S. Venugopal Rao

Studies of third-order optical nonlinearity and nonlinear absorption in tetra tolyl porphyrins using degenerate four wave mixing and Z-scan

We present our experimental results on the measurement of third-order optical nonlinearity in the ns and ps domain, in several Tetra Tolyl Porphyrin molecules (TTP), using degenerate four wave mixing (DFWM) and Z-Scan techniques. Our results indicate a very high value of nonlinearity for these molecules in the ns domain and reasonably high values in the ps domain. They are found to exhibit a strong nonlinear absorption at both 532 nm and 600 nm. The high value of nonlinearity for ns pulses is attributed to higher excited singlet and triplet states. Time-resolved studies indicate an ultra-fast temporal evolution of the nonlinearity in these molecules.

Dispersion studies of non-linear absorption in C60 using Z-scan

Abstract We present our experimental and theoretical results on the dispersion studies of non-linear absorption in C60 solution. Open-aperture Z-scans are performed over the visible region (440–660 nm) using an ns OPO. Our results are interpreted using a 5-level model taking into account both the excited state absorption and two-photon absorption processes. Results indicate that the excited state absorption dominates in the shorter-wavelength region (440–560 nm) whereas the two-photon absorption dominates in the longer-wavelength region (580–660 nm).

Laser-induced breakdown spectroscopy-based investigation and classification of pharmaceutical tablets using multivariate chemometric analysis

We report the effectiveness of laser-induced breakdown spectroscopy (LIBS) in probing the content of pharmaceutical tablets and also investigate its feasibility for routine classification. This method is particularly beneficial in applications where its exquisite chemical specificity and suitability for remote and on site characterization significantly improves the speed and accuracy of quality control and assurance process. Our experiments reveal that in addition to the presence of carbon, hydrogen, nitrogen and oxygen, which can be primarily attributed to the active pharmaceutical ingredients, specific inorganic atoms were also present in all the tablets. Initial attempts at classification by a ratiometric approach using oxygen (∼777 nm) to nitrogen (742.36 nm, 744.23 nm and 746.83 nm) compositional values yielded an optimal value at 746.83 nm with the least relative standard deviation but nevertheless failed to provide an acceptable classification. To overcome this bottleneck in the detection process, two chemometric algorithms, i.e. principal component analysis (PCA) and soft independent modeling of class analogy (SIMCA), were implemented to exploit the multivariate nature of the LIBS data demonstrating that LIBS has the potential to differentiate and discriminate among pharmaceutical tablets. We report excellent prospective classification accuracy using supervised classification via the SIMCA algorithm, demonstrating its potential for future applications in process analytical technology, especially for fast on-line process control monitoring applications in the pharmaceutical industry.

Dinaphthoporphycenes: synthesis and nonlinear optical studies.

Naphthobipyrrole-derived porphycenes are synthesized for the first time via McMurry coupling of the β-alkylated 2,9-diformylnaphthobipyrrole derivatives, which in turn were prepared easily from 2,3-naphthalene bishydrazine in four steps. Insertion of nickel into the porphycene core results in transformation of the rectangular N4-core into a square type geometry owing to the fusion of naphthalene moiety onto the bipyrrole entities. These porphycenes show large, intensity dependent three-photon absorption.

Ultrafast nonlinear optical properties of alkyl-phthalocyanine nanoparticles investigated using Z-scan technique

We report our results on the femtosecond nonlinear optical studies of alkyl-phthalocyanine nanoparticles dispersed in water and dissolved in chloroform. Nonlinear refractive and absorptive properties were investigated using the closed and open aperture Z-scan techniques. The nonlinear optical coefficients obtained for nanoparticles suspended in water are compared with those dissolved in chloroform, which resulted in simple phthalocyanine solutions. Our studies clearly demonstrate the nonlinear refractive index of nanoparticles to be positive, while that of the nanoparticles in chloroform was negative. Our results and analysis point out a high nonlinearity in the ultrafast domain for both the nanoparticles and solutions indicating their potential in optical signal processing and limiting applications.

Nonlinear absorption and excited state dynamics in Rhodamine B studied using Z-scan and degenerate four wave mixing techniques

Abstract We report our experimental results of nonlinear absorption and excited state dynamics in Rhodamine B solutions at 532 nm, on resonance of the absorption band and 600 nm at the absorption edge, studied using the standard Z-scan and degenerate four wave mixing techniques. We observed saturable absorption at 532 nm and a transition from saturable absorption to reverse saturable absorption at 600 nm with increase in either intensity or concentration. Time-resolved degenerate four wave mixing studies using incoherent light from a broadband dye laser also confirmed the presence of excited state absorption through resonant two-photon absorption. Concentration dependent studies indicate a complex behavior of reverse saturable absorption within saturable absorption. Results are explained using an effective five-level model. The value of two-photon absorption coefficient evaluated using the same model was ∼0.58×10−8 cm/W.

Second-harmonic generation through optimized modal phase matching in semiconductor waveguides

We report optical second-harmonic generation (SHG) through modal phase matching in GaAs/AlGaAs semiconductor waveguides. Using femtosecond pulses, both type-I and type-II SHG is demonstrated for fundamental wavelengths near 1.55 μm.

Nonlinear frequency conversion in semiconductor optical waveguides using birefringent, modal and quasi-phase-matching techniques

We describe the use of GaAs-based semiconductor optical waveguides for nonlinear frequency conversion of femtosecond pulses in the infrared. Different techniques for the realization of phase matching, including artificial birefringence, quasi-phase-matching and modal dispersion, are reported and analysed and key issues relating to efficiency, practicality and applicability are discussed. Using the data obtained from these experiments, an overall comparison is made with other prominent techniques in order to identify the most promising and viable route to the development of efficient semiconductor waveguide frequency conversion devices for incorporation in the future generation of integrated photonic networks.

Fabrication of buried channel waveguides in photosensitive glass using proton beam writing

We report our results on the fabrication and characterization of buried, channel optical waveguides in photosensitive Foturan™ glass using a high energy proton beam. Waveguides were fabricated with varying fluence, and the propagation loss and refractive index change were measured. Near-field mode data measured at 632.8nm showed that waveguiding could be achieved for all fluences ranging from 1014to1016protons∕cm2. The maximum positive refractive index change of 1.6×10−3 was measured for the highest fluence. The waveguide propagation losses measured using the scattering technique were estimated to be in the range of 8.3–12.9dB∕cm, increasing with proton fluence.

Excited state dynamics in phthalocyanines studied using degenerate four wave mixing with incoherent light

We present our experimental results on the measurements of excited state dynamics in phthalocyanines (Pc, CuPc, FePc, and ZnPc) using degenerate four wave mixing with incoherent light (DFWM-IL). Three different timescales were observed, for all samples, in the phase conjugate signal recorded as a function of delay in probe beam in the standard backward geometry. The fastest component (< 170 fs) is associated to the phase relaxation in the Sn states and the slowest components (∼3-5 ps and 35-55 ps) to the vibration and population relaxation times from the S1 state. The present results are compared with those obtained with shorter pulses and various techniques for similar molecules. The consequences of such short lifetimes are also discussed in view of the strong optical limiting properties of these molecules.