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Journal ArticleDOI

Two-photon-assisted excited state absorption in nanocomposite films of PbS stabilized in a synthetic glue matrix.

21 Feb 2007-Nanotechnology (IOP Publishing)-Vol. 18, Iss: 7, pp 075708
TL;DR: Free carrier absorption due to two-photon-assisted excited state absorption appears to be the predominant mechanism of optical nonlinearity in nanocomposite films containing PbS nanocrystals.
Abstract: Strong nonlinear absorption is observed in nanocomposite films containing PbS nanocrystals of mean size of 3.3 nm stabilized in a commercial poly(vinyl acetate) glue by a novel and simple chemical route of synthesis. A significant blueshift of the optical absorption edge indicates strong quantum confinement. The mean nanocrystal size was characterized by x-ray diffraction and transmission electron microscopy. The surface structure of nanocrystals is analysed using infrared spectroscopy. The excitonic transitions are probed by photoacoustic spectroscopy and the results are analysed on the basis of theoretical calculations using envelope function formalism. Results of open aperture z-scan experiments suggest a model involving saturable absorption followed by two-photon absorption at a lower concentration while the data for a higher concentration fitted saturable absorption followed by three-photon absorption. Free carrier absorption due to two-photon-assisted excited state absorption appears to be the predominant mechanism of optical nonlinearity.
Citations
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Journal Article
TL;DR: In the limit of a QD much smaller than the bulk exciton size, the linear spectrum will be a series of lines, and the phonon broadening of these lines is considered.
Abstract: We analyze theoretically the optical properties of ideal semiconductor crystallites so small that they show quantum confinement in all three dimensions [quantum dots (QD's)]. In the limit of a QD much smaller than the bulk exciton size, the linear spectrum will be a series of lines, and we consider the phonon broadening of these lines. The lowest interband transition will saturate like a two-level system, without exchange and Coulomb screening. Depending on the broadening, the absorption and the changes in absorption and refractive index resulting from saturation can become very large, and the local-field effects can become so strong as to give optical bistability without external feedback. The small QD limit is more readily achieved with narrow-band-gap semiconductors.

788 citations

Journal ArticleDOI
TL;DR: In this paper, an open-aperture (OA) Z scan and nonlinear transmission theory of two-photon-induced excited-state absorption were developed under the excitation of spatial Gaussian laser pulses with temporal Gaussian and hyperbolic secant profiles.
Abstract: We develop an open-aperture (OA) Z scan and nonlinear transmission theory of two-photon-induced excited-state absorption, under the excitation of spatial Gaussian laser pulses with temporal Gaussian and hyperbolic secant profiles. The found analytic expressions allow us to straightforwardly fit the OA Z-scan trace and the nonlinear transmission curve, for convenient extraction of the nonlinear absorption coefficients. As a test, the two-photon-induced excited-state absorption in a chalcone derivative of 3,4-dimethoxy-4′-fluorochalcone is explored by performing femtosecond Z-scan measurement and is analyzed by our theory.

98 citations

Journal ArticleDOI
TL;DR: In this article, a versatile approach to obtain MoO3 nanostructures such as nanorods, nanowires, nanobelts, and nanotubes in thin film form on glass substrates, by incorporating ZnO, via RF magnetron sputtering and controlled subsequent oxidation.
Abstract: We report a versatile approach to obtain MoO3 nanostructures such as nanorods, nanowires, nanobelts, and nanotubes in thin film form on glass substrates, by incorporating ZnO, via RF magnetron sputtering and controlled subsequent oxidation. The nanostructures growth mechanism has been elucidated on the basis of strain field associated with defect-oriented partial screw dislocation induced by ZnO for the drastic variation of the morphology with respect to ZnO incorporating levels from initial tiny nanorods (pure MoO3) to larger nanorods (at 1%), then to aligned and tilted nanowire arrays (at 3 and 5% respectively), afterward to nanobelts (at 7%), and finally to nanotubes (at 10%). Novel properties of ZnO-incorporated MoO3 nanostructures like enhanced photoluminescence and optical limiting have been brought out. This study opens the door to the potentiality of ZnO-added MoO3 nanostructures to be used as luminescent transparent conducting materials, saturable absorbers, and optical limiters.

73 citations

Journal ArticleDOI
TL;DR: In this paper, stable films containing CdS quantum dots of mean size 3.4 nm embedded in a solid host matrix are prepared using a room temperature chemical route of synthesis using high resolution transmission electron microscopy, infrared spectroscopy, differential scanning calorimetry and thermogravimetric analysis.
Abstract: Stable films containing CdS quantum dots of mean size 3.4 nm embedded in a solid host matrix are prepared using a room temperature chemical route of synthesis. CdS/synthetic glue nanocomposites are characterized using high resolution transmission electron microscopy, infrared spectroscopy, differential scanning calorimetry and thermogravimetric analysis. Significant blue shift from the bulk absorption edge is observed in optical absorption as well as photoacoustic spectra indicating strong quantum confinement. The exciton transitions are better resolved in photoacoustic spectroscopy compared to optical absorption spectroscopy. We assign the first four bands observed in photoacoustic spectroscopy to 1se–1sh, 1pe–1ph, 1de–1dhand 2pe–2phtransitions using a non interacting particle model. Nonlinear absorption studies are done using z-scan technique with nanosecond pulses in the off resonant regime. The origin of optical limiting is predominantly two photon absorption mechanism.

70 citations

Journal ArticleDOI
TL;DR: In this article, the effect of reverse saturable absorption in solid films of a new organic polymer nanocomposite, cast by doping Biebrich Scarlet dye in a vinyl polymer host polyvinyl alcohol for various concentrations, was studied employing the Z-scan technique at 442nm under different peak incident intensities ranging from 9.37 to 104.18.

55 citations

References
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Journal ArticleDOI
TL;DR: In this paper, a single-beam technique for measuring both the nonlinear refractive index and nonlinear absorption coefficient for a wide variety of materials is reported, including a comprehensive theoretical analysis.
Abstract: A sensitive single-beam technique for measuring both the nonlinear refractive index and nonlinear absorption coefficient for a wide variety of materials is reported. The authors describe the experimental details and present a comprehensive theoretical analysis including cases where nonlinear refraction is accompanied by nonlinear absorption. In these experiments, the transmittance of a sample is measured through a finite aperture in the far field as the sample is moved along the propagation path (z) of a focused Gaussian beam. The sign and magnitude of the nonlinear refraction are easily deduced from such a transmittance curve (Z-scan). Employing this technique, a sensitivity of better than lambda /300 wavefront distortion is achieved in n/sub 2/ measurements of BaF/sub 2/ using picosecond frequency-doubled Nd:YAG laser pulses. >

7,717 citations

Journal ArticleDOI
TL;DR: In this paper, the excited electronic states of semiconductor crystallites sufficiently small (∼50 A diam) that the electronic properties differ from those of bulk materials were modeled, and an approximate formula was given for the lowest excited electronic state energy.
Abstract: We model, in an elementary way, the excited electronic states of semiconductor crystallites sufficiently small (∼50 A diam) that the electronic properties differ from those of bulk materials. In this limit the excited states and ionization processes assume a molecular‐like character. However, diffraction of bonding electrons by the periodic lattice potential remains of paramount importance in the crystallite electronic structure. Schrodinger’s equation is solved at the same level of approximation as used in the analysis of bulk crystalline electron‐hole states (Wannier excitons). Kinetic energy is treated by the effective mass approximation, and the potential energy is due to high frequency dielectric solvation by atomic core electrons. An approximate formula is given for the lowest excited electronic state energy. This expression is dependent upon bulk electronic properties, and contains no adjustable parameters. The optical f number for absorption and emission is also considered. The same model is applied to the problem of two conduction band electrons in a small crystallite, in order to understand how the redox potential of excess electrons depends upon crystallite size.

4,322 citations

Journal ArticleDOI
TL;DR: In this article, a quantitative derivation for the acoustic signal in a photoacoustic cell in terms of the optical, thermal, and geometric parameters of the system is presented. And the theory predicts the dependence of the signal on the absorption coefficient of the solid, thereby giving a theoretical foundation for the technique of photoacoustical spectroscopy.
Abstract: When chopped light impinges on a solid in an enclosed cell, an acoustic signal is produced within the cell. This effect is the basis of a new spectroscopic technique for the study of solid and semisolid matter. A quantitative derivation is presented for the acoustic signal in a photoacoustic cell in terms of the optical, thermal, and geometric parameters of the system. The theory predicts the dependence of the signal on the absorption coefficient of the solid, thereby giving a theoretical foundation for the technique of photoacoustic spectroscopy. In particular, the theory accounts for the experimental observation that with this technique optical absorption spectra can be obtained for materials that are optically opaque.

2,278 citations

Journal ArticleDOI
TL;DR: In this paper, a growth technique of the semiconductor microcrystals in a glassy dielectric matrix has been developed, which permits to vary the size of the grown microcrystal in a controlled manner from some tens to thousands of angstroms.

1,039 citations

Journal ArticleDOI
TL;DR: In this article, the optical properties of ideal semiconductor crystallites so small that they show quantum confinement in all three dimensions [quantum dots (QD's)] were analyzed theoretically, and the phonon broadening of these lines was considered.
Abstract: We analyze theoretically the optical properties of ideal semiconductor crystallites so small that they show quantum confinement in all three dimensions [quantum dots (QD's)]. In the limit of a QD much smaller than the bulk exciton size, the linear spectrum will be a series of lines, and we consider the phonon broadening of these lines. The lowest interband transition will saturate like a two-level system, without exchange and Coulomb screening. Depending on the broadening, the absorption and the changes in absorption and refractive index resulting from saturation can become very large, and the local-field effects can become so strong as to give optical bistability without external feedback. The small QD limit is more readily achieved with narrow-band-gap semiconductors.

843 citations