Effect of silver nanoparticles on photo-induced reorientation of azo groups in polymer films

A series of polymer films containing azo groups and silver nanoparticles were prepared. Photo-induced reorientation of the film was conducted under irradiation of polarized light with wavelength at 365 nm, 442 nm and 532 nm, respectively. The influence of the concentration of dopant silver on the reorientation of the azo groups was studied. An enhancement of about 50% for the reorientation rate and about 70% for the reorientation amplitude was achieved. From a comparison of the enhancement obtained by irradiating with three different light sources, it was realized that the mechanism for enhancement of reorientation of azo groups is due to plasmon resonance of silver nanoparticles doped in the polymer films.     

Tuning the optical properties of metamaterials based on gold nanowire arrays embedded in alumina

We fabricated a series of gold nanowires/alumina composite films with different wire lengths. Optical transmission measurements confirmed that the composite films exhibit transverse and longitudinal surface plasmon resonances. We show that the wavelength of the longitudinal resonance is sensitive to nanowire length, while that of the transverse resonance is not. The experimental results are in agreement with the modeled results based on the Maxwell Garnett effective medium theory. Moreover, the window for negative refraction of the samples can be tuned in synchronism with the longitudinal resonance by the nanowire length.     

石墨烯动态调控太赫兹表面等离激元

太赫兹表面等离激元(SPPs)是利用亚波长周期性结构在太赫兹频段模拟的具有与可见光频段表面等离激元相似的光学特性的电磁波,分为传输型和局域型2种。本文将石墨烯引入太赫兹表面等离激元结构作为动态激励源,通过外加偏压改变石墨烯的电导率,分别实现了对传输型表面等离激元的幅度、频率、相位和对局域表面等离激元共振强度的动态调控。本文方法为表面等离激元的动态调控提供了新的思路,拓宽了表面等离激元在太赫兹频段的应用。     

Real time spectroscopic ellipsometry of Ag/ZnO and Al/ZnO interfaces for back-reflectors in thin film Si:H photovoltaics

Real time spectroscopic ellipsometry (RTSE) has been applied to analyze the optical characteristics of Ag/ZnO and Al/ZnO interfaces used in back-reflector (BR) structures for thin film silicon photovoltaics. The structures explored here are relevant to the substrate/BR/Si:H(n-i-p) solar cell configuration and consist of opaque Ag or Al films having controllable thicknesses of microscopic surface roughness, followed by a ZnO layer up to ~ 3000 Å thick. The thicknesses of the final surface roughness layers on both Ag and Al have been varied by adjusting magnetron sputtering conditions in order to study the effects of metal film roughness on interface formation and interface optical properties. The primary interface loss mechanisms in reflection are found to be dissipation via absorption through localized plasmon modes for Ag/ZnO and through intraband and interband transitions intrinsic to metallic Al for Al/ZnO.     

Polaron State Screening by Plasmons in a Spherical Nanocrystal

We establish that the screening due to plasmons reduces the effective electron-phonon coupling in the nanocrystal appreciably, and that the contribution of the self-action potential energy to the polaron ground state energy reduces with increasing plasmon density. We find that higher plasmon density leads to smaller Debye radius and greater screening of the electron-phonon interaction. We point out the existence of a threshold value of the Debye radius beyond which the electron-phonon interaction is redundant. The Debye screening radius provides a good estimate of the distance beyond which the electron-phonon interactions can be essentially ignored. The screened self-action potential energy is found to have a dominant effect on the polaron ground state energy. The Green function and the modified electron-phonon coupling amplitudes are principally expressed in terms of a function which we call the F-function, one of the arguments of which determines its zero.     

Substrate interaction effects on optical and acoustical plasmons in bi-waveguides based on graphene

We theoretically study the dynamic dielectric response function of a gas of massless Fermions embedded in a coupled double quantum wire structure based on graphene. We write the dielectric function within the random phase approximation (RPA). We approach the system using the two-dimensional (2D) Dirac-like Hamiltonian in the first place, where a parameter β, accounting for the interaction between the substrate and the graphene sheet, is considered in an ad-hoc manner. We study the weak tunneling regime between the two ribbons and find the energy dispersion of the acoustical and optical plasmon modes. Our results show that different choices for the parameter β in the structure should induce spatial anisotropy effects on the plasmon modes.     

Composition dependence of transition temperature in some ceramic superconductors

The composition dependence of transition temperature in some ceramic superconductors (La_2−x (Ba, Sr) _x CuO₄) was studied by modifying our earlier approach and developing a Fourier-transformed effective potential which involves the effect of two-dimensional (2D) acoustic plasmons. This potential was used to obtain the pairing (electron-electron attraction) parameter (λ), the averaged Coulomb repulsive parameter (μ*) and the cut-off 2D acoustic plasmon frequency (ω _c) required to compute the superconducting transition temperature (T _c) from the strong coupling theory. The variations ofT _c with compositions (x) obtained for La_2−x (Ba, Sr) _x CuO₄ show reasonably good agreement with experimental data.     

Safe fabrication of sharp gold tips for light emission in scanning tunnelling microscopy

Gold is the optimal tip metal for light emission in scanning tunnelling microscopy (LESTM) under ambient conditions. Sharp Au-tips of 10 nm radius were produced reliably using a safe, two-step etching method in 20% (w/w) CaCl₂ solution. Previous CaCl₂-based methods have tended to produce blunter tips, while other etching techniques that do produce sharp Au-tips, do so with the use of toxic or hazardous electrolytes. The tips are characterised using scanning electron microscopy and their efficacy in LESTM is evidenced by high-resolution, simultaneous topographic and photon mapping of Au(1 1 1)- and polycrystalline Au-surfaces. Spectra of the optical emission exhibit only one or two peaks with etched tips in contrast to the more complex spectra typical of cut tips; this feature, together with the highly symmetric geometry of the tips, facilitates a definitive analysis of the light emission process.