Multiphoton Transitions in a Spin System Driven by Strong Bichromatic Field

TL;DR

This paper investigates multiphoton transitions in a spin system driven by strong bichromatic fields, using EPR spectroscopy to measure effective fields and analyzing higher-order effects beyond first-order perturbation theory.

Contribution

It provides experimental and theoretical analysis of multiphoton Rabi oscillations in a spin system under strong bichromatic driving, including second and third order corrections.

Findings

Effective field amplitude behavior is characterized under strong MW fields.

Second-order corrections cause a Bloch-Siegert shift affecting nutation amplitude.

Third-order corrections modify the nutation frequency in inhomogeneously broadened lines.

Abstract

EPR transient nutation spectroscopy is used to measure the effective field (Rabi frequency) for multiphoton transitions in a two-level spin system bichromatically driven by a transverse microwave (MW) field and a longitudinal radio-frequency (RF) field. The behavior of the effective field amplitude is examined in the case of a relatively strong MW field, when the derivation of the effective Hamiltonian cannot be reduced to first-order perturbation theory in w_{1} / w_{rf} (w_{1} is the microwave Rabi frequency, w_{rf} is the RF frequency). Experimental results are consistently interpreted by taking into account the contributions of second and third order in w_{1} / w_{rf} evaluated by Krylov-Bogolyubov-Mitropolsky averaging. In the case of inhomogeneously broadened EPR line, the third-order correction modifies the nutation frequency, while the second-order correction gives rise to a change in the nutation amplitude due to a Bloch-Siegert shift.