http://hdl.handle.net/123456789/13 Assistatnt Professor 2026-06-05T22:25:11Z http://hdl.handle.net/123456789/95 Title: Size-dependent enhancement of nonlinear optical properties in nanocolloids of ZnO Authors: litty irimpan, Bindu Krishnan; V.P.N. Nampoori, A Deepthi; P Radhakrishnan Abstract: We have investigated the third-order nonlinearity in ZnO nanocolloids with particle sizes in the range 6–18 nm by the z-scan technique. The third-order optical susceptibility 3 increases with increasing particle size R within the range of our investigations. In the weak confinement regime, an R2 dependence of 3 is obtained for ZnO nanocolloids. The optical limiting response is also studied against particle size. 2008-02-11T00:00:00Z http://hdl.handle.net/123456789/94 Title: Size dependent fluorescence spectroscopy of nanocolloids of ZnO Authors: litty irimpan, P. Radhakrishnan; V.P.N. Nampoori, A Deepthi; Bindhu Krishnan Abstract: n this article we present size dependent spectroscopic observations of nanocolloids of ZnO. ZnO is reported to show two emission bands, an ultraviolet UV emission band and another in the green region. Apart from the known band gap 380 nm and impurity 530 nm emissions, we have found some peculiar features in the fluorescence spectra that are consistent with the nanoparticle size distribution. Results show that additional emissions at 420 and 490 nm are developed with particle size. The origin of the visible band emission is discussed. The mechanism of the luminescence suggests that UV luminescence of ZnO colloid is related to the transition from conduction band edge to valence band, and visible luminescence is caused by the transition from deep donor level to valence band due to oxygen vacancies and by the transition from conduction band to deep acceptor level due to impurities and defect states. A correlation analysis between the particle size and spectroscopic observations is also discussed. 2007-09-01T00:00:00Z http://hdl.handle.net/123456789/93 Title: Nonlinear optical characteristics of self-assembled films of ZnO Authors: litty irimpan, A. Deepthy; Bindu Krishnan, V P N Nampoori; R Radhakrishnan Abstract: In the present work, we have investigated the nonlinear optical properties of self-assembled films formed from ZnO colloidal spheres by z-scan technique. The sign of the nonlinear component of refractive index of the material remains the same; however, a switching from reverse saturable absorption to saturable absorption has been observed as the material changes from colloid to self-assembled film. These different nonlinear characteristics can be mainly attributed to ZnO defect states and electronic effects when the colloidal solution is transformed into self-assembled monolayers. We investigated the intensity, wavelength and size dependence of saturable and reverse saturable absorption of ZnO self-assembled films and colloids. Values of the imaginary part of third-order susceptibility are calculated for particles of size in the range 20–300 nm at different intensity levels ranging from 40 to 325 MW/cm2 within the wavelength range of 450–650 nm. 2008-11-01T00:00:00Z http://hdl.handle.net/123456789/92 Title: Excitation wavelength dependent fluorescence behaviour of nano colloids of ZnO Authors: Litty Irimpan, Bindu Krishnan; A Deepthy, V P N Nampoori; P Radhakrishnan Abstract: In this paper, the fluorescence behaviour of nano colloids of ZnO has been studied as a function of the excitation wavelength. We have found that excitation at the tail of the absorption band gives rise to an emission that shifts with the change of the excitation wavelength. The excitation wavelength dependent shift of the fluorescence maximum is measured to be between 60 and 100 nm. This kind of excitation wavelength dependent fluorescence behaviour, which may appear to be in violation of Kasha’s rule of excitation wavelength independence of the emission spectrum, has been observed for nano ZnO colloids prepared by two different chemical routes and different capping agents. It is shown that the existence of a distribution of energetically different molecules in the ground state coupled with a low rate of the excited state relaxation processes, namely, solvation and energy transfer, are responsible for the excitation wavelength dependent fluorescence behaviour of the systems. 2007-07-21T00:00:00Z