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Upon illumination by ultraviolet light, many animal species emit light through fluorescence processes arising from fluorophores embedded within their biological tissues. Fluorescence studies in living organisms are however relatively scarce and so far limited to the linear regime. Multiphoton excitation fluorescence analyses as well as non-linear optical techniques offer unique possibilities to investigate the effects of the local environment on the excited states of fluorophores. Herein these techniques are applied for the first time to the study of insects' natural fluorescence. The case of the male Hoplia coerulea beetle is investigated because the scales covering the beetle's elytra are known to possess an internal photonic structure with embedded fluorophores, which controls both the beetle's colouration and the fluorescence emission. An intense two-photon excitation fluorescence signal is observed, the intensity of which changes upon contact with water. A Third-Harmonic Generation signal is also detected, the intensity of which depends on the light polarisation state. The analysis of these non-linear optical and fluorescent responses unveils the multi-excited states character of the fluorophore molecules embedded in the beetle's elytra. The anisotropy of the photonic structure, which causes additional tailoring of the beetle's optical responses, is confirmed by circularly polarised light and non-linear optical measurements. arXiv:1801.07639v1 [physics.optics]

In this perspective article, we review the optical study of different biophotonic geometries and biological structures using classical light in linear and nonlinear regime, especially highlighting the link between these morphologies and modern biomedical research. Additionally, the importance of nonlinear optical study in biological research, beyond traditional cell imaging is also highlighted and described. Finally, we present a short introduction regarding nonclassical light and describe the new future perspective of quantum optical study in biology, revealing the link between quantum realm and biological research.

The wings of some insect species are known to fluoresce under illumination by ultraviolet light. Their fluorescence properties are however, not comprehensively documented. In this article, the optical properties of one specific insect, the Trictenotoma childreni yellow longhorn beetle, were investigated using both linear and nonlinear optical (NLO) methods, including one‐ and two‐photon fluorescence and second harmonic generation (SHG). These three distinct optical signals discovered in this beetle are attributed to the presence of fluorophores embedded within the scales covering their elytra. Experimental evidence collected in this study indicates that the fluorophores are non‐centrosymmetric, a fundamental requirement for SHG. This study is the first reported optical behavior of this type in insects. We described how NLO techniques can complement other more convenient approaches to achieve a more comprehensive understanding of insect scales and integument properties.

The synthesis and characterization of small monodisperse multimodal iron oxide nanoparticles (IONPs), functionalized with boron‐dipyrromethene (BODIPY) derivatives is described. Optical and relaxometric properties of the nanoparticles have been evaluated, suggesting a potential applicability of the particles as bimodal contrast agent for magnetic resonance and optical imaging. The BODIPY funcionalized IONPs showed bright fluorescence and high transverse relaxivity r2 which was in the range 47‐49 mM−1 s−1. Two‐photon fluorescence microscopy experiments performed at 1100 nm revealed that BODIPY‐functionalized IONPs showed significant increase in two‐photon fluorescence compared to the bare nanoparticles, suggesting their potential for applications in multimodal imaging. Cell viability studies on rat tumor (AR42J) and human ovarian cancer (SKOV3) cells showed no significant cytotoxicity at any of the tested concentrations, while greater sensitivity was observed for human hepatic stellate (GRX), human umbilical vein (HUVEC) and rat insulinoma (INS1) cells at higher nanoparticle concentrations.

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