Ph.D Thesis

Ph.D StudentSidorenko Pavel
SubjectAspects in Pulse Generation, Imaging and Pulse
Characterization in Ultrafast Optics
DepartmentDepartment of Physics
Supervisor PROF. Oren Cohen
Full Thesis textFull thesis text - English Version


In my doctoral thesis, I studied several topics in ultrafast optics, including nonlinear effects in propagation and generation of high harmonics, structure based resolution enhancement in Coherent Diffraction Imaging (CDI), single-shot (ultrafast) ptychography, novel algorithmical method for ultrafast laser pulse diagnostics and structure based resolution enhancement in spectroscopy. My thesis comprises of both experimental work, mostly on imaging (CDI and ptychography), and theoretical work, which include both numerical modeling and analytics.

The three (already published) aspects of this thesis are:

•           Efficient generation of extreme ultraviolet (EUV) and soft X-ray radiation through High Harmonic Generation (HHG). HHG is a process where useful coherent light at very short wavelengths, in the EUV or x-ray regions of the spectrum are generated by spectrally upconverting strong IR or visible lasers. However the conversion efficiency of HHG process is small (10-6).I proposed to significantly increase the conversion efficiency of HHG at short wavelengths (10 - 1 nm) by optimizing a technique, which is called Sawtooth-Grating-Assisted-Phase-Matching (SGAPM), that quasi-phase matched the HHG process.  I demonstrate numerically sawtooth GAPM in high-order harmonics, displaying the highest conversion efficiency of all QPM methods in HHG.

•           Coherent diffractive imaging of one dimensional objects with super resolution. CDI is a “lensless” technique for imaging.  In CDI, a highly coherent beam of radiation is incident on an object. The beam scattered by the object produces a diffraction pattern which is recorded by a camera. This recorded pattern is then used to reconstruct an image of the object via an iterative algorithm. However, one-dimensional (1D) signals are known to suffer from ambiguity that hampers their recovery from measurements of their Fourier magnitude. As part of my research, I demonstrated sparsity-based coherent diffraction imaging of one-dimensional objects using EUV radiation produced by high harmonic generation. Using sparsity as prior information removes the ambiguity (associated with phase retrieval problem of 1D object) and enhances the resolution beyond the physical limit of the microscope.

•           Ptychography is a scanning coherent diffractive imaging technique in ptychography, a complex-valued object is scanned in a step-wise fashion through a localized coherent beam. In each scanning step, the intensity of the diffraction pattern of the object is measured. Critically, the spatial support of illumination spot needs to be bigger than the step size so that neighboring diffraction patterns result from different, but overlapping, regions of the object. The set of measured diffraction patterns is used for reconstructing the complex valued object. The fact that ptychography is based on scanning results with long acquisition time, in the order of a second or more. Also, scanning limited resolution, vibration stability, drift and dynamic range weaken the performances of ptychographic microscopes. I propose and analyze single-shot ptychography, where tens or hundreds of quasi-localized partially-overlapping beams illuminate the object simultaneously. Various schemes for single-shot ptychography, in both transmission and reflection modes, with coherent and partially coherent illumination are proposed. Experimentally, I demonstrate single-shot ptychography with 180 millisecond acquisition time, using a sub-milliwatt blue diode laser that simultaneously illuminates the object with 49 partially overlapping beams.