Unusual Nonlinear Optical Absorption Dispersion in a:SiN_x Aperiodic Photonic Structures

The strong confinement of electromagnetic fields in photonic crystals influences light-matter interactions and is utilized for the generation, control, and manipulation of light at desired frequency bands. Here we demonstrate the unusual nonlinear optical responses of a:SiN_x-based spatially inhomogeneous quasi-periodic optical microcavity during intense femtosecond laser propagation. Aperiodic microcavities are realized using two quarter-wavelength nonstoichiometric Si- and N-rich a:SiN_x layers having contrasting refractive indices and bandgaps. The photonic miniband region of the aperiodic microcavity was probed with high-intensity femtosecond laser using wavelength-dependent (740 to 900 nm) and angle-resolved studies at the miniband edge (∼750 nm). The studies reveal unusual switching from one-photon-induced saturable absorption (SA) of defect-mediated energy levels to two-photon-induced reverse saturable absorption (RSA) around the photonic miniband edge. The strong overlap of the symmetric photonic confinement with the highly dense defect energies of fractal N-rich low refractive index a:SiN_x layers (3L) directly expedites such anomalous nonlinear optical switching. Extensive optical field simulations and comparative studies with its periodic microcavity counterpart strongly support the unusual nonlinearities. The fractal effect-induced resonant photonic modes in an aperiodic microcavity lead to unpredictable nonlinear optical responses, which establishes a novel and versatile route for photon manipulation and control. Thus, the present study provides deep insights into many different geometrical dominances in the evolution of photonic modes and consequent nonlinear responses, which further modulate photonic device capabilities.