Infrared antennas coupled to lithographic Fresnel zone plate lenses

Several designs for Fresnel zone plate lenses (FZPLs) to be used in conjunction with antenna-coupled infrared detectors have been fabricated and tested. The designs comprise square and circular FZPLs with different numbers of Fresnel zones working in transmissive or reflective modes designed to focus infrared energy on a square-spiral antenna connected to a microbolometer. A 163x maximum increase in response was obtained from a 15-zone circular FZPL in the transmissive mode. Sensor measurements of normalized detectivity D* resulted in a 2.67x increase with FZPLs compared with measurements made of square-spiral antennas without FZPLs. The experimental results are discussed and compared with values obtained from theoretical calculations.     

Polypyrrole-derived N-doped carbon nanoribbon for broadband microwaves absorption

In this work, we successfully synthesize N-doped carbon nanoribbon (NCNR) from polypyrrole precursor and investigate their dielectric and microwaves absorption (MA) properties. NCNR appears as two-dimensional ribbon-like microstructure with tunable N-doping ratio. The dielectric property of NCNR can be tuned by N-doping content controlling. The results demonstrate that NCNR exhibits excellent MA performance at a filler-loading ratio of only 5 wt%. When the sample thickness is 3.3 mm, the maximal absorption reaches ??73.76 dB at 10.48 GHz. The maximum efficient bandwidth gets to 7.4 GHz (10.6–18 GHz), under a sample thickness of 2.7 mm. A model that refers to conductive loss, polarization relaxation, and impedance match is adopted to explain the MA mechanism of NCNR. This research opens up the exploration of NCNR in the field of MA, and provides a new idea for the design of carbon-related broad band MA materials.

     

Alternative method for measuring effective focal length of lenses using the front and back surface reflections from a reference plate

We present a simple method for measuring the effective focal length without determining the location of principle plane of the lens. The method is based on the measurement of confocal backreflection axial responses from the front and back surfaces of a reference plate with known refractive index and thickness. We proved the concept by measuring the effective focal lengths of thin singlet lenses and complex microscope objectives. The theoretical limit of measurement precision varies depending on the numerical aperture of the lens. This method can provide an alternative focal length measurement method for complex lenses or lenses that are permanently attached to other structures. Measurement errors were analyzed theoretically and improvements in measurement accuracy were discussed.     

Particular solution for geodesic lenses

On the basis of the analytical method of designing aspherical optical waveguide geodesic lenses [J. Opt. Soc. Am. 69, 1248 (1979)], a particular solution for geodesic lenses without any curvature singularity was obtained.     

Paraxial theory for birefringent lenses

The generalized ray tracing for the extraordinary ray through uniaxial crystals developed by Avenda?o-Alejo and Stavroudis [J. Opt. Soc. Am. A 19, 1674 (2002)] has been applied to derive paraxial refracting equations. Paraxial equations are derived for three cases where the incident, ordinary, and extraordinary rays lie in the incident plane: (a) the crystal axis is parallel to the optical axis, (b) the crystal axis is orthogonal to the optical axis and lies in the plane of incidence, and (c) the crystal axis is orthogonal to both the optical axis and the incident plane. The paraxial ray-tracing equations for the extraordinary ray are represented by matrix operators. The elements of the matrix system give all the information of the focal points and of the principal points. Gaussian formulas are derived, and some examples are presented.     

Use of microwaves for the synthesis and processing of materials

An overview of the synthesis of materials under microwave irradiation has been presented based on the work performed recently. A variety of reactions such as direct combination, carbothermal reduction, carbidation and nitridation have been described. Examples of microwave preparation of glasses are also presented. Great advantages of fast, clean and reduced reaction temperature of microwave methods are emphasized. The example of ZrO₂-CeO₂ ceramics has been used to show the extraordinarily fast and effective sintering which occurs in microwave irradiation.     

Strong ultrasonic microwaves in ferroelectric ceramics

It is well known that ferroelectric materials have piezoelectric properties which allow the transformation of electrical signals into mechanical signals and vice versa. The transducer action normally is restricted to frequencies up to the mechanical resonance frequency of the sample. There are, however, two mechanisms which allow transducer action in ferroelectric ceramics at much higher frequencies: one is the normal piezoelectric effect in a ferroelectric ceramic in which the crystallites have periodic domain structures, the other is a domain wall effect in which ferroelastic domain walls in a periodic domain structure are powerful shear wave emitters. Both mechanisms give rise to extensive dielectric losses in ceramics at microwave frequencies.     

Diffractive lenses for chromatic confocal imaging

A diffractive zone plate provides a highly linear wavelength-to-depth coding, allowing for nonmechanical depth scanning in a confocal microscope. This chromatic confocal microscope, constructed with 40x and 60x objectives, achieves axial position changes of 55 and 25 mum, respectively, for a wavelength tuning range of 100 nm. The corresponding longitudinal point-spread functions are measured and shown to possess full-width half-maximums of 2.52 and 2.23 mum, respectively. Two-dimensional profiles of a two-phase-level grating and a four-phase-level diffractive structure are given. The performance of the chromatic confocal microscope is consistent with that of the conventional confocal operation of the microscope.     

Evanescent microwaves: a novel super-resolution noncontactnondestructive imaging technique for biological applications

Scanning tunneling microscopes (STM) and atomic force microscopes (AFM) are used to study biological materials. These methods, often capable of achieving atomic resolutions, reveal fascinating information regarding the inner workings of these materials. However, both STM and AFM require physical contact to the specimen. In the case of STM the specimen needs to be conducting as well. Here we introduce a new method for imaging biological materials through air or a suitable liquid using decaying or evanescent fields at the tip of a properly designed microwave resonator. This novel method involves the use of an evanescent microwave probe (EMP) and it is capable of imaging a variety of nonuniformities in biological materials including conductivity, permittivity, and density variations. EMP is a noncontact and nondestructive sensor and it does not require conducting specimens. Its spatial resolution is currently around 0.4 μm at 1 GHz. We have used this probe to map nonuniformities in a variety of materials including metals, semiconductors, insulators, and biological and botanical samples. Here we discuss applications of EMP imaging in bone, teeth, botanical, and agricultural specimens     

Desired concentration-dependent ion exchange for micro-optic lenses

The required concentration-dependent diffusion coefficients for both ideal one-dimensional and ideal radial gradient-index profiles are determined. The modified quasi-chemical diffusion model is used to relate the diffusion coefficient to optimum glass composition. Adding aluminum to sodium silicate glasses facilitates the approach to the desired concentration dependence of the diffusion coefficient for silver-sodium ion exchange. A parabolic one-dimensional index profile is fabricated in one of the glasses. It deviates from ideal values by less than 2%.     

Millimeter-wave antireflection coating for cryogenic silicon lenses

We have developed and tested an antireflection (AR) coating method for silicon lenses used at cryogenic temperatures and millimeter wavelengths. Our particular application is a measurement of the cosmic microwave background. The coating consists of machined pieces of Cirlex glued to the silicon. The measured reflection from an AR-coated flat piece is less than 1.5% at the design wavelength. The coating has been applied to flats and lenses and has survived multiple thermal cycles from 300 to 4 K. We present the manufacturing method, the material properties, the tests performed, and estimates of the loss that can be achieved in practical lenses.     

Model eyes for evaluation of intraocular lenses

In accordance with the present international standard for intraocular lenses (IOLs), their imaging performance should be measured in a model eye having an aberration-free cornea. This was an acceptable setup when IOLs had all surfaces spherical and hence the measured result reflected the spherical aberration of the IOL. With newer IOLs designed to compensate for the spherical aberration of the cornea there is a need for a model eye with a physiological level of spherical aberration in the cornea. A literature review of recent studies indicated a fairly high amount of spherical aberration in human corneas. Two model eyes are proposed. One is a modification of the present ISO standard, replacing the current achromat doublet with an aspheric singlet cut in poly(methyl methacrylate) (PMMA). The other also has an aspheric singlet cut in PMMA, but the dimensions of it and the entire model eye are close to the physiological dimensions of the eye. They give equivalent results when the object is at infinity, but for finite object distances only the latter is correct. The two models are analyzed by calculation assuming IOLs with different degrees of asphericity to elucidate their sensitivity to variation and propose tolerances. Measured results in a variant of the modified ISO model eye are presented.     

Modified phase function model for kinoform lenses

A more computationally tractable model of the kinoform lenses in hybrid refractive-diffractive systems is proposed by taking into consideration the actual phase function of the kinoform lenses for every wavelength. The principle and outline of this modified model are explained. We compare the results of this approach with the more conventional single order calculation and with the standard diffraction-order expansion by using a practical hybrid optical system example.     

Field conjugate acoustic lenses for ultrasound hyperthermia

Multipoint foci have been synthesized by applying the pseudoinverse field conjugation method to a single ultrasonic transducer coupled to a polystyrene lens. The lens design is based on phased array calculations are then fabricated on a computer-controlled milling machine. The measured beam patterns from the lenses agree closely with the beam patterns predicted by theory for the equivalent phased arrays. Temperature distributions from thermal modeling and those measured in tissue equivalent phantoms show that the lens system is capable of generating strongly localized, controlled temperature fields for hyperthermia.     

Variable frequency field conjugate lenses for ultrasoundhyperthermia

This paper describes variable frequency focusing with field conjugate lenses designed to mimic the multiple-focusing capabilities of large two-dimensional phased arrays. Simulations, experiments, and Fresnel diffraction analysis are used to show that both the size and the depth of a field conjugate lens focus may vary with frequency. Examples are given for field conjugate lens focusing with planar transducers, focused transducers, and ordinary refracting lenses