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.

Search for Hermite-Gauss mode rotation in cholesteric liquid crystals.

In theory, there are analogous transformations of light's 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).

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.

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.

Giant optical transmission of a subwavelength slit optimized using the magnetic field phase.

In this Letter, we show that the energy equivalent to that incident on a 4.7 microm wide strip can be squeezed through a 50 nm wide slit in a metal film surrounded by grooves. This corresponds to a transmission efficiency of 9400%, which can be even further enhanced by increasing the number of grooves. We use the phase of the magnetic field to explain that the ideal slit-to-groove distance is just over half the plasmon wavelength. In addition, we also optimize the groove depth and width. Such optimized transmission enhancement is very important for near-field devices.

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 Young's 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.

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.

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.

On the phase of plasmons excited by slits in a metal film.

The excitation of surface plasmons by subwavelength slits in metal films is studied using a rigorous diffraction model. It is shown that the plasmon is launched by a slit in antiphase with the incident magnetic field. This is true independent of slit width and of the metal used. Using this phase information, maxima and minima in transmission are explained in the case of two and more slits.

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.

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.

From amplified spontaneous emission to laser oscillation: dynamics in a short-cavity polymer laser.

We present an experimental study of the input-output characteristics of an ultrashort pumped pi -conjugated polymer in solution. By comparing the results for configurations with zero, one, and two mirrors around the polymer, we show that the physics is driven by amplified spontaneous emission and not by cooperative emission. This finding is substantiated by picosecond-time-scale measurements of the evolution of the emission of the polymer. For the two-mirror configuration a sharply defined threshold for laser oscillation is found; the output of the laser exhibits strong pulsations.

Tomographic image reconstruction from optical projections in light-diffusing media.

The recent developments in light generation and detection techniques have opened new possibilities for optical medical imaging, tomography, and diagnosis at tissue penetration depths of ~10 cm. However, because light scattering and diffusion in biological tissue are rather strong, the reconstruction of object images from optical projections needs special attention. We describe a simple reconstruction method for diffuse optical imaging, based on a modified backprojection approach for medical tomography. Specifically, we have modified the standard backprojection method commonly used in x-ray tomographic imaging to include the effects of both the diffusion and the scattering of light and the associated nonlinearities in projection image formation. These modifications are based primarily on the deconvolution of the broadened image by a spatially variant point-spread function that is dependent on the scattering of light in tissue. The spatial dependence of the deconvolution and nonlinearity corrections for the curved propagating ray paths in heterogeneous tissue are handled semiempirically by coordinate transformations. We have applied this method to both theoretical and experimental projections taken by parallel- and fan-beam tomography geometries. The experimental objects were biomedical phantoms with multiple objects, including in vitro animal tissue. The overall results presented demonstrate that image-resolution improvements by nearly an order of magnitude can be obtained. We believe that the tomographic method presented here can provide a basis for rapid, real-time medical monitoring by the use of optical projections. It is expected that such optical tomography techniques can be combined with the optical tissue diagnosis methods based on spectroscopic molecular signatures to result in a versatile optical diagnosis and imaging technology.

Image quality in time-resolved transillumination of highly scattering media.

Using a photon-counting setup and a streak-camera arrangement with time resolutions of 35 and 6 ps, respectively, we have investigated the spatial resolution of a time-gated transillumin tion technique applied to turbid media. In the case of large relative amounts of unscattered light, it is found that small detection angles improve the spatial resolution. For large concentrations of scatterers and large sample thicknesses, i.e., when the amount of unscattered light is negligible, the best time-gate position is found to be at times that are later than the minimum transit time. In this case (minimum transit time), temporal resolutions from small values up to approximately 50 ps yield almost the same image resolution. The only advantage of measuring systems with a higher than 50-ps temporal resolution is their ability to distinguish the diffused from the unscattered light, when a significant amount of the latter is present.