Harald Giessen

The Fano resonance in plasmonic nanostructures and metamaterials

Since its discovery, the asymmetric Fano resonance has been a characteristic feature of interacting quantum systems. The shape of this resonance is distinctively different from that of conventional symmetric resonance curves. Recently, the Fano resonance has been found in plasmonic nanoparticles, photonic crystals, and electromagnetic metamaterials. The steep dispersion of the Fano resonance profile promises applications in sensors, lasing, switching, and nonlinear and slow-light devices.

Infrared perfect absorber and its application as plasmonic sensor.

We experimentally demonstrate a perfect plasmonic absorber at lambda = 1.6 microm. Its polarization-independent absorbance is 99% at normal incidence and remains very high over a wide angular range of incidence around +/-80 degrees. We introduce a novel concept to utilize this perfect absorber as plasmonic sensor for refractive index sensing. This sensing strategy offers great potential to maintain the performance of localized surface plasmon sensors even in nonlaboratory environments due to its simple and robust measurement scheme.

Plasmonic analogue of electromagnetically induced transparency at the Drude damping limit

In atomic physics, the coherent coupling of a broad and a narrow resonance leads to quantum interference and provides the general recipe for electromagnetically induced transparency (EIT). A sharp resonance of nearly perfect transmission can arise within a broad absorption profile. These features show remarkable potential for slow light, novel sensors and low-loss metamaterials. In nanophotonics, plasmonic structures enable large field strengths within small mode volumes. Therefore, combining EIT with nanoplasmonics would pave the way towards ultracompact sensors with extremely high sensitivity. Here, we experimentally demonstrate a nanoplasmonic analogue of EIT using a stacked optical metamaterial. A dipole antenna with a large radiatively broadened linewidth is coupled to an underlying quadrupole antenna, of which the narrow linewidth is solely limited by the fundamental non-radiative Drude damping. In accordance with EIT theory, we achieve a very narrow transparency window with high modulation depth owing to nearly complete suppression of radiative losses.

Nanoantenna-enhanced gas sensing in a single tailored nanofocus

We demonstrate antenna-enhanced hydrogen sensing at the single-particle level. We place a single palladium nanoparticle near the tip region of a gold nanoantenna and detect the changing optical properties of the system upon hydrogen exposure.

THREE-DIMENSIONAL PLASMON RULERS

The optical response of a fabricated nanostructure can be used to determine distances in three dimensions. Plasmon rulers can be used to determine nanoscale distances within chemical or biological species. They are based on the spectral shift of the scattering spectrum when two plasmonic nanoparticles approach one another. However, the one-dimensionality of current plasmon rulers hampers the comprehensive understanding of many intriguing processes in soft matter, which take place in three dimensions. We demonstrated a three-dimensional plasmon ruler that is based on coupled plasmonic oligomers in combination with high-resolution plasmon spectroscopy. This enables retrieval of the complete spatial configuration of complex macromolecular and biological processes as well as their dynamic evolution.

Transition from isolated to collective modes in plasmonic oligomers.

We demonstrate the transition from isolated to collective optical modes in plasmonic oligomers. Specifically, we investigate the resonant behavior of planar plasmonic hexamers and heptamers with gradually decreasing the interparticle gap separation. A pronounced Fano resonance is observed in the plasmonic heptamer for separations smaller than 60 nm. The spectral characteristics change drastically upon removal of the central nanoparticle. Our work paves the road toward complex hierarchical plasmonic oligomers with tailored optical properties.

3D optical Yagi-Uda nanoantenna array.

We fabricated three-dimensional arrays of optical Yagi-Uda nano-antennas. Due to the high directivity of the array structure the incoming light is received efficiently at the resonant wavelength in the near-infrared (around λ = 1.3 µm).

Plasmonic oligomers: The role of individual particles in collective behavior

We demonstrate the transition from isolated to collective optical modes in plasmonic oligomers. Specifically, we investigate the resonant behavior of planar plasmonic hexamers and heptamers with gradually decreasing the inter-particle gap separation.

XFROG-A New Method for Amplitude and Phase Characterization of Weak Ultrashort Pulses

We present a new method to characterize the amplitude and phase of weak ultrashort pulses. Our method is based on the spetrally resolved crosscorrelation signal of a weak test pulse with a fully characterized intensive reference pulse and requires no spectral overlap between the signal and the reference. To retrieve the amplitude and phase of the test pulse, we use an iterative Fourier transform algorithm with generalized projections.

Three-dimensional chiral plasmonic oligomers.

We demonstrate a chiral optical response in stacked arrangements of resonantly coupled plasmonic nanostructures possessing the capability to encode their 3D arrangement in unique spectra making then ideal candidates for a 3D chiral plasmonic ruler.