High-contrast nonlinear spiral phase contrast imaging via four-wave mixing in atomic medium.

Nonlinear spiral phase contrast imaging serves as a powerful tool for high-performance image edge detection in optical imaging. Compared to conventional computer-based digital imaging methods, it offers numerous possibilities for optical image processing with superior speed, lower energy consumption, and high information capacity. Here, we experimentally demonstrate the high-contrast nonlinear spiral phase contrast imaging in a diamond-type atomic system. A pump vortex-filtered beam (780 nm) and a signal beam with object image (776 nm) simultaneously interact with Rb atomic medium. As a result, a 420 nm beam is generated via the nonlinear four-wave mixing process, carrying the edge information of asymmetric Arabic numeral patterns. The geometric patterns such as triangle, circle, and square are further utilized to validate the effectiveness of nonlinear spiral phase contrast imaging. The high image contrast of ∼95.8% is achieved owing to the stringent phase matching conditions via the atomic four-wave mixing process. Moreover, the directional nonlinear spiral phase contrast imaging of circle and square patterns at 420 nm are realized by employing a Laguerre-Gaussian composite vortex filter on the 780 nm pump beam. This work establishes a versatile platform for multi-wavelength optical image analysis and provides a robust foundation for developing optical information processing methods.