A quantitative assessment approach of feasible optical mechanisms contributing to structural color of golden-like Chrysina aurigans scarab beetles

Light Reflection by Cuticles of Chrysina Jewel Scarabs: Optical Measurements, Morphology Characterization, and Theoretical Modeling

“…As also shown elsewhere [12] [13], the presence of defects through the chiral structures contributes with left-handed non-coherent light to the reflected spectra. The measured left-handed circularly polarized reflection spectra have a contribution of predominantly coherent light with a small fraction of non-coherent radiation.…”

“…Due to the monotonically increasing pitch of the structure with the depth z through the procuticle, the reflection beyond the plateau is due to multiple reflections from the deepest spatial periods. Following a calculation method similar to that previously reported to analyze the spectral variation with the depth z in the reflection spectra of the C. aurigans [13], we have observed that for the silver-like C. chrysargyrea the reflection in the near infrared is determined by the deepest pitches located beyond 0.7 7.6 m s h ⋅ = µ . We conclude that through the last 3.3 μm of the structure the volume fraction of uric acid is larger than a value close to 0.59 which characterizes the rest of the structure, with larger fluctuations in the average refractive index.…”

“…The agreement between these behaviors of Δλ as a function of z and the spectral ranges of the measured broad reflection bands (see Figure 2(a) and Figure 2(b)) is remarkable. There is also a noticeable correlation between the z-dependence of Δλ and the evaluation of the reflection in terms of z [R(z)] recently reported by Azofeifa et al [13] for the golden-like C. aurigans. Other noteworthy feature is this: the Δλ b and Δλ a shown in Figure 9(a) are the wavelength band gaps at those depths where the spatial period displays its maximum and minimum values respectively.…”

“…As seen from the previous section, the key aspects to apply Berreman's framework are the knowledge of the optical constants and birefringence of the materials, as well as the pitches of the structures which in this case are dependent on the depth z through the cuticles. In the second paragraph of the Introducción we briefly mentioned how the pitch is obtained from SEM images, with additional details published elsewhere [12] [13]. We report here a revised version of the method previously followed.…”

“…Helicoidal structures characterized by a constant pitch P o have been considered in literature [10] as well as arrangements with a small step in the pitch from some depth through the cuticle [11]. Our group has recently reported for Chrysina aurigans scarab beetles the presence of a pitch P that shows a smooth dependence with depth z through the cuticle [7]- [13], i.e. P = P(z).…”