Nano-optics of complex media

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• R. Sapienza, King's College London
• Friday 15 February 2013, 14:00-15:00
• Small Lecture Theatre, Cavendish Laboratory.

If you have a question about this talk, please contact Alessio Zaccone.

Central to modern nanophotonics is spontaneous emission control which can be attained by local density of states (LDOS) engineering in dielectric and metallic nano structures. I will discuss our recent studies of fluorescence from a nano-sized emitter embedded in complex photonic media, such as photonic crystals and random powders. I will report nanoscale mapping of the local density of states by cathodo-luminescence microscopy, a combination of electron-beam scanning and optical spectroscopy [1]; it relies on scanning a transient dipolar emitter induced by electron beam bombardment with respect to its photonic environment while measuring the total emitted power. Each individual electron traversing the photonic structure generates a nanoscale transient dipole by the accelerated charge which we exploit as a local probe of the LDOS .

With unprecedented resolution (∼10 nm) we image localized photonics crystal cavity modes in a nano-structured silicon nitride membrane, over the visible spectrum into the near-IR. We identify individual cavity modes that are spatially different and we map their LDOS . Also, our measurements reveal extended Bloch modes which are delocalized over the crystal and periodically modulated. Moreover, by momentum spectroscopy, we resolve the angular emission pattern of the radiation emitted, which exhibits complex diffraction patterns [1].

In addition, by fluorescence dynamics we measure LDOS distributions in 3D disordered dielectric powders. We observe a surprisingly long-tailed distribution of the LDOS with Purcell factor up to ~10 [2]. I will discuss how our experimental results contribute to shed light onto the controversial dependence of the C0 correlation function on macroscopic disorder parameters.

[1] R. Sapienza, et al. Deep-subwavelength imaging of the modal dispersion of light, Nature Materials 11, 781–787 (2012). [2] R. Sapienza, et al. Long-Tail Statistics of the Purcell Factor in Disordered Media Driven by Near-Field Interactions, Phys. Rev. Lett. 106, 163902 (2011).

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