Nonuniform Wave Momentum Band Gap in Biaxial Anisotropic Photonic Time Crystals

Photonic time crystals (PTCs) host momentum band gaps, enabling intriguing nonresonant light amplification in propagating waves, but opening substantial band gaps demands refractive index changes too extreme for conventional nonlinear optics. Here, we introduce momentum band gaps for nonuniform waves, including evanescent and ghost types, by extending PTCs to biaxial anisotropic photonic time crystals that periodically alternate between uniform biaxial anisotropy and isotropic media over time. We show that ghost waves, unlike evanescent waves, sustain only momentum band gaps, opening wide band gaps at even the smallest modulation depths. Moreover, we demonstrate momentum band-gap effects on nonuniform waves that can be amplified or, through decaying modes, selectively attenuated. We find that ghost wave momentum band gaps uniquely boost refracted over reflected waves under one-way incidence, in stark contrast to the balanced amplification seen in both propagating and evanescent waves. Our approach expands time-varying metamaterials by integrating wave characteristics, bridging the gap between conventional nonlinear optics and PTC momentum band gaps, and shedding new light on extreme manipulation of surface polaritons.