We demonstrate the capabilities of ion-beam lithography (IBL) for the manufacturing of the X-ray refractive micro-optics. For the first time with the help of IBL, the hardest of current materials – diamond – was milled, and microscale diamond half-lenses were produced. Lenses have a rotationally parabolic profile with radii of parabola apexes in the range from 3 to 10 μm. As has been confirmed with SEM, the surface of produced lenses was free of low- and high-frequency modulations: figure errors of fabricated lenses were < 200 nm, while the surface roughness was estimated to be 30 nm. The optical performance of the lens was successfully tested at a third-generation synchrotron, where the lenses provided diffraction-limited focusing of X-ray radiation and demonstrated intensity profiles with Gaussian distributions at every measured longitudinal position (along the optical axis) downstream of the optics.
X-ray reflecto-interferometry technique based on compound refractive lenses using an x-ray laboratory source was proposed to study thin-film structures. The setup for this experiment is very simple: a focused x-ray beam is reflected from parallel flat surfaces, which creates an interference pattern in a wide angular range, therefore the interference pattern can be obtained in a single shot without the need to rotate the sample or the detector. The reflecto-interferograms for Si3N4 membranes were obtained using the MetalJet Excillium micro-focus laboratory source with GaKα emission line at 9.25 keV. The experimentally obtained film thickness is in good agreement with the declared characteristics.
A new X-ray Reflecto-Interferometry (XRI) technique is proposed and realized for thin-film characterization. The XRI employs refractive optics that produce a converging fan of radiation, incident onto a sample surface, and a high-resolution CCD detector, which simultaneously collects the reflecto-interferogram over a wide angular range. The functional capabilities of the new method were experimentally tested at the ESRF ID06, and ID10 beamlines in the X-ray energy range from 14 keV to 22 keV. The free-standing Si3N4 membranes with different thickness were studied. The main advantages and possible future applications of the proposed reflecto-interferometry are discussed.
Optical properties (reflection, refractive index, real and imaginary part of permittivity function) of rough titanium surfaces fabricated by anodizing method at different anodic voltage have been studied. It is shown that a negative region in the visible wavelength range is observed on a rough titanium surface in the refractive index spectrum; its minimum appeared to be red-shifted shifted with surface roughness increase. These optical-nonlinear effects are studied by means s- and p-polarized light reflection coefficients spectra and permittivity spectra registration. It is also shown that the generation of surface plasmon oscillations in the visible spectral region on the rough titanium surface is possible. Excitation of surface plasmons is found to be accompanied by redistribution of the incident electromagnetic energy on the surface and leads to various nonlinear effects including negative values of the refractive index.
X-ray microscopy is advantageous over conventional optical microscopy because of its high resolution and capability to study the inner structure of materials opaque to visible light. Furthermore, this method does not require metallization and vacuum and therefore it can be used to visualize fragile biological samples that cannot be studied by scanning electron microscopy. Focusing X-ray optics may be roughly divided into three groups based on the physical principle of focusing: reflection, diffraction and refraction. The reflection optics includes curved mirrors, multilayers and capillaries; the diffractive optics includes Fresnel zone plates. Refractive optics comprises X-ray compound refractive lenses (CRLs) that are widely used nowadays because of their compactness and ease of fabrication. Focusing performance of the CRL is determined by the refractive index, absorption, the inner structure of the CRL material and the geometry of the lens. The optimal shape for the lens is parabolic with a small radius of curvature, because the smaller radius of the parabola leads to shorter focal distance and therefore allows to achieve higher resolution. The common choice of the CRL material is beryllium. However the resolution of Be lenses is far below theoretically predicted limits because of the parasitic scattering introduced by the grains in the material. Moreover the existing manufacturing technologies do not allow to achieve radius of curvature less than 50 μm. Polymer materials are also popular for the CRL microfabrication because of their amorphous nature, ease of structuring and low price. Among the advanced lithographic techniques the two-photon polymerization lithography (2PP) holds a special place. It is based on polymer solidification by means of two-photon absorption. Nonlinear character of two-photon absorption leads to the transparency of the out-of focus material, while presence of polymerization threshold reduces resolution far below diffraction limit. Therefore 2PP can be used for fabrication 3D structures of almost arbitrary shape including overhanging and self-intersecting structures.
In this work we introduce the 3D X-ray CRL fabricated by 2PP from the commercially available photoresist ORMOCOMP. Hundred double concave individual lenses formed a CRL with the 60 μm distance between adjacent lenses. Radius of curvature of a single parabolic surface was 3 μm that is comparable to radius of 2D silicon nano-lens made by conventional lithography and much less than achievable radius of 3D Be lens. Physical aperture was 28 μm. The optimal processing parameters (power, incident on the sample, and velocity of the laser beam waist movement) were determined. The fabricated CRL was studied by scanning electron microscopy. It was shown that surface of the lens is smooth and the geometrical parameters do not deviate significantly from that of the model.
Focusing performance of lenses was studied by the knife-edge technique. It was obtained that the focal distance is not larger than 2 cm at the energy of 9.25 keV. The radiation resistance of the CRL was tested at the synchrotron DESY: PETRA-III. The CRL was exposed at the non-focused X-ray radiation with the standard power and the energy of 12 keV for more than 10 hours without visible degradation.