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Physics and applications of nanocrystal - polymer nanophotonic devices

Final Report Summary - NANOPHOTONIC DEVICES (Physics and applications of nanocrystal - polymer nanophotonic devices)

Project context and objectives

Subwavelength optics is becoming a prominent field for both new physical effects and new device functionalities. In particular, the response of sub-wavelength metallic corrugated structures in the optical regime reveals many physically interesting, potentially useful and sometime initially counterintuitive phenomena. One of the most widely known effects is that of resonant ‘extraordinary optical transmission’ in arrayed sub-wavelength holes or slits in metallic sheets. Carefully engineered sub-wavelength slit arrays were previously shown to exhibit various other interesting qualities, such as beaming and focusing light, large local field enhancements and many suggestions for use in various device realisations.

Main results

We developed an approximated, simple closed-form model for predicting and explaining the general emergence of enhanced transmission resonances through metallic gratings, in various configurations and polarisations. This model is based on an effective index approximation and it unifies in a simple way the underlying mechanism of all forms of enhanced transmission in such structures as emerging from standing wave resonances of the different diffraction orders of periodic structures.

We demonstrated a highly directional beaming of the photons that are emitted from nanocrystal quantum dots embedded in a sub-wavelength metallic nanoslit array. We showed that the photon beaming is achieved via the enhanced resonant coupling of the quantum dots to the resonant Bragg cavity modes of the structure, and that the emission probability to those modes is 20 times larger than that to non-directional leaky modes. This enhanced coupling dominates the emission properties of the quantum dots. These Bragg cavity modes, which are the eigenmodes of the structure, result in localised electromagnetic field enhancements at the Bragg cavity resonances, which can be controlled and engineered. This can be used to engineer wavelength and angular selective emission of nanoemitters using the polarisation, spatial and angular selectivity of such resonant standing modes. We showed such selectivities in two different nanoslit array configurations, supporting different types of resonant modes. The simple calculations support the physical picture of enhanced optical dipole coupling due to a large enhancement of electromagnetic density of states, which cause a preferred coupling of the optical dipoles to those modes that have a well-defined angular directionality as well as a well-defined polarisation. We showed that this beaming effect occurs on the single quantum dot level, which could be useful for exploiting such effects for future single photon-based devices for quantum information or other quantum optics applications. Such devices can be engineered by the selective binding of single quantum dots to specific areas on similar nanoslit array samples.

We also showed a large enhancement of two-photon absorption processes in nanocrystal quantum dots and of light upconversion efficiency from the infrared to the near-infrared spectral regime, using a hybrid optical device in which quantum dots were embedded on top of a metallic nanoslit array. The resonant enhancement of these nonlinear optical processes is again due to the strong local electromagnetic field enhancements inside the nanoslit array structure at the extraordinary transmission resonances. Different high-field regions were identified for different polarisations, which can be used for designing and optimising efficient nonlinear processes in such hybrid structures. Combining nanocrystal quantum dots with sub-wavelength metallic nanostructures is therefore a promising way for a range of possible nonlinear optical devices.