# Complete energy conversion between light beams carrying orbital angular momentum using coherent population trapping for a coherently driven double-\Lambda atom-light coupling

## TL;DR

This paper presents a method for complete energy transfer between light beams with orbital angular momentum using atomic coherence in a double-or Lambda scheme, achieving near-perfect efficiency under adiabatic conditions.

## Contribution

It introduces a novel scheme for total energy conversion between vortex beams in cold atoms with a double-or Lambda configuration, utilizing spatially dependent control fields and adiabatic evolution.

## Key findings

- Achieves near-unity energy conversion efficiency with spatially dependent control fields.
- Demonstrates the feasibility of transferring OAM between light beams in atomic media.
- Shows that efficiency drops to 1/2 with spatially independent control fields.

## Abstract

We propose a procedure to achieve a complete energy conversion between laser pulses carrying orbital angular momentum (OAM) in a cloud of cold atoms characterized by a double-\Lambda atom-light coupling scheme. A pair of resonant spatially dependent control fields prepare atoms in a position-dependent coherent population trapping state, while a pair of much weaker vortex probe beams propagate in the coherently driven atomic medium. Using the adiabatic approximation we derive the propagation equations for the probe beams. We consider a situation where the second control field is absent at the entrance to the atomic cloud and the first control field goes to zero at the end of the atomic medium. In that case the incident vortex probe beam can transfer its OAM to a generated probe beam. We show that the efficiency of such an energy conversion approaches the unity under the adiabatic condition. On the other hand, by using spatially independent profiles of the control fields, the maximum conversion efficiency is only 1/2.