Second harmonic generation at the probe tip for background-free near-field optical imaging

Area of Science:

• Optical Imaging
• Nonlinear Optics
• Condensed Matter Physics

Background:

• Second harmonic generation (SHG) is used to reduce background noise in near-field optical imaging.
• Current SHG techniques are limited to samples with high second-order nonlinear susceptibility.
• A need exists for background-free imaging applicable to a wider range of samples.

Purpose of the Study:

• To present a versatile background-free SHG configuration for near-field optical imaging.
• To overcome the limitations of existing SHG techniques for samples with weak nonlinearities.
• To enable high-contrast near-field optical imaging.

Main Methods:

• Utilized the second-order nonlinear susceptibility of a probe as a near-field polarizer.
• Modeled probe-sample optical interactions using the coupled dipole method at fundamental and second harmonic frequencies.
• Analyzed a deuterated potassium dideuterium phosphate (DKDP) crystal probe (symmetry class 42m) numerically.

Main Results:

• The proposed configuration requires s-polarized incident light and specific crystal symmetry classes for the probe (222, 622, 422, 42m, 43m, 23).
• Numerical analysis demonstrated over a tenfold improvement in imaging contrast for samples with negligible nonlinear susceptibility.
• Performance was investigated concerning probe-sample distance, relative size, and crystal tilt angle.

Conclusions:

• The developed versatile background-free SHG configuration effectively filters background signals.
• This technique significantly enhances imaging contrast, particularly for samples with low second-order nonlinear susceptibility.
• The proposed method holds potential for widespread application in high-contrast near-field optical imaging.