Reji Philip

White light Z-scan measurements of ultrafast optical nonlinearity in reduced graphene oxide nanosheets in the 400–700 nm region

Wavelength dispersion of optical power limiting is an important factor to be considered while designing potential optical limiters for laser safety applications. We report the observation of broadband, ultrafast optical limiting in reduced graphene oxide (rGO), measured by a single open aperture Z-scan using a white light continuum (WLC) source. WLC Z-scan is fast when the nonlinearity is to be measured over broad wavelength ranges, and it obviates the need for an ultrafast tunable laser making it cost-economic compared to conventional Z-scan. The nonlinearity arises from nondegenerate two-photon absorption, owing mostly to the crystallinity and extended π conjugation of rGO.

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.

Laser fluence dependence on emission dynamics of ultrafast laser induced copper plasma

The characteristic emission features of a laser-produced plasma depend strongly on the laser fluence. We investigated the spatial and temporal dynamics of neutrals and ions in a femtosecond laser (800u2009nm, ∼40 fs, Ti:Sapphire) induced copper plasma in vacuum using both optical emission spectroscopy (OES) and spectrally resolved two-dimensional (2D) imaging over a wide fluence range of 0.5–77.5u2009J/cm2. 2D fast gated monochromatic images showed a distinct plume splitting between the neutrals and ions, especially at moderate to higher fluence. OES studies at low to moderate laser fluence confirm intense neutral line emission over ion emission, whereas this trend changes at higher laser fluence with dominance of the latter. This evidences a clear change in the physical processes involved in the femtosecond laser-matter interaction at high input laser intensity. The obtained ion dynamics resulting from OES and spectrally resolved 2D imaging are compared with charged particle measurement employing Faraday cup and...

Nonlinear transmittance and optical power limiting in magnesium ferrite nanoparticles: effects of laser pulsewidth and particle size

We report comparative measurements of size dependent nonlinear transmission and optical power limiting in nanocrystalline magnesium ferrite (MgFe2O4) particles excited by short (nanosecond) and ultrashort (femtosecond) laser pulses. A standard sol–gel technique is employed to synthesize particles in the size range of 10–50 nm, using polyvinyl alcohol as the chelating agent. The structure and morphology of the samples are studied using X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Growth of the particles in time is tracked through Fourier transform infrared spectroscopy. Nonlinear transmission measurements have been carried out using the open aperture Z-scan technique employing 532 nm, 5 nanosecond pulses and 800 nm, 100 femtosecond pulses, respectively. The measured optical nonlinearity is primarily of a reverse saturable absorption (RSA) nature, arising mostly from excited state absorption for nanosecond excitation, and two-photon absorption for femtosecond excitation. The optical limiting efficiency is found to increase with particle size for both cases. The calculated nonlinear parameters indicate that these materials are potential candidates for optical limiting applications.

Dopant-configuration controlled carrier scattering in graphene

Controlling optical and electronic properties of graphene via substitutional doping is central to many fascinating applications. Doping graphene with boron (B) or nitrogen (N) has led to p- or n-type graphene; however, the electron mobility in doped-graphene is severely compromised due to increased electron-defect scattering. Here, we demonstrate through Raman spectroscopy, nonlinear optical and ultrafast spectroscopy, and density functional theory that the graphitic dopant configuration is stable in graphene and does not significantly alter electron–electron or electron–phonon scattering, that is otherwise present in doped graphene, by preserving the crystal coherence length (La).

An organic dye-polymer (phenol red-poly (vinyl alcohol)) composite architecture towards tunable -optical and -saturable absorption characteristics

Herein, we demonstrate that blending an organic dye (guest/filler), with a vinyl polymer (host template), is an inexpensive and simple approach for the fabrication of multifunctional photonic materials which could display an enhancement in the desirable properties of the constituent materials and, at the same time provide novel synergistic properties for the guest-host system. A new guest-host nanocomposite system comprising Phenol Red dye and poly (vinyl alcohol) as guest and host template, respectively, which exhibits tunable optical characteristics and saturable absorption behavior, is introduced. The dependence of local electronic environment provided by the polymer template and the interactions of the polymer molecules with the encapsulated guest molecules on the observed optical/nonlinear absorption behavior is discussed. An understanding of the tunability of the optical/ photophysical processes, with respect to the filler content, as discussed herein could help in the design of improved optical mat...

Spatio-temporal optimization of a laser produced Al-plasma: Generation of highly ionized species

Laser produced plasmas are transient in nature, and their properties, which depend on the laser parameters as well as the material properties and the irradiation conditions, can be tailored for different applications. Highly ionized Al plasmas generated using 7 ns and negatively chirped 60 ps pulses are optimized for the purpose of generating Al IV and Al III, respectively. The plasma is optimized spatio-temporally for Al IV or Al III with irradiation energy as the control parameter using time-resolved optical emission spectroscopy. Plasmas attuned for higher charged states could be utilized as a good alternative source for the generation of high order harmonics.

Ultrafast laser produced zinc plasma: Stark broadening of emission lines in nitrogen ambient

The effect of ambient pressure on Stark broadening of emission lines from neutrals and ions in an ultrafast laser (100 fs, 800u2009nm) produced zinc plasma is investigated. Measured spectra reveal that the full width at half maximum (δλ) of neutral lines remains unchanged in the pressure range of 10−6 to 10−1u2009Torr, shows an even fluctuation in the pressure range of 0.1 to 100u2009Torr, and then increases with pressure. On the other hand, δλ of ion lines is nearly a constant from 10−6 to 10−3u2009Torr, and then increases consistently with ambient pressure. A line narrowing of neutral emissions observed in the region of 1 to 100u2009Torr can be attributed to larger plasma temperatures, whereas the consistent increase in δλ with pressure seen for ion emission results from the prevalence of additional broadening mechanisms related to Coulomb interactions, ion-ion interaction, and Debye shielding. An accurate knowledge of emission line width is crucial for unambiguously calculating number density values for any given ambient ...