K. V. Anil Kumar

Wavelength dependent nonlinear optical switching in electron beam irradiated CuTTBPc thin film

The main aim of the present work is to investigate the role of 8 MeV electron beam interaction in modifying the linear and nonlinear optical (NLO) properties of thermally evaporated copper tetra tert-butyl phthalocyanine (CuTTBPc) thin films. The irradiation dose dependent change in linear optical band gap and roughness changes after electron irradiation have been explored. Third order NLO investigations were performed for unirradiated and 10 kGy irradiated CuTTBPc films with the femtosecond (fs) Z-scan technique at wavelengths of 800 nm and 900 nm. The unirradiated film depicted both reverse saturable and saturable kinds of nonlinear absorption behaviour as expected from the band gap restrictions on two photon absorption. A switch over to reverse saturable absorption within saturable absorption of the irradiated film at 900 nm laser excitation is explained using the irradiation induced band gap shifts of the material due to localized defects formation within the band gap region.

Formation Mechanism of Mud Bank Along the Southwest Coast of India

Mud bank formation during the southwest monsoon along the southwest coast of India remains an enigma to the researchers and coastal community in spite of several earlier studies. The present study attempts to unravel the mystery through a high-frequency, season-long time-series observation at Alappuzha, located at the southern part of the west coast of India, a region of frequent occurrence of mud bank. Using 7-month-long weekly time-series observation, we identified strong winds and high waves associated with onset of the southwest monsoon and subsequent three episodic atmospheric low-pressure events (LPEs).With the help of in situ time-series data, we show that the strong winds and high waves associated with southwest monsoon pre-conditions the near shore bottom sediment to bring it into suspension. The high amplitude waves associated with the southwest monsoon, while propagating from the deep water to shallow water region, interact with the bottom initiating bottom-sediment movement and its suspension due to wave refraction and shoaling. The sporadic occurrence of the atmospheric LPEs enhances the process of suspension of bottom sediment in the near shore region leading to the formation of fluid mud. Simulations with a cohesive sediment transport model yielded realistic estimates of sediment transport, in the presence of an onshore current, a pre-requisite for transporting the fluid mud toward the coast. The prevailing onshore upwelling current during the southwest monsoon provides the favorable pre-requisite conditions for transporting the fluid mud through depression channel network towards the coast. Once sufficient quantity and thickness of fluid mud is accumulated in the near shore region, it acts as a wave damper for subsequent high monsoon waves, as indicated by the time-series wave data, leading to the formation of tranquil mud bank region. Depression channel networks extending from the shelf to the coast off Alappuzha, Kochi, Ponnani, Beypore, and Ullal were found in the bathymetric charts, thus explaining why mud banks occur only at few locations in spite of the prevalence of similar monsoon conditions.

Phase-dependent ultrafast third-order optical nonlinearities in metallophthalocyanine thin films

We present a comprehensive study on the impact of phase transformations of metallophthalocyanine thin films on their third-order nonlinear optical (NLO) properties. The metallophthalocyanine thin films are prepared by thermally evaporating the commercially available Copper(II)2,9,16,23-Tetra-tert-butyl-29H,31H-phthalocyanine (CuPc) and Zinc(II) 2,9,16,23-Tetra-tert-butyl-29H,31H-phthalocyanine (ZnPc) powder on glass substrate. Thermal annealing causes a phase transformation which has a distinct signature in powder X-ray diffraction and UV-Vis-NIR spectroscopy. The NLO characteristics which include nonlinear refractive index n2, as well as nonlinear absorption coefficient (βeff), were measured by using a single beam Z-scan technique. An ultrashort pulsed fiber laser emitting femtosecond pulses (Δτ ≈ 250 fs) at 1064 nm central wavelength is used as a source for the Z-scan experiment. The βeff values in as prepared thin films were ascertained to be smaller as compared to the annealed one due to the smaller v...

Correction to: Formation Mechanism of Mud Bank Along the Southwest Coast of India

In the original article, V. Samiksha’s family name was incorrect. It is correct as reflect in this erratum. The original article has also been updated.

Femtosecond nonlinear optical properties of as grown and annealed Phthalocyanines thin films

The nonlinear optical (NLO) properties of Tetra-tert-butyl-phthalocyanine (H2PC), Copper (II) Tetra-tert-butyl-phthalocyanine (CuPc) and Zinc (II) Tetra-tert-butyl-Phthalocyanine (ZnPC) thin films have been measured by using a single beam Z-scan technique with 250 fs laser pulse of 1064nm central wavelength at a repetition rate of 664 KHz. The choice of Phthalocyanines (Pc) amongst other organic thin films for NLO properties is essentially motivated by architectural flexibility, high thermal stability and the ease of tailoring their physical and chemical properties. The observed nonlinear absorption (NLA) in Pc thin films at 1064 nm may be explained by using a model that includes Q band transition. The results showed that NLA increases on annealing the as prepared Pc films. This could be because of aggregation of Pc molecules on annealing. We found that nonlinear refractive index is affected by the nature of the central metal atom. Nonlinear refractive index values for CuPc are negative as compared to positive values for ZnPc and H2Pc. We also observed nonlinear optical switching and the presence of large fifth order nonlinearity in as prepared H2Pc films which could be attributed to two photon absorption (TPA) generated excited state refraction (ESR).