Effect of metal/dielectric substrates on photopolymerization of BITh thin films.

We have studied effects of metal-dielectric substrates on photopolymerization of [2,2'-Bi-1H-indene]-1,1'-dione-3,3'-diyl diheptanoate (BITh) monomer. We synthetized BITh and spin-coated it onto a variety of dielectric, metallic, and metal-dielectric substrates. The films were exposed to radiation of a UV-Visible Xe lamp, causing photo-polymerization of monomer molecules. The magnitude and the rate of the photo-polymerization were monitored by measuring the strength of the ~ 480 nm absorption band, which existed in the monomer but not in the polymer. Expectedly, the rate of photo-polymerization changed nearly linearly with the change of the pumping intensity. In contrast with our early study of photo-degradation of semiconducting polymer P3HT, the rate of photo-polymerization of BITh is getting modestly higher if the monomer film is deposited on top of silver separated from the monomer by a thin insulating MgF2 layer preventing a charge transfer. This effect is partly due to a constructive interference of the incident and reflected light waves, as well as known in the literature effects of metal/dielectric substrates on a variety of spectroscopic and energy transfer parameters. At the same time, the rate of photopolymerization is getting threefold larger if monomer is deposited on Ag film directly and charge transfer is allowed. Finally, Au substrates cause modest (~ 50%) enhancement of both monomer film absorption and the rate of photo-polymerization.

Directional emission of rhodamine 6G on top of a silver grating.

We have observed directional spontaneous emission of rhodamine 6G dye deposited on top of a silver grating and found that its angular distribution patterns were very different in TE and TM polarizations. The latter was related to the dispersion curves determined based on the polarized reflection spectra measured at multiple incidence angles. The most intriguing finding of this Letter was a resonance, which was coupled with TE-polarized light and determined the characteristic double-crescent patterns in the TE-polarized spontaneous emission. This observation, as well as nearly similar resonance observed in TM polarization, was tentatively explained in terms of leaky waveguide modes supported by a film of dye-doped polymer.

Thermal radiation of Er doped dielectric crystals: Probing the range of applicability of the Kirchhoff's law.

Kirchhoff's law of thermal radiation, relating emissivity and absorptance is commonly formulated for opaque bodies in thermodynamic equilibrium with the environment. However, in many systems of practical importance, both assumptions are often not satisfied. We revisit the century-old law and examine the limits of its applicability in an example of Er:YAG and Er:YLF dielectric crystals-potential radiation converters for thermophotovoltaic applications. The (80 at.%) Er:YAG crystal is opaque between 1.45 µm and 1.64 µm. In this spectral range, its absorptance α(λ) is spectrally flat and differentiates from unity only by a small amount of reflection. The shape of the emissivity spectrum ɛ(λ) closely matches that of absorptance α(λ), implying that the Kirchhoff's law can adequately describe thermal radiation of opaque bodies, even if thermodynamic equilibrium is not satisfied. The (20 at.%) Er:YLF crystal had smaller size, lower concentration of Er ions, and it was not opaque. Nevertheless, its spectrum of emissivity had almost the same shape (between 1.45 µm and 1.62 µm) as the absorptance derived from the transmission measurements. Our results are consistent with the conclusion that the Kirchhoff's law of thermal radiation can be extended (with caution) to not-opaque bodies away from the thermodynamic equilibrium.

Strong coupling of localized surface plasmons and ensembles of dye molecules.

We have studied strong exciton-plasmon coupling in the films of Ag nanoislands as well as in the layer-by-layer (LBL) deposited films of Au nanoparticles (NPs) coated with highly concentrated rhodamine 6G (R6G) dye. Their absorbance and the reflectance spectra featured the peaks or dips, which were not characteristic of dye or NPs/nanoislands taken separately. The positions of the spectral maxima (or minima) in the dye-doped films, plotted against those in pristine Ag nanoislands films, resulted in the dispersion curves comprised of three branches. They could be described by the analytical model based on the Hamiltonian accounting for the unperturbed energies of the surface plasmon (SP) resonance, the two bands composing the absorption spectrum of R6G dye, and the exciton-plasmon coupling energy Δ. Its value was larger in Ag nanoislands films deposited on hyperbolic metamaterials (0.221 eV) than on glass (0.165 eV). The minimal gap between the upper and the lower branches was equal to ≈3Δ. The dispersion curves in the Au NPs LBL films could be described with the Hamiltonian equation at relatively small dye concentrations. At larger concentrations of R6G molecules, the spectral peaks shifted and became more pronounced. The corresponding dispersion curve could not be described in terms of the existing model, indicating the need for further theoretical studies.

Study of the effect of excited state concentration on photodegradation of the p3ht polymer.

We have studied photoinduced reduction of absorption and emission in p3ht, a semiconducting polymer, and found that the rate of photodegradation (destruction of the constituent thiophene rings) does not correlate with the luminescence intensity and, correspondingly, does not depend on the excited state concentration. This conclusion rules out Purcell enhancement of radiative decay rate as a possible explanation of the recently discovered reduction of the p3ht photodegradation rate in the vicinity of metallic substrates and lamellar metal-dielectric metamaterials.

Long-range wetting transparency on top of layered metal-dielectric substrates.

It has been recently shown that scores of physical and chemical phenomena (including spontaneous emission, scattering and Förster energy transfer) can be controlled by nonlocal dielectric environments provided by metamaterials with hyperbolic dispersion and simpler metal/dielectric structures. At this time, we have researched van der Waals interactions and experimentally studied wetting of several metallic, dielectric and composite multilayered substrates. We have found that the wetting angle of water on top of MgF2 is highly sensitive to the thickness of the MgF2 layer and the nature of the underlying substrate that could be positioned as far as ~100 nm beneath the water/MgF2 interface. We refer to this phenomenon as long range wetting transparency. The latter effect cannot be described in terms of the most basic model of dispersion van der Waals-London forces based on pair-wise summation of dipole-dipole interactions across an interface or a gap separating the two media. We infer that the experimentally observed gradual change of the wetting angle with increase of the thickness of the MgF2 layer can possibly be explained by the distance dependence of the Hamaker function (describing the strength of interaction), which originates from retardation of electromagnetic waves at the distances comparable to a wavelength.

Strong coupling of surface plasmon polaritons and ensembles of dye molecules.

We demonstrate the strong coupling of dye molecules to surface plasmon polaritons (SPPs) excited in the Kretschmann geometry and propagating at the interface of silver and dye-doped polymer. The dispersion curve of such a system, studied in the reflectometry experiments, is split into three branches and demonstrates an avoided crossing - the signature of a strong coupling. We have further studied the excitation spectra of the dye emission and found that the positions of the excitation peaks have a good match with the points in the dispersion curve determined by the reflectometry. At the same time, the analysis of the spectra of the plasmon-mediated spontaneous emission, decoupled to the prism and acquired at multiple collection angles, has resulted in a quite different dispersion curve exhibiting a non-trivial splitting into multiple branches. This suggests that the same plasmonic environment couples differently to absorbing and emitting dye molecules.

Control of a chemical reaction (photodegradation of the p3ht polymer) with nonlocal dielectric environments.

Proximity to metallic surfaces, plasmonic structures, cavities and other inhomogeneous dielectric environments is known to control spontaneous emission, energy transfer, scattering, and many other phenomena of practical importance. The aim of the present study was to demonstrate that, in spirit of the Marcus theory, the rates of chemical reactions can, too, be influenced by nonlocal dielectric environments, such as metallic films and metal/dielectric bilayer or multilayer structures. We have experimentally shown that metallic, composite metal/dielectric substrates can, indeed, control ordering as well as photodegradation of thin poly-3-hexylthiophene (p3ht) films. In many particular experiments, p3ht films were separated from metal by a dielectric spacer, excluding conventional catalysis facilitated by metals and making modification of the nonlocal dielectric environment a plausible explanation for the observed phenomena. This first step toward understanding of a complex relationship between chemical reactions and nonlocal dielectric environments is to be followed by the theory development and a broader scope of thorough experimental studies.

Control of Förster energy transfer in the vicinity of metallic surfaces and hyperbolic metamaterials.

Optical cavities, plasmonic structures, photonic band crystals and interfaces, as well as, generally speaking, any photonic media with homogeneous or spatially inhomogeneous dielectric permittivity (including metamaterials) have local densities of photonic states, which are different from that in vacuum. These modified density of states environments are known to control both the rate and the angular distribution of spontaneous emission. In the present study, we question whether the proximity to metallic and metamaterial surfaces can affect other physical phenomena of fundamental and practical importance. We show that the same substrates and the same nonlocal dielectric environments that boost spontaneous emission, also inhibit Förster energy transfer between donor and acceptor molecules doped into a thin polymeric film. This finding correlates with the fact that in dielectric media, the rate of spontaneous emission is proportional to the index of refraction n, while the rate of the donor-acceptor energy transfer (in solid solutions with a random distribution of acceptors) is proportional to n(-1.5). This heuristic correspondence suggests that other classical and quantum phenomena, which in regular dielectric media depend on n, can also be controlled with custom-tailored metamaterials, plasmonic structures, and cavities.

Angular distribution of emission from hyperbolic metamaterials.

We have studied angular distribution of emission of dye molecules deposited on lamellar metal/dielectric and Si/Ag nanowire based metamaterials with hyperbolic dispersion. In agreement with the theoretical prediction, the emission pattern of dye on top of lamellar metamaterial is similar to that on top of metal. At the same time, the effective medium model predicts the emission patterns of the nanowire array and the dye film deposited on glass to be nearly identical to each other. This is not the case of our experiment. We tentatively explain the nearly Lambertian (∝cosθ) angular distribution of emission of the nanowire based sample by a surface roughness.

Blue shift of spontaneous emission in hyperbolic metamaterial.

Spontaneous emission is one of the most fundamental quantum phenomena in optics. Following the seminal work of Purcell and in agreement with the Fermi's Golden Rule, its rate can be controlled with the photonic density of states (PDOS). In recent years, this effect has been demonstrated in metamaterials with hyperbolic dispersion--highly anisotropic composite materials, which have a broad-band singularity of the density of photonic states. At this time, we show that hyperbolic metamaterials can control spontaneous emission spectra as well. Experimentally, DCM laser dye has been embedded into lamellar metal/dielectric metamaterial. The observed 18 nm blue shift of emission is explained by strong dispersion of the density of photonic states. On the other hand, practically no spectral shift has been observed in the excitation spectra of the same dye. This suggests that the effect of PDOS on spontaneous emission is very different from its effect on excitation and absorption.

Toward parametric amplification in plasmonic systems: second harmonic generation enhanced by surface plasmon polaritons.

Having in mind parametric amplification of surface plasmon polaritons (SPPs) as the final goal, we took the first step and studied in the Kretschmann geometry a simpler nonlinear optical process - second harmonic generation (SHG) enhanced by SPPs propagating at the interface between gold film and 2-methyl-4-nitroaniline (MNA). The experimentally demonstrated SHG efficiency was nearly 10(6) times larger than the one reported previously in the SPP system with different nonlinear optical material. The experimentally measured nonlinear conversion efficiency is estimated to be sufficient for parametric amplification of surface plasmon polaritons at ultra-short laser pumping.

Modification of electric and magnetic dipole emission in anisotropic plasmonic systems.

In order to investigate the effects of plasmonic environments on spontaneous emission of magnetic and electric dipoles, we have studied luminescence of Eu³âº ions in close vicinity to gold nanostrip arrays. Significant changes in the emission kinetics, emission polarization, and radiation patterns have been observed in the wavelength range corresponding to the plasmonic resonance. The effect of the plasmonic resonance on the magnetic dipole transition 5D0-->7F1 is found to be very different from its effect on the electric dipole transitions. This makes Eu³âºâ‚‹ containing complexes promising for mapping local distributions of magnetic and electric fields in metamaterials and plasmonic systems.

Reduced reflection from roughened hyperbolic metamaterial.

We show that roughened surfaces of hyperbolic metamaterials scatter light preferentially inside the media, resulting in a very low reflectance. This phenomenon of fundamental importance, demonstrated experimentally in arrays of silver nanowires grown in alumina membranes, is consistent with a broad-band singularity in the density of photonic states. It paves the road to a variety of applications ranging from the stealth technology to high-efficiency solar cells and photodetectors.

Hyperbolic and plasmonic properties of silicon/Ag aligned nanowire arrays.

The hyperbolic and plasmonic properties of silicon nanowire/Ag arrays have been investigated. The aligned nanowire arrays were formed and coated by atomic layer deposition of Ag, which itself is a metamaterial due to its unique mosaic film structure. The theoretical and numerical studies suggest that the fabricated arrays have hyperbolic dispersion in the visible and IR ranges of the spectrum. The theoretical predictions have been indirectly confirmed by polarized reflection spectra, showing reduction of the reflection in p polarization in comparison to that in s polarization. Studies of dye emission on top of Si/Ag nanowire arrays show strong emission quenching and shortening of dye emission kinetics. This behavior is also consistent with the predictions for hyperbolic media. The measured SERS signals were enhanced by almost an order of magnitude for closely packed and aligned nanowires, compared to random nanowire composites. These results agree with electric field simulations of these array structures.

Stimulated emission of surface plasmon polaritons in a microcylinder cavity.

We have observed stimulated emission of surface plasmon polaritons (SPPs) in dye-doped polymeric microcylinder cavities deposited onto gold and silver wires. The stimulated emission spectra featured a characteristic series of laser modes, with modal spacing corresponding to SPPs propagating at the interface between the metal and dielectric. A plasmonic microlaser adds to the toolbox of plasmonic devices and plasmonic metamaterials and enables on-chip plasmonic generation and loss compensation.

Controlling spontaneous emission with metamaterials.

We have observed, in metamaterial with hyperbolic dispersion (an array of silver nanowires in alumina membrane), a sixfold reduction of the emission lifetime of dye deposited onto the metamaterial's surface. This serves as evidence of an anomalously high density of photonic states in hyperbolic metamaterials, demonstrates the feasibility of an earlier-predicted single-photon gun, and paves the road for the use of metamaterials in quantum optics.

Demonstration of a spaser-based nanolaser.

One of the most rapidly growing areas of physics and nanotechnology focuses on plasmonic effects on the nanometre scale, with possible applications ranging from sensing and biomedicine to imaging and information technology. However, the full development of nanoplasmonics is hindered by the lack of devices that can generate coherent plasmonic fields. It has been proposed that in the same way as a laser generates stimulated emission of coherent photons, a 'spaser' could generate stimulated emission of surface plasmons (oscillations of free electrons in metallic nanostructures) in resonating metallic nanostructures adjacent to a gain medium. But attempts to realize a spaser face the challenge of absorption loss in metal, which is particularly strong at optical frequencies. The suggestion to compensate loss by optical gain in localized and propagating surface plasmons has been implemented recently and even allowed the amplification of propagating surface plasmons in open paths. Still, these experiments and the reported enhancement of the stimulated emission of dye molecules in the presence of metallic nanoparticles lack the feedback mechanism present in a spaser. Here we show that 44-nm-diameter nanoparticles with a gold core and dye-doped silica shell allow us to completely overcome the loss of localized surface plasmons by gain and realize a spaser. And in accord with the notion that only surface plasmon resonances are capable of squeezing optical frequency oscillations into a nanoscopic cavity to enable a true nanolaser, we show that outcoupling of surface plasmon oscillations to photonic modes at a wavelength of 531 nm makes our system the smallest nanolaser reported to date-and to our knowledge the first operating at visible wavelengths. We anticipate that now it has been realized experimentally, the spaser will advance our fundamental understanding of nanoplasmonics and the development of practical applications.

Effect of metallic surface on electric dipole and magnetic dipole emission transitions in Eu3+ doped polymeric film.

Spontaneous emission of Eu(3+) ions is studied in thin organic films deposited onto several different substrates. It has been demonstrated that the presence of a metallic surface in close vicinity to emitting Eu(3+) ions causes modifications of their spontaneous emission spectra, in particular, the change in the relative strengths of magnetic-dipole and electric-dipole transitions. The character and the magnitude of the effect depend on the polarization and the observation angle. The experimental data are discussed in terms of modification of transition probabilities and account for the interference between directly emitted and reflected light waves.

Stimulated emission of surface plasmon polaritons.

We have observed laserlike emission of surface plasmon polaritons (SPPs) decoupled to the glass prism in an attenuated total reflection setup. SPPs were excited by optically pumped molecules in a polymeric film deposited on the top of a silver film. Stimulated emission was characterized by a distinct threshold in the input-output dependence and narrowing of the emission spectrum. The observed stimulated emission and corresponding compensation of the metallic absorption loss by gain enables many applications of metamaterials and nanoplasmonic devices.