Nanoscale optical coherence tomography using extreme ultraviolet radiation produced with a laser plasma source based on a gas puff target

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.