The synthesis of metal (Au,Ag) and semiconductor (PbS) nanoparticles of specific morphology and shape is reported. The shape of PbS nanoparticles has been varied from spherical to oval to cubic, by use of poly(vinyl alcohol) (PVA), DNA and ethylene glycol as stabilisers respectively. For the first time, a seeding method has been used to successfully prepare PVA stabilised gold and silver nanoparticles. Characterisation of the third order optical nonlinearity of the nanoparticles has been carried out using the Z-scan technique with values of Im ÷ (3) as large as 10-10. Modulation of the magnitude of the nonlinear optical response with morphology in the case of the PbS nanoparticles is presented.
The nonlinear optical properties of multiwalled carbon nanotubes (MWNT) was investigated using femtosecond, picosecond and nanosecond laser pulses by the Z-scan and Degenerate four wave mixing techniques. Measurements show a significant third order nonlinear response in the both visible and near-infrared wavelengths regions, with Χ(3) values as high as 10-10 esu obtained on nanosecond and picosecond excitation and somewhat diminished Χ(3) values of the order of 10-12 esu obtained on femtosecond excitation. The temporal response at both picosecond and femtosecond excitation show a significant fast component indicating that electronic processes contribute to the third order nonlinear optical response of MWNT. This electronic role is highlighted by the observation of Van Hove singularities (VHS) in the density of states of MWNT. Unexpected visible luminescence from MWNT, observed on photo excitation at 1064 nm, is believed to arise from radiative transitions between energy states in the VHS. Our results shows that the presence of VHS enable efficient optical transitions in MWNT and furthermore the enhancement of the third order nonlinear optical response.
Visible photoluminescence from multiwalled carbon nanotubes (MWNT) was observed on excitation at 1064 nm. Strong nonlinear behavior of the photoluminescence was shown using power law dependence studies. The nonlinear response in MWNT was further investigated using degenerate four wave mixing. An ultrafast response was observed and the magnitude of the third order optical susceptibility, (chi) , was determined to be in the region of 1.2x10-10 esu. Van Hove singularities in the density of states were identified for the first time in MWNT using optical absorption spectroscopy. Optical transitions between the singularities coincide with the spectral region of the photoluminescence. We propose that a multiphoton absorption process, followed by up conversion luminescence, is responsible for nonlinear photoluminescence in MWNT. Photoluminescence from graphitic particles (GP) was also investigated. This is shown to result mainly from thermal behavior and well-known optical centers. Blackbody radiation was observed in the near infrared region in both materials with MWNT exhibiting lower blackbody temperatures than graphite under the same irradiation conditions.