Physical modeling of interference enhanced imaging and characterization of single nanoparticles.

Interferometric imaging schemes have gained significant interest due to their superior sensitivity over imaging techniques that are solely based on scattered signal. In this study, we outline the theoretical foundations of imaging and characterization of single nanoparticles in an interferometric microscopy scheme, examine key parameters that influence the signal, and benchmark the model against experimental findings.

Engineered absorption enhancement and induced transparency in coupled molecular and plasmonic resonator systems.

Coupled plasmonic resonators have become the subject of significant research interest in recent years as they provide a route to dramatically enhanced light-matter interactions. Often, the design of these coupled mode systems draws intuition and inspiration from analogies to atomic and molecular physics systems. In particular, they have been shown to mimic quantum interference effects, such as electromagnetically induced transparency (EIT) and Fano resonances. This analogy also been used to describe the surface-enhanced absorption effect where a plasmonic resonance is coupled to a weak molecular resonance. These important phenomena are typically described using simple driven harmonic (or linear) oscillators (i.e., mass-on-a-spring) coupled to each other. In this work, we demonstrate the importance of an essential interdependence between the rate at which the system can be driven by an external field and its damping rate through radiative loss. This link is required in systems exhibiting time-reversal symmetry and energy conservation. Not only does it ensure an accurate and physically consistent description of resonant systems but leads directly to interesting new effects. Significantly, we demonstrate this dependence to predict a transition between EIT and electromagnetically induced absorption that is solely a function of the ratio of the radiative to intrinsic loss rates in coupled resonator systems. Leveraging the temporal coupled mode theory, we introduce a unique and intuitive picture that accurately describes these effects in coupled plasmonic/molecular and fully plasmonic systems. We demonstrate our approach's key features and advantages analytically as well as experimentally through surface-enhanced absorption spectroscopy and plasmonic metamaterial applications.

Reusable nanostencils for creating multiple biofunctional molecular nanopatterns on polymer substrate.

In this paper, we demonstrate a novel method for high throughput patterning of bioprobes with nanoscale features on biocompatible polymer substrate. Our technique, based on nanostencil lithography, employs high resolution and robust masks integrated with array of reservoirs. We show that the smallest pattern size can reach down to 100 nm. We also show that different types of biomolecules can be patterned on the same substrate simultaneously. Furthermore, the stencil can be reused multiple times to generate a series of identical patterns at low cost. Finally, we demonstrate that biomolecules can be covalently patterned on the surface while retaining their biofunctionalities. By offering the flexibility on the nanopattern design and enabling the reusability of the stencil, our approach significantly simplifies the bionanopatterning process and therefore could have profound implications in diverse biological and medical applications.

Fano-resonant asymmetric metamaterials for ultrasensitive spectroscopy and identification of molecular monolayers.

Engineered optical metamaterials present a unique platform for biosensing applications owing to their ability to confine light to nanoscale regions and to their spectral selectivity. Infrared plasmonic metamaterials are especially attractive because their resonant response can be accurately tuned to that of the vibrational modes of the target biomolecules. Here we introduce an infrared plasmonic surface based on a Fano-resonant asymmetric metamaterial exhibiting sharp resonances caused by the interference between subradiant and superradiant plasmonic resonances. Owing to the metamaterial's asymmetry, the frequency of the subradiant resonance can be precisely determined and matched to the molecule's vibrational fingerprints. A multipixel array of Fano-resonant asymmetric metamaterials is used as a platform for multispectral biosensing of nanometre-scale monolayers of recognition proteins and their surface orientation, as well as for detecting chemical binding of target antibodies to recognition proteins.

Rational design and optimization of plasmonic nanoarrays for surface enhanced infrared spectroscopy.

We present an approach for rational design and optimization of plasmonic arrays for ultrasensitive surface enhanced infrared absorption (SEIRA) spectroscopy of specific protein analytes. Motivated by our previous work that demonstrated sub-attomole detection of surface-bound silk fibroin [Proc. Natl. Acad. Sci. U.S.A. 106, 19227 (2009)], we introduce here a general framework that allows for the numerical optimization of metamaterial sensor designs in order to maximize the absorbance signal. A critical feature of our method is the explicit compensation for the perturbative effects of the analyte's refractive index which alters the resonance frequency and line-shape of the metamaterial response, thereby leading to spectral distortion in SEIRA signatures. As an example, we leverage our method to optimize the geometry of periodic arrays of plasmonic nanoparticles on both Si and CaF2 substrates. The optimal geometries result in a three-order of magnitude absorbance enhancement compared to an unstructured Au layer, with the CaF2 substrate offering an additional factor of three enhancement in absorbance over a traditional Si substrate. The latter improvement arises from increase of near-field intensity over the Au nanobar surface for the lower index substrate. Finally, we perform sensitivity analysis for our optimized arrays to predict the effects of fabrication imperfections. We find that <20% deviation from the optimized absorbance response is readily achievable over large areas with modern nanofabrication techniques.

Ultra-sensitive vibrational spectroscopy of protein monolayers with plasmonic nanoantenna arrays.

Infrared absorption spectroscopy enabling direct access to vibrational fingerprints of the molecular structure is a powerful method for functional studies of bio-molecules. Although the intrinsic absorption cross-sections of IR active modes of proteins are nearly 10 orders of magnitude larger than the corresponding Raman cross-sections, they are still small compared to that of fluorescence-label based methods. Here, we developed a new tool based on collective excitation of plasmonic nanoantenna arrays and demonstrated direct detection of vibrational signatures of single protein monolayers. We first tailored the geometry of individual nanoantennas to form resonant structures that match the molecular vibrational modes. The tailored nanoantennas are then arranged in such a way that their in-phase dipolar coupling leads to a collective excitation of the ensemble with strongly enhanced near fields. The combined collective and individual plasmonic responses of the antenna array play a critical role in attaining signal enhancement factors of 10(4)-10(5). We achieved measurement of the vibrational spectra of proteins at zeptomole levels for the entire array, corresponding to only 145 molecules per antenna. The near-field nature of the plasmonic enhancement of the absorption signals is demonstrated with progressive loading of the nanoantennas with varying protein film thicknesses. Finally, an advanced model based on nonequilibrium Green's function formalism is introduced, which explains the observed Fano-type absorption line-shapes and tuning of the absorption strengths with the antenna resonance.

On chip plasmonic monopole nano-antennas and circuits.

Analogues of many radio frequency (RF) antenna designs such as the half-wave dipole and Yagi-Uda have been successfully adapted to the optical frequency regime, opening the door for important advances in biosensing, photodetection, and emitter control. Examples of monopole antennas, however, are conspicuously rare given the element's extensive use in RF applications. Monopole antennas are attractive as they represent an easy to engineer, compact geometry and are well isolated from interference due the ground plane. Typically, however, the need to orient the antenna element perpendicular to a semi-infinite ground plane requires a three-dimensional structure and is incompatible with chip-based fabrication techniques. We propose and demonstrate here for the first time that monopole antenna elements can be fashioned out of single element nanoparticles fabricated in conventional planar geometries by using a small nanorod as a wire reflector. The structure offers a compact geometry and the reflector element provides a measure of isolation analogous to the RF counterpart. This isolation persists in the conductive coupling regime, allowing multiple monopoles to be combined into a single nanoparticle, yet still operate independently. This contrasts with several previous studies that observed dramatic variations in the spectral response of conductively coupled particles. We are able to account for these effects by modeling the system using circuit equations from standard RF antenna theory. Our model accurately describes this behavior as well as the detailed resonance tuning of the structure. As a specific practical application, the monopole resonances are precisely tuned to desired protein absorption bands, thereby enhancing their spectroscopic signatures. Furthermore, the accurate modeling of conductive coupling and demonstrated electronic isolation should be of general interest to the design of complex plasmonic circuits incorporating multiple antennas and other current carrying elements.

Angle-and polarization-dependent collective excitation of plasmonic nanoarrays for surface enhanced infrared spectroscopy.

Our recent work has showed that diffractively coupled nanoplasmonic arrays for Fourier transform infrared (FTIR) microspectroscopy can enhance the Amide I protein vibrational stretch by up to 10(5) times as compared to plain substrates. In this work we consider computationally the impact of a microscope objective illumination cone on array performance. We derive an approach for computing angular- and spatially-averaged reflectance for various numerical aperture (NA) objectives. We then use this approach to show that arrays that are perfectly optimized for normal incidence undergo significant response degradation even at modest NAs, whereas arrays that are slightly detuned from the perfect grating condition at normal incidence irradiation exhibit only a slight drop in performance when analyzed with a microscope objective. Our simulation results are in good agreement with microscope measurements of experimentally optimized periodic nanoplasmonic arrays.

High-throughput nanofabrication of infrared plasmonic nanoantenna arrays for vibrational nanospectroscopy.

The introduction of high-throughput and high-resolution nanofabrication techniques operating at low cost and low complexity is essential for the advancement of nanoplasmonic and nanophotonic fields. In this paper, we demonstrate a novel fabrication approach based on nanostencil lithography for high-throughput fabrication of engineered infrared plasmonic nanorod antenna arrays. The technique relying on deposition of materials through a shadow mask enables plasmonic substrates supporting spectrally sharp collective resonances. We show that reflectance spectra of these antenna arrays are comparable to that of arrays fabricated by electron beam lithography. We also show that nanostencils can be reused multiple times to fabricate a series of infrared nanoantenna arrays with identical optical responses. Finally, we demonstrate fabrication of plasmonic nanostructures in a variety of shapes with a single metal deposition step on different substrates, including nonconducting ones. Our approach, by enabling the reusability of the stencil and offering flexibility on the substrate choice and nanopattern design, could facilitate the transition of plasmonic technologies to the real-world applications.

Radiative engineering of plasmon lifetimes in embedded nanoantenna arrays.

It is generally accepted that the lifetimes of the localized plasmonic excitations are inherently controlled by the type of the metals and the shape of the nanoparticles. However, extended plasmonic lifetimes and enhanced near-fields in nanoparticle arrays can be achieved as a result of collective excitation of plasmons. In this article, we demonstrate significantly longer plasmon lifetimes and stronger near-field enhancements by embedding the nanoantenna arrays into the substrate. Our approach offers a more homogeneous dielectric background allowing stronger diffractive couplings among plasmonic particles leading to strong suppression of the radiative damping. We observe near-field enhancements well beyond than those achievable with isolated nanoparticles. Enhanced fields obtained in these structures could be attractive for biosensing and non-linear photonics applications.

Design principles for optoelectronic applications of extraordinary light transmission effect in plasmonics nanoapertures.

The extraordinary light transmission effect (EOT) through sub-wavelength nanoapertures in opaque metal films has lead to observation of a wide variety of exciting new optical phenomena. This remarkable effect is generally related to the interaction of the light with the extended plasmonic resonances on the surface of the metal film and localized surface plasmons in the apertures. On the other hand, there is little conceptual understanding for controlling the localized surface plasmonic behavior of the individual apertures and their coupling to the extended surface plasmons. In this letter, we present an intuitive and straightforward picture of the extra-ordinary light transmission phenomena based on basic antenna principles for plasmonic excitations and coupling of these plasmonic excitations in complex nano-apertures. Our quasi-static model remarkably well explains our experimental measurements in shape anisotropic structures with unique properties controlled by adjusting the size and the geometry of the apertures. Our approach puts forward new design principles for potential applications ranging from subwavelength optoelectronics and data storage to bio/chemical sensing.

Hybridized nanocavities as single-polarized plasmonic antennas.

We experimentally demonstrate that hybridized nanocavities in optically thick metal films radiate in coherence, and act as an efficient single-polarized plasmonic nano-antenna array. We employ propagating and localized plasmons to enhance polarization control along one axis, with total suppression of the perpendicular polarization component. The relationship between the near-field and far-field radiation is established through a quasi-static model connecting the individual nano-antenna behavior to the phenomenon of extraordinary light transmission. Hybridized nanocavity antennas, with length scales below the conventional diffraction limit, present opportunities for potential applications in photovoltaics, optoelectronic devices and optical sensors.

Control of 2D plasmon-polariton mode with dielectric nanolayers.

We introduce two nanoscale thickness dielectric layers on the top and bottom sides of a finite width and finite thickness metal microstrip and have calculated the fundamental symmetric plasmon-polariton mode of the 2D metal-dielectric layer waveguide. The dielectric nanolayers provide an additional degree of freedom to control the plasmon-polariton mode. When the dielectric constant of the nanolayers is smaller than that of the cladding dielectric, the travel range of the fundamental symmetric plasmon-polariton mode is extended with the trade-off of the mode confinement. The figure of merit of the mode increases as the travel range extends.

Nanoimaging and Control of Molecular Vibrations through Electromagnetically Induced Scattering Reaching the Strong Coupling Regime.

Optical resonators can enhance light-matter interaction, modify intrinsic molecular properties such as radiative emission rates, and create new molecule-photon hybrid quantum states. To date, corresponding implementations are based on electronic transitions in the visible spectral region with large transition dipoles yet hampered by fast femtosecond electronic dephasing. In contrast, coupling molecular vibrations with their weaker dipoles to infrared optical resonators has been less explored, despite long-lived coherences with 2 orders of magnitude longer dephasing times. Here, we achieve excitation of molecular vibrations through configurable optical interactions of a nanotip with an infrared resonant nanowire that supports tunable bright and nonradiative dark modes. The resulting antenna-vibrational coupling up to 47 ± 5 cm-1 exceeds the intrinsic dephasing rate of the molecular vibration, leading to hybridization and mode splitting. We observe nanotip-induced quantum interference of vibrational excitation pathways in spectroscopic nanoimaging, which we model classically as plasmonic electromagnetically induced scattering as the phase-controlled extension of the classical analogue of electromagnetically induced transparency and absorption. Our results present a new regime of IR spectroscopy for applications of vibrational coherence from quantum computing to optical control of chemical reactions.

Characteristics of ultra-long range surface plasmon waves at optical frequencies.

It has been reported earlier that ultra-long range surface plasmon waves can be supported if dielectric layers with lower index of refraction than that of the dielectric cladding are placed on either side of the thin metal film. In this paper, we report a further investigation of the ultra-long range surface plasmon modes and the condition to support such ultra-long propagation distances at optical frequencies. We studied the effects of the index of refraction contrast between the inner layer and the cladding dielectrics, the metal film thickness, and the dispersion with wavelength. We present a condition which must be satisfied by the waveguide structure to support the bound ultra-long range surface plasmon mode.

Extended long range plasmon waves in finite thickness metal film and layered dielectric materials.

In this paper, we show that the propagation distance of the long range plasmon wave mode guided by a finite thickness gold metal film can be extended several orders of magnitude longer if we place intermediate dielectric layers on both sides of the metal film and choose the layer thickness properly. The propagation distance goes to infinite if the intermediate layer thickness approaches a critical thickness.

In-situ ultra-sensitive infrared absorption spectroscopy of biomolecule interactions in real time with plasmonic nanoantennas.

Infrared absorption spectroscopy is a powerful biochemical analysis tool as it extracts detailed molecular structural information in a label-free fashion. Its molecular specificity renders the technique sensitive to the subtle conformational changes exhibited by proteins in response to a variety of stimuli. Yet, sensitivity limitations and the extremely strong absorption bands of liquid water severely limit infrared spectroscopy in performing kinetic measurements in biomolecules' native, aqueous environments. Here we demonstrate a plasmonic chip-based technology that overcomes these challenges, enabling the in-situ monitoring of protein and nanoparticle interactions at high sensitivity in real time, even allowing the observation of minute volumes of water displacement during binding events. Our approach leverages the plasmonic enhancement of absorption bands in conjunction with a non-classical form of internal reflection. These features not only expand the reach of infrared spectroscopy to a new class of biological interactions but also additionally enable a unique chip-based technology.

Dual-band perfect absorber for multispectral plasmon-enhanced infrared spectroscopy.

Metamaterial-based perfect absorbers utilize intrinsic loss, with the aid of appropriate structural design, to achieve near unity absorption at a certain wavelength. For most of the reported absorbers, the absorption occurs only at a single wavelength where plasmon resonances are excited in the nanostructures. Here we introduce a dual-band perfect absorber based on a gold nanocross structure. Two bands of maximum absorption of 94% are experimentally accomplished by breaking the symmetry of the cross structure. Furthermore, we demonstrate the two bands can be readily tuned throughout the mid-infrared with their associated resonances giving rise to large near-field enhancements. These features are ideal for multiband surface-enhanced infrared spectroscopy applications. We experimentally demonstrate this application by simultaneously detecting two molecular vibrational modes of a 4 nm thick polymer film utilizing our proposed absorber. Furthermore, in response to variations in the interaction strength between the plasmonic and molecular dipoles, we observe an anticrossing behavior and modification in the spectral line shape of the molecular absorption peak, which are characteristic of the coupling between the two modes.

Single-shot subpixel response measurement with an aperture array pixel mask.

We first point out that the subpixel response function is another kernel function in digital imaging. Then we show that the subpixel response function of CMOS imaging sensor pixels can be measured with an aperture array pixel mask in a single-shot image capture. Our technique permits high-resolution subpixel response function measurement of imaging pixels for superresolution imaging applications.