Interferometric spectroscopy and high-speed orientation detection of individual gold nanorods.

Although 3D positional tracking of single nanoparticles in suspension is now an established technique, the small size of the particles compared to the wavelength of light has meant it is still challenging to optically characterize individual diffusing particles in other ways. Here we introduce Quantitative Optical Anisotropy Imaging (QOAI), an interferometric technique that fills some of this gap by allowing for real-time tracking of orientation as well as spectroscopic characterization of polarizability in nanoparticles at the microsecond timescale. Applying this to gold nanorods, we demonstrate measurement of nanorod orientation with high precision with simultaneous spectroscopic characterization of the rods' longitudinal plasmon resonance. We also show that we can quantify rotational diffusion in individual particles in both the azimuthal and polar directions near a solid wall, as well as detecting binding of particles to that wall. The simple optical configuration of QOAI will make combining it with positional nanoparticle tracking techniques straightforward, and this opens the door to measurements that are not reachable with current techniques, such as detailed characterization of correlations between rotational and translational diffusion in nanoparticles, real-time observation of particle aggregation and assembly, and measurements of fluctuations in the plasmon resonance in metal nanoparticles as they encounter a changing or heterogeneous environment.

Tunable gap plasmons in gold nanospheres adsorbed into a pH-responsive polymer film.

HYPOTHESIS: Plasmon nanorulers are exquisitely sensitive distance sensors that are based on the electromagnetic interaction between metal nanoparticles and surfaces. We hypothesize that nanorulers can act as quantitative probes of processes such as particle aggregation and adsorption, and deploy them to investigate particle adsorption onto stimulus-responsive polymer films. While such systems have previously been qualitatively investigated with plasmon nanorulers, our quantitative analysis should provide deeper insights. EXPERIMENT: Gold nanospheres are adsorbed from solution onto pH-responsive, amine-rich polyelectrolyte multilayer (PEM) films that are either directly deposited on a gold substrate or onto an intermediate self-assembled monolayer (SAM) of charged thiols. Fitting the optical scattering spectrum to a full-wave calculation, we quantify the sphere-substrate gap distance with good accuracy. FINDINGS: We find that the gold spheres partially embed into the PEMs rather than ride on top of them, and that although the amount of actuation of the spheres afforded by tuning the pH is well controlled, it is significantly smaller than the corresponding thickness changes in unstrained films. Further, the presence of a SAM below the PEM increases the amount of polymer in the PEM, except for the thickest and most highly charged films, where the SAM instead appears to displace from the area below the nanospheres.

Multi-photon patterning of photoactive o-nitrobenzyl ligands bound to gold surfaces.

We quantitatively investigate lithographic patterning of a thiol-anchored self-assembled monolayer (SAM) of photocleavable o-nitrobenzyl ligands on gold through a multi-photon absorption process at 1.7 eV (730 nm wavelength). The photocleaving rate increases faster than the square of the incident light intensity, indicating a process more complex than simple two-photon absorption. We tentatively ascribe this observation to two-photon absorption that triggers the formation of a long-lived intermediate aci-nitro species whose decomposition yield is partially determined either by absorption of additional photons or by a local temperature that is elevated by the incident light. At the highest light intensities, thermal processes compete with photoactivation and lead to damage of the SAM. The threshold is high enough that this destructive process can largely be avoided, even while power densities are kept sufficiently large that complete photoactivation takes place on time scales of tens of seconds to a few minutes. This means that this type of ligand can be activated at visible and near infrared wavelengths where plasmonic resonances can easily be engineered in metal nanostructures, even though their single-photon reactivity at these wavelengths is negligible. This will allow selective functionalization of plasmon hotspots, which in addition to high resolution lithographic applications would be of benefit to applications such as Surface Enhanced Raman Spectroscopy and plasmonic photocatalysis as well as directed bottom-up nanoassembly.

Light-Directed Patchy Particle Fabrication and Assembly from Isotropic Silver Nanoparticles.

We demonstrate the creation of anisotropic patchy silver nanospheroids (AgNSs) using linearly polarized UV light and a photo-uncaging o-nitrobenzyl-based ligand, which anchors to the AgNSs by two gold-sulfur bonds. Exposure to a 1 J/cm2 dose of UV light induces a photo-uncaging reaction in the ligand that reveals a primary amine on the surface. By using linearly polarized UV light, we meter the exposure dose such that only the poles of the nanoparticle receive a full dose, limiting the photo-uncaging reaction primarily to the particle's plasmonic hot spots. We reveal this anisotropy by preferentially adhering negatively charged gold nanospheres (AuNSs) to the AgNSs' poles by using the electrostatic attraction between them and the positively charged primary amines generated by photo-uncaging. When the assembly is performed onto silver particles that are immobilized on a substrate, it results in nanoscale structures with a strong tendency to align with the polarization of the exposing light. This manifests in polarimetric spectroscopy as a linear dichroism aligned with the polarization direction.

Fluorescence lifetime based characterization of active and tunable plasmonic nanostructures.

We report a non-contact method that utilizes fluorescence lifetime (FL) to characterize morphological changes of a tunable plasmonic nanostructure with nanoscale accuracy. The key component of the plasmonic nanostructure is pH-responsive polyelectrolyte multilayers (PEMs), which serve as a dynamically tunable "spacer" layer that separates the plasmonic structure and the fluorescent materials. The validity of our method is confirmed through direct comparison with ellipsometry and atomic force microscopy (AFM) measurements. Applying the FL-based approach, we find that a monolayer polycation film responds to pH changes with significantly less hysteresis than a thicker multilayer film with polyelectrolytes of both charges. Additionally, we characterize an active and tunable plasmonic nanostructure composed of self-assembled fluorescent dye (Texas Red), pH-sensitive PEMs, and gold nanospheres adsorbed on the PEM surface. Our results point towards the possibility of using stimulus-sensitive polymers to construct active and tunable plasmonic nanodevices.

Impact of lithography on the fluorescence dynamics of self-assembled fluorophores.

Micro- and nano-patterned fluorescent materials are important for many photonic devices and applications. In this paper, we investigate the impact of three common lithographical techniques, deposition and removal of sacrificial masks, ultraviolet ablation, and focused ion beam milling, on self-assembled fluorophores. We find that different patterning techniques can dramatically change the fluorescence lifetime of the fluorophores and that the degree of modification depends on the patterning techniques.

High quality factor silica microspheres functionalized with self-assembled nanomaterials.

With extremely low material absorption and exceptional surface smoothness, silica-based optical resonators can achieve extremely high cavity quality (Q) factors. However, the intrinsic material limitations of silica (e.g., lack of second order nonlinearity) may limit the potential applications of silica-based high Q resonators. Here we report some results in utilizing layer-by-layer self-assembly to functionalize silica microspheres with nonlinear and plasmonic nanomaterials while maintaining Q factors as high as 10(7). We compare experimentally measured Q factors with theoretical estimates, and find good agreement.

Two-photon activated two-photon fluorescence and binding of azidocoumarin in a gelatin matrix.

We study the creation of fluorescence patterns inside a gelatin gel by way of two-photon photoactivation of 7-azido-4-trifluoromethyl-1,2-benzopyrone (azidocomarin 151) contained in the gel matrix. As ultrafast light pulses are focused into the gel, onset of two-photon fluorescence, highly nonlinear in the applied optical power, is observed as azidocoumarin is converted into a fluorescent dye that binds to the gelatin. We fit the time dependence of the fluorescence to a model that incorporates the competition between coumarin photoactivation and photobleaching as well as the gradual degradation of the gel when it is exposed to the high intensity laser light. The model predicts that the initial rate of fluorescence onset should scale as the P (4), where P is laser power, while the signal at long exposure time should scale as P (3/2). The observed exponents are 4.18 and 1.34, respectively. The model allows us to estimate the cross section and quantum yield of two-photon induced photobleaching of azidocoumarin 151. The numerous technical uses of gelatin and the collagen from which it derives in areas ranging from photography to tissue engineering provide possible applications for the techniques described in this paper.

Irreversible adsorption of gold nanospheres on fiber optical tapers and microspheres.

We study the adsorption of gold nanospheres onto cylindrical and spherical glass surfaces from quiescent particle suspensions. The surfaces consist of tapers and microspheres fabricated from optical fibers and were coated with a polycation, enabling irreversible nanosphere adsorption. Our results fit well with theory, which predicts that particle adsorption rates depend strongly on surface geometry and can exceed the planar surface deposition rate by over two orders of magnitude when particle diffusion length is large compared to surface curvature. This is particularly important for plasmonic sensors and other devices fabricated by depositing nanoparticles from suspensions onto surfaces with non-trivial geometries.

Amine-rich polyelectrolyte multilayers for patterned surface fixation of nanostructures.

We describe a lithographic method for directly patterning the adhesive properties of amine-rich layer-by-layer assembled polymer films, useful for positioning metal and other nanostructures. The adhesive properties of the films are sufficiently robust that the films can be patterned with standard as opposed to soft lithographic methods. We perform the patterning with a lithographically defined evaporated aluminum mask which protects selected regions of the substrate, passivating adhesion in the exposed regions with acetic anhydride. When the aluminum is removed with a HCl etch, the protected regions retain their adhesion, whereas particle adsorption is almost completely eliminated in the passivated areas, making it possible to guide adsorption to the protected areas. The high degree of adhesion comes about because of uncoordinated amine groups that pervade the film. Cycling the pH from high values to low and back causes the amines to be rearranged, rejuvenating the adhesive properties of the surface, which is the likely origin of the robustness of the adhesive properties to processing. pH adjustment also causes reversible swelling and deswelling of the film, so that the vertical position and dielectric environment of the nanostructure can be dynamically adjusted, which can be particularly beneficial for tuning the plasmonic resonances of metallic structures.

Restricted meniscus convective self-assembly.

Convective (or evaporation-induced) self-assembly is a standard technique for depositing uniform, poly-crystalline coatings of nanospheres across multiple square centimeters on the timescale of minutes. In this paper, we present a variation of this technique, where the drying meniscus is restricted by a straight-edge located approximately 100 microm above the substrate adjacent to the drying zone. Surprisingly, we find this technique to yield films at roughly twice the growth rate compared to the standard technique. We attribute this to differing rates of diffusion of vapor from the drying crystal in the two cases. We also investigate the crystal growth rate dependence on ambient relative humidity and find, contrary to some previous reports, that the growth rate depends strongly on the humidity. We introduce a model which indicates that while the length of the drying zone may increase with humidity, this alone cannot compensate for the simultaneous reduction in evaporation rate, so a lower humidity must always lead to a higher growth speed. Comparing the model to our experimental results, we find that the length of the drying zone is constant and mostly independent of parameters such as humidity and surface tension.

Plasmon-enhanced second-harmonic generation from ionic self-assembled multilayer films.

We have demonstrated large enhancements of the effective second-order nonlinear susceptibility (chi(2)) of ionic self-assembled multilayer (ISAM) films, causing a film with just 3 bilayers to be optically equivalent to a 700-1000 bilayer film. This was accomplished by using nanosphere lithography to deposit silver nanoparticles on the ISAM film, tuning the geometry of the particles to make their plasmonic resonances overlap the frequency of optical excitation. An enhancement in the efficiency of second harmonic generation (SHG) by as much as 1600 times was observed. Even though this is already a large value, we suggest that further refinements of the techniques are expected to lead to additional enhancements of similar or larger magnitude.