Optical coherence tomography (OCT) is a well-established non-destructive imaging technique providing high-resolution cross-sectional views of objects. The axial resolution of OCT is limited to single micrometers when using infrared and optical wavelengths. Recently, optical coherence tomography using broadband soft X-rays and extreme ultraviolet has been proposed to improve axial resolution. This variant of OCT, known as X-ray coherent tomography (XCT), enables axial resolution of a few nanometers. The paper presents OCT with the use of extreme ultraviolet in the wavelength range of 10–20 nm generated using a compact laser-produced plasma (LPP) source based on a double-stream gas puff target. The use of a gas puff target enables efficient extreme ultraviolet emission without producing target debris by laser ablation from a solid target. Two axisymmetric ellipsoidal grazing-incidence mirrors were used to focus the radiation from the source to the sample, and then to focus the radiation reflected from the sample to the spectrometer. We also present measurements on Silver/Zirconium multilayer periodic structures with a periodicity of 60 nm.
The optical coherence tomography (OCT), typically used in the visible wavelength range, due to relatively long wavelength is limited by the axial resolution to approximately a few hundreds of nanometers to a micron. Visible light is also incapable of resolving multilayered structures of tens of nanometers periods. Thus, the extension of the OCT to shorter wavelengths, such as the extreme ultraviolet (EUV) and soft X-ray (SXR), in the so-called X-ray coherence tomography (XCT), allows mitigating those problems. We present a nanometer resolution XCT using broadband SXR radiation with 2 nm axial resolution using a compact laser plasma soft X-ray source. The laser-produced Kr/He plasma was formed by the interaction of nanosecond laser pulses with a gaseous target in a double stream gas puff target approach, emitting 2 to 5 nm wavelength broadband radiation. The coherence parameters of the SXR radiation allowed for the OCT measurements of a bulk multilayer structure composed of Mo/Si multilayers with 10 nm period, with an axial resolution of about 2 nm, the interface position accuracy of sub-nm and detection of multilayer interfaces up to a depth of about 100 nm. The experimental data were compared to the OCT simulations.
Tomography is a 3-D imaging method, which allows producing 3-D images by image stacking and numerical refocusing, a spatially localized probing or by sample rotation. Usually, those methods are employed at visible range wavelengths of electromagnetic radiation. It is the simplest, most developed and most common approach since visible light is the part of electromagnetic radiation, which is the closest to humans. There are, however, certain limitations to the visible light methods, such as diffraction limit in the range of hundreds of nanometers, the incapability of direct imaging low-density objects, such as gasses, or the objects being completely opaque to the visible light radiation. Thus, the extension of those methods to the extreme ultraviolet (EUV) and soft X-ray (SXR) spectral ranges allows mitigating those problems. A few examples of such tomographic 3-D imaging experiments employing EUV and SXR compact, tabletop laser plasma sources will be presented and discussed.
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