Enhanced fidelity in nonlinear structured light by virtual light-based apertures.

Tailoring the degrees of freedom (DoF) of light for a desired purpose, so-called structured light, has delivered numerous advances over the past decade, ranging from communications and quantum cryptography to optical trapping and microscopy. The shaping toolkit has traditionally been linear in nature, only recently extended to the nonlinear regime, where input beams overlap in a nonlinear crystal to generate a structured output beam. Here we show how to enhance the fidelity of the structured output by aligning light with light. Using orbital angular momentum modes and difference frequency generation as an example, we demonstrate precise control of the spatial overlap in both the transverse and longitudinal directions using the structure of one mode as a virtual structured (in amplitude and phase) light-based aperture for the other. Our technique can easily be translated to other structured light fields as well as alternative nonlinear processes such as second harmonic generation and sum frequency generation, enabling advancements in communication, imaging, and spectroscopy.