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Published on: August 6, 2018
Measuring the molecular second hyperpolarizability in absorptive solutions by the third harmonic generation ratio
Danielle Tokarz1, Richard Cisek, Nicole Prent
1Department of Chemical and Physical Sciences, University of Toronto Mississauga, Mississauga, ON, Canada.
Analytica Chimica Acta
|November 14, 2012
Summary
Measuring molecular hyperpolarizability (γ) aids understanding third harmonic generation (THG). A new method using a nonlinear optical microscope and precise refractive index measurements revealed significant differences in γ values for chlorophylls and carotenoids.
Area of Science:
- Nonlinear optics
- Molecular spectroscopy
- Photochemistry
Background:
- Third harmonic generation (THG) signal intensity is influenced by molecular hyperpolarizability (γ).
- Accurate measurement of γ is crucial for understanding structure-property relationships in nonlinear optical materials.
- Previous methods for γ determination faced challenges, particularly with compounds exhibiting absorption near the fundamental or third harmonic wavelengths.
Purpose of the Study:
- To develop a convenient method for measuring the second hyperpolarizability (γ) of compounds in solution.
- To investigate the influence of refractive index differences at fundamental and third harmonic wavelengths on γ calculations.
- To determine the γ values of carotenoids and chlorophylls within photosynthetic pigment-protein complexes.
Main Methods:
- Implemented the THG ratio method using a nonlinear optical microscope to measure THG intensity from two interfaces.
- Developed a tunable refractometer (UV to near-IR) to accurately measure solution refractive indices at fundamental and third harmonic wavelengths.
- Calculated γ values by accounting for refractive index differences, especially for compounds with absorption near the tested wavelengths.
Main Results:
- Demonstrated that differences in refractive index at fundamental and third harmonic wavelengths significantly impact calculated γ values.
- Chlorophylls exhibited γ values over two orders of magnitude larger than carotenoids.
- Chlorophylls displayed negative γ values, while carotenoids showed positive γ values.
Conclusions:
- The developed THG ratio method with precise refractive index correction offers a reliable approach for measuring molecular hyperpolarizability.
- Significant differences in the sign and magnitude of γ values were observed between chlorophylls and carotenoids.
- These findings provide insights into the molecular structural requirements for enhanced THG signals in photosynthetic pigments.
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