K. P. Unnikrishnan

Nonlinear optical absorption in silver nanosol

Nonlinear optical absorption in silver nanosol was investigated at selected wavelengths (456 nm, 477 nm and 532 nm) using open aperture Z-scan technique. It was observed that nature of nonlinear absorption is sensitively dependent on input fluence as well as on excitation wavelength. Besides, the present sample was found to exhibit reverse saturable absorption (RSA) and saturable absorption (SA) at these wavelengths depending on excitation fluence. RSA is attributed to enhanced absorption resulting from photochemical changes. SA observed for fluence values lower and higher than those corresponding to RSA are, respectively, attributed to plasmon bleach and saturation of RSA.

Nonlinear absorption in certain metal phthalocyanines at resonant and near resonant wavelengths

Abstract Nonlinear absorption present in two bis-phthalocyanines viz. Sm(Pc)2, and Eu(Pc)2, dissolved in dimethyl formamide, is studied for a number of wavelengths in the lower wavelength side of the Q-band of their linear absorption spectrum using open aperture Z-scan technique. It was observed that while Eu(Pc)2 exhibited reverse saturable absorption at 604 nm, Sm(Pc)2 showed saturable absorption though both samples have similar structures. Saturation intensity Is, nonlinear absorption coefficient β and imaginary part of third order nonlinear susceptibility Im[χ(3)], were also measured at different wavelengths for these samples. It was also observed that resonant enhancement effect is higher in the case of Sm(Pc)2 than that in Eu(Pc)2.

Third order nonlinear optical studies in europium naphthalocyanine using degenerate four wave mixing and Z-scan

Third order nonlinear susceptibility v ð3Þ and second hyperpolarizabilityhci of a bis–naphthalocyanine viz. europium naphthalocyanines, EuðNcÞ 2 , were measured in dimethyl formamide solution using degenerate four wave mixing at 532 nm under nanosecond pulse excitation. Effective nonlinear absorption coefficient, beff and imaginary part of nonlinear susceptibility, Imðv ð3Þ Þ were obtained using open aperture Z-scan technique at the same wavelength. Optical limiting property of the sample was also investigated. The role of excited state absorption in deciding the nonlinear properties of this material is discussed. 2002 Elsevier Science B.V. All rights reserved.

Degenerate four wave mixing in some metal phthalocyanines and naphthalocyanines

Here we report measurements of third-order susceptibility v ð3Þ , figure of merit F defined as v ð3Þ =a (where a is the absorption coefficient) and second hyperpolarizability hci of some metal substituted phthalocyanines and a naphthalocyanine in solutions of dimethyl formamide using degenerate four wave mixing at 532 nm under nanosecond excitation. It was found that among samples investigated, bis-naphthalocyanine possessed the highest value of hci followed by the bis-phthalocyanine. This observation is explained on the basis that bis-naphthalocyanine followed by bis-phthalocyanine has higher degree of p electron conjugation. 2002 Elsevier Science B.V. All rights reserved.

Thermal lens spectrum of organic dyes using optical parametric oscillator

The wavelength dependence of thermal lens signal from organic dyes are studied using dual beam thermal lens technique. It is found that the profile of thermal lens spectrum widely differ from the conventional absorption spectrum in the case of rhodamine B unlike in the case of crystal violet. This is explained on the basis of varying contribution of nonradiative relaxations from the excited vibronic levels.

Evanescent wave fibre optic sensors for trace analysis of Fe3+ in water

In this communication, we discuss the details of fabricating an off-line fibre optic sensor (FOS) based on evanescent wave absorption for detecting trace amounts of Fe3+ in water. Two types of FOS are developed; one type uses the unclad portion of a multimode silica fibre as the sensing region whereas the other employs the microbent portion of a multimode plastic fibre as the sensing region. Sensing is performed by measuring the absorption of the evanescent wave in a reagent medium surrounding the sensing region. To evaluate the relative merits of the two types of FOS in Fe3+ sensing, a comparative study of the sensors is made, which reveals the superiority of the latter in many respects, such as smaller sensing length, use of a double detection scheme (for detecting both core and cladding modes) and higher sensitivity of cladding mode detection at an intermediate range of concentration along with the added advantage that plastic fibres are inexpensive. A detection limit of 1 ppb is observed in both types of fibre and the range of detection can be as large as 1 ppb–50 ppm. All the measurements are carried out using a LabVIEW set-up.

Studies on Two-Photon Absorption of Aniline Using Thermal Lens Effect

Two-photon absorption spectrum of aniline is recorded using thermal lens effect with optical parametric oscillator as the pump source. Studies show that the two-photon absorption spectrum observed corresponds to 1A1 → 1B2 transition of aniline.

NONLINEAR ABSORPTION AND OPTICAL LIMITING IN SOLUTIONS OF SOME RARE EARTH SUBSTITUTED PHTHALOCYANINES

Effective nonlinear absorption coefficient, βeff, of solutions of some rare earth substituted phthalocyanines (viz: Sm(Pc)2, Eu(Pc)2 and LaPc) was measured using open aperture Z-scan technique under nanosecond pulse excitation at 532 nm. The effect of nonlinear absorption on optical limiting in these samples was also investigated. Both nonlinear absorption and optical limiting phenomena in these samples are attributed to sequential two-photon absorption (STPA) process.

STUDY OF ENERGY TRANSFER IN ORGANIC DYE PAIRS USING THERMAL LENS TECHNIQUE

It is demonstrated that the dual beam thermal lens technique can be very effectively used for the study of energy transfer processes in organic dye mixtures. The energy transfer rate and critical transfer radius in Rhodamine 6G — Rhodamine B are calculated using this technique. The observed results support Forster type resonance transfer due to long-range dipole–dipole interaction as reported by earlier workers using other techniques.

Realization of Optical Logic Gates Using the Thermal Lens Effect

A sensitive method based on the principle of photothermal phenomena to realize optical logic gates is presented. A dual beam thermal lens method using low power cw lasers in a dye-doped polymer can be very effectively used as an alternate technique to perform the logical function such as NAND, AND and OR.