Exploring the superhydrophilicity of nanosecond laser textured silicon: a Raman analysis.

We present Raman analysis of nanosecond laser textured silicon. The samples have also been characterized by field emission scanning electron microscopy (FESEM) and x ray diffraction. Contact angles (CAs) are measured to trace the hydrophilic nature. Characterization of the textured samples in argon and air shows that cleavage cracks are developed during texturing. CA measurements reveal the superhydrophilic nature of textured samples obtained in the presence of ambient oxygen and argon. In vacuum, however, the hydrophilicity is decreased. Micro-Raman analysis indicates the formation of nano-sized cleavage cracks that impart stable superhydrophilic properties to textured silicon is supported from FESEM images also. On the other hand, in vacuum textured silicon, evidence of such cracks is not noticed, which is also supported by Raman analysis. Further, the hydrophilicity is decreased. A definitive trend appears to exist between Raman signatures and hydrophilicity. We believe that the study will further the understanding of the mechanistic aspect in designing textured silicon with a high degree of self-cleaning capability.

Third-order non-linear optical switching and threshold limiting of NiO thin films.

This study discusses the nonlinear optical properties of as-grown and annealed NiO thin films using the Z-scan technique utilizing the femtosecond laser pulses at a repetition rate of 100 kHz of wavelength 1030 nm with a pulse duration of 370 fs. In open aperture measurements, pristine reveal saturable absorption (SA) with a negative sign of nonlinearity, while annealed samples exhibit reverse saturable absorption (RSA) with a positive sign of nonlinearity. In closed aperture conditions, all samples show prefocal minima and postfocal maxima, indicating self-focusing behavior. The observed RSA phenomena are attributed to the indirect two-photon absorption (TPA) process. The influence of surface plasmon resonance on nonlinear absorption is successfully ruled out due to insufficient excitation energy (1.20 eV) to induce resonance in the samples. The increase in bandgap with annealing temperature, associated with indirect TPA, is elucidated with the shifting of Ni- d x 2 - y 2 and Ni- d z 2 orbitals in the conduction band, under the framework of Density Functional Theory (DFT). The DFT results are correlated with the observed nonlinear SA and RSA processes. The Ni d-d and Ni-d to O-p transitions contribute to the modulation of nonlinearity in the indirect TPA process. The study suggests that the sample annealed at 400 °C exhibits superior optical limiting properties with the highest nonlinear absorption coefficients compared to the other annealed sample, while the as-grown sample is suitable for passive Q-switching applications.

O,S-Chelated bis(pentafluorophenyl)boron and diphenylboron-ß-thioketonates: synthesis, photophysical, electrochemical and NLO properties.

Boron-ß-diketonates are classical emissive materials that have been utilized in various fields, however, boron monothio-ß-thioketonates, where one oxygen atom is exchanged for a sulphur atom, have not been explored in detail. To gain a better understanding of this class of materials, we synthesised various aryl substituted monothio-ß-diketonate boron complexes with two different aryl substitutions on the boron center and studied their structural, optical and electrochemical properties. Single crystal X-ray analysis revealed that there is considerable deviation in B-O and B-S bond lengths for bis(pentafluorophenyl)boron complexes against diphenyl boron complexes. The bis(pentafluorophenyl)boron complexes have a relatively high absorption coefficient over diphenyl boron complexes. More importantly, a striking difference was observed for the emission behaviour of these compounds. The bis(pentafluorophenyl)boron complexes exhibit weak emission in the solution as well as in the solid state, whereas diphenyl boron complexes do not show any emission in either solution or the solid state. Further, the electrochemical study reveals that diphenyl boron complexes show a reduction potential that is more negative compared to the bis(pentafluorophenyl)boron complexes. The high absorption coefficient of the compounds pointed towards the possibility of high first order hyperpolarizability upon optical excitation, which motivated us to ascertain the nonlinear optical coefficients in the near infrared range, towards applicability of such compounds in optical limiting and switching. The open aperture Z-scan measurements at ultrashort time scales elucidated a few critical features of such compounds towards optical limiting applications.

Octupolar Cyclotriphosphazene-Cored Self-Standing Covalent Organic Framework Membranes as Nonlinear Optical Materials: Impact of Linkage Types and Material Forms.

Conjugated and processable self-standing vinylene-linked covalent organic framework membranes (COFMs) are highly demanding for photonics and optoelectronics. In this work, we have fabricated the first cyclotriphosphazene (CTP) cored vinylene-linked self-standing COFM (CTP-PDAN). For comparison purposes, we have successfully fabricated the imine-linked congener (CTP-PDA). Leveraging the inherent nonlinear optical (NLO) response of the CTP core, both membranes were directly mounted to evaluate NLO parameters using the open-aperture (OA) Z-scan technique. Direct measurement of NLO responses on membranes is advantageous and free from solvent and scattering effects, making it a more practical approach compared to the conventional dispersion mode. The OA Z-scan transmission yields a reverse saturable absorption signature exhibiting a higher NLO absorption coefficient (ß) of 58.37 cm/GW for CTP-PDAN, compared to that of the imine-linked CTP-PDA COFM (ß = 8.5 cm/GW). These results can be correlated to the efficient conjugation through the vinylene linkage in CTP-PDAN compared to the imine linked congener.