Resonant Nanoantennas Research Articles

Resonant slot nanoantennas for surface plasmon radiation in optical frequency range

We demonstrate resonant slot nanoantennas on Ag film for surface plasmon (SP) radiation in optical frequency range. Radiation spectra of the nanoantennas and resonant mode patterns for different mode orders with high spatial resolution are obtained using cathodoluminescence spectroscopy. The dispersions are obtained from the experimental data, and the far-field angular distributions of the radiation are simulated for the four lowest modes. The nanoantennas, which can realize resonant conversion between free space light and SPs at nanoscale, should find applications in nanophotonic devices and integrated optical circuits.

Dynamics of Four-Photon Photoluminescence in Gold Nanoantennas

Two-pulse correlation is employed to investigate the temporal dynamics of both two-photon photoluminescence (2PPL) and four-photon photoluminescence (4PPL) in resonant and nonresonant nanoantennas excited at a wavelength of 800 nm. Both 2PPL and 4PPL data are consistent with the same two-step model already established for 2PPL, implying that the first excitation step in 4PPL is a three-photon sp → sp direct interband transition. Considering energy and parity conservation, we also explain why 4PPL behavior is favored over, for example, three- and five-photon photoluminescence in the power range below the damage threshold of our antennas. Since sizable 4PPL requires larger peak intensities of the local field, we are able to select either 2PPL or 4PPL in the same gold nanoantennas by choosing a suitable laser pulse duration. We thus provide a first consistent model for the understanding of multiphoton photoluminescence generation in gold nanoantennas, opening new perspectives for applications ranging from the characterization of plasmonic resonances to biomedical imaging.

Giant and uniform fluorescence enhancement over large areas using plasmonic nanodots in 3D resonant cavity nanoantenna by nanoimprinting

Using a new nanoplasmonic architecture and an optimized spacer, we observed the following: (a) the average fluorescence of an infrared dye (indocyanine green) is enhanced by 2970 fold uniformly (variation < 11%) over a large sample area and over a wide range of dye concentrations (380 to 380 000 molecule µm−2), laser excitation powers and laser beam sizes; and (b) for a single molecule placed at a ‘hot spot’, the fluorescence enhancement is 4.5 × 106 fold. The giant and uniform enhancements (orders of magnitude higher than before), plus easy and inexpensive large area fabrication ( > 4″ wafers), should open up wide applications.

Metal–Polymer–Metal Split‐Dipole Nanoantennas

The conjugated polymer semiconductor poly(3-hexylthiophene), (P3HT), is integrated directly into the slot region of resonant plasmonic split-dipole nanoantennas. The P3HT radiative emission rate is enhanced by a factor of up to 29, in experiment, and 550 for the ideal case, due to the large local density of optical states in the nanoantenna slot region. Additionally, the theoretical modified luminescence quantum efficiency is shown to increase from 1% to 45% for optimized nanoantenna parameters.

Enhancement of Terahertz Pulse Emission by Optical Nanoantenna

Bridging the gap between ultrashort pulsed optical waves and terahertz (THz) waves, the THz photoconductive antenna (PCA) is a major constituent for the emission or detection of THz waves by diverse optical and electrical methods. However, THz PCA still lacks employment of advanced breakthrough technologies for high-power THz emission. Here, we report the enhancement of THz emission power by incorporating optical nanoantennas with a THz photoconductive antenna. The confinement and concentration of an optical pump beam on a photoconductive substrate can be efficiently achieved with optical nanoantennas over a high-index photoconductive substrate. Both numerical and experimental results clearly demonstrate the enhancement of THz wave emission due to high photocarrier generation at the plasmon resonance of nanoantennas. This work opens up many opportunities for diverse integrated photonic elements on a single PCA at THz and optical frequencies.

Extremely large extinction efficiency and field enhancement in terahertz resonant dipole nanoantennas

The distinctive ability of nanometallic structures to manipulate light at the nanoscale has recently promoted their use for a spectacular set of applications in a wide range of areas of research including artificial optical materials, nano-imaging, biosensing, and nonlinear optics. Here we transfer this concept to the terahertz spectral region, demonstrating a metal nanostructure in shape of a dipole nanoantenna, which can efficiently resonate at terahertz frequencies, showing an effective cross section >100 times larger than its geometrical area, and a field enhancement factor of ~280, confined on a lateral section of ~λ/1,000. These results lead to immediate applications in terahertz artificial materials exhibiting giant dichroism, suggest the use of dipole nanoantennas in nanostructure-based terahertz metamaterials, and pave the way for nanoantenna-enhanced terahertz few-molecule spectroscopy and localized terahertz nonlinear optics.

Relating localized nanoparticle resonances to an associated antenna problem

We conceptually unify the description of resonances existing at metallic nanoparticles and optical nanowire antennas. To this end the nanoantenna is treated as a Fabry-Perot resonator with arbitrary semi-nanoparticles forming the terminations. We show that the frequencies of the quasi-static dipolar resonances of these nanoparticles coincide with the frequency where the phase of the complex reflection coefficient of the fundamental propagating plasmon polariton mode at the wire termination amounts to $\pi$. The lowest order Fabry-Perot resonance of the optical wire antenna occurs therefore even for a negligible wire length. This approach can be used either to easily calculate resonance frequencies for arbitrarily shaped nanoparticles or for tuning the resonance of nanoantennas by varying their termination.

Intrinsic Quantum Beats of Atomic Populations and Their Nanoscale Realization Through Resonant Plasmonic Antenna

In the coherently trapped populations of a four-level atom, we demonstrated the quantum beats with different mechanism, which originate from the interference between transition channels with different dipole moments. The beat frequency is determined by the intrinsic atomic parameters, i.e., the spacing of upper levels and ratio of dipole moments. The resonant plasmonic nanoantenna, as a candidate for the creation of anisotropic vacuum, was proposed to achieve the nanoscale realization of the quantum beats, spontaneous emission cancellation, and Rabi oscillation in two-photon correlations through the enhanced near-field and modified decay rates.

Electromagnetic properties of resonant magnetoplasmonic core-shell nanostructures

We present a numerical model we have created and verified to characterize the frequency dependence of the effective magnetic permeability and permittivity of a core-shell (CS) nanostructure composed of a magnetic core and a plasmonic shell with well-controlled dimensions for different geometries and polarizations. Two principal ingredients in our model are as follows: (i) we consider two-dimensional (or cross sections of infinite three-dimensional parallel, infinitely long, identical, cylinders, where the properties and characteristics are invariant along the perpendicular cross sectional plane) three-phase heterostructure, and (ii) while strictly valid only in a dc situation, our analysis can be extended to treat electric fields that oscillate with time provided that the wavelengths associated with the fields are much larger than the microstructure dimension in order that the homogeneous (effective medium) representation of the composite structure makes sense. Such nanostructures simultaneously possess both magnetic gyromagnetic resonance and plasmonic resonance (PLR) resonances. To illustrate the effects of shape anisotropy of the CS structure, we analyze several possible shell shapes involving sharp edges and tips. Geometric parameters of the CS nanostructures and excitation polarized parallel and perpendicular to the antenna axis permit to finely tune the PLR. Changing the internal geometry of the nanostructure not only shifts its resonance frequencies but can also strongly modify the relative magnitudes of the electric field enhancement, independently of nanoparticle shape. The model sets the foundation of quantitatively determining the spatial confinement of the electric field in regions ≈20 nm in linear dimension. Because of its resonant nature, we found nanolocalized terahertz fields corresponding to large electric field enhancement two orders of magnitude higher in amplitude than the excitation optical field. The simulations in this paper are important because magnetoplasmonic CS nanostructures are currently being explored as candidates for resonant optical nanoantennas for biosensing applications.

Doubly resonant optical nanoantenna arrays for polarization resolved

We report that rhomb-shaped metal nanoantenna arrays support multiple plasmonic resonances, making them favorable bio-sensing substrates. Besides the two localized plasmonic dipole modes associated with the two principle axes of the rhombi, the sample supports an additional grating-induced surface plasmon polariton resonance. The plasmonic properties of all modes are carefully studied by far-field measurements together with numerical and analytical calculations. The sample is then applied to surface-enhanced Raman scattering measurements. It is shown to be highly efficient since two plasmonic resonances of the structure were simultaneously tuned to coincide with the excitation and the emission wave- length in the SERS experiment. The analysis is completed by measuring the impact of the polarization angle on the SERS signal.

Optical nanorod antennas as dispersive one-dimensional Fabry–Pérot resonators for surface plasmons

Resonant optical nanoantennas exhibit a different length scaling due to the surface plasmons compared to their radio frequency counterparts. In this letter, we address this difference by calculating the wavelength-dependent effective mode index neff for a cylindrical one-dimensional gold nanowire waveguide. Our results show that nanorod optical antennas act as dispersive and lossy Fabry–Pérot resonators for surface plasmons.

Understanding near/far-field engineering of optical dimer antennas through geometry modification

Numerical investigations based on the boundary element method (BEM) have been carried out to two-dimensional (2-D) silver dimer nano-antennas of various geometries. The near-field and far-field properties are mainly determined by the local geometry at the gap and the global shape of the antenna shafts respectively. A hybrid dimer antenna, which mixes the geometry ingredients of the rod dimer and the bowtie, benefits in both near and far field. Using a microcavity representation, the resonance in dimer nano-antennas is explained in a common and semi-analytical manner. The plasmonic enhancement and the wavelength mismatching in the optical dimer antenna are naturally embodied in this model. The quality factor of the resonance, which can be influenced by the wavelength and the geometry, is discussed intuitively. The understanding presented in this work could guide the future engineering of the optical dimer antenna.

Enhanced light extraction from emitters close to clusters of resonant plasmonic nanoantennas

We perform time-resolved fluorescence spectroscopy on clusters of plasmonic nanoantennas covered with a dye–polymer mixture. Dimer antenna structures were fabricated consisting of two interacting gold nanorods with varying lengths and interparticle separation. By combining four individual antennas into a cluster within a diffraction limited spot size, we can couple out half of the dye molecule fluorescence via antenna plasmons. Two-dimensional confocal fluorescence lifetime scans visualize the spontaneous emission enhancement of the molecular fluorescence around the antenna clusters.

Strong Enhancement of the Radiative Decay Rate of Emitters by Single Plasmonic Nanoantennas

We demonstrate a strong, 5-fold enhancement of the radiative decay rate from highly efficient fluorescent dye molecules around resonant optical nanoantennas. The plasmonic modes of individual gold dimer antennas are tuned by the particle length and the antenna gap, providing control over both the spectral resonance position and the near-field mode profile of the nanoantenna. Resonant enhancement of the radiative and nonradiative decay rates of a fluorescent dye is observed, resulting in an increase of the internal quantum efficiency from 40% up to 53% for single antennas, and up to 59% for antenna clusters. This improvement of the already high quantum efficiency of the dye molecules is in agreement with electrodynamic model calculations that predict a maximum attainable efficiency around 80% due to nonradiative losses in the metal.

Near-field excitation of nanoantenna resonance

An array of paired elliptic nanoparticles designed to enhance local fields around the particle pair is fabricated with gold embedded in quartz. Light excites a coupled plasmon resonance in the particle pair and the system acts like a plasmonic nanoantenna providing an enhanced electromagnetic field. Near-field scanning optical microscopy and finite element modeling are used to study the local field effects of the nanoantenna system. Local illumination shows similar resonant properties as plane wave illumination: a strong, localized optical resonance for light polarized parallel to the main, center-to-center axis.

λ/4 Resonance of an Optical Monopole Antenna Probed by Single Molecule Fluorescence

We present a resonant optical nanoantenna positioned at the end of a metal-coated glass fiber near-field probe. Antenna resonances, excitation conditions, and field localization are directly probed in the near field by single fluorescent molecules and compared to finite integration technique simulations. It is shown that the antenna is equivalent to its radio frequency analogue, the monopole antenna. For the right antenna length and local excitation conditions, antenna resonances occur that lead to an enhanced localized field near the antenna apex. Direct mapping of this field with single fluorescent molecules reveals a spatial localization of 25 nm, demonstrating the importance of such antennas for nanometer resolution optical microscopy.

Toward nanometer-scale optical photolithography: utilizing the near-field of bowtie optical nanoantennas.

Optically resonant metallic bowtie nanoantennas are utilized as fabrication tools for the first time, resulting in the production of polymer resist nanostructures <30 nm in diameter at record low incident multiphoton energy densities. The nanofabrication is accomplished via nonlinear photopolymerization, which is initiated by the enhanced, confined optical fields surrounding the nanoantenna. The position, size, and shape of the resist nanostructures directly correlate with rigorous finite-difference time-domain computations of the field distribution, providing a nanometer-scale measurement of the actual field confinement offered by single optical nanoantennas. In addition, the size of the photoresist regions yields strong upper bounds on photoacid diffusion and resist resolution in SU-8, demonstrating a technique that can be generalized to the study of many current and yet-to-be-developed photoresist systems.