We have investigated low frequency guiding polariton modes in finite linear chains of closely packed dielectric spherical particles of different optical materials. These guiding (chain bound) modes cannot decay radiatively, because photon emission cannot take place with simultaneous conservation of energy and momentum. For extending previous work on infinite chains of spherical particles [1] and infinite rods [2, 3], we were able to apply the multisphere Mie scattering formalism to finite chains of dielectric particles to calculate quality factors of most bound modes originating from the first two Mie resonances depending on the number of particles N and the material's refractive index nr. We found that, in agreement with the earlier work [4], guiding modes exist for n(r) > 2 and the quality factor of the most bound mode scales by N(3). We interpreted this behavior as the property of "frozen" modes near the edges of guiding bands with group velocity vanishing as N increases. In contrast with circular arrays, longitudinal guiding modes in particle chains possess a higher quality factor compared to the transverse ones.

No abstract

Low-dimensional ordered arrays of optical elements can possess bound modes having an extremely high quality factor. Typically, these arrays consist of metal elements which have significantly high light absorption thus restricting performance. In this paper we address the following question: can bound modes be formed in dielectric systems where the absorption of light is negligible? Our investigation of circular arrays of spherical particles shows that (1) high quality modes in an array of 10 or more particles can be attained at least for a refractive index nr > 2, so optical materials like TiO2 or GaAs can be used; (2) the most bound modes have nearly transverse polarization perpendicular to the circular plane; (3) in a particularly interesting case of TiO2 particles (rutile phase, nr = 2.7), the quality factor of the most bound mode increases almost by an order of magnitude with the addition of 10 extra particles, while for particles made of GaAs the quality factor increases by almost two orders of magnitude with the addition of ten extra particles. We hope that this preliminary study will stimulate experimental investigations of bound modes in lowdimensional arrays of dielectric particles. 1,2,3,4 They can even serve as a 1 − d nano-waveguide. 5The strong dependence of light scattering and absorption on 1 − d array geometry suggests that the quasi- bound optical modes exist in extended 1 − d arrays. The resonant interaction of light with those modes is responsible for the optical properties of arrays. Each mode can be characterized by the complex eigenfrequency z = ω − iγ/2, with the real part ω representing the mode frequency and the imaginary part γ representing the mode decay rate.6 Modes of interest must have a large enough quality factor Q = ω/γ so that they are bound to the whole 1-d structure.Indeed, 1−d arrays of N identical particles can possess bound modes with a quality factor approaching infinity for N → ∞, 6,7,8,9,10 so γ(N = ∞) = 0. The case Q → ∞ takes place in a linear chain of identical particles separated by a small distance a that is less than half of the resonant wavelength λ a < λ/2 = c/(2ω).(If Eq. (1) is satisfied, then there exists at least one quasibound mode having a maximum wavevector q max = π/a exceeding the wavevector of the resonant photon k = 2π/λ. When the chain is infinite the decay of such mode is forbidden by the momentum conservation law requiring k ≥ q. In the finite system of N particles, the decay rate is finite, but it tends to zero when N → ∞ in accordance with the power law γ(N ) ∝ N −3 (see Ref. 6 ). Quasi-bound modes of the finite system possess a very narrow frequency resonance that can be used in a variety of photonics applications including waveguides, 5 antennas and detectors. 7,8,9 These applications involve the emission or absorption of light in a very narrow frequency range, and the capability of functioning in the single photon regime.11 This narrow resonance leads to a well resolved far field emission pattern that can be used as a guiding signal for aircr...

A kinetics model is designed to investigate the charge separation (CT) process in stilbene-capped DNA hairpins composed of AT base pairs. This model combines standard tunneling and hopping electron transport with exciplex formation upon photoexcitation of the acceptor stilbene and its neighboring adenine and is capable of interpreting the CT rate and yield data within experimental accuracy. An analysis of hopping transport within the framework of a 1-D diffusion model results in a calculation of the nearest-neighbor CT rate to be approximately 1.2 ns(-1). In agreement with previous experimental and theoretical work, it is ascertained through a novel application of an extension to classical Marcus theory that the nearest-neighbor CT is adiabatic with reorganization energy lambda approximately 0.83 eV. The kinetics model can be extended to accurately characterize CT in other poly(A)-poly(T) systems with different hole donors (naphthaldiimide and 2-aminopurine) and acceptors (phenothiazine and guanine).