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Near-field enhancement on photonic nanostructures

Designing the optical properties of slab-type photonic crystals (PhCs) in view of the interaction with external light fields enables the control of e.g. the reflectance and field energy enhancement. The latter causes the field energy in a specific region close to the PhC surface to be largely increased compared to the incident field [1], providing a platform to interact with surface-near emitters such as quantum dots (QDs) [2,3]. The possibility to tailor the optical properties, e.g. by numerical techniques, can thus lead to major improvements in the fields of biosensors [4] or light emitting diodes [5,6].

Our group combines in-house fabrication facilities and know-how with high accuracy finite element based simulations using the Maxwell solver JCMsuite, performed directly in our group within the framework of the BerOSE joint lab.

Using near-field enhancement to increase fluorescence of quantum dots

Near field energy enhancement occurs if leaky modes of the PhC are excited using external radiation and it, hence, depends on the direction of incidence and the wavelength. We showed that the numerically determined field enhancement can be used to gain insight into fundamental properties of the PhC, such as anticrossing phenomena [7], but also to directly increase the fluorescence of lead sulfide QDs on the PhC surface [2]. For the latter case, figure 1 shows the results of the fluorescence enhancement experiment in the upper row, and compares it to numerically obtained field energy enhancement maps in the lower row. We found fluorescence enhancement values of up to 50% which draw a clear correlation to the leaky mode bands from the lower plots. Some bands are clearly visible in the fluorescence enhancement, while others do not seem to affect the QDs at all. This is explained using 3D local field energy enhancement plots (insets) revealing the spatial distribution of the modes. The QDs can only be influenced if the field enhancement occurs in a suitable volume for interaction (*mode B*), so that modes which bound closely to the PhC surface (*mode A*) are inappropriate. (See also the supplementary material of [2].)

Increased-fluresence-quantum-dots - enlarged view

Measured fluorescence enhancement maps (upper row) and simulated electric field energy enhancement maps (lower row). The insets show 3-dimensional field energy distributions for the two selected points A and B. The white lines in the simulation data mark the experimental data limits. (from Barth et al.)