Showing papers on "Zone plate published in 2014"

Ultra-high aspect ratio high-resolution nanofabrication for hard X-ray diffractive optics.

Abstract: Although diffractive optics have played a major role in nanoscale soft X-ray imaging, high-resolution and high-efficiency diffractive optics have largely been unavailable for hard X-rays where many scientific, technological and biomedical applications exist. This is owing to the long-standing challenge of fabricating ultra-high aspect ratio high-resolution dense nanostructures. Here we report significant progress in ultra-high aspect ratio nanofabrication of high-resolution, dense silicon nanostructures using vertical directionality controlled metal-assisted chemical etching. The resulting structures have very smooth sidewalls and can be used to pattern arbitrary features, not limited to linear or circular. We focus on the application of X-ray zone plate fabrication for high-efficiency, high-resolution diffractive optics, and demonstrate the process with linear, circular, and spiral zone plates. X-ray measurements demonstrate high efficiency in the critical outer layers. This method has broad applications including patterning for thermoelectric materials, battery anodes and sensors among others.

Optimization-free superoscillatory lens using phase and amplitude masks

Abstract: A superoscillatory focusing lens has been experimen- tally demonstrated by optimizing Fresnel zone plates (FZP), with limited physical insight as to how the lens feature contributes to the focal formation. It is therefore imperative to establish a generalized viable account for both FZP (amplitude mask) and binary optics (phase mask). Arbitrary superoscillatory spots can now be customized and realized by a realistic optical device, without using optimization. It is counterintuitively found that high spatial frequency with small amplitude and destructive interfer- ence are favorable in superfocusing of a superoscillation pat- tern. The inevitably high sidelobe is pushed 15λ away from the central subwavelength spot, resulting in significantly enlarged field of view for viable imaging applications. This work therefore not only reveals the explicit physical role of any given metallic/

Generation of annular beam by a novel class of Fresnel zone plate

Abstract: We introduce a type of Fresnel zone plate (FZP) in which its phase is shifted radially outward. This FZP can easily and precisely turn an incident plane wave into an annular beam at its focal plane. High efficiency and flexibility are the advantages of generating doughnut beams by this method. By performing direct calculations and using Bessel function properties, it is shown that a radially shifted zone plate produces annulus focus. Furthermore, by simulating the modified phase structure, in addition to demonstrating the formation of a ring-shaped focus, we also showed that its radius merely depends on the amount of the shift. We also showed that the width of the annulus is a function of focal length. Simulation results were thoroughly examined by experiments. Finally, it is clearly revealed that at a certain distance from the focal plane along the beam propagation, an annular beam is transformed into a Bessel beam, and a focal line is formed.

Table-top soft x-ray microscope using laser-induced plasma from a pulsed gas jet.

Abstract: An extremely compact soft x-ray microscope operating in the "water window" region at the wavelength λ = 2.88 nm is presented, making use of a long-term stable and nearly debris-free laser-induced plasma from a pulsed nitrogen gas jet target. The well characterized soft x-ray radiation is focused by an ellipsoidal grazing incidence condenser mirror. Imaging of a sample onto a CCD camera is achieved with a Fresnel zone plate using magnifications up to 500x. The spatial resolution of the recorded microscopic images is about 100 nm as demonstrated for a Siemens star test pattern.

High‐efficiency zone‐plate optics for multi‐keV X‐ray focusing

Abstract: High-efficiency nanofocusing of hard X-rays using stacked multilevel Fresnel zone plates with a smallest zone width of 200 nm is demonstrated. The approach is to approximate the ideal parabolic lens profile with two-, three-, four- and six-level zone plates. By stacking binary and three-level zone plates with an additional binary zone plate, the number of levels in the optical transmission function was doubled, resulting in four- and six-level profiles, respectively. Efficiencies up to 53.7% focusing were experimentally obtained with 6.5 keV photons using a compact alignment apparatus based on piezoelectric actuators. The measurements have also been compared with numerical simulations to study the misalignment of the two zone plates.

Monolithic focused reference beam X-ray holography

Abstract: There is a trade-off between image contrast and spatial resolution in Fourier transform holography, which limits its application in single-shot X-ray imaging Here Geilhufe et al use a Fresnel zone plate to decouple these two factors, which improves the efficiency of high-resolution holography imaging

Computational modelling of a graphene Fresnel lens on different substrates

Abstract: Graphene being the thinnest material known has a lot of potential applications in compact systems, where the size and weight are limited. Here we study through computational modelling a graphene based Fresnel zone plate lens. The optical properties of graphene and the focusing response of the lens are studied for the 850 and 1550 nm wavelengths. The lens performance can be tuned by adjusting the Fermi level of graphene and the number of layers. The effects of substrates (such as glass and SiO2/Si) on the performance of graphene lens are also analysed. The result presented can also be used in optimising the performance of other graphene based optical devices, such as scattering from graphene based transparent electrodes.

Fresnel zone plate stacking in the intermediate field for high efficiency focusing in the hard X-ray regime

Abstract: Focusing efficiency of Fresnel zone plates (FZPs) for X-rays depends on zone height, while the achievable spatial resolution depends on the width of the finest zones. FZPs with optimal efficiency and sub-100-nm spatial resolution require high aspect ratio structures which are difficult to fabricate with current technology especially for the hard X-ray regime. A possible solution is to stack several zone plates. To increase the number of FZPs within one stack, we first demonstrate intermediate-field stacking and apply this method by stacks of up to five FZPs with adjusted diameters. Approaching the respective optimum zone height, we maximized efficiencies for high resolution focusing at three different energies, 10, 11.8, and 25 keV.

Three-dimensional structured on-chip stacked zone plates for nanoscale X-ray imaging with high efficiency

Abstract: Fresnel zone plates are the key optical elements for nanoscale focusing of X-ray beams with high spatial resolution Conventional zone plates manufactured by planar nanotechnology processes are limited by the achievable aspect ratios of their zone structures Additionally, ultra-high resolution X-ray optics with high efficiency requires three-dimensional (3-D) shaped tilted zones The combination of high spatial resolution and high diffraction efficiency is a fundamental problem in X-ray optics Based on electrodynamical simulations, we find that the optimized zone plate profile for volume diffraction is given by zone structures with radially increasing tilt angles and decreasing zone heights On-chip stacking permits the realization of such advanced 3-D profiles without significant loss of the maximum theoretical efficiency We developed triple layer on-chip stacked zone plates with an overlay accuracy of sub-2 nm which fulfills the nanofabrication requirements Efficiency measurements of on-chip stacked zone plates show significantly increased values compared to conventional zone plates

Experimental verification 3D subwavelength resolution beyond the diffraction limit with zone plate in millimeter wave

Abstract: The results of experimental investigations of resolution power of millimeter-wave phase reversal binary Fresnel zone plate with short focal distance are described. It has been experimentally shown that in the near-field of single diffractive lens with subwavelength focal distance without the immersion medium a resolution of 0.3–0.4 were obtained. The “Abbe barrier” can be completely broken by such diffractive lenses with unique three-dimensional super resolution. Also it was experimentally shown the focal spot in the high numerical aperture diffractive lenses is no longer an Airy pattern and focal intensity distribution is not circularly symmetric. © 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:2436–2439, 2014

Focusing Performance of Terahertz Zone Plates with Integrated Cross-shape Apertures

Abstract: Search of novel breakthrough concepts of room temperature compact terahertz (THz) imaging systems [1–3] remains as one of sensitive topics in THz photonics. As a rule, a particular attention is attributed to the development of compact THz sources and detectors, however, compact optic components – such as focusing lenses, mirrors, beam splitters, etc – as well plays important role in reducing size of the systems. Preferences here are usually given to inexpensive and relatively simple technology-based approaches, for instance, binary lenses [4], focusing elements of a combined design structures [5] or narrow-band laser-ablated filters [6]. These solutions can be found well-suited in THz heterodyne [7] and spectroscopic imaging [8] when only several specific spectral lines are used to record image, but still enabling to indentify an object under test. Very recently, terahertz zone plate with integrated resonant filters (TZP) was proposed and experimentally demonstrated for efficient radiation focusing and spectral selection in a single device at selected THz frequency [9]. In this letter, we extended the previously presented concept of the TZP showing routes for their optimal design revealing the effect of zone number and influence of the plate thickness. It was demonstrated using three-dimensional finite-difference time-domain simulations and experimental confirmation of the effect of THz radiation focusing via laser beam profiling. Development of the TZP with a short focal length and high numerical aperture is of prime interest employing multi-mode THz lasers for imaging with up to sub-wavelength resolution [1]. Two designs of zone plates were studied (Fig. 1) – the first contained regular Fresnel’s zones and used for reference purpose, while the second one – the TZP – was designed in such a way that cross-shape apertures of the resonant THz filter were placed in open areas of the regular zone plate providing an additional THz frequencies selection. The first type of the samples were produced using standard photolithography on a 30 μm thick copper coated printed circuit board, after pealing out processed the metallic layer from the substrate. Meanwhile the TZP was fabricated using laser direct writing technique of 30 J Infrared Milli Terahz Waves (2014) 35:699–702 DOI 10.1007/s10762-014-0086-8

Influence of random zone positioning errors on the resolving power of Fresnel zone plates

Abstract: Fresnel zone plates produced by electron beam lithography and planar etching techniques provide a resolving power of about 10 nm. An alternative zone plate fabrication approach is based on alternately coating a micro-wire with two different materials. With this process, very thin zone layers with very high aspect ratios can be deposited. However, depending on the fabrication method, random zone positioning errors may introduce strong aberrations. We simulate the effect of positioning errors using different random fluctuations and study their influence on zone plate resolution. We find that random errors significantly decrease the contrast transfer of X-ray microscopes. Additionally, we derive an upper bound for the mean acceptable variance of the deposition rate.

Blue Phase LC/Polymer Fresnel Lens Fabricated by Holographics

Abstract: A novel fabrication method using holographic exposure for switchable blue phase (BP) liquid crystal (LC)/polymer Fresnel lens was demonstrated. The Fresnel pattern can be achieved by interfering a planar and a spherical wave fronts with a 532-nm laser. The BPLC/polymer Fresnel lens can thus be fabricated without photo mask and controlled with a simpler driving compared with the conventional LC Fresnel lens.

Ronchi test for characterization of X-ray nanofocusing optics and beamlines.

Abstract: A Ronchi interferometer for hard X-rays is reported in order to characterize the performance of the nanofocusing optics as well as the beamline stability. Characteristic interference fringes yield qualitative data on present aberrations in the optics. Moreover, the visibility of the fringes on the detector gives information on the degree of spatial coherence in the beamline. This enables the possibility to detect sources of instabilities in the beamline like vibrations of components or temperature drift. Examples are shown for two different nanofocusing hard X-ray optics: a compound refractive lens and a zone plate.

Fabrication of Fresnel zone plate lens in fused silica glass using femtosecond laser lithography technology

Abstract: This paper expresses maskless formation of Fresnel zone plate (FZP) lens on fused silica glass surface using femtosecond laser lithography technology. The FZP lens consists of a series of concentric rings that has been encoded on a glass surface using femtosecond laser irradiation followed by chemical etching and stripping. To compare the performance of the FZP lens with traditional laser induced FZP, we also fabricated FZP on the surface of and inside fused silica glass using femtosecond laser direct writing. In all the cases, the FZP lenses have a focal length of 50 mm. In addition, we fabricated a 25-mm focal length FZP lens by means of femtosecond laser lithography. Compared to traditional femtosecond laser direct writing, femtosecond laser lithography technique offers smooth patterning of materials. Consequently, femtosecond laser lithography engraved FZP lens yields considerably higher diffraction efficiency. Besides, we investigated the diffraction pattern of the fabricated FZP lenses. Using these FZP lenses, we are able to observe microletters encoded on an aluminum coated poly-methylmethacrylate surface indicating excellent focusing and imaging capability of the FZP lenses. The proposed maskless technology is simple compared to other lithography techniques, representing great potential for small-scale manufacturing of similar kinds of optical/photonics devices.

Refractive and diffractive neutron optics with reduced chromatic aberration

Abstract: Thermal neutron beams are an indispensable tool in physics research. The spatial and the temporal resolution attainable in experiments are dependent on the flux and collimation of the neutron beam which remain relatively poor, even for modern neutron sources. These difficulties may be mitigated by the use of optics for focusing and imaging. Refractive and diffractive optical elements, e.g. compound refractive lenses and Fresnel zone plates, are attractive due to their low cost, and simple alignment. These optical elements, however, suffer from chromatic aberration, which limit their effectiveness to highly monochromatic beams. This paper presents two novel concepts for focusing and imaging non-monochromatic thermal neutron beams with well-known optical elements: (1) a fast mechanical transfocator based on a compound refractive lens, which actively varies the number of individual lenses in the beam path to focus and image a time-of-flight beam, and (2) a passive optical element consisting of a compound refractive lens, and a Fresnel zone plate, which may focus and image both continuous and pulsed neutron beams. (C) 2014 Elsevier B.V. All rights reserve (Less)

Study of polarization properties of fiber-optics probes with use of a binary phase plate.

Abstract: We conduct a theoretical and experimental study of the distribution of the electric field components in the sharp focal domain when rotating a zone plate with a π-phase jump placed in the focused beam. Comparing the theoretical and experimental results for several kinds of near-field probes, an analysis of the polarization sensitivity of different types of metal-coated aperture probes is conducted. It is demonstrated that with increasing diameter of the non-metal-coated tip part there occurs an essential redistribution of sensitivity in favor of the transverse electric field components and an increase of the probe’s energy throughput.

Diffraction theory of high numerical aperture subwavelength circular binary phase Fresnel zone plate.

Abstract: An analytical model of vector formalism is proposed to investigate the diffraction of high numerical aperture subwavelength circular binary phase Fresnel zone plate (FZP). In the proposed model, the scattering on the FZP's surface, reflection and refraction within groove zones are considered and diffraction fields are calculated using the vector Rayleigh-Sommerfeld integral. The numerical results obtained by the proposed phase thick FZP (TFZP) model show a good agreement with those obtained by the finite-difference time-domain (FDTD) method within the effective extent of etch depth. The optimal etch depths predicted by both methods are approximately equal. The analytical TFZP model is very useful for designing a phase and hybrid amplitude-phase FZP with high-NA and short focal length.

Quasi-achromatic Fresnel zone lens with ring focus.

Abstract: The phase of a standard Fresnel zone lens (FZL) is periodically modulated in the radial direction using the phase of a binary fraxicon. The resulting element (rf-FZL) focuses light into a ring. The ring is found to be quasi-achromatic, in that the diameter is wavelength independent but its location is not. The binary rf-FZL is fabricated using electron beam direct writing. Experimental results confirm the generation of a wavelength-independent ring pattern at the focus of the rf-FZL. An efficiency of 24% was obtained. The variation in radius of ring pattern was reduced from 61 μm to less than 10 nm for a corresponding wavelength variation from 532 to 633 nm.

On-chip integration of laser-ablated zone plates for detection enhancement of inGaAs bow-tie terahertz detectors

Abstract: One order of magnitude detection enhancement of bow-tie-shaped InGaAs-based terahertz detectors by on-chip incorporation of the secondary diffractive optics is reported. Zone plates were produced directly on the bottom surface of 500 µm-thick InP substrate using the direct laser write technique after an array of InGaAs detectors was formed on the front surface of the wafer. Measurements of detected signal dependence on the angle of the incident wave were performed at 0.76 THz and compared with finite-difference time-domain simulation results. Good correlation of the results shows that the observed enhancement is indeed caused by the focusing performance of the zone plate rather than by the exceptional sensitivity of the single detector.

Studying ATHENA optics with divergent and collimated x-ray beams

Abstract: An open question in the measurement of X-ray optics for satellite experiments is what the PSF (point spread function) looks like in orbit and what the focal length for a source at infinite distance is. In order to measure segmented optics as proposed for ATHENA a collimated X-ray beam with a size of several square centimeters is necessary. We showed that by using a zone plate such a collimated beam can be achieved. We discuss here the requirements such a zone plate collimator has to comply in order to characterize with this collimator an ATHENA type optic. Additional we can present results obtained with a first version of such a collimator and can show so the proof of principle.

Diffractive performance of a photon-sieve-based axilens

Abstract: An axilens is a combination of an axicon and a Fresnel zone plate to provide a long focal diffractive lens. However, the photon sieve has been known as a high-resolution version of the Fresnel zone plate. Therefore, construction of an axilens on the basis of a photon sieve may yield a high-resolution axilens. To this end, circular zones of an axilens were replaced by a given distribution of pinholes. It is shown that lateral resolution of the axilens is increased due to the replacement. The impact of different distributions of pinholes on the focusing properties of the axilens were also examined. Results indicate that by implementing different distributions of pinholes resolution and focal depth are changed and hence, are manageable. Theoretical results are verified by experimental work.

Design and optimization of a parallel spectrometer for ultra-fast X-ray science

Abstract: In the present work, different varied line space (VLS) and reflection zone plate (RZP) gratings are analyzed for their suitability in low-signal femtosecond soft X-ray spectroscopy. The need for high efficiency suggests a straightened focal line whose sharpness and residual curvature will determine the quality. One- and two-dimensional VLS structures feature an attractive trade-off between a sufficient optical performance and a strongly relaxed fabrication, due to moderate line densities which are easily accessible by e-beam lithography. Based on fanned-out RZP arrays, their continuous limit version is identified to generate an almost perfect focal line however, with an aberration level three orders of magnitude better than for the VLS gratings and well below the diffraction limit over large acceptance angles.

Metal-only Fresnel zone plate antenna for millimetre-wave frequency bands

Abstract: A metal-only Fresnel zone plate antenna (FZPA) to operate at millimetre wave frequency bands is presented in this work. The proposed antenna is a very attractive alternative to parabolic reflectors and reflectarrays because of its several advantages such as low profile, low cost, easy to manufacture and lossless. A centre-fed Fresnel zone plate whose diameter is 300 mm has been designed and analysed at 75 GHz providing a gain of 44.29 dBi, an aperture efficiency of 48.42% and a cross-polarised level lower than −40 dB for the diagonal plane (ϕ = 45°). A second prototype has been designed and analysed from 54 to 68 GHz to evaluate the performance of the proposed antenna compared with an already published FZPA.

Diffraction theory for azimuthally structured Fresnel zone plate

Abstract: A conventional Fresnel zone plate (FZP) consists of concentric rings with an alternating binary transmission of zero and one. In an azimuthally structured Fresnel zone plate (aFZP), the light transmission of the transparent zones is modulated in the azimuthal direction, too. The resulting structure is of interest for extreme ultraviolet and x-ray imaging, in particular, because of its improved mechanical stability as compared to the simple ring structure of an FZP. Here, we present an analysis of the optical performance of the aFZP based on scalar diffraction theory and show numerical results for the light distribution in the focal plane. These will be complemented by calculations of the optical transfer function.

Light diffraction by concentrator Fresnel lenses

Abstract: Fresnel lenses are widely used in concentrating photovoltaic (CPV) systems as primary optical elements focusing sunlight onto small solar cells or onto entrance apertures of secondary optical elements attached to the solar cells. Calculations using the Young-Maggi-Rubinowicz theory of diffraction yield analytical expressions for the amount of light spilling outside these target areas due to diffraction at the edges of the concentrator Fresnel lenses. Explicit equations are given for the diffraction loss due to planar Fresnel lenses with small prisms and due to arbitrarily shaped Fresnel lenses. Furthermore, the cases of illumination by monochromatic, polychromatic, totally spatially coherent and partially spatially coherent light (e.g. from the solar disc) are treated, resulting in analytical formulae. Examples using realistic values show losses due to diffraction of up to several percent.

Recording oscillations of sub-micron size cantilevers by extreme ultraviolet Fourier transform holography

Abstract: We recorded the fast oscillation of sub-micron cantilevers using time-resolved extreme ultraviolet (EUV) Fourier transform holography. A tabletop capillary discharge EUV laser with a wavelength of 46.9 nm provided a large flux of coherent illumination that was split using a Fresnel zone plate to generate the object and the reference beams. The reference wave was produced by the first order focus while a central opening in the zone plate provided a direct illumination of the cantilevers. Single-shot holograms allowed for the composition of a movie featuring the fast oscillation. Three-dimensional displacements of the object were determined as well by numerical back-propagation, or "refocusing" of the electromagnetic fields during the reconstruction of a single hologram.

Spectral purification and infrared light recycling in extreme ultraviolet lithography sources

Abstract: We present the design of a novel collector mirror for laser produced plasma (LPP) light sources to be used in extreme ultraviolet (EUV) lithography. The design prevents undesired infrared (IR) drive laser light, reflected from the plasma, from reaching the exit of the light source. This results in a strong purification of the EUV light, while the reflected IR light becomes refocused into the plasma for enhancing the IR-to-EUV conversion. The dual advantage of EUV purification and conversion enhancement is achieved by incorporating an IR Fresnel zone plate pattern into the EUV reflective multilayer coating of the collector mirror. Calculations using Fresnel-Kirchhoff’s diffraction theory for a typical collector design show that the IR light at the EUV exit is suppressed by four orders of magnitude. Simultaneously, 37% of the reflected IR light is refocused back the plasma.