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This opens the route towards space-time-resolved spectroscopy with direct observation of nanoscopic energy transport. The challenge lies in the optimization of a number of near-field observables, exploiting properly shaped laser pulses, to achieve ultimate spatial control over linear and nonlinear electromagnetic flux, the local spectrum, and the local temporal intensity profile.
much richer. First, one has to take into account the plasmonic properties of metals at optical frequency. Second, nanoscale optical sources are atoms, organic molecules or semiconductor quantum dots (Q-dots), i.e. quantum systems; therefore one enters the quantum regime of single photon emitters. Finally optical nanoantennas are truly small, with dimensions between 30 and 500 nm, posing challenges both to fabrication and novel methods to drive and tune such antennas.
2.5 Imaging and sensing In recent years “nanoscopy” optical microscopy with 10-30 nm detail, has become a reality. By proper engineering of the microscopical point spread function, in combination with non-linear response, the effective resolution is now an order of magnitude below the diffraction limit. Particularly STimulated Emission Depletion (STED) microscopy has moved into active applications, mainly in biology.
Nano-optical antennas offer unique new opportunities. They do allow confining and controlling optical fields truly on the nanometer scale. Even more, in close proximity to photon emitters, such as molecules, Q-dots or color centers, nano-antennas are particularly promising. First, they boost the radiative rate far over intrinsic non-radiative decay, thus with the potential to generate super-emitters with ps photo-cycling times. Indeed 100-fold lifetime reduction to 10 ps regime was reported recently. Second, nanoantennas funnel the incident far field efficiently to dedicated antenna mode maxima thus nanofocusing the incident light on e.g. a Q-dot. Last, not least, antennas redirect all photon emission in a dedicated direction with narrow angle. Indeed complete redirection of radiation patterns over 90 degrees was reported recently.
In parallel the controlled photo activation of single molecules has allowed the concept of Photo-Activation Localisation Microscopy (PALM), with effective resolution reaching the 20 nm, a method finding applications extremely rapidly. At the same time strong attention is on resonant metallic particles that enhance the local field enhancement and on nano-antenna configurations which afford improved coupling efficiency. Several types of antenna geometries are being pioneered for nanoscaling imaging in biology and technical application, again with resolution in the 20 nm range. In parallel, new physics routes are explored through superlensing by negative index (meta)materials, where the evanescent decay is locally inverted to gain; unfortunately, material losses are competing heavily with the superlensing efficiency.
2.4 Phase control of nanoscale optical fields By exploiting the interplay between the nanostructure and the spatio-temporal light field a high degree of control is attainable. Size and shape of the nanostructure play a vital role. For example theoretical modelling has shown that the field distribution of a tapered nanostructure depends directly on the linear chirp of a femtosecond excitation pulse. Thus the light is slowed down and ultimately stopped or trapped. Recently first results were reported that shaping allows specific control over the spatio-temporal nanoscopic field. Thus, pulse sequences can be generated in which local excitations occur at specific time and position with sub-diffraction resolution.
2.6 Nanophotonic manipulation Small particles can be trapped by optical fields, the so-called “optical tweezers”. The near field photonic forces generated at nanostructures or arrays of nanoholes provide a novel route of control, to trap nanoparticles in nanochannels,
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