D. van Oosten

Role of interactions in 87Rb-40K Bose-Fermi mixtures in a 3D optical lattice.

We investigate the effect of interspecies interaction on a degenerate mixture of bosonic 87Rb and fermionic 40K atoms in a three-dimensional optical lattice potential. Using a Feshbach resonance, the 87Rb-40K interaction is tuned over a wide range. Through an analysis of the 87Rb momentum distribution, we find a pronounced asymmetry between strong repulsion and strong attraction. In the latter case, we observe a marked shift in the superfluid to Mott insulator transition, which we attribute to a renormalization of the Bose-Hubbard parameters due to self-trapping.

Ultracold atoms in optical lattices

Bosonic atoms trapped in an optical lattice at very low temperatures can be modeled by the Bose-Hubbard model. In this paper, we propose a slave-boson approach for dealing with the Bose-Hubbard model, which enables us to analytically describe the physics of this model at nonzero temperatures. With our approach the phase diagram for this model at nonzero temperatures can be quantified.

Nanohole chains for directional and localized surface plasmon excitation

Arrangements of subwavelength sized holes in metal films are often used to launch surface plasmon polaritons (SPPs) onto metal-dielectric interfaces. They are readily fabricated and can also be used to generate a variety of near- and far-field intensity patterns. We use a short chain of equally spaced subwavelength sized holes to launch SPPs onto a gold-air interface in complex patterns of hotspots. With a phase-sensitive near-field microscope, we visualize the electric field of the excited SPPs. We observe self-images of the chain that we attribute to the Talbot effect. Far from the chain we observe the SPP diffraction orders. We find that when the spacing of the holes is of the order of the wavelength, the revivals do not occur on the well-known Talbot distance as derived in the paraxial limit. We present an alternative expression for the Talbot distance that does hold for these small spacings. We study the behavior of both the revivals and the diffraction orders as a function of the number of holes. We find that the Talbot revivals become more pronounced as the number of holes is increased, which is in accordance with numerical calculations. We anticipate that our findings are interesting for multiplexing sensor applications, where control over the local intensity of SPPs is crucial.

Ultrafast rerouting of light via slow modes in a nanophotonic directional coupler

We acknowledge funding through the EU FP6-FET “SPLASH” project. This work is also part of the research program of FOM, which is financially supported by the NWO.

Characterization of bending losses for curved plasmonic nanowire waveguides

We characterize bending losses of curved plasmonic nanowire waveguides for radii of curvature ranging from 1 to 12 microm and widths down to 40 nm. We use near-field measurements to separate bending losses from propagation losses. The attenuation due to bending loss is found to be as low as 0.1 microm(-1) for a curved waveguide with a width of 70 nm and a radius of curvature of 2 microm. Experimental results are supported by Finite Difference Time Domain simulations. An analytical model developed for dielectric waveguides is used to predict the trend of rising bending losses with decreasing radius of curvature in plasmonic nanowires.

Feshbach resonances in an optical lattice

We present a theory for ultracold atomic gases in an optical lattice near a Feshbach resonance. In the single-band approximation the theory describes atoms and molecules that can both tunnel through the lattice. Moreover, an avoided crossing between the two-atom and the molecular states occurs at every site. We determine the microscopic parameters of the generalized Hubbard model that describes this physics, using the experimentally known parameters of the Feshbach resonance in the absence of the optical lattice. As an application we also calculate the zero-temperature phase diagram of an atomic Bose gas in an optical lattice.

Inelastic light scattering from a Mott insulator

We propose to use Bragg spectroscopy to measure the excitation spectrum of the Mott-insulator state of an atomic Bose gas in an optical lattice. We calculate the structure factor of the Mott insulator taking into account both the self-energy corrections of the atoms and the corresponding dressing of the atom-photon interaction. We determine the scattering rate of photons in the stimulated Raman transition and show that by measuring this scattering rate in an experiment, in particular, the excitation gap of the Mott insulator can be determined.

Measurements of modal symmetry in subwavelength plasmonic slot waveguides

We excite a guided plasmonic mode in slot waveguides of subwavelength width. With a phase- and polarization-sensitive near-field microscope, we measure the electric field of the mode for a range of slot widths from 40 to 120 nm. The field is experimentally found to be antisymmetric across the slot gap. Numerical calculations confirm this symmetry. Calculations also show a confinement of the field to a lateral size ∼10 times smaller than the free-space wavelength.

Mott insulators in an optical lattice with high filling factors

We discuss the superfluid to Mott insulator transition of an atomic Bose gas in an optical lattice with high filling factors. We show that in this multiband situation, the long-wavelength physics is described by a single-band Bose-Hubbard model. We determine the many-body renormalization of the tunneling and interaction parameters in the effective Bose-Hubbard Hamiltonian, and consider the resulting model at nonzero temperatures. We show that, in particular, for a one- or two-dimensional optical lattice, the Mott-insulator phase is more difficult to realize than anticipated previously.

Saturation effects in femtosecond laser ablation of silicon-on-insulator

We report a surface morphology study on single-shot submicron features fabricated on silicon on insulator by tightly focused femtosecond laser pulses. In the regime just below single-shot ablation threshold nano-tips are formed, whereas in the regime just above single-shot ablation threshold, a saturation in the ablation depth is found. We attribute this saturation by secondary laser absorption in the laser-induced plasma. In this regime, we find excellent agreement between the measured depths and a simple numerical model. When the laser fluence is further increased, a sharp increase in ablation depth is observed accompanied by a roughening of the ablated hole.