Contents lists available at ScienceDirect Optics Communications journal homepage: www.elsevier.com/locate/optcom Nanoscale imaging with table-top coherent extreme ultraviolet source based on high harmonic generation Khuong Ba Dinh a,b, ⁎ , Hoang Vu Le a , Peter Hannaford a , Lap Van Dao a a Centre for Quantum and Optical Science, Swinburne University of Technology, Melbourne, Vic 3122, Australia b University of Science and Technology, University of Danang, Vietnam ARTICLE INFO Keywords: Laser application High harmonic generation Coherent diffractive imaging ABSTRACT A table-top coherent diffractive imaging experiment on a sample with biological-like characteristics using a focused narrow-bandwidth high harmonic source around 30 nm is performed. An approach involving a beam stop and a new reconstruction algorithm to enhance the quality of reconstructed the image is described. 1. Introduction Microscopy is a critical enabling technology for visualizing objects with high resolution imaging down to the nanometer scale in order to study dynamic processes in material and biological systems. Conventional visible light microscopy can image living cells with a resolution as high as 200 nm [1]. However, its resolution is typically limited to λ/2NA, where λ is the wavelength of the light source and NA is the numerical aperture. To significantly improve resolution, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) have been widely used and atomic resolution has been demon- strated [2]. Unfortunately, electron microscopes are limited by the mean free path of the charged particles, and therefore this technique is restricted to imaging thin samples, typically < 500 nm. For thickness as larger than 500 nm, because of inelastic scattering, this technique produces a blurred image and low resolution. Therefore, new techni- ques for high resolution imaging of thick samples are of great interest. Coherent x-ray diffractive imaging (CDI) using short wavelength light in the extreme ultraviolet or soft x-ray regions has emerged as a promising alternative approach to address the above problem [3–7]. Especially, CDI is a very useful method to investigate biological samples. Coherent diffractive imaging (CDI) is a powerful tool for imaging in which the optical lens used to reconstruct the sample's image is replaced by a computer-based reconstruction algorithm [3–7]. When an object is exposed to light from a coherent source, a diffraction pattern of the object is captured and based on diffraction and propagation theory the complex electric field of the light diffraction can be considered as a Fourier transform of the object. The object's image is then reconstructed by performing an inverse Fourier trans- form. Because only the intensity of the diffraction pattern is recorded a Fourier-based iterative phase-retrieval algorithm combined with an over-sampling method is used to recover the phase for image recon- struction process. This “lens-less” technique is aberration-free so that it is suitable for use at extreme UV and soft X-ray wavelengths and the theoretical spatial resolution is limited only by the radiation wave- length. In addition, because x-ray radiation can penetrate thicker samples, the CDI technique can overcome the limitations of an electron microscope and can be used as a promising approach for high resolution imaging of thick samples. Besides the radiation from synchrotrons [4] and free electron x-ray lasers [5], high harmonic generation (HHG) sources which are generally produced by focusing a high intensity laser beam into a nonlinear medium [8–10] provide a new illuminating source for XUV and soft x-ray imaging, with their ultra-short pulses, excellent coher- ence properties and high degree of tunability [6,11–16]. Moreover, the generation of this source only requires a compact table-top setup, which enables small scale x-ray microscopy. The radiation of the harmonics can be explained by a three-step model [8–10], in which, free electrons produced by ionization by the laser field and then accelerated by the laser field recombine with their parent ions, releasing energy as single high energy photons. Basically, in order to meet the requirements of the image reconstruction algorithm, a monochromatic wave field CDI is conducted with a single harmonic order which can be selected using XUV focusing mirrors. In addition, since the single harmonic beam is focused into a tiny area ( < 50 μm) comparable to the sample size the effective photon flux illuminating the sample strongly increases. Consequently, the acquisition time of a high-dynamic range diffraction image can be dramatically reduced. By http://dx.doi.org/10.1016/j.optcom.2017.03.046 Received 3 December 2016; Received in revised form 13 March 2017; Accepted 20 March 2017 ⁎ Corresponding author. Present address: Quantum Optics and Laser Science group, Department of Physics, Imperial College London, South Kensington campus, London SW7 2AZ, UK. E-mail address: kdinh@imperial.ac.uk (K. Ba Dinh). Optics Communications 396 (2017) 100–104 Available online 23 March 2017 0030-4018/ © 2017 Elsevier B.V. All rights reserved. MARK