Identification of multi-photon transitions between magnetic dipole states using electrically detected magnetic resonant excitation with variable drive-field helicities.*

For magnetic resonance conditions where the amplitude of the resonant driving field B₁ is close to the static magnetic Zeeman field B₀, quantum-optical effects linked to the two photon helicities, including the Bloch-Siegert shift [1,2] or multiple photon effects [3,4,5], emerge. In order to study the latter, we built a radio frequency domain electron spin resonance setup which allows for driving fields with arbitrary polarization. For the detection of magnetic resonance at B₀ in the mT-range where spin polarization is all but vanishing, we used spin-dependent recombination currents in a fully deuterated form of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (d-MEH-PPV), in which proton-induced random hyperfine fields are minimized. We report results obtained for circularly, linearly, and elliptically polarized excitation states. [1] J. J. Sakurai, Modern Quantum Mechanics, Revised Edition. Addison-Wesley (1994); [2] J. Romhányi et al., Phys. Rev. B 92(5), 054422 (2015); [3] J. H. Shirley, Phys. Rev. 138 (4B), B979 (1965); [4] M. Göppert-Mayer, Annalen der Physik, 18(7-8), 466 (1931); [5] J. I. Kaplan and S. Meiboom, Phys. Rev. 106(3), 499 (1957); [6] D. P. Waters et al., Nature Phys. 11(11), 910 (2015).