Mario Hentschel

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.

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.

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.

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.

Cavity-enhanced localized plasmon resonance sensing

We present a method to enhance the sensing properties of a localized plasmon sensor. The concept is based on the combination of localized plasmons in nanostructures and a photonic microcavity. Metal nanorods that are placed at Bragg distance above a metal mirror form a Fabry–Perot microcavity and constitute a coupled photonic-plasmonic system. The localized plasmon resonances of the nanorods and the phase shifts upon plasmon excitation are extremely sensitive to changes in the refractive index of the material surrounding the nanorods. Compared to the plasmonic nanorods alone, the coupled photonic-plasmonic system allows for a much more sensitive detection of small refractive index changes.

Classical analog of electromagnetically induced absorption in plasmonics

We present the classical analog of electromagnetically induced absorption which is achieved by tuning the coupling phase between a bright and a dark plasmonic resonance in the intermediate regime and thus obtaining constructive interference.

Quantitative modeling of the third harmonic emission spectrum of plasmonic nanoantennas.

Plasmonic dimer nanoantennas are characterized by a strong enhancement of the optical field, leading to large nonlinear effects. The third harmonic emission spectrum thus depends strongly on the antenna shape and size as well as on its gap size. Despite the complex shape of the nanostructure, we find that for a large range of different geometries the nonlinear spectral properties are fully determined by the linear response of the antenna. We find excellent agreement between the measured spectra and predictions from a simple nonlinear oscillator model. We extract the oscillator parameters from the linear spectrum and use the amplitude of the nonlinear perturbation only as scaling parameter of the third harmonic spectra. Deviations from the model only occur for gap sizes below 20 nm, indicating that only for these small distances the antenna hot spot contributes noticeable to the third harmonic generation. Because of its simplicity and intuitiveness, our model allows for the rational design of efficient plasmonic nonlinear light sources and is thus crucial for the design of future plasmonic devices that give substantial enhancement of nonlinear processes such as higher harmonics generation as well as difference frequency mixing for plasmonically enhanced terahertz generation.

Nanoantenna-enhanced ultrafast nonlinear spectroscopy of a single gold nanoparticle

Optical nanoantennas, just like their radio-frequency equivalents, enhance the light-matter interaction in their feed gap. Antenna enhancement of small signals promises to open a new regime in linear and nonlinear spectroscopy on the nanoscale. Without antennas especially the nonlinear spectroscopy of single nanoobjects is very demanding. Here we present the first antenna-enhanced ultrafast nonlinear optical spectroscopy. In particular, we use the antenna to determine the nonlinear transient absorption signal of a single gold nanoparticle caused by mechanical breathing oscillations. We increase the signal amplitu-de by an order of magnitude, which is in good agreement with our analytical and numerical models. Our method will find applications in linear and nonlinear spectroscopy of single nanoobjects, especially in simplifying such challenging experiments as transient absorption or multiphoton excitation.