Plasmonic Nanoantennas Research Articles

The Role of Electron Transfer in the Nonlinear Response of Ge2Sb2Te5-Mediated Plasmonic Dimers

Here, we study the possibility of exquisitely selective harmonic generation based on the concept of charge transfer plasmons (CTPs) in bridged nanoparticle assemblies. By choosing plasmonic dimer nanoantenna, as a fundamental member of the nanocluster family, and bridging the capacitive gap space between the proximal nanoparticles with an optothermally controllable substance, we judiciously showed that variations in the generation of third harmonic light in the visible regime can be possible by considering distinct states of the functional bridge. To this end, the conductive connection between the nanoparticles is mediated with Ge2Sb2Te5 (GST) with inherently opposite optical and electrical properties below (dielectric, amorphous state) and above 477 °C (conductive, crystalline state). This helped to actively control the transition of charges across the bridge and thereby control the excitation of CTP resonances and provide a switching feature between dipolar and CTP modes. This versatile approach also allowed for production of the intended harmonic signal at different wavelengths depending on the conductivity of the interparticle nanojunction.

In Situ Plasmonic Nanospectroscopy of the CO Oxidation Reaction over Single Pt Nanoparticles.

The ongoing quest to develop single-particle methods for the in situ study of heterogeneous catalysts is driven by the fact that heterogeneity in terms of size, shape, grain structure, and composition is a general feature among nanoparticles in an ensemble. This heterogeneity hampers the generation of a deeper understanding for how these parameters affect catalytic properties. Here we present a solution that in a single benchtop experimental setup combines single-particle plasmonic nanospectroscopy with mass spectrometry for gas phase catalysis under reaction conditions at high temperature. We measure changes in the surface state of polycrystalline platinum model catalyst particles in the 70 nm size range and the corresponding bistable kinetics during the carbon monoxide oxidation reaction via the peak shift of the dark-field scattering spectrum of a closely adjacent plasmonic nanoantenna sensor and compare these changes with the total reaction rate measured by the mass spectrometer from an ensemble of nominally identical particles. We find that the reaction kinetics of simultaneously measured individual Pt model catalysts are dictated by the grain structure and that the superposition of the individual nanoparticle response can account for the significant broadening observed in the corresponding nanoparticle ensemble data. In a wider perspective our work enables in situ plasmonic nanospectroscopy in controlled gas environments at high temperature to investigate the role of the surface state on transition metal catalysts during reaction and of processes such as alloying or surface segregation in situ at the single-nanoparticle level for model catalysts in the few tens to hundreds of nanometer size range.

Control of the emission from electric and magnetic dipoles by gold nanocup antennas.

The control of the emission from electric and magnetic dipoles is highly desired for the development of optic chips. Although the emission of electric dipole has been successfully controlled by plasmonic nanoantenna, the control of magnetic dipole emission is relatively difficult. Here, we systematically study the effect of electric and magnetic modes of Au nanocups on the emission of electric and magnetic dipoles. The emission of electric dipole can be enhanced by both the electric and magnetic mode of the Au nanocup, while the emission of the magnetic dipole is only increased by the magnetic mode. The enhancement exhibits wavelength dependence. The wavelength of the largest enhancement is determined by the resonance wavelength of electric and magnetic modes. The enhancement values for electric and magnetic dipoles are determined by the near-field electric and magnetic field enhancements, respectively. More importantly, the emission pattern of magnetic dipole is greatly modified by the magnetic mode of Au nanocup. The directional emission of magnetic dipole is first time realized by use of the magnetic mode of the Au nanocup. Our findings deepen the understanding of the plasmon-controlled emission of electric and magnetic dipoles and will be very helpful to the development of the nanophotonic chips.

All-Silicon On-Chip Optical Nanoantennas as Efficient Interfaces for Plasmonic Devices

Plasmonic technology promises to unfold new advanced on-chip functionalities with direct applications in photovoltaics, light–matter interaction, and the miniaturization of optical interconnects at the nanoscale. In this scenario, it is crucial to efficiently drive light to/from plasmonic devices. However, typically used plasmonic wires introduce prohibitive losses, hampering their use for many applications. Recently, plasmonic nanoantennas have been proposed to overcome this drawback, not only providing a notable loss reduction, but also an enhanced on-chip flexibility and reconfigurability. Nevertheless, these devices still perform poorly for long-reach interconnects, owing to their low-directive radiation and low efficiency. Here, we introduce a class of slot-waveguide-based silicon nanoantennas that lift all these limitations and show their feasibility to be connected directly and efficiently to plasmonic devices. To test the performance of these antennae, an on-chip plasmonic-dielectric interconnect ...

SERS Detection via Individual Bowtie Nanoantennas Integrated in Si3N4 Waveguides

Plasmonic resonances in metallic nanostructures provide a way for broadband enhanced light–matter interaction in subwavelength regions, which can be used to boost a variety of physical phenomena, notably including Raman scattering. Such nanostructures can be integrated on silicon chips and driven via dielectric waveguides, which may improve the performance of photonic integrated circuits in terms of foot-print, efficiency, sensitivity or power consumption, amongst other figures of merit. Here, we show that an isolated plasmonic nanoantenna can be efficiently integrated into a silicon nitride waveguide to detect surface-enhanced Raman scattering (SERS) spectra from molecular monolayers. We study numerically and experimentally two different configurations, both enabling the recording of Raman spectra at the output: nanoantenna on top of the waveguide and nanoantenna inserted in a subwavelength gap built in the waveguide. We also compare both configurations, which may pave the way toward massive integration of SERS devices for lab-on-a-chip applications.

Waveguide interrogation of a compound plasmonic nanoantenna

While long-range propagating plasmons have been extensively investigated for implementing on-chip optical sensing platforms, waveguide-excited localized surface plasmon resonance-based sensing systems have not yet received much attention. Waveguide excitation and readout, an alternative to free-space single-particle spectroscopy, are particularly attractive. A numerical investigation of the optical response of a waveguide-excited plasmonic dolmen-shaped compound nanoantenna is presented. The dolmen shape is found to offer improved refractive index sensing performance in comparison to simpler shapes for both spectral-shift and intensity-shift measuring modalities. The compound plasmonic nanoantenna configuration presented is of interest in hybrid photonic-plasmonic sensing approaches with high-throughput capabilities.

Ultra-Wideband Photonic Hybrid Plasmonic Horn Nanoantenna with SOI Configuration

The current study proposes a hybrid plasmonic horn-like nanoantenna with a silicon-on-insulator (SOI) configuration. This nanoantenna can be utilized for broadband nanophotonic applications and can operate in a wide optical frequency range of 160–400 THz. It includes optical communication wavelengths of 850, 1310 and 1550 nm. This nanoantenna can directionally radiate and receive signals with about 74% bandwidth within the operational wavelength spectrum. The proposed nanoantenna is simulated using the finite element method and produced nanoantenna gains of 4.7, 7.3 and 4.8 dB and a reflection coefficient (S11) of −15.7, −12.8 and −15.6 dB at optical wavelengths of 1550, 1310 and 850 nm, respectively.

Indium⁻Tin⁻Oxide Nanostructures for Plasmon-Enhanced Infrared Spectroscopy: A Numerical Study.

Plasmonic nanoantennas can significantly enhance the light–matter interactions at the nanoscale, and as a result have been used in a variety of applications such as sensing molecular vibrations in the infrared range. Indium–tin–oxide (ITO) shows metallic behavior in the infrared range, and can be used for alternative plasmonic materials. In this work, we numerically studied the optical properties of hexagonal ITO nanodisk and nanohole arrays in the mid-infrared. Field enhancement up to 10 times is observed in the simulated ITO nanostructures. Furthermore, we demonstrated the sensing of the surface phonon polariton from a 2-nm thick SiO2 layer under the ITO disk arrays. Such periodic arrays can be readily fabricated by colloidal lithography and dry etching techniques; thus, the results shown here can help design efficient ITO nanostructures for plasmonic infrared applications.

Magnetization Dynamics Induced by Nanoconfined Magnetic-Field Pulse Generated by Resonant Plasmonic Nanoantennas

The interaction between light and matter is key in both basic science and technology. The strength of the light-magnet interaction, however, is very weak, which has been the main challenge in optically probing or controlling magnetism. This study shows that the magnetic field pulses generated by plasmonic nanoantennas can effectively manipulate magnetism, on a very short time scale. Here the excited magnetization undergoes coherent oscillation at ~100 THz, followed by the emission of propagating spin waves with frequencies of a few GHz. These results reveal the strong potential of plasmonic nanoantennas in spintronics.

Increased electric field enhancement and broad wavelength tunability by plasmonic bow-tie nano-antenna based on fractal geometry with grid

We propose and analyze a new type of plasmonic bow-tie nano-antenna based on fractal geometry with grid. The optical properties of the first-, second- and third-iteration of the proposed fractal nano-antennas including absorption spectra, gap enhancement, electric field distribution and charge density are numerically investigated by means of finite-difference time-domain (FDTD) method. We demonstrate the bonding and anti-bonding modes arising from the plasmon hybridization modes of the proposed fractal nano-antennas. Our simulation results show that these nano-antennas can be tuned by changing the refractive index of the surrounding medium or antenna thickness. We also observe a strong electric field hotspot inside the gap region as the fractal iteration increases. Accordingly, the gap enhancement of the third-iteration is achieved up to 144.5 v/m within the nano-antenna gap. We find that the proposed second- and third-iteration bow-tie fractal nano-antennas are good candidates for strong electric field localization and tunable broadband applications.

Time-effective strategies for the fabrication of poly- and single-crystalline gold nano-structures by focused helium ion beam milling

Milling with the focused helium ion beam of a helium ion microscope is one of the most accurate ways to produce nano-structures such as plasmonic nanoantennas. In addition to good and immediate control of the dimensions, features in the sub-10 nm regime are achievable. Especially small gaps and sharp tips in this regime may lead to very high field enhancement under excitation. However, the milling rate of 30 keV helium ions is rather low, making it time-consuming to cut nano-structures out of a gold film. We present two processes to work around the low milling rate to obtain arrays of nano-structures with maximum precision within a reasonable time. These strategies can both be adapted to either poly-crystalline gold films or single-crystalline gold flakes. Using single crystals from a fabrication point of view enables even higher precision due to constant etch rates over the whole crystal as well as straight edges and vertical side-walls due to the uniform crystalline structure.

Broadband coherent hyperspectral near-field imaging of plasmonic nanostructures.

We develop a coherent hyperspectral near-field microscope using a combined nano-Fourier Transform Infra-Red (FTIR) spectroscope and a scattering Scanning Near-field Optical Microscope (s-SNOM) illuminated by an ultra-broadband few-cycle femtosecond source, spanning a spectrum from 660 to 1050 nm. Using this spatio-spectral approach, we resolve hyperspectral near-field response of a single plasmonic nano-antennas over 450 nm bandwidth with a spatial resolution of 40 nm and a spectral resolution of 50 cm-1. In particular, we identify the electric near-field spatial distribution of the dipole resonant mode of various nano-antennas and observe, in accordance with previous theoretical reports, that those are spectrally red-shifted from their far-field response. Moreover, we are able to spectrally and spatially differentiate the near-field distribution of the dipole and quadrupole modes at the single nanoparticle level. Being coherent and short-pulsed, our technique opens the path for optical ultrafast characterization and control of light-matter interaction at the nanoscale.

Watching a Single Fluorophore Molecule Walk into a Plasmonic Hotspot

Plasmonic nanoantennas allow for enhancing the spontaneous emission, altering the emission polarization, and shaping the radiation pattern of quantum emitters. A critical challenge for the experimental realizations is positioning a single emitter into the hotspot of a plasmonic antenna with nanoscale accuracy. We demonstrate a dynamic light-matter interaction nanosystem enabled by the DNA origami technique. A single fluorophore molecule can autonomously and unidirectionally walk into the hotspot of a plasmonic nanoantenna along a designated origami track. Successive fluorescence intensity increase and lifetime reduction are in situ monitored using single-molecule fluorescence spectroscopy, while the fluorophore walker gradually approaches and eventually enters the plasmonic hotspot. Our scheme offers a dynamic platform, which can be used to develop functional materials, investigate intriguing light-matter interaction phenomena, and serve as prototype system for examining theoretical models.

Silver Columnar Thin-Film-Based Half-Wavelength Antennas for Bright Directional Emission from Nanodiamond Nitrogen-Vacancy Centers

A plasmonic nanoantenna offers directionality and a large photonic density of states to a nearby emitter. Here researchers create antennas based on silver columnar thin film for bright, directional emission from N-$V$ centers in nanodiamonds (NDs). By simply drop-casting the NDs from aqueous solution on top of the film, order-of-magnitude enhancement of emittance is attained. The silver nanocolumns couple to an N-$V$ center's emission via the optical near field, and out-couple the excitation energy effectively into the far field. As the approach is not restricted to these emitters, it offers a broad way to practical implementation of solid-state single-photon sources.

Phase-matched nonlinear second-harmonic generation in plasmonic metasurfaces

Abstract The phase matching between the propagating fundamental and nonlinearly generated waves plays an important role in the efficiency of the nonlinear frequency conversion in macroscopic crystals. However, in nanoscale samples, such as nanoplasmonic structures, the phase-matching condition is often ignored due to the sub-wavelength nature of the materials. Here, we first show that the phase matching of the lattice plasmon modes at the fundamental and second-harmonic frequencies in a plasmonic nanoantenna array can effectively enhance the surface-enhanced second-harmonic generation. Additionally, a significant enhancement of the second-harmonic generation is demonstrated using stationary band-edge lattice plasmon modes with zero phase.

An effective triple-band enhanced-infrared-absorption detection by honeycomb-shaped metamaterial-plasmonic absorber

Nowadays, accessing to new electromagnetic properties can be realized by fabricating metamaterials consist of artificial functional and subwavelength-scaled structures. Promoting artificial and functional metamaterial designs with large electromagnetic near-field enhancement especially over throughout multiband spectral range can provide critical contributions for infrared spectroscopy applications. In this context, the design and experimental realization of a photonic metamaterial are presented which can be utilized as an infrared biosensor. We utilize a honeycomb-shaped plasmonic nanoantenna for the design of the proposed novel absorber metamaterial. The fabrication and optical characterization of the structures are carried out and the sensor potential of the metamaterial is demonstrated through the detection of a polymer nanofilm.

Optofluidic control using plasmonic TiN bowtie nanoantenna

The performance of plasmonic titanium nitride (TiN) nanoantennas for the manipulation of fluidic flow and suspended particles in an optofluidic chip is studied. A unified theoretical framework is utilized to model the multidisciplinary problem that comprises optics, thermodynamics, and hydrodynamics. Using multiphysics finite element analysis, we simulate the temperature rise resulting from the photothermal heating of a plasmonic TiN bowtie nanoantenna (BNA) and the accompanying hydrodynamic flow generated in a microfluidic channel. We show that the TiN BNA enables over three times higher electrothermoplasmonic flow velocity in comparison to a gold BNA under similar signal conditions. Our analysis shows that TiN BNAs at near-IR biological transparency wavelengths can be utilized to initiate strong microfluidic flow for directed transport and trapping of target nanoscale objects. This makes TiN an excellent plasmonic material choice for optically controlling heat, fluidic dynamics and heat-induced forces in microfluidic devices.

Metasurface based on cross-shaped plasmonic nanoantennas as chemical sensor for surface-enhanced infrared absorption spectroscopy

Infrared spectroscopy is an effective technique extensively used in research and industry for the label-free and unambiguous identification of molecular species. However, the sensitivity of this technique is severely limited as a result of Beer’s law and, the small infrared absorption cross-section that make prohibitively weak the absorption signals, of minute amounts of analyte as those present in monolayers. This limitation can be overcome by enhancing the infrared vibration of molecules through the enhancement of the electromagnetic (EM) field. Surface Enhanced InfraRed Absorption (SEIRA) using resonant metal Nano-scale Antennas (NAs) can provide huge electromagnetic fields on the nanometer scale featuring localized collective oscillations of electrons, an effect named Localized Surface Plasmonic Resonances (LSPRsWe here report on a series of 2D arrays of cross-shaped NAs having several mm2 area coverage (metasurface) as SEIRA optimized antennas, which can be used in practical applications such as the vibrational sensing of chemical and biological analytes. Cross-shape designed NAs are insensitive to the polarization of the electromagnetic radiation impinging the active area. Due to the random orientation of the dipole moments of molecules they are particularly suitable for the construction of bio-molecular sensors. At the same time, the 2D-array configuration ensures a good near-field signal enhancement arising from the coupling between neighbour NAs Moreover, SEIRA NAs can be easily integrated with micrometre-sized channels and be suitable for the high sensitivity, real time analysis of IR emitting samples, in matrices where IR spectroscopy is severely limited due to absorption bands of liquid water. We present the design, fabrication and experimental characterization of large-area metasurfaces based on cross-shaped plasmonic NAs for the spectroscopic characterization of various types of compounds and for sensing applications in the mid-infrared range. The cross-shaped NAs we have designed exhibit SEIRA phenomena which are very sensitive to both refractive index changes in the surrounding medium and to the specific molecular vibration band emerging from surface adsorbed molecules. To test this effect on our device, we have used as model compounds small molecules (molecular weight (MW) < 500 g/mol) containing triple bond groups resonating at about 2100 cm−1 and a large polymer (MW ˜ 950,000 g/mol) containing carbonyl groups resonating at wavenumbers of about 1700 cm−1. We show a sensitivity of 600 nm/RIU at different wavelengths at a maximum amount of immobilized small molecule of 0.7 fmoles and a SEIRA enhancement factor of 48,000. We also show the device potential to reveal chemical reactions, occurring on the sensor surface at the same scale, where the nitrile group is converted to a triazole ring.

Nano‐Optical Tweezing of Single Proteins in Plasmonic Nanopores

AbstractSingle‐molecule sensing technologies aim to detect and characterize single biomolecules, but generally need labeling while the measurement times and throughput are severely restricted by a lack of positional control over the molecule. Here, a plasmonic nanopore biosensor is reported where single molecules can be electrophoretically delivered into a nanopore sensor with a plasmonic nanoantenna that is used to optically trap single molecules for extended measurement times. Using the light transmission through the antenna as read‐out, optical trapping of 20 nm diameter polystyrene nanoparticles and individual beta‐amylase proteins, a 200 kDa enzyme, in the plasmonic nanoantenna are demonstrated. Application of an electrical bias voltage allows the increase of the event rate over an order of magnitude as well as shorten the residence time of the proteins in the plasmonic nanopore as they can controllably be drawn out of the trap by electrical forces. Trapping is found to be assisted by protein–surface interactions and trapped proteins can denature on the nanopore surface. The integration of two single‐molecule sensors, a plasmonic nanoantenna and solid‐state nanopore, creates independent control handles at the single‐molecule level—the optical trapping force and electrophoretic force—which provides augmented control over single molecules.

10-million-elements-per-second printing of infrared-resonant plasmonic arrays by multiplexed laser pulses.

We report on high-quality infrared (IR)-resonant plasmonic nanoantenna arrays fabricated on a thin gold film by tightly focused femtosecond (fs) laser pulses coming at submegahertz repetition rates at a printing rate of 10 million elements per second. To achieve this, the laser pulses were spatially multiplexed by fused silica diffractive optical elements into 51 identical submicrometer-sized laser spots arranged into a linear array at periodicity down to 1μm. The demonstrated high-throughput nanopatterning modality indicates fs laser maskless microablation as an emerging robust, flexible, and competitive lithographic tool for advanced fabrication of IR-range plasmonic sensors for environmental sensing, chemosensing, and biosensing.