A fully optical equivalent of nuclear magnetic resonance in quantum light fluids has been unveiled.

The rotation of the linear polarization of emitted light is directly correlated with the mixing of the polariton's spin. The rate of such temporal modulation can reach the gigahertz range due to the ultrafast dynamics of the polariton system. Scientists have established that this type of precession occurs only under specific resonant conditions of external "mixing" with the internal parameters of the system. The results of the researchers' study have been published in the journal Optica.

One of the most effective methods for controlling spins is Larmor precession, which occurs in magnetic materials placed in a transverse magnetic field, causing their spins to stably rotate (precess) around the magnetic field lines at a frequency proportional to the strength of the applied field.

“The use of an additional radiofrequency magnetic field that is in resonance with the precession frequency leads to the emergence of a resonant response in the studied system (for example, nuclear magnetic resonance (NMR) or electron magnetic resonance (EMR)), which can be measured and utilized. A vivid example of such application is the visualization of human body tissues in MRI medical devices,” noted co-author Stepan Baryshev, a research associate at the Skoltech Laboratory of Hybrid Photonics.

Physicists from the Skoltech Laboratory of Hybrid Photonics have discovered an effect analogous to traditional NMR in the so-called "liquid light" — polariton condensates. Notably, this effect was achieved using only optical fields, without magnetic fields.

The Skoltech researchers uncovered a resonance effect in the case of complete optical pumping of spin precession in microresonators at cryogenic temperatures. In previous studies, a group of scientists from the Skoltech Laboratory of Hybrid Photonics, led by Professor Pavlos Lagoudakis, demonstrated that in microresonator polaritons, the characteristic energy splitting induced by laser excitation with elliptical polarization functions as a magnetic field.

As a result, self-induced Larmor precession of the spins in polariton condensates occurs. By employing a newly developed methodology for gigahertz rotation of polariton condensates, the researchers achieved gigahertz spin precession with high phase stability. Similar to traditional NMR, spin precession occurs only when the rotation frequency is in resonance with the frequency of the self-induced Larmor precession.

“It is important to note that when resonance occurs, the polariton spin precession exhibits an extremely long spin dephasing time of 174 ns, which is twenty times greater than previously recorded values. This figure indicates the exceptionally high stability of the precession. Resonance was observed while varying different system parameters, such as rotation frequency, ellipticity of polarization, and laser pump power,” continued Stepan Baryshev.

The researchers also developed a rigorous numerical model that replicates the results of the experimental studies. Furthermore, for the first time in polariton condensates, the researchers were able to determine the spin coherence time T2, equal to 320 ps, based on the shape of the observed spin resonance. T2 is an important temporal indicator regarding potential applications of polaritons, as it characterizes the possible speed of spin manipulation of the polariton and allows for comparisons with other physical systems.

The resonance mechanism discovered by the scientists opens up new exciting possibilities for developing innovative spintronic devices that enable control over sources of coherent, nonlinear, and twisted light. Additionally, the new mechanism may be beneficial for creating a source of coherent light with rotating linear polarization at gigahertz frequencies. The ability to control high-speed spins also paves the way for creating innovative probing methods and quantum systems with continuous variables based on polariton condensates. The obtained results may also provide the opportunity for coherent control of the spin state of the condensate analogously to traditional NMR methods, and potentially the use of this new method at room temperature with materials that have more stable excitonic resonances.

The experimental part of the study was conducted at the Skoltech Center for Photonics and Photonic Technologies. The Skoltech research group included not only the first author of the article, Skoltech graduate Ivan Gnusov, but also research associate Stepan Baryshev, senior lecturer Sergey Alyatkin, junior researcher Kirill Sitnik, and Professor Pavlos Lagoudakis. Significant contributions to the theoretical part of the work were made by Dr. Helgi Sigurdsson (University of Warsaw and University of Iceland).