M.Sc Thesis

M.Sc StudentWengrowicz Omri
SubjectTime-Resolved Imaging by Multiplexed Ptychography (TIMP)
DepartmentDepartment of Physics
Supervisor PROF. Oren Cohen
Full Thesis textFull thesis text - English Version


Imaging of non-repetitive ultrafast dynamical objects require ultrahigh-speed cameras. The frame rate is an important property of these systems. The introduction of electronic imaging sensors revolutionized high-speed imaging, enabling acquisition rates of up to 107 frames per second. However, further increase is limited by on-chip storage and electronic readout speed.

Over the last decade, various optical imaging techniques have been developed to allow frame rates that exceed the limits posed by the detectors. While all these techniques opened new opportunities in imaging, they add significant complexity to the imaging system. Moreover, the frame rate in these techniques is strongly and negatively coupled with the spatial resolution and field of view, and it is technically challenging to apply them in the short wavelengths’ spectral regions, including EUV and x-rays.

Recently, our group proposed Time-resolved Imaging by Multiplexed Ptychography (TIMP) as a promising approach to obtain ultrahigh-speed high-resolution imaging of complex-valued objects. In TIMP, multiple frames of a complex-valued object are reconstructed algorithmically from data recorded in a single camera exposure of a single shot ptychography (SSP) system. In order to retrieve separated frames from the single multiplexed measurement, ptychographic information multiplexing (PIM) is exploited so the pulses must differ in one of their properties (e.g. spatial profile). Since the frame rate and temporal resolution in TIMP are determined by the illumination source and not by the imaging system, this method is very flexible and can be applied across the electromagnetic spectrum. In this thesis, I demonstrate experimentally TIMP, reconstructing up to 36 complex-valued frames of an object (still at low spatial resolution and low frame rate).

The measured frames consist of digits and letters displayed on a spatial light modulator (SLM), and the laser source output was switched on and off in synchronization with the SLM’s updating rate. The probe pulses were then orthogonalized and temporally encoded using another SLM. I demonstrate two different encoding approaches to acquire mutually orthogonal pulses - an orbital angular momentum encoding which is based on the orthogonality of the Laguerre-Gaussian beams, and a phase gradient encoding which shifts transversely the probe pattern. Another encoding approach combines them both in order to increase the number of measurable frames in a single camera snapshot.

This thesis is an important first step in our group’s 5-year project to develop TIMP with sub-micron spatial resolution and THz frame rate.