Optical properties of SOI waveguides functionalized with close-packed quantum dot films

It is shown that dipolar coupling between neighboring quantum dots enhances the absorption of light in close packed monolayers of colloidal quantum dots. Based on this concept, the experimentally determined losses in planarized waveguides coated by a quantum dot monolayer can be successfully simulated. These simulations rely on replacing the quantum dot layer by an effective medium with a dielectric function determined by dipolar coupling and use the dielectric constant of the quantum dot host medium as the only adjustable parameter. This leads to a generic approach for the simulation of optical materials including close packed quantum dot layers

Absorption enhancement in 2D nanocrystal superlattices through near-field dipolar coupling: a novel optical phenomenon at the nanoscale

We demonstrate giant and broadband enhancement of the nanocrystal absorption cross section in close packed nanocrystal superlattices, which is the first report on a collective optical phenomenon in this type of self-assembled metamaterials to date

Modelling colloidal quantum dots on SOI photonic waveguides

We studied the optical absorption of a colloidal PbS/CdS quantum dot (Qdot) monolayer integrated on SOI waveguides (WGs). Using arefractive index value determined from effective medium theory including dipolar coupling between Qdots, we numerically determine loss values

Assessing colloidal quantum dot optical properties through free standing layers on photonic waveguides

We present a novel way of assessing the dielectric function of thin Qdot mono- to multilayers by creating free standing layers of PbS/CdS core shell Qdots on top of 450nm wide oxide planarized SOI waveguides (PWGs). Through transmission measurements in the PWGs we can sensitively monitor the absorption coefficient at the bandgap energy of thin free standing Qdot layers. Based on effective medium theory including dipolar coupling between neighbouring Qdots we predict a layer dielectric constant yielding simulated waveguide losses that are in excellent agreement with the experimental results. We show an increase of the host permittivity with the thickness of the free standing Qdot layer, indicating a change of the Qdot environment from air to mainly oleic acid ligand molecules. Considering the emerging use of colloidal Qdots due to their optical properties and the facile (wet) processing, PbS/CdS hybrid PWGs can be used as a model system to assess the fundamental Qdot optical properties of other types of colloidal Qdot photonic hybrid systems

Modeling the optical properties of low-cost colloidal quantum dot functionalized Strip SOI waveguides

We studied the optical absorption of silicon-on-insulator strip waveguides functionalized by a monolayer of colloidal PbS/CdS core/shell quantum dots. The integration is done by Langmuir-Blodgett deposition, which results in a monolayer of quantum dots (QDs) on the waveguides. Experimental absorption coefficients of QD functionalized strip waveguides range from 10-30 cm-1. These values are about five times larger than the absorption coefficient of QD functionalized planarized waveguides. Using a refractive index as determined from effective medium theory including dipolar coupling between the QDs, we obtain simulated values for the absorption coefficient that are in quantitative agreement with the experimental values and we find that difference with planarized waveguides results from an increased overlap between the QD layer and the guided optical mode in the case of strip waveguides. The modeling of the absorption coefficients of more complex strip waveguides functionalized by colloidal QDs as demonstrated in this study will enable the development and simulation of QD-based photonic devices integrated in silicon

Near-field resonance at far-field-induced transparency in diffractive arrays of plasmonic nanorods

We numerically demonstrate that a periodic array of metallic nanorods sustains a maximum near-field enhancement and a far field (FF)-induced transparency at the same energy and in-plane momentum. The coupling of bright and dark plasmonic lattice resonances, and electromagnetic retardation along the nanorod length, are responsible for this effect. A standing wave with a quadrupolar field distribution is formed, giving rise to a collective suppression of FF scattering and simultaneously enhanced local fields.IST/Imaging Science and TechnologyApplied Science

Size-dependent optical properties of zinc blende cadmium telluride quantum dots

We analyze the optical properties of CdTe quantum dots, including the sizing curve, the absorption coefficient, and the oscillator strength of the band gap transition, by combining absorption spectroscopy, elemental analysis, and electron microscopy imaging. At short wavelengths, the absorption coefficient spectrum is still affected by quantum confinement, yet a largely constant value, close to that of bulk CdTe, is found at around 410 nm. At shorter wavelengths, remaining quantum confinement effects on the CdTe E-1 transition are present even for the largest quantum dots studied (11 nm). For the band gap transition, we find an integrated absorption coefficient mu(gap) that scales almost proportionally to the inverse of the quantum dot volume. Especially for the smaller diameters, deviations up to a factor of 3 are found as compared to widely used literature values. The corresponding oscillator strength f(gap) is almost size-independent in the diameter range 3-7 nm. The correspondence between radiative lifetimes predicted based on f(gap) and literature values is discussed