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Bridging the gap between natural photonics, nonlinear optics and quantum optics

3 May | By Biophotonics.World
Bridging the gap between natural photonics, nonlinear optics and quantum optics
The male Hoplia coerulea beetle exhibit a blue visual appearance. This colouration (top left) is due to the presence of scales covering its body (bottom left). Upon illumination by ultraviolet light, these scales exhibit a turquoise colour through fluorescence processes (top centre). Nonlinear techniques such as two-photon excitation microscopy (bottom centre) also allow to image the scales and to collect additional information e.g., related to the fluorophores embedded in the scales of the beetle. Both colour and fluorescence emission are controlled by photonic structures found inside the scales (top and bottom right).
Image source: Reproduced from Mouchet S.R. et al., 2019, Interface Focus 9, 20180052 (top left, bottom centre, top right and bottom right) and Mouchet S.R. et al., 2016, Proc. R. Soc. B 283, 20162334 (bottom left and top centre).

Whereas pigments give rise to the majority of colours in biological systems by selective absorption of incident light,biological photonic structures found for instance in the tissues of some insects, birds and plants are optical devices giving rise to brilliant visual effects. Examples of these effects encompass structural colours (i.e., colours due to interference between incident light and these structures), water-induced colour changes, colour sensitivity to gases or vapours or polarisation effects. These structures consist in sophisticated materialsthat have evolved adapted characteristics for millions of years through natural selection. The striking colours they exhibit often play crucial roles in biological functions such as the organisms’ intra- and interspecific communication. Butterflies’ and beetles’ wings are among the most beautiful examples (figure).

In addition to pigments and photonic structures, many natural organisms emit light and display colours when they are illuminated by ultraviolet radiationthat cannot ordinarily be perceived by humans. This phenomenon is called fluorescence emission and arises from the presence of fluorophores embedded within the biological tissues of these organisms. This can be found in many species, terrestrial as well as aquatic, including arthropods (butterflies, beetles and scorpions), fish, marine invertebrates (from corals to worms), birds and plants. Moreover, the confinement of fluorophores within photonic structures gives rise to modifications of the associated fluorescence emission in terms of intensity, emitted wavelength or spatial distribution (i.e., direction of emission).

So far, the investigation of fluorescent or structurally coloured tissues from natural organisms has been exclusively performed with linear classical optical techniques. In a perspective article published in the Journal of Biophotonics [1], researchers from KU Leuven, the University of Namur and the University of Mons in Belgium, the University of Exeter in the UK and the University of Belgrade in Serbia demonstrated the importance of studying the optical properties of such biological photonic structures using nonlinear optical analyses and quantum techniques.

Nonlinear optics is the science of light propagating in media where the dielectric polarisation is not proportional to the electric field. This usually happens with very high light intensities such as that produced by lasers. Nonlinear optical techniques includes two-photon excitation fluorescence, second-harmonic generation and third-harmonic generation.

Quantum optics is the branch of optics for which light is considered as streams of particles called photons. Quantum techniques encompass ghost imaging, sub-Rayleigh imaging and squeezed light. In their article, these researchers showed that bridging the gap between natural photonics, nonlinear optics and quantum optics opens the road to new possibilities in biology and biomedical studies.

For example, nonlinear optical techniques were applied for the first time to study of the naturally controlled fluorescence in insects. The results related to case of the male Hoplia coeruleabeetle (figure) were recently published in Interface Focus [2]. The scales covering the beetle's wings are known to possess internal photonic structures containing naturally occurring fluorophores. These structures control both the beetle's colouration and the fluorescence emission. The analysis of these tissues by nonlinear optical techniques revealed undiscovered properties of the photonic structures and the embedded fluorophores.

 

Author:Dr Sébastien R. Mouchet, University of Exeter & University of Namur

 

Related journal articles:[1] https://doi.org/10.1002/jbio.201800262and [2] https://doi.org/10.1098/rsfs.2018.0052


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