Nonlinear optics with full three-dimensional illumination

Nonlinear optical interactions play a crucial role in modern technology and lead to important applications such as optical switching, optical harmonic generation, and the characterization of ultrafast material processes. Nonlinear interactions are enhanced by using a tightly focused laser beam, but nonetheless they are typically excited by a loosely focused (that is, paraxial) laser beam. Here we investigate a specific process, third-harmonic generation, excited by a highly nonparaxial beam that illuminates an interaction region from a nearly full solid angle. We elucidate the influence of the focal volume and the pump intensity on the number of frequency-tripled photons by varying the solid angle from which the pump light is focused, and we find good agreement between the experiments and numerical calculations. As the pump light is focused to a spot size much smaller than the laser wavelength, the Gouy phase does not limit the yield of frequency-converted photons, in stark contrast to the paraxial regime. We believe that our findings are generic and apply to many other nonlinear optical processes when the pump light is focused from a full solid angle.