Gold Nanodisk Arrays Research Articles

Broadband Tunable, Polarization-Selective and Directional Emission of (6,5) Carbon Nanotubes Coupled to Plasmonic Crystals.

We demonstrate broadband tunability of light emission from dense (6,5) single-walled carbon nanotube thin films via efficient coupling to periodic arrays of gold nanodisks that support surface lattice resonances (SLRs). We thus eliminate the need to select single-walled carbon nanotubes (SWNTs) with different chiralities to obtain narrow linewidth emission at specific near-infrared wavelengths. Emission from these hybrid films is spectrally narrow (20–40 meV) yet broadly tunable (∼1000–1500 nm) and highly directional (divergence <1.5°). In addition, SLR scattering renders the emission highly polarized, even though the SWNTs are randomly distributed. Numerical simulations are applied to correlate the increased local electric fields around the nanodisks with the observed enhancement of directional emission. The ability to control the emission properties of a single type of near-infrared emitting SWNTs over a wide range of wavelengths will enable application of carbon nanotubes in multifunctional photonic devices.

Normal and anomalous plasmonic lattice modes of gold nanodisk arrays in inhomogeneous media

We study plasmonic lattice modes in two dimensional arrays of large metallic nanodisks in strongly inhomogeneous environments with controlled dielectric asymmetries. This is done within the two limits of positive (air/substrate) and negative (Si/substrate) asymmetries. In the former, the nanodisks are exposed to air, while in the latter, they are fully embedded in a dielectric material with a refractive index much higher than that of the glass substrate (Si). Our results show that in the air/substrate limit, the arrays can mainly support two distinct visible and infrared peaks associated with the optical coupling of multipolar plasmonic resonances of nanodisks in air and substrate (normal modes). As the nanodisks are gradually embedded in Si, i.e., going from the positive to negative asymmetry limit, the visible peak undergoes more than 200 nm red shift without significant mode degradation. Our results show that as this transition happens, a third peak (anomalous mode) becomes dominant. The amplitude and wavelength of this peak increase quadratically with the thickness of the Si layer, indicating formation of a unique collective mode. We study the impact of this mode on the emission semiconductor quantum dots, demonstrating they become much brighter as the result of the long-reach plasmonic fields of the nanodisks when the arrays are in this mode.

Strong coupling between J-aggregates and surface plasmon polaritons in gold nanodisks arrays

Recently, much attention has been paid to an interesting subject, i.e., the interactions between surface plasmon polaritons (SPPs) and molecules. The interactions between SPPs and molecules often appear in two opposite cases, namely weak and strong coupling. When the interaction is weak, the absorption maximum simply coincides with the electronic transition energy of the molecule. In the weak coupling regime, the wave functions of the molecule and the SPP modes are considered to be unperturbed, only leading to enhancement of the absorption or fluorescence of the molecules. On the other hand, when the interaction is strong enough, the SPPs and molecules can form a coherent hybrid object, thus the excitation energy is shared by and oscillates between the SPPs and molecular systems (Rabi oscillations), leading to vacuum Rabi splitting of energy levels at the resonance frequency. Due to the fact that both the SPPs and the molecule components can be confined into the nanometer scale, the work on strong coupling with SPPs offers a very good opportunity to realize nanoplasmonic devices, such as thresholdless laser and room temperature B-E condensates.In this work, we investigate a hybrid system formed by strong coupled gold nanodisk arrays and J-aggregate molecules. Smooth gold nanodisk arrays are fabricated by a template-stripping process. In such an experimentally simple replicate process, mass-production of gold nanodisk arrays with the same morphology can be transferred from patterned indium tin oxides (ITO) glass. The structures on ITO glass are milled with a focused ion beam. Periodic gold nanodisk arrays have the capability of converting light into SPPs modes, resulting in a significant field confinement at the patterned metal surface. In particular, the desired SPP mode can be chosen by changing the nanodisk array period to match the absorbance peak of the J-aggregate molecule. On the other hand, J-aggregate molecule is chosen due to its large dipole moments and absorption coefficient, which makes it attractive for designing the strong exciton-plasmon interaction system. The strong coupled system is formed when the J-aggregate molecule is spin-coated on the gold nanodisk arrays. Through reflection measurements, Rabi splitting energy value 200 meV is observed when the period of the nanodisk array is 350 nm. Through tuning the coupling strength by changing the lattice period from 250 nm to 450 nm, the typical signature of strong coupling:anticrossing of energies is found in reflection spectra. This simple replicate process for strong coupling hybrid system fabrication should play an important role in designing novel ultrafast nanoplasmonic devices with coherent functionalities.

Investigation of gold/GaN nanorod arrays for hypersonic detection: The effect of periodicity

Gold nanodisk arrays on the upper surface of GaN nanorods were developed with the aim of achieving hypersonic imaging with a detection frequency above 10 GHz. In this paper, we report the interesting phenomenon that the hypersonic signal detected by a single gold nanodisk in this structure might be dependent on the array periodicity and rod length. When the array periodicity is of the same order of or longer than the surface hypersonic wavelength, the detected signal would be enhanced by the period-dependent resonance of the surface hypersonic waves scattered by the nanorod/substrate interface. When the array periodicity is smaller than the surface hypersonic wavelength, the coupling of extensional modes between neighboring nanorods, as the detection frequency approaches the coupled-extensional-mode frequency, would enhance the detected signals. Although these enhancements are beneficial to heat transport at nanorod/substrate interfaces, they will cause cross-talk between the neighboring detection rod unit and should be avoided by choosing the correct period for imaging array applications. Our result further indicates that the extensional mode coupling could be avoided by increasing the rod-length to shift the extensional mode frequency away from the detection frequency. This work not only investigates the transport behavior of hypersonic-frequency acoustic phonons at the interface between a bulk material and a nanostructure, but also suggests that the effects of the periodicity and nanorod length need to be taken into consideration for the design of future hypersonic imaging arrays.

Polarization Selective Electromagnetic-Induced Transparency in the Disordered Plasmonic Quasicrystal Structure

We report a resonant plasmonic quasicrystal structure that support multiple bands of electromagnetically induced transparency (EIT) at visible frequencies. The quasicrystal structure is formed by a planar array of thin gold nanodisks arranged in a square lattice. The essential aspect of this quasicrystal is the periodic disorder that couples higher order modes to the broad background resonant mode. It can be noted that the EIT response of disordered quasicrystals having diagonally symmetric unit cells is polarization independent. Conversely, the EIT responses in the quasicrystals are “polarization selective”, that can be useful for spectral filtering application. Dispersion engineering through light–matter interactions in quasicrystals will be interesting for various applications including sensing, display, switching, nonlinear, and slow-light devices.

Plasmonic Fano interference produced by gold nano-disks on a dielectric Bragg stack

Fano interference is predicted in a structure consisting of a periodic array of uncoupled gold nano-disks (AuNDs) supported by a Bragg stack. The phenomenon occurs due to resonant interference between the reflection spectrum of the Bragg stack and localized surface plasmon resonances (LSPRs) on the AuNDs. Fitting the computational results to the Fano lineshape formula yields the Fano parameters of the resonances including their linewidth. One of the most important advantages of the structure is that the linewidth of the Fano resonances (∼9 nm) is much narrower than the bandwidth of the LSPR nano-disks (∼80 nm) and the reflection bandwidth of the Bragg stack on which the disks are supported (∼100 nm). The narrow Fano linewidth and the ease with which they can be interrogated using perpendicularly incident light makes the structure attractive for biosensing applications. The bulk sensitivities and figure of merit of Fano resonances in a proposed design are 44 nm/RIU (redshift) and −52 nm/RIU (blueshift), and ∼4 RIU−1, respectively.

Acousto-plasmofluidics: Acoustic modulation of surface plasmon resonance in microfluidic systems.

We acoustically modulated the localized surface plasmon resonances (LSPRs) of metal nanostructures integrated within microfluidic systems. An acoustically driven micromixing device based on bubble microstreaming quickly and homogeneously mixes multiple laminar flows of different refractive indices. The altered refractive index of the mixed fluids enables rapid modulation of the LSPRs of gold nanodisk arrays embedded within the microfluidic channel. The device features fast response for dynamic operation, and the refractive index within the channel is tailorable. With these unique features, our "acousto-plasmofluidic" device can be useful in applications such as optical switches, modulators, filters, biosensors, and lab-on-a-chip systems.

Optically-Triggered Nanoscale Memory Effect in a Hybrid Plasmonic-Phase Changing Nanostructure

Nanoscale devices, such as all-optical modulators and electro-optical transducers, can be implemented in heterostructures that integrate plasmonic nanostructures with functional active materials. Here we demonstrate all-optical control of a nanoscale memory effect in such a heterostructure by coupling the localized surface plasmon resonance (LSPR) of gold nanodisk arrays to a phase-changing material (PCM), vanadium dioxide (VO2). By latching the VO2 in a distinct correlated metallic state during the insulator-to-metal transition (IMT), while concurrently exciting the hybrid nanostructure with one or more ultraviolet optical pulses, the entire phase space of this correlated state can be accessed optically to modulate the plasmon response. We find that the LSPR modulation depends strongly but linearly on the initial latched state, suggesting that the memory effect encoded in the plasmon resonance wavelength is linked to the strongly correlated electron states of the VO2. The continuous, linear variation of ...

Magneto-optical response of two-dimensional magnetic plasmon structures based on gold nanodisks embedded in a ferrite garnet layer

Resonance optical effects in ordered two-dimensional arrays of gold nanodisks embedded in an insulating magnetic film are investigated. The angle-resolved transmission spectra exhibit features related to the excitation of local surface plasmons and lattice plasmon polaritons in the gold nanoparticles and waveguide modes in the magnetic insulator layer. Resonance enhancement of the linear magneto-optical intensity effect in transmission configuration is found in the vicinity of the lattice plasmon resonance and the waveguide mode in the structures under study.

Contribution of temperature to deformation of adsorbed vesicles studied by nanoplasmonic biosensing.

With increasing temperature, biological macromolecules and nanometer-sized aggregates typically undergo complex and poorly understood reconfigurations, especially in the adsorbed state. Herein, we demonstrate the strong potential of using localized surface plasmon resonance (LSPR) sensors to address challenging questions related to this topic. By employing an LSPR-based gold nanodisk array platform, we have studied the adsorption of sub-100-nm diameter 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) lipid vesicles on titanium oxide at two temperatures, 23 and 50 °C. Inside this temperature range, DPPC lipid vesicles undergo the gel-to-fluid phase transition accompanied by membrane area expansion, while DOPC lipid vesicles remain in the fluid-phase state. To interpret the corresponding measurement results, we have derived general equations describing the effect of deformation of adsorbed vesicles on the LSPR signal. At the two temperatures, the shape of adsorbed DPPC lipid vesicles on titanium oxide remains nearly equivalent, while DOPC lipid vesicles become less deformed at higher temperature. Adsorption and rupture of DPPC lipid vesicles on silicon oxide were also studied for comparison. In contrast to the results obtained on titanium oxide, adsorbed vesicles on silicon oxide become more deformed at higher temperature. Collectively, the findings demonstrate that increasing temperature may ultimately promote, hinder, or have negligible effect on the deformation of adsorbed vesicles. The physics behind these observations is discussed, and helps to clarify the interplay of various, often hidden, factors involved in adsorption of biological macromolecules at interfaces.

Experimental demonstration of sharp Fano resonance within binary gold nanodisk array through lattice coupling effects.

Fano resonance originating from the interaction of anti-phased and in-phased lattice collective resonances supported by a binary gold nanodisk array is investigated both numerically and experimentally. As the magnitude of particle radii in the two arrays approaches gradually, the Q factor of such resonance increases while the spectral contrast reduces. In the experiment, a high spectral contrast of 0.7 and a large Q factor of 14 could be achieved at the same time due to the excitation of the pure dipole resonance of each particle resulting from the weak near-field coupling. Optimal calculation gives rise to a higher Q factor of up to 85 and a corresponding spectral contrast of 0.86, which is very promising in refractive sensors. The sensitivity and the figure of merit are 337 nm/RIU and 116, respectively.

Metal-insulator-metal plasmonic absorbers: influence of lattice.

We experimentally demonstrate three kinds of metal-insulator-metal based plasmonic absorbers consisting of arrays of gold nanodisks distributed in different lattices, including square, triangular and honeycomb lattices. It's found that resonances originated from localized surface plasmon undergo little changes with respect to different lattice distributions of the nanodisks. The interparticle coupling results in a minor bandwidth broadening of the fundamental mode. Different from square- and triangular-lattice absorbers, honeycomb-lattice absorber possesses a unique red-shifting (with respect to incident angles) narrow-band high-order mode, which originates from coupling of incident light to propagating surface plasmon polariton (SPP) waves. Similar high-order mode can also be generated in square-lattice absorber by increasing the period so that a smaller reciprocal lattice vector can be introduced to excite the SPP mode. Furthermore, we show that two types of resonances can interact and create Fano-type resonances. The simulation results agree well with the experimental results.

Mapping electroactivity at individual catalytic nanostructures using high-resolution scanning electrochemical-scanning ion conductance microcopy.

Combined scanning electrochemical-scanning ion conductance microcopy (SECM-SICM) has been used to map the electroactivity of surfaces decorated with individual features at the 100-150 nm scale. Dual channel capillary probes consisting of an open SICM barrel, and a solid carbon SECM electrode enabled correlation of surface activity with accurate topographical information. Measurements were validated by approach curve analysis and imaging of model systems in feedback and substrate generation-tip collection modes and then applied to the examination of two nanostructured test substrates. First, electronically isolated gold nanodisk arrays were imaged using a simple electrochemical redox mediator, in which a clear positive feedback signal was observed at the SECM electrode, and the topographical channel compared well with AFM imaging. Second, platinum nanosphere ensembles were mapped using platinum-modified carbon probes to detect oxygen consumption in a redox competition mode, demonstrating the means to study electrocatalytic processes at individual nanoparticles. This work demonstrates the value of high-resolution SECM-SICM for low-current amperometric imaging of nanosystems, and is a step toward quantitative measurement of electrokinetics at the single particle level.

Enhanced detection sensitivity of higher-order vibrational modes of gold nanodisks on top of a GaN nanorod array through localized surface plasmons

We report a method that enables the excitation of localized surface plasmons (LSPs) in a gold nanodisk array by placing each nanodisk on top of a GaN nanorod. When the rod length was much longer than the plasmon penetration depth inside the nanorod, the plasmonic field was found to be localized, and coupling between neighboring gold nanodisks was eliminated. The interaction between LSPs and acoustic vibrations in gold nanodisks was then investigated. Owing to the strong localization of the plasmonic field, weak, higher-order vibrational modes of gold nanodisk could be optically observed. Furthermore, such an LSP-based acoustic sensor could be operated at any angle of incident light. Our study not only provides an approach to excite LSPs in high-density metallic arrays, but also opens one of the possible solutions for the development of highly sensitive sub-terahertz hypersonic sensors with high angle tolerance of incident light.

Gold crescent nanodisk array for nanoantenna-enhanced sensing in subwavelength areas.

Benefitting from the antenna effect and localized surface plasmon resonance (LSPR), a metal nanoparticle with a designed morphology has the amazing ability to confine light energy into the required extremely small volume, whose refractive index largely affects the optical properties of the whole metal nanoparticle. In this work, the optical spectra and near-field distribution of a gold nanocrescent array were investigated both experimentally and theoretically. To find out the LSPR wavelength and the enhancement using different morphologies of sharp tips, the spectra of gold nanocrescent arrays with different waist widths (d) were first measured, which were then confirmed and analyzed using the finite difference time-domain method and the hybridization theory. At last, the LSPR of this array with 100nm diameter dielectric nanodisks was studied for sensing in subwavelength areas. Our results showed that because of its giant nanoantenna-enhanced electromagnetic field at the two tips, this gold nanocrescent array could be a suitable local senor to sense the variation of a local medium in a subwavelength area.

Tuning the interaction between propagating and localized surface plasmons for surface enhanced Raman scattering in water for biomedical and environmental applications

With a view to biomedical and environmental applications, we investigate the plasmonic properties of a rectangular gold nanodisk array in water to boost surface enhanced Raman scattering (SERS) effects. To control the resonance wavelengths of the surface plasmon polariton and the localized surface plasmon, their dependence on the array period and diameter in water is studied in detail using a finite difference time domain method. A good agreement is obtained between calculated resonant wavelengths and those of gold nanodisk arrays fabricated using electron beam lithography. For the optimized structure, a SERS enhancement factor of 7.8 × 107 is achieved in water experimentally.

An enhanced LSPR fiber-optic nanoprobe for ultrasensitive detection of protein biomarkers

A miniaturized, localized surface plasmon resonance (LSPR)-coupled fiber-optic (FO) nanoprobe is reported as a biosensor that is capable of label-free, sensitive detection of a cancer protein biomarker, free prostate specific antigen (f-PSA). The biosensor is based on the LSPR at the reusable dielectric-metallic hybrid interface with a robust, gold nano-disk array at the fiber end facet that is directly fabricated using EBL and metal lift-off process. The f-PSA has been detected with a mouse anti-human PSA monoclonal antibody (mAb) as a specific receptor linked with a self-assembled monolayer at the LSPR-FO facet surfaces. Experimental investigation and data analysis found near field refractive index (RI) sensitivity at ~226nm/RIU with current LSPR-FO nanoprobe, and demonstrated the lowest limit of detection (LOD) at 100fg/mL (~3fM) of f-PSA in PBS solutions. The control experimentation using 5mg/mL bovine serum albumin in PBS and nonspecific surface test shows the excellent specificity and selectivity in the detection of f-PSA in PBS. These results present important progress towards a miniaturized, multifunctional fiber-optic technology that integrates informational communication and sensing function for developing a high performance, label-free, point-of-care (POC) device.

Enhancing DNA binding rate using optical trapping of high-density gold nanodisks

We present the dynamic study of optical trapping of fluorescent molecules using high-density gold nanodisk arrays. The gold nanodisks were fabricated by electron beam lithography with a diameter of 500 nm and a period of 1 μm. Dark-field illumination showed ∼15 times enhancement of fluorescence near edges of nanodisks. Such enhanced near-field generated an optical trapping force of ∼10 fN under 3.58 × 103 W/m2 illumination intensity as calculated from the Brownian motions of 590 nm polystyrene beads. Kinetic observation of thiolated DNA modified with Cy5 dye showed different binding rates of DNA under different illumination intensity. The binding rate increased from 2.14 × 103 s−1 (I = 0.7 × 103 W/m2) to 1.15 × 105 s−1 (I = 3.58 × 103 W/m2). Both enhanced fluorescence and binding rate indicate that gold nanodisks efficiently improve both detection limit and interaction time for microarrays.

Direct imaging of the near field and dynamics of surface plasmon resonance on gold nanostructures using photoemission electron microscopy

Localized surface plasmon resonance (LSPR) can be supported by metallic nanoparticles and engineered nanostructures. An understanding of the spatially resolved near-field properties and dynamics of LSPR is important, but remains experimentally challenging. We report experimental studies toward this aim using photoemission electron microscopy (PEEM) with high spatial resolution of sub-10 nm. Various engineered gold nanostructure arrays (such as rods, nanodisk-like particles and dimers) are investigated via PEEM using near-infrared (NIR) femtosecond laser pulses as the excitation source. When the LSPR wavelengths overlap the spectrum of the femtosecond pulses, the LSPR is efficiently excited and promotes multiphoton photoemission, which is correlated with the local intensity of the metallic nanoparticles in the near field. Thus, the local field distribution of the LSPR on different Au nanostructures can be directly explored and discussed using the PEEM images. In addition, the dynamics of the LSPR is studied by combining interferometric time-resolved pump-probe technique and PEEM. Detailed information on the oscillation and dephasing of the LSPR field can be obtained. The results identify PEEM as a powerful tool for accessing the near-field mapping and dynamic properties of plasmonic nanostructures. A novel form of microscopy has been used to map the field dynamics of surface plasmon resonances. Hiroaki Misawa and co-workers in Japan used high-resolution (10 nm) multiphoton photoemission electron microscopy (MP-PEEM) to image the field distribution of a variety of metallic nanostructures, such as arrays of gold rods, nanodisks and dimer pairs. MP-PEEM works by recording the electrons emitted from a sample in response to illumination with a series of ultrashort pulses. Illumination with near-infrared pulses caused excited plasmon resonances in the gold nanostructures to show up in the resulting MP-PEEM images as local hot spots, from which the field distribution was determined. The short duration of the pulses made it possible to probe the dynamics of the plasmon resonance field distribution on the femtosecond timescale.

Optical properties of a two-dimensional nanodisk array with super-lattice defects

Surface plasmon resonance modes in a two-dimensional gold nanodisk array incorporated with super-lattice defects are investigated in this paper. Both transmission and absorption spectra of the super-lattice metal nanodisk array are calculated. The transmission spectrum exhibits an asymmetric Fano resonance lineshape and a narrow linewidth peak between two transmission dips. The electric field and polarization charge distributions at the transmission peak and dip wavelengths are also calculated. It is explained that the surface plasmon resonance modes associated with the large period nanodisks and small period nanodisks give rise to the Fano resonance lineshape and the anomalous transmission peak. We also retrieve the effective optical constants of the metal nanodisk array film. The effective optical constants exhibit strong chromatic dispersion and a small attenuation at the peak transmission wavelength. The strong dispersion and small attenuation can potentially be used for slow light.