Showing papers on "Zone plate published in 2008"

Ellipsoidal and parabolic glass capillaries as condensers for x-ray microscopes

Abstract: Single-bounce ellipsoidal and paraboloidal glass capillary focusing optics have been fabricated for use as condenser lenses for both synchrotron and tabletop x-ray microscopes in the x-ray energy range of 2.5-18 keV. The condenser numerical apertures (NAs) of these devices are designed to match the NA of x-ray zone plate objectives, which gives them a great advantage over zone plate condensers in laboratory microscopes. The fabricated condensers have slope errors as low as 20 μrad rms. These capillaries provide a uniform hollow-cone illumination with almost full focusing efficiency, which is much higher than what is available with zone plate condensers. Sub-50 nm resolution at 8 keV x-ray energy was achieved by utilizing this high-efficiency condenser in a laboratory microscope based on a rotating anode generator.

Imaging with neutral atoms: a new matter-wave microscope.

Abstract: Summary Matter-wave microscopy can be dated back to 1932 when Max Knoll and Ernst Ruska published the first image obtained with a beam of focussed electrons. In this paper a new step in the development of matter-wave microscopy is presented. We have created an instrument where a focussed beam of neutral, ground-state atoms (helium) is used to image a sample. We present the first 2D images obtained using this new technique. The imaged sample is a free-standing hexagonal copper grating (with a period of about 36μm and rod thickness of about 8μm). The images were obtained in transmission mode by scanning the focussed beam, which had a minimum spot size of about 2.0 μm in diameter (full width at half maximum) across the sample. The smallest focus achieved was 1.9 ± 0.1 μm. The resolution for this experiment was limited by the speed ratio of the atomic beam through the chromatic aberrations of the zone plate that was used to focus. Ultimately the theoretical resolution limit is set by the wavelength of the probing particle. In praxis, the resolution is limited by the source and the focussing optics.

Near-field focusing properties of zone plates in visible regime--new insights.

Abstract: Near-field focusing properties of zone plates are investigated in the visible regime by a 3-dimensional finite-difference time-domain method. It is shown that Frensel zone plates (FZPs) with metallic coatings can achieve subwavelength focusing in the visible wavelength. The characteristics of subwavelength focusing are found to be independent of the type of metal coatings used. All the FZPs exhibit similar shift in focal length and depth of focus when compared with classical calculations. These results indicate that plasmonic waves do not contribute to subwavelength focusing. Instead the subwavelength focusing characteristic is attributed to the interference of diffracted evanescent waves from a large numerical aperture. It is found that the near-field focusing of FZPs suppresses higher order foci such that the corresponding diffraction efficiency is improved. The use of phase zone plate structured on glass without opaque coating is proposed to improve the diffraction efficiency of subwavelength focusing.

Wedged multilayer Laue lens

Abstract: A multilayer Laue lens (MLL) is an x-ray focusing optic fabricated from a multilayer structure consisting of thousands of layers of two different materials produced by thin-film deposition. The sequence of layer thicknesses is controlled to satisfy the Fresnel zone plate law and the multilayer is sectioned to form the optic. An improved MLL geometry can be created by growing each layer with an in-plane thickness gradient to form a wedge, so that every interface makes the correct angle with the incident beam for symmetric Bragg diffraction. The ultimate hard x-ray focusing performance of a wedged MLL has been predicted to be significantly better than that of a nonwedged MLL, giving subnanometer resolution with high efficiency. Here, we describe a method to deposit the multilayer structure needed for an ideal wedged MLL and report our initial deposition results to produce these structures.

Beam-shaping condenser lenses for full-field transmission X-ray microscopy.

Abstract: A new type of diffractive X-ray optical elements is reported, which have been used as beam-shaping condenser lenses in full-field transmission X-ray microscopes. These devices produce a square-shaped flat-top illumination on the sample matched to the field of view. The size of the illumination can easily be designed depending on the geometry and requirements of the specific experimental station. Gold and silicon beam-shapers have been fabricated and tested in full-field microscopes in the hard and soft X-ray regimes, respectively.

Microfabricated water immersion zone plate optical tweezer

Abstract: We demonstrate the trapping of beads in water with a microfabricated Fresnel zone plate. Beads are loaded onto the microfabricated optical traps using conventional optical tweezers and fluorescence microscopy is used to track bead position. Analysis of the bead position as a function of time is used to determine trapping stiffness. We present experiments showing the three-dimensional trapping of 2μm diameter beads with trapping stiffnesses that are comparable to conventional optical tweezers when the zone plate efficiency is taken into account.

Phase contrast soft x-ray microscopy using Zernike zone plates.

Abstract: Soft x-ray Zernike phase contrast microscopy was implemented using a “Zernike zone plate” (ZZP) without the use of a separate phase filter in the back focal plane. The ZZP is a single optic that integrates the appropriate ±π/2 radians phase shift through selective zone placement shifts in a Fresnel zone plate. Imaging using a regular zone plate, positive ZZP, and negative ZZP was performed at a wavelength of λ=2.163 nm. Contrast enhancement with the positive ZZP and contrast reversal with the negative ZZP were observed.

The application of Fresnel zone plate based projection in optofluidic microscopy.

Abstract: Optofluidic microscopy (OFM) is a novel technique for low-cost, high-resolution on-chip microscopy imaging. In this paper we report the use of the Fresnel zone plate (FZP) based projection in OFM as a cost-effective and compact means for projecting the transmission through an OFM's aperture array onto a sensor grid. We demonstrate this approach by employing a FZP (diameter = 255 µm, focal length = 800 µm) that has been patterned onto a glass slide to project the transmission from an array of apertures (diameter = 1 µm, separation = 10 µm) onto a CMOS sensor. We are able to resolve the contributions from 44 apertures on the sensor under the illumination from a HeNe laser (wavelength = 633 nm). The imaging quality of the FZP determines the effective field-of-view (related to the number of resolvable transmissions from apertures) but not the image resolution of such an OFM system -- a key distinction from conventional microscope systems. We demonstrate the capability of the integrated system by flowing the protist Euglena gracilis across the aperture array microfluidically and performing OFM imaging of the samples.

Planar infrared binary phase reflectarray.

Abstract: A reflective, binary phase reflectarray is demonstrated in the infrared, at a wavelength of 10.6 μm. The unique aspect of this work, at this frequency band, is that the specific desired phase shift is achieved using an array of subwavelength metallic patches on top of a ground-plane-backed dielectric stand-off layer. This is an alternative to the usual method of constructing a reflective Fresnel zone plate by means of a given thickness of dielectric. This initial demonstration of the reflectarray approach at infrared is significant in that there is inherent flexibility to create a range of phase shifts by varying the dimensions of the patches. This will allow for a multilevel phase distribution, or even a continuous variation of phase, across an optical surface with only two-dimensional lithography, avoiding the need for dielectric height variations.

Near-field behavior of zone-plate-like plasmonic nanostructures

Abstract: The near-field behavior of a new plasmonic structure, the plasmonic micro-zone-plate (PMZP), is presented. The PMZP can realize superfocusing at a working distance on the micrometer scale and a resolving power beyond the diffraction limit. Compared with conventional Fresnel zone plates (CFZPs), its unique characteristics of a significantly elongated depth of focus (DOF) and focal length will make autofocusing easier for the relevant optical systems. These characteristics imply that it is possible to realize a free feedback control system for autofocusing systems in which probe scanning is performed with a constant working distance from the probe to the sample surface, provided that the flatness variation of the sample substrate is within the DOF. Moreover, unlike the CFZPs, there is no series of focal points appearing for beam propagation in the near-field region with a propagation distance ranging from λ to 8λ or even longer. In addition, transmission properties in the near-field region are investigated by means of a computational simulation based on a finite-difference time-domain numerical algorithm. Peak transmission wavelength shifts were observed while the metal film thickness was changed. Focusing characteristics were analyzed for different numerical apertures of the PMZPs.

Photoacoustic imaging using an ultrasonic Fresnel zone plate transducer

Abstract: A photoacoustic (PA) imaging system based on an ultrasonic Fresnel zone plate (FZP) transducer is developed for the purpose of imaging biological tissue. This FZP transducer has a two-zone negative zone plate piezoelectric material pattern, and an optical fibre is integrated with the transducer on the symmetric axis of the zone plates to deliver laser pulses to the sample. The focal characteristic of the FZP transducer is analysed by theoretical prediction and experimental measurement, and the measured results are in good agreement with the predicted results. The limited-field back-projection deconvolution algorithm combined with the coherence-factor weighting technique is used to reconstruct the optical absorption distribution. The experiments were performed with phantoms and the blood vessels of chicken embryo chorioallantoic membrane. The results demonstrate that PA imaging using the FZP transducer has the ability to image biological tissue and has potential application in monitoring neovascularization in tumour angiogenesis.

Neutral atom and molecule focusing using a Fresnel zone plate

Abstract: Focusing of neutral atoms and molecules has several potential applications. The very first microscopy images using helium as an imaging probe were published earlier this year. Another possible application is to study the diffusion of atoms and molecules through materials with high spatial resolution by stepping a porous or permeable sample across the focused beam. With this application in mind, the authors present the best resolution transmission images hitherto achieved with helium atoms (less than 2 μm) of a thin carbon film with 2 μm holes. Furthermore, they present the first experiment using a Fresnel zone plate to focus neutral molecules. They used a beam of deuterium (D2) which was focused down to 15.2±0.5 μm. D2 was chosen because it fits in mass to the geometry of our system, which is optimized for helium. However, the method can be extended to hydrogen (H2) or other molecules by using a suitably adapted zone plate. In both cases the focus was limited by chromatic aberrations, caused by the veloci...

Fourier plasmonics: Diffractive focusing of in-plane surface plasmon polariton waves

Abstract: An in-plane Fresnel zone plate for focusing surface plasmon polariton (SPP) fields has been designed, fabricated and tested. Diffractive SPP fields from each Fresnel zone constructively interfere and focus at the designed focal point.

Fabrication of high-aspect-ratio Fresnel zone plates by e-beam lithography and electroplating

Abstract: The fabrication of gold Fresnel zone plates, by a combination of e-beam lithography and electrodeposition, with a 30 nm outermost zone width and a 450 nm-thick structure is described. The e-beam lithography process was implemented with a careful evaluation of applied dosage, tests of different bake-out temperatures and durations for the photoresist, and the use of a developer without methylisobutylketone. Electrodeposition with a pulsed current mode and with a specially designed apparatus produced the desired high-aspect-ratio nanostructures. The fabricated zone plates were examined by electron microscopy and their performances were assessed using a transmission X-ray microscope. The results specifically demonstrated an image resolution of 40 nm.

Real image display device with wide viewing angle

Abstract: A 3-dimensional image display device having wide viewing angles using Fresnel lenses is disclosed. According to an embodiment of the invention, the 3-dimensional image display device includes an image source providing part for providing an image source, a first Fresnel lens refracting and transmitting the image source that is incident from the image source providing part, and a second Fresnel lens for generating a 3-dimensional image by refracting and transmitting the image source transmitted from the first Fresnel lens. At least one of the first Fresnel lens and the second Fresnel lens is a curved type Fresnel lens surface. The present invention can maximize the display area for a 3-dimensional image and realize the 3-dimensional image having wide viewing angles without distortion on the left and right boundaries.

Miniaturization of a Fresnel spectrometer

Abstract: The core optical parts of an ultra-small spectrometer with less than 1 mm diameter were constructed using Fresnel diffraction. The fabricated spectrometer grating has a diameter of 750 µm and a focal length of 2.4 mm at 533 nm wavelength. The micro-spectrometer was built with a simple negative zone plate that has an opaque center with an ecliptic shadow to remove the zero-order direct beam to the aperture slit. Unlike conventional approaches, detailed optical calculation indicates that the ideal spectral resolution and resolving power in the Fresnel regime do not depend on the miniaturized sizes but only on the total number of rings. We calculated the 2D and 3D photon distributions around the aperture slit and confirmed that improved micro-spectrometers below 1 mm in diameter can be built with Fresnel diffraction. The comparison between mathematical simulation and measured data demonstrates the theoretical resolution, measured performance, misalignment effect, and improvement for the ultra-small Fresnel spectrometer. We suggest the utilization of an array of Fresnel spectrometers for tunable multi-spectral imaging in the ultra-violet range.

Analysis of extreme ultraviolet microscopy images of patterned nanostructures based on a correlation method

Abstract: A method to analyze extreme ultraviolet microscopy images of nanostructures that allows for the simultaneous determination of an object’s feature size and image resolution is presented. It is based on the correlation between the image and a set of templates of known resolution generated from the original image using Gaussian filters. The analysis was applied to images obtained with a Fresnel zone plate microscope that uses a 13.2 nm wavelength laser light for illumination. The object’s feature size and the resolution obtained with this method are shown to be in very good agreement with independent measurements of both magnitudes. © 2008 Optical Society of America OCIS codes: 100.2000, 100.2960, 110.0180.

Spring constant modulation in a zone plate tweezer using linear polarization

Abstract: At large NAs a micro-Fresnel zone plate produces a focal spot that is more elliptical than that produced by an objective lens with the same NA. Using this anisotropy we demonstrate a method for modulating the spring constant of an optical trap by rotating the linear input polarization. The focal spot ellipticity is enhanced by the apodization factor of the zone plate and its extremely high NA. By measuring the positions of trapped particles we obtain two-dimensional histograms of particle position. These indicate that the trap spring constant is 2.75 times larger perpendicular to the incident polarization than along it. The elliptical focal spot distribution can be rotated by rotating the incident polarization, allowing the spring constant along a given direction to be modulated.

A new Fresnel zone antenna with beam focused in the Fresnel region

Abstract: A new Fresnel Zone (FZ) Antenna to focus the microwave power in the radiation near field region is presented. FZ Antennas, in comparison with array antennas or reflector antennas, conventional focused antennas, take the advantages of design simplicity as well as lower sidelobe levels (SLL) in both axial and transverse directions. Simulation results and comparisons made between the new structure and aperture antennas based on the quadratic phase distribution show a reduction of sidelobe levels in both axial and transverse directions.

Compact Zernike phase contrast x-ray microscopy using a single-element optic

Abstract: We demonstrate Zernike phase contrast in a compact soft x-ray microscope using a single-element optic. The optic is a combined imaging zone plate and a Zernike phase plate and does not require any additional alignment or components. Contrast is increased and inversed in an image of a test object using the Zernike zone plate. This type of optic may be implemented into any existing x-ray microscope where phase contrast is of interest.

Optimization of polygonal Fresnel zone plates

Abstract: The performance of Fresnel zone plates having polygonal boundaries between zones has been studied for polygons with a low number of sides. An optimized polygonal shape has been analytically defined by minimizing the mismatched area between the circular and the polygonal pseudo-Fresnel zones. The square polygon has been analyzed in more depth than the others because of its simple symmetry. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 536–541, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23125

Nondestructive Reconstruction and Analysis of Solid Oxide Fuel Cell Anodes using X-ray Computed Tomography at sub-50 nm Resolution

Abstract: X-ray computed tomography (XCT) is applied to non-destructively image the three-dimensional (3D) microstructure of a solid oxide fuel cell (SOFC) with a solid yttria-stabilized zirconia (YSZ) electrolyte and porous nickel and YSZ (Ni-YSZ) anode. The x-ray microscope uses the 8 keV Cu-Kα line from a laboratory x-ray source with a condenser optic lens and an objective Fresnel zone plate lens providing spatial resolution of 42.7 nm. The data is visualized as 3D images and post-processed as binary images to obtain structural parameters. Porosity is calculated using a voxel counting method and tortuosity is evaluated by solving the Laplace equation. The 3D representation of the microstructure is used to calculate true structural and transport parameters and optimization through a detailed mass transfer and electrochemical study. Simulation of multi-component mass transport and electrochemical reactions in the microstructure using XCT data as geometric input illustrates the impact of this technique on SOFC modeling.

Light receiver and Fresnel lens used therein

Abstract: A light receiver comprises a Fresnel lens for collecting light signals, and a light receiving element disposed closer to the Fresnel lens than the focal point of the Fresnel lens for receiving the light signals collected by the Fresnel lens. The Fresnel lens comprises a lens surface group having a plurality of lens surfaces, and a back cut surface group having a plurality of back cut surfaces connecting the lens surfaces. The back cut surfaces are inclined with respect to the center axis of the Fresnel lens. Thus, the light receiver has a high light collection efficiency of light signals incident within a certain acceptance angle.

Variable focus lens system for eyeglasses

Abstract: A variable focus lens system that can be used in eyeglasses is disclosed. The system consists of superimposed first and second transparent thin lenses with surfaces divided into Fresnel zones and zone boundaries. The lenses are relatively slidable for focus adjustment. The Fresnel zone boundary surfaces are made parallel to the user's optical line of sight to reduce visual obstructions, and the zone boundaries may be restricted to areas near the lens periphery to further reduce visual obstructions. An opaque light absorbing coating may be applied to the zone boundaries to reduce light scattering. A fixed prescription lens, having the user's distance and astigmatism corrections, may be superimposed on the other two lenses.

Fresnel lenses based on dye-doped liquid crystals

Abstract: We demonstrated transmissive- and reflective-type Fresnel lenses based on dye-doped liquid crystal using photoalignment technique. The former is a polarization-independent and electrically tunable. The maximum diffraction efficiency reaches 37%, which approaches the theoretical limit ~ 41 %. Such a lens functions as a half-wave plate, and this feature could be well preserved under the applied voltage. The reflective-type Fresnel lens is based on dye-doped cholesteric liquid crystals (DDCLC). The formed lens persists without any external disturbance, and its focusing efficiency, analyzed using circularly polarized light, is ~ 23.7 %, which almost equals the measured diffraction efficiency of the used Fresnel-zone-plate mask (~ 25.6 %). The lens is thermally erasable, and rewritable. Notably, both of the transmissive- and reflective-type Fresnel lenses are switchable between focusing and defocusing states, upon application of a voltage. In addition, these devices are simple to fabricate, and have fast switching responses between focusing and defocusing state.

Optical focusing of plasmonic Fresnel zone plate-based metallic structure covered with a dielectric layer

Abstract: By modulating the zone width of a plasmonic Fresnel zone plate, consisting of metallic nanostructures covered with a dielectric layer, the authors demonstrated numerically that a focused beam can be achieved with higher intensity and smaller spot size than the diffraction-limited conventional Fresnel zone plate. Rigorous electromagnetic simulation predicts a full width at half maximum of 162nm (equivalent to an effective numerical aperture of 1.30) at 0.5μm focal length, using 405nm wavelength illumination. This sub-diffraction-limit focusing has potential in applications such as maskless nanolithography, high resolution scanning optical microscopy, optical data storage, and optical antenna. This focusing capability is related to extraordinary optical transmission, which is explained by the complex propagation constant in the zones afforded by higher refractive index dielectric layer and surface plasmons.

Toward sub-10-nm resolution zone plates using the overlay nanofabrication processes

Abstract: Soft x-ray zone plate microscopy has proven to be a valuable imaging technique for nanoscale studies. It complements nano-analytic techniques such as electron and scanning probe microscopies. One of its key features is high spatial resolution. We developed an overlay nanofabrication process which allows zone plates of sub-20 nm zone widths to be fabricated. Zone plates of 15 nm outer zones were successfully realized using this process, and sub-15 nm resolution was achieved with these zone plates. We extend the overlay process to fabricating zone plates of 12 nm outer zones, which is expected to achieve 10 nm resolution. In addition, we have identified a pathway to realizing sub-10 nm resolution, high efficiency zone plates with tilted zones using the overlay process.

Selection of a single femtosecond high-order harmonic using a zone plate based monochromator

Abstract: We report the use of zone plate optics as a monochromator for the spectral selection of a single high-order harmonic of a femtosecond laser generated in a rare gas medium. We show that this is a convenient way of monochromatizing the vacuum ultraviolet (VUV) pulses in the photon energy range from 30 up to 70 eV while keeping the pulse duration in the femtosecond range. We measure the emission spectra with a typical energy resolution of E/ΔE=60. Comparably high transmission and diffraction efficiencies allow the record of images of different harmonics using a camera within a few seconds of acquisition time. This is our first step toward a VUV microscope with a submicrometer spatial resolution and femtosecond time resolution. Possible applications of this setup are also discussed.