Pankaj Srivastava

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.

The alterations in the magnetic properties and electronic structure of chemical vapor deposition (CVD) grown nano-dimensional molybdenum disulfide (MoS2) after low-energy ion irradiation are thoroughly investigated. The formation of pure hexagonal 2H phase has been identified by Raman spectroscopy, Transmission electron microscopy (TEM), and X-ray diffraction (XRD). The pristine samples are irradiated by Argon (Ar) ions with low energy at different fluences. A comprehensive analysis of Raman spectroscopy data manifests the formation of lattice defects like S-vacancies across the samples after irradiation. Triangular-flake formation in the pristine sample is confirmed by field emission scanning electron microscopy (FESEM) images. After increasing the irradiation fluences, the large flakes began to fragment into smaller ones, thereby increasing the number of edge-terminated structures. The electron probe microanalyzer (EPMA) analysis verifies the absence of any magnetic impurity. Rutherford backscattering spectrometry (RBS) and X-ray photoelectron spectroscopy (XPS) studies confirm the formation of S-vacancies after irradiation. The pristine sample exhibits diamagnetic behavior at room temperature. The saturation magnetization value increases with increasing ion irradiation fluences, and the sample irradiated with 1×1015 ions/cm2 demonstrates the highest saturation magnetization value of 4.18 emu/g at room temperature. The impact of edge-terminated structure and point defects like S-vacancies to induce room-temperature ferromagnetism (RTFM) is thoroughly examined.

Meticulously engineered nonstoichiometric a:SiN_x-based photonic structures, such as optical microcavities, facilitate the emergence of pronounced photonic states that are predominantly influenced by defect and interface energies, resulting in diverse and complex electron-photon interactions. In this study, we present third-order nonlinear optical investigations on a:SiN_x-based optical microcavities employing femtosecond lasers across a broad wavelength range of 740-900 nm to examine the effects associated with photonic band edges, which are contingent upon defect and interface energies. The dispersion of nonlinear absorption (β) and the nonlinear refractive index (n₂) through wavelength- and angle-dependent studies strongly correlate with linear angular photonic mode dispersion. Intensity-dependent nonlinear assessments provide a variation of impact with the photonic band edge effects. The strong optical field confinements and layer-by-layer nonlinearities show substantial effects on the effective optical nonlinearities in the spectral domain. Such a significant impact of photonic modes and the marked dependence of the nonlinear optical properties on optical microcavity structures highlight promising avenues for designing various photonic interconnects suited for both linear and nonlinear optical applications.

We report the effect of the stoichiometry (from Si-rich to N-rich) of PECVD-grown a-SiNx:H thin films on their optical properties in the soft x-ray regime. The stoichiometry (x = [N]/[Si]) of thin films varied from 0.25 to 1.48 with the control of the flow of precursor gases. The soft x-ray reflectivity was performed in the vicinity of the Si L2,3 absorption edge energy region to evaluate the soft x-ray optical behavior of these non-stoichiometric thin films. Reflectivity analysis suggests the possible tunability of optical constants of a-SiNx:H thin films with stoichiometry. N-rich a-SiNx:H shows promising optical constant values comparable to widely used SiC for the application of soft x-ray/extreme ultraviolet (EUV) mirrors and for masks in next-generation EUV-based 13.5 nm lithography technology.

Exposure to intense electronic excitation in PECVD-grown amorphous hydrogenated silicon nitride (a-SiNx:H) thin films with 100 MeV Ni7+ results in the formation of continuous nanoscale ion track structures at the lower fluence of 5 × 1012 ions/cm2, whereas at the higher fluence of 1 × 1014 ions/cm2, the track structures fragment into discontinuous ion track-like structures. Concomitantly, the diameter of the ion tracks is also found to increase with fluence. The observation of the discontinuous ion track-like structures at the high fluence of 1 × 1014 ions/cm2 suggests that higher fluence irradiation may not always lead to dissolution of the microstructure formed at lower fluence. The results are understood on the basis of a dynamic electronic energy loss (Se) during the course of irradiation and a continuously evolving density resulting from the out-diffusion of hydrogen from the a-SiNx:H films. The proposed mechanism of ion track evolution is supported by the thermal spike simulations.