Non-Bloch-Siegert-type power-induced shift of two-photon electron paramagnetic resonances of charge-carrier spin states in an OLED

TL;DR

This study combines Floquet theory and EDMR experiments to analyze two-photon magnetic resonance shifts in charge-carrier spins within OLED materials, revealing power-dependent shifts that are independent of drive helicity and confirming theoretical predictions.

Contribution

The paper provides the first combined theoretical and experimental analysis of two-photon resonance shifts in organic semiconductors under strong magnetic drive conditions, demonstrating the robustness of Floquet theory.

Findings

Two-photon resonance shifts are nearly drive-helicity independent.

Floquet theory accurately predicts nonlinear magnetic resonance behaviors.

Experimental results confirm theoretical predictions under strong drive conditions.

Abstract

We present Floquet theory-based predictions and electrically detected magnetic resonance (EDMR) experiments scrutinizing the nature of two-photon magnetic resonance shifts of charge-carrier spin states in the perdeuterated $\pi$-conjugated polymer poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene vinylene] (d-MEH-PPV) under strong magnetic resonant drive conditions (radiation amplitude $B_1$ ~ Zeeman field $B_0$). Numerical calculations show that the two-photon resonance shift with power is nearly drive-helicity independent. This is in contrast to the one-photon Bloch-Siegert shift that only occurs under non-circularly polarized strong drive conditions. We therefore treated the Floquet Hamiltonian analytically under arbitrary amplitudes of the co- and counter-rotating components of the radiation field to gain insight into the nature of the helicity dependence of multi-photon resonance shifts. In addition, we tested Floquet-theory predictions experimentally by comparing one-photon and two-photon charge-carrier spin resonance shifts observed through room-temperature EDMR experiments on d-MEH-PPV-based bipolar injection devices [i.e., organic light emitting diode structures (OLEDs)]. We found that under the experimental conditions of strong, linearly polarized drive, our observations consistently agree with theory, irrespective of the magnitude of $B_1$, and therefore underscore the robustness of Floquet theory in predicting nonlinear magnetic resonance behaviors.