High Performance Reflector Research Articles

HfO2/SiO2 nanolaminate-based composites prepared by ion beam sputtering for low-loss optics

HfO2 prepared by ion beam sputtering (IBS) is widely used as a high-refractive index material for making low-loss laser films. The challenge of a high-performance HfO2 film is to simultaneously obtain an amorphous morphology with low scattering and a stoichiometric structure with low absorption. Furthermore, nanometer-sized voids are commonly present in IBS films due to excessive oxygen and argon adsorption during deposition, which is the primary barrier to achieve a smooth surface and low optical loss of HfO2. Thin amorphous SiO2 layers were added periodically into HfO2 coatings using the IBS process to synthesize amorphous HfO2 / SiO2 nanolaminate-based composites. The resulting composites exhibited excellent comprehensive performance with a dense amorphous microstructure and a void-free smooth surface. High-temperature annealing was performed to ensure superior stoichiometry and lower absorption. However, the crystalline states and microstructure of some composites evolved during the gradual annealing. We present a detailed study of the crystallization, surface topography, and absorption evolution in HfO2 / SiO2 nanolaminates as a function of HfO2 sublayer thickness and thermal annealing temperature. Moreover, the interplay between crystallization, surface topography, and absorption is elucidated. The HfO2 / SiO2 nanolaminate with 19 thin layers maintained a dense amorphous structure with low absorption after annealing. Finally, a 1064-nm HfO2 / SiO2 nanolaminate-SiO2 high-performance reflector was prepared and achieved lower absorption with a smooth surface after annealing, which demonstrated the great potential of the HfO2 / SiO2 nanolaminates for considerably improving optical loss.

Hf1-xSixO2 Nanocomposite Coatings Prepared by Ion-Assisted Co-Evaporation Process for Low-Loss and High-LIDT Optics.

Hf1−xSixO2 nanocomposites with different SiO2 doping ratios were synthesized using an ion-assisted co-evaporation process to achieve dense amorphous Hf1−xSixO2 coatings with low loss and a high laser-induced damage threshold (LIDT). The results showed that the Hf1−xSixO2 nanocomposites (x ≥ 0.20) exhibited excellent comprehensive performance with a wide band gap and a dense amorphous microstructure. High-temperature annealing was carried out to ensure better stoichiometry and lower absorption. Precipitation and regrowth of HfO2 grains were observed from 400 °C to 600 °C during annealing of the Hf0.80Si0.20O2 nanocomposites, resulting in excessive surface roughness. A phenomenological model was proposed to explain the phenomenon. The Hf1−xSixO2 nanocomposites (x = 0.3 and 0.4) maintained a dense amorphous structure with low absorption after annealing. Finally, a 1064-nm Hf0.70Si0.30O2/SiO2 high-performance reflector was prepared and achieved low optical loss (15.1 ppm) and a high LIDT (67 J/cm2).

Development of broad-band high-reflectivity multilayer film for positron emission tomography system

The use of non-ideal reflective materials and low-precision manual manufacturing technologies is a fundamental technical obstacle blocking the positron emission tomography (PET) systems from achieving better performances. We propose to address that long-standing obstacle with advanced multilayer dielectric coating technologies. We designed an broad-band multilayer high-reflectivity (HR) film that can be coated directly on the surface of ultra-precision polished lutetium-yttrium oxyorthosilicate (LYSO) scintillators. The film consists of 48 layers of TiO2/SiO2/HfO2 which are deposited on LYSO scintillator crystal alternately using the electron beam evaporation method. The overall thickness of the HR film is about 3μm. The HR film combines 3 quarter wavelength reflective films, with the central wavelengths of 365 nm, 430 nm and 570 nm respectively, to match the emission spectrum of the LYSO scintillator. The optical experimental results show that the HR film achieved an excellent average reflectivity of 99.50% at 8̂ incident angle for incident lights with wavelengths between 360 to 620 nm. The average reflectivity at 60̂ incident angle is higher than 90%. The results of the hardness experiments and the adhesive strength experiments show that the HR film has an excellent mechanical strength. The HR coating technology developed in this study is very attractive because it allows to “print” high-performance reflectors on a scintillator directly with high-precision, instead of manually gluing reflective films on the scintillator. Thus, we conclude that the HR film provides a viable solution to the long standing technical bottleneck that limits the development of high-performance detectors for advanced PET imaging.

Omnidirectional Reflector based on Transparent Oxide Materials for Light Extraction Enhancement of Organic and Inorganic Solid-state Light Emitting Devices

High performance reflector is significant structure for efficiency improvement of organic and inorganic solid-state light emitting devices because a large difference in the refractive index between the light source and air leads to multiple internal reflections at the reflector. Then we focused on wide angular light incidence and designed a total internal omnidirectional reflector (TI-ODR) based on transparent oxide layers. The simulation results showed that light extraction structure with TI-ODR gives 17% and 4% efficiency enhancement compared to typical Ag based mirror for organic LED (n=1.8) and inorganic LED (n=2.4), respectively. Then the TI-ODR on blue LED structure was fabricated and that showed enhancement effect of about 8%. The layers are composed of conductive layer for current distribution, low-index thick layer for total internal reflection (TIR) of wide angular light, and high-index layer for the reinforcement of adhesion of layers.

A silicon-based wideband multisubpart profile grating reflector

In this paper, a multilayer configuration high-performance reflector utilizing a multisubpart profile grating structure is presented. Rigorous coupled-wave analysis (RCWA) for multilayered grating is adopted to design and optimize the structure. And experimental verification of theoretical design is accomplished. It is shown that, for transverse magnetic (TM) polarization, over a broadband spectrum from 1.65 to 1.72μm, the reflector experimentally demonstrates combined merits of high reflectivity (>97%) and good angular insensitivity of about 24.6°. Moreover, it is found by RCWA that the reflector proposed here has a reasonably good tolerance of fabrication error, which provides a favorable advantage in the fabrication process.

Optimization of the UWB Feed Antenna Position in Reflector Applications

In reflector system design, achieving high stability of phase center position with changes in frequency in reflector feed antennas is highly desired. However, obtaining highly stable phase center is not possible for UWB feed antennas, specially for planar ones. Thus, an optimum positioning for the UWB feed antenna should be defined. Optimization of the positioning of the feed antenna is essential since this process lowers resulting phase error losses significantly. In this work, a novel method for optimizing the UWB feed position of a prime focus reflector antenna from phase and amplitude recordings of the measured radiated field is introduced. An automatic and fast design procedure, based on Genetic Algorithms, is described. The proposed methodology has been numerically and experimentally assessed. The procedure is introduced by an application example to one of the most commonly used UWB feed antennas in high-performance reflector antenna systems: Linear Tapered Slot Antenna (LTSA). A LTSA antenna operating in 6–25 GHz frequency band is designed and manufactured. The performance of the method is quantified in terms of its phase error losses inE- andH-planes for reflector illumination.

Diffused Reflection Performance of Microcellular Foamed Polyethylene Naphthalate

Due to the increasing demand for high-performance diffuse reflectors in applications including LCD displays and lighting, many studies on the manufacturing of high-performance reflectors have been conducted. Microcellular foamed plastics are characterized as having a structure with a cell size of 10 µm or less along with a cell density of at least 109 cells/cm3. Since the structural characteristics of these materials were determined to result in diffused reflection, the use of polymers as diffuse reflectors has been studied. In this study, a diffuse reflector specimen was manufactured by applying a microcellular foaming process to polyethylene naphthalate (PEN). The optimum foaming conditions of PEN were determined through preliminary experiments using the Batch process and the specimen was manufactured using supercritical CO2. Additionally, to improve the low productivity, which is one of the disadvantages of the Batch process, additional specimens were manufactured while varying the saturation conditions. Through observation of the cross-sections of the specimens, the cell morphology depending on the process conditions was confirmed and then, the diffusion reflectivity was measured.

Fano-Resonance Photonic Crystal Membrane Reflectors at Mid- and Far-Infrared

We report here single-layer ultracompact Fano-resonance photonic crystal membrane reflectors (MRs) at mid-infrared (IR) and far-IR (FIR) bands, based on single layer crystalline Si membranes. High-performance reflectors were designed for surface-normal incidence illumination with center operation wavelengths up to the 75-μm FIR spectral band. Large-area patterned MRs were also fabricated and transferred onto glass substrates based on membrane transfer processes. Close to 100% reflection was obtained at the ~ 76-μm spectral band, with a single-layer Si membrane thickness of 18 μm. Such Fano-resonance-based membranes reflectors offer great opportunities for high-performance ultracompact dielectric reflectors at IR and THz regions.

Resonant transport and giant persistent currents in double-asymmetric rings

Occurrence of resonant transport due to Fabry-Perot like effect through cascade double asymmetric rings is proposed. We find that high performance reflectors with wide bands can be generated by two asymmetrical open rings with an arm ratio of 1/3. At the resonances, giant persistent currents exist in both rings of the system. The resonance energy is variant and dependent on the length of the connected wire. However, the confined space of the system at the resonance is not equal to the integral multiple of a half-wavelength of traditional Fabry-Perot etalons.

Investigation of an X-band over-moded bent circular waveguide TM01-HE11 mode converter

In the application of low loss beam waveguides and high performance reflector antennas, high power microwave should be converted to quasi-Gaussian HE11 mode, to decrease side lobe level and increase feed coefficient. Based on the fact that TM01 mode converts to TE11 and TM11 mode at the same time in bent overmode circular waveguide, a TM01 to HE11 mode converter was proposed in this paper, which can convert TM01 mode to HE11 mode directly using a bent circular waveguide, the converter has higher power compacity than corrugated TE11-HE11 mode converter and more compact than TE11-TM01 mode converter added with smooth wall TE11-HE11 mode converter. The geometric parameters of the convertor were analyzed and optimized by using coupled mode theory and Taguchi method, to output the target modes content, 80% TE11 and 20% TM11 modes, which are primarily composed of HE11 mode. The optimized converter was analyzed using full electro-magnetic simulation method, the scalar Gauss ratio of the output beam is higher than 99% from 9.05 GHz to 9.8 GHz, the power capacity is as high as 4.5 GW.

A generalized diffraction synthesis technique for high performance reflector antennas

Stringent requirements on reflector antenna performances in modern applications such as direct broadcast satellite (DBS) communications, radar systems, and radio astronomy have demanded the development of sophisticated synthesis techniques. Presented in the paper is a generalized diffraction synthesis technique for single- and dual-reflector antennas fed by either a single feed or an array feed. High versatility and accuracy are achieved by combining optimization procedures and diffraction analysis such as physical optics (PO) and physical theory of diffraction (PTD). With this technique, one may simultaneously shape the reflector surfaces and adjust the positions, orientations, and excitations of an arbitrarily configured array feed to produce the specified radiation characteristics such as high directivity, contoured patterns, and low sidelobe levels, etc. The shaped reflectors are represented by a set of orthogonal global expansion functions (the Jacobi-Fourier expansion), and are characterized by smooth surfaces, well-defined (superquadric) circumferences, and continuous surface derivatives. The sample applications of contoured beam antenna designs and reflector surface distortion compensation are given to illustrate the effectiveness of this diffraction synthesis technique.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Study on optimization of mechanical and electronic synthesis for the antenna structural system

Integrated optimization design of mechanics and electronics for the high performance reflector antenna is considered in this paper and an efficient synthetic method is developed. A general mathematical expression of the phase centre for reflector antenna feeds is given. A very useful formula of a mathematical relationship between the electromagnetic field and the structural deformation of the antenna is then described, which provides a good complement for Rutz's formula. Moreover, the paper submits three typically mathematical models for synthetic optimization design of mechanics and electronics (SODME) for the antenna structural system, on which a good foundation is laid for the realization of SODME in practical engineering. As a demonstration, research work is applied to the design of an 8 m antenna used in a satellite-earth station and useful results have been achieved.