Nonlinear Conformal Transformation for In Situ IR-Visible Detection of Orbital Angular Momentum

Linear conformal transformation provides an effective way to detect orbital angular momentum (OAM) of photons, and particularly, their coherent superposition states that are significant for OAM-based technologies. However, these methods have limited applicability-they are applied to single or few separated wavelengths and cannot achieve nondestructive detection-although those features are attractive for practical applications. Here, the second-harmonic spiral transformation is theoretically described and experimentally demonstrated through IR-visible detection of OAM states from 900 to 1400 nm, with a low energy loss of approximate to 10(-6). Remarkably, a record high optical finesse of approximate to 5.52 is predicted and observed, indicating that a nonlinear enhancement factor resulting from OAM conservation significantly improves separation efficiency. Additionally, this scheme allows flexibility to achieve lower energy losses or higher sensitivity by adjusting phase-matching conditions. These results can be applicable to classical and quantum areas and promote conformal transformation into nonlinear regions.