arXiv subjects

Mohammad Mojtaba Sadafi

Publications and source records attributed to Mohammad Mojtaba Sadafi.

2 recordsLinked to original sources

arXiv2026

Floquet-Bloch Theory for Dispersive Time-varying Metasurfaces

Light-matter interaction in time-varying metasurfaces brings about phenomena that transcend the limits of static systems. Temporal modulation enables energy exchange between light and matter, and electromagnetic fields are coupled in both momentum and frequency, with implications across a broad range of photonic applications. Nevertheless, a precise description of such systems necessitates a theory that captures the intertwined effects of spatiotemporal variations while accounting for dispersion in a causal manner, as realistic optical materials exhibit frequency-dependent response and finite temporal memory. Here, a self-contained Floquet-Bloch theory is developed to capture the response of dispersive, time-varying metasurfaces, respecting causality through physically consistent constitutive relations. The theory treats spatial periodicity, nonadiabatic temporal modulation, and material dispersion within a unified formalism, providing access not only to the metasurface's scattering response but also to its inherent modal structure. The formulation is validated against full-wave simulations. As illustrative examples, it is first applied to asymmetric Floquet harmonic generation in an excitonic time-varying metasurface, where excitonic dispersion enables selective harmonic enhancement. It is then used to investigate metasurface-based photonic time crystals, revealing how modal dispersion governs momentum-bandgap formation and dynamics. This work establishes a comprehensive platform for understanding dispersive, time-varying metasurfaces and their underlying physical mechanisms.

physics.optics↗

arXiv2025

Electrically Tunable Excitonic-Hyperbolicity in Chirality-Pure Carbon Nanotubes

Metamaterials exhibiting hyperbolic dispersion enable unprecedented control over light-matter interactions, from sub-diffraction imaging to enhanced spontaneous emission. However, conventional plasmonic hyperbolic metamaterials suffer from limited tunability and lack intrinsic emission capabilities, constraining their utility for active photonic devices. Here, we demonstrate the first room-temperature, electrically tunable, excitonic hyperbolic metamaterial using aligned films of chirality-pure semiconducting carbon nanotubes. Unlike plasmonic systems, these excitonic metamaterials of aligned nanotubes combine strong optical anisotropy with dynamic electrostatic tunability. Spectroscopic ellipsometry reveals that the hyperbolic dispersion window can be electrically shifted by 53 meV, enabling real-time switching between hyperbolic and elliptical regimes. Theory predicts that this tunability translates to the propagation angle being modulated by 34°, driven by a momentum enhancement 3.11 times that of free space, limited primarily by material losses that can be mitigated through improved alignment. In addition, simulations of the system exhibit a high Purcell factor of 1550 and a modulation of 37 % without an optical cavity for a dipole placed 5 nm above the aligned nanotubes. These findings establish excitonic carbon nanotubes as a versatile platform for dynamically reconfigurable photonic metamaterials, opening pathways for adaptive optical devices, electrically-controlled spontaneous emission, and tunable hyper-lenses operating at room temperature.

physics.optics↗