Figure 2: Transparent and compressible aerogel with measured and calculated properties.

(a) A photograph of a compressible transparent bulk aerogel of 8 cm in diameter, and 1 cm in thickness. The inset shows an SEM image of a nanoporous network structure with 84% porosity. The size of the scale bar, 20 mm (inset 200 nm). (b) The measured optical transmittance and reflectance show the good transparency of a 3-mm-thick aerogel in the visible spectrum. (c) The measured attenuation coefficients of aerogels at 633 (red), 589 (yellow), 523 (green), 473 (blue) and 400–700 (black) nm are 0.039, 0.055, 0.080, 0.109 and 0.069 mm⁻¹, respectively. (d) An optical image of white-light propagation in an aerogel. The size of the scale bar, 20 mm. (e) Photographs of a compressible aerogel (red dashed box) before (left) and after (right) deformation. The compression ratios (J) are 1 and 0.5. The size of the scale bar, 10 mm. (f) Using the retrieval method with COMSOL, the calculated effective refractive index (n_eff(J)) of the deformed aerogel versus the compression ratios (J) for TE (red circle) and TM (black square) polarizations are obtained. The unit cell consists of a dielectric material cubic shell (blue) around an air void with a side length of 60 nm. The dielectric volume fraction is 0.16, the Poisson’s ratio is 0.12, and the undeformed refractive index is 1.074. The simulated shapes of the deformed unit cell are shown for the specific compression ratios of J=0.2, 0.6 and 1. n_eff(J) from the approximate Clausius-Mossotti (blue line) relation precisely agrees with the retrieval method. The experimentally measured effective indices (red diamond) versus compression ratios of our aerogel chunks agree well with the approximate Clausius-Mossotti relation.