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Progress in the study of coherence and vortex dynamics of random phase scattered light
[ Instrument R&D of Instrument Network ] Recently, the Information Optics and Optoelectronics Technology Laboratory of Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences and the Department of Electronic Engineering of Princeton University in the United States collaborated on the speckle formed by the nonlinear propagation of random phase beams with different coherence lengths The field statistics were measured experimentally, and it was observed that with the generation of free light vortices, the autocorrelation function of the speckle field degraded from power law attenuation to exponential attenuation. This study uses photonics system to quantify experimental verification of Berezinskii-Kosterlitz-Thouless (BKT) phase transition theory in condensed matter physics, which proves that there are some common between nonlinear optics and condensed matter physics, cold atom physics, etc. The theoretical basis of the paper reveals the deep and complex relationship between the correlation of random light fields and vortex dynamics, and provides a new basis for further exploration of coherent-vortex dynamics in non-equilibrium states. Related research results are published in "Nature Photonics" under the title of Dynamics of the Berezinskii-Kosterlitz-Thouless transition in a photon fluid.
In the photon BKT experiment, the researchers used the randomness of the input light waves to simulate the "temperature" of a two-dimensional system. This can be achieved by random phase encoding of incident light waves: a higher "temperature" means a shorter coherence length of the random phase. The light beam with random phase encoding is input into a photorefractive crystal SBN applying an electric field, and its polarization state is adjusted to be consistent with the optical axis of the crystal. Under the action of an external electric field, light propagates in the crystal and self-defocusing occurs Or the self-focusing effect, this process is the same as the evolution of the wave function of the two-dimensional system with time, and can be described by the nonlinear Schrödinger equation. Through digital holographic imaging of the light field of the crystal output surface, the phase distribution can be reconstructed, and then the position and number of all vortices can be determined.
Under different applied negative voltages, the function relationship between the number of newly generated free vortices in the output speckle field and the "temperature" of the input field, the experimental data is completely consistent with the BKT theoretical prediction: when the "temperature" of the system is lower than a certain At a critical temperature, no free vortex is generated. At this time, the output speckle field remains quasi-long-range ordered, and its correlation function satisfies the power-law decay. When the "temperature" is higher than the critical value, the entropy of the system increases, and it tends to produce a new free vortex (because the energy of the vortex and entropy are natural logarithmic functions). As the "temperature" increases, the number of vortices increases, the vortices cause distortion of the field, the coherence of the system is destroyed, and its correlation function degenerates into exponential decay. By calculating the correlation index of the speckle field, we can clearly see the conversion of the system between these two "states".
The research work was supported by the National Frontier Science Key Research Project of the Chinese Academy of Sciences and the China-Germany Cooperation Group of the National Natural Science Foundation of China.