In this paper, we present results from our theoretical and experimental exploration of tailoring the absorption spectrum of a type of metamaterial absorber through manipulating the symmetricity and uniformity of the metallic submicron particle array on the top layer. The absorber under study is a metal-insulator-metal (MIM) trilayer structure made up of a top layer of engineered metallic submicron particles, a middle insulator spacer layer and an opaque ground metal reflector layer. We first studied the structure with a top layer consisting of a uniform array of raindrop-shaped gold (Au) submicron disks. We designed the raindrop shape with a reflectional symmetry on the 45° line. We compared the spectrum generated with that of a similar structure but the top layer which is filled with uniformly arranged circular submicron discs. It has been well reported that an array of circular particles each with both reflectional and rotational symmetries usually generates a spectrum with one absorption spike. By changing the circular shape to raindrop shape, the MIM absorber has been predicted to generate two absorption peaks with significantly broadened absorption bandwidth. Subsequently, we found that even wider spectra could be achieved if the top layer is built with a periodic arrangement of the unit cells containing differently sized raindrop-shaped disks. This leads to a wider bandwidth of higher than 50% absorbance ranging from 2.80 μm to 3.90 μm.
Incorporating resonant optical properties of metal nanostructures into nanoscale applications such as ultrahigh density storage devices, nanoelectronics, and nanophotonics has gained considerable interest within the last years. Recent advances in hybrid and molecular plasmonics are presented. The approach relies on near-field energy transfer between metal nanoparticles and other molecular material, and is not diffraction-limited. We will see that optical nanosources supported by metal nanoparticles can be used for controlling/triggering photochemical and photo physical processes involving photons, charges and motion transfers at the nanoscale. In particular, three examples will be presented and commented: free radical photopolymerization, photo isomerization and nanoscale strong coupling. These examples open new routes including optical near-field photography of ultra confined fields, mode hybridizing in single nanoparticles, molecular optical nanomotors, and new anisotropic nanoemitters.
We studied the near-field optical properties of colloidal gold nanocubes (GNCs) using a photochemical imaging method.
This method is based on the vectorial molecular displacements, of photosensitive azo-dyes, which are sensitive to the
polarization of the optical near-field of the GNCs. We analyzed the spatial confinement of both electromagnetic hot and
"cold" spots with a spatial resolution up to 15nm (λ/35). The new concept of cold spot presents valuable and
complementary electromagnetic information to the well known electromagnetic hot spot. We demonstrated that cold
spots are highly sensitive to polarization and can be much more confined than hot spots enabling them to be applied in
high resolution imaging and spectroscopy.
Nanoscale materials absorb, propagate, and dissipate energy very differently than their bulk
counterparts. Furthermore, hybrid nanostructures consisting of molecular and plasmonic materials with
strongly coupled electronic states can produce new optical states and decay pathways that provide additional
handles with which to externally control energy flow in complex nanostructured systems. In this talk, we
discuss our recent studies of electromagnetic coupling and associated temporal dynamics of molecular
excitations with plasmonic resonances supported by either localized or extended planar geometries. Recent
experimental results and theoretical analysis for observing and controlling coherences between molecular
excitations and plasmonic polarizations are shown. Advances will explore new directions in ultrafast
manipulation of energy dissipation processes in hybrid plasmonic structures, as well as ultrafast addressing
and switching in plasmonics-based circuit architectures. Also discussed are recent synthetic advances in the
creation of hybrid materials. Ultimately, these studies may impact a range of next-generation optical materials
and devices, of relevance to new energy conversion materials, nanoscale photocatalysis, or plasmon-enhanced
The effect of field component perpendicular to the surface (longitudinal fields) on the photo-induced molecular
migration and surface deformations in azobenzene polymer films are investigated. Case of tip-enhanced near-field
illumination of the polymer surface is first discussed. In order to rule out the possible influence of mechanical
interaction between tip and polymer, tightly focused higher-order laser beams are then used. We demonstrate
that the surface topography is principally induced by longitudinal fields. Our findings can be explained by the
translational diffusion of isomerized chromophores when the constraining effect of the polymer-air interface is
Local surface plasmons resonances are widely accepted to be the basis for improving the efficiency of absorption and emission processes through a local electromagnetic field enhancement. Nonlinear processes in gold surfaces such as second harmonic generation or two-photon induced photoluminescence are particularly sensitive to this local effect due to their quadratic dependence on the intensity. Isolated regions of enhanced photoluminescence yield on rough gold surfaces were identified emphasizing the physical similarities with surface enhanced Raman scattering (SERS) substrates. In this vein, we investigated luminescence from individual gold nanorods and found that their emission characteristics closely resemble surface plasmon behavior. In particular, we observed spectral similarities between the scattering spectra of individual nanorods and their photoluminescence emission. We also measured a blue-shift of the photoluminescence peak wavelength with decreasing aspect ratio of the nanorods as well as an optically tuneable shape-dependent spectrum of the photoluminescence. The emission yield of single nanorods strongly depends on the orientation of the incident polarization consistent with the properties of surface plasmons.
Plasmonics applications will benefit if reliable means to alter plasmon absorption and damping properties via external inputs are found. We are working towards this goal by functionalizing noble metal films with polarizable, excitonic molecular films. Examples include molecular j-aggregates, whose excitonic absorptions can be photobleached to modify plasmon absorption properties. We report two developments in this area. The first is the observation of coherent polarization coupling between the exciton of a molecular J-aggregate and the electronic polarization of noble metal nanoparticles. The second is a new far-field method to directly observe surface plasmon propagation, demonstrating that the lateral intensity decay length is affected by a change of the interface property. The method relies on the detection of the intrinsic lossy modes associated with plasmon propagation in thin films. We also uniquely introduce a method to excite a broad spectral distribution of surface plasmon simultaneously throughout the visible spectrum allowing surface plasmon based spectroscopy to be performed.
The surface plasmons of metal films and nanostructures are increasingly well-known for applications in sensor technologies and photonics applications. Their potential is largely due to the plasmons' characteristic as an interface phenomenon and the generation of an optical near-field at the interface. In many cases, the spatial dimensions of the near-field lie significantly below the diffraction limit of conventional optics in at least one dimension. This requires novel methods means for imaging their spatial profile and propagation properties. We present recent methods ongoing in our laboratory for imaging plasmonic features of metal nanostructures
A new method for optically exciting and visualizing surface plasmons in thin metal films is described. The technique relies on the use of a high numerical aperture objective lens to locally launch surface plasmons with an area much smaller than their lateral decay length. We visualize directly the intensity distribution of the surface plasmons by detecting the intrinsic lossy modes associated with plasmon propagation in thin films. Our approach allowed us to excite simultaneously a broad spectral continuum of surface waves and to describe for the first time surface plasmon rainbow jets. We quantified the attenuation of the jet as a function of wavelength and film thickness and compared it to the different propagation damping mechanisms. We demonstrated the influence of the interface on the surface plasmon propagation length and demonstrated surface plasmon spectral filtering using molecular excitonic adsorbates.
High resolution, conformable phase masks provide a means to fabricate, in an experimentally simple manner, classes of three dimensional (3D) nanostructures that are technologically important but difficult to generate in other ways. This proposed approach can answer the greatest challenge of photonic system; a simple and reliable fabrication method of periodic and aperiodic structure. Those unique advantages of proximity field nanopatterning originated from direct conformal contact and a further application to multi-photon process, and a representative waveguiding structure are investigated. The patterning capability in a broad range of wavelengths (from UV to near-IR) and unusual structures place this method as a key technique for photonic system.
We report on the study of the optical properties of hybrid systems made from molecular aggregates (J-aggregates) and metallic nanoparticles (Au and Ag). Both entities have outstanding optical properties that have been extensively addressed both through experimental and theoretical efforts. The J-aggregates were chosen for their linear and non-linear excitonic response that shows among other intriguing properties, superradiant emission, ultrafast optical switching, and electroluminescence. These J-aggregates form spontaneously on the surface of Au and Ag nanoparticles, used primarily to excite the organic shell through optical near-field interaction. Steady state as well as ultrafast spectroscopic measurements was used to characterize the nature of the interaction between the two constituents as well as to follow the dynamics of the exciton in the aggregate upon near-field excitation from the particle. For both Au and Ag, Mie scattering theory calculations of the hybrid ground state absorption spectra account for the experimental observations and reflect the coherent coupling between the excitonic states in the aggregate and the nanoparticles electronic transition dipoles. Furthermore we show that the dipole-dipole coupling strength between the individual molecules in the aggregate is increased by ~30% on the surface of the particle compared to its value in solution. The different dynamics of these nanosystems have been probed using femtosecond spectroscopy and reveals contrasted relaxation pathways for the exciton when interacting with Ag with respect to Au as well as a delay dependent excitonic coherent length.
Illumination of metal nanoparticles at the plasmon resonance produces enhanced evanescent fields on the nanoparticles’ surfaces. The unusual strength of the field make it a target for exploring photoinduced phenomena at the nanoscale, if efficient functionalization or coating of the nanoparticle surface with appropriate chromophores is possible. One direction is to use cyanine dyes that form monolayers of J-aggregates on the surface of noble metal nanoparticle colloids. The unique, collective electronic properties of J-aggregates produce excitons with enormous
extinction coefficients that are of interest for their efficient energy transfer, electron transfer, and nonlinear optical
properties. In that vein, we report our results on time-resolved spectroscopy and near-field scanning optical microscopy (NSOM) of J-aggregate exciton dynamics on Ag and Au nanoparticle colloids. Ultrafast transient absorption studies show that J-aggregate exciton lifetimes on Ag nanoparticles are much longer than on Au nanoparticles, with a 300 ps lifetime that is two orders of magnitude longer than the electronic processes in the nanoparticles themselves. Complementary NSOM studies of the colloids show that fluorescence from the J-aggregates on the Ag nanoparticles is induced by the scanning probe. These results may be significant for improving
the nanophotonic performance of hybrid materials for nanoscale applications.
The first observation of photorefractivity in ferroelectric liquid crystals (flcs) is reported. The flcs are doped with the easily oxidized chromophore perylene, which also functions as the sensitizer. The electron acceptor di-buty1-pyromellitimide is added to induce photoconductivity through an efficient intermolecular electron transfer reaction to produce mobile ions. A strong dependence on the orientation of the wavevector of the optical interference pattern and polarization of the writing beams relative to the orientation of the lfc molecules is observed. The results are interpreted as an orientational photorefractive effect in which the net polarization of the flc couples linearly to the space-charge field as opposed to nematic liquid crystals in which the dielectric anisotrophy couples to the square of the space charge field.
Photoconductive polymers are doped into liquid crystals to create a new mechanism for space-charge field formation in photorefractive liquid crystal composites. The composites contain poly(2,5-bis(2'-ethylhexyloxy)-1,4- phenylenevinylene) (BEH-PPV) and the electron acceptor N,N'- dioctyl-1,4:5,8-naphthalenediimide, NI. Using asymmetric energy transfer (beam coupling) measurements that are diagnostic for the photorefractive effect, the direction of beam coupling as a function of grating fringe spacing inverts at a spacing of 5.5 micrometers . We show that the inversion is due to a change in the dominant mechanism for space-charge field formation. At small fringe spacings, the space-charge field is formed by ion diffusion in which the photogenerated anion is the more mobile species. At larger fringe spacings, the polarity of the space charge field inverts due to dominance of a charge transport mechanism in which photogenerated holes are the most mobile spaces due to hole migration along the BEH-PPV chains coupled with interchain hole hopping. Control experiments are presented, which use composites that can access only one of the two charge transport mechanisms. The results show that charge migration over long distances leading to enhanced photorefractive effects can be obtained using conjugated polymers dissolved in liquid crystals.
Polymer-stabilized liquid crystals, consisting of low concentrations of a polymeric electron acceptor, are shown to exhibit significantly enhanced photorefractive properties. The charge generation and transport properties of these composite systems are strongly modified from nematic liquid crystals doped with electron donors and acceptors. The new composites are produced by polymerizing a small quantity of a 1,4:5,8-naphthalenediimide electron acceptor functionalized with an acrylate group in an aligned nematic liquid crystal. Photopolymerization creates an anisotropic gel-like medium in which the liquid crystal is free to reorient in the presence of a space charge field, while maintaining charge trapping sites in the polymerized regions of the material. The presence of these trapping sites results in the observation of longer lived, higher resolution holographic gratings in the polymer-stabilized liquid crystals than observed in nematic liquid crystals alone. These gratings display Bragg regime diffraction. Asymmetric beam coupling, photo-conductivity, and four-wave mixing experiments are performed to characterize the photophysics of these novel materials.
Ultrafast transient absorption studies of intramolecular photoinduced charge separation and thermal charge recombination were carried out on a molecule consisting of a 4-(N-pyrrolidino)naphthalene-1,8-imide donor (PNI) covalently attached to a pyromellitimide acceptor (PI) dissolved in the liquid crystal 4'-(n-pentyl)-4- cyanobiphenyl (5CB). The temperature dependencies of the charge separation and recombination rates were obtained at temperatures above the nematic-isotropic phase transition of 5CB, where ordered microdomains exist and scattering of visible light by these domains is absent. We show that excited state charge separation is non-adiabatic, and obtain the unexpected result that charge separation is dominated by molecular reorientation of 5CB perpendicular to the director within the liquid crystal microdomains relative to the orientation of PNI+-PI-. We also report the result of time resolved electron paramagnetic resonance studies of photoinduced charge separation in a series of supramolecular compounds dissolved in oriented liquid crystal solvents. These studies permit the determination of the radical pair energy levels as the solvent reorganization energy increase from the low temperature crystalline phase, through the soft glass phase, to the nematic phase of the liquid crystal.
We report recent improvements in the photorefractive performance of liquid crystalline thin film composites containing electron donor and acceptor molecules. The improvements primarily result from optimization of the exothermicity of the intermolecular charge transfer reaction and improvement of the diffusion characteristics of the photogenerated ions. Intramolecular charge transfer dopants produce greater photorefractivity and a 10-fold decrease in the concentration of absorbing chromophores. The mechanism for the generation of mobile ions is discussed.
The polar low frequency vibrational and relaxational modes crucial to understanding the structure and phase transitions of ferroelectric perovskite crystals are examined using impulsive stimulated Raman scattering (ISRS). At wavevectors near the Brillouin zone center, these modes are strongly coupled to light, and the pure optic mode behavior is deduced from the wavevector dependence of the mixed phonon-polaritons.
Timed sequences of femtosecond pulses produced by pulse-shaping techniques
have been used to achieve improved optical control over molecular motion in crystalline
solids. Selected lattice vibrational modes in an organic molecular crystal have been driven
repetitively by appropriately timed pulse sequences in a manner analogous to that in which
a child on a swing is pushed repetitively with timed mechanical forces. Repetitive driving
with a pulse sequence results in larger lattice vibrational amplitudes and improved modeselectivity
compared to driving with a single pulse. Numerous applications of pulseshaping
techniques in femtosecond spectroscopy are anticipated.