A survey of polarization-dependent optical phenomena in semiconductor and metal nanowires and nanorods is presented. Due to a large dielectric constant mismatch between nanostructures and their environment, the amplitude of the optical electric field inside the former depends drastically on the angle between the direction of light polarization and the nanostructure axis. As a result, optical absorption, photoconductivity, and nonlinear photoresponse in semiconductor structures are strongly anisotropic, with the maximal value for the parallel light polarization. In metal structures, absorption anisotropy depends on the light frequency, and for that close to the transverse plasmon frequency is maximal for the perpendiculat light polarization. Luminescence emitted by semiconductor nanowires and nanorods is strongly polarized along their axis. Joint action of polarization effects in absorption and luminescence results in the polarization memory, when luminescence of a random ensemble of nanorods is polarized in the same direction as the exciting light.
In recent years, semiconductor nanodots have been actively used for biolabeling. We propose using alternate composite nanostructures consisting of a semiconductor size-quantized core covered by a nanometer-thick Au shell, having two principal advantages over purely semiconducting nanodots: (i) reduction of toxicity due to a complete Au coverage of the cores containing potentially poisonous Cd, Se, or Pb; (ii) amplification of exciting and/or emitted light by plasmon effects in a metallic shell which will increase the imaging efficiency. Theoretical calculations show that the optical absorption and emission spectra have several peaks corresponding to interband transitions in the core, and the two plasmon modes in the Au shell. When the energy of interband transitions coincides with one of the plasmon peaks, the resonant electromagnetic field in the core is enhanced which should result in amplification of the luminescence
intensity. Especially effective amplification can be reached if the frequencies of the exciting and emitting light both match two plasmon peaks. Experimental measurements were performed with composite nanostructures containing CdSe-ZnS cores fabricated by the organo-metallic method, followed by deposition of the gold shell using thermal decomposition of a Au (I) precursor. These revealed a multimodal structure of the absorption and luminescence spectra, good tunability, high intensity, and narrow emission linewidth. The dependence of spectra on the thickness of Au shell was investigated. The measurements were performed in different biological media and demonstrated stability and environment-insensitivity - a prerequisite for biolabeling.
Polarization phenomena in the optical absorption and emission of metallic, semiconducting or composite nanowires are considered theoretically. Most nanowire-based structures are characterized by a dramatic difference in dielectric constant ε between the nanowire material and environment. Due to image forces caused by such ε mismatch in nanowire structures, coefficients of their absorption and emission become essentially different for light polarized parallel or perpendicular to the nanowire axis. As a result, the intensity and spectra of absorption, luminescence, luminescence excitation, and photoconductivity in nanowires or arrays of parallel nanowires are strongly polarization-sensitive. In light-emitting nanowire core-shell structures, the re-distribution of a.c. electric field caused by the image forces may result in essential enhancing of core luminescence in frequency regions corresponding to luminescence from the semiconducting core or when the frequency of optical excitation coincides to the frequency of the plasmon resonance in the metallic shell. Random nanowire arrays acquire some properties typical for nematic liquid crystals. In such arrays, the effect described above may result in "polarization memory", where polarization of luminescence is determined by the polarization of the exciting light.