Observation of Anomalous Orbital Angular Momentum Conservation in Parametric Nonlinearity

TL;DR

This paper reports an unexpected anomaly in orbital angular momentum conservation during parametric upconversion, revealing hidden OAM transfer mechanisms that could advance control of light's angular momentum in nonlinear and quantum optics.

Contribution

It uncovers a novel anomalous OAM conservation phenomenon in parametric nonlinear interactions involving specially engineered pump modes.

Findings

Anomalous OAM conservation observed during sum-frequency generation.

Hidden OAM transfer occurs in residual pump modes.

Insights may lead to new control methods for OAM in quantum optics.

Abstract

Orbital angular momentum (OAM) conservation plays an important role in shaping and controlling structured light with nonlinear optics. The OAM of a beam originating from three-wave mixing should be the sum or difference of the other two inputs because no light-matter OAM exchange occurs in parametric nonlinear interactions. Here, we report anomalous OAM conservation during parametric upconversion, in which a Hermite-Gauss mode signal interacts with a specially engineered pump capable of astigmatic transformation in a crystal, resulting in Laguerre-Gaussian mode sum-frequency generation (SFG). The anomaly here refers to the fact that the pump and signal carry no net OAM, while their SFG does. We show that the lost OAM with the opposite sign that maintains OAM conservation in the system is hidden in the residual pump. This unexpected OAM selection rule improves our understanding of OAM conservation in parametric nonlinear systems and may inspire new ideas for controlling OAM states via nonlinear optics, especially in quantum applications.