The development of flat acoustic lenses for different applications such as biomedical engineering is a topic of great interest. Flat lenses like Fresnel Zone Plates (FZPs) are capable of focusing energy beams without the need of concave or convex geometries, which are more difficult to manufacture. One of the possible applications of these type of lenses is tumor ablation through High Intensity Focused Ultrasound (HIFU) therapies with real time Magnetic Resonance Imaging (MRI) monitoring. In order to be MRI compatible, the FZP material cannot have electromagnetic interaction. In this work, a Phase-Reversal FZP (PR-FZP) made of Polylactic Acid (PLA) manufactured with a commercial 3D printer is proposed as a better, more efficient and MRI compatible alternative to conventional Soret FZPs. Phase-Reversal lenses, unlike traditional FZPs, take advantage of all the incident energy by adding phase compensation regions instead of pressure blocking regions. The manufactured PR-FZP achieves 21.9 dB of focal gain, which increases the gain compared to a Soret FZP of its same size by a factor of 4.0 dB. Both numerical and experimental results are presented, demonstrating the improved focusing capabilities of these types of lenses.

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Acoustic Focusing Enhancement In Fresnel Zone Plate Lenses.

http://xrm.phys.northwestern.edu/research/pdf_papers/2011/mayer_ultramic_2011.pdf

Multilayer Fresnel zone plate for soft X-ray microscopy resolves sub-39 nm structures

W/Si multilayers having 5, 7, 9, 13, 17, and 25 layers have been deposited on c-Si substrates by Ion Beam Sputtering technique and have been characterized by specular and diffused grazing incidence X-ray reflectivity measurements. Information regarding the density, thickness and interface widths of individual layers of the multilayer stacks have been obtained from the theoretical fitting of the specular reflectivity spectra while fitting of the diffused X-ray reflectivity have yielded information regarding roughness and diffusivity at the individual interfaces along with the in-plane correlation lengths of roughness of the individual layers and the vertical correlation length of the whole multilayer structure. Investigations have been carried out on the different behavior of W-on-Si and Si-on-W interfaces and on the variation of the above parameters with the increase in number of layers in the multilayer structures.

Correlation of interface roughness for ion beam sputter deposited W/Si multilayers

We present our recent results, related to nanoscale imaging in the extreme ultraviolet (EUV) and soft X-ray (SXR) spectral ranges and demonstrate three novel imaging systems recently developed for the purpose of obtaining high spatial resolution images of nanoscale objects with the EUV and SXR radiations. All the systems are based on laser-plasma EUV and SXR sources, employing a double stream gas puff target. The EUV and SXR full field microscopes—operating at 13.8 nm and 2.88 nm wavelengths, respectively—are currently capable of imaging nanostructures with a sub-50 nm spatial resolution with relatively short (seconds) exposure times. The third system is a SXR contact microscope, operating in the “water-window” spectral range (2.3–4.4 nm wavelength), to produce an imprint of the internal structure of the investigated object in a thin surface layer of SXR light sensitive poly(methyl methacrylate) photoresist. The development of such compact imaging systems is essential to the new research related to biological science, material science, and nanotechnology applications in the near future. Applications of all the microscopes for studies of biological samples including carcinoma cells, diatoms, and neurons are presented. Details about the sources, the microscopes, as well as the imaging results for various objects will be shown and discussed.

Bioimaging Using Full Field and Contact EUV and SXR Microscopes with Nanometer Spatial Resolution

We have demonstrated ultrafast direct soft x-ray microscopy imaging. Microscopy images were acquired using an objective zone plate and a single strong high harmonic burst generated from He in a two-color laser field. This zone-plate-based microscopy system delivered real-space images directly without any data processing. The spatial resolution of the microscope was estimated to be about 140 nm from the image of nanoscale grating patterns.

Ultrafast direct imaging using a single high harmonic burst.

A soft x-ray microscope based on a phase-reversal zone plate was constructed and tested using high harmonic radiation as a coherent light source. The 61st harmonic centered at 13.3 nm was optimized in spectral sharpness and intensity by controlling the incident laser energy and chirp. A phase-reversal zone plate made of polymethyl methacrylate more than doubled the first-order efficiency. The nano patterns, imaged on an x-ray CCD with a magnification of 650, showed that the measured resolution of the microscope was better than 100 nm.

Soft x-ray microscope constructed with a PMMA phase-reversal zone plate

Using high harmonic radiation as an X-ray light source, a soft X-ray microscope with nanometer-scale spatial resolution was investigated. A transmission soft X-ray microscope was constructed using a Mo/Si multilayer concave mirror as a condenser and a Fresnel zone plate as a microscope objective. The high-order harmonic source at 13 nm, emitted from neon atoms driven by intense femtosecond laser pulses, was optimized by controlling laser chirp. Objects, patterned on 160-nm-thick hydrogen silsesquioxane coated on a 100-nm-thick Si3N4 membrane, were used for imaging. The analysis of object images, captured on an X-ray charge-coupled device, showed that the spatial resolution of the microscope was about 130 nm, verifying the usefulness of the tabletop soft X-ray source.

http://iopscience.iop.org/article/10.1143/JJAP.48.026506/pdf

Soft X-ray Microscope Constructed with 130-nm Spatial Resolution Using a High Harmonic X-ray Source

A table-top soft x-ray microscope, adopting a phase-reversal zone plate (PRZP) made of PMMA and high harmonic x-ray source, was constructed. The PMMA PRZP showed enhanced imaging efficiency compared to a metallic Fresnel zone plate.

Table-top soft x-ray microscope adopting a PMMA phase-reversal zone plate

Compact soft X-ray microscopes, using high harmonics as a light source, were constructed. A phase-reversal zone plate fabricated with polymethyl methacrylate was adopted to improve the focusing efficiency. Microscope images at 13 nm were acquired with sub-100 nm resolution, and the exposure time was reduced by a half, compared to that of the one using a gold Fresnel zone plate. A single-shot microscope image could be obtained using a strong harmonic at 22 nm generated in the two-color laser field consisting of the fundamental and the second harmonic of a femtosecond laser.

Highly efficient soft X-ray microscope using a PMMA phase-reversal zone plate

Fresnel zone plates (FZPs) for soft X-ray microscopy with an energy range of 284 eV to 540 eV are designed and fabricated in a simple method. An adequate aspect ratio of the resist mold for electroplating was obtained by the proximity effect correction technology for an incident electron beam on a single thick layer resist. Without additional complicated reactive ion etching, a sufficient electro plating mold for nickel structures was fabricated. The overall fabrication procedures which involve a mix-and-match overlay technique for electron beam lithography and an optic exposure system that centers the membrane on the nanostructures, and hybrid silicon etching technology in junction with deep anisotropy and a KOH wet method in order to release the backside Si substrates of the Si3N4 membranes with no deformation of FZPs are introduced. High quality nanostructures with minimum outermost zone widths of 50 nm and diameters of 120 microm were fabricated with simplified fabrication process and with cost-effective.

Deuk Su Kim

Papers

Soft x-ray microscope constructed with a PMMA phase-reversal zone plate

Soft X-ray Microscope Constructed with 130-nm Spatial Resolution Using a High Harmonic X-ray Source

Table-top soft x-ray microscope adopting a PMMA phase-reversal zone plate

Highly efficient soft X-ray microscope using a PMMA phase-reversal zone plate

A simplified fabrication process of Fresnel zone plates with controlling proximity effect correction.