Optical Properties of Mesoscopic Systems of Coupled Microspheres

Abstract

Two mechanisms of optical coupling between spherical cavities, tight-binding be- tween their whispering gallery modes and focusing produced by periodically coupled microlenses, are directly observed using spatially resolved scattering spectroscopy and imaging. The results can be used for developing device concepts of lasers, optical fllters, microspectrometers and sen- sors based on mesoscopic systems of coupled microspheres. DOI: 10.2529/PIERS060908001922 In this paper we consider structures formed by coupled spherical cavities which can be regarded as mesoscopic systems due to the fact that the size of their building blocks (spheres) is comparable to the characteristic wavelength. In contrast to metamaterials or photonic crystals conceptualized through the process of homogenization, the optical phenomena in such mesoscopic systems are essentially based on the properties of the constituting cavities. These include their ultra high quality whispering gallery mode (WGM) resonances (1) and their ability to focus plane waves into \nanoscale photonic jets (2) at the shadow-side. These properties lead to two difierent mechanisms of optical transport between the cavities: (i) tight-binding between WGMs, (ii) propagation in a series of periodically coupled microlenses. These mechanisms are directly studied in the present work. 1. TIGHT-BINDING BETWEEN WGMS Previously we observed WGM-related propagation efiects (3) in one-dimensional (1D) chains of slightly disordered spheres using evanescently coupled dye-doped spherical cavities pumped above the lasing threshold for WGMs. In the present work we integrated spherical cavities into 3D closed packed structures with the thickness varying from one monolayer up to » 50 monolayers. The samples were obtained by self-assembly of spheres (with sizes in 2-10 micron range with standard 3% size dispersion) directed by hydro-dynamic ∞ow in a specially designed cuvette placed in ultrasonic bath. The spheres were dye-doped and locally excited to create a built-in source of whispering gallery modes (WGMs). In the scattering spectra of such samples we observed fringes due to light propagation via coupled WGMs, as illustrated in Fig. 1. The study of pump dependence of scattering spectra indicated that above WGM lasing threshold the emission is provided in localized modes formed by multiple spheres. This is conflrmed by observation of double peak structures, the spectral signature of strong coupling regime between multiple cavities. Although size disorder plays a negative role in the e-ciency of such transport, we show that due to existence of multiple paths for photons the optical transport can be very e-cient. The study of thickness dependence of scattering spectra indicates that attenuation length of light in such 3D samples exceeds 50m. The results show that the optical transport in such systems is provided along particular conflgurations of cavities according to a principle of minimization of the total WGM detuning.