Nonlinear Optical Properties of Organic and Molecular Materials

Nonlinear optical phenomena in organic and molecular materials arise from the interplay of electron delocalisation, molecular asymmetry and supramolecular order, yielding large hyperpolarizabilities and ultrafast response times. Key processes include second-harmonic generation, two-photon absorption and the optical Kerr effect, which underpin applications in frequency conversion, optical modulation, bioimaging and environmental sensing. Architectures such as donor–π–acceptor small molecules, conjugated polymers, dendrimers and metal–organic frameworks have been tailored to maximise macroscopic noncentrosymmetric alignment and transparency across UV–visible–infrared regions. Rational design employing computational prediction of molecular hyperpolarizabilities guides synthetic efforts, while film processing and crystal growth techniques address phase-matching, thermal stability and long-term durability. Hybrid systems that couple organic chromophores to photonic cavities or plasmonic elements further boost local fields and nonlinearity. Together, these advances are broadening the practical impact of organic nonlinear optics in compact laser sources, all-optical switching devices and multiphoton imaging probes.

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Technical terms

Second-harmonic generation (SHG): Coherent process where two photons at a fundamental frequency combine to yield one photon at twice that frequency.

Two-photon absorption (2PA): Simultaneous absorption of two photons by a molecule, enabling deeper penetration and reduced photodamage in imaging.

Hyperpolarizability: Molecular parameter quantifying the extent of polarization induced by an applied electric field, central to second- and third-order nonlinear effects.

Kerr effect: Change in refractive index proportional to optical intensity, driving self-phase modulation and all-optical switching.

Multiphoton photoluminescence (MPL): Light emission following nonlinear absorption of multiple photons, used in high-resolution microscopy and sensing.

Phase matching: Alignment of propagation constants among interacting waves, essential for efficient frequency conversion.

Donor–π–acceptor motif: Molecular structure combining electron-donating and electron-accepting groups across a conjugated bridge to enhance charge transfer and nonlinearity.

References

1. Nonlinear plasmonics: second-harmonic generation and multiphoton photoluminescence. PhotoniX (2023).
2. Nonlinear optics in plasmonic nanostructures. Journal of Optics (2018).
3. Enhanced second-harmonic generation from double resonant plasmonic antennae.. Optics Express (2012).
4. A full vectorial model for pulse propagation in emerging waveguides with subwavelength structures part I: Kerr nonlinearity.. Optics Express (2009).