Substrate Lens Research Articles

Terahertz Direct Detection in ${\hbox{YBa}}_{2}{\hbox{Cu}}_{3}{\hbox{O}}_{7}$ Microbolometers

A high sensitivity broadband terahertz direct detector based on YBa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sub> high-Tc superconductor microbolometers is presented. At 77 K, the responsivity of the spiral antenna-integrated microbolometers (1.5 μm ×1.5 μm) is 190 V/W, referenced to the input of the silicon substrate lens, across the frequency range of 330 GHz-1.63 THz in a single device. The response time is approximately 300 ps. Using a room temperature readout, we measure an optical noise equivalent power (NEP) of 20 pW/Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sup> (readout noise limited) for modulation frequencies ranging from 500 Hz to 100 kHz.

ULTRAWIDEBAND ANTENNA EXCITED BY A PHOTOMIXER FOR TERAHERTZ BAND

An end-flre linear tapered slot antenna is presented in the terahertz band. The operation frequency goes from 0.6THz to 2THz, with symmetric radiation patterns from 1THz to 1.7THz, and gains up to 15dB. The gaussicity of the beam is over 80% in the whole operation band and an e-ciency at 1THz of 85%. This antenna gives enough directivity avoiding the use of a typical substrate lens, which introduce 30% of losses. This new design is a good candidate for new applications in the THz range in Radioastronomy and Imaging.

Strong interference enhancement of terahertz emission from a photoexcited semiconductor surface

To enhance terahertz emission from an optically excited semiconductor surface, we propose to sandwich a thin (as compared to the terahertz wavelength) semiconductor layer between a dielectric hyperhemispherical lens and metal substrate. The layer is excited through the lens. The substrate provides constructive interference of terahertz waves emitted to the lens directly from the layer and reflected by the substrate. The lens outcouples terahertz radiation into free space. For InAs layer sandwiched between MgO (or sapphire) lens and metal substrate, our theory predicts order of magnitude increase in the terahertz yield as compared to the previous schemes of terahertz emission from semiconductor surfaces.

Influence of substrate configuration on the angular response pattern of infrared antennas

The far-field angular response pattern for dipole antenna-coupled infrared detectors is investigated. These devices utilize an asymmetric metal-oxide-metal diode that is capable of rectifying infrared-frequency antenna currents without applied bias. Devices are fabricated on both planar and hemispherical lens substrates. Measurements indicate that the angular response can be tailored by the thickness of the electrical isolation standoff layer on which the detector is fabricated and/or the inclusion of a ground plane. Electromagnetic simulations and analytical expressions show excellent agreement with the measured results.

A structure for enhanced terahertz emission from a photoexcited semiconductor surface

A structure that can provide enhancement of terahertz emission from a semiconductor surface excited with femtosecond laser pulses is proposed. The structure consists of a semiconductor layer on a Si substrate with metal coating on the upper surface of the layer and a Si lens attached to the bottom of the substrate. The semiconductor is excited through a hole in the coating and emits terahertz radiation through the substrate lens. We demonstrate theoretically that the proposed structure can increase the terahertz yield by orders of magnitude as compared to the previously used schemes of terahertz emission from a semiconductor surface.

A study of coating mechanical and optical losses in view of reducing mirror thermal noise in gravitational wave detectors

Mirror coatings play a crucial role in the performance of laser interferometers devoted to gravitational wave detection such as Virgo and LIGO. Mechanical losses in the coating material limit the sensitivity of the detectors due to the associated mirror thermal noise. The absorption of light in the coating induces a thermal lens in the mirror substrate which reduces the quality of the optical interference and requires sophisticated thermal compensation systems. This paper describes the work ongoing at LMA in order to reduce mechanical losses and optical absorption in the coating. The results obtained by doping Ta2O5 layers and testing different high-index materials are described. Finally the performances of different potential coatings are compared and the results obtained with a 40 kg mirror are reported. Titania doped Ta2O5 shows mechanical losses of 2 × 10−4 and absorption below 0.5 ppm. Nb2O5 appears to be the best competitor from the thermal noise point of view but it has an optical absorption four to five times larger.

A full electromagnetic analysis of a filter substrate lens for spatiotemporal terahertz pulse shaping

A novel filter substrate lens for THz pulse is presented, which could efficiently couple the THz radiation in a desired frequency range into free space. The band-pass and band-stop filter lens is designed and studied by difference time method. The results show that the design is well supported by the theoretical analyses.

A New Low Loss Rotman Lens Design Using a Graded Dielectric Substrate

In this paper, a new low loss graded dielectric Rotman lens design is presented. The graded dielectric allows for the enhanced focusing of the energy between the beam ports and the array ports of the lens, thereby minimizing the energy spill-over to the Rotman lens sides. New lens design equations are presented, which account for the change in effective wavelength and ray bending within the lens. The graded dielectric within the lens was fabricated by forming a periodic lattice of voids, with a variable density, in the lens substrate. Compared to a conventional Rotman lens, the new graded dielectric Rotman lens shows a measured insertion loss improvement of up to 2.1 dB. In addition, the degradation in peak power at the extreme scan angles is improved from 1.5 to 0.7 dB.

A 220 GHz Single-Chip Receiver MMIC With Integrated Antenna

This letter presents the design and characterization of a 220 GHz microstrip single-chip receiver monolithic microwave integrated circuit (MMIC) with an integrated antenna in a 0.1 mum GaAs metamorphic high electron mobility transistor technology. The receiver MMIC consists of a novel slot-square substrate lens feed antenna, a three-stage low noise amplifier, and a sub-harmonically pumped resistive mixer. The receiver MMIC is mounted on a 12 mm silicon substrate lens which focuses the radiation from the calibration loads to the on-chip antenna through an opening in the backside metallization of the MMIC. The double sideband noise figure of this quasioptical receiver is as low as 8.4 dB (1750 K) at 220 GHz including the losses in the antenna and in the lens. To the best of the authors' knowledge, this work demonstrates the highest integration level versus operating frequency for a MMIC ever published, regardless of technology.

Synthesis of novel UV-curable difunctional thiourethane methacrylate and studies on organic–inorganic nanocomposite hard coatings for high refractive index plastic lenses

This study relates to the development of ultraviolet (UV)-curable, organic–inorganic nanocomposite hard coatings for plastic lens substrates, especially for polythiourethane (PTU) and polycarbonate (PC). Novel difunctional thiourethane methacrylate (mercaptoethylsulfide-thiourethane methacrylate: coded MES-TUMA and isophorone diisocyanate-mercaptoethylsulfide-thiourethane methacrylate: coded IPDI-MES-TUMA) was synthesized to enhance the adhesive strength for PTU. On the basis of IR, 1H NMR, electron spray ionisation-mass spectrometry (ESI-MS) analysis and gel permeation chromatography (GPC), the expected structures were confirmed. These difunctional thiourethane methacrylates were easily mixed with multifunctional urethane acrylate, surface-modified ZrO 2–TiO 2 nanoparticles and photoinitiator in coating formulations. The UV-cured organic–inorganic nanocomposites were very useful as hard coatings for high refractive index plastic lenses such as PTU and PC.

Mechanism of Small Heat Shock Protein Function in Vivo: A KNOCK-IN MOUSE MODEL DEMONSTRATES THAT THE R49C MUTATION IN αA-CRYSTALLIN ENHANCES PROTEIN INSOLUBILITY AND CELL DEATH

alphaA-crystallin (Cryaa/HSPB4) is a small heat shock protein and molecular chaperone that prevents nonspecific aggregation of denaturing proteins. Several point mutations in the alphaA-crystallin gene cause congenital human cataracts by unknown mechanisms. We took a novel approach to investigate the molecular mechanism of cataract formation in vivo by creating gene knock-in mice expressing the arginine 49 to cysteine mutation (R49C) in alphaA-crystallin (alphaA-R49C). This mutation has been linked with autosomal dominant hereditary cataracts in a four-generation Caucasian family. Homologous recombination in embryonic stem cells was performed using a plasmid containing the C to T transition in exon 1 of the cryaa gene. alphaA-R49C heterozygosity led to early cataracts characterized by nuclear opacities. Unexpectedly, alphaA-R49C homozygosity led to small eye phenotype and severe cataracts at birth. Wild type littermates did not show these abnormalities. Lens fiber cells of alphaA-R49C homozygous mice displayed an increase in cell death by apoptosis mediated by a 5-fold decrease in phosphorylated Bad, an anti-apoptotic protein, but an increase in Bcl-2 expression. However, proliferation measured by in vivo bromodeoxyuridine labeling did not decline. The alphaA-R49C heterozygous and homozygous knock-in lenses demonstrated an increase in insoluble alphaA-crystallin and alphaB-crystallin and a surprising increase in expression of cytoplasmic gamma-crystallin, whereas no changes in beta-crystallin were observed. Co-immunoprecipitation analysis showed increased interaction between alphaA-crystallin and lens substrate proteins in the heterozygous knock-in lenses. To our knowledge this is the first knock-in mouse model for a crystallin mutation causing hereditary human cataract and establishes that alphaA-R49C promotes protein insolubility and cell death in vivo.

Influence of frozen capillary waves on contact mechanics

Free surfaces of liquids exhibit thermally excited (capillary) surface waves. We show that the surface roughness which results from capillary waves when a glassy material is cooled below the glass transition temperature can have a large influence on the contact mechanics between the solids. The theory suggest a new explanation for puzzling experimental results [L. Bureau, T. Baumberger and C. Caroli, European Physical Journal E19, 163 (2006)] about the dependence of the frictional shear stress on the load for contact between a glassy polymer lens and flat substrates. It also lend support for a recently developed contact mechanics theory.

Design and optimisation of axisymmetric millimetre-wave shaped lens antennas with directive, secant-squared and conical beams

The paper investigates the optimisation of rotationally-symmetric shaped dielectric substrate lenses at millimetre wave frequencies. The aim is to determine the best lens profile that complies with an arbitrary desired far-field power template. The design procedure is briefly described in three main steps. First, determination of the starting lens shape using an analytical geometrical optics (GO) formulation; secondly computation of the radiation characteristics of the lens using a hybrid GO-PO (physical optics) technique accounting for diffraction effects and, thirdly, iterative optimisation of trial lens profiles using a local minimisation procedure based on a multidimensional conjugate gradient method (M-CGM). The characteristics of the optimisation algorithm are then studied numerically, in terms of convergence speed and analysis of sensitivity. To this end, the authors design, discuss and compare several shaped lenses radiating directive, secant-squared and conical beams. These various examples demonstrate very good performance and promising potentialy of optimised lens antennas for shaped-beam applications.

A Novel Formulation for the Modification of Transition Metal Oxide Surfaces for Antifogging Optical Application

Formation of coatings with combined properties of being superhydrophilic / superhydrophobic is of immense applications in the field of optics. In our present work, we developed a hydrophilic, antifogging, transparent, and stable coating by employing an anionic electrolyte, Tiron and dip-coating of organic-inorganic mixture. One of the attempts was to adhere Tiron on metal oxide surface by chelating effect, and the other one was to apply an organicinorganic coating on glass or lens substrate. The latter was formulated by mixing appropriate ratios of organic silanol coupling agents and diacrylates component for strong adhesion to the substrate, and inorganic component of a hybrid mixture of silica, titania and Tiron for the antifogging behavior. The surface chemistry of the coated films was studied by taking ATR-IR measurements, and the wetting behavior was characterised by contact angle measurement

Ultraprecision Machining Characteristics of Poly-Crystalline Germanium

Germanium is an excellent infrared optical material. On most occasions, single-crystalline germanium is used as optical lens substrate because its homogeneous structure is beneficial for fabricating uniform optical surfaces. In this work, we attempt to use poly crystals as lens substrates instead of single crystals, which may lead to a significant reduction in production cost. We conducted ultraprecision cutting experiments on poly-crystalline germanium to examine the microscopic machinability. The crystal orientations of specific crystal grains were characterized, and the machining characteristics of these crystal grains including surface textures, cutting forces, and grain boundary steps were investigated under various machining conditions. It was possible to produce uniformly ductile-cut surfaces cross all crystal grains by using an extremely small undeformed chip thickness (∼ 80nm) under negative tool rake angles (∼ -45°). This work indicates the possibility of fabricating high-quality infrared optical components from poly-crystalline germanium.

Performance of reduced size substrate lens antennas for Millimeter-wave communications

This paper presents the theoretical performance (input impedance, -10 dB return-loss bandwidth, radiation patterns and surface efficiencies) of reduced size substrate lenses fed by aperture-coupled microstrip patch antennas. The diameter of the extended hemispherical homogeneous dielectric (/spl epsiv//sub r,lens/) lenses varies between one and five wavelengths in free-space, in order to obtain radiating structures whose directivity is comprised between 10 and 25 dB. A lot of configurations of lenses are investigated using the finite-difference time-domain methods technique and compared in the 47-50 GHz band as a function of their diameter, extension length and dielectric constant. In particular, the analysis of internal reflections-in time and frequency domains-shows that the latter have potentially a strong influence on the input impedance of small lens antennas, even for low values of /spl epsiv//sub r,lens/(2.2), whereas the usual limit (beyond which anti-reflection coatings are required) is /spl epsiv//sub r,lens/=4. We also demonstrate that the diffraction limit of reduced size lenses is reached for extension lengths varying between 50% and 175% of the extension of synthesized ellipses, depending on the lens material and diameter. Finally, we show that superdirective structures with surface efficiencies reaching 250% can be obtained with small lens diameters, justifying the interest in reduced size lens antennas.

A new accurate design method for millimeter-wave homogeneous dielectric substrate lens antennas of arbitrary shape

The synthesis and the optimization of three-dimensional (3-D) lens antennas, consisting of homogeneous dielectric lenses of arbitrary shape and fed by printed sources, are studied theoretically and experimentally at millimeter(mm)-wave frequencies. The aim of the synthesis procedure is to find a lens profile that transforms the radiation pattern of the primary feed into a desired amplitude shaped output pattern. This synthesis problem has been previously applied for dielectric lenses and reflectors. As far as we know, we propose, for the first time, to adapt and implement it for the design of substrate lens antennas. The inverse scattering problem is solved in two steps. In the first one, the geometry of the 3-D lens is rigorously derived using geometrical optics (GO) principles. The resulting second-order partial-differential equation is strongly nonlinear and is of the Monge-Ampe/spl grave/re (M.A) type. The iterative algorithm implemented to solve it is described in detail. Then, a surface optimization of the lens profile combined with an analysis kernel based on physical optics (PO) is performed in order to comply with the prescribed pattern. Our algorithms are successfully validated with the design of a lens antenna radiating an asymmetric Gaussian pattern at 58.5 GHz whose half-power beamwidth equals 10/spl deg/ in H plane and 30/spl deg/ in E plane. The lens is illuminated by a microstrip 2/spl times/2 patch antenna array. Two lens prototypes have been manufactured in Teflon. Before optimization, the measured radiation patterns are in very good agreement with the predicted ones; nevertheless, the -12 dB side lobes and oscillations appearing in the main lobe evidence a strong difference between the desired and measured patterns. This discrepancy is significantly reduced using the optimized lens.

Ultraprecision Machining Characteristics of Poly-crystalline Germanium(Ultra-precision machining)

Germanium is an excellent infrared optical material. On most occasions, single-crystalline germanium is used as optical lens substrate because its homogeneous structure is beneficial for fabricating uniform optical surfaces. In this work, we attempt to use poly crystals as lens substrates instead of single crystals, which may lead to a significant reduction in production cost. We conducted ultraprecision cutting experiments on poly-crystalline germanium to examine the microscopic machinability. The crystal orientations of specific crystal grains were characterized, and the machining characteristics of these crystal grains including surface textures, cutting forces, and grain boundary steps were investigated under various machining conditions. It was possible to produce uniformly ductile-cut surfaces cross all crystal grains by using an extremely small undeformed chip thickness (〜80nm) under high negative tool rake angles (〜45°). This work indicates the possibility of fabricating high-quality infrared optical components from poly-crystalline germanium.

A Plane-Wave Beam-Steering Lens Design Using Microstrip Switched-Line Phase Shifters

A compact beam-steering lens design appropriate for millimeter-wave and submillimeter-wave applications is experimentally verified with an X-band test model. The lens achieves coupling to plane-wave beams through arrays of patch antennas placed on its two outer surfaces. The isolation between input and output antennas is accomplished by inserting a metal ground plane in the middle of the lens. The two closest patch antennas on the front and the backside are connected together with microstrip circuits that include switched-line phased shifters and interconnecting vias through the lens substrate. Three different X-band 100-element plane-wave microstrip lenses that use passive delay lines instead of actual phase-shifters were fabricated to successfully demonstrate the beam-steering angles of 20 and 40 degrees. From a separate waveguide measurement on the unit-cell element only, the insertion loss of the lens was estimated to be approximately 3.5 dB with bandwidth of 2% at 10 GHz.

A V-band quasi-optical GaAs HEMT monolithic integrated antenna and receiver front end

A single-chip monolithic integrated V-band folded-slot antenna with two Schottky-barrier diodes and a local oscillator source is developed as a quasi-optical receiver for the first time. The monolithic microwave integrated circuit consists of a voltage-controlled oscillator (VCO), a coplanar waveguide (CPW)-to-slotline transition, a low-pass filter, a folded-slot antenna, and a 180/spl deg/ single balanced mixer. The chip is fabricated based on the 0.15-/spl mu/m GaAs high electron-mobility transistor technology and the overall chip size is 3/spl times/1.5 mm/sup 2/. A finite-difference time-domain method solver is also developed for analyzing the embedded impedance characteristics of the folded-slot antenna to design the mixer. The chip is placed on an extended hemispherical silicon substrate lens to be a quasi-optical receiver. The performance of the receiver is verified by experimental measurements. The VCO has achieved a tuning range from 61.9 to 62.5 GHz and approximately 9.3-dBm output power. The CPW-to-slotline transition has bandwidth from 50 to 70 GHz. The mixer results in 15-dB single-sideband conversion loss and the receiving patterns of the IF power are also measured.