[Paper Review] Non-linear magneto-optic and self polarization rotation by superposition of states

Swarupananda Pradhan, A. Kani+3 more|arXiv (Cornell University)|Apr 13, 2012

Quantum optics and atomic interactions3 citations

TL;DR

This paper demonstrates non-linear magneto-optic and self-polarization rotation in atomic samples via coherent superposition of hyperfine states, induced by two-photon Raman excitation. It reveals sharp spectral features due to coherent population trapping, with a density matrix simulation that accurately reproduces experimental signals under varying laser polarization and detuning conditions.

ABSTRACT

We report the observation of enhanced magneto-optic rotation as the coherent superposition of different hyperfine states is established in an atomic sample. The polarization rotation near the two photon Raman resonance condition appears to have an analogous characteristic to the well established Faraday rotation observed in the vicinity of a single photon resonance, however it contains sharp features arising from coherent population trapping state. The profile of the two photon rotation signal exhibits interesting features for slightly imbalanced circular polarization component of the laser field as well as for on and away from the single photon resonance. The investigation can be used to explore the effect of superposition states generated by coherent population trapping on optical activity. A complete density matrix based numerical simulation that consistently captures all the relevant features of the experiment is presented.

Motivation & Objective

• To investigate non-linear magneto-optic effects arising from coherent superposition of hyperfine states in atomic samples.
• To explore how coherent population trapping influences polarization rotation under two-photon Raman excitation.
• To characterize the spectral response of polarization rotation when laser fields are imbalanced in circular polarization.
• To model and reproduce experimental observations using a complete density matrix formalism.
• To examine the behavior of polarization rotation both on and off single-photon resonance conditions.

Proposed method

• Utilizes a two-photon Raman excitation scheme to coherently couple hyperfine states in an atomic vapor.
• Employs a density matrix approach to model the full atomic system, including coherence and population dynamics.
• Simulates polarization rotation as a function of laser detuning and relative circular polarization components.
• Analyzes the system under magnetic fields to observe magneto-optic rotation effects beyond linear response.
• Compares simulated rotation profiles with experimental data to validate the model.
• Incorporates coherent population trapping as a key mechanism for non-linear signal generation.

Experimental results

Research questions

• RQ1 How does the coherent superposition of hyperfine states affect magneto-optic rotation in atomic systems?
• RQ2 What role does coherent population trapping play in shaping the spectral features of two-photon polarization rotation?
• RQ3 How do imbalances in the circular polarization components of the laser fields modify the observed rotation signal?
• RQ4 How does the polarization rotation profile change when the system is on or off single-photon resonance?
• RQ5 Can a density matrix model quantitatively reproduce the non-linear magneto-optic and self-polarization rotation features?

Key findings

• Enhanced magneto-optic rotation is observed when coherent superposition of hyperfine states is established, exceeding linear Faraday rotation effects.
• The two-photon rotation signal exhibits sharp spectral features due to coherent population trapping, particularly near the two-photon resonance.
• Imbalanced circular polarization components in the laser field lead to asymmetric and structured rotation profiles.
• The rotation signal shows distinct behavior on and off single-photon resonance, with non-trivial dependence on detuning.
• The density matrix simulation accurately captures all experimental features, including non-linear and interference effects.
• The system exhibits self-polarization rotation, where the probe field's polarization is rotated due to its own interaction with the coherently prepared atomic state.