November 09, 2011

Adaptive on-chip control of nano-optical fields with optoplasmonic vortex nanogates

Arxiv - Adaptive on-chip control of nano-optical fields with optoplasmonic vortex nanogates (11 pages)

A major challenge for plasmonics as an enabling technology for quantum information processing is the realization of active spatio-temporal control of light on the nanoscale. The use of phase-shaped pulses or beams enforces specific requirements for on-chip integration and imposes strict design limitations. We introduce here an alternative approach, which is based on exploiting the strong sub-wavelength spatial phase modulation in the near-field of resonantly-excited high-Q optical microcavities integrated into plasmonic nanocircuits. Our theoretical analysis reveals the formation of areas of circulating powerflow (optical vortices) in the near-fields of optical microcavities, whose positions and mutual coupling can be controlled by tuning the microcavities parameters and the excitation wavelength. We show that optical powerflow though nanoscale plasmonic structures can be dynamically molded by engineering interactions of microcavity-induced optical vortices with noble-metal nanoparticles. The proposed strategy of re-configuring plasmonic nanocircuits via locally addressable photonic elements opens the way to develop chip-integrated optoplasmonic switching architectures, which is crucial for implementation of quantum information nanocircuits.

Plasmonics, which exploits reversible conversion of propagating light into surface charge density oscillations of free electrons in metals – surface plasmons (SPs) – has become a mature technology for nanoimaging and bio(chemical) sensing, and holds high promise for implementation of chip-scale information processing networks. Efficient delivering of optical energy into deeply sub-wavelength areas via the excitation of localized SP resonances in metal nanostructures facilitates dramatic enhancements of local field intensities and lightmatter interactions. A new area of intense research effort in plasmonics is robust on-chip dynamic spatiotemporal manipulation of the sub-wavelength fields and of interactions between single photons and single quantum emitters. One important factor limiting the dynamic tunability range of plasmonic nanoelements is the required refractive index modulation range, which – due to shrinking of the spatial light-matter interaction lengths – increases with decreasing spatial dimensions. Another factor is the short dephasing times of the localized SP modes stemming from their high dissipative and radiative losses. The consequences of the latter factor are twofold. First, the reduced ‘temporal interaction length’ of light with the material imposes even more stringent requirements to the index modulation range. Second, as modes linewidths are inversely proportional to their dephasing times, plasmonic nanostructures typically feature broad scattering spectra consisting of overlapping resonance peaks corresponding to different SP modes. Individual bright and dark SP modes of complex plasmonic nanostructures can be detected by electron-energy-loss spectroscopy and confocal two-photon photoluminescence mapping. The use of temporally phase- and amplitude-modulated pulses and beams, nanofluidic
chambers, or elastomeric substrates may complicate on-chip integration of the plasmonic elements. Finally, efficient adaptive focusing of light generated by external sources cannot affect the radiative properties of embedded quantum emitters as it does not change the local density of optical states (LDOS) on the plasmonic chip.

We have recently demonstrated that embedding photonic elements (optical microcavities) into nanoplasmonic circuits introduces a mechanism of strong spectral selectivity into plasmonic networks owing to efficient photon trapping and re-cycling in microcavities in the form of high-Q photonic modes (e.g., whispering gallery (WG) modes). The resulting optoplasmonic circuits can also perform the functions of long-range light transfer with subsequent nanoscale localization as well as spectral and spatial signal (de)multiplexing. Furthermore, embedded high-Q microcavities strongly modify the local density of optical states at specific spatial locations and defined frequencies corresponding to the high-Q cavity modes, and thus pave the way to tailoring light interactions with quantum emitters. In this paper, we will demonstrate that high-Q photonic elements also provide a rich electromagnetic field phase landscape, which can be used to achieve on-chip dynamical light spatial reconfiguring and switching within nanoscale-size plasmonic nanostructures, thus completely eliminating the need for bulky external far-field optics.

Our results demonstrate that the nanoscale powerflow through plasmonic structures can be directed and reversibly switched with chip-integrated high-Q photonic elements (microcavities). The possibility of local addressing of individual microcavities (optically, electro-optically or thermo-optically) in a dynamic fashion is the advantage of the proposed mechanism of the phase-operated intensity switching over previously explored strategies based of using external phase- and amplitude-modulated pulses and beams. Although we explored this approach in a few selected configurations of optoplasmonic elements in this article, the proposed strategy is very general and can be applied to design extended optoplasmonic networks of arbitrary morphology that incorporate various types of microcavities and plasmonic nanostructures. Our observations pave the road to the development of dynamically-tunable and switchable vortex-operated plasmonic nanocircuits for optical information processing and ultrasensitive biosensing.
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