Second harmonic generation theory for a helical macromolecule with high sensitivity to structural disorder

Area of Science:

• Theoretical Chemistry
• Nonlinear Optics
• Molecular Spectroscopy

Background:

• Second harmonic generation (SHG) is a crucial nonlinear optical process.
• Understanding SHG in helical systems requires detailed microscopic theories.
• Previous models often lack comprehensive inclusion of all relevant interactions.

Purpose of the Study:

• To develop a microscopic theory for SHG in helical molecular systems.
• To incorporate non-local interaction effects and minimal coupling.
• To analyze contributions from dipolar, quadrupolar, and magnetic terms, including a novel K coupling.

Main Methods:

• Developed a theoretical framework based on minimal coupling representation.
• Extended the theory to second order in the applied electromagnetic field.
• Derived a compact expression for the SHG response, isolating quadratic coupling terms (K coupling).

Main Results:

• A unified expression for SHG incorporating dipolar, quadrupolar, and magnetic contributions was derived.
• The K coupling term was identified and analyzed.
• Dipolar and quadrupolar contributions were shown to correlate with system symmetry and homogeneity, while K coupling indicates inhomogeneities.

Conclusions:

• The developed theory provides a comprehensive description of SHG in helical systems.
• The K coupling term offers a new way to probe molecular system inhomogeneities.
• This work advances the understanding of light-matter interactions in complex chiral structures.