Fig. 2

Optical characterization of single cocoon fibers. a Integrated hemispherical reflectance and emissivity (calculated by 1−reflectance−transmittance) spectra of a single comet moth cocoon fiber from the visible to the mid-infrared (λ = 400 nm–13.5 μm). The normalized spectral intensity of the AM 1.5 solar spectrum, the blackbody radiation spectrum at 300 K, and the atmospheric transparency window are plotted in the background. b Integrated hemispherical reflectance spectra of a single cocoon fiber illuminated with transverse electric (TE) and transverse magnetic (TM) polarized light at normal incidence, where TE polarization is defined with the electric field aligned with the longitudinal direction of the fiber. c Time-of-flight measurements of a single cocoon fiber. IRF represents the instrument response function, which is the cross-correlation of ultra-short reference (λ = 800 nm) and probe (λ = 600 nm) pulses. Cross-correlation between the reference pulse and a TE polarized probe pulse passing through the fiber (blue curve) shows a longer decaying tail compared with that in the case of TM polarization (red curve), indicating that TE polarized light interacts more strongly with the nanostructured fiber. Dashed curves are fits to the experimental data (solid curves) to extract photon lifetimes. d Schematic showing a focused laser beam at λ = 633 nm passing through a single cocoon fiber oriented in the vertical direction. Measured scattering pattern is shown on the right. Filamentary voids along the fiber prevents excessive scattering in the vertical direction; thus, the scattering pattern forms a horizontal narrow band. e Schematic showing the focused laser beam passing through a regenerated silk fiber bundle (as a control) containing a high density of nanoscale particulate voids (Fig. 3b). Measured scattering pattern on the right shows that there is no preferential scattering direction due to the 3D nature of the voids