Subwavelength Metallic Nanostructures Research Articles

Scalable, Green Fabrication of Single-Crystal Noble Metal Films and Nanostructures for Low-Loss Nanotechnology Applications.

The confinement of spatially extended electromagnetic waves to nanometer-scale metal structures can be harnessed for application in information processing, energy harvesting, sensing, and catalysis. Metal nanostructures enable negative refractive index, subwavelength resolution imaging, and patterning through engineered metamaterials and promise technologies that will operate in the quantum plasmonics regime. However, the controlled fabrication of high-definition single-crystal subwavelength metal nanostructures has remained a significant hurdle due to the tendency for polycrystalline metal growth using conventional physical vapor deposition methods and the challenges associated with placing solution-grown nanocrystals in desired orientations and locations on a surface to manufacture functional devices. Here, we introduce a scalable and green wet chemical approach to monocrystalline noble metal thin films and nanostructures. The method enables the fabrication of ultrasmooth, epitaxial, single-crystal films of controllable thickness that are ideal for the subtractive manufacture of nanostructures through ion beam milling and additive crystalline nanostructure via lithographic patterning for large-area, single-crystal metasurfaces and high aspect ratio nanowires. Our single-crystal nanostructures demonstrate improved feature quality, pattern transfer yield, reduced optical and resistive losses, and tailored local fields to yield greater optical response and improved stability compared to those of polycrystalline structures-supporting greater local field enhancements and enabling practical advances at the nanoscale.

Multiphoton Absorption and Graphitization in Poly(methyl methacrylate)-Coated Aluminum Nanoantenna Arrays

Collective electron oscillations, termed surface plasmons, in subwavelength metallic nanostructures can be excited by light, giving rise to an enhanced optical near-field at the surface of the meta...

THz Near-Field Imaging of Extreme Subwavelength Metal Structures

Modern scattering-type scanning near-field optical microscopy (s-SNOM) has become an indispensable tool in material research. However, as the s-SNOM technique marches into the far-infrared (IR) and terahertz (THz) regimes, emerging experiments sometimes produce puzzling results. For example, anomalies in the near-field optical contrast have been widely reported. In this Letter, we systematically investigate a series of extreme subwavelength metallic nanostructures via s-SNOM near-field imaging in the GHz to THz frequency range. We find that the near-field material contrast is greatly impacted by the lateral size of the nanostructure, while the spatial resolution is practically independent of it. The contrast is also strongly affected by the connectivity of the metallic structures to a larger metallic ground plane. The observed effect can be largely explained by a quasi-electrostatic analysis. We also compare the THz s-SNOM results to those of the mid-IR regime, where the size-dependence becomes significant only for smaller structures. Our results reveal that the quantitative analysis of the near-field optical material contrasts in the long-wavelength regime requires a careful assessment of the size and configuration of metallic (optically conductive) structures.

Multiwavelength achromatic microlens through phase compensation based on the subwavelength metallic nanostructures

Chromatic aberration has been a functional distortion for the emerging microlens owing to phase mismatch, thus an extensive challenge is to achieve achromatic microlens over a broad wavelength range because it is difficult to achieve engineer simultaneous phase profiles at distinct wavelengths. For practical applications, a further challenge is to create an achromatic microlens with uncomplicated structure that can work in the transmission mode for incident light waves. In this work, we design a special and uncomplicated structure to achieve achromatic focusing based on the metallic silver subwavelength nanostructures, grating bars in our work, the structure can be regarded as a metal–insulator–metal (MIM) waveguide via exciting the surface plasmon polaritons (SPPs) mode to produce the phase retardation that could compensate and correct phase, thus we can obtain an achromatic microlens with uncomplicated structure over the whole visible, which do not need excessive requirement for the fabrication technology. The low-cost and manufacture compatibility make our achromatic microlens suitable for applications in nanophotonics and integrated optics in the visible.

Fully Integrated Fluorescence Biosensors On-Chip Employing Multi-Functional Nanoplasmonic Optical Structures in CMOS

Integrated optical system-on-chip in silicon operating in the visible range can have a tremendous impact on enabling new applications in sensing and imaging through the ultra-miniaturization of complex optical instrumentation. CMOS technology has allowed an integration of optical detection circuitry for image sensors with massively large number of pixels. In this paper, we focus on techniques to realize complex optical-field processing elements inside CMOS by exploiting optical interaction with sub-wavelength metal nanostructures realized in the electrical interconnects layers, whose feature sizes are now in the sub-100-nm range. In particular, we present a fully integrated fluorescence-based bio-molecular sensor in 65-nm CMOS with integrated nanoplasmonic waveguide-based filters capable of more than 50 dB of rejection ratio across a wide range of incident angles. Co-designed with the integrated photo-detection circuitry, capacitive TIAs, and correlated double-sampling circuitry, the sensor is capable of detecting 48 zeptomoles of quantum dots on the surface with 52 fA of photodetector current with a fluorescence/excitation ratio of nearly −62 dB without any post-fabrication, external optical filters, lenses, or collimators. The ability to integrate complex nanoplasmonic metal structures with unique optical properties in CMOS with no post-processing creates the opportunity to enable large multiplexed assays on a single chip and a wide variety of applications, from in vitro to in vivo .

Experimental measurement of plasmonic nanostructures embedded in silicon waveguide gaps.

In this work, we report numerical simulations and experiments of the optical response of a gold nanostrip embedded in a silicon strip waveguide gap at telecom wavelengths. We show that the spectral features observed in transmission and reflection when the metallic nanostructure is inserted in the gap are extremely different than those observed in free-space excitation. First, we find that interference between the guided field and the electric dipolar resonance of the metallic nanostructure results in high-contrast (> 10) spectral features showing an asymmetric Fano spectral profile. Secondly, we reveal a crossing in the transmission and reflection responses close to the nanostructure resonance wavelength as a key feature of our system. This approach, which can be realized using standard semiconductor nanofabrication tools, could lead to a full exploitation of the extreme properties of subwavelength metallic nanostructures in an on-chip configuration, with special relevance in fields such as biosensing or optical switching.

One-step, subwavelength patterning of plasmonic gratings in metal–polymer composites

Two photon patterning of sub-wavelength resolved metallic nanostructures using single photon absorbing photo initiator.

Fano Interference between Localized Plasmons and Interface Reflections

Layers of subwavelength metal nanostructures that support localized surface plasmon resonances are of broad interest in applied nanotechnology, for example, in optical sensor development and solar energy harvesting devices. We measured specular reflection spectra as a function of incidence angle for two-dimensional layers of gold nanodisks on glass and found highly asymmetric line-shapes and a spectral red-shift of up to 0.2 eV, or 10% of the plasmon resonance energy, as the angle changed from normal toward grazing incidence. This dramatic angular dispersion is the result of a tunable Fano interference between the spectrally narrow plasmon emission and a "white" continuum caused by the interface reflection. The data are found to be in excellent agreement with predictions based on a theory for Fresnel reflection coefficients of an interface with subwavelength inclusions. The theory can also be used to derive analytical expressions for the Fano parameters.

Highly Directional Emission and Photon Beaming from Nanocrystal Quantum Dots Embedded in Metallic Nanoslit Arrays

We demonstrate a directional beaming of photons emitted from nanocrystal quantum dots that are embedded in a subwavelength metallic nanoslit array with a divergence angle of less than 4°. We show that the eigenmodes of the structure result in localized electromagnetic field enhancements at the Bragg cavity resonances, which could be controlled and engineered in both real and momentum space. The photon beaming is achieved using the enhanced resonant coupling of the quantum dots to these Bragg cavity modes, which dominates the emission properties of the quantum dots. We show that the emission probability of a quantum dot into the narrow angular mode is 20 times larger than the emission probability to all other modes. Engineering nanocrystal quantum dots with subwavelength metallic nanostructures is a promising way for a range of new types of active optical devices, where spatial control of the optical properties of nanoemitters is essential, on both the single and many photons level.

Enhancement of two photon processes in quantum dots embedded in subwavelength metallic gratings

We show a large enhancement of two-photon absorption processes in nanocrystal quantum dots and of light upconversion efficiency from the IR to the near-IR spectral regime, using a hybrid optical device in which near-IR emitting InAs quantum dots were embedded on top a metallic nanoslit array. The resonant enhancement of these nonlinear optical processes is due to the strong local electromagnetic field enhancements inside the nanoslit array structure at the extraordinary transmission resonances. A maximal two-photon absorption enhancement of more than 20 was inferred. Different high field regions were identified for different polarizations, which can be used for designing and optimizing efficient nonlinear processes in such hybrid structures. Combining nanocrystal quantum dots with subwavelength metallic nanostructures is therefore a promising way for a range of possible nonlinear optical devices.

Influence of hole geometry and lattice constant on extraordinary optical transmission through subwavelength hole arrays in metal films

We fabricate a series of square-lattice subwavelength circular, rectangular, and trapezoidal air-hole arrays drilled in opaque gold thin films and measured the transmission spectra of light passing through these metallic nanostructures in the near-infrared range. The measured results show strong dependence of extraordinary optical transmission on the lattice constant, hole size, and hole shape. The wavelengths of the transmission peaks and dips are mainly determined by the lattice constant. However, they are also influenced by the parameters of the hole size and hole shape. The experimental data agree well with numerical calculation results by means of a plane-wave transfer-matrix method when the detailed geometry of the air holes is fully considered. The results indicate that the extraordinary light transmission through subwavelength metallic nanostructures of air-hole arrays are governed by excitation of surface plasmon polaritons on the metal surface and their scattering by periodic air-hole arrays.

Functional Nanostructured Plasmonic Materials

Plasmonic crystals fabricated with precisely controlled arrays of subwavelength metal nanostructures provide a promising platform for sensing and imaging of surface binding events with micrometer spatial resolution over large areas. Soft nanoimprint lithography provides a robust, cost-effective method for producing highly uniform plasmonic crystals of this type with predictable optical properties. The tunable multimode plasmonic resonances of these crystals and their ability for integration into lab-on-a-chip microfluidic systems can both be harnessed to achieve exceptionally high analytical sensitivities down to submonolayer levels using even a common optical microscope, circumventing numerous technical limitations of more conventional surface plasmon resonance techniques. In this article, we highlight some recent advances in this field with an emphasis on the fabrication and characterization of these integrated devices and their demonstrated applications.

Effect of target localization on the sensitivity of a localized surface plasmon resonance biosensor based on subwavelength metallic nanostructures

A localized surface plasmon resonance (LSPR) biosensor using surface relief nanostructures was investigated to evaluate the importance of target localization on the sensitivity enhancement. The LSPR device was modeled as periodic metallic nanowires with a square profile on a gold film and the target as a self-assembled monolayer in buffer solution. The numerical results using rigorous coupled-wave analysis and the finite-difference time domain method demonstrated localized plasmonic fields induced by the surface nanostructure from which the effect of target localization on the sensitivity was quantitatively analyzed. Interestingly, it was found that target localization on nanowire sidewalls improves sensitivity significantly because of strong overlap with localized plasmonic fields. An LSPR structure optimized for a localized target on sidewalls provides sensitivity enhancement per unit target volume by more than 20 times in water ambience.

Optical Forces on a Quantum Dot in Metallic Bowtie Structures

We present a numerical scheme to calculate the electromagnetic force on a quantum dot in the near-field of subwavelength metallic nano-structures. As an application, we examine the forces on a quantum dot placed in the center of a gold nano-structure. The forces on the quantum dot are calculated using an adaptive mesh refinement finite-difference time-domain code combined with microscopic material equations. Is is shown that the dot may be laterally confined in the proposed geometry.

Quantitative multispectral biosensing and 1D imaging using quasi-3D plasmonic crystals.

We developed a class of quasi-3D plasmonic crystal that consists of multilayered, regular arrays of subwavelength metal nanostructures. The complex, highly sensitive structure of the optical transmission spectra of these crystals makes them especially well suited for sensing applications. Coupled with quantitative electrodynamics modeling of their optical response, they enable full multiwavelength spectroscopic detection of molecular binding events with sensitivities that correspond to small fractions of a monolayer. The high degree of spatial uniformity of the crystals, formed by a soft nanoimprint technique, provides the ability to image binding events over large areas with micrometer spatial resolution. These features, together with compact form factors, low-cost fabrication procedures, simple readout apparatus, and ability for direct integration into microfluidic networks and arrays, suggest promise for these devices in label-free bioanalytical detection systems.