Erwin Altewischer

Plasmon-assisted transmission of entangled photons

The state of a two-particle system is said to be entangled when its quantum-mechanical wavefunction cannot be factorized into two single-particle wavefunctions. This leads to one of the strongest counter-intuitive features of quantum mechanics, namely non-locality. Experimental realization of quantum entanglement is relatively easy for photons; a starting photon can spontaneously split into a pair of entangled photons inside a nonlinear crystal. Here we investigate the effects of nanostructured metal optical elements on the properties of entangled photons. To this end, we place optically thick metal films perforated with a periodic array of subwavelength holes in the paths of the two entangled photons. Such arrays convert photons into surface-plasmon waves—optically excited compressive charge density waves—which tunnel through the holes before reradiating as photons at the far side. We address the question of whether the entanglement survives such a conversion process. Our coincidence counting measurements show that it does, so demonstrating that the surface plasmons have a true quantum nature. Focusing one of the photon beams on its array reduces the quality of the entanglement. The propagation of the surface plasmons makes the array effectively act as a ‘which way’ detector.We investigate whether entanglement is conserved when polarization entangled photons are converted to surface plasmons, propagating along subwavelength metal hole arrays. We find that entanglement survives, but is limited by spatial dispersion of the hole arrays.

Polarization tomography of metallic nanohole arrays.

We report polarization tomography experiments on metallic nanohole arrays with square and hexagonal symmetry. As a main result we find that a fully polarized input beam is partly depolarized after transmission through a nanohole array. This loss of polarization coherence is found to be anisotropic; i.e., it depends on the polarization state of the input beam. The depolarization is ascribed to a combination of two factors: (i) the nonlocal response of the array as a result of surface-plasmon propagation and (ii) the non-plane-wave nature of a practical input beam.

Nonreciprocal reflection of a subwavelength hole array

We present measurements of the wavelength-dependent reflectivity of a subwavelength metal hole array on a glass substrate. We compare the observed resonant structures with those found in transmission and note a nonreciprocity under illumination from the air versus the glass side. This can be used to verify on which interface the surface plasmons are resonantly excited and to estimate the losses in the subwavelength channels.

Fano-type interference in the point-spread function of nanohole arrays

Measurement of the point-spread function of metal nanohole arrays by using microscopic imaging reveals two contributions. The first of these is due to propagating resonant surface plasmons and the second to nonresonant transmission through the holes. We observe a Fano-type interference between these contributions.

Resonant Bragg scatter of surface plasmons on nanohole arrays

Using microscopic imaging, we study the generation and propagation of beams of surface plasmons (SPs) on a hexagonal metal nanohole array. We discuss the wavelength-dependent propagation, a Fano-type interference and the possibility to generate focused SP beams. Prominent forking of these beams is attributed to resonant (Bragg) scattering from consecutive lattice planes. This claim is supported by a coupled-mode model.

Coherent Polarization Transfer through Sub-wavelength Hole Arrays

We review a series of experiments on the optical properties of metal films perforated with arrays of sub-wavelength holes. A key experiment is the transfer of polarization entanglement under plane-wave/focused illumination, where we observed a conservation/degradation of the quantum entanglement. Surface plasmons play a prominent role in the observed extra-ordinary large transmission. This is demonstrated with two supporting experiments performed with classical light, investigating: (i) the polarization and angular dependent transmission through the arrays, (ii) the generation of coherent beams of surface plasmons. Both experiments prove the directionality and TM-character expected for surface plasmon modes on a hole array.

Direct optical imaging of surface plasmon propagation in nanohole arrays

Measurements of the optical near-field transmission of square and hexagonal nanohole arrays are performed, by making a magnified image of the backside of the hole array with a normal microscope objective. In these measurements it can be observed the surface plasmon (SP) propagation directly by strongly focussing the illumination spot on the array down to a size smaller than the SP propagation length.

Plasmon-assisted transmission of quantum entanglement

We investigate whether entanglement is conserved when polarization entangled photons are converted to surface plasmons, propagating along subwavelength metal hole arrays. We find that entanglement survives, but is limited by spatial dispersion of the hole arrays.