Distributed Bragg Reflector Stack Research Articles

Strong Coupling of Tamm Plasmons and Fabry-Perot Modes in a One-Dimensional Photonic Crystal Heterostructure

The coupling between multiple modes in subwavelength scale has attracted a lot of attention due to the interesting physics favored by the enhancement of light-matter interaction. In this contribution, we propose a planar hybrid plasmonic-photonic crystal structure, which enables the tunable coupling between Tamm plasmon-polariton and Fabry-Perot cavity modes. Then, a strong coupling can be achieved through evoking the Tamm-mode dependence of the top-layer thickness of the hybrid structure, which is indicated by an anticrossing with a spectrum splitting of 9.298 meV. The effect enables the spatial confinement of polaritons and the formation of hybrid one-dimensional plasmon-polariton modes. In order to account for the splitting of the orthogonally polarized resonances, a detailed investigation on the TE-TM splitting is also performed through the angular-dependence analysis. We note that the value of the TE-TM splitting strongly depends on the thicknesses of dielectric layer and the distance between two distributed Bragg reflector (DBR) stacks. We show that the cavity modes are extremely sensitive to the distance between the DBR stacks and propose using this feature for strain sensing. Thus, our structure may not only offer a platform for study of interesting physics (e.g., spin optronics), but may also provide a route for high-accuracy strain sensors.

Reflectivity simulation by using transfer matrix method

Efficiency of optoelectronic devices can be enhanced by using Distributed Bragg Reflector (DBR). Distributed Bragg Reflector (DBR) is a multipair layered structure of high and low refractive indices. High reflectivity of DBR is due to constructive interference of reflected light from interfacial layers of DBR. In the present paper, reflectivity of DBR composed of II-VI compound semiconductor is simulated using Transfer Matrix Method (TMM). In this paper, reflectivity of 4, 8, 12, 16 and 20 periods for the incidence angle of 45+ is simulated. Also, the effect of interchange of constituent layers in DBR stack, on reflectivity is simulated.

Effect of optical parameters on design of highly reflecting distributed Bragg reflectors based on compound semiconductors for space applications

Comprehensive model for the design of highly reflecting distributed Bragg reflectors (DBRs) is investigated for spectral band, ranging from ultraviolet to short wavelength infrared (SWIR). The reflectivity calculation is done using thin film multiple reflection technique. The variation of reflectivity as a function of wavelength, number of grown epilayers, angle of incidence and difference between refractive indices of constituent epilayers is examined. The target wavelengths for reflectivity simulation include 1.55 and 1.3 μm for optical communication, 850 and 980 nm for infrared (IR) imaging devices and chip-scale atomic clocks, and 412 and 443 nm for UV light emitting diodes and antireflection coatings for charge-coupled devices and solar cells. The reflectivity values calculated for compound semiconductor DBRs lie between 95 and 99%. It is also inferred that the reflectivity of grown epilayers and broadness of operational spectrum increases with number of epilayers. If DBR stack is grown on lattice mismatched substrate, increment of the reflectivity is found only at single wavelength. As the angle of incidence increases from 0° to 80°, blue shift in reflectance (towards low wavelength) is also observed. The increase in reflectivity is also observed with increase in difference in refractive indices of epilayers.

Optimising porous silicon electrical properties for thermal sensing applications

Present day sensing elements used in thermo-resistive type uncooled detectors or bolometers either suffer from material incompatibility issues or have relatively poor performance in the long wave infrared region (LWIR). This work explores the potential of meso-porous silicon films as the base material for the sensing element in these bolometer applications. The performance of these highly sensitive bolometers depends on the electrical properties, specifically the electrical resistivity and temperature coefficient of resistance (TCR), of the sensing element. In this work, we explore the electrical properties of meso-porous silicon films and optimize these properties through fabrication conditions and surface treatment. We further compare the TCR values of single and multi-layer porous silicon films for use in multilayers to fabricate highly selective distributed Bragg reflectors (DBR) in LWIR detector applications. The experimental results show that a low value of resistance along with high value of TCR can be obtained by altering porosity and surface passivation in single and multi-layer porous silicon films. We measured the TCR within a three-layer porous silicon film (DBR stack) to be 6%/K, which is significantly higher than the TCR from contemporary sensing element materials. Our studies indicate that the use of meso-porous silicon films for the sensing element in bolometers can potentially overcome the performance and the material compatibility issues associated with existing technologies such as amorphous silicon and vanadium oxide in LWIR.

Study of optical and structural properties of sputtered aluminum nitride films with controlled oxygen content to fabricate Distributed Bragg Reflectors for ultraviolet A

Aluminum nitride (AlN) films with controlled oxygen content were deposited on silicon substrates, and optical properties studied with dependency on film morphology. Combinations of argon (Ar) and nitrogen (N2) gases were used in RF magnetron sputtering of an AlN target. The resulting refractive index ranging from 1.6 to 2.0 at 400 nm was tuned by controlling the sputter gas flow rate ratio of Ar and N2. The resulting refractive index is associated with density and aluminum nitride content of the thin films. Distributed Bragg Reflectors (DBRs) optimized for ultraviolet-A reflectivity were fabricated with pairs of alternating AlN thin films using an explicit combination of low-n and high-n to further investigate the thin film optical properties. The effect of structural transformation in the DBR stack on the progress of optical properties was studied. The DBRs exhibit a negligible extinction coefficient, utilizing precise control of oxygen incorporation with one sputtering target.

Comprehensive structural and optical characterization of AlAs/GaAs distributed Bragg reflector

30-pair AlAs/GaAs distributed Bragg reflector (DBR), which has 1030 nm center reflectivity, is studied extensively by means of High Resolution X-ray Diffraction (HR-XRD) and reflectivity measurements. Theta/2-Theta measurements and dynamical simulations have been done for (002), (004) and (006) planes to determine strain and thickness of AlAs and GaAs layers in the DBR stack. Reciprocal space mappings (RSMs) are measured for same planes and also for (224) plane to find out tilt and relaxation of the DBR stack. Relaxation is not observed and it is confirmed with symmetric in-plane (400) Theta/2-Theta and RSM measurements. This is a first study in the literature according to the best of our knowledge. Finally, we have shown sensitivity of high angle diffraction planes to disorders in crystal. Angle-dependent reflectivity simulations have been also done and compared with measurements. 99.99% reflectivity is obtained with 99.5 nm stop bandwidth and 482.7 nm penetration depth.

Design and fabrication of double AlGaN/GaN distributed Bragg reflector stack mirror for the application of GaN-based optoelectronic devices

In this work, a near-ultraviolet (380 nm) double Al0.2Ga0.8N/GaN distributed Bragg reflectors (DBRs) stack mirror was designed and fabricated. The double DBRs stack mirror consists of a 30-pair Al0.2Ga0.8N/GaN DBRs centered at 375 nm and a 20-pair Al0.2Ga0.8N/GaN DBRs centered at 385 nm. Our simulation results show that the method of double DBRs stack mirror design can broaden the stopband width greatly and increase the reflected angle efficiently, compared with the single Al0.2Ga0.8N/GaN DBRs mirror. In experiment, the double Al0.2Ga0.8N/GaN DBRs stack mirror and the reference Al0.2Ga0.8N/GaN DBRs mirror were grown on sapphire substrate by metalorganic chemical vapor deposition. The measured stopband width of the double DBRs stack mirror (~ 25 nm) is more than two times that of the reference DBRs mirror (~ 11 nm), which consists well with our simulation results. It is reasonable to believe that this work could provide a valuable information to obtain AlGaN/GaN DBRs with wide stopband width that can be used in the fabrication of GaN-based resonant cavity light-emitting diodes and vertical cavity surface emitting lasers.

Effect of Dielectric Distributed Bragg Reflector on Electrical and Optical Properties of GaN-Based Flip-Chip Light-Emitting Diodes.

We demonstrated two types of GaN-based flip-chip light-emitting diodes (FCLEDs) with distributed Bragg reflector (DBR) and without DBR to investigate the effect of dielectric TiO2/SiO2 DBR on optical and electrical characteristics of FCLEDs. The reflector consisting of two single TiO2/SiO2 DBR stacks optimized for different central wavelengths demonstrates a broader reflectance bandwidth and a less dependence of reflectance on the incident angle of light. As a result, the light output power (LOP) of FCLED with DBR shows 25.3% higher than that of FCLED without DBR at 150 mA. However, due to the better heat dissipation of FCLED without DBR, it was found that the light output saturation current shifted from 268 A/cm2 for FCLED with DBR to 296 A/cm2 for FCLED without DBR. We found that the use of via-hole-based n-type contacts can spread injection current uniformly over the entire active emitting region. Our study paves the way for application of DBR and via-hole-based n-type contact in high-efficiency FCLEDs.

High Reflectivity Hybrid AlGaN/Silver Distributed Bragg Reflectors for Use in the UV-Visible Spectrum

Indium-free AlGaN-based distributed Bragg reflectors (DBRs) in the UV spectrum are known to have very low reflectivities due both to the low refractive index contrast as well as limitations imposed by the critical thickness of AlGaN alloys (tensile strain of ~2.41% for AlN on GaN). Near-bandedge excitonic resonances influence the real part of AlGaN's dielectric function, which sharply increases its refractive index as the photon energy approaches the bandgap. Furthermore, heavy doping (Si: 10 20 cm -3 ) can modify the plasma frequency of AlGaN, leading to a reduction in its refractive index. Hence, judiciously choosing the high index material to exploit excitonic resonances and using heavy doping to reduce the refractive index of the low index material can enhance the index contrast and enable growth of epitaxial DBRs with higher reflectivities. We have demonstrated this technique both experimentally and by simulations for wavelengths ranging from 240 to 370 nm. Typically, over 50 epitaxial pairs are needed to achieve a mirror whose reflectivity exceeds 99%, but this can be shrunk down to 20-30 epitaxial pairs by depositing silver/aluminum underneath the epitaxial DBR stack. Silver and aluminum exhibit 90% reflectivity at the AlGaN/metal interface between wavelengths ranging from >360 to 180-670 nm, respectively. A thinner DBR stack also reduces the thermal resistance, which would allow the VCSEL to achieve higher peak output powers, and simultaneously reduce overall tensile strain.

Numerical simulation and experimental investigation of GaN-based flip-chip LEDs and top-emitting LEDs.

We demonstrate a GaN-based flip-chip LED (FC-LED) with a highly reflective indium-tin oxide (ITO)/distributed Bragg reflector (DBR) ohmic contact. A transparent ITO current spreading layer combined with Ta2O5/SiO2 double DBR stacks is used as a reflective p-type ohmic contact in the FC-LED. We develop a strip-shaped SiO2 current blocking layer, which is well aligned with a p-electrode, to prevent the current from crowding around the p-electrode. Our combined numerical simulation and experimental results revealed that the FC-LED with ITO/DBR has advantages of better current spreading and superior heat dissipation performance compared to top-emitting LEDs (TE-LEDs). As a result, the light output power (LOP) of the FC-LED with ITO/DBR was 7.6% higher than that of the TE-LED at 150mA, and the light output saturation current was shifted from 130.9 A/cm2 for the TE-LED to 273.8 A/cm2 for the FC-LED with ITO/DBR. Owing to the high reflectance of the ITO/DBR ohmic contact, the LOP of the FC-LED with ITO/DBR was 13.0% higher than that of a conventional FC-LED with Ni/Ag at 150mA. However, because of the better heat dissipation of the Ni/Ag ohmic contact, the conventional FC-LED with Ni/Ag exhibited higher light output saturation current compared to the FC-LED with ITO/DBR.

Polymer microcavities by room temperature electron-beam evaporation of TiO x and SiO x

We report on monolithic symmetric vertical microcavities exploiting two TiC x/SiO x distributed Bragg reflectors (DBRs), realized by means of electron-beam evaporation. The whole nanofabrication process is carried out without heating the chamber atmosphere, thus allowing the direct evaporation of the top dielectric mirror onto the organic layer previously spin-cast on the bottom DBR stack. The high adhesion to the substrate, the possibility of employing active media of low softness temperature, and the absence of any degradation of the emission properties of the organic compound opens the way for novel vertical emitting resonators and monolithic photo-and electroluminescent microcavities. Our mirrors exhibit a stop-band reflectivity higher than 95% and a spectral width of more than 170 nm.

Design of a 364 nm Electrically Pumped Multi-Quantum Well Continuous Wave Nitride Vertical Cavity Surface Emitting Laser

ABSTRACTShort-wavelength vertical cavity surface emitting lasers (VCSELS) are typically important for high-density optical memory and optical imaging systems. An AlGaN/GaN multiquantum well (MQW) VCSEL was designed for operation at wavelength of 364 nm. The design process consisted of careful selection of materials and optimization of the parameters for distributed Bragg reflector (DBR) stacks, active region, and calculation of the threshold current density, external quantum efficiency, and threshold carrier concentration. InGaN/AlGaN material system was selected for constructing DBR stacks in order to minimize power dissipation and to achieve high reflectivity. A reflectivity of 99% was calculated for 33 pairs in the bottom DBR stack and 97% was calculated for 30 pairs in the top DBR stack. An external quantum efficiency of 85% has been achieved through parametric optimization. Results were comparable to that of the similar structures based on the review of recent literatures.

MOCVD Growth of ZnSe/ZnS Distributed Bragg Reflectors on ZnSe(100) and GaAs(100) Substrates

The possibility to increase the reflectivity of ZnS/ZnSe distributed Bragg reflectors (DBR) grown on GaAs(100) and ZnSe(100) substrates was studied by using metallorganic chemical vapour deposition (MOCVD) technique. As a result of growth optimization, a maximum reflectivity of the DBR mirror on GaAs substrate as high as 99% was achieved at a wavelength of 478 nm. The maximum reflectivity of the 20-pair ZnS/ZnSe DBR stack on transparent ZnSe substrate was 91% at wavelength of 495 nm while the transmission coefficient was 5% at maximum reflectivity. Scattering by surface roughness limited the increase of the ZnS/ZnSe DBR mirror reflectivity in this case. The ZnCdSe/ZnSe QW structure grown on the ZnS/ZnSe DBR mirror showed intense low temperature cathodoluminescence.

Electron Beam Pumping in Nitride Vertical Cavities with GaN/ Al0.25 Ga0.75 N Bragg Reflectors

Electron beam pumped surface emitting lasers are of great interest for a variety of applications, such as Laser Cathode Ray Tubes (LCRT) in projection display technology or high power UV light sources for photolithography.Two distributed Bragg reflector (DBR) samples were grown by plasma assisted molecular beam epitaxy (PAMBE). The active regions of the samples are a GaN:Si bulk layer and a multihetero (MH) structure, respectively.Also, a separately grown single DBR stack was studied to find optical transmission and reflection properties which were compared to transfer matrix simulations.

Distributed Bragg reflectors based on AlN/GaN multilayers

A 20.5 period distributed Bragg reflector stack based on AlN/GaN has been grown on (0001) sapphire by electron cyclotron resonance plasma-assisted molecular-beam epitaxy. Peak reflectance up to 95% was observed at a wavelength of 392.5 nm. Cross-section transmission electron microscopy studies indicate that the interface of GaN-on-AlN is always sharper and more distinct than the interface of AlN-on-GaN. This is attributed to the different growth modes of AlN and GaN. When AlN grows on GaN, it tends to grow in a two-dimensional mode (Frank–van der Merwe mode) whereas GaN grows on AlN in a three-dimensional mode (Stranski–Krastanov mode). Based on these findings, the experimental reflectance data were simulated using the transmission matrix method.

Electron Beam Pumping in Nitride Vertical Cavities with GaN/ Al0.25 Ga0.75 N Bragg Reflectors

AbstractElectron beam pumped surface emitting lasers are of great interest for a variety of applications, such as Laser Cathode Ray Tubes (LCRT) in projection display technology or high power UV light sources for photolithography.Two distributed Bragg reflector (DBR) samples were grown by plasma assisted molecular beam epitaxy (PAMBE). The active regions of the samples are a GaN:Si bulk layer and a multihetero (MH) structure, respectively.Also, a separately grown single DBR stack was studied to find optical transmission and reflection properties which were compared to transfer matrix simulations.Scanning electron beam pumping at 80 K with an excitation energy of 40 keV at varying beam currents revealed luminescence emission maxima located at about 3.45 eV for the sample with the MH structure active region. Optical modes appeared for excitation powers greater than 0.85 MW/ cm2. Further increasing the excitation power density the number of modes increased and a broadening and redshift of the luminescence spectrum could be observed.Based on our experimental results, we discuss the dependence of optical parameters of the nitride vertical cavity and sample surface reactions on primary electron beam power.

A comparison of 1.55 mu m distributed Bragg reflector stacks for use in multi quantum well micro resonator modulators

We have investigated p- and n-doped distributed Bragg reflector stacks (DBRs) in several material systems in order to evaluate their use in multi quantum well (MQW) micro resonator modulators. We have shown that over 80% reflectivity can be obtained using 12 periods in a p-type Ga0.35In0.65As0.75P0.25/InP or Al0.08Ga0.39In0.53As/Al0.48In0.52As. Lattice-matched n-doped Ga0.47In0.53As/InP and Ga0.47In0.53As/Al0.48In0.52As DBRs are shown to give 98% reflectivity with only 20 periods. We have found that n doping of greater than 1*1018 cm-3 shifts the absorption edge of lattice-matched GaInAs to enable its use in 1.55 mu m DBRs.

Highly doped 1.55 µm GaxIn1-xAs/InP distributed Bragg reflector stacks

The authors have investigated 20 period lattice matched mirror stacks doped to 5*10/sup 18/ cm/sup -1/ and 10/sup 19/ cm/sup -1/ at 1.55 mu m in Ga/sub x/In/sub 1-x/As/InP and have obtained greater than 95% reflectivity over 100 nm and peak reflectivities up to 98%. >