Plasmon resonance in a nanosphere

The phenomenon of plasmon resonance in a nanosphere and a nanospheroid is considered. As is known, this phenomenon does not depend explicitly on the size of these nanoparticles. However, it is shown that, actually, the resonance conditions are determined by the diameter of the nanosphere and by the length of the major axis of the nanospheroid.     

Enhancement of Raman scattering for an atom or molecule near a metal nanocylinder: quantum theory of spontaneous emission and coupling to surface plasmon modes

An analytic expression for the electromagnetic enhancement of the spontaneous emission rate and Raman scattering cross-section for an excited atom or molecule in close proximity to a metal nanocylinder has been derived by quantum theory. Coupling of the atomic or molecular optical radiation into the TM0 surface plasmon mode of the nanocylinder results in reradiation by the cylinder, a process that is most efficient when the incident radiation is linearly polarized, with the electric field oriented parallel to the axis of the nanocylinder. For a silver cylinder having a radius and length of 5 and 20 nm, respectively, the enhancement in the spontaneous emission rate is >10(7) for variant Planck's over 2pi omega0 approximately 2.4 eV (lambda=514 nm), which corresponds to an increase of approximately 10(14) in the Raman scattering cross section. This result, as well as the prediction that the atomic dipole generates broadband, femtosecond pulses, are in qualitative agreement with previously reported experiments involving metal nanoparticle aggregates. The theoretical results described here are expected to be of value in guiding future nonlinear optical experiments in which carbon nanotubes or metal nanowires with controllable physical and electrical characteristics are patterned onto a substrate and coupled with emitting atoms or molecules.     

Once again on the enhanced Raman scattering of light from a molecule near a small object

The effect of a significant (by a factor of more than 10⁷) increase in the rate of a one-photon spontaneous radiative transition is calculated for an excited atom (molecule) located near a metal (silver) nanocylinder. The atom radiates into a surface plasmon wave of the TM⁰ type. The TM⁰ wave is concentrated near the nanocylinder surface rather than spreads infinitely in space. Having a limited length, the nanocylinder efficiently radiates into free space, in contrast to the case of an infinite surface (plane or cylindrical). Under the conditions considered, the duration of the atomic dipole radiation is about a femtosecond. The situation discussed, which can be easily realized in experiments, may help in understanding the significant increase in the intensity of Raman scattering observed experimentally.     

Principal waves in multiply connected waveguide light concentration on subwavelength areas

The use of multiply connected waveguides for localizing and concentrating optical electromagnetic radiation onto areas of subwavelength transverse dimension is substantiated. In optical microscopes, this makes it possible to reach subwavelength resolution along all three spatial coordinates. In experiments where interaction of laser radiation with matter is studied, the subwavelength multiply connected waveguides allow the field intensity in the optical wave to be significantly increased. Applications of subwavelength waveguides in submicrometer photolithograpy, submicrosecond microelectronics, and photogrammetry is discussed, as well as for optical information recording and readout.     

On the Effect of Increasing the Radiative-Transition Rate Near a Nanosize Point

A short review of theoretical and experimental studies concerning the photoexcited florescence and Raman scattering of light for a substance in a space containing small material bodies is presented. Calculations of the radiativetransition probability for atoms (molecules) in the vicinity of bodies with a size much smaller than the light wavelength are performed. The probabilities of the singlephoton and doublephoton transitions are shown to increase by factors of 9 and 81 in the vicinity of a nanosize sphere with dielectric constant ||\ 1. The probability of a radiative transition in the vicinity of the vertex of a conic needle bearing up against a plane (both with || 1) increases by factors of (/R _in)² and (/R _in)⁴ for singlephoton and doublephoton transitions, respectively (R _in is the curvature radius for the needle vertex). This theoretical result is suggested as an explanation of the effect of increasing the radiation process intensity in the experiments carried out in the studies cited below.     

Electrical transport and magnetic ordering in R 2Ti3Ge4 (R=Dy, Ho and Er) compounds

New R ₂Ti₃Ge₄ (R=Dy, Ho and Er) intermetallic compounds have been synthesized and characterized by X-ray diffraction and low temperature ac magnetic susceptibility, electrical resistivity and thermoelectric power measurements were carried out. The compounds crystallize in the parent, Sm₅Ge₄-type orthorhombic structure (space group Pnma) and lanthanide contraction is observed as one moves along the rare-earth series. The changeover from paramagnetic to antiferromagnetic phase happens at low temperatures and the ordering temperature scales with the de Gennes factor. The electrical resistivity is metallic with a negative curvature above 100 K. Thermopower displays a weak maximum at temperatures less than 50 K signifying the possible phonon and magnon drag effects.     

A possible interpretation of enhanced raman scattering in the vicinity of a nanotip

Theoretical and experimental studies on Raman scattering in a space with small bodies are briefly reviewed. Probabilities of radiative transitions for atoms (molecules) in the vicinity of bodies with dimensions much smaller than a wavelength are calculated. It is shown that in the vicinity of a nanosphere with |ε|?1, the probability of a single-photon electric dipole transition increases by a factor of 9, and the probability of a two-photon transition—by a factor of 81. In the vicinity of a conical needle resting on a plane (the needle and plane have |ε|?1), the radiative transition probability increases by a factor (λ/R _in)² and (λ/R _in)⁴ for single-and two-photon transitions, respectively (R _in is the radius of the needle tip curvature). This theoretical result is offered to interpret the enhancement of radiative processes experimentally observed in the referenced studies.     

Subwavelength Electromagnetic-Field Narrowing

This paper is primarily aimed at experimental verification of results of theoretical studies of the possibility of concentration of an electromagnetic wave on areas of a size much smaller than the wavelength λ with the help of doubly connected narrowing waveguides. Fundamentally important experimental results, in both the microwave and optical ranges, have been obtained on an installation containing a biconical horn in the form of a conical needle and a metal plane. A fundamental convergent mode has been excited. A reflected fundamental mode appeared and changed sharply as the vertex of the conical needle approached the plane at a distance of the order of several nanometers and closer. The predictions of the theory concerning the concentration of electromagnetic (microwave and optical) radiation in a biconical horn onto objects with a size of the order of a nanometer with almost no losses have been confirmed experimentally. The possibility of increasing the sensitivity of the methods of spectroscopy of individual impurity sites using a biconical horn for coupling with the near field of a quantum oscillator (atom, molecule) in a quasi-stationary region is also investigated. The efficiency of electric-dipole radiation emission into a biconical horn increases by a factor of (λ/r₀)⁴ compared to spontaneous radiation into free space (here λ is the wavelength and r₀ is the distance from the dipole to the horn input). We have shown experimentally that it is possible in principle to create a device functioning as a sensor (a near-field electromagnetic microscope) and as an instrument of the action by a strong electromagnetic field (simultaneously at several frequencies) with a spatial resolution of the order of 1 nm in the optical and microwave ranges. Results of experiments of other authors are discussed in terms of concepts of convergent and divergent waves in a biconical horn. The feasibility of extending these methods to the extreme UV and soft x-ray ranges is pointed out.