Tunable nonlinear effects through focused spatially phase-shaped beams

Polarization of light is of high importance when it is used to probe and characterize optical materials. When studying nanostructures using microscopy techniques, it is necessary to control the three-dimensional (3D) vectorial fields under strong focusing. This allows the coupling of incident light to the nano-object to be controlled, opening new perspectives in, e.g., unambiguous characterization of molecular orientations. The 3D focal fields depend on proper balance between the transverse and longitudinal field components. Such opportunities have been demonstrated for radially and azimuthally polarized incident beams [1-3] and Hermite Gaussian HG10 laser mode [4]. Here, we take this approach a significant a step further by demonstrating that spatially phase-shaped incident beams can drive second-harmonic generation (SHG) from a single nanostructure in a controlled manner.
For our demonstration, we excited a single vertically aligned semiconductor (GaAs) nanowire from an ensemble of nanowires that were grown on top of a semiconductor substrate [5]. We used a custom-built point scanning SHG microscope powered by a femtosecond laser source. For selective phase-shaping of the input beam, we digitally addressed a phase-only spatial light modulator (SLM) to modify one or two lobes of the HG10 laser mode from 0 to Pi. Without phase delay applied, we obtained a highly symmetric SHG image from the nanowire. It arises from the mainly longitudinal fields produced at the focus of a HG10 laser mode and is consistent with previous results [4]. Interestingly, when the phase difference of one of the two lobes is adjusted to Pi/2 and further to Pi, we observed significant changes in the SHG image. Our work opens new avenues for controlling nonlinear interactions in individual nanostructures through polarization-encoded incident beams.

References
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