Polymer microwire devices for the exploration of the non-linear magnetic resonance domain*

Electric currents in organic thin-films under bipolar charge carrier injection conditions are sensitive probes for non-linear magnetic resonance phenomena as they are controlled by the singlet−triplet ratios of recombining electron−hole pairs. By probing these current, magnetic resonance becomes detectable at very low magnetic fields where spin polarization is absent. We use this to study ultrastrong light-to-matter coupling regimes, in which charge carrier (polaron) spin Rabi frequencies are in the same magnitude range as the Zeeman frequencies of these systems. To achieve such an experiment, we have raised the AC driving field B₁ of the magnetic resonant excitation above the static resonance field B₀ [1]. Technologically, this is achieved by scaling of the thin films to the micrometer domain and by fabrication directly on top of RF microwires, within entirely monolithic thin-film device structures. When ultrastrong light−matter coupling is achieved, the individual resonant spin transitions of electron−hole pairs become indistinguishable and a collective triplet state formation occurs, causing a pronounced quenching of the recombination current. [1] S. Jamali, et al., Nano Lett., 17, 4648 (2017).