Now showing 1 - 9 of 9
  • Placeholder Image
    Publication
    Effective Medium-Based Plasmonic Waveguides for Tailoring Dispersion
    (15-09-2015)
    Balasubrahmaniyam, M.
    ;
    Abhilash, T.
    ;
    ;
    We propose a waveguide configuration with a plasmonic grating for tailoring dispersion characteristics. The unit-cell of the plasmonic grating encompasses the subwavelength distribution of metal nanowires forming a highly resonant effective medium. The configuration enables independent control of the coupling between the plasmonic and waveguide modes via the resonant strength of the effective medium. Numerical simulations show that the line shapes of the coupled modes can be varied from Fano to electromagnetically induced transparency-like. Furthermore, we use the structure to enhance the group index from 250 to 850 and to broaden the associated band from 40 to 180 meV.
  • Placeholder Image
    Publication
    SPR: A promising platform for thermal transport studies
    (18-07-2013)
    Abhilash, T.
    ;
    Balasubrahmaniyam, M.
    ;
    ;
    A theoretical model has been adopted to simulate the effect of temperature on surface plasmon resonance (SPR) of a gold film by modeling its dielectric functions at various temperatures. A two-prism experimental arrangement that uses Kretschmann configuration in angular interrogation mode has been used to study SPR. Reflectivity measurements as a function of temperature have been performed at different positions of the resonance spectrum. Observed intensity variations reveal a considerable sensitivity of the reflectivity towards temperature which can further be utilized for extracting thermal transport properties. © 2013 AIP Publishing LLC.
  • Placeholder Image
    Publication
    Localized surface plasmon resonance in Au nanoparticles embedded dc sputtered ZnO thin films
    (01-02-2015)
    Patra, Anuradha
    ;
    Balasubrahmaniyam, M.
    ;
    Laha, Ranjit
    ;
    Malar, P.
    ;
    Osipowicz, T.
    ;
    Manivannan, A.
    ;
    The plasmonic behavior of metallic nanoparticles is explicitly dependent on their shape, size and the surrounding dielectric space. This study encompasses the influence of ZnO matrix, morphology of Au nanoparticles (AuNPs) and their organization on the optical behavior of ZnO/AuNPs-ZnO/ZnO/GP structures (GP: glass plate). These structures have been grown by a multiple-step physical process, which includes dc sputtering, thermal evaporation and thermal annealing. Different analytical techniques such as scanning electron microscopy, glancing angle X-ray diffraction, Rutherford backscattering spectrometry and optical absorption have been used to study the structures. In-situ rapid thermal treatment during dc sputtering of ZnO film has been found to induce subtle changes in the morphology of AuNPs, thereby altering the profile of the plasmon band in the absorption spectra. The results have been contrasted with a recent study on the spectral response of dc magnetron sputtered ZnO films embedded with AuNPs. Initial simulation results indicate that AuNPs-ZnO/Au/GP structure reflects/absorbs UV and infrared radiations, and therefore can serve as window coatings.
  • Placeholder Image
    Publication
    Effective medium based optical analysis with finite element method simulations to study photochromic transitions in Ag-TiO2 nanocomposite films
    (28-03-2016)
    Abhilash, T.
    ;
    Balasubrahmaniyam, M.
    ;
    Photochromic transitions in silver nanoparticles (AgNPs) embedded titanium dioxide (TiO2) films under green light illumination are marked by reduction in strength and blue shift in the position of the localized surface plasmon resonance (LSPR) associated with AgNPs. These transitions, which happen in the sub-nanometer length scale, have been analysed using the variations observed in the effective dielectric properties of the Ag-TiO2 nanocomposite films in response to the size reduction of AgNPs and subsequent changes in the surrounding medium due to photo-oxidation. Bergman-Milton formulation based on spectral density approach is used to extract dielectric properties and information about the geometrical distribution of the effective medium. Combined with finite element method simulations, we isolate the effects due to the change in average size of the nanoparticles and those due to the change in the dielectric function of the surrounding medium. By analysing the dynamics of photochromic transitions in the effective medium, we conclude that the observed blue shift in LSPR is mainly because of the change in the dielectric function of surrounding medium, while a shape-preserving effective size reduction of the AgNPs causes decrease in the strength of LSPR.
  • Placeholder Image
    Publication
    Size-dependent persistent photocurrent and its origin in dc sputtered indium oxide films under UV and sub-band gap illuminations
    (14-05-2017)
    Sen, Prabal
    ;
    Balasubrahmaniyam, M.
    ;
    Kar, Durgesh
    ;
    The size and spectral dependence of the persistent photocurrent (PPC) of dc sputtered indium oxide (IO) films has been studied under UV and sub-band gap illuminations. PPC follows bi-exponential decay with a fast and a slow process having time constants (denoted by τf and τs, respectively) that differ by about two orders of magnitude. τs is associated with carrier scattering from an initial surface state to a surface or bulk state with the former dominating below a characteristic length scale of ∼60 nm. On the other hand, τf is characterized by the process where both the initial and final states are surface related. Treating the IO film surface with tetramethyl tetraphenyl trisiloxane (TTTS) decreases τs by a factor of 5 with τf remaining almost unaffected, which is a clear indication of reduction of defects specific to the slow relaxation process. Based on the molecular structure and chemical activity of TTTS, it is suggested that TTTS may passivate mainly the dangling oxygen-bonds at the film surface. The spectral dependence of τs indicates that the associated surface states exhibit a maximum around 2.5 eV above the level from where strong optical transitions are allowed.
  • Placeholder Image
    Publication
    Observation of subwavelength localization of cavity plasmons induced by ultra-strong exciton coupling
    (24-04-2017)
    Balasubrahmaniyam, M.
    ;
    Kar, Durgesh
    ;
    Sen, Prabal
    ;
    ;
    In condensed matter systems, there exists a class of exotic localized electronic states wherein the localization is induced, not by a disorder or a defect, but by extremely strong interactions, for example, Kondo-insulator and Mott-insulator. In this work, we investigate and experimentally implement the photonic analog of localization induced by ultra-strong interactions in a coupled three-mode system. We show that the localization of a propagating mode can be achieved without the aid of an underlying spatial disorder, a defect, or even periodicity. We demonstrate the same by realizing ultra-strong coupling between a highly dispersive cavity plasmon mode and dimer excitons of Rhodamine B. Using a photon tunneling arrangement, we map the dispersion of the hybrid modes and provide evidence for the existence of a quasi-dispersionless hybrid mode with the sub-wavelength localization length and cavity plasmon-like characteristics.
  • Placeholder Image
    Publication
    Metal-dielectric composite for dispersion free optics
    (15-03-2013)
    Balasubrahmaniyam, M.
    ;
    Patra, Anuradha
    ;
    ;
    Bergman formulation has been used to extract effective dielectric function of Au nanoparticles (AuNPs) embedded metal oxide (MO) composite thin films. The extracted composite dielectric function helps further understanding ofthe resonance features in optical far-field responses. Detailed analysis on the extracted dielectric functions point towards the possibility of using them for dispersion free optics. © 2013 American Institute of Physics.
  • Placeholder Image
    Publication
    Hot electron mediated enhancement in the decay rates of persistent photocurrent in gold nanoparticles embedded indium oxide films
    (27-05-2019)
    Sen, Prabal
    ;
    Kar, Durgesh
    ;
    Laha, Ranjit
    ;
    Balasubrahmaniyam, M.
    ;
    A major factor that hinders the realization of indium oxide (IO) as a potential gas/photosensor is the response time, which is quite poor. For instance, the photoresponse of dc sputtered IO films under UV and sub-bandgap illumination is known to exhibit bi-exponential decay with fast (10-100 min) and slow (∼3000 min) time constants. We demonstrate here that the response time can be greatly improved by the hot carriers generated through localized surface plasmon decay. Our study shows that when IO films are incorporated with gold nanoparticles (AuNPs), the fast and slow decay time constants are reduced by a factor of 20 and 30, respectively, at excitation wavelengths close to the localized surface plasmon resonance (LSPR) wavelength. We also show the possibility to achieve wavelength tunable reduction in the time constants by tuning the LSPR wavelength. We attribute the changes to the strong enhancement in the recombination rates facilitated by plasmon decay-mediated excess hot electrons injected into the conduction band of IO. We use a simple analytical model to explain the role of plasmon-mediated hot electrons in enhancing the recombination rate. We believe that the present results are of great significance to improve the response time of metal oxide based photodetectors/sensors, in general, since the underlying physical process depends primarily on the plasmonic nature of the AuNPs.
  • Placeholder Image
    Publication
    Plasmon resonance mediated enhancement in Fabry-Pérot cavity modes
    (16-06-2014)
    Abhilash, T.
    ;
    Balasubrahmaniyam, M.
    ;
    Patra, Anuradha
    ;
    Fabry-Pérot (FP) modes of a dielectric cavity with a planar distribution of metal nanoparticles at one end have been studied numerically. An unusual enhancement in the strength of FP modes is observed close to the strong localized surface plasmon resonance of the metal nanoparticles. Experimental evidence for the numerical results has been provided by studying ZnO thin film cavities with Ag (Au) nanoparticles at one end. The enhancement in FP resonances has been interpreted in terms of a redefined cavity having a strongly dispersive adjacent medium. © 2014 AIP Publishing LLC.