Au Antennas Research Articles

Strong coupling in an Au plasmonic antenna– SiO2 layer system: A hybrid-mode analysis

A detailed analysis of the optical response of a system accommodating several coupled modes is needed for the complete understanding of the strong coupling effect. In this paper, we report on the analysis of scattering cross-section spectra of Au antennas on a ${\mathrm{SiO}}_{2}$ layer on a Si substrate in the IR region. A classical model of coupled oscillators is used for determining the resonant energies, damping rates, and coupling strengths of four phonon polariton modes in the ${\mathrm{SiO}}_{2}$ layer coupled to a localized surface plasmon mode in a Au antenna. The calculated Hopfield mixing coefficients then show the contribution of the individual uncoupled modes to the hybrid modes of the coupled system.

Bioinspired patterned photonic junctions for plasmon-enhanced metal photoluminescence and fluorescence: design of optical cavities for near-infrared electronics

Photonic junctions with their plasmonic components can outstandingly amplify the light-matter interactions, boost the localized surface plasmon resonance and tightly concentrate the optical electromagnetic fields. The mechanisms of plasmon-enhanced metal photoluminescence (PL) and especially fluorescence (FL) emission enhancement of plasmonic junctions (PJs) in optoelectronic devices are still undisclosed in many aspects. Herewith, various bioinspired designs of colonial PJs are investigated, where the geometrical features of Au nanoantennas are originated from diatoms patterns. Developed PJs are based on few nanometer thick heterostructured metal oxide spacer semiconductors sandwiched between Au antennas. PL and dynamic FL spectroscopy measurements are carried out to investigate and understand the underlying effects behind the tangible increase of absorption of IR-lights (70%), outstanding increase of PL emissions (43 times) and dynamic FL characteristics of optical cavities. Considerable high-level PL emission of dot-like PJs is attributed to the development of horizontal Fabry–Pérot cavities and the higher Purcell factors of fabricated photonic structures. The obtained data suggest that two-dimensional (2D) heterostructured metal oxide spacer constructively improves the photonic characteristics of PJs. Tailoring plasmonic dot-like PJs into functional photodetectors resulted in 27.4 A/W photoresponsivity, 44% external quantum efficiency (EQE) and ultra-fast microsecond photo-response at near-infrared (IR) light illumination (∼800 nm). Findings demonstrate the outstanding plasmonic structures’ capability for further development of novel IR optoelectronics.

Bright CdSe/CdS Quantum Dot Light-Emitting Diodes with Modulated Carrier Dynamics via the Local Kirchhoff Law.

Addressing the interactions between optical antennas and ensembles of emitters is particularly challenging. Charge transfer and Coulomb interactions complicate the understanding of the carrier dynamics coupled by antennas. Here, we show how Au antennas enhance the luminescence of CdSe/CdS quantum dot assemblies through carrier dynamics control within the framework of the local Kirchhoff law. The Au antennas inject hot electrons into quantum dot assemblies via plasmon-induced hot electron transfer that increases the carrier concentration. Also, the localized surface plasmon resonances of Au antennas favorably tilt the balance between nonradiative Auger processes and radiative recombination in the CdSe core. Eventually, a high bright (125,091.6 cd/m2) deep-red quantum dot light-emitting diode is obtained by combining with Au antennas. Our findings suggest a new understanding of light emission of assembled emitters coupled by antennas, which is of essential interest for the description of light-matter interaction in advanced optoelectronics.

Near-field enhancement by plasmonic antennas for photocatalytic Suzuki-Miyaura cross-coupling reactions

It is well documented that placing a plasmonic antenna close to catalytically active nanoparticles can enhance their catalytic activity in chemical reactions via the near-field enhancement effect. Less known is whether and how the near-field enhances the reactivity of the reactant substrates involved in these reactions. Herein, we prepared an “antenna-reactor” catalyst with Au nanoparticles absorbing light as an optical antenna and the adjacent Pd nanoparticles acting as a chemical reactor to study the near-field enhancement effect in Suzuki–Miyaura cross-coupling reaction involving various substrates. The results showed that the activity of Pd nanoparticles were significantly enhanced in the presence of Au antennas. Excessively increasing the density of Au antennas, however, suppressed the reaction due to the interaction between neighboring electromagnetic hot spots. Moreover, the near-field affected different substrates in different extents, enhancing more to reactions that involve substrates with higher electron cloud density at the reactive center. The overall effect of the near-field to the catalytic reactions was proved to be an integrated result of the effect of Au nanoparticle density and reactive center electron cloud density. This study provides enriched understanding for the near-field enhancement effect in photocatalytic reactions with various substrates, and deepened insights for new “antenna-reactor” photocatalyst design.

SiC THz Sensors for Harsh Environment Applications

Gold (Au) and ITO bow-tie antenna sensors built on silicon carbide (SiC) substrates are designed and analyzed using the Comsol Multiphysics program. The study shows that the output electric field across the antenna gap decreases with gap separation but increases with metal thickness. The antenna width does not directly affect the output, but it indirectly increases the output as a result of increased radiation absorption and conductivity. The length does not affect the strength of the output, but it affects the resonant frequency. The output field from ITO antennas is found to be much lower than that from Au antennas, which can be attributed, in part, to the lower conductivity of ITO. Using the bow-tie antenna structure, it is possible to build either narrowband or wideband antenna or sensor arrays, for THz applications.

Nano-particle transport and the prediction of a valid area to be trapped based on a plasmonic antenna array.

Optical antennas are promising for optical trapping and particle manipulation, when converting light between localized energy and freely propagating radiation. In this paper, we proposed a numerical method for the transport of nanoparticles using the optical force field over a plasmonic Au antenna array. The plasmonic Au antenna array is designed to produce strong near-field hot spots when illuminated by a plane wave. The hot spots function as optical traps, separately addressable by their resonant wavelengths. By changing the traps sequentially, the nanoparticles can be handed off between adjacent traps. We also demonstrated a valid area in which the nanoparticles could be trapped and transferred stably by discussing the trapping potential that particles encountered. The simulated and calculated results showed that this method had promising applications in the field of biochemical diagnoses and high-accuracy optical manipulation.

Hybrid thermal Yagi-Uda nanoantennas for directional and narrow band long-wavelength IR radiation sources.

We investigate the possibility of spatially and spectrally controlling the thermal infrared emission by exploitation of the Yagi-Uda antenna design. Hybrid antennas composed of both SiC and Au rods are considered and the contributions of emission from all the elements, at a given equilibrium temperature, are taken into account. We show that the detrimental effect due to thermal emission from the not ideal parasitic elements drastically affect the performances of conventional thermal Au antennas in the 12 µm wavelength range. Nevertheless, our results show that the hybrid approach allows the development of efficient narrow-band and high directivity sources. The possibility of exploiting the Yagi-Uda design both in transmission and in reception modes, may open the way to the realization of miniaturized, efficient, robust and cheap sensor devices for mass-market applications.

Low‐Loss Hybrid High‐Index Dielectric Particles on a Mirror for Extreme Light Confinement

AbstractThe quest for enhancing light–matter interactions at the nanoscale has led scientists to nanophotonic systems that support the smallest possible modes, which are currently realized via particle‐on‐mirror (PoM) geometries using plasmonic particles. The drawback of metallic PoM systems is the large absorption/scattering ratio due to the significant Ohmic losses inherent to plasmonics. Here, an alternative dielectric PoM composed of a high‐index dielectric nanodisk on top of a metallic mirror is realized. Custom‐shaped high‐quality dielectric colloidal nanoparticles are fabricated and dispersed on a mirror to fully unlock the potential of PoM systems. This hybrid device combines the large field enhancement and extreme localization of plasmonic systems with the low absorption of dielectric nanoresonators. By means of far‐field scattering and near‐field cathodoluminescence spectroscopy, the nature of the modes supported by the hybrid PoM are revealed. Utilizing enhanced spectroscopies, such as Raman and fluorescence, these hybrid‐PoM systems are used in proof‐of‐principle applications. A comparison with Au antennas indicates that the hybrid PoM system presents efficiencies and optical characteristics comparable or superior to those of more conventional purely plasmonic systems, opening new avenues for low‐loss light control at the deep nanoscale level.

Radiation properties of graphene‐based optically transparent dipole antenna

AbstractThe radiation properties of a graphene‐based optically transparent half‐wavelength dipole antenna in microwave bands was investigated. The resonance frequencies of a reference Au antenna and a graphene antenna having the same antenna structures were 9.5 and 21.6 GHz, respectively, owing to the difference in conductivity. Despite the low conductivity of graphene, the gain of the graphene antenna was 1 dB higher than that of the Au antenna. We also showed the feasibility of using graphene‐based optically transparent antenna through the measurements of transmission characteristics between two graphene antenna.

High sensitivity high Tc superconducting Josephson junction antenna for 200 GHz detection

ABSTRACTA high sensitivity rounded bow-tie antenna was optimized based on electromagnetic simulations. The bicrystal Josephson junction detector with a thin layer of YBCO was located exactly at the bicrystal misorientation point at the center of the antenna. The Au antenna was fabricated on an (100) MgO bicrystal substrate. In the presence of 200 GHz RF radiation, a noise equivalent power (NEP) of around 6×10−12 W Hz−1/2 was measured, with a voltage sensitivity, , of 30 dB with the detector at 60 K. Even though no coupling lens was used, these results are as good as those reported in systems in which the impinging radiation was focused with lenses over the detectors. Thus, important simplification of the system was obtained without loss of sensitivity.

Plasmon Resonances of Mid-IR Antennas on Absorbing Substrate: Optimization of Localized Plasmon-Enhanced Absorption upon Strong Coupling Effect

We report on the surface plasmon resonances of mid-infrared Au antennas deposited on an absorbing silicon-rich oxynitride (SRON) thin film, and on their utilization for enhancement of a spatially l...

Nanogapped Au Antennas for Ultrasensitive Surface-Enhanced Infrared Absorption Spectroscopy.

Surface-enhanced infrared absorption (SEIRA) spectroscopy has outstanding potential in chemical detection as a complement to surface-enhanced Raman spectroscopy (SERS), yet it has historically lagged well behind SERS in detection sensitivity. Here we report a new ultrasensitive infrared antenna designed to bring SEIRA spectroscopy into the few-molecule detection range. Our antenna consists of a bowtie-shaped Au structure with a sub-3 nm gap, positioned to create a cavity above a reflective substrate. This three-dimensional geometry tightly confines incident mid-infrared radiation into its ultrasmall junction, yielding a hot spot with a theoretical SEIRA enhancement factor of more than 107, which can be designed to span the range of frequencies useful for SEIRA. We quantitatively evaluated the IR detection limit of this antenna design using mixed monolayers of 4-nitrothiophenol (4-NTP) and 4-methoxythiolphenol (4-MTP). The optimized antenna structure allows the detection of as few as ∼500 molecules of 4-NTP and ∼600 molecules of 4-MTP with a standard commercial FTIR spectrometer. This strategy offers a new platform for analyzing the IR vibrations of minute quantities of molecules and lends insight into the ultimate limit of single-molecule SEIRA detection.

Design and Fabrication for the Construction of MIR HEB Mixers

In order to design antennas for mid-infrared (MIR) hot electron bolometers (HEBs), the surface impedance of Au thin films at cryogenic temperatures was evaluated using Fourier transform infrared spectroscopy with a sample cooling system. We fabricated resonator arrays that were constructed by gold thin film strips. The length, width, and thickness of the resonators were set at 1300, 200, and 55 nm, respectively. As the temperature was lowered, the resonant frequency was shifted to the low-frequency side. By fitting the resonant frequency to simulated results, the corrected surface reactance was established for designing MIR nano-antennas. MIR HEBs with a twin-slot antenna were designed and fabricated using the corrected surface impedance. The design frequency was 61 THz. The HEBs were formed as a superconducting strip with Au antennas and electrodes. Here, the superconductivity under the Au electrodes was suppressed by a Ni thin film. For evaluating the mixer properties of the MIR HEBs, measurement setup without beam splitter was constructed. In this setup, local oscillator power was applied directly to the HEB, and we confirmed that the critical current of the HEB can be reduced to be almost zero by the LO irradiation. The HEB responses with clear polarization dependencies were also observed.

Deterministic radiative coupling between plasmonic nanoantennas and semiconducting nanowire quantum dots

We report on the deterministic coupling between single semiconducting nanowire quantum dots emitting in visible and plasmonic Au nanoantennas. Both systems are separately and carefully characterized through micro-photoluminescence and cathodoluminescence. A two-step realignment process using cathodoluminescence allows for electron-beam lithography of Au antennas near individual nanowire quantum dots with a precision of 50 nm. A complete set of optical properties was measured before and after antenna fabrication. They evidence both an increase of the nanowire absorption, and an improvement of the quantum dot emission rate up to a factor of two in presence of the antenna.

Antenna coupled niobium bolometers for 10 /spl mu/m wavelength radiation detection

We report on the fabrication and testing of antenna-coupled niobium bolometers for 10 /spl mu/m wavelength radiation detection. We use 20 nm thick Nb films on oxidized Si substrates. The bolometer design consists of a 1 /spl mu/m/spl times/1 /spl mu/m Nb microbridge embedded in either a dipole or a spiral Au antenna. We have performed dark dc tests on devices in vacuum at 4.2 K. Current biased dark measurements correspond well to established theory. This result allows us to extract an upper limit for the value of thermal conductance g (340 nWK/sup -1/)-hence an indirect measure of the device bandwidth (0.2 GHz). Measurements under incident black body radiation show that the radiation perturbs the return switching characteristics of the devices.

Detection mechanisms in microstrip dipole antenna-coupled infrared detectors

We compare the detection mechanisms employed in microstrip dipole antenna-coupled infrared detectors. The electrical currents induced along the antenna arms are detected by a rectangular niobium (Nb) microsensor placed at the center of the antenna. The ohmic nature of the Au–Nb contact determines the detection mechanism. Devices with linear contacts between the Au antenna arms and the Nb microsensor exhibit bolometric response. A nonlinear Au–insulator–Nb junction rectifies the induced antenna currents. Devices with nonlinear contacts also exhibit a bolometric response. The devices with nonlinear contacts are 1/ f noise limited while the devices with linear contacts are Johnson noise limited. The rectification mechanism is 5.3 times faster than the thermal detection. The current–voltage ( I– V) characteristic of the devices exhibiting bolometric response is linear, while that of the rectifying devices is cubic. For devices with nonlinear contacts excellent agreement is obtained between the measured detector response and the ratio between the second and the first derivative of the I– V characteristic.

Observation of Terahertz Photon-Assisted Tunneling in Triple-Barrier Resonant Tunneling Diodes Integrated with Patch Antenna

Terahertz (THz) photon-assisted tunneling was observed in GaInAs/InAlAs triple-barrier resonant tunneling diodes (RTDs) integrated with patch antennas on InP substrates. The conduction loss of the antenna was reduced by a structure that consists of an Au-coated ground plane, an Au antenna electrode and a thick benzo-cyclo-butane layer as a dielectric between them. Large THz voltages were induced across the RTD with this structure by THz irradiation (frequency f=1.4–3.1 THz), e. g. , 122 mV at an incident power of 76 mW at f=1.4 THz. Current changes due to photon-assisted tunneling with photon absorption and emission were determined. The peak voltage of the current change shifts due to the multiphoton process with increasing incident power. The multiphoton process was remarkable at low irradiation frequency. These measurements were in reasonable agreement with theory.

Submillimeter-wave mixing and noise in HTS Josephson junctions

A Josephson quasioptical detector comprising YBaCuO junction on bicrystal MgO substrate, integrated log-periodic Au antenna, MgO extended hyperhemisphere lens attached to the substrate and 1.4 GHz cold amplifier with cold circulator has been designed, fabricated and experimentally studied. We fabricated several integrated Josephson junctions 1-2 /spl mu/m wide which at 4.2 K demonstrate RSJ IV curves with critical currents 100-300 /spl mu/A and normal resistance of 10-20 /spl Omega/. The noise temperature (T/sub N/) at 4.2 K and 330 GHz measured by a hot/cold load method in fundamental mixing mode with external BWO local oscillator brings T/sub N/=1100 K for the junction shunted by low-inductance resistive shunt and T/sub N/=1400 K without shunting. At bath temperatures of 20-40 K the noise temperature was in the range 1500-3000 K, at 50 K the noise temperature was below 2000 K, and at 60-70 K increased over 3000 K in the frequency range 310-586 GHz. The noise temperatures measured in the self-pumping mode vary between 580-2300 K with dc bias in the range 0.1-0.5 mV. The linewidth of Josephson oscillations has been extracted from a selective detector response and IF noise voltage dependence. The measured excess noise can be attributed to shot noise and multiple Andreev reflection in pinholes with Ss'S structure. We observed small subharmonic steps in IV curve and noise dependencies, which are typical for SNS junctions.