P. Sreekanth

Enhanced multi-phonon Raman scattering and nonlinear optical power limiting in ZnO:Au nanostructures

We report the synthesis of ZnO:Au hetero structures, their Raman scattering and optical power limiting efficiencies. ZnO:Au nanostructures with flower-like morphology were obtained by the self-assembly of ZnO nano entities using Au nanoparticles as seeds. Compared to ZnO nanostructures, ZnO:Au nanocomposites show enhanced UV emission and decrease in green emission. Increases in the intensities of Raman bands and multiphonon Raman bands of nano ZnO are observed substantially in the nanocomposites. Raman results further indicate that lattice vibrations of semiconductors are sensitive to the presence of metal nanoparticles. Open aperture Z-scan measurements carried out at 532 nm using 5 ns laser pulses reveal metal nanoparticle induced changes in the optical limiting properties of the nanocomposites.

Nanosecond and ultrafast optical power limiting in luminescent Fe2O3 hexagonal nanomorphotype

Nonlinear optical absorption and optical power limiting properties of Fe2O3 hexagonal nanomorphotype are investigated using open aperture Z-scan technique with the 5 ns and 100 fs laser pulses, at 532 nm and 800 nm excitation domains. At relatively low pulse energies (below 5 μJ), sample shows saturable absorption (SA), but on going to the higher energies an interesting switchover from saturable absorption to effective two photon absorption is observed in both excitation domains. The magnitude of effective two photon absorption coefficients is calculated to be in the range of 10−10 m/W for nanosecond and 10−15 m/W for femtosecond laser pulse energies, respectively. XRD and TEM study reveals the polycrystalline nature, hexagonal morphology, and size of the nanostructure. The luminescence emission property is examined by photoluminescence spectroscopy (PL). It is found that some strange features exist in the luminescence spectra that are consistent with the nanoparticles size distribution. The PL emission l...

Morphology dependent nanosecond and ultrafast optical power limiting of CdO nanomorphotypes

A versatile approach to the fabrication of cadmium oxide (CdO) nanospheres, nanoflakes and nanoparallelepiped shapes through chemical precipitation and hydrothermal methods is studied. The growth mechanism of nanostructures is elucidated on the basis of capping agent and reaction time. The crystalline nature and various geometrical shapes of nanostructures are examined by X-ray diffraction and high resolution transmission electron microscopic analysis. Nonlinear optical absorption and optical power limiting studies of nanostructures are carried out using the open aperture z-scan technique in the nanosecond (ns) and femtosecond (fs) excitation time scales. Optical nonlinearity is found to arise from effective two- photon absorption involving excited state absorption. The magnitude of the effective two-photon absorption coefficient is calculated and found to be in the range of 10−10 m W−1 for ns and 10−15 m W−1 for fs. The nonlinear optical study discloses that the nonlinear optical absorption and optical limiting threshold values are strongly related to nanomaterial morphology. The Brunauer–Emmett–Teller (BET) surface area analysis also confirms our claim that the optical nonlinearity is strongly dependent on the morphology as well as surface area of the nanomaterial. Thus the present nanomorphotypes have potential applications in devices for optical limiting, optical switching, pulse shaping, pulse compression and optical diode action.

Electric, magnetic and optical limiting (short pulse and ultrafast) studies in phase pure (1 − x)BiFeO3–xNaNbO3 multiferroic nanocomposite synthesized by the pechini method

The perovskite (1 − x)BiFeO3–xNaNbO3 nanocomposite was successfully synthesized by Pechini method and crystallographic information was obtained from XRD and TEM analysis. Structural analysis using XRD and TEM shows that phase pure samples were obtained with reduced particle size. The observations reveal that particle size plays a crucial role in deciding the electric and magnetic properties. The multiferroic character of nanoparticles is confirmed through magneto electric (ME) coupling studies. The good coexistence of ferroelectric and ferromagnetic behaviors in the composite provides the possibility to achieve a measurable ME effect. The highest value of the magneto electric coefficient (α) is observed for x = 0.1 (α = 0.13 V cm−1 Oe−1) and it reduces for higher x values. Magnetization measurements for x = 0.1 show a small hysteresis at 6 K which also confirms the presence of the magnetic phase at low temperature while the usual antiferromagnetic behavior of BFO is found at 300 K. The reduced size and absence of impurity could be the reason for the enhancement in electrical properties. Low loss tangent values observed in the samples display a remarkable improvement. Open aperture Z-scan measurements reveal a nonlinear absorption behavior, which results in good optical limiting when excited with short pulse (nanosecond) as well as ultrafast (femtosecond) laser pulses.

Nonlinear optical interactions of Co: ZnO nanoparticles in continuous and pulsed mode of operations

Nanoparticles of Co: ZnO were synthesized by a co-precipitation method and characterized structurally by XRD (wurtzite), FTIR and TEM (spherical, 6–13 nm). The oxidation state of the Co ions is confirmed as +2 from electron paramagnetic resonance (EPR). UV-Vis absorption spectra represent a complete transparency in the visible region. A zinc vacancy is identified by PL measurements at 410 and 465 nm. The intensity dependent third order nonlinearity is studied using two frequency doubled (532 nm) pumping sources namely nanosecond pulsed Nd: YAG and CW Nd: YAG lasers. The nonlinear optical coefficients are obtained by analyzing the z-scan curve on the basis of a thermal lensing model for CW excitation whereas two photon absorption (2PA) along with a absorption saturation process for ns excitation.

Composition-structure–physical property relationship and nonlinear optical properties of multiferroic hexagonal ErMn1−xCrxO3 nanoparticles

Nanocrystalline hexagonal ErMn1−xCrxO3 (0 ≤ x ≥ 0.15) samples have been synthesized by a citrate assisted sol–gel method to study their structural and physical properties. X-ray diffraction studies show that ErMn1−xCrxO3 samples crystallize in the hexagonal phase when the doping concentration is low, while they crystallize into mixed orthorhombic and hexagonal phases at high doping concentrations. The photopyroelectric (PPE) technique is used to determine the thermal parameters of ErMn1−xCrxO3 nanoparticles. Thermal studies indicate that phonon transport through ErMn1−xCrxO3 nanocrystallites reduces as a function of Cr concentration. Temperature dependent magnetic studies show that Cr-doped ErMnO3 have charge ordered canted (C) type antiferromagnetism (AFM) with a spin-glass transition at low temperature. The striving ferromagnetic (FM) and AFM states are expected to be responsible for the spin-glass type behaviour in Cr doped ErMnO3. Strong nonlinear optical absorption has been found when the samples are irradiated by laser pulses. The optical limiting response studied against Cr doping shows that nonlinearity varies with dopant concentration.

Synthesis, characterization and nonlinear optical studies of novel blue-light emitting room temperature truxene discotic liquid crystals

A new series of discotic liquid crystals based on a truxene core has been synthesized to study the structure–property relationship in view of the self-assembling property and their linear and nonlinear optical properties. All these branched alkyl chain truxene derivatives show a mesogenic property in a columnar hexagonal fashion at room temperature which is studied by a combination of different techniques. The newly synthesized truxene discotic 7a possesses a clearing temperature of 117 °C, one of the lowest clearing temperatures known in truxene discotic liquid crystals. We have discovered that the introduction of branching near to the core even in a small alkyl chain drastically reduces the isotropic temperature. Due to their C3 symmetry and their large first hyperpolarizability, these truxene derivatives show long-lived emissions in the solution state at room temperature. We also report large effective three-photon absorption in these materials under nanosecond laser pulse excitation at 532 nm, which makes them suitable candidates for optical limiting applications.