A broad‐band and omnidirectional antireflection coating consisting of semiconductor nanowires is fabricated. The reflection is reduced due to a graded refractive index, which is achieved by a certain nanorod‐length distribution of cylindrical and conically shaped rods. An increased transmission and, accordingly, a reduced reflection are demonstrated for the visible and near‐infrared parts of the spectrum.

We have studied a GaAs-AlAs planar microcavity with a resonance near 1300 nm in the telecom range by ultrafast pump-probe reflectivity. By the judicious choice of pump frequency, we observe a ultimate fast and reversible decrease of the resonance frequency by more than half a linewidth due to the instantaneous electronic Kerr effect. The switch-on and switch-off of the cavity is only limited by the cavity storage time of τ cav = 0.3ps and not by intrinsic material parameters. Our results pave the way to supra-THz switching rates for on-chip data modulation and real-time cavity quantum electrodynamics.Switches are widely applied and necessary ingredients in modulation and computing schemes 1 . The recent progress on photonic integrated circuits 2,3 promises to overtake boundaries set by conventional switching technology. To do so, ultrafast switching of photonic cavities is crucial as it allows the capture or release on demand of photons 4-6 , which is relevant to on-chip communication with light as information carrier 7 , and to high-speed miniature lasers 8 . Ultrafast switching would also permit the quantum electrodynamical manipulation of coupled cavity-emitter systems 9 in real-time. Switching the optical properties of photonic nanostructures is achieved by changing the refractive index of the constituent materials. To date, however, the switching speed has been limited by material properties 11-14 , but not by optical considerations. To achieve ultimate fast switching of a cavity two challenges arise. Firstly, both the switch-on and switch-off times τ on and τ of f must be shorter than all other relevant time scales for the system, i.e., the cavity storage time in photon capture/release experiments τ cav , or the vacuum Rabi period τ Rabi for a strongly coupled emitter-cavity system 10 . Secondly, the refractive index change must be large enough to switch the cavity resonance by at least half a linewidth.Here, we demonstrate the ultimate fast switching of the resonance of a planar cavity in the well-known GaAs/AlAs system in the telecom wavelength range. We exploit the instantaneously fast electronic Kerr effect by the judicious tuning of the pump and probe frequencies relative to the semiconductor bandgap. We observe that the speed of the switching is then only limited by the dynamics of the light in our cavity (τ cav = 0.3 ps), but not by the intrinsic material parameters.Instantaneous on-and off-switching with vanishing τ on and τ of f is feasible with the well-known nonlinear rea) Electronic mail: g.ctistis@utwente.nl b) Electronic mail: W.L.Vos@tnw.utwente.nl fractive index from nonlinear optics 15 . Physically the electronic Kerr effect is the fastest Kerr phenomenon on account of the small electron mass. In many practical situations, however, non-degenerate two-photon absorption overwhelms any instantaneous effect and therefore also the dispersive electronic Kerr effect 13,15 . In order to avoid two-photon absorption and to access the electronic Kerr switching regime, we designed our experiment to op...

We have performed white light reflectivity measurements on GaAs/AlAs micropillar cavities with diameters ranging from 1 m up to 20 m. We are able to resolve the spatial field distribution of each cavity mode in real space by scanning a small-sized beam across the top facet of each micropillar. We spectrally resolve distinct transverse-optical cavity modes in reflectivity. Using this procedure we can selectively address a single mode in the multimode micropillar cavity. Calculations for the coupling efficiency of a small-diameter beam to each mode are in very good agreement with our reflectivity measurements.