Nanoplasmonics

Plasmonic metasurfaces, optical antennas and magnetic sub-wavelength near-field.Nanoplasmonics

My field of interest is the physics of very small structures (~50-100 nm a ten-thousandth of a hair diameter) in which it is possible to confine and guide light. As light in those structures can be described as so called propagating plasmons, more precise, surface-plasmon-polaritons (w), the field of research is called “plasmonics”, referring to the size scale “nanoplasmonics”. My field of research can be subsumed under the larger field of “nanosciences”.

More precisely, I am in my projects working on fundamental research, investigating the basic building blocks of nanoplasmonic “optical” circuitry with a special focus on subwavelength plasmonic (SPP) waveguides, directional couplers, discrete diffraction and negative diffraction in planar plasmonic systems. Up to now I have, together with my colleagues, realized efficient antennas for transferring light from a free beam into those structures. We were also able to demonstrate and fabrucate nanostrucutures which offer coupling between nanoplasmonic waveguides on the shortest lengths, achieved up to now.

Subwavelength SPP waveguides for optical nanocircuitry

Subwavelength, plasmonic waveguides open the way to the manipulation of light in photonic circuits at the nanoscale. Surface Plasmon Polaritons (SPP) allow for focusing light to scales below the Abbe diffraction limit. Highly integrated waveguiding becomes possible, whereas the diffraction limit in classical waveguides inhibits integration in the size scales of current CPU generations (down to ~50 nm). Compared with competing techniques as dielectric waveguides, plasmonic slab waveguides or dielectric loaded SPP waveguides, gap SPP waveguides allow for the strongest field confinement, achieving real subwavelength operation. At the same time, in gap SPP waveguides, compared to alternative SPP waveguide geometries, losses are reduced though still higher than in waveguides with lower confinement. Existing and new waveguide geometries are first simulated with Finite Elements Method (FEM) calculations. In the second step we fabricate such waveguides with Focused Ion Beam (FIB) milling into thin metal films and finally investigate them with near field (Near Field Scanning Optical Microscope, NSOM) and far field technologies.

Directional couplers and discrete diffraction

Directional couplers (w), connecting different transmission lines, are fundamental building blocks in macroscopic fiber optics and in the microwave domain. Transferring this concept into the nanoworld is a promising challenge as it allows for transferring light from one subwavelength waveguide to the other. Coupling in the size of a wavelength has recently been realized in dielectric waveguides. We successfully fabricate subwavelength SPP waveguides with pitch around 120 nm. Combined with loaded optical antennas and near field as well as far field measurement techniques, those allow for the detailed investigation of small scale coupling and serve as a component for discrete diffraction in larger waveguide arrays.

SPP circuits with loss tunability

SPP gap waveguides have one major disadvantage: Strong propagation loss. Typical propagation lengths only reach less than 10 nm with confinement of below 200 nm. However, we realized, that a complete SPP circuit does not need to consist only of high-confinement waveguides. High confinement rather is necessary for certain functional plasmonic units. Therefore we developed a new design and fabrication approach, based on SPP gap waveguides, embedded in a matrix of dielectric. The design allows us to adiabatically cross over from low-confinement, low-loss waveguides to high-confinement, increased-loss waveguides.

Nonlinear effects in plasmonic nanocircuitry

Subwavelength plasmonic circuits lead to to extraordinary field enhancement. The optical response in presence of high electromagnetic fields is not described by linear approximations any more. Metals are usually assumed to match the response of a Drude free electron gas (w).  A perfect free electron gas would not show any nonlinearity in its response. Real metals, different to this, show effects as loss and interband-transitions. Experimentally, two-photon absorption has been observed. Quantitatively measuring the nonlinear response of metals nevertheless still remains difficult due to the fact that all electromagnetic fields decay exponentially in metals, whereas there are propagating solutions of Maxwell's equations for dielectrics. Thus, intrinsically, the field overlap with metals in almost all available geometries is limited and therefore the nonlinearity of the electromagnetic response is limited to values which are hard to measure experimentally. There have, independent of this, been strongly diverging statements on the magnitude of the nonlinearity of metals, based on theoretical assumptions, reaching to an expected extremely high nonlinearity, which exceeds that of most dielectric materials. In some of our smalles plasmonic nanostructures we recently observed strong nonlinear optical effects, even nonlinear switching, based on the Kerr nonlinearity of the system. Those nonlinearities are strongly influenced by the structure geometry, but we claim that, to a certain degree, they also allow conclusions on the intrinsic bulk nonlinearity of metals.

Nearfield scanning optical microscopy to extend the optical resolution limit

A nearfield scanning optical microscope (NSOM) (w) operates as an optically extended atomic force microscope (AFM). It is an invasive method of measurement. A sharp, oscillating tip in the nanometers-close vicinity to the surface draws the nearfield from a structure under investigation, converts part of it into a propagating wave and guides it or scatters it to a detector. Imaging becomes feasible with subwavelength resolution and electromagnetic waves become visible that usually would decay evanescently into free space on a length scale of tens of nanometers. Plasmonic structures, carrying guided waves are some of those structures. During my thesis I have also been building a setup involving a fiber based NSOM system and enabling it to probe such devices. With the machine we are now able to probe nearfields with a lateral resolution down to about 100 nm. Applying an NSOM with high resolution on the other hand requires a basic understanding of the electromagnetic field effects on the tip itself, which are governed by plasmonic enhancement and lightning-rod-effects. Therefore we also investigate tip manipulation with a focused ion beam system (FIB), polarization effects in NSOM tips, polarization analysis and preparation.

Collaboration with Daniel Ploss, Jing Wen, IOIP Uni Erlangen / MPL and Stanley Burgos, Harry A. Atwater, Thomas J. Watson Laboratory of Applied Physics, California Institute of Technology.

References

Kriesch, A., Burgos, S. P., Ploss, D., Pfeifer, H., Atwater, H. A., Peschel, U., 
"Functional Plasmonic Nanocircuits with Low Insertion and Propagation Losses"
Nano Letters, 13 (9), 4539-4545 (2013); PDF, or from the journal doi:10.1021/nl402580c.

Wen, J., Banzer, P., Kriesch, A., Ploss, D., Schmauss, B., & Peschel, U. (2011). "Experimental cross-polarization detection of coupling far-field light to highly confined plasmonic gap modes via nanoantennas." Applied Physics Letters, 98(10), 101109. AIP. doi:10.1063/1.3564904.

Wen, J., Romanov, S., & Peschel, U. (2009). Excitation of plasmonic gap waveguides by nanoantennas. Optics Express, 17(8), 5925. OSA. doi:10.1364/OE.17.005925.

Romanov, S., Vogel, Bley, Landfester, Weiss, Orlov, Korovin, Chuiko, Regensburger, A., Romanova, A., Kriesch, A. & Peschel, U.,
"Probing guided modes in a monolayer colloidal crystal on a flat metal film"
Physical Review B, 86, 195145 (2012) [10 pages]; doi:10.1103/PhysRevB.86.195145.