Photonic funnels, conical waveguides with hyperbolic metamaterial (HMM) cores, efficiently focus mid-IR light to spatial areas much smaller than the free space wavelength. These devices were originally conceived as having a perfect electric conductor (PEC) coating to confine light as it propagates to their subwavelength tip, however, recent numerical analysis demonstrates that funnels without a conductive cladding exhibit peak intensities over 1,000 times greater than their clad counterparts while maintaining a confinement scale determined by their tip radius. The funnel's conical surface provides an oblique interface between the highly anisotropic HMM and an isotropic medium. This oblique interface enables anomalous reflections which reshape and redirect the incident beam towards the funnel tip. In this work we analyze how the gold cladding suppresses the field enhancement and demonstrate the importance of the anomalous reflection to the funnel's performance.
We aim to address one of the fundamental limitations of machine learning (ML): its reliance on extensive training datasets by incorporating physics-based intuition and Maxwell-equation-based constraints into ML process. We show that physics-guided networks require significantly smaller datasets, enable learning outside the original training data, and provide improved prediction accuracy and physics consistency. The proposed approaches are illustrated on examples of photonic composites, from photonic crystals to hyperbolic metamaterials.
Type-II superlattice (T2SL)-based detectors as a new platform for mid-IR detectors, aiming to bring the performance of state-of-the-art HgCdTe based designs to room-temperature operation. T2SLs provide a way to highly reduce detector thickness when a thin T2SL layer is implemented in a multilayer core combined with a dielectric metasurface that uses guided mode resonance (GMR) to couple the incoming light into the detector. However, supporting a GMR requires the detector to have significantly expanded spatial area, potentially preventing the creation of truly compact finite detectors. In this work we propose a possible solution to this problem. The proposed design of a detector relies on the reflective metasurface to couple incident light (λ_0) into a guided mode that overlaps T2SL absorber layer. Our analysis demonstrates that the width of the detector must be at least ~20λ_0-wide to achieve ~60% external quantum efficiency (EQE), with narrower detectors exhibiting decreased performance. To counteract this, we propose a design that combines the benefits of GRM and Fabry-Perot cavity enhancement, resulting in 2λ_0-wide, subwavelength-thick detector that in theory achieves ~51% EQE.
We demonstrate that incorporating physics-based intuition and Maxwell-equation-based constraints into machine learning process reduces the required amount of the training data and improves prediction accuracy and physics consistency. In addition, physics-based provides an avenue to extend the range of the model applicability outside the space of the original labeled dataset. The proposed approaches are illustrated on examples of photonic composites, from photonic crystals to hyperbolic metamaterials.
Photonic funnels, conical waveguiding structures with hyperbolic metamaterial cores, have been proposed – and recently demonstrated at mid-infrared frequencies – as optical links between macro- and nano-scales. All recent realizations of the funnels utilize highly conductive claddings to prevent the leakage of light out of the core. Here we demonstrate that clad-less funnels can significantly outperform their PEC-clad counterparts due to excitation of novel surface modes. We also analyze funnel-light interaction in the time domain and demonstrate temporal separation between diffraction-limited and nano-confined signals. Perspectives of temporal shaping of the nano-confined radiation are also discussed.
KEYWORDS: Mid-IR, Near field scanning optical microscopy, Waveguides, Diffraction, Dielectrics, Wave propagation, Signal to noise ratio, Shape analysis, Scattering, Metamaterials
Waveguides with extremely anisotropic metamaterial cores support propagating modes even when their cross section is deeply subwavelength. A funnel-shaped structure with microscale base, nanoscale tip, and hyperbolic core can therefore act as an efficient optical link between the diffraction-limited and subwavelength domains. In this work we analyze the potential applications of this platform for mid-IR near-field scanning optical microscopy. We present the relationship between the shape and composition of the funnels and the resulting compression of light, characterized by both the intensity and the spread of electromagnetic field in the vicinity of the funnel tip.
Machine learning is widely used for optimization or classification tasks. Unfortunately, extensive labeled datasets are often required for training machine learning models. In this work we demonstrate that incorporating physics-driven constraints into machine learning algorithms can dramatically improve both accuracy and extendibility of resulting models, simultaneously reducing the size of the required training set and enabling training on unlabeled data. Physics-informed machine learning is illustrated on example of predicting optical modes supported by periodic layered composites. The approach can be readily utilized for analysis of electromagnetic modes in composites with 2D periodic geometry or in complex waveguiding structures.
In this talk I will discuss our group’s work on the design, growth, fabrication and characterization of a new class of all-epitaxial plasmonic optoelectronic devices with enhanced performance when compared to state-of-the-art infrared optoelectronics. Specifically, we demonstrate that highly doped semiconductors, serving as ‘designer’ plasmonic materials, can be monolithically integrated with a range of infrared optoelectronic device architectures to provide strong field confinement, and enhanced emission, detection, and potentially modulation capabilities in the mid-infrared. We will present results from long-wave infrared detectors with thickness of only 350 nm, capable of over 50% external quantum efficiency and state-of-the-art detectivity, as well as dual color detectors, spectrally-selective detectors, and enhanced efficiency emitters leveraging our designer plasmonic materials with a range of novel device architectures.
Photonic funnels, conical structures with hyperbolic cores, that have been recently demonstrated at mid-IR frequencies, provide a platform to avoid the diffraction limit and enable a smooth optical link between the nanoscale and microscale. Orbital angular momentum (OAM) of beams play important role in optical manipulation, microscopy, and potentially optical communications.. In this work we analyze theoretically the interaction structured light (light that has non-zero OAM) with photonic funnels. In particular, we study the effect of light confinement, facilitated through the geometric profile of the funnel, on spatial structure of the mode, and its local intensity.
We present phonic funnels, a novel material platform, that enables a smooth optical link between the diffraction-limited and deep subwavelength areas. Photonic funnels comprise conical structures with hyperbolic cores that enable highly confined propagation of light and perfectly conducting walls that isolate the core of the funnel from the surroundings. We demonstrate realization of the funnels with semiconductor metamaterial platform, with minimum diameter of the opening of the order of 1/30-th of free space wavelength and characterize propagation of light through the funnels experimentally and theoretically. We also analyze funnel-induced modulation of emission.
We analyze the mid-infrared emission resulting from the interplay between a type-II superlattice (T2SL) material and semiconductor-based plasmonic “designer metals”. We demonstrate an order of magnitude emission enhancement, accompanied by spectral reshaping, relative to all-dielectric T2SL counterparts and provide a theoretical description of the underlying physics. The all-semiconductor LWIR emitters with integrated plasmonic components, developed in this work, represent novel approach to broadband room-temperature mid-IR sources.
We develop photonic funnels, structures that provide efficient optical coupling between nano- and micro-worlds. The funnels represent conical waveguides with highly anisotropic cores and highly conductive cladding that have one opening with crossection of the order of free space wavelength and the second opening with deep subwavelength crossection. We fabricate all-semiconductor photonic funnels at mid-infrared frequency range and demonstrate, theoretically and experimentally, efficient confinement of mid-infrared light to wavelength/30 areas. Theoretically, we predict efficient out-coupling of light from ultra-small areas to diffraction-limited domain.
We develop an approach that enables characterization of wavelength-scale objects with deep subwavelength resolution. The technique combines diffractive imaging that out-couples the information about the subwavelength features of the object into the far-field zone with machine learning that analyzes the resulting patterns. Recovery of complex objects with 120-nm resolution with ~530-nm light is demonstrated experimentally. Our theoretical analysis suggests that the same objects can be recovered with up to 2-micron-wavelength light. Our work opens the door for new characterization tools that combine high spatial resolution, fast data acquisition, and artificial intelligence
KEYWORDS: Luminescence, Spectroscopy, Process control, Electromagnetism, Metamaterials, Composites, Biophysics, Quantum optics, Nanostructures, Resonance energy transfer
The control of photoluminescence processes, via the design of composite materials with engineered electromagnetic properties, is of great interest for the development of many application areas ranging from biophysics to quantum optical technologies. Approaches providing broadband enhancements of emission, not limited to resonant nanostructures, are particularly advantageous. We discuss how various photoluminescence processes, including conventional and dipolar-forbidden spontaneous emission, as well as Förster resonance energy transfer, are altered nearby and inside plasmonic hyperbolic metamaterials. They provide a flexible platform for engineering broadband Purcell enhancements due to their peculiar electromagnetic mode structure controlled by the nonlocal response of the metamaterial.
Enhancement of local electromagnetic fields is instrumental for engineering of light absorption, emission, scattering, chemical reactions, and other processes. Nanostructured composites with plasmonic inclusions have been shown as promising candidates to concentrate electromagnetic waves in nanometer-sized “hot spots”. Unfortunately, majority of high-performance plasmonic structures are resonance-based, and therefore their performance is relatively narrow-band. Here we present a novel material system that has potential to realize broadband enhancement of local intensity and explain the origin of this behavior.
The proposed material platform comprises an array of aligned plasmonic cones arranged in a periodic planar lattice. From the effective medium standpoint, such structure represents a uniaxial material whose effective permittivity varies along the cone. Importantly, there exists a relatively wide range of wavelengths where one component of the effective permittivity tensor crosses zero within the composite. According to previous research, strong enhancement of local field is expected in the vicinity of epsilon-near-zero point in homogeneous materials with spatially varying permittivity, often called transitional metamaterials. We show, however, that due to strong structural nonlocality electromagnetic response of nanocone media does not follow this recipe. In fact, we demonstrate that the incoming radiation is coupled into an additional electromagnetic wave that propagates towards the tip of the cone causing a strong enhancement to the local field. We present a comprehensive description of this phenomenon.
Second harmonic generation is one of the fundamental nonlinear optical processes that is at the heart of communication and sensing applications. Due to the underlying crystal symmetry, second harmonic generation in noble metal-nanostructures is dominated by metal/dielectric interfaces with only weak (magneto-dipole and quadrupolar) contributions coming from the bulk of the metal inclusions. Here we demonstrate that, in metamaterials, nonlinear contributions from individual plasmonic inclusions can add up together, resulting in the bulk nonlinear polarization. The resulting nonlinear response can be described in terms of volumetric second harmonic polarizability that relates unit-cell averaged nonlinear polarization to a product of unit-cell averaged fundamental fields. The amplitude of this effective nonlinear polarizability is comparable to that of common nonlinear crystals.
In order to analyze nonlinear response of the plasmonic nanowire arrays we compare experimental results to numerical solutions of Maxwell equations where second harmonic response is calculated using nonlinear hydrodynamic model. Numerical solutions of Maxwell equations are also used to analyze the spatial and spectral distributions of fundamental and nonlinear fields across the composites and, in the end, to guide and validate the development of analytical description of effective second harmonic polarizability. The developed analytical description of the second harmonic generation in plasmonic composites opens new avenues for engineering of nonlinear response.
The control of spontaneous emission via the design of composite materials with engineered electromagnetic properties is important for the development of new faster and brighter sources of illumination with applications ranging from biophysics to quantum optical technologies. In particular, the fabrication of nanostructures leading to broadband enhancement of emission is of great interest. Hyperbolic plasmonic metamaterials have recently emerged as a very flexible platform for this purpose as they provide a high local density of electromagnetic states available for the radiative relaxation of emitters. This is due to their peculiar mode structure governed by both the structural nonlocal response and the dispersion properties.
Here, we investigate the modification of the spontaneous emission rate and intensity enhancement of emitters located inside a nanorod-based hyperbolic metamaterial. We experimentally show the coupling of the radiated emission to the waveguided mode of a planar hyperbolic metamaterial with finite thickness. The emitters located inside this planar hyperbolic metamaterial waveguide exhibit an almost 50-fold reduction of the decay rate and 3-fold intensity enhancement of the fluorescence coupled to the mode. We also discuss the effect of nanostructuring the nanorod-based metamaterial on the spontaneous emission properties of emitters located inside it, where suitable designs can lead to further enhancement of the radiative rate and improved light extraction of the emission coupled to the high-wavevector modes of the metamaterial to the far-field, useful for the development of efficient and fast free-space light-emitting devices.
Pixel size in cameras and other refractive imaging devices is typically limited by the free-space diffraction. However, a vast majority of semiconductor-based detectors are based on materials with substantially high refractive index. We demonstrate that diffractive optics can be used to take advantage of this high refractive index to reduce effective pixel size of the sensors below free-space diffraction limit. At the same time, diffractive systems encode both amplitude and phase information about the incoming beam into multiple pixels, offering the platform for noise-tolerant imaging with dynamical refocusing. We explore the opportunities opened by high index diffractive optics to reduce sensor size and increase signalto- noise ratio of imaging structures.
Optical characterization of subwavelength objects is important for biology, nanotechnology, chemistry, and materials science. Unfortunately, the information about interaction of an isolated subwavelength object with light is contained in evanescent waves that exponentially decay away from the source. Numerous techniques have been proposed to access or restore this information. In interscale mixing microscopy (IMM), a diffraction grating positioned in the near field proximity of the object is used to convert the originally-evanescent waves into propagating modes that can be detected with far-field measurements. However, far-field signal needs to be post-processed to un-couple the diffraction-limited and subwavelength responses. Several techniques, based on multiple measurements, have been previously proposed. Here, we show that with simple Fourier-transform based post processing can be used to characterize position, and optical size of the object based on a single measurement. To verify the proposed formalism, three finite diffraction gratings were fabricated. Two of these gratings contained pre-engineered “defects” that played the role of “unknown objects”, while the remaining grating was used as a reference. We demonstrate that we can identify the position and size of ~wavelength/10 object with far-field characterization. The same measurement provides a platform to analyze optical spectrum of the object. Although demonstrated in this work on example of 1D grating, IMM can be extended to 2D subwavelength imaging
Plasmonic nanowire metamaterials, arrays of aligned plasmonic nanowires grown inside an insulating substrate, have recently emerged as a flexible platform for engineering refraction, diffraction, and density of photonic states, as well as for applications in bio- and acoustic sensing. Majority of unique optical phenomena associated with nanowire metamaterials have been linked to the collective excitation of cylindrical surface plasmons propagating on individual nanowires. From the effective medium standpoint, this collective excitation can be described as an additional electromagnetic wave, emanating from nonlocal effective permittivity of metamaterial. The electromagnetic fields associated with such mode can are strongly inhomogeneous on the scale of the unit cell.
In this work we analyze the effect of the strong field variation inside nanowire metamaterial on second harmonic generation (SHG). We show that second harmonic generation is strongly enhanced in the frequency region where metamaterial is nonlocal. Overall, the composite is predicted to outperform its homogeneous metal counterparts by several orders of magnitude. Quantitative description of SHG in nanowire medium is developed. The results suggest that bulk second harmonic polarizability emerges as result of collective surface-enhanced SHG by individual components of the composite.
Fluorescence-based processes are strongly modified by the electromagnetic environment in which the emitters are placed. Hence, the design of nanostructured materials with appropriate electromagnetic properties opens up a new route in the control of, for instance, the spontaneous rate of emission or the energy transfer rate in donor-acceptor pairs. In particular, hyperbolic plasmonic metamaterials have emerged as a very flexible and powerful platform for these applications as they provide a high local density of electromagnetic states due to their peculiar mode structure which is governed by both the structural nonlocal response and the dispersion properties. Here, we will discuss an experimental and theoretical study of the influence of a hyperbolic metamaterial comprised of an array of gold nanorods on the radiative properties of quantum emitters and the energy-transfer processes between them.
In Extraordinary Optical Transmission (EOT), a metallic film perforated with an array of [periodic] apertures exhibits transmission over 100% normalized to the total aperture area, at selected frequencies. EOT devices have potential applications as optical filters and as couplers in hybrid electro-optic contacts/devices. Traditional passive extraordinary optical transmission structures, typically demonstrate un-normalized transmission well below 50%, and are typically outperformed by simpler thin-film techniques. To overcome these limitations, we demonstrate a new breed of extraordinary optical transmission devices, by “burying” an extraordinary optical transmission grating in a dielectric matrix via a metal-assisted-chemical etching process. The resulting structure is an extraordinary optical transmission grating on top of a dielectric substrate with dielectric nano-pillars extruded through the grating apertures. These structures not only show significantly enhanced peak transmission when normalized to the open area of the metal film, but more importantly, peak transmission greater than that observed from the bare semiconductor surface. The structures were modeled using three-dimensional rigorous coupled wave analysis and characterized experimentally by Fourier transform infrared reflection and transmission spectroscopy, and the good agreement between the two has been demonstrated. The drastic enhancement of light transmission in our structures originates from structuring of high-index dielectric substrate, with pillars effectively guiding light through metal apertures.
Plasmonic metamaterial composites are often considered to be promising building blocks for a number of applications that include subwavelength light manipulation, imaging, and quantum optics engineering. These applications often rely on effective medium response of metamaterial composites and require metamaterial to operate in exotic (hyperbolic, or epsilon-near-zero) regimes. However, the behaviour of metamaterials is often different from the predictions of effective medium. In this work we aim to understand the implications of composite nature of metamaterials on their optical properties. Plasmonic nanowire metamaterials are a convenient metamaterial platform that is capable of realization of ENZ, hyperbolic, and elliptic responses depending on light frequency and metamaterial geometry. In this work we show that the response of metamaterial in elliptical regime may be strongly affected by the additional electromagnetic wave that represents collective excitation of cylindrical surface plasmons in nanowire arrays. We present an analytical description of optical properties of additional wave and analyse the effect of this mode on quantum emitters inside nanorod metamaterials.
Here we demonstrate a new class of designer plasmonic materials for use in the mid-infrared (mid-IR) region of the electromagnetic spectrum. By heavily doping epitaxially-grown semiconductor materials, we are able to grow single-crystal materials whose optical properties in the mid-IR mimic those of metals at shorter wavelengths. We demonstrate materials with plasma frequencies from 5.5-15μm and low losses, compared to their shortwavelength counterparts. In addition, we demonstrate the ability of subwavelength particles formed from our materials to support localized surface plasmon resonances, and measure the near-field absorption of these structures using a novel nanoscale infrared spectroscopy technique. Finally, we show good agreement between our observed results and analytical and finite-element models of our materials and structures. The results presented offer a path towards nanoscale confinement of light with micron-scale wavelengths.
We demonstrate reduced reflectance and a corresponding enhancement of transmittance in lamellar hyperbolic
metamaterials, with scatterers deposited on the top. The effect is much more significant in curvilinear hyperbolic
metamaterials. We also show that absorption strengths of dyes on the top of hyperbolic metamaterials can be tuned and
enhanced (nearly threefold). The effect can be controlled by interplay of the substrate geometry, composition and
location of the absorbing medium. Our observations pave the way for a variety of applications, including broadband
enhancements of light trapping and absorption in solar cells.
We demonstrate the steering of coherent mid-infrared radiation through plasmonic structures consisting of a
single sub-wavelength slit flanked by a periodic array of grooves, fabricated on GaAs substrates. We demonstrate
control of steering angle by tuning the incident radiation, and study beam quality for the transmitted light. In
addition, we demonstrate that small shifts in the refractive index of the GaAs substrate can actively control the
steering angle of the transmitted light, opening a path towards the development of no-moving-parts plasmonic
beam steering devices.
We present a technique for subwavelength far-field focusing of light in planar non-resonant structures. The approach
combines the diffraction gratings that generate high-wavevector waves and planar slabs of homogeneous anisotropic
metamaterials that propagate these waves and combine them at the subwavelength focal spots. The technique has all the
benefits of Fresnel lens, near-field zone plate, hyperlens, and superlens, and at the same time resolves their fundamental
limitations. Several realizations of hypergratings for visible, near-IR, and mid-IR frequencies are proposed, and their
performance is analyzed. Generalization of the developed approach for sub-diffractional imaging and on-chip photonics
is suggested.
We consider the propagation of surface plasmon polaritons in anisotropic metamaterial systems. It is shown that material
anisotropy can be used as an efficient tool to independently control effective modal index and spatial profile of the
surface mode. In particular, it is possible to utilize anisotropic media to completely eliminate the out-of-plane scattering
of surface plasmons, realizing the paradigm of truly two-dimensional optics where surface modes are completely
uncoupled from their volume counterparts. The developed formalism yields a mapping between the familiar laws of 3D
optics and the behavior of two-dimensional surface optics. The mapping is illustrated on examples of plasmonic refractor
and plasmonic Bragg reflector. The tolerance of the surface optics paradigm with respect to material imperfections is
assessed with perturbation theory and with numerical solutions of Maxwell's equations. Practical realizations of
dynamical plasmonic circuits and extensions of the developed framework to volume-guiding structures are discussed.
We explore the perspectives offered by nanoplasmonic metamaterials for manipulation of optical signals at the
nanoscale. It is shown that in contrast to conventional dielectric waveguides, plasmonic and anisotropy-based
waveguides support a number of highly-confined optical modes even when the waveguide size is much smaller than the
wavelength. The effective modal indices in these systems can be either positive or negative and are strongly affected by
material composition and waveguide size, providing a mechanism for manipulating the phase velocity and diffraction
limit at the nanoscale. In active metamaterials, the combined effect of waveguide- and material-induced dispersions leads
to a versatile control over the group velocity which can be changed from negative to large or small (in comparison with
the speed of light in vacuum) positive values by a relatively weak modulation of material properties. In the end, the
active metamaterial provide a unique platform for independent manipulation of group and phase velocities of
electromagnetic radiation in sub-diffraction areas.
In this work, we report the substantial compensation of loss of propagating SPPs at the interface between silver film and
optically pumped polymer with dye. The large magnitude of the effect, nearly threefold change of the reflectivity,
enables a variety of applications of "active" nanoplasmonics. In order to quantify the observed phenomenon, we have
extended the theoretical formalism relating the reflectivity in ATR experiment and the SPP propagation length to the
case of active dielectric media.
It has been recently shown that a planar waveguide structure with a strongly anisotropic core can be used as non-magnetic medium with negative refractive index. In such a system, the optical radiation propagating in the plane of the waveguide is effectively confined in deep subwavelength space perpendicular to this plane, leading to the strong enhancement of energy density inside the system. We demonstrate the possibility of using the high energy-density waveguide as a planar lens, present a detailed study of imaging properties of the proposed system, and consider the perspectives of energy confinement beyond the diffraction limit.
We develop a new approach to materials with negative refraction index which can be implemented for optical and infrared frequencies. In contrast to conventional designs which require simultaneously negative dielectric permittivity and magnetic permeability, our system is intrinsically non-magnetic and makes use of an anisotropic dielectric constant to provide negative refractive index in waveguide geometry. The proposed approach is not limited to the proximity of a resonance and thus allows for low loss, critical for super-lensing applications.
We demonstrate that a nanostructured plasmonic composite material
can show negative index of refraction at infrared and optical frequencies. In contrast to conventional negative refraction materials, our design does not require periodicity and thus is highly tolerant to fabrication defects. Moreover, since the proposed material is intrinsically non-magnetic (μ ≡ 1), its performance is not limited to proximity of a resonance so that the resulting structure has relatively low loss. We develop the analytical description of the relevant electromagnetic phenomena and justify our analytic results via numerical solutions of Maxwell equations.
We demonstrate that the phenomenon of Dynamical Anderson Localization of light leads to high-Q whispering-gallery modes in microcylinder and microdisk resonators with substantial surface roughness, and determines their lifetimes and emission patterns.
High-Q whispering-gallery modes with unidirectional emission are present in spiral - shaped microresonators despite completely chaotic ray dynamics. We demonstrate that formation of such modes is due to dynamical localization, and develop the theory of this effect.
Optical properties of metal nanowires and nanowire composite materials are studied experimentally and theoretically. We suggest that a nanowire composite, constructed from parallel pairs of nanowires has both effective magnetic permeability and dielectric permittivity negative in the visible and near-infrared spectral ranges due to resonant excitation of surface plasmon polaritons.
Experimental results confirm excitation of surface plasmons polaritons in periodical array of nanowires.
Dielectric spheres and cylinders can support high-Q modes due to internal reflection of the light. However, small deviations of the shape of these resonators away from symmetry lead to the onset of chaotic ray dynamics and the resulting suppression of the mode lifetimes. We demonstrate, that in such conditions even the regular modes are strongly affected by small deformations, due to the phenomenon of chaos-assisted tunneling, and develop a
quantitative theory of this effect.
An analytical theory for extraordinary light transmittance through an optically thick metal film with sub-wavelength holes is developed. It is shown that the film transmittance has sharp peaks that are due to the Maxwell-Garnet resonances in the holes. At resonances electric and magnetic fields are dramatically enhanced in the holes. These resonances are proposed to guide light over a metal film at a nanoscale.
We show that plamonic nanomaterials allow the localization and guiding light, with high efficiency, and molecule sensing, with unsurpassed sensitivity. Two types of palsmonic naomaterials are considered: metal nanowires and fractal colloid aggregates. The electromagnetic field distribution for thin metal nanowires is found, by using the discrete dipole approximation. The plasmon polariton modes in wires are numerically simulated. These modes are found to be dependent on the incident light wavelength and direction of propagation. The existence of localized plasmon modes and strong local field enhancement in percolation nanowire composites is demonstrated. Novel left-handed materials in the near-infrared and visible are proposed based on nanowire composites. Dramatic enhancement in fractal colloid aggregates and, especially, in fractal-microcavity composites are discussed along with new potential applications of these plasmonic materials.
This report presents the discovery of greatly enhanced, broad-range, multiphoton excited emission from Ag aggregate- adsorbate complexes seeded into a cylindrical microcavity. The emission spectrum contains descrete peaks spanning the wavelength range from the 632 nm HeNe laser exciting wavelength down to 200 nm. Observation of multiphoton processes at the low exciting light intensity (20 W/cm2) became possible due to using a fractal-microcavity composite, where coupling the localized plasmon modes in fractal aggregates with microcavity resonances is provided. The important role of the multiphoton resonant transitions between discrete states of a finite-size metal particle in enhanced local fields is shown. Analysis, based on the model of a spherical potential well, shows that the observed spectra contain fingerprints of the quantum size effect.
We describe the characteristics of a novel optical material developed in our laboratory, a fractal/microcavity composite. Both components of this composite exhibit resonance behavior whereby the amplitudes of spectral emissions, generated by molecules either adsorbed onto the composite or located remotely from it, are significantly enhanced. In the composite, the individual enhancement factors combine multiplicatively with the result that spectral emissions are enhanced by an extremely large factor. In particular, the extremely large enhancement factors facilitate the generation of nonlinear optical processes in the composite, which may be exploited in the fabrication of ultra-sensitive detectors.
Cermet is mixture of nano-sized metallic grains and insulating matrix. When concentration in metal is large enough, the conducting particles are strongly interacting between each other. As predicted by V. Shalaev for fractal metallic clusters and experimentally verified, in the vicinity of the percolation threshold, the local electromagnetic field can be very large because of plasmon resonance in the metallic grains which may happen in a wide spectrum of frequencies. In this enhanced field region, the micro-crystallites of the matrix are immersed in these huge fields. In the spectral range of the optical phonon of the matrix there can be a very large absorption band because of the possible coupling between phonon of the matrix and resonance of the metallic grains. We experimentally observe this huge absorption in gold alumina cermets. The theoretical model is in good agreement with experimental results.
KEYWORDS: Near field optics, Silver, Nanoparticles, Particles, Transmission electron microscopy, Fractal analysis, Laser ablation, Thin films, Laser beam diagnostics, Near field
Thin films of randomly distributed silver nanoparticles are studied experimentally using photon scanning tunneling microscopy and theoretically using real-space renormalization group method. The studies reveal large variations of local optical intensity at sub-wavelength scales. In addition, irradiation of the film by nanosecond laser pulses is observed to yield substantial changes in the local optical response. The threshold for the photomodification is less than 10 mJ/cm2. It is believed that particles within some areas of nanometer scales are restructured during nanosecond laser irradiation. The geometric changes in turn result in modification of the local optical intensity.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.