Microscopic second harmonic generation (SHG) from tilt-placed collagen fibrils

Area of Science:

• Biophysics
• Nonlinear Optics

Background:

• Collagen's nonlinear optical properties are crucial for bio-imaging.
• Understanding second-harmonic generation (SHG) in collagen requires analyzing fibril orientation.
• Previous studies have not fully explored the combined effects of fibril tilt and light polarization on SHG.

Purpose of the Study:

• To theoretically investigate second-harmonic generation (SHG) from collagen fibrils.
• To determine the influence of collagen fibril tilt angle (Φ) and light polarization angle (α) on SHG emission.
• To analyze how these parameters affect the strength and spatial distribution of SHG signals.

Main Methods:

• Theoretical modeling of SHG from collagen fibrils.
• Simulation of SHG emission under varying fibril tilt angles (Φ) and polarization angles (α).
• Analysis of the total SHG strength (E(2ω), P(2ω)) and its components (E(2ω,p), P(2ω,p), E(2ω,s), P(2ω,s)).

Main Results:

• SHG strength (E(2ω), P(2ω)) peaks at a fibril tilt angle (Φ) of approximately 27° for all polarization angles (α).
• SHG emission significantly decreases for Φ > 27°, becoming undetectable at Φ ≥ 45°.
• Fibril tilt angle (Φ) influences the spatial patterns (lobes, shape) of SHG emission, while polarization angle (α) affects overall strength and distribution, with significant decreases when deviating from the x-axis.

Conclusions:

• Collagen fibril tilt angle (Φ) is a critical factor determining SHG signal strength and spatial distribution.
• Optimal SHG signal is observed around Φ = 27°, with rapid signal loss at higher tilt angles.
• Polarization angle (α) modulates SHG intensity and pattern, highlighting the importance of polarization control in collagen-based nonlinear imaging.