Bioinspired Air‐Gap Self‐Rolled Bragg Reflectors

In this paper, for the first time, a novel physical structure of planar spiral inductors fabricated on a multilayered Bragg reflector is proposed. The multilayered Bragg reflector (BR) was fabricated on Si substrate. The effects of the multilayered Bragg reflector and inductor patterns on the characteristics of inductors are studied. The results show that the inductors fabricated on the Bragg reflector result in a significant improvement in terms of the S11‐parameter. This approach seems highly feasible and promising for future Si‐based RF IC applications. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 1296–1298, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21621

In this paper, we, for the first time, present the effects of the thermal annealing of the W/SiO2 multi-layer Bragg reflectors on the resonance characteristics of the ZnO-based film bulk acoustic wave resonator (FBAR) devices. In order to improve the resonance characteristics of the FBAR devices, we employed a thermal annealing process after the Bragg reflectors were formed on a silicon substrate using a radio frequency (RF) magnetron sputtering technique. As a result, the resonance characteristics of the FBAR devices were observed to strongly depend on the annealing conditions applied to the Bragg reflectors. The FBAR devices with the Bragg reflectors annealed at 400°C/30 min showed excellent resonance characteristics as compared to those with the non-annealed (as-deposited) Bragg reflectors. The newly proposed simple thermal annealing process will be very useful to more effectively improve the resonance characteristics of the future FBAR devices with multi-layer Bragg reflectors.

GaInAs/InP MQW and DBR growth for surface emitting lasers by CBE

Solidly Mounted Resonators (SMR) are alternatives to membrane resonators to realize Bulk Acoustic Wave (BAW) filters for RF communication systems. SMR decoupling from substrate, which is realized with a multi-layered Bragg reflector, has a major influence on resonator performance (quality factor, spurious modes). A finite element-boundary element (FEM-BEM) method is presented for two-dimensional SMR simulation. The substrate is described by an elastodynamic half-space Green function which is discretized and implemented in the ATILA finite element software. Results are presented for an aluminum nitride resonator with molybdenum electrodes operating at 2.14 GHz and decoupled from the substrate by a tungsten (W)/silicon oxide (SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) or a silicon nitride (SiN)/silicon oxycarbide (SiOC) Bragg reflector. When compared to one-dimensional Mason's model results, FEM-BEM computed values show a decrease of the quality factor and of the effective coupling coefficient. Lowered quality factors at parallel resonance are attributed to elastodynamic radiation into the substrate at the vicinity of the electrode boundary

We for the first time present a new fabrication technique of ZnO-based FBAR devices using a multi-layered Bragg reflector. To improve the resonance performance, 0.03?m-thick chromium (Cr)-adhesion layers were inserted into the Bragg reflector and also thermal treatments were made to the devices. At operating frequency of about 2.75 GHz, very high return loss values and quality factor (Q) were observed under an optimum thermal annealing condition.

The effects on reflectivity of a statistical variation in the thickness of layers in a multilayered Bragg reflector are studied. Analytic expressions are obtained for 〈ρ〉 and 〈ρρ*〉, the expected value of the reflection and reflectivity coefficients as a function of σ, the standard deviation in layer thickness. These expressions are then compared with values obtained using a computer routine which builds a reflector with the desired parameters and σ value, and then calculates the reflection. The results of the computer experiment are presented in the form of p(ρρ*), the probability distribution function of a statistical Bragg reflector. Finally, simple phenomenological expressions are presented for the reflectivity probability distribution.

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.

Nanowire superconducting single photon detectors (SSPDs) [1] are characterized by very high sensitivity in the near infrared (detection efficiency η up to 30%, for a dark count rate DK of few Hz), speed (up to ∼1 GHz repetition rate) and time resolution (jitter of 20 ps full width at half maximum, FWHM). They can be operated at temperatures near 4 K, so they can be packaged in cryogenic dipsticks or cryogen-free refrigerators. These features make SSPDs the most promising detectors for telecom-wavelength single-photon counting applications. The basic structure of an SSPD is a narrow (w=50 to 120 nm), thin (th∼4-10 nm) NbN superconducting nanowire folded in a meander pattern. The typical detector active area (i.e. the size of the pixel) is Ad=10 × 10 µm2 (which allows an efficient coupling with the core of optical fibers at telecom wavelengths) with filling factor (f, the ratio of the area occupied by the superconducting meander to the device total area) ranging from 40% to 60%. The meanders are embedded in a 50 Ω coplanar transmission line. At present, the SSPD detection efficiency is limited by its absorbance (α, the ratio of the number of photons absorbed in the nanowire to the number of incident photons on the device active area). Indeed, it has been shown that in the classic front-illumination configuration α cannot exceed 30%. Our approach to increase α consists in integrating SSPDs with advanced optical structures such as distributed Bragg reflectors (DBRs) and optical waveguides. This requires to transfer the challenging SSPD technology (i.e. the deposition of high-quality few-nm thick NbN films and the nano-patterning by electron beam lithography, EBL) from the usual comfortable substrates, i.e. sapphire and MgO, which are known to allow the deposition of few-nm thick NbN films of excellent quality, to an optical substrate like GaAs, on which DBRs and waveguides can be easily obtained. Our first task was then to optimize a process for the deposition of high-quality few-nm thick NbN films on GaAs and AlAs/GaAs-based DBRs. Because of the requirement of compatibility with GaAs, the substrate temperature used for the depositions is 400°C, in order to prevent As evaporation. As GaAs and DBRs are highly mismatched substrates, the deposition parameters were first optimized with respect to the superconducting properties of NbN films on MgO substrates, which allow the growth of high crystal quality NbN films at low temperature. This made easier to separate the influence of stoichiometry from that of microstructure. The optimized deposition parameters were then used to grow NbN films on GaAs and DBRs, under the reasonable assumption (later checked and confirmed) that changing the substrate would not produce a change in film stoichiometry, but only in its microstructure. NbN films ranging from 150nm to 3nm in thickness were then deposited on epitaxial-quality single crystal MgO, GaAs and DBRs structures. The deposition technique is the current controlled DC magnetron sputtering (planar, circular, balanced configuration) of Nb in an Ar + N2 plasma. NbN films deposited on MgO exhibit superconducting critical temperature ΤC=10 Κ, superconducting transition width ΔΤC=0.8 Κ and residual resistivity ratio RRR=R(20K)/R(300K)=0.8 for th=4 nm, which are state of the art values, proof of the excellent quality of our low-temperature deposition process. The quality of films deposited on GaAs and on DBRs is lower than that of NbN deposited on MgO, as for any thickness they systematically exhibit higher ΔΤC and lower ΤC and RRR. However, 5.5 nm-thick NbN films on GaAs still exhibit ΤC=10.7 Κ, ΔΤC=1.1 Κ and RRR=0.7, which compares with 4.5 nm thick films on MgO, making them suitable for device fabrication. To our knowledge, the growth of such high quality thin NbN films on GaAs and DBRs, has never been reported in literature. The degradation of the superconducting properties exhibited by NbN films on GaAs and DBRs was attributed to a highly defected microstructure, due both to a higher lattice misfit between NbN and GaAs and to a poorer quality of the substrate surface. Encouraging preliminary results show that the quality of these films can be improved either cleaning the GaAs/DBR substrate surface more effectively or adding an MgO buffer layer. SSPDs were fabricated on thin NbN films (th=3-7 nm) deposited under optimal conditions on MgO and GaAs by EBL and reactive ion etching. The geometrical parameters of our detectors are: Ad=5×5 µm2, w=60-200 nm, f=40%-60%. The devices were then characterized both electrically and optically. I-V curves of test structures were measured, from which it was possible to deduce important physical parameters used as figures of merit to estimate the superconducting properties of the nanowires, or for the design and the simulation of the devices. The quality of the devices fabricated on GaAs is poorer than those on MgO, most likely due to the lower quality of NbN films deposited on GaAs and to issues related to the EBL nano-patterning step. Measurements of η and of DK as a function of the bias current were performed on SSPDs fabricated on MgO and GaAs. The best performance was exhibited by a w=100 nm, f=40%, th=4 nm meander, showing η=20% and noise equivalent power NEP=10-16 W/Hz1/2 (at λ=1.3 µm and T=4.2 K), which are state of the art values. This result showed for the first time that high performance NbN SSPDs can be realized on a different substrate and from a deposition process at lower temperature than previously reported. High detection efficiencies could not be measured with SSPDs fabricated on GaAs, but it should be noted that at present only first-generation devices (fabricated on GaAs substrates of poor surface quality) have been tested. Better results are expected from devices fabricated on the improved NbN films grown on clean or MgO-buffered GaAs substrates. Although SSPDs on MgO have shown high detection efficiency, the fabrication yield of high performance detectors has to be improved. Variations of the critical current along a nanowire are responsible for the wide distribution in efficiency values of nominally identical SSPDs. In order to understand the physical origin of the nanowire constrictions (i.e. regions of suppressed superconductivity) we performed a spatially-resolved characterization of η of a long straight nanowire, followed by a high resolution SEM (scanning electron microscope) scan on its whole length. Two types of inhomogeneities were evidenced, corresponding to localized efficiency dips and peaks. The peaks likely correspond to constrictions. SEM observations did not evidence any width narrowing at the position of the efficiency peaks, which suggests that constrictions might be due to thickness or quality inhomogeneities of the film occurring during the film deposition or later in the process. On the other hand, the efficiency dips have been correlated with lithography problems discovered on SEM images. Finally, a new photon number resolving detector, the Parallel Nanowire Detector (PND), has been demonstrated, which significantly outperforms existing approaches in terms of sensitivity, speed and multiplication noise in the telecommunication wavelength range. In particular, it provides a repetition rate (80 MHz) three orders of magnitude larger than any existing detector at telecom wavelength, and a sensitivity (NEP=4.2×10-18 W/Hz1/2) one-two orders of magnitude better, with the exception of transition-edge sensors (which require a much lower operating temperature). An electrical equivalent model of the device was developed in order to study its operation. The modeling predicts a physical limit to the reset time of the PND, which is lower than initially estimated. Furthermore, the figures of merit of the device performance in terms of efficiency, speed and sensitivity were defined and their dependency on the design parameters analyzed. Additionally, we developed modeling tools to fully characterize the device and an algorithm to estimate the photon number statistics of an unknown light using the PND. The reconstruction proved to be successful only for low photon fluxes, most likely due to the limited counting capability and the poor calibration of the detector. The PND, with its high repetition rate and high sensitivity, is then suitable for measuring an unknown photon number probability distribution assuming accurate calibration and sufficient counting capability. ______________________________[1] G. N. Gol'tsman, O. Okunev, G. Chulkova, A. Lipatov, A. Semenov, K. Smirnov, B. Voronov, A. Dzardanov, C. Williams, and R. Sobolewski, Appl. Phys. Lett. 79, 705 (2001).

A distributed Bragg reflector (DBR) composed of Y2O3-doped HfO2 (YDH)/SiO2 layers with high reflectivity spectrum centered at a wavelength of ∼240 nm is fabricated using radio-frequency magnetron sputtering. Before the DBR deposition, optical properties for a single layer of YDH, SiO2, and HfO2 thin films were studied using spectroscopic ellipsometry and spectrophotometry. To investigate the performance of YDH as a material for the high refractive index layer in the DBR, a comparison of its optical properties was made with HfO2 thin films. Due to larger optical bandgap, the YDH thin films demonstrated higher transparency, lower extinction coefficient, and lower absorption coefficient in the UV-C regime (especially for wavelengths below 250 nm) compared to the HfO2 thin films. The fabricated YDH/SiO2 DBR consisting of 15 periods achieved a reflectivity higher than 99.9% at the wavelength of ∼240 nm with a stopband of ∼50 nm. The high reflectivity and broad stopband of YDH/SiO2 DBRs will enable further advancement of various photonic devices such as vertical-cavity surface-emitting lasers, resonant-cavity light-emitting diodes, and resonant-cavity photodetectors operating in the UV-C wavelength regime.

Tunable semiconductor lasers with narrow spectral linewidths at specific wavelengths are of interest for both communication and spectroscopy applications. In communications, narrowlinewidth lasers are necessary to reduce optical dispersion, enabling high-speed data rates. Such lasers can also be used for spectroscopic applications that require tuning to the specific wavelengths of an atomic transition. The optical pumping of a cesium transition at 852nm is of particular interest for applications like optical inertial guidance systems. Both types of applications require lasers that operate with a single optical mode and that exhibit spectral linewidths narrower than 1MHz. Most edge-emitting semiconductor lasers, however, have multiple longitudinal modes spaced closely together. Incorporating a distributed Bragg reflector (DBR) or distributed feedback (DFB) grating selects a single longitudinal mode, which allows the laser to operate in a single spectral mode. DBR and DFB gratings are typically located at the interface between the core and cladding of a laser to provide the necessary feedback for narrow-linewidth performance. However, locating the grating between the core and cladding requires epitaxial regrowth, which is difficult for devices with AlGaAs barriers due to the rapid oxidation of Al-containing compounds. Surface-grating ridge-waveguide DBR lasers have been developed as a method for achieving narrow-linewidth lasers with a single epitaxial-growth step. By incorporating an asymmetric cladding, one can reduce the etch depth and form first-order gratings in the DBR section. 3 These devices exhibit narrow-linewidth operation with a minimum spectral linewidth of 36kHz, as determined by the self-heterodyning measurement technique. We recently reported fabricating narrow-linewidth 852nm asymmetric-cladding ridge-waveguide DBRs with firstorder gratings in the AlGaAs/GaAs material system. Figure 1. Scanning electron micrographs of the (a) top view and (b) cross section of the first-order distributed Bragg reflector (DBR) grating. The schematic diagram shows structure of the asymmetriccladding DBR laser diode.

The effects of thermal annealing of W/SiO2 multilayer Bragg reflectors on the resonance characteristics of ZnO-based film bulk acoustic resonator (FBAR) devices are presented for the first time. The resonance characteristics could be significantly improved due to thermal annealing. FBAR devices with Bragg reflectors annealed at 400°C/30 min show excellent resonance characteristics in terms of return loss and Q-factor.

Numerical investigations into a THz-driven dielectric accelerator with a Bragg reflector

Free-standing multilayer distributed Bragg reflectors (DBR) porous silicon dielectric mirrors, prepared by electrochemical etching of crystalline silicon using square wave currents are treated with polystyrene to produce flexible, stable composite materials in which the porous silicon matrix is covered with caffeine-impregnated polystyrene. Optically encoded DBR PSi/polystyrene composite films retain the optical reflectivity. Optical characteristics of DBR PSi/polystyrene composite films are stable and robust for 2 hrs in a pH=7 aqueous buffer solution. The appearance of caffeine and change of DBR peak were simultaneously measured by UV-vis spectrometer and Ocean optics 2000 spectrometer, respectively.

Absorption loss influence on optical characteristics of multilayer distributed Bragg reflector: wavelength-scale analysis by the method of single expression

Comparisons of InP/InGaAlAs and InAlAs/InGaAlAs distributed Bragg reflectors grown by metalorganic chemical vapor deposition