Nonlinear optics has long relied on high-power lasers, whose spatial and temporal coherence ensures efficient nonlinear frequency conversion. However, a new study challenges the traditional assumption that coherence is merely a byproduct of light sources, instead treating it as a physical property that can be actively shaped and controlled.
In a paper published in Light: Science & Applications, a research team led by Prof. Ady Arie from Tel Aviv University introduces a method to synthesize spatial coherence during nonlinear interactions. Using periodically poled nonlinear photonic crystals, the researchers demonstrated the transfer of spatial coherence patterns—such as a smiley-face image—from the infrared spectrum to visible light through second-harmonic generation.
This approach provides an innovative way to encode and transfer spatial coherence patterns in nonlinear systems, enabling advanced imaging techniques. For instance, the ability to engineer coherence unlocks new possibilities in infrared imaging for medical diagnostics, environmental monitoring, and security applications.
The researchers also applied their method to generate structured light beams, such as incoherent vortex beams and Airy beams, which retained their signature properties, including orbital angular momentum conservation and self-acceleration, respectively. These results expand the potential of structured beams in fields such as optical trapping, secure communication, and biomedical imaging.
The process involves modulating the speckle fields of a fully incoherent light source to encode spatial coherence patterns. Figure (below) illustrates this experimental process, where a smiley-face pattern induced by incoherent light is transferred to the visible spectrum via second-harmonic generation.
The researchers summarize their work:
"Our method demonstrates that coherence is not just a byproduct of coherent light sources but a fundamental property that can be actively shaped. By harnessing this property, we provide a new framework for nonlinear optics, enabling structured light generation and advanced imaging."
This innovative approach invites future investigations into higher-order nonlinear processes, such as third-harmonic generation and four-wave mixing, in the context of coherence shaping. It sets the stage for further exploration of systems involving incoherent or partially coherent light interacting with structured or disordered nonlinear media.
The team forecasts:
"This technique opens new possibilities in optical imaging, structured beam shaping, and communications, paving the way for applications in medical diagnostics, smart sensors, and advanced microscopy."
