Controlled fluorescence in a beetle's photonic structure and its sensitivity to environmentally induced changes

Abstract

The scales covering the elytra of the male Hoplia coerulea beetle contain fluorophores embedded within a porous photonic structure. The photonic structure controls both insect colour (reflected light) and fluorescence emission. Herein, the effects of water-induced changes on the fluorescence emission from the beetle were investigated. The fluorescence emission peak wavelength was observed to blue-shift on water immersion of the elytra whereas its reflectance peak wavelength was observed to red-shift. Time-resolved fluorescence measurements, together with optical simulations, confirmed that the radiative emission is controlled by a naturally engineered photonic bandgap while the elytra are in the dry state, whereas non-radiative relaxation pathways dominate the emission response of wet elytra.

Nonlinear optical spectroscopy and two-photon excited fluorescence spectroscopy reveal the excited states of fluorophores embedded in a beetle’s elytra

“…The peak shift may seem, however, to contrast with the previously reported 17-nm blue-shift of the OPEF peak wavelength (from 463 nm to 446 nm) [14]. In the dry state, with a 355 nm excitation wavelength, the emission peak was positioned within the photonic band gap, a range where the local density of optical state is very sensitive to changes in the refractive index contrast induced upon contact with water [14]. This opposite and smaller shift observed with TPEF can be explained by the fact that the selection rules are entirely different for the one-and two-photon absorption processes [33].…”

“…At emission wavelengths far from the photonic band gap, τ is almost constant. Similar but non-polarised measurements reported previously gave rise to much shorter radiative decay times [14] ranging from 3.9 ns in the dry state and inside the gap to 1.4 ns in the wet state and outside the gap ( Supplementary Table 1). This clearly means that the real excited states and their related radiative decay times are affected by the polarisation state of the excitation light.…”

Unveiling the nonlinear optical response of Trictenotoma childreni longhorn beetle

“…Interestingly, no SHG signal was detected in the case of Hoplia coerulea scarab beetle , where the absence of detected SHG signal may be explained by a centrosymmetric distribution of molecules within that be quite unexpected considering the fluorescence response of H. coerulea was shown, most beetle's nanostructured scales. This centrosymmetry would likely, to be due to complex fluorescent molecules that allow multiple resonance transfers between donor and acceptor groups . Unlike the case of H. coerulea , THG signal was not observed for T. childreni , but we believe this is due to the low laser intensity, which was necessary to prevent damage to the samples.…”

Linear and nonlinear optical effects in biophotonic structures using classical and nonclassical light

“…In these scales, a photonic periodic multilayer (Figure B) is found giving rise to its blue‐violet appearance (Figure C) . Fluorophores embedded within this multilayer emit a turquoise coloration (Figure D) under incident ultraviolet light . Both the color and the fluorescence emission are controlled by the photonic structure: upon contact with fluids, the fluorescence emission peak is observed to blue‐shift, leading to a navy blue color (Figure F) whereas the reflectance peak wavelength undergoes a red‐shift, giving rise to a green appearance (Figure F) , following liquid penetration into the scale structure and the filling of its air pores.…”

“…Fluorophores embedded within this multilayer emit a turquoise coloration (Figure D) under incident ultraviolet light . Both the color and the fluorescence emission are controlled by the photonic structure: upon contact with fluids, the fluorescence emission peak is observed to blue‐shift, leading to a navy blue color (Figure F) whereas the reflectance peak wavelength undergoes a red‐shift, giving rise to a green appearance (Figure F) , following liquid penetration into the scale structure and the filling of its air pores. Very recently, two‐photon excitation fluorescence and Third Harmonic Generation (THG) analyses (as detailed further in the next section) highlighted the multi‐excited states character of H. coerulea 's fluorophores as well as the anisotropy of its photonic structure .…”