# Dressed Rabi oscillation in a crystalline organic radical

## TL;DR

This paper reports on the observation of frequency shifts during Rabi oscillations in a crystalline organic radical, attributed to spin-spin interactions, modeled with a nonlinear semi-classical approach, advancing understanding of spin dynamics without cavity effects.

## Contribution

It introduces the concept of dressed Rabi oscillation in a crystalline organic radical and provides a semi-classical model that explains the observed frequency shifts.

## Key findings

- Frequency shifts up to 11 MHz observed during Rabi oscillations.
- Nonlinear semi-classical model reproduces experimental results.
- Frequency shifts attributed to spin-spin interactions and demagnetizing fields.

## Abstract

Free electron laser-powered pulsed electron paramagnetic resonance (EPR) experiments performed at 240 GHz / 8.56 T on the crystalline organic radical BDPA reveal a tip-angle dependent resonant frequency. Frequency shifts as large as 11 MHz (45 ppm) are observed during a single Rabi oscillation. We attribute the frequency shifts to a "dressing" of the nutation by spin-spin interactions. A nonlinear semi-classical model which includes a temperature- and sample-geometry-dependent demagnetizing field reproduces experimental results. Because experiments are performed without a cavity, radiation damping -- the most common nonlinear interaction in magnetic resonance -- is negligible in our experiments.