Polarization-resolved haze in transparent wood - towards wooden glass
Transparent wood (TW) composites possess residual haze that limits image clarity despite their high light transmission, yet the physical origin of this scattering has not been established. Here, polarization-resolved measurements reveal a pronounced polarization dependence of scattering by a wood-polymer composite with improved refractive index matching. Polarization- and spectrally-resolved haze as low as 21% (40-45% at 550 nm for unpolarized light) with transmittance of approximately 92% for polarization perpendicular to the fibers was detected. It is found that the low-haze spectrum closely follows an inverse-square relation over the 400-800 nm range, well described by a weak-index-contrast Rayleigh-Gans-Debye (RGD) model invoking subwavelength birefringent heterogeneities within the cell wall. This physical mechanism was previously obscured by haze-limiting strong scattering from debonding gaps or refraction at the lumen-cell wall interface. Scanning electron diffraction independently reveals spatially distinct domains with markedly different cellulose diffraction responses within the infiltrated cell walls. The observed polarization-dependent refractive-index mismatch is attributed to inherent cellulose birefringence. Time-resolved measurements further support nearly scattering-free quasi-ballistic transport of the sampled transmitted light. It is demonstrated that selecting only a preferential polarization markedly improves image visibility through TW without further material modification. These findings connect polarization-dependent haze to the internal structure of transparent wood and identify ways to engineer a refractive-index match in hierarchical anisotropic composites towards glass-like clarity.