Two-Photon Absorption Phenomena in Organic Materials

Two-photon absorption (2PA) is a third-order non-linear optical process in which a molecule simultaneously absorbs two photons of lower energy to reach an excited electronic state. This phenomenon has garnered considerable interest in organic materials owing to its potential for deep-tissue imaging, three-dimensional microfabrication and optical data storage. In organic chromophores, the magnitude of the 2PA cross section depends critically on molecular architecture, conjugation length and donor–acceptor interactions. Quadrupolar, octupolar and dendritic frameworks often yield enhanced cross sections by promoting cooperative electronic coupling and delocalisation. Precise measurement of 2PA requires reference standards and ultrafast laser systems, while the design of high-performance chromophores demands careful balancing of photostability, solubility and synthetic accessibility. Applications extend from two-photon excited fluorescence microscopy and photodynamic therapy to two-photon induced polymerisation and optical limiting. Recent advances in macromolecular scaffolds such as dendrimers and in small-molecule squaraine or indolic dyes have significantly expanded the accessible range of excitation wavelengths, improved quantum yields and enabled real-time sensing of chemical vapours. The global significance of 2PA research lies in its capacity to drive innovations in medical diagnostics, additive manufacturing and secure optical communications.

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Two-Photon Absorption Phenomena in Organic Materials publication trend

The graph below shows the total number of articles in two-photon absorption phenomena in organic materials across all publications each year (not limited to Nature Index journals).

Technical terms

Two-photon absorption (2PA): A non-linear optical process in which two photons are absorbed simultaneously to excite a molecule to a higher energy state.

Cross section (δ): A quantitative measure of the probability of two-photon absorption, typically expressed in Göppert–Mayer units (GM).

Chromophore: A structural unit within a molecule responsible for light absorption and electronic excitation.

Dendrimer: A highly branched, tree-like macromolecule with controlled generation and architecture, often used to enhance optical properties.

Reference standard: A well characterised material with established two-photon absorption parameters used for calibration and validation of experimental measurements.

References

1. Two-Photon Absorbing Dendrimers and Their Properties—An Overview. International Journal of Molecular Sciences (2024).
2. Two-photon absorption standards in the 550-1600 nm excitation wavelength range. Optics Express (2008).
3. Rational design of small indolic squaraine dyes with large two-photon absorption cross section. Chemical Science (2015).
4. Cooperative enhancement versus additivity of two-photon-absorption cross sections in linear and branched squaraine superchromophores. Physical Chemistry Chemical Physics (2016).