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We present the first angle-resolved measurements of spatial photon correlations that are induced by multiple scattering of light. The correlation relates multiple scattered photons at different spatial positions and depends on incident photon fluctuations.

We present a new broadband technique for the measurement of di®use light transport throughopaque scattering media. Using the spectral correlations introduced by a scattering medium ontoa white-light supercontinuum spectrum, the di®usion constant of light is determined over a widespectral range in the visible and near infrared. Independent broadband measurements of both thetransport mean free path and the di®usion constant are used to calculate the spectral dependenceof the energy velocity in a porous GaP slab. Broadband correlation spectroscopy is found to be anexcellent tool for the characterization of random scattering media.

We present a theoretical study of the spontaneous emission of an optical emitter close to a metal nanostructure of arbitrary shape. The modification of the corresponding radiative and nonradiative decay rates and resulting quantum efficiencies, expressed on the basis of a semiclassical dipole model in terms of the local plasmonic mode density, is calculated by means of the rigorous formulation of the Green's theorem surface integral equations. Metal losses and the intrinsic nonradiative decay rate of the molecules are properly considered, presenting relationships valid in general for arbitrary intrinsic quantum yields. Resonant enhancement of the radiative and nonradiative decay rates of a fluorescent molecule is observed when coupled to an optical dimer nanoantenna. Upon varying the dipole position, it is possible to obtain a predominant enhancement of radiative decay rates over the nonradiative counterpart, resulting in an increase of the internal quantum efficiency. For emitters positioned in the gap, quantum efficiency enhancements from an intrinsic value of 1% to ~75% are possible.

We demonstrate that highly tunable nanowire arrays with optimized diameters, volume fractions, and alignment form one of the strongest optical scattering materials to date. Using a new broad-band technique, we explore the scattering strength of the nanowires by varying systematically their diameter and alignment on the substrate. We identify strong Mie-type internal resonances of the nanowires which can be tuned over the entire visible spectrum. The tunability of nanowire materials opens up exciting new prospects for fundamental and applied research ranging from random lasers to solar cells, exploiting the extreme scattering strength, internal resonances, and preferential alignment of the nanowires. Although we have focused our investigation on gallium phosphide nanowires, the results can be universally applied to other types of group III-V, II-VI, or IV nanowires.

A novel class of optically anisotropic materials is presented. Layers of semiconductor nanowires fabricated in a bottom-up process exhibit a large in-plane birefringence and show quarter-wavelength retardation for a wavelength of 690 nm. These nanowire metamaterials are promising materials for optical gas- and biosensing.

Optical absorption spectroscopy of a single metal nanoparticle is used to characterize its properties and to obtain quantitative information on its local environment. Experiments were performed using the spatial modulation spectroscopy (SMS) technique on 16 nm mean diameter gold nanoparticles embedded in different medium (i.e., deposited on glass or embedded in a polymer layer). Extraction of the nanoparticle characteristics and determination of the dielectric constant of its environment are discussed, focusing on the impact of the particle shape assumption. The refractive index of the local environment deduced from these measurements shows large particle-to-particle variation, yielding information about fluctuations of the dielectric properties of the surrounding medium on a nanometric scale, inaccessible in ensemble measurements. The influence of the environment of a nanoparticle on its optically extracted geometry and its surface plasmon resonance broadening by surface effect are also studied at a single-particle level.

A novel class of optical metamaterials is presented consisting of high densities of aligned gallium phosphide (GaP) nanowires fabricated using metal-organic vapor phase-epitaxy. Starting from a gold island film as a catalyst for nanowire growth, a sequential combination of vapor-liquid-solid and lateral growth modes is employed to obtain a continuous tunability of the nanowire volume fraction from 7% to over 35%. By choosing different crystallographic orientations of the GaP substrate, metamaterials are designed with different nanowire orientations. The anisotropy of the nanowire building blocks results in strong optical birefringence. Polarization interferometry demonstrates a very large polarization extinction contrast of 4 × 10 3 combined with a sharp angular resonance which holds promise for optical sensing. Nanowire metamaterials may find applications in photonics, optoelectronics, non-linear and quantum optics, microfluidics, bio-, and gas sensing.

We report on a new experimental method for enhanced backscattering spectroscopy (EBS) of strongly scattering media over a bandwidth from 530-1000 nm. The instrument consists of a supercontinuum light source and an angle-dependent detection system using a fiber-coupled grating spectrometer. Using a combination of two setups, the backscattered intensity is obtained over a large angular range and using circularly polarized light. We present broadband EBS of a TiO2 powder and of a strongly scattering porous GaP layer. In combination with theoretical model fits, the EBS system yields the optical transport mean free path over the available spectral window.

We perform time-resolved fluorescence spectroscopy on clusters of plasmonic nanoantennas covered with a dye–polymer mixture. Dimer antenna structures were fabricated consisting of two interacting gold nanorods with varying lengths and interparticle separation. By combining four individual antennas into a cluster within a diffraction limited spot size, we can couple out half of the dye molecule fluorescence via antenna plasmons. Two-dimensional confocal fluorescence lifetime scans visualize the spontaneous emission enhancement of the molecular fluorescence around the antenna clusters.

Here, we present the first broadband birefringence measurements on samples of GaP nanowires. The samples are grown vertically in the metal-organic vapor phase epitaxy (MOVPE) using the vapor-liquid-solid (VLS) growth mode.

We demonstrate a strong, 5-fold enhancement of the radiative decay rate from highly efficient fluorescent dye molecules around resonant optical nanoantennas. The plasmonic modes of individual gold dimer antennas are tuned by the particle length and the antenna gap, providing control over both the spectral resonance position and the near-field mode profile of the nanoantenna. Resonant enhancement of the radiative and nonradiative decay rates of a fluorescent dye is observed, resulting in an increase of the internal quantum efficiency from 40% up to 53% for single antennas, and up to 59% for antenna clusters. This improvement of the already high quantum efficiency of the dye molecules is in agreement with electrodynamic model calculations that predict a maximum attainable efficiency around 80% due to nonradiative losses in the metal.

Optical absorption spectra of small single metal nanoparticles are measured using a far-field technique combining a spatial modulation microscope with a broadband light source. Quantitative determination of the spectral and polarization dependencies of the absorption cross section of individual gold nanoparticles permits precise determination of their geometrical properties in excellent agreement with transmission electron microscopy measurements.

Optical absorption spectroscopy of a single nanoparticle is performed using a spatial-modulation microscope combined with a white-light supercontinuum source. The absolute values of the extinction cross-section of isolated 16nm size gold nanoparticles are determined around their surface plasmon resonance. Measurements performed as a function of the linear light polarization permit to optically identify the properties of an individual particle: size, shape and orientation on the substrate. The statistics of this optically deduced properties are found in excellent agreement with those obtained with transmission electron microscopy.

Far-field optical detection of a single metal nanoparticle using a space modulation technique is modeled and the results compared to experimental data in the case of gold nanospheres. The measured size and wavelength dependence of the absolute absorption cross-section of single nanospheres deposited on a transparent substrate is discussed and compared to theoretical predictions.

We examine the interaction between coherent, terahertz strain solitons, and the metastable $29\text{\ensuremath{-}}{\mathrm{cm}}^{\ensuremath{-}1}$ electronic two-level systems in photoexcited ruby. A large difference in soliton-induced ${R}_{2}$ luminescence is observed for soliton propagation along the crystallographic $c$ axis with respect to earlier experiments along the $a$ direction [Phys. Rev. Lett. 92, 035503 (2004)], that can be explained by the strong anisotropy of the crystal field interaction strength. The behavior of the soliton-induced population of terahertz two-level systems is further investigated in several configurations and explained successfully in the context of a model of nonlinear propagation of the supersonic soliton packets and their coherent interaction with the two-level medium.

Ultrashort strain pulses are a promising tool for the analysis and manipulation of condensed matter, thin films, and nanostructures. We present a new and unconventional way to generate coherent longitudinal acoustic wavepackets of high amplitude in the THz frequency range using the nonlinear development of picosecond strain pulses in a crystal. Our work [PRL 89, 285504 (2002)] demonstrated breakup of an initial wavepacket into a train of ultrashort strain solitons, using position-dependent Brillouin scattering. We extend in this paper the interpretation of the Brillouin scattering data in terms of optical Bragg reflections off the moving soliton train, using the analogy with an N-slit diffraction grating. Finally, we show that these short pulses can excite an electronic two-level system at THz resonance frequency, allowing for coherent amplification and even the development of a phonon laser.

We observe coherent interactions between an ultrashort, longitudinal acoustic soliton train and the 29-cm(-1) electronic transition in photoexcited ruby. Propagation of the strain pulses over millimeter distance through an excited zone reveals striking behavior of the induced electronic population, which has been explained by impulsive excitation of the two-level systems, combined with the nonlinear properties of the solitons in the resonant medium. This opens up new possibilities for coherent manipulation of ultrashort acoustic pulses by local electronic centers.

We demonstrate the development of high-amplitude picosecond strain pulses in a sapphire single crystal into an ultrafast compressional soliton train. For this purpose, large-intensity light pulses were used to excite a metal film, yielding a 2 orders of magnitude higher strain than that achieved in earlier studies. Propagation of the packets is monitored over a distance of several millimeters by means of Brillouin light scattering. A one-parameter model, based on the Korteweg-de Vries-Burgers equation, simultaneously explains the observed behavior at all strains and temperatures under study. We predict up to 11 solitons in the train, reaching pressures as high as 40 kbar and 0.5 ps temporal widths.

Recent experiments on propagation of picosecond acoustic wave packets in condensed matter have opened up a new, exciting area of soliton physics. Single cycle strain pulses as short as several picoseconds can be generated in a thin metallic film, yielding local strain fields of the order of 10−4. The combination of phonon dispersion and anharmonicity of the atomic interaction potential may give rise to strongly nonlinear, but stable propagation of the wave packets over a distance of the order of several millimeters in a single crystalline material. We present new results on nonlinear propagation of acoustic wave packets created by nJ femtosecond optical pulses in a lead molybdate single crystal, employing the Brillouin Scattering technique as a local probe of acoustic strain. Studies of diffraction of narrow discs of acoustic strain show anomalous diffraction of the various Fourier components constituting the wave packet. Propagation of virtually one-dimensional nature is studied by exciting the metal film over a large area using an amplified femtosecond laser. We show that these data can be interpreted by means of the Korteweg-de Vries equation and strongly suggest the development of acoustic solitons.

We experimentally observe the spatial intensity statistics of light transmitted through three-dimensional isotropic scattering media. The intensity distributions measured through layers consisting of zinc oxide nanoparticles differ significantly from the usual Rayleigh statistics associated with speckle, and instead are in agreement with the predictions of mesoscopic transport theory, taking into account the known material parameters of the samples. Consistent with the measured spatial intensity fluctuations, the total transmission fluctuates. The magnitude of the fluctuations in the total transmission is smaller than expected on the basis of quasi-one-dimensional (1D) transport theory, which indicates that quasi-1D theories cannot fully describe these open three-dimensional media., Comment: 4 pages 3 figures

Plasmonic nanoantennas are of interest for their capacity of controlling light on the nanoscale [1]. In particular, precise control of plasmonic near-field interactions on sub-nanometer scales is of interest for achieving strongly enhanced local fields as well as for exploring the fundamental limits of nonlocal and quantum plasmonic effect [2]. Such applications require pushing the limits of nanofabrication to new levels of precision and control. Helium ion microscopy (HIM) is a surface imaging technique which involves scanning a helium ion beam across a sample surface and forming an image from secondary electron emission which is similar to an SEM. The larger mass and therefore smaller de Broglie wavelength of a helium ion compared to that of an electron enables the HIM to focus the charged beam to a smaller spot on a sample. The latest commercially available version of the HIM are rated at an edge resolution of 0.35nm[3]. The helium ion microscope is also a patterning tool which can be used to pattern resist like electron beam lithography or can be used for direct writing on a substrate like a focused ion beam. In our recent work, we have demonstrated the ultrafine control of milling partial antenna gaps and narrow conducting bridges with nanometer precision using a helium ion beam microscope (HIM) [4]. A conducting bridge of nanometer height is found sufficient to shift the antenna from the capacitive to conductive coupling regime, in agree with the circuit theory. In addition to new fabrication tools, we are developing new materials systems to combine plasmonics in electrically and optically controlled environments. I will present recent results showing how we can use new hybrid nonlinearities to achieve tunable and reconfigurable plasmonic devices. [1] Novotny, L.; van Hulst, N. Nat. Photonics 2011, (5) 83-90 [2] Savage, K. J.; Hawkeye, M. M.; Esteban, R.; Borisov, A. G.; Aizpurua, J.; Baumberg, J. J. Nature 2012, (491) 574-577. [3] Bell, D. C.; Lemme, M. C.; Stern, L. A.; Williams, J. R. and Marcus, C. M. 2009 Nanotechnology (20) 455301 [4] Wang, Y.; Abb, M.; Boden, S. A.; Aizpurua, J.; de Groot, C. H.; Muskens, O. L. Nano Lett. 2013, 13(11), 5647-5653 Figure 1 (left panel) Illustration showing analogy between radiowave antenna and plasmonic nanoantenna. Bottom: SEM image of antenna fabricated using e-beam lithography. (right panel) SEM image showing nano-gap obtained by helium ion beam milling.

The potential of small metal nanoparticles for converting resonant light into local heat has given rise to new applications in biomedicine [1]. While many studies focus on destructive laser hyperthermia of malignant cancer cells, the impact of photothermal therapies using plasmonic nanoparticles on healthy, primary human cells is of interest for controlling biological functions and for combined, dual-action treatments. Here, we present results on the controlled laser treatment of human endothelial cells (HUVECs) using oligo-ethylene glycol (OEG-)coated nanoparticles, which are either specifically targeted using peptide-functionalization, or which are nonspecifically taken up by the cells without further functionalization [2-4]. Peptide-functionalized nanoparticles were found to specifically bind to VEGF-receptors in the cell membrane [2]. Subsequent laser treatment at mild conditions of up to 30 W/cm2laser intensity resulted in partially reversible damage of the cellular membrane. A remarkable recovery of cells was found within 24 hours following treatment [3]. For non-functionalized nanoparticles, combined ICP-OES analysis and numerical modeling indicated that irreversible cell apoptosis was caused by collective heating effects over the illumination spot (Fig. 1, left panel) [3]. It is argued that the multiscale effects of nanoparticle hyperthermia range from single nanoparticles to collective heating of macroscopic areas, and on time scales from picoseconds to seconds, are an important factor in designing effective nanoparticle treatments. New studies explore plasmonic laser treatment on in-vitro angiogenesis, the organisation of endothelial cells into a network of microvessels (Fig.1, right panel). [1] Dreaden, E. C., Alkilany, A. M., Huang, X., Murphy, C. J., and El-Sayed, M. A., Chem. Soc. Rev., 41, 2740-2779 (2012); Barreto, J. A., O'Malley, W., Kubeil, M., Graham, B., Stephan, H., Spiccia, L., Adv. Mater., 23, H18-H40 (2011). [2] Bartczak D, Sanchez-Elsner T., Louafi F., Millar T., Kanaras A. G., Small 7, 388–394 (2011). [3] D. Bartczak, O. L. Muskens, T. M. Millar, T. Sanchez-Elsner, A. G. Kanaras, Nano Lett. 11, 1358-1363 (2011). [4] D. Bartczak, O. L. Muskens, S. Nitti, T. Sanchez-Elsner, T. M. Millar, and A. G. Kanaras, Small 8, 122-130 (2012). Figure 1 (left panel) Calculated temperature increase caused by a single nanoparticle (left) and integrated over the 500mm illumination spot (right) [4]. (right panel) In-vitro angiogenesis of HUVECs in a MDA cancer-cell medium.

The performance of plasmonic nanoantenna structures on top of SOI wire waveguides as coherent perfect absorbers for modulators and all-optical switches is explored. The absorption, scattering, reflection and transmission spectra of gold and aluminum nanoantenna-loaded waveguides were calculated by means of 3D finite-difference time-domain simulations for single waves propagating along the waveguide, as well as for standing wave scenarios composed from two counterpropagating waves. The investigated configurations showed losses of roughly 1% and extinction ratios greater than 25 dB for modulator and switching applications, as well as plasmon effects such as strong field enhancement and localization in the nanoantenna region. The proposed plasmonic coherent perfect absorbers can be utilized for ultracompact all-optical switches in coherent networks as well as modulators and can find applications in sensing or in increasing nonlinear effects., Comment: 10 pages, 6 figures

Semiconductor nanowires exhibit large polarization anisotropy for the absorption and emission of light, making them ideal building blocks for novel photonic metamaterials. Here, we demonstrate that a high density of aligned nanowires exhibits giant optical birefringence, a collective phenomenon observable uniquely for collections of wires. The nanowire material was grown on gallium phosphide (GaP) (111) in the form of vertically standing GaP nanowires. We obtain the largest optical birefringence to date, with a difference between the in-plane and out-of-plane refractive indices of 0.80 and a relative birefringence of 43%. These values exceed by a factor of 75 the natural birefringence of quartz and a by more than a factor of two the highest values reported so far in other artificial materials. By exploiting the specific crystallographic growth directions of the nanowires on the substrate, we further demonstrate full control over the orientation of the optical birefringence effect in the metamaterial.

Optical solar reflectors are devices that combine high reflection for visible wavelengths with a strong emissivity in the infrared. Compared to the conventional rigid quartz tiles used on spacecraft since the 1960s, thin-film solutions can offer a significant advantage in weight, assembly, and launch costs. Here, we present a metasurface-based approach using an Al-doped ZnO (AZO) transparent conducting oxide as infrared plasmonic material. The AZO is patterned into a metasurface to achieve broad plasmonic resonances with an enhanced absorption of electromagnetic radiation in the thermal infrared. In the visible range, the transparent conducting oxide provides low losses for solar radiation, while intrinsic absorption losses in the ultraviolet range are effectively suppressed using a multilayer reflecting coating. The addition of high-emissivity layers to the stack eventually results in comparable emissivity values to the thin plasmonic device, thus defining a window of opportunity for plasmonic absorption as a design strategy for ultrathin devices. The optimized experimental structure achieves solar absorptance (α) of 0.16 and thermal emissivity (ε) of 0.79. Our first prototype demonstrator paves the way for further improvement and large-area fabrication of metasurface solar reflectors and ultimately their application in space missions.

Recently metal oxides have been introduced as promising materials for infrared and active plasmonics. By designing nanoantennas and metamaterials using transparent conducting oxides (TCOs), we can achieve strong light-matter interactions in the infrared while maintaining high transparency in the visible range. These properties have been used to design new types of infrared active surfaces for optical sensing and metamaterials [1, 2]. Compared to noble-metals, the TCOs offer a strongly reduced negative permittivity which allows for much more compact resonant nanostructures and hence a higher density of elements per square wavelength. Up to 80 resonant elements per square wavelength were realized using ITO split-ring resonators.

Plasmonics holds promise for achieving enhanced light-matter interaction for nonlinear transistor-type optical devices. Here, I will discuss several approaches to obtain active control over the optical near-field and far-field modes of plasmonic antennas.

Metal-infiltrated protein crystals form a novel class of bio-nanomaterials of great interest for applications in biomedicine, chemistry, and optoelectronics. As yet, very little is known about the internal structure of these materials and the interconnectivity of the metallic network. Here, the optical response of individual Au- and Ag-infiltrated cross-linked lysozyme crystals is investigated using angle- and polarization-dependent spectroscopy. The measurements unequivocally show that metallic inclusions formed inside the nanoporous solvent channels do not connect into continuous nanowires, but rather consist of ensembles of isolated spheroidal nanoclusters with aspect ratios as high as a value of four, and which exhibit a pronounced plasmonic response that is isotropic on a macroscopic length scale. Fluorescence measurement in the visible range show a strong contribution from the protein host, which is quenched by the Au inclusions, and a weaker contribution attributed to the molecule-like emission from small Au-clusters.

Plasmonic resonance shift between capacitive and conductive coupling of a partially loaded dimer antenna has been achieved by the ultrafine control of milling partial antenna gaps with nanometer precision using a helium ion beam microscope.

Spatial modulation microscopy is a technique originally developed for quantitative spectroscopy of individual nano-objects. Here, a parallel implementation of the spatial modulation microscopy technique is demonstrated based on a line detector capable of demodulation at kHz frequencies. The capabilities of the imaging system are shown using an array of plasmonic nanoantennas and dendritic cells incubated with gold nanoparticles., 3 pages, 4 figures