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Large molecular third-order optical nonlinearities in polarized carotenoids
S R Marder1, W E Torruellas, M Blanchard-Desce
1Beckman Institute, California Institute of Technology, Pasadena, CA 91125, USA.
Science (New York, N.Y.)
|May 23, 1997
Summary
Researchers enhanced molecular hyperpolarizability (gamma) in pi-conjugated molecules by increasing intramolecular charge transfer (ICT). This led to a significant 35-fold increase in gamma, demonstrating a new mechanism for optimizing nonlinear optical properties.
Area of Science:
- Nonlinear optics
- Molecular engineering
- Materials science
Background:
- Garito and co-workers proposed a mechanism to enhance molecular second hyperpolarizability (gamma) in linear pi-electron-conjugated systems.
- Molecular hyperpolarizability is crucial for advanced optical materials and technologies.
Purpose of the Study:
- To investigate the relationship between intramolecular charge transfer (ICT) and the second hyperpolarizability (gamma) in carotenoids.
- To explore a mechanism for dramatically increasing gamma in pi-electron-conjugated molecules.
Main Methods:
- Synthesis and characterization of carotenoids with varying degrees of ICT.
- Measurement of gamma as a function of wavelength using third-harmonic generation (THG).
- Stark spectroscopic measurements to determine ground and excited state dipole moments (Delta mu).
- Quantum-chemical calculations to support experimental findings.
Main Results:
- A series of carotenoids with increasing ICT were examined.
- The compound with the highest ICT showed a 35-fold enhancement in peak gamma (gammamax) compared to beta-carotene.
- Stark spectroscopy confirmed a large difference dipole moment (Delta mu) between ground and excited states.
- Quantum-chemical calculations highlighted the role of states with large Delta mu.
Conclusions:
- Intramolecular charge transfer significantly enhances the second hyperpolarizability (gamma) in linear pi-electron-conjugated molecules.
- A large difference dipole moment (Delta mu) between ground and excited states is key to achieving high gamma values.
- This mechanism offers a pathway for designing molecules with superior nonlinear optical properties.