Browsing by Author "Srivastava, Ajit"

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    Dynamic Franz-Keldysh effect and its applications
    (2003) Srivastava, Ajit; Kono, Junichiro
    In this dissertation we study the dynamic Franz-Keldysh effect (DFKE) in GaAs. We observe the salient features of DFKE viz., below bandgap absorption and oscillations above the band gap, in our experiments. We also report the first observation of a large photoinduced transparency which represents a novel coherent phenomenon based on the DFKE and agrees well with the theoretical predictions. Finally we propose schemes making use of this effect to realize all-optical wavelength conversion required in high speed optical networks based on wavelength division multiplexing (WDM).
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    Micro-photoluminescence spectroscopy of excitons in individual single-walled carbon nanotubes
    (2009) Srivastava, Ajit; Kono, Junichiro
    Single-walled carbon nanotubes (SWNTs) are fascinating materials to study one-dimensional photophysics. Their optical properties are strongly affected by strong Coulomb interactions and are determined by "excitons" which represent the quantum of polarization in non-metallic solids. In this thesis dissertation we have experimentally investigated both the structure and the dynamics of excitons in non-metallic SWNTs. In particular, we have performed micro-photoluminescence spectroscopy of individual semiconducting SWNTs at low temperature to study their intrinsic optical properties and investigate the excitonic fine structure. Using magnetic field parallel to the tube axis we were able to directly observe theoretically predicted dark states for the first time in SWNTs. In addition, we found that the inter-valley and exchange energy, which determines the energy separation between the dark and the bright state, to be very sensitive to the surrounding environment of the nanotube. We have also studied the temperature dependent lineshape of SWNT photoluminescence in order to gain insight into the dynamics of exciton-phonon interaction, finding evidence for acoustic phonon scattering. For the rest of this thesis dissertation, we have developed a model based on reaction-diffusion processes to theoretically explain the observation of photoluminescence saturation in SWNTs. Our model shows that efficient exciton-exciton annihilation under high pumping conditions can explain this observed behavior quantitatively.