Surface plasmon polaritons

We study the propagation, confinement and dispersion of surface plasmon polaritons on metal nanostructures. The taper shown in the figure is used to concentrate light with a wavelength of 1500 nm to a "hot spot" with a diameter smaller than 100 nm. We study the process of adiabatically guiding towards the taper tip and use these tapers to couple light into metallic nanowire waveguides.

Surface plasmon cavities

We study the confinement of light in the smallest possible optical cavities. The ring shown in the figure is made by focused ion beam milling in single-crystalline gold, and has a diameter of only 600 nm. Light is confined in a whispering-gallery mode that propagates at the bottom of the groove. Such plasmonic nanocavities have a very high ratio of Q/V (quality factor / mode volume), which is of interest in the control of spontaneous emission and low-threshold lasing.

Plasmonic solar cells

We study the integration of metal nanostructures with thin film photovoltaic solar cells. Light is scattered from the metal nanoparticles and subsequently coupled into the thin-film semiconductor layer over a wide range of angles, thereby enhancing the effective path length (and thus absorption) in the layer. We study fundamental aspects of this effect and apply it in experiments on thin-film silicon solar cells.

Nanofabrication

in order to perform leading experiments in the field of nanophotonics, it is essential to develop develop novel methods to fabricate optical materials at the nanoscale. The figure shows a plasmonic nanolense that is made by assembling Au nanoparticles (diameters 5, 8 and 15 nm) by using a DNA templating technique. Other nanofabrication tools that we continuously develop and improve are focussed ion beam milling, electron beam lithography and colloidal self-assembly.

Surface plasmon devices

Turning optical phenomena that w study in our research into practical applications is an important part of our research. The image shows an electrically excitable source of surface plasmon polaritons, that operates by the electrical excitation of silicon quantum dots that subsequently decay by the generation of surface plasmons.

Optical metamaterials

Optical metamaterials are materials with a nanoscale structure that is engineered to lead to novel/unusual optical properties. For example, we have studied metal-insulator-metal waveguides that, for a particular geometry shows a negative refractive index for light propagating in the dielectric. Negative-index materials find applications in optical imaging below the diffraction limit, or invisibility cloaking.

Cathodoluminescence plasmon microscopy

Cathodoluminescence imaging spectroscopy is a new technique that we have developed to study optical phenomena at the nanoscale. An electron beam, incident on the surface of a metal nanostructure will generate transition radiation into the far field, and surface plasmons that propagate over the surface. By detecting the emitted light as a function of position of the electron beam, we are able to determine two-dimensional images of the optical density of states. The resolution of the technique is only determined by the electron-beam spot size, typically < 10 nm.

Microcavity lasers

Together with the group of Vahala (CALTECH) and Kippenberg (MPQ, Munich), we study the optical microcavities based on a silica glass ring or disk on a Si post. y using ion implantation techniques we study Er-doped microcavity lasers that operate at 1500 nm, as well as upconversion lasing in the green. Most recently, we have studied optical scattering from Si nanocrystals that are embedded in the cavity, in particular the relation between scattering and the Purcell factor.

• erbium-doped crystalline Si LED (1994), with Coffa & Priolo groups (Catamia)
• erbium-doped silicon-rich oxide LED (1995), with Campisano group (Catania)
• erbium-doped amorphous Si LED (1996), with Schropp group (Utrecht)
• miniature Al2O3 waveguide optical amplifier on Si (1996), with Smit group (Delft)
• miniature silica-based optical waveguide amplifier on Si (1997), with Faber group (TNO, Eindhoven)
• Si photonic crystal fabrication technology (1999), with Van der Drift group (Delft)
• Nd-doped polymer infrared LED (2001), with van Veggel group (twente) and Friend group (Cambridge)
• erbium-doped Si waveguide photodetector (2002), with Coffa & Libertino groups (Catania)
• ultra-low threshold Er-doped toroidal microcavity laser on Si (2004), with Vahala group (CALTECH)
• ultra-low threshold Er-doped microresonator laser on Si (2006), with Vahala group (CALTECH)
• nanoscale optical antenna (2007)
• On-chip green silica upconversion laser (2009), with Vahala group (CALTECH)

In selecting the topics on which we perform fundamental research, industrial roadmaps often play a role. We have interaction with several industrial partners worldwide to transfer knowledge acquired from our research program and we regularly file joint patents with industry.

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