Molecular orientation sensitive second harmonic microscopy by radially and azimuthally polarized light

Area of Science:

• Optics and Photonics
• Microscopy
• Nonlinear Optics

Background:

• High numerical aperture (NA) objective lenses are crucial for achieving high resolution in microscopy.
• Controlling light polarization at the focus of high-NA objectives is essential for advanced imaging techniques.
• Understanding the influence of polarization on nonlinear optical effects is key for novel microscopy applications.

Purpose of the Study:

• To demonstrate the switching of the z-polarization component in high-NA objective lenses using radially and azimuthally polarized light.
• To investigate the impact of incident light polarization on nonlinear optical effects and signal generation.
• To experimentally evaluate the differences in nonlinear signal generation between radial and azimuthal polarization in biological tissue.

Main Methods:

• Simulations were performed to analyze the field distribution and nonlinear effects for various incident polarizations (linear, circular, radial, azimuthal) with high-NA objectives.
• The intensity contributing to nonlinear signal generation was computed based on input polarization.
• Experiments were conducted using radially and azimuthally polarized vortex beams on native collagen tissue (porcine cornea) to evaluate signal differences.

Main Results:

• High-NA focusing generates a significant z-polarization component for linear, circular, and radially polarized light.
• Azimuthal polarization is exceptional, producing no z-component even under strong focusing.
• No distinct differences in nonlinear signal generation intensity were found between comparable input polarization states above chosen thresholds.

Conclusions:

• The study demonstrates the ability to control the z-polarization component at the focus of high-NA objectives by selecting incident light polarization.
• This control allows for probing molecular orientation along the optical axis in samples with known nonlinear properties.
• Combining simulations and experiments can enhance the understanding of nonlinear signal formation in various materials.