Research / Coherence / Fundamental concepts

Vector-field coherence: The foundations of coherence theory of random vectorial light fields and the development of various interferometric schemes to measure such vector-light coherence form a long-standing, major research theme at our center. Over the years we have extended several results of scalar light fields into the domain of electromagnetic optics providing new insights into the physical interpretation of coherence in vectorial light beams and its interplay with the polarization characteristics. Among the most important results is the consistent formulation of the degree of coherence for vector-light fields and the measurement of the so-called polarization time in terms of two photon absorption. Very recent steps include, e.g., the development of ghost polarimetry and phase-contrast imaging, as well as the introduction of new techniques for measuring spectral and temporal coherence in stationary light. This subtopic contains collaboration with the University of Miami (USA), University of Groningen (The Netherlands), Technical University of Darmstadt (Germany), Tampere University (Finland), and Aalto University (Finland).

Left: Illustration of the polarization-state evolution on the Poincarè sphere and the polarization-sensitive Michelson interferometer employed in the measurement of polarization time in terms of two-photon absorption (from [2]). Right: Setup for measuring the spectral coherence Stokes parameters of a partially polarized light beam (from [5], cover picture).

Geometric phase: The Pancharatnam-Berry phase is an additional phase contribution that a light beam acquires when its polarization state undergoes a cyclic in-phase evolution. Besides of its fundamental importance in optics and physics in general, the functionality of some novel optical elements is based on its emergence. Such elements can perform, e.g., focusing of light beams and more sophisticated operations such as generation of various helical beams by utilizing a form of spin-orbit coupling. In our works, we have demonstrated theoretically and experimentally that the geometric phase appears also in both spatial and temporal interference patterns of vectorial beams with different polarization states. This is a completely new context for the geometric phase and its consideration is expected to be important in controlled two-beam interference. This topic contains collaboration with Aalto University (Finland).

Experimental setup used for studying the geometric phase in Young’s two-pinhole interference (from [1]).

Contact persons:
Dr. Atri Halder
Dr. Kimmo Saastamoinen
Asst. Prof. Andreas Norrman
Assoc. Prof. Tommi Hakala
Prof. Tero Setälä
Prof. Jari Turunen
Prof. Ari T. Friberg

1. A. T. Friberg and T. Setälä, “Electromagnetic theory of optical coherence [Invited]”, J. Opt. Soc. Am. A 33, 2431 (2016).
2. A. Shevchenko, M. Roussey, A. T. Friberg, and T. Setälä, “Polarization time of unpolarized light”, Optica 4, 64 (2017).
3. H. Partanen, B. J. Hoenders, A. T. Friberg, and T. Setälä, “Young’s interference experiment with electromagnetic narrowband light”, J. Opt. Soc. Am. A 35, 1379 (2018); “Editor’s pick”.
4. A. Hannonen, B. J. Hoenders, W. Elsässer, A. T. Friberg, and T. Setälä, “Ghost polarimetry using Stokes correlations”, J. Opt. Soc. Am. A 37, 714 (2020).
5. A. Hannonen, A. Shevchenko, A. T. Friberg, and T. Setälä, “Temporal phase-contrast ghost imaging”, Phys. Rev. A 102, 063524 (2020).
6. H. Partanen, A. T. Friberg, T. Setälä, and J. Turunen, “Spectral measurement of coherence Stokes parameters of random light beams”, Phot. Res. 7, 669 (2019). Cover article of Vol. 7, Issue 6, June 2019.
7. A. Hannonen, H. Partanen, J. Tervo, T. Setälä, and A. T. Friberg, “Pancharatnam-Berry phase in electromagnetic double-pinhole interference”, Phys. Rev. A 99, 053826 (2019).
8. A. Hannonen, K. Saastamoinen, L.-P. Leppänen, M. Koivurova, A. Shevchenko, A. T. Friberg, and T. Setälä, “Pancharatnam-Berry phase in polarization beating of optical beams”, New J. Phys. 21, 083030 (2019).
9. A. Halder, H. Partanen, A. Leinonen, M. Koivurova, T. K. Hakala, T. Setälä, J. Turunen, and A. T. Friberg, “Mirror-based scanning wavefront-folding interferometer for universal coherence measurements”, Opt. Lett. 45, 4260 (2020).
10. A. Hannonen, H. Partanen, A. Leinonen, J. Heikkinen, T. K. Hakala, A. T. Friberg, and T. Setälä, “Measurement of the Pancharatnam-Berry phase in two-beam interference”, Optica 7, 1435 (2020).
11. J. Laatikainen, A. T. Friberg, O. Korotkova, and T. Setälä, “Poincaré sphere of electromagnetic spatial coherence”, Opt. Lett. 46, 2143 (2021).
12. A. Halder, A. Norrman, and A. T. Friberg, “Poincaré sphere representation of scalar two-beam interference under spatial unitary transformations”, Opt. Lett. 46, 5619 (2021).