalls of complementary use of these techniques were discussed.

Stimulated emission properties of an organic salt-doped polymer film in microcavity

An organic vertical cavity surface emitting laser has been fabricated, using a Forster energy transfer system of organic pyridium salt dye ASPI and Alq3 doped in PMMA film as the gain medium, which was sandwiched between a high-reflectance distributed Bragg reflector and a silver reflector. The stimulated emission properties were investigated by nanosecond-pulsed laser pumping. The lasing phenomenon was observed with a full width at half maximum of 243 nm at the lasing wavelength of 600 nm, and the threshold energy for lasing was estimated to be about 8?J per pulse.

Novel blue continuous-wave current-injected light source

Vertical-cavity surface-emitting lasers (VCSELs) contain two high-reflectivity distributed-Bragg reflectors (DBRs) as well as a short cavity characterized by a small optical-mode volume. They emit a single longitudinal mode in a circularly symmetric beam with little beam divergence. Both characteristics are superior to those achieved with edge-emitting lasers and desirable for many practical applications in, for example, high-density optical storage and laser printing. In addition, the strong field in the microcavity facilitates exploration of cavity quantum-electrodynamics effects, such as single-photon and polariton emission, controlled spontaneous emission, and low threshold (or thresholdless) lasing. Over the past decade, two developments have affected the realization of electrically pumped gallium nitride (GaN)-based VCSELs. First, high-reflectivity, high-quality DBRs composed of AlxGa1−xN and GaN exhibit a large lattice mismatch that forms cracks in the epitaxially grown DBR structure. These cracks could result in a reduction in optical reflectivity and become a leakage-current path. The second is the fabrication of novel, high-transparency, high-conductivity contacts for current injection, which was triggered by the difficulties encountered in constructing low-resistance p-type GaN layers. In 1996, GaN-based edge-emitting laser diodes were first realized at room temperature by Nakamura’s team.1 Arakawa and co-workers subsequently fabricated an In0.1Ga0.9N VCSEL and observed lasing at 77K for the first time in 1998.2 We recently fabricated a crack-free high-reflectivity DBR by incorporating AlN/GaN superlattice layers between the DBR structures to reduce V-shaped defects and act as strain buffers.3 Figure 1 shows the device structure employing hybrid mirrors. It consists of 29 pairs of AlN/GaN DBRs and a 5λ (where λ refers to the operating wavelength) active region with a Figure 1. Schematic diagram of an electrically pumped vertical-cavity surface-emitting laser. DBR: Distributed-Bragg reflector. GaN, AlN: Gallium, aluminum nitride. MQW: Multiquantum well.

Enhanced Light Extraction from Gallium Nitride Optoelectronics on Si Substrates

This letter presents a technique to enhance the light extraction from a III-nitride light emitting diode on a Si substrate by inserting a high-reflectance distributed Bragg reflector below the active layer of the device to reflect the emitted light away from the opaque substrate. Additionally, the alternating layer sequence in the distributed Bragg reflector absorbs strain and filters dislocations generated at the GaN / Si interface. Theoretical results show that optimal placement of the light source is critical to achieve enhanced emission from the optical cavity.

Al As ∕ Ga As micropillar cavities with quality factors exceeding 150.000

The authors report on AlAs∕GaAs micropillar cavities with unprecedented quality factors based on high reflectivity distributed Bragg reflectors (DBRs). Due to an increased number of mirror pairs in the DBRs and an optimized etching process record quality (Q) factors up to 165.000 are observed for micropillars with diameters of 4μm. Optical studies reveal a very small ellipticity of 5×10−4 of the pillar cross section. Because of the high Q factors, strong coupling with a vacuum Rabi splitting of 23μeV is observed for micropillars with a diameter of 3μm.

Polarization splitting in organic-based microcavities working in the strong coupling regime

Abstract The emission features and the polarization splitting are investigated in organic microcavities operating in the strong coupling regime. The microcavity relies on a J -aggregate of cyanine dyes as active layer, and on high reflectivity distributed Bragg reflectors, fabricated by low-temperature reactive electron-beam evaporation. The operation of the microcavities in the strong coupling regime is demonstrated both by reflectivity and photoluminescence characterization. A vacuum Rabi splitting in the range 30–100 meV is reported. Important, both polariton branches exhibit a TM–TE polarization splitting depending on the incident angle, θ , and exhibiting a maximum of 35 meV. The angular behaviour of the splitting in the strong coupling regime is investigated and described by the sin 2 θ dependence predicted by the polarization splitting model.

Semiconductor quantum dot microcavity pillars with high-quality factors and enlarged dot dimensions

Vertical-emitting AlAs∕GaAs microcavity pillars with a type of GaInAs quantum dots within a one λ cavity have been realized based on high reflectivity distributed Bragg reflectors. High-quality factors were achieved due to an improved fabrication technology with a maximum quality factor of 27 700 for a micropillar with a diameter of 4μm. The dot dimensions could be enlarged by one order of magnitude using a low strain Ga0.7In0.3As nucleation layer.

Optically pumped lasing from organic microcavity

An optically pumped organic laser has been fabricated using a DCM (4-dicyanmethylene-2-methyl-6-4H-pyran)-doped Alq (tris (8-hydroxyquinoline) aluminum) film as the gain medium sandwiched between a high-reflectance distributed Bragg reflector and a silver reflector. The stimulated emission properties were investigated by short-pulse laser pumping. The intensity ratio of the resonant peak at 604 nm to the DBR (distributed Bragg reflector)-modulated free space emission band changed greatly with increasing pump energy density. The dependence on the pump energy density of the output properties including the cavity mode output, the total output, and the ratio of the cavity output versus total output, indicates a clear threshold energy density of 400 ± 20 µJ/cm2. The effective gain coefficient at the threshold was about 250 ± 14 cm–1. PACS No.: 42.55.Sa

Silicon substrates with buried distributed Bragg reflectors for resonant cavity-enhanced optoelectronics

We report on a commercially reproducible silicon wafer with a high-reflectance buried distributed Bragg reflector (DBR). The substrate consists of a two-period DBR fabricated using a double silicon-on-insulator (SOI) process. The buried DBR provides a 90% reflecting surface. We have fabricated resonant cavity-enhanced Si photodetectors with 40% quantum efficiency at 860 nm and a full-width at half-maximum of 29 ps suitable for 10 Gbps data communications. We have also implemented double-SOI substrates with 90% reflectivity covering 1300 and 1550 nm for use in Si-based optoelectronics.

High reflectivity and crack-free AlGaN/AlN ultraviolet distributed Bragg reflectors

High reflectivity and crack-free ultraviolet distributed Bragg reflectors (DBRs), based on AlGaN/AlN quarter-wave layers, have been designed and grown on (0001) sapphire by plasma-assisted molecular beam epitaxy. To minimize the tensile stress and thus prevent nucleation and propagation of cracks in the DBRs, the substrate was coated first with an AlN film, of thickness approximately equal to the total thickness of all the AlN layers in the epitaxially grown DBR structure. In such a configuration the AlN layers are stress-free, while the AlGaN layers are under compressive stress. Peak reflectivity higher than 99% was obtained in all samples. The design calculations and simulations of measured reflectivity data were performed using the transmission matrix method. The measured reflectivity spectra have a bandwidth of around 20% smaller than the simulated spectra, a result attributed to a combination of changes of the thickness and variations in the Al content in the quarter-wave layers.

GaInAsP/InP Distributed Reflector Lasers Consisting of Deeply Etched Vertical Gratings

Deep-etching technology, which is a very simple fabrication process and requires only one lithography and one etching step without any regrowth, is applied to realize 1.5–1.55 µm wavelength GaInAsP/InP vertical grating (VG) distributed reflector (DR) lasers, which consist of a distributed feedback (DFB) region with the VG and a high-reflectivity distributed Bragg reflector (DBR) region. First, the coupling coefficient of the VG is theoretically estimated and fundamental lasing characteristics of the VG-DR lasers are investigated. Then, the fabrication process of the VG-DR lasers and the experimentally achieved fundamental lasing characteristics are given. A threshold current of 12.4 mA and a differential quantum efficiency of 42% from the front cleaved facet are achieved for a 220-µm-long and 5-µm-wide device. A submode suppression ratio (SMSR) of 33 dB at a bias current of 2.4 times the threshold is obtained.

High finesse organic microcavities

We have constructed high-finesse light-emitting microcavities from the conjugated polymer poly( p-phenylene-vinylene), PPV, and high reflectivity distributed Bragg reflectors. The modifications to the photoluminescence and electroluminescence of the polymer arising from confinement of the photon field by these mirrors were investigated. Spontaneous emission enhancements of 2 orders of magnitude were found in the forward direction. The total integrated quantum yield for photoluminescence from PPV in such devices was 8.5 ± 1%. Stimulated emission from these devices was also investigated using short-pulse laser excitation. The emission line width narrowed by a factor of 2 above the lasing threshold. Short-term polarisation memory effects were observed as the life time of the excited states were reduced by coupling to stimulated emission. The lasing threshold for pulsed excitation was found to have been reduced by an order of magnitude, to 15 μJ/cm 2, compared to previous studies on microcavities incorporating metal mirrors. Findings were complemented by simulations of the optical fields using multi-layer stack theory and transfer matrices.

ZnSe/ZnS Distributed Bragg Reflectors in the Blue Region Grown on (311)B GaAs Substrates

High-reflectivity distributed Bragg reflectors (DBRs) in the blue region consisting of II–VI semiconductors were grown on (311)B GaAs substrates for the first time using metalorganic vapor phase epitaxy. ZnSe/ZnS alternative layers were used as the DBR. To realize atomically flat surfaces, the conditions of the thermal cleaning and the growth of ZnSe and ZnS on (311)B GaAs surfaces were investigated using atomic force microscopy and X-ray diffraction. The supply of sufficient organic As flow during the thermal cleaning led to extremely-flat (311)B GaAs surfaces, and it was attributed to the suppression of As desorption from (311)B surfaces. Through the examination of the growth conditions for ZnSe and ZnS layers on (311)B GaAs substrates, a DBR with a high reflectivity was fabricated. The maximum reflectivity of the ZnSe/ZnS DBR grown on the (311)B GaAs substrate, measured at room temperature, was 94.5% with only 10 periods at a wavelength of 468 nm, which is in good agreement with the calculated reflectivity of 94.8%.

Organic–inorganic dielectric multilayer systems as high reflectivity distributed Bragg reflectors

Unprecedentedly high reflectivity distributed Bragg reflectors consisting of fluorocarbon polymer (CFx) and high refractive index inorganic oxide multilayers have been fabricated by means of assisted ion-beam sputtering at room temperature. Multilayer stacks consisting of a CFx/TiOx and CFx/HfOx pairs exhibit reflectivities larger than 98% in the infrared and ultraviolet spectral region, respectively. The superior wide tunability and the high efficiency, with respect to inorganic–inorganic multilayers is due to the exceptionally low refractive index of the fluorcarbon polymer (n≃1.35) on a very large spectral region (300–2000 nm).

Isotype heterojunctions with flat valence or conduction band

We show that isotype heterojunctions with a perfectly flat majority or minority carrier band edge can be realized by modulation doping of arbitrary continuous alloy grading. The required impurity distribution is obtained analytically from the knowledge of the compositional grading and band structure parameters in the grading. This analytic relationship is exact for heterojunctions in which the grading fields are negligible in comparison with the atomic fields. We illustrate the design of flat valence-band heterojunctions for application in high-reflectivity low-resistance distributed Bragg reflectors for vertical-cavity lasers. The presented formalism enables the design of isotype heterojunctions with arbitrary band-edge profiles.

Wide-bandwidth distributed Bragg reflectors using oxide/GaAs multilayers

The authors have designed and fabricated a wide bandwidth high reflectivity distributed Bragg reflector (DBR) using the native oxide of AlAs as the low refractive index layer and GaAs as the high refractive index layer. Results show that the refractive index of the native oxide is 1.55±0.01, the peak reflectivity of the DBR is over 99%, and the reflectivity bandwidth is ~460 nm.