Asymmetric Nanoantennas Research Articles

Strong Field Enhancement and Unidirectional Scattering Based on Asymmetric Nanoantenna

Dielectric-metal nanostructures have lately emerged as one of the most promising approaches to modulating light at the optical frequency. Their remarkable electric and magnetic resonances give them a one-of-a-kind ability to augment local field enhancements with negligible absorption losses. Here, we propose a hybrid metal-dielectric-metal (MDM) nanoantenna that contains a dimer of three-layers of shell nanoparticles. In addition, we only theoretically and numerically show the optical properties of the hybrid dimer nanoantenna. We found that the nanoantenna sustained unidirectional forward scattering with narrow beamwidth (30.9 deg) and strong scattering intensity (up to 5 times larger than the single MDM particle). Furthermore, when the hybrid asymmetric dimer was excited by the plane wave with different electric polarization directions, our findings revealed that the hybrid nanoantenna boosted the gap’s electric near-field while also supporting unidirectional forward scattering. Finally, we analyzed the hybrid dimer with substrates of different materials. It supported strong electric high-order moments along the z-axis and x-axis in gaps between MDM nanoparticles and between MDM nanoparticles and the Ge substrate, owing to the intense displacement currents inside of the dielectric layer. We found that the local electric field of this MDM hybrid dimer nanoantenna with Ge substrate was well improved and attained 3325 v/m.

Distinct local angular chiroptical effects with unidirectional emission based on asymmetric plasmonic nanopillar antennas

Here, we observed the local angular chiroptical effects with unidirectional emission based on exquisitely designed plasmonic nanopillar antennas. Firstly, the scattering spectra for the asymmetric nanoantennas show double resonant peaks when illuminated by circular polarized light. The calculated circular dichroism reaches the maximum value at one resonant peak while almost disappears at the other peak. Secondly, when we turn to the momentum space for the CD-disappeared peak, the asymmetric nanoantennas exhibit different angular emission patterns that are sensitive to the handedness of light, which indicate the existence of the local angular chiroptical effects. The emission patterns also show unidirectional emission lobes in momentum space, which is distinct to other chiral phenomenon. Multipolar expansion and Green’s function method in stratified medium is used to demonstrate the inner mechanism of the observed local angular chiroptical effects, and we found the phase difference of the multipolar resonance is the origin of the distinct local angular chiroptical effects. Our work investigating the local angular chiroptical effects based on nanopillar will shed new light on optical chirality and will be widely applied to wavelength router, photon-detection, and other optoelectronic devices.

Engineering single-molecule fluorescence with asymmetric nano-antennas

As a powerful tool for studying molecular dynamics in bioscience, single-molecule fluorescence detection provides dynamical information buried in ensemble experiments. Fluorescence in the near-infrared (NIR) is particularly useful because it offers higher signal-to-noise ratio and increased penetration depth in tissue compared with visible fluorescence. The low quantum yield of most NIR fluorophores, however, makes the detection of single-molecule fluorescence difficult. Here, we use asymmetric plasmonic nano-antenna to enhance the fluorescence intensity of AIEE1000, a typical NIR dye, by a factor up to 405. The asymmetric nano-antenna achieve such an enhancement mainly by increasing the quantum yield (to ~80%) rather than the local field, which degrades the molecules’ photostability. Our coupled-mode-theory analysis reveals that the enhancements stem from resonance-matching between antenna and molecule and, more importantly, from optimizing the coupling between the near- and far-field modes with designer asymmetric structures. Our work provides a universal scheme for engineering single-molecule fluorescence in the near-infrared regime.

From symmetric to asymmetric bowtie nanoantennas: electrostatic conformal mapping perspective

Abstract Plasmonic nanoantennas have revolutionized the way we study and modulate light–matter interaction. Due to nanofabrication limitations, dimer-type nanoantennas always exhibit some degree of asymmetry, which is desirable in some cases. For instance, in sensing applications, asymmetry is sometimes induced by design in plasmonic nanoantennas to favor higher order nonradiative modes with sharp Fano line shapes. Regardless of the actual origin of the asymmetry, unintentional or intentional, an analytical frame that can deal with it in a seamless manner would be beneficial. We resort to conformal mapping for this task and we track the influence of the degree of asymmetry of the circular sectors composing gold bowtie nanoantennas on the nonradiative Purcell enhancement of a nearby nanoemitter. This manuscript reviews the contributions of conformal mapping to plasmonic nanoantennas and illustrates the advantages of the elegant analytical solution provided by conformal mapping to grasp physical insights, which can serve as a springboard for new plasmonic asymmetric nanoantenna designs.

Above-Threshold Photoemission of Compound Nanoantennas Irradiated by Femtosecond Laser Pulses

A theoretical study of electron emission processes that occur at interaction of femtosecond laser pulses with bowtie compound nanoantennas is performed. A numerical analysis is conducted of the dependences of the spectra of ejected electrons on the features of the light pulses for different geometric parameters of the antennas. The calculation results show that asymmetric nanoantennas have distinct advantages over symmetric configurations of the elements of compound antennas.

Tunable unidirectional surface plasmon polariton launcher utilizing a graphene-based single asymmetric nanoantenna

We design and numerically investigate a graphene-based asymmetric nanoantenna microstructure that can be used to realize electrically controllable, unidirectionally propagating broadband surface plasmon polaritons. The device geometry facilitates the simultaneous excitation of two localized surface plasmons resonances in the whole structure, and consequently, the asymmetric nanoantenna can be considered as being composed of two oscillating magnetic dipoles, wherein the interference of the radiated electromagnetic waves leads to a unidirectional propagation effect. Our results indicate that our proposed active device is promising for realizing compactable, tunable, terahertz plasmonic light sources.

Tunable unidirectional surface plasmon polariton coupler utilizing graphene-based asymmetric nanoantenna pairs

Surface plasmon polaritons (SPPs), the electromagnetic waves traveling along metal-dielectric or metal-air interface, which originate from the interactions between light and collective electron oscillations on metal surface, have received considerable attention for their promising applications in the future optical field, such as image, breaking diffraction limit, subwavelength-optics microscopy, lithography, etc. However, one of the fundamental issues in plasmonics is how to actively manipulate the propagation direction of SPPs. In this paper, we propose and numerically investigate a graphene-based unidirectional SPP coupler, which is composed of asymmetric plasmonic nanoantenna pairs with a graphene sheet separated by a SiO2 spacer from the gold substrate. The device geometry facilitates the simultaneous excitation of two localized surface plasmon resonances in the entire structure, and consequently, the asymmetric nanoantenna pairs can be considered as being composed of two oscillating magnetic dipoles or as two SPP sources. Because the resonance of the plasmonic antenna pairs depends on the bias voltage applied across graphene sheet and back-gated Au, the phase difference between radiated electromagnetic waves induced by the antenna can be tuned through varying the Fermi level of graphene. Here, approximately a n/2 phase difference between radiated electromagnetic (EM) waves can be acquired at EF 0.81 eV, which indicates that the radiated EM waves can interfere constructively along the direction of the x-axis while interfere destructively along the opposite direction. This directional propagation of EM wave leads to the unidirectional propagation of SPPs. Furthermore, electric field distribution of the cavity demonstrates that the tunability of plasmonic antenna is proportional to the electric field intensity in the vicinity of the graphene region. For our designed structure, the left cavity can provide a significantly larger tunable range than the right one. With this result, we can quantitatively analyze the tuning behavior of graphene-loaded plasmonic antenna based on equivalent circuit model, and draw the conclusions that the unidirectional SPP propagation effect originates from the interference mechanism. In addition, compared with the device reported previously, our proposed device possesses a huge extinction ratio (2600) and more broadband tunable wavelength range (6.3-7.5 m). In addition, it is possible to make up for the deficiencies of current nanofabrication technologies by utilizing its actively controlled capability. All the above results indicate that the proposed active device promises to realize a compactable, tunable, and broadband terahertz plasmonic light source. It will play an important role in future photonic integrations and optoelectronics.

Asymmetric split H-shape nanoantennas for molecular sensing.

In this paper we report on a very sensitive biosensor based on gold asymmetric nanoantennas that are capable of enhancing the molecular resonances of C-H bonds. The nanoantennas are arranged as arrays of asymmetric-split H-shape (ASH) structures, tuned to produce plasmonic resonances with reflectance double peaks within the mid-infrared vibrational resonances of C-H bonds for the assay of deposited films of the molecule 17β-estradiol (E2), used as an analyte. Measurements and numerical simulations of the reflectance spectra have enabled an estimated enhancement factor on the order of 105 to be obtained for a thin film of E2 on the ASH array. A high sensitivity value of 2335 nm/RIU was achieved, together with a figure of merit of approximately 8. Our experimental results were corroborated using numerical simulations for the C-H stretch vibrational resonances from the analyte, superimposed on the plasmonic resonances of the ASH nanoantennas.

A sub-terahertz broadband detector based on a GaN high-electron-mobility transistor with nanoantennas

We report a sub-terahertz (THz) detector based on a 0.25-µm-gate-length AlGaN/GaN high-electron-mobility transistor (HEMT) on a Si substrate with nanoantennas. The fabricated device shows excellent performance with a maximum responsivity (Rv) of 15 kV/W and a minimal noise equivalent power (NEP) of 0.58 pW/Hz0.5 for 0.14 THz radiation at room temperature. We consider these excellent results as due to the design of asymmetric nanoantennas. From simulation, we show that indeed such nanoantennas can effectively enhance the local electric field induced by sub-THz radiation and thereby improve the detection response. The excellent results indicate that GaN HEMTs with nanoantennas are future competitive detectors for sub-THz and THz imaging applications.

Switchable directional scattering of electromagnetic radiation with subwavelength asymmetric silicon dimers.

High refractive index dielectric nanoparticles show high promise as a complementary nanophotonics platform due to compared with plasmonic nanostructures low absorption losses and the co-existence of magnetic and electric resonances. Here we explore their use as resonantly enhanced directional scatterers. We theoretically demonstrate that an asymmetric dimer of silicon nanoparticles shows tuneable directional scattering depending on the frequency of excitation. This is due to the interference between electric and magnetic dipoles excited in each nanoparticle, enabling directional control of the scattered light. Interestingly, this control can be achieved regardless of the polarization direction with respect to the dimer axis; however, difference in the polarization can shift the wavelengths at which the directional scattering is achieved. We also explore the application of such an asymmetric nanoantenna as a tuneable routing element in a nanometer scale, suggesting applications in optical nanocircuitry.

Hot spots in different metal nanostructures for plasmon-enhanced Raman spectroscopy

Noble metal nanostructures are able to concentrate light into small volumes, which enhances greatly the local electromagnetic (EM) field near the metal nanostructures. The areas with greatly enhanced EM field become "hot spots" for surface-enhanced Raman spectroscopy (SERS), which utilizes the field enhancement properties of metal nanostructures to amplify the usually weak Raman scattering signals. The preparation of metal nanostructures with superior SERS performances is one of the main topics in the SERS field and is important for the applications of SERS in sensing and analysis. In this feature article, we review several different kinds of metal structures for SERS, including coupled metal nanostructures, nanostructure arrays, nano/micro structures with textured surfaces and graphene-mediated SERS substrates. Tip-enhanced Raman spectroscopy (TERS) using the metal tip of the scanning tunneling microscope is also discussed. Besides the intensity enhancement in SERS and TERS, some less studied aspects are highlighted; for example, the remote excitation technique, tuning the nanogap of gold nanorod dimers through strain control on an elastomeric substrate, manipulating Raman emission polarization using asymmetric nanoantennas, and investigating plasmon-enhanced chemical reactions by TERS.

Unidirectional radiation of a magnetic dipole coupled to an ultracompact nanoantenna at visible wavelengths

The optical radiation of a magnetic dipole coupled to a plasmonic nanoantenna composed of stacked gold nanodisks is investigated. The nanoantenna is fed by a point magnetic dipole properly situated between two nanodisks. It is found that the magnetic dipole efficiently couples to the antenna due to the excitation of an antiphase localized surface plasmon resonance mode. This leads to a remarkable enhancement of the optical radiation at the resonant wavelength associated with the hybridized mode of the nanoantenna. By introducing a slight structural asymmetry in the nanoantenna, the optical radiation is substantially altered. Interestingly, it is shown that the omnidirectional radiation of the magnetic dipole becomes almost unidirectional, further enhanced, and accessible in three-dimensions by the asymmetric nanoantenna. Based on the classical model, quasistatic theory, and electrodynamics calculations, the electromagnetic interaction of the magnetic dipole and the nanoantennas is studied. A remarkable forward-to-backward directionality is achieved by the proposed nanoantenna at visible wavelengths.

Interference, Coupling, and Nonlinear Control of High-Order Modes in Single Asymmetric Nanoantennas

We investigate theoretically and experimentally the structure of plasmonic modes in individual asymmetric dimer antennas. Plasmonic near-field coupling of high-order modes results in hybridization of bright and dark modes of the individual nanorods, leading to an anticrossing of the coupled resonances. For two bright modes, hybridization results in a capacitive red shift and super-radiant broadening. We show that the properties of asymmetric dimers can be used for nonlinear control of spectral modes and demonstrate such a nonlinear effect by measuring the modulation of a hybrid asymmetric dimer--ITO antenna. With use of full electrodynamical calculations, we find that the properties of the near-field nonlinear responses are distinctly different from the far-field, which opens up new routes for nonlinear control of plasmonic nanosystems.