J. P. Woerdman

Helical-wavefront laser beams produced with a spiral phaseplate

We demonstrate experimentally that a spiral phaseplate can convert a TEM00 laser beam into a helical-wavefront beam with a phase singularity at its axis. The diffractive-optical effect of the spiral phaseplate is implemented by index matching a macroscopic structure in an optical immersion. We discuss the optical properties of a helical wavefront beam produced this way by means of a mode analysis and by Fraunhofer diffraction calculations.

Astigmatic laser mode converters and transfer of orbital angular momentum

Abstract We present the design of a mode converter which transforms a Hermite-gaussian mode of arbitrarily high order to a Laguerre-gaussian mode and vice versa. The converter consists of two cylindrical lenses and is based on appropriate use of the Gouy phase. We demonstrate mode conversion experimentally and consider where the concomitant transfer of orbital angular momentum is localized.

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.

Fano-type interpretation of red shifts and red tails in hole array transmission spectra

We present a unifying point of view which allows to understand spectral features reported in recent experiments with two-dimensional arrays of subwavelength holes in metal films. We develop a Fano analysis of the related scattering problem by distinguishing two interfering contributions to the transmission process, namely a non-resonant contribution (direct scattering) and a resonant contribution (surface plasmon excitation). The introduction of a coupling strength between these two contributions naturally induces resonance shifts and asymmetry of profiles which satisfy simple scaling relations. We also report an experiment to confirm this analysis.

Production and characterization of spiral phase plates for optical wavelengths

We describe the fabrication and characterization of a high-quality spiral phase plate as a device to generate optical vortices of low (3-5) specified charge at visible wavelengths. The manufacturing process is based on a molding technique and allows for the production of high-precision, smooth spiral phase plates as well as for their replication. An attractive feature of this process is that it permits the fabrication of nominally identical spiral phase plates made from different materials and thus yielding different vortex charges. When such a plate is inserted in the waist of a fundamental Gaussian beam, the resultant far-field intensity profile shows a rich vortex structure, in excellent agreement with diffraction calculations based on ideal spiral phase plates. Using a simple optical test, we show that the reproducibility of the manufacturing process is excellent.

Role of beam propagation in Goos-Hänchen and Imbert-Fedorov shifts.

We derive the polarization-dependent displacements parallel and perpendicular to the plane of incidence for a Gaussian light beam reflected from a planar interface, taking into account the propagation of the beam. Using a classical-optics formalism we show that beam propagation may greatly affect both Goos-Hänchen and Imbert-Fedorov shifts when the incident beam is focused.

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.

Experimental demonstration of fractional orbital angular momentum entanglement of two photons.

The singular nature of a noninteger spiral phase plate allows easy manipulation of spatial degrees of freedom of photon states. Using two such devices, we have observed very high-dimensional spatial entanglement of twin photons generated by spontaneous parametric down-conversion.

Elasto-optic anisotropy and polarization orientation of vertical-cavity surface-emitting semiconductor lasers

We report a new technique to apply strain to a vertical‐cavity surface‐emitting semiconductor laser. This has allowed us to study the relation between strain and birefringence. We have found that the corresponding tensor is anisotropic, with a measured anisotropy 2p44/(p11−p12)=4.7±0.6. This anisotropy explains the natural preference of the polarization for the [110]/[110] axes.

How orbital angular momentum affects beam shifts in optical reflection

It is well known that reflection of a Gaussian light beam (TEM{sub 00}) by a planar dielectric interface leads to four beam shifts when compared to the geometrical-optics prediction. These are the spatial Goos-Haenchen (GH) shift, the angular GH shift, the spatial Imbert-Fedorov (IF) shift, and the angular IF shift. We report here, theoretically and experimentally, that endowing the beam with orbital angular momentum leads to coupling of these four shifts; this is described by a 4x4 mixing matrix.