Plasmonic Bowtie Nanoantennas Research Articles

Near-field plasmonic coupling for enhanced nonlinear absorption by femtosecond pulses in bowtie nanoantenna arrays

Plasmonic bowtie nanoantennas (BNAs) can exhibit a strong enhancement of optical field, leading to large nonlinear effects. We investigated the nonlinear optical absorption of an array of BNA by femtosecond pulses, using the open-aperture Z-scan technique. The BNA array composed of paired gold nanotriangles was fabricated by nanosphere lithography. We experimentally demonstrated that upon decreasing the gap width, nonlinear absorption is enhanced due to both the enhancement of near-field coupling of nanoantennas and the minimum of the spectral detuning between the center wavelength of the laser for excitation and the localized surface plasmon resonances. The role of near-field resonant plasmonic coupling in BNA is analyzed theoretically and confirmed by our simulations.

Optical properties and interparticle coupling of plasmonic bowtie nanoantennas on a semiconducting substrate

We present the simulation, fabrication, and optical characterization of plasmonic gold bowtie nanoantennas on a semiconducting GaAs substrate as geometrical parameters such as size, feed gap, height, and polarization of the incident light are varied. The surface-plasmon resonance was probed using white light reflectivity on an array of nominally identical, 35-nm-thick gold antennas. To elucidate the influence of the semiconducting, high-refractive-index substrate, all experiments were compared using nominally identical structures on glass. Besides a linear shift of the surface-plasmon resonance from 1.08 to 1.58 eV when decreasing the triangle size from 170 to 100 nm on GaAs, we observed a global redshift by 0.25 $\ifmmode\pm\else\textpm\fi{}$ 0.05 eV with respect to nominally identical structures on glass. By performing polarization-resolved measurements and comparing results with finite-difference time-domain simulations, we determined the near-field coupling between the two triangles composing the bowtie antenna to be $\ensuremath{\sim}8$ times stronger when the antenna is on a glass substrate compared to when it is on a GaAs substrate. The results obtained have strong relevance for the integration of lithographically defined plasmonic nanoantennas on semiconducting substrates and therefore for the development of novel optically active plasmonic-semiconducting nanostructures.

Plasmonic Nanopore for Electrical Profiling of Optical Intensity Landscapes

We present a novel method for sensitive mapping of optical intensity distributions at subdiffraction-limited resolution. This is achieved with a novel device, a plasmonic nanopore, which combines a plasmonic bowtie nanoantenna with a 10 nm-in-diameter solid-state nanopore. Variations in the local optical intensity modulate the plasmonic heating, which we measure electrically through changes in the ionic conductance of the nanopore. We demonstrate the method by profiling the focal volume of a 10 mW laser beam that is tightly focused by a high-numerical-aperture microscope objective. The results show a complex three-dimensional intensity distribution that closely matches predictions obtained by theoretical calculations of the optical system. In addition to laser profiling, the ionic conductance of a nanopore is also shown to provide quantitative estimates of the temperature in the proximity of single plasmonic nanostructures.

Fluorescence correlation spectroscopy at high concentrations using gold bowtie nanoantennas

Fluorescence correlation spectroscopy (FCS) measures the fluorescence fluctuations of fluorophores in solution, but is restricted to extremely low concentrations. Plasmonic gold bowtie nanoantennas enhance a single molecule’s fluorescence relative to a large background of unenhanced molecules, and here we show that bowties can extend FCS measurements to much higher concentrations. In this demonstration, the bowtie-FCS signal is dominated by molecules that transiently stick to the substrate near the bowtie gap, and photobleaching/photoblinking dynamics for two fluorophores are measured on the 10–100ms time scale.

Application of Plasmonic Bowtie Nanoantenna Arrays for Optical Trapping, Stacking, and Sorting

We present the use of Au bowtie nanoantenna arrays (BNAs) for highly efficient, multipurpose particle manipulation with unprecedented low input power and low-numerical aperture (NA) focusing. Optical trapping efficiencies measured are up to 20× the efficiencies of conventional high-NA optical traps and are among the highest reported to date. Empirically obtained plasmonic optical trapping "phase diagrams" are introduced to detail the trapping response of the BNAs as a function of input power, wavelength, polarization, particle diameter, and BNA array spacing (number density). Using these diagrams, parameters are chosen, employing strictly the degrees-of-freedom of the input light, to engineer specific trapping tasks including (1) dexterous, single-particle trapping and manipulation, (2) trapping and manipulation of two- and three-dimensional particle clusters, and (3) particle sorting. The use of low input power densities (power and NA) suggests that this bowtie nanoantenna trapping system will be particularly attractive for lab-on-a-chip technology or biological applications aimed at reducing specimen photodamage.

Localized radiative energy transfer from a plasmonic bow-tie nano-antenna to a magnetic thin film stack

Localized radiative energy transfer from a near-field emitter to a magnetic thin film structure is investigated. A magnetic thin film stack is placed in the near-field of the plasmonic nano-antenna to utilize the evanescent mode coupling between the nano-antenna and magnetic thin film stack. A bow-tie nano-optical antenna is excited with a tightly focused beam of light to improve near-field radiative energy transfer from the antenna to the magnetic thin film structure. A tightly focused incident optical beam with a wide angular spectrum is formulated using Richards–Wolf vector field equations. Radiative energy transfer is investigated using a frequency domain 3D finite element method solution of Maxwell’s equations. Localized radiative energy transfer between the near-field emitter and the magnetic thin film structure is quantified for a given optical laser power at various distances between the near-field emitter and magnetic thin film.

A New Type of Optical Antenna: Plasmonics Nanoshell Bowtie Antenna with Dielectric Hole

We first investigate plasmonic nanoshell bowtie nanoantenna with dielectric hole that interact with incident plane wave of transverse-magnetic polarization by use of the finite element method. The influence of the dielectric hole in nanoshell on the antenna field enhancement and spectral response is discussed. Results show that the proposed plasmonic nanoshell bowtie antenna can confine the field in a volume well below the diffraction limit and exhibit tunable surface plasmon resonances in a wider range of wavelength that are not observed for the conventional solid-gold case with the same volume. This offers a new possibility to further optimize the performance of optical nanoantenna.

Engineering the optical response of plasmonic nanoantennas

The optical properties of plasmonic dipole and bowtie nanoantennas are investigated in detail using the Green's tensor technique. The influence of the geometrical parameters (antenna length, gap dimension and bow angle) on the antenna field enhancement and spectral response is discussed. Dipole and bowtie antennas confine the field in a volume well below the diffraction limit, defined by the gap dimensions. The dipole antenna produces a stronger field enhancement than the bowtie antenna for all investigated antenna geometries. This enhancement can reach three orders of magnitude for the smallest examined gap. Whereas the dipole antenna is monomode in the considered spectral range, the bowtie antenna exhibits multiple resonances. Furthermore, the sensitivity of the antennas to index changes of the environment and of the substrate is investigated in detail for biosensing applications; the bowtie antennas show slightly higher sensitivity than the dipole antenna.