G. W. 't Hooft

Observation of Goos-Hänchen shifts in metallic reflection

We report the first observation of the Goos-Hänchen shift of a light beam incident on a bare metal surface. This phenomenon is particularly interesting because the Goos-Hänchen shift for p polarized light in metals is negative and much bigger than the positive shift for s polarized light. The experimental result for the measured shifts as a function of the angle of incidence is in excellent agreement with theoretical predictions. In an energy-flux interpretation, our measurement shows the existence of a backward energy flow at the bare metal surface when this is excited by a p polarized beam of light.

Shannon dimensionality of quantum channels and its application to photon entanglement.

We introduce the concept of Shannon dimensionality D as a new way to quantify bipartite entanglement as measured in an experiment. This is applied to orbital-angular-momentum entanglement of two photons, using two state analyzers composed of a rotatable angular-sector phase plate that is lens coupled to a single-mode fiber. We can deduce the value of D directly from the observed two-photon coincidence fringe. In our experiment, D varies between 2 and 6, depending on the experimental conditions. We predict how the Shannon dimensionality evolves when the number of angular sectors imprinted in the phase plate is increased and anticipate that D approximately 50 is experimentally within reach.

Enhancement of spatial coherence by surface plasmons

We report on a method to generate a stationary interference pattern from two independent optical sources, each illuminating a single slit in Youngs interference experiment. The pattern arises as a result of the action of surface plasmons traveling between subwavelength slits milled in a metal film. The visibility of the interference pattern can be manipulated by tuning the wavelength of one of the optical sources.

Surface plasmon dispersion in metal hole array lasers

We experimentally study surface plasmon lasing in a series of metal hole arrays on a gold-semiconductor interface. The sub-wavelength holes are arranged in square arrays of which we systematically vary the lattice constant and hole size. The semiconductor medium is optically pumped and operates at telecom wavelengths (λ ~ 1.5 μm). For all 9 studied arrays, we observe surface plasmon (SP) lasing close to normal incidence, where different lasers operate in different plasmonic bands and at different wavelengths. Angle- and frequency-resolved measurements of the spontaneous emission visualizes these bands over the relevant (ω, k||) range. The observed bands are accurately described by a simple coupled-wave model, which enables us to quantify the backwards and right-angle scattering of SPs at the holes in the metal film.

Bouncing surface plasmons

Employing an interferometric cavity ring-down technique we study the launching, propagation and reflection of surface plasmons on a smooth gold-air interface that is intersected by two parallel, sub-wavelength wide slits. Inside the low-finesse optical cavity defined by these slits the surface plasmon is observed to make multiple bounces. Our experimental data allow us to determine the surface-plasmon group velocity (v(groug) = 2.7+/-0.3x10(-8) m/s at lambda = 770 nm) and the reflection coefficient (R approximately 0.04) of each of our slits for an incident surface plasmon. Moreover, we find that the phase jump upon reflection off a slit is equal to the scattering phase acquired when light is converted into a plasmon at one slit and back-converted to light at the other slit. This allows us to explain fine details in the transmission spectrum of our double slits.

Search for Hermite-Gauss mode rotation in cholesteric liquid crystals

In theory, there are analogous transformations of lights spin and orbital angular momentum [Allen and Padgett, J. Mod. Opt. 54, 487 (2007)]; however, none have been observed experimentally yet. In particular, it is unknown if there exists for the orbital angular momentum of light an effect analogous to the spin angular momentum-based optical rotation; this would manifest itself as a rotation of the corresponding Hermite-Gauss mode. Here we report an experimental search for this effect in a cholesteric liquid crystal polymer, using strongly focussed, spin-orbit coupled light. We find that the relative phase velocities of the orbital modes constituting the Hermite-Gauss mode agree to within 10(-5).

Loss compensation of extraordinary optical transmission

In the past decade, metal hole arrays have been studied intensively in the context of extraordinary optical transmission (EOT). Recently it was shown that surface plasmons on optically pumped hole arrays can show laser action. So far, however, it is not demonstrated that the optical transmission of these arrays can also be increased using gain. In this Letter, we present a dramatic increase of the EOT via loss compensation of surface plasmons, accompanied by spectral narrowing of the resonance. These experiments allow us to quantify the modal gain experienced by the surface plasmon. Interestingly, the transmission minimum of the Fano-resonance becomes smaller.

Enhanced coupling of plasmons in hole arrays with periodic dielectric antennas

We compare the angle-dependent transmission spectra of a metal hole array with dielectric pillars in each hole with that of a conventional metal hole array. The pillars enhance the optical transmission as well as the interaction between surface plasmon modes. This results in an observed splitting Delta omega/omega as large as 6%, at normal incidence, for the modes on the pillar side of the array.

Photonic bandstructure of a metal hole array with pillars

We have studied the angle and wavelength dependent extra-ordinary transmission of novel metal hole arrays that contain a dielectric pillar in each hole [1]. Similar to transmission spectra of conventional hole arrays, the measured transmission contains asymmetric resonances related to the excitation of surface plasmons on either side of the metal film. The pillars enhance the coupling of light to surface plasmons and introduce avoided crossings between the plasmon modes. We find that the dispersion of these plasmon modes can be described by the photonic bandstructure of a two-dimensional array of SiO2 pillars.

Coupling surface plasmons: Index matching and dielectric pillar arrays

We study coupling between surface plasmons in metal-hole arrays. We observe avoided crossings in the measured transmission spectra for metal-hole arrays with a dielectric pillar in each hole and for arrays in index matching liquid.