Biomimetic,
lamellar, and highly porous transition-metal carbide
(MXene) embedded cellulose nanofiber (CNF) aerogels are assembled
by a facile bidirectional freeze-drying approach. The biopolymer aerogels
have large-scale, parallel-oriented micrometer-sized pores and show
excellent mechanical strength and flexibility, tunable electrical
properties, and low densities (2.7–20 mg/cm3). The
CNF, MXene, and lamellar pores are efficiently utilized to endow the
aerogels with exceptionally high birefringence in the terahertz (THz)
regime. Birefringence values as high as 0.09–0.27 at 0.4 THz
are achieved, which is comparable to most commercial THz birefringent
materials such as liquid crystals, which suffer from fast disintegration,
high cost, and complicated preparation processes. Empirical modeling
for different MXene contents and an experimental comparison with silver
nanowire or carbon nanotube embedded CNF aerogels suggest that the
intrinsic conductivity and content of embedded nanomaterials, the
aerogel porosity, and the lamellar cell walls can affect the optical
properties such as the THz birefringence and absorption. The determination
of optical anisotropy in the biopolymer aerogels lays a foundation
for further exploration of ultralight, freestanding, and low-cost
biomimetic porous architecture-based THz devices.