Azmat Iqbal Bashir

Employing the first-principles density functional theoretical approach, this study first reports systematic investigation on the electronic and topological properties, and nonlinear optical phenomena of Ge-doped graphene. These include energy bands spectra, valley Hall effect, second-harmonic generation, and the Berry curvature. The substitutionally doped graphene supercells of the functional quantum materials were modeled with Ge concentrations of 12.5%, 5.5%, 3.125%, and 2%. The computed results show that Ge doping breaks the inversion symmetry of graphene and opens a tunable bandgap ranging from 0.17 eV to 1.06 eV. The calculated Berry curvature exhibits opposite signs at the K and K′ valleys, indicating valley-contrasting transportation and the emergence of a valley Hall effect. Pronounced valley polarization is observed for the 12.5% and 3.125% doped Ge-graphene quantum systems, while the 5.5% doped structure exhibits weakened valley contrast associated with a nearly closed bandgap. Spin–orbit coupling lifts band degeneracies near the valley points, whereas the calculated Chern numbers remain close to zero, confirming a topologically trivial phase. Furthermore, inversion-symmetry breaking induces strong second-order nonlinear optical responses and pronounced second-harmonic generation peaks correlated with the optical absorption spectra. These findings demonstrate that Ge doping is an effective strategy for tailoring the valleytronic and nonlinear optical properties of graphene.

Superluminal propagation of light in various materials has attracted tremendous research for fundamental and practical interest. This study is aimed at investigating the low-frequency off-resonance optical response, gain-assisted superluminal photon propagation, and optical drag through GaZnO, an innovative TCO (transparent conductor oxide) material. TCO materials are among the novel class of optical materials, simultaneously showing high transparency and conductivity, leading to milestone fundamental and practical device implications. Earlier, no study reported superluminal propagation of light and rotary photon drag in GaZnO in the low-frequency range. In particular, gain-assisted superluminal propagation of light and photon drag is studied employing the Drude-Lorentz model. The phenomenon of gain-assisted superluminal propagation is studied in light of negative/positive refractive index and group index that lead to superluminal phase velocity and group velocity. TCO nature of GaZnO is also justified in light of large absorption coefficient and real conductivity at the highest transmission coefficient. The findings may leads to potential applications in the advanced applications in laser and optics, photonics, optical communication systems, signal processing, and quantum information processing.

Controlling the basic properties of light such as polarization, angular momentum, and speed through various optical media leads to interesting phenomena in various fields such as quantum optics, photonics, plasmonics, and quantum information technology. Few of the phenomenon include, for instance, light storage, ultra-slow and ultra-fast propagation, optical transparency, and quantum entanglement via quantum coherence or superposition. The understanding of these phenomenon of milestone implications paved the way for the invention of novel classical and quantum technology. In this article, we present detailed theoretical and numerical demonstration on the dispersion-dependent phenomenon of slow and fast light, optically induced transparency, and photon drag. For this, we employ a three-level scheme of an ensemble of GaAs\AlGaAs double quantum dot molecule in cascade configuration. In particular, by tuning the medium dispersion we predict interesting interplay between ultra-slow and ultra-fast light propagation with vanishing, infinite, ultra-positive and ultra-negative group velocities, and optical-assisted quantum transparency. The quantum coherence of the dressed states modifies considerably the optical properties of the system in view of dispersion, absorption, electron energy loss, transmission, and photon drag. An interesting interplay between ultra-slow to ultra-fast propagation is found by tuning various optical parameters. The calculated superluminal value of group velocity is ± 6 × 1 0 11 m s⁻¹. We also predict light propagation at vanishing and infinite group velocity. The calculated light drag is ± 6 × 10 − 6 rad/m.