Daniel Dregely

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.

Palladium-based plasmonic perfect absorber in the visible wavelength range and its application to hydrogen sensing.

We report on the experimental realization of a palladium-based plasmonic perfect absorber at visible wavelengths and its application to hydrogen sensing. Our design exhibits a reflectance <0.5% and zero transmittance at 650 nm and the operation wavelength of the absorber can be tuned by varying its structural parameters. Exposure to hydrogen gas causes a rapid and reversible increase in reflectance on a time scale of seconds. This pronounced response introduces a novel optical hydrogen detection scheme with very high values of the relative intensity response.

Nonreciprocal plasmonics enables giant enhancement of thin-film Faraday rotation

Light propagation is usually reciprocal. However, a static magnetic field along the propagation direction can break the time-reversal symmetry in the presence of magneto-optical materials. The Faraday effect in magneto-optical materials rotates the polarization plane of light, and when light travels backward the polarization is further rotated. This is applied in optical isolators, which are of crucial importance in optical systems. Faraday isolators are typically bulky due to the weak Faraday effect of available magneto-optical materials. The growing research endeavour in integrated optics demands thin-film Faraday rotators and enhancement of the Faraday effect. Here, we report significant enhancement of Faraday rotation by hybridizing plasmonics with magneto-optics. By fabricating plasmonic nanostructures on laser-deposited magneto-optical thin films, Faraday rotation is enhanced by one order of magnitude in our experiment, while high transparency is maintained. We elucidate the enhanced Faraday effect by the interplay between plasmons and different photonic waveguide modes in our system.

Near-Field Dynamics of Optical Yagi-Uda Nanoantennas

We present near-field measurements of optical Yagi-Uda nanoantennas that are used in receiving mode. The eigenmode imaging of amplitude and phase by apertureless scanning near-field optical microscopy allows us to investigate the dynamics of the local out-of-plane electric field components and to visualize the temporal evolution of this time-harmonic reception process. The antenna directionality manifests itself by the dependence of the local field enhancement at the feed element on the illumination direction. Simulations taking into account the substrate confirm our observation of the directionality. Our work demonstrates the possibility to characterize multielement nanoantennas by electromagnetic antenna near-field scanners.

Vibrational near-field mapping of planar and buried three-dimensional plasmonic nanostructures

Nanoantennas confine electromagnetic fields at visible and infrared wavelengths to volumes of only a few cubic nanometres. Assessing their near-field distribution offers fundamental insight into light–matter coupling and is of special interest for applications such as radiation engineering, attomolar sensing and nonlinear optics. Most experimental approaches to measure near-fields employ either diffraction-limited far-field methods or intricate near-field scanning techniques. Here, using diffraction-unlimited far-field spectroscopy in the infrared, we directly map the intensity of the electric field close to plasmonic nanoantennas. We place a patch of probe molecules with 10 nm accuracy at different locations in the near-field of a resonant antenna and extract the molecular vibrational excitation. We map the field intensity along a dipole antenna and gap-type antennas. Moreover, this method is able to assess the near-field intensity of complex buried plasmonic structures. We demonstrate this by measuring for the first time the near-field intensity of a three-dimensional plasmonic electromagnetically induced transparency structure.

Imaging and steering an optical wireless nanoantenna link

Optical nanoantennas tailor the transmission and reception of optical signals. Owing to their capacity to control the direction and angular distribution of optical radiation over a broad spectral range, nanoantennas are promising components for optical communication in nanocircuits. Here we measure wireless optical power transfer between plasmonic nanoantennas in the far-field and demonstrate changeable signal routing to different nanoscopic receivers via beamsteering. We image the radiation pattern of single-optical nanoantennas using a photoluminescence technique, which allows mapping of the unperturbed intensity distribution around plasmonic structures. We quantify the distance dependence of the power transmission between transmitter and receiver by deterministically positioning nanoscopic fluorescent receivers around the transmitting nanoantenna. By adjusting the wavefront of the optical field incident on the transmitter, we achieve directional control of the transmitted radiation over a broad range of 29°. This enables wireless power transfer from one transmitter to different receivers.

Strong coupling of localized and surface plasmons to microcavity modes

We strongly couple surface plasmon modes on a thin metal layer via localized plasmons of nanowires to photonic microcavity modes. In particular, we place an array of nanowires close to a mirror and position a second mirror at Bragg distance. The coupling becomes evident from an anticrossing of the resonances in the dispersion diagram. We experimentally determine the dispersion by applying external pressure to the microcavity and find excellent agreement with simulations.

Octave-wide photonic band gap in three-dimensional plasmonic Bragg structures and limitations of radiative coupling

We demonstrate that a three-dimensional arrangement of particle plasmonic oscillators at Bragg distance leads to a superradiant plasmon mode. We observe the formation of a very broad photonic band gap that spans almost one octave.

Emission properties of an oscillating point dipole from a gold Yagi-Uda nanoantenna array

We investigate numerically the interaction of an oscillating point dipole with a periodic array of optical Yagi-Uda nanoantennas in the weak coupling limit. A very strong near-field enhancement of the dipole emission by the resonant plasmon mode in the feed element is predicted in this structure. It is shown that the enhancement strength depends strongly on the dipole position, the direction of the dipole moment, and the oscillation frequency. The radiative intensity of the point dipole from appropriate places next to one feed element may exceed the radiative intensity of an equivalent dipole in free-space by a factor of hundred. In spite of only one director used in each nanoantenna of the array, the far-field emission pattern is highly directed. The radiative efficiency (the ratio of the radiative to the full emission) appears to be around 20%.