3D printing of X-ray optics
In many rapidly developing fields of science – biology, chemistry, medicine, or fundamental physics, there is a need to study objects or processes on a nanometer scale. Most often, at the moment, this problem is solved using an electron microscope.
However, working with an electron microscope is not an easy task: a vacuum must be maintained in the microscope chamber, and metallic film must cover the sample itself to obtain a high-quality image. This leads to the fact that it is tough to examine living samples, for example, cells, using an electron microscope – a “living” cell in a vacuum can burst due to internal pressure, and attempts to metalize it can damage the cell or distort its shape. In addition, the electron beam can penetrate only a few microns deep into the sample, which makes it difficult to study the internal structure.
Nevertheless, it is still necessary to study small biological objects. To do this, you can use X-rays. It can penetrate very deeply into biological tissues and does not require additional special operations with the sample, which often leads to its destruction. If you design a microscope that works using X-ray radiation, you can get images not only from the surface but also the internal structure of the samples with a resolution of several nanometers without using vacuum and metal sputtering of the samples, including in “real-time.”
To make such a microscope, it is necessary to focus the X-ray radiation. Systems of mirrors, diffraction plates, or refractive optics (lenses) are used to focus radiation. X-ray focusing lenses have several differences from visible lenses. First, since the material’s refractive index for X-ray radiation is less than unity, the focusing lenses should not be convex but concave. Secondly, since the difference in the refractive indices of materials and air is very small (from a thousandth to a millionth), it is necessary to make entire arrays of tens or even hundreds of concave lenses, which are arranged in a row – on the same optical axis. Such designs are called composite refractive x-ray lenses (SPRL).
At the moment, SPRL is most often made from beryllium and silicon in the form of biconcave paraboloids of revolution by stamping and ultraviolet lithography with subsequent etching. The resolution of the image obtained with an X-ray microscope is determined by the size of the focal spot, that is, the transverse dimensions of the beam at the focus. Traditional technologies are rather crude, and, as a rule, with their help, focal spot sizes are obtained not less than 50 nm. In addition, for some tasks, a reconfigurable focal length is required, for which SPRL is used with a variable number of lenses – zoom lenses. Zoom lenses based on beryllium SPRL are very bulky devices – the characteristic size of zoom lenses is a few meters.
2PP is based on the effect of “two-photon absorption” – the laser illuminates the resist at a wavelength in the near-IR range, for which the resist is transparent. In the focal region, the radiation intensity is so high that the nonlinear process of two-photon absorption becomes noticeable. In this process, there is a simultaneous absorption of two photons in the IR region, equivalent to the absorption of one photon with a doubled frequency, i.e., already from the UV range. Since the resist is not transparent to UV radiation, absorption occurs at this wavelength, solidifying the resist.
The 2PP method allows printing products from photopolymer resins with a resolution of up to 100 nm. Moreover, since only those areas of the photoresist that are exposed to laser radiation solidify, this method allows printing complex overhanging and self-intersecting structures even through an already polymerized material. Using special filled photoresists and annealing after printing, it is possible to produce refractive X-ray optics from resistant, stable, and weakly absorbing materials such as glassy carbon or silicon dioxide. At the same time, the lenses produced are compact and lightweight. The photo below shows an array of 100 lenses, occupying no more than 3 mm on a glass substrate and weighing in tenths of a gram.
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