High-harmonic generation by resonant plasmon field enhancement

Light funneling mechanism explained by magnetoelectric interference

We investigate the mechanisms involved in the funneling of optical energy into subwavelength grooves etched on a metallic surface. The key phenomenon is unveiled thanks to the decomposition of the electromagnetic field into its propagative and evanescent parts. We unambiguously show that the funneling is not due to plasmonic waves flowing toward the grooves, but rather to the magnetoelectric interference of the incident wave with the evanescent field, this field being mainly due to the resonant wave escaping from the groove.

Nanoscale plasmonic contour bowtie antenna operating in the mid-infrared

A plasmonic antenna design is proposed and investigated numerically over a large parameter space. By considering the contour of a bowtie antenna and introducing an additional design parameter, the contour thickness, it is demonstrated that the resonant wavelength of the antenna may be tuned over a broad spectral range while maintaining a constant antenna footprint. These new antennas allow for a factor of 3.6 reduction in the antenna footprint and an increase in the gap enhancement by 28%.

Universal scaling and Fano resonance in the plasmon coupling between gold nanorods

The plasmon coupling between metal nanocrystals can lead to large plasmon shifts, enormous electric field enhancements, and new plasmon modes. Metal nanorods, unlike spherical ones, possess a transverse and a longitudinal plasmon mode owing to their geometrical anisotropy. Consequently, the plasmon coupling between metal nanorods is much more complicated than that between nanospheres. For the latter, experimental approaches, simple scaling relationships, and exact analytic solutions have been developed for describing the plasmon coupling. In this study, we have carried out extensive finite-difference time-domain simulations to understand the plasmon coupling in the dimers of Au nanorods that are aligned along their length axes. The effects of the gap distance, longitudinal plasmon energy, and end shape of the nanorod monomers on the plasmon coupling have been scrutinized. The coupling energy diagrams show a general anticrossing behavior. All of them can be rescaled into one simple and universal hyperbolic formula. A theoretical model based on two interacting mechanical oscillators has been developed to understand the plasmon coupling between two arbitrarily varying Au nanorods. This model, together with the universal equation, allows for the determination of the coupled plasmon energies of Au nanorod dimers with high accuracies. Furthermore, the Fano interference has been observed in the nanorod heterodimers, with its behavior being dependent on the gap distance and plasmon energies of the nanorod monomers. Our results will be useful for predicting the coupled plasmon energies of metal nanorod dimers in a variety of plasmonic applications and understanding the Fano resonance in plasmonic nanostructures.

Scalable manufacturing of plasmonic nanodisk dimers and cusp nanostructures using salting-out quenching method and colloidal lithography

Localization of large electric fields in plasmonic nanostructures enables various processes such as single-molecule detection, higher harmonic light generation, and control of molecular fluorescence and absorption. High-throughput, simple nanofabrication techniques are essential for implementing plasmonic nanostructures with large electric fields for practical applications. In this article we demonstrate a scalable, rapid, and inexpensive fabrication method based on the salting-out quenching technique and colloidal lithography for the fabrication of two types of nanostructures with large electric field: nanodisk dimers and cusp nanostructures. Our technique relies on fabricating polystyrene doublets from single beads by controlled aggregation and later using them as soft masks to fabricate metal nanodisk dimers and nanocusp structures. Both of these structures have a well-defined geometry for the localization of large electric fields comparable to structures fabricated by conventional nanofabrication techniques. We also show that various parameters in the fabrication process can be adjusted to tune the geometry of the final structures and control their plasmonic properties. With advantages in throughput, cost, and geometric tunability, our fabrication method can be valuable in many applications that require plasmonic nanostructures with large electric fields.

Ultrasmall bullets of light--focusing few-cycle light pulses to the diffraction limit

We demonstrate an essentially dispersion-free and diffraction-limited focusing of few-cycle laser pulses through all-reflective microscope objectives. By transmitting 6-fs-pulses from a Ti:sapphire oscillator through an all-reflective 0.5 NA objective, we reach a focus with a beam diameter of 1.0 µm, preserving the time structure of the pulses. The temporal and spatial pulse profile is recorded simultaneously using a novel tip-enhanced electron emission autocorrelator, indicating a focal volume of these pulses of only 1.8 µm3. We anticipate that the demonstrated technique is of considerable interest for inducing and probing optical nonlinearities of individual nanostructures.

Omnidirectional 3D nanoplasmonic optical antenna array via soft-matter transformation

Inspired by the natural processes during morphogenesis, we demonstrate the transformation capability of active soft-matter to define nanoscale metal-on-polymer architectures below the resolution limit of conventional lithography. Specifically, using active polymers, we fabricate and characterize ultradense nanoplasmonic antenna arrays with sub-10 nm tip-to-tip nanogaps. In addition, the macroscale morphology can be independently manipulated into arbitrary three-dimensional geometries, demonstrated with the fabrication of an omnidirectional nanoplasmonic optical antenna array.

Seeing protein monolayers with naked eye through plasmonic Fano resonances

We introduce an ultrasensitive label-free detection technique based on asymmetric Fano resonances in plasmonic nanoholes with far reaching implications for point-of-care diagnostics. By exploiting extraordinary light transmission phenomena through high-quality factor (Q(solution) ∼ 200) subradiant dark modes, we experimentally demonstrate record high figures of merits (FOMs as high as 162) for intrinsic detection limits surpassing that of the gold standard prism coupled surface-plasmon sensors (Kretschmann configuration). Our experimental record high sensitivities are attributed to the nearly complete suppression of the radiative losses that are made possible by the high structural quality of the fabricated devices as well as the subradiant nature of the resonances. Steep dispersion of the plasmonic Fano resonance profiles in high-quality plasmonic sensors exhibit dramatic light intensity changes to the slightest perturbations within their local environment. As a spectacular demonstration of the extraordinary sensitivity and the quality of the fabricated biosensors, we show direct detection of a single monolayer of biomolecules with naked eye using these Fano resonances and the associated Wood's anomalies. To fabricate high optical-quality sensors, we introduce a high-throughput lift-off free evaporation fabrication technique with extremely uniform and precisely controlled nanofeatures over large areas, leading to resonance line-widths comparable to that of the ideally uniform structures as confirmed by our time-domain simulations. The demonstrated label-free sensing platform offers unique opportunities for point-of-care diagnostics in resource poor settings by eliminating the need for fluorescent labeling and optical detection instrumentation (camera, spectrometer, etc.) as well as mechanical and light isolation.

Highly plasmon-enhanced upconversion emissions from Au@β-NaYF4:Yb,Tm hybrid nanostructures

Unselectively enhanced multicolour upconversion (UC) emissions and low pumping threshold were achieved in Au@β-NaYF(4):Yb,Tm hybrid nanostructures. This demonstrates that the plasmon field enhancement effect is the main reason for the improved UC emission efficiency.

Resonances on-demand for plasmonic nano-particles

A method for designing plasmonic particles with desired resonance spectra by exploiting the interaction of local geometry with surface charge distribution and applying evolutionary algorithm is presented. The method is based on repetitive perturbations of an initial particle's shape while calculating the eigenvalues of the various quasistatic resonances. Novel family of particles with collocated dipole-quadrupole resonances was designed, as an example for the unique power of the method.

Sierpiński fractal plasmonic antenna: a fractal abstraction of the plasmonic bowtie antenna

A new class of bowtie antennas with Sierpiński fractal features is proposed for sensing molecular vibration modes in the near- to mid-infrared. These antennas offer a compact device footprint and an enhanced confinement factor compared to a bowtie antenna. Through extensive simulations, it is shown that these characteristics are related to the ability of this fractal geometry to become polarized. Simulation results demonstrate that these antennas may be tuned between 700 nm ≤ λ ≤ 3.4 µm and that electric field enhancement by 56 is possible at the center of the antenna gap.

Light focusing by slot Fabry-Perot photonic crystal nanoresonator on scanning tip

We numerically investigate the propagation of light through the photonic crystal (PhC) waveguide on low refraction index material for near-field light focusing at the visible wavelength (635 nm) by incorporating a center air slot and Fabry-Perot resonator on the scanning tip. Perturbations by water and substrate refraction index changes of the PhC are analyzed by the finite-difference time-domain method to show minimal impact on light confinement and throughput. Such a total dielectric probe tip design has great potential to complement the current widely used metal-coated optical-fiber-based light confinement probe.

Plasmonic hybridization between nanowires and a metallic surface: a transformation optics approach

The interaction between metallic nanowires and a metal substrate is investigated by means of transformation optics. This plasmonic system is of particular interest for single molecule detection or nanolasers. By mapping such a plasmonic device onto a metal-insulator-metal infinite structure, its optical response can be fully derived analytically. In this article, the absorption cross-section of a nanowire placed close to a metallic surface is derived within and beyond the quasi-static limit. The system is shown to support several modes characterized by a different angular momentum and whose resonance red-shifts when the nanoparticle approaches the metal substrate. These resonances give rise to a drastic field enhancement (>10(2)) within the narrow gap separating the nanoparticle from the metal surface. The case of a nanowire dimer is also investigated and is closely related to the previous configuration. More physical insights are provided especially with respect to the invisibility dips appearing in the radiative spectrum. Numerical simulations have also been performed to confirm our analytical predictions and determine their range of validity.

Plasmon resonance in individual nanogap electrodes studied using graphene nanoconstrictions as photodetectors

We achieve direct electrical readout of the wavelength and polarization dependence of the plasmon resonance in individual gold nanogap antennas by positioning a graphene nanoconstriction within the gap as a localized photodetector. The polarization sensitivities can be as large as 99%, while the plasmon-induced photocurrent enhancement is 2-100. The plasmon peak frequency, polarization sensitivity, and photocurrent enhancement all vary between devices, indicating the degree to which the plasmon resonance is sensitive to nanometer-scale irregularities.

Multi-gigawatt ultrashort pulses at high repetition rate and average power from two-stage nonlinear compression

We present simple and compact (1.5m x 0.5m footprint) post-compression of a state-of-the-art fiber chirped pulse amplification system. By using two stage nonlinear compression in noble gas filled hollow core fibers we shorten 1 mJ, 480 fs, 50 kHz pulses. The first stage is a 53 cm long, 200 µm inner diameter fiber filled with xenon with subsequent compression in a chirped mirror compressor. A 20 cm, 200 µm inner diameter fiber filled with argon further broadens the spectrum in a second stage and compression is achieved with another set of chirped mirrors. The average power is 24.5 W/19 W after the first/second stage, respectively. Compression to 35 fs is achieved. Numerical simulations, agreeing well with experimental data, yield a peak power of 5.7 GW at a pulse energy of 380 µJ making this an interesting source for high harmonic generation at high repetition rate and average power.

Second harmonic generation based on strong field enhancement in nanostructured THz materials

The THz response of slit structures and split-ring resonators (SRRs) featuring extremely small gaps on the micro- or nanoscale is investigated numerically. Both structures exhibit strong field enhancement in the gap region due to light-induced current flows and capacitive charging across the gap. Whereas nanoslits allow for broadband enhancement the resonant behavior of the SRRs leads to narrowband amplification and results in significantly higher field enhancement factors reaching several 10,000. This property is particularly beneficial for the realization of nonlinear THz experiments which is exemplarily demonstrated by a second harmonic generation process in a nonlinear substrate material. Positioning nanostructures on top of the substrate is found to result in a significant increase of the generation efficiency for the frequency doubled component.

16 fs, 350 nJ pulses at 5 MHz repetition rate delivered by chirped pulse compression in fibers

We demonstrate a simple approach for broadening and compression of intense pulses at megahertz repetition rates by self-phase modulation in nonlinear photonic crystal fibers. In order to avoid damage by self-focusing, we positively chirp the input pulses, which allows coupling of significantly more energy into the fiber, while maintaining the same spectral bandwidth and compression as compared to the Fourier-limited case at lower energy. Using a commercial long-cavity Ti:sapphire oscillator with 55 fs, 400 nJ pulses at 5 MHz, we generate 16 fs, 350 nJ pulses, which is a factor of 4 more energy than possible with unchirped input pulses. Self-phase-modulated spectra supporting 11 fs duration are also shown with 350 nJ pulse energy. Excellent stability is recorded over at least 1 h.

Active control over nanofocusing with nanorod plasmonic antennas

Active control over light nanofocusing in a nanorod plasmonic antenna coupled to a photonic crystal cavity is proposed and demonstrated by means of full-vectorial 3D simulations. By varying the excitation of the cavity with laser beam spot size allows us to achieve a gradual control over light nanofocusing at the tip of the nanoantenna. The demonstrated control mechanism eliminates the need for nonlinear effects or mechanical reconfiguration and represents a step towards the implementation of reliable tunable subwavelength light sources.

Plasmonic oligomers: the role of individual particles in collective behavior

We present a comprehensive experimental study of the optical properties of plasmonic oligomers. We show that both the constitution and configuration of plasmonic oligomers have a large influence on their resonant behavior, which draws a compelling analogy to molecular theory in chemistry. To elucidate the constitution influence, we vary the size of individual nanoparticles and identify the role of the target nanoparticle from the spectral change. To illustrate the configuration influence, we vary the positions and numbers of nanoparticles in a plasmonic oligomer. Additionally, we demonstrate experimentally a large spectral redshift at the transition from displaced nanoparticles to touching ones. The oligomeric design strategy opens up a rich pathway for the implementation of optimized optical properties into complex plasmonic nanostructures for specific applications.

High-resolution mapping of electron-beam-excited plasmon modes in lithographically defined gold nanostructures

We demonstrate the use of high-resolution electron beam lithography to fabricate complex nanocavities with nanometric spatial and positional control. The plasmon modes of these nanostructures are then mapped using electron energy-loss spectroscopy in a scanning transmission electron microsope. This powerful combination of patterning and plasmon mapping provides direct experimental verification to theoretical predictions of plasmon hybridization theory in complex metal nanostructures and allows the determination of the full mode spectrum of such cavities.

Direct Metal Nano-patterning Using Embossed Solid Electrolyte

The recent growth in optoelectronics, nanoelectronics, nanooptics, and chemical and biological sensing has been fueled by the ability to fabricate nanostructures with ever smaller features. However, several significant constraints still remain in terms of cost, limited pattern size, processing conditions, pattern flexibility, and so on. Fabrication of features as small as 50 nm at ambient conditions with high pattern flexibility and low cost remains a serious challenge. Here we report a new solid-state electrochemical imprinting process that is carried out at ambient conditions, requires nominal pressure and very low electric potential, eliminates any liquid electrolytes, shows very high reproducibility, and promises the capability to scale up for large area patterning while retaining a significant cost advantage. Through combination of the best merits of nanoimprint lithography, micro forming, and the solid-state electrochemical imprinting technique, S4, (recently introduced by Hsu et al., Nano Lett., 2007, 7, 446; and Schultz et al., J. Vac. Sci. Techol. B, 2007, 25, 2419), we show a very high pattern flexibility to create nano-scale metallic features.

As a first step, we use a micro-forming-like embossing process to engrave nano-scale features onto a solid electrolyte tool surface using an e-beam fabricated Si mold. Silver sulfide, Ag2S, is used as a solid electrolyte because of its favorable mechanical properties for micro forming and its excellent electrochemical properties. This ionic compound is ductile and has a relatively low yield stress at 80MPa. Followed by embossing, the patterned solid electrolyte surface is then used to carry out the S4 process, creating a negative image on a metallic substrate. This process eliminates the costly Focused Ion Beam milling used by Hsu et al. to create features on the electrolyte tool. It is also highly favorable for large-area patterning as well as mass-production of metallic substrates restricted only by the capability to fabricate the mold at first step. The embossed solid-electrolyte tool surface can be easily trimmed off with a microtome; the tool can then be re-used for embossing and patterning metallic substrates.

Using this process we demonstrate the ability to fabricate silver nanostructures with features <15 nm. Such small features are critical in metal nanostructures for field enhancement that finds applications in SERS and other biological and chemical sensing. So far, a line edge roughness of <10 nm is observed which is significant in the sense that silver is highly mobile and has the tendency to granulate. Finally, we show how this methodology has the capability to fabricate large area patterns at low cost and ambient conditions. As a proof of concept, we demonstrate the ability to fabricate areas >30 sq. mm. Such large scale fabrication is highly desired for applications like biomimetics and patterning for superhydrophobic surfaces.

Plasmonic interaction between overlapping nanowires

The plasmonic interaction between overlapping nanowires with and without structure singularities is studied analytically and numerically. A conformal transformation approach is adopted to predict analytically the optical response of overlapping nanowires in the quasi-static limit. Surface plasmon excitations are shown to exhibit a lower bound cutoff frequency, which blue-shifts when the overlap distance increases. Between this cutoff and the surface plasmon frequencies, overlapping nanowires are capable of a strong and broad-band harvesting of light. This band gap feature is shown to be robust to radiative losses and to the bluntness of the structure singularities. Hence, the light harvesting performance of overlapping nanowires would not be damaged by nanofabrication imperfections. These remarkable features might be beneficial to the realization of plasmonic band gap filters.

Shot noise limited characterization of ultraweak femtosecond pulse trains

Ultrafast science is inherently, due to the lack of fast enough detectors and electronics, based on nonlinear interactions. Typically, however, nonlinear measurements require significant powers and often operate in a limited spectral range. Here we overcome the difficulties of ultraweak ultrafast measurements by precision time-domain localization of spectral components. We utilize this for linear self-referenced characterization of pulse trains having ∼ 1 photon per pulse, a regime in which nonlinear techniques are impractical, at a temporal resolution of ∼ 10 fs. This technique does not only set a new scale of sensitivity in ultrashort pulse characterization, but is also applicable in any spectral range from the near-infrared to the deep UV.

Optical switching in metal-slit arrays on nonlinear dielectric substrates

We propose a scheme for an optical limiter and switch of the transmitted light intensity in an array of subwavelength metallic slits placed on a nonlinear Kerr-type dielectric substrate of finite thickness, where the geometrical parameters are designed for operation at telecom wavelengths. Our approach is based on the abrupt changes of the output light intensity observed in these systems near transmission minima.

Cascaded optical field enhancement in composite plasmonic nanostructures

We present composite plasmonic nanostructures designed to achieve cascaded enhancement of electromagnetic fields at optical frequencies. Our structures were made with the help of electron-beam lithography and comprise a set of metallic nanodisks placed one above another. The optical properties of reproducible arrays of these structures were studied by using scanning confocal Raman spectroscopy. We show that our composite nanostructures robustly demonstrate dramatic enhancement of the Raman signals when compared to those measured from constituent elements.

Free-standing optical gold bowtie nanoantenna with variable gap size for enhanced Raman spectroscopy

We describe plasmonic interactions in suspended gold bowtie nanoantenna leading to strong electromagnetic field (E) enhancements. Surface-enhanced Raman scattering (SERS) was used to demonstrate the performance of the nanoantenna. In addition to the well-known gap size dependence, up to 2 orders of magnitude additional enhancement is observed with elevated bowties. The overall behavior is described by a SERS enhancement factor exceeding 10(11) along with an anomalously weak power law dependence of E on the gap size in a range from 8 to 50 nm that is attributed to a plasmonic nanocavity effect occurring when the plasmonic interactions enter a strongly coupled regime.

Large enhancement of nonlinear optical phenomena by plasmonic nanocavity gratings

Enhancing nonlinear processes at the nanoscale is a crucial step toward the development of nanophotonics and new spectroscopy techniques. Here we demonstrate a novel plasmonic structure, called plasmonic nanocavity grating, which is shown to dramatically enhance surface nonlinear optical processes. It consists of resonant cavities that are periodically arranged to combine local and grating resonances. The four-wave mixing signal generated in our gold nanocavity grating is enhanced by a factor up to ≈2000, 2 orders of magnitude higher than that previously reported.

Interaction between plasmonic nanoparticles revisited with transformation optics

The interaction between plasmonic nanoparticles is investigated by means of transformation optics. The optical response of a dimer can be decomposed as a sum of modes whose resonances redshift when the nanoparticles approach each other. The extinction and scattering cross sections as well as the field enhancement induced by the dimer are derived analytically taking into account radiation damping. Interestingly, some invisibility dips occur in the scattering spectrum and originate from a destructive interference between each surface plasmon mode.

Plasmon-modulated light scattering from gold nanocrystal-decorated hollow mesoporous silica microspheres

Localized surface plasmon resonances of noble metal nanocrystals are powerful in enhancing a variety of linear and nonlinear optical signals and photorelated processes. Here we demonstrate the plasmonic enhancement of the light scattering from hollow mesoporous silica microspheres by attaching a dense layer of gold nanocrystals onto the outer surface of the microspheres. The attachment of gold nanocrystals induces both the shift and intensity increase in the resonant scattering peaks of the microspheres. The spectral region of the resonant scattering enhancement can be controlled by using gold nanocrystals with different plasmon resonance wavelengths. The spectral region of the enhancement is independent of the microsphere diameter. The scattering enhancement factor ranges from 20 to 130, depending on the plasmonic properties and surface coverage of the attached gold nanocrystals. The systematic evolution of the scattering spectra of the individual microspheres is also revealed by chemically etching away the attached gold nanocrystals gradually.

Localized surface plasmon resonance interaction with Er3+-doped tellurite glass

We show the annealing effect on silver and Erbium-doped tellurite glasses in the formation of nanoparticles (NPs) of silver, produced by the reduction of silver (Ag+ → Ag0), aiming to an fluorescence enhancement. The absorption spectra show typical Localized Surface Plasmon Resonance (LSPR) band of Ag0 NP in addition to the distinctive absorption peaks of Er3+ ions. Both observations demonstrate that the photoluminescence enhancement is due to the coupling of dipoles formed by NPs with the Er3+ 4I(13/2) → 4I(15/2) transition. This plasmon energy transfer to the Er3+ ions was observed in the fluorescence spectrum with a blue-shift of the peaks.

A photonic-crystal optical antenna for extremely large local-field enhancement

We propose a novel design of an all-dielectric optical antenna based on photonic-band-gap confinement. Specifically, we have engineered the photonic-crystal dipole mode to have broad spectral response (Q~70) and well-directed vertical-radiation by introducing a plane mirror below the cavity. Considerably large local electric-field intensity enhancement~4,500 is expected from the proposed design for a normally incident planewave. Furthermore, an analytic model developed based on coupled-mode theory predicts that the electric-field intensity enhancement can easily be over 100,000 by employing reasonably high-Q (~10,000) resonators.

Observation of few-cycle, strong-field phenomena in surface plasmon fields

We present experimental evidence of the generation of few-cycle propagating surface plasmon polariton wavepackets. These ultrashort plasmonic pulses comprised of only 2-3 field oscillations were characterized by an autocorrelation measurement based on electron photoemission. By exploiting plasmonic field enhancement, we achieved plasmon-induced tunnelling emission from the metal surface at low laser intensity, opening perspectives for strong-field experiments with low pulse energies. All-optical electron acceleration up to keV kinetic energy is also demonstrated in these surface-confined, few-cycle fields with only 1.35×10(12) W/cm2 focused laser intensity. The experimental results are found to be in excellent agreement with the model.

A scanning tunneling microscope as a tunable nanoantenna for atomic scale control of optical-field enhancement

The high stability of a low temperature (9 K) scanning tunneling microscope junction is used to precisely adjust the enhancement of an external pulsed vacuum ultraviolet (VUV) laser. The ensuing VUV optical-field strength is mapped on an hydrogenated Si(100) surface by imprinting locally one-photon atomic scale hydrogen desorption. Subsequent to irradiation, topography of the Si(100):H surface at the reacted area revealed a desorption spot with unprecedented atomic precision. Our results show that the shapes, positions, and sizes of the desorption spots are correlated to the calculated optical-field structure, offering real control of the optical-field distribution at molecular scale.

Tip-enhanced strong-field photoemission

Nonlinear photoelectron emission from metallic nanotips is explored in the strong-field regime. The passage between the multiphoton and the optical field emission regimes is clearly identified. The experimental observations are in agreement with a quantum mechanical strong-field model.

High harmonic generation by novel fiber amplifier based sources

Significant progress in high repetition rate ultrashort pulse sources based on fiber technology is presented. These systems enable operation at a high repetition rate of up to 500 kHz and high average power in the extreme ultraviolet wavelength range via high harmonic generation in a gas jet. High average power few-cycle pulses of a fiber amplifier pumped optical parametric chirped pulse amplifier are used to produce µW level average power for the strongest harmonic at 42.9 nm at a repetition rate of 96 kHz.

Continuous-wave and modelocked Yb:YCOB thin disk laser: first demonstration and future prospects

Yb:YCOB is a very attractive material for femtosecond pulse generation given its broad emission bandwidth. We demonstrate continuous-wave power scaling in the thin disk geometry to the 100-W level with a 40% optical-to-optical efficiency in multi-mode operation. Furthermore, we present initial modelocking results in the thin disk geometry, achieving pulse durations as short as 270 fs. The modelocked average power is, however, limited to less than 5 W because of transverse mode degradation. This is caused by anisotropic thermal aberrations in the 15% Yb-doped thin disks which were 300 to 400 µm thick. This result confirms the potential of Yb:YCOB to generate short femtosecond pulses in the thin disk geometry but also makes clear that significantly thinner disks are required to overcome the thermal limitations for high power operation.

Colloidal quantum dots as probes of excitation field enhancement in photonic antennas

Optical antennas are essential devices to interface light to nanoscale volumes and locally enhance the electromagnetic intensity. Various experimental methods can be used to quantify the antenna amplification on the emission process, yet characterizing the antenna amplification at the excitation frequency solely is a challenging task. Such experimental characterization is highly needed to fully understand and optimize the antenna response. Here, we describe a novel experimental tool to directly measure the antenna amplification on the excitation field independently of the emission process. We monitor the transient emission dynamics of colloidal quantum dots and show that the ratio of doubly to singly excited state photoluminescence decay amplitudes is an accurate tool to quantify the local excitation intensity amplification. This effect is demonstrated on optical antennas made of polystyrene microspheres and gold nanoapertures, and supported by numerical computations. The increased doubly excited state formation on nanoantennas realizes a new demonstration of enhanced light-matter interaction at the nanoscale.

Metallization of nanofilms in strong adiabatic electric fields

We introduce an effect of metallization of dielectric nanofilms by strong, adiabatically varying electric fields. The metallization causes optical properties of a dielectric film to become similar to those of a plasmonic metal (strong absorption and negative permittivity at low optical frequencies). This is a quantum effect, which is exponentially size-dependent, occurring at fields on the order of 0.1 V/Å and pulse durations ranging from ∼1 fs to ∼10 ns for a film thickness of 3-10 nm.

Subwavelength electromagnetic dynamics in stacked complementary plasmonic crystal slabs

Resonant electromagnetic fields in stacked complementary plasmonic crystal slabs (sc-PlCSs) are numerically explored in subwavelength dimensions. It is found that the local plasmon resonances in the sc-PlCSs are composite states of locally enhanced electric and magnetic fields. Two sc-PlCSs are analyzed in this paper and it is shown that each sc-PlCS realizes a resonant electromagnetic state suggested by one of Maxwell equations. It is moreover clarified that the local plasmons open efficient paths of Poynting flux, those result in high-contrast polarized transmission.

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.

Near-field nanofocusing through a combination of plasmonic Bragg reflector and converging lens

We report the near-field nanofocusing through a plasmonic lens containing a Bragg reflector and a converging lens, which consist of semitransparent annular grooves milled into a gold film with different periods along the radial direction. By illuminating the structure with a linearly polarized light, two tightly focal spots were detected by scanning near-field optical microscope. This plasmonic lens has considerably reduced direct light transmission, making the focal spots obvious. By raising the radius of half of every groove, one single spot was obtained. Furthermore, theoretical simulations prove that the light intensity of the focal spots can be doubled through adding the Bragg reflector surrounding the converging lens.

Angle- and energy-resolved plasmon coupling in gold nanorod dimers

The plasmon coupling in the dimers of Au nanorods linked together at their ends with dithiol molecules has been studied. The plasmon coupling in the dimers composed of similarly sized nanorods gives antibonding and bonding plasmon modes. The plasmon wavelengths of the two modes have been found to remain approximately unchanged, with the scattering intensity ratio between the antibonding and bonding modes decaying rapidly as the angle between the nanorods is increased. This plasmon coupling behavior agrees with that obtained from both electrodynamic calculations and modeling on the basis of the dipole-dipole interaction. The electric field in the gap region is largely enhanced for the bonding mode, while that for the antibonding mode is even smaller than the far field, highlighting the importance of selecting appropriate plasmon modes for plasmon-enhanced spectroscopies. An anti-crossing-like behavior in the plasmon coupling energy diagram has further been revealed for linearly end-to-end assembled dimers composed of differently sized nanorods. This result will be useful for plasmonic applications where the plasmon wavelength is required to be controllable but without sacrificing the electric field enhancement.

Vertically oriented sub-10-nm plasmonic nanogap arrays

Nanometric gaps in noble metals can harness surface plasmons, collective excitations of the conduction electrons, for extreme subwavelength localization of electromagnetic energy. Positioning molecules within such metallic nanogaps dramatically enhances light-matter interactions, increasing absorption, emission, and, most notably, surface-enhanced Raman scattering (SERS). However, the lack of reproducible high-throughput fabrication techniques with nanometric control over the gap size has limited practical applications. Here we show sub-10-nm metallic nanogap arrays with precise control of the gap's size, position, shape, and orientation. The vertically oriented plasmonic nanogaps are formed between two metal structures by a sacrificial layer of ultrathin alumina grown using atomic layer deposition. We show increasing local SERS enhancements of up to 10(9) as the nanogap size decreases to 5 nm. Because these sub-10-nm gaps can be fabricated at high densities using conventional optical lithography over an entire wafer, these results will have significant implications for spectroscopy and nanophotonics.

Mode imaging and selection in strongly coupled nanoantennas

The number of eigenmodes in plasmonic nanostructures increases with complexity due to mode hybridization, raising the need for efficient mode characterization and selection. Here we experimentally demonstrate direct imaging and selective excitation of the "bonding" and "antibonding" plasmon mode in symmetric dipole nanoantennas using confocal two-photon photoluminescence mapping. Excitation of a high-quality-factor antibonding resonance manifests itself as a two-lobed pattern instead of the single spot observed for the broad "bonding" resonance in accordance with numerical simulations. The two-lobed pattern is observed due to the fact that excitation of the antibonding mode is forbidden for symmetric excitation at the feedgap, while concomitantly the mode energy splitting is large enough to suppress excitation of the "bonding" mode. The controlled excitation of modes in strongly coupled plasmonic nanostructures is mandatory for efficient sensors, in coherent control as well as for implementing well-defined functionalities in complex plasmonic devices.

Subwavelength broadband splitters and switches for femtosecond plasmonic signals

Numerical simulations and an analytic approach based on transmission line theory are used to design splitters for nano-plasmonic signal processing that allow to arbitrarily adjust the ratio of transmission from an input into two different output arms. By adjusting the geometrical parameters of the structure, either a high bandwidth or a sharp transmission resonance is obtained. Switching between the two arms can be achieved by modulating the effective refractive index of the waveguide. Employing the instantaneous Kerr effect, switching rates in the THz regime are potentially feasible. The suggested devices are of interest for future applications in nanoplasmonic information processing.

Nonlocal ponderomotive nonlinearity in plasmonics

We analyze an inherent nonlinearity of surface plasmon polaritons at the interface of Fermi-Dirac metal plasma, stemming from the depletion of electron density in high-intensity regions. The derived optical nonlinear coefficients are comparable with the experimental values for metals. We calculate the dispersion relations for the nonlinear propagation of high-intensity surface plasmon polaritons, predicting a nonlinearity-induced cutoff and vanishing group velocity.