THz imaging of Talbot carpets and Orbital Angular Momentum Beams
The aims of this Master thesis are
- to demonstrate Talbot imaging of a THz wave and its evaluation in two dimensions.
- to generate and measure Orbital Angular Momentum (OAM) beams in the near- and farfield.
Results of this Master thesis will be published in a peer-reviewed journal.
Contributors and collaborators
Krispin Dettlaff, Master Student, ETH Zurich, D-MAVT
- The Talbot effect is a self-imaging of periodic gratings illuminated by monochromatic radiation. It describes the periodic repetition of planar field distributions at the so called fundamental Talbot length
behind a grating. To determine is crucial for applications that exploit this kind of lensless imaging. For that purpose, a reliable method to detect within a stack of recorded images was developed. The method is based on band-pass filtering and image correlation. The advantage over the conventional technique of imaging the Talbot carpet in one dimension is that it is applicable also for two-dimensional gratings with radial and azimuthal periodicities. - A fork grating is an optimized form of linear amplitude gratings to generate Orbital Angular Momentum (OAM) carrying beams in its diffraction orders. OAM is an additional quality of light represented by a twisted phasefront of the beam. These beams also called vortex beam opened a new perspective in optics. While several methods exist to detect OAM in farfield, a technique to measure OAM in the near-field of fork gratings was still lacking. Therefore the aim of this part of the project was to verify the typical helical phasefront in the near-field diffraction of fork gratings. To this end a method based on Talbot self-imaging and phase retrieval was developed and successfully tested in experiments with THz radiation.
Bilder auf der nächsten Seite:
Talbot carpet of a linear grating measured with THz radiation
An OAM carrying beam in the far-field (left) and its detection in the focal plane of a cylindrical lens (right)