Update – 07-2020
Activefiber Systems HHG source arrives in Delft
A Soft X-Ray source based on High Harmonic Generation was recently installed at the Optics Research Group as part of the LINX-consortium by partner Active Fiber Systems. The aim of this consortium is to generate new techniques to image extremely small features in 3D, for example on next generation computer chips. The newly installed Soft X-Ray source, at the moment the brightest of its kind, has a key role to achieve these ambitious goals.
A unique Soft X-Ray source was recently installed at the Optics Research Group by Active Fiber Systems, a spin-off company from University of Jena. The Soft X-Ray source, at the moment the brightest of its kind, has a key role within the LINX consortium where TU Delft has the lead. LINX will use this platform to realise a tailored coherent Soft X-Ray source for high resolution 3D microscopy and retrieve structural features at sub-wavelength dimensions.
Generating coherent Extreme UV (EUV) or Soft X-Ray (SXR) light is not an easy task. Conventionally the academic world and the industry relied on enormous and expensive facilities (synchrotrons) with some having a size of hundreds meters in diameter. Although these synchrotrons are extremely bright and nice coherent type of sources, they are not within the reach of many academics or of the industry.
However, immense progress in High Harmonic Generation (HHG) led to the availability of matured, commercial, coherent Soft X-Ray sources which fit on a typical optical table. HHG is an extreme non-linear process where, in our case, a gas is illuminated by a very short and bright infrared laser pulse. The atoms in the noble gas are ionized because of the high energy infrared pulse, and the ejected electron follows a closed trajectory generated by the high electric field, so that upon recombination of the ejected electron with the ionized atom, characteristic SXR radiation is emitted as high index odd harmonics of the optical frequency of the infrared light used. These sources do not compete with synchrotrons in terms of brightness but for many experiments these extremely high levels of brightness are not required. This new piece of equipment enables a lot of new opportunities by opening up a new high energy wavelength range for optical metrology at the Delft Optics group.
Lensless Microscopy with Soft X-Rays
A microscope uses a set of lenses to image extremely small objects. The smallest object that can be imaged by a microscope depends, in general, on the quality of the lenses and the wavelength of the light used to observe the tiny object. So the resolution can be improved by going to shorter wavelengths and by using higher quality lenses. This new source allows us to go to very small wavelengths in the Soft X-Ray regime (8 to 30 nanometer). However, conventional lenses do not exist anymore for this wavelength regime, making it an extremely hard challenge to make a microscope set-up.
It turns out that images can be generated without the need for any lenses, i.e. lensless imaging, solving two problems at the same time. We are not limited anymore by the quality of the imaging optics and in addition images can be made with wavelengths where conventional lenses do not exist. Lensless imaging does not make an image directly like conventional microscopes, but rather records the scattered light by the object of interest, so called diffraction patterns. A computer algorithm resolves a synthetic image by processing a set of these diffraction patterns. Lensless imaging allows us to to “see” very small features with dimensions of the SXR wavelength in the range of 8-30 nm. The LINX project even aims to retrieve sub-wavelength resolution by optimizing these lensless imaging techniques with prior information. The video below shows a rendered impression of the source itself and its application.