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Sub-wavelength chiral resonators formed from artificial structures exhibit exceedingly large chiroptical responses compared to those observed in natural media. Owing to resonant excitation, chiral near fields can be significantly enhanced for these resonators, holding great promise for developing enantioselective photonic components such as biochemical sensors based on circular dichroism (CD) and spin-dependent nonlinear imaging. In the present work, strong linear and nonlinear chiroptical responses (scattering CD > 0.15 and nonlinear differential CDs > 0.4) at visible and near infrared frequencies are reported for the first time for individual micrometer-scale plasmonic and dielectric helical structures. By leveraging dark-field spectroscopy and nonlinear optical microscopy, the circular-polarization-selective scattering behavior and nonlinear optical responses (e.g., second harmonic generation and two-photon photoluminescence) of 3D printed micro-helices with feature sizes comparable to the wavelength (total length is ∼5λ) are demonstrated. These micro-helices provide potential for readily accessible photonic platforms, facilitating an enantiomeric analysis of chiral materials. One such example is the opportunity to explore ultracompact photonic devices based on single, complex meta-atoms enabled by state-of-the-art 3D fabrication techniques. [ABSTRACT FROM AUTHOR]

The photoluminescence (PL) mechanisms of gold nanorods following nonlinear excitation are described. Using single-particle nonlinear optical measurements, we compare PL signals resulting from both the plasmon-resonant and non-resonant excitations. In both cases, spectrally broad interband PL emission was observed. However, we also show that resonant excitation of the longitudinal plasmon mode leads to an increased photonic density of states at energies corresponding to the transverse plasmon resonance. This increased density of states is achieved by a multi-step mechanism, which is initiated by three-photon excitation and followed by an Auger relaxation process. Importantly, the results show that nonlinear excitation can lead to energy and polarization modulation of nanoparticle optical signals that are not observed using linear excitation. This work also demonstrates the effectiveness of single-nanoparticle PL studies for understanding how plasmon-resonant excitations can be used to modify hot carrier distributions. [ABSTRACT FROM AUTHOR]

Electronic spin-state dynamics were studied for a series of Au25(SC8H9)18q and Au24Pd(SC8H9)18 monolayer-protected clusters (MPCs) prepared in a series of oxidation states, q, including q = −1, 0, +1. These clusters were chosen for study because Au25(SC8H9)18−1 is a closed-shell superatomic cluster, but Au25(SC8H9)180 is an open-shell (7-electron) system; Au25(SC8H9)18+1 and PdAu24(SC8H9)180 are isoelectronic (6-electron) closed-shell systems. Carrier dynamics for electronic fine structure spin states were isolated using femtosecond time-resolved circularly polarized transient-absorption spectroscopy (fs-CPTA). Excitation energies of 1.82 eV and 1.97 eV were chosen for these measurements on Au25(SC8H9)180 in order to achieve resonance matching with electronic fine structure transitions within the superatomic P- and D-orbital manifolds; 1.82-eV excited an unpaired Pz electron to D states, whereas 1.97-eV was resonant with transitions between filled Px and Py subshells and higher-energy D orbitals. fs-CPTA measurements revealed multiple spin-polarized transient signals for neutral (open shell) Au25(SC8H9)18, following 1.82-eV excitation, which persisted for several picoseconds; time constants of 5.03 ± 0.38 ps and 2.36 ± 0.59 ps were measured using 2.43 and 2.14 eV probes, respectively. Polarization-dependent fs-CPTA measurements of PdAu24(SC8H9)18 clusters exhibit no spin-conversion dynamics, similar to the isoelectronic Au25(SC8H9)18+1 counterpart. These observations of cluster-specific dynamics resulted from spin-polarized superatom P to D excitation, via an unpaired Pz electron of the open-shell seven-electron Au25(SC8H9)18 MPC. These results suggest that MPCs may serve as structurally well-defined prototypes for understanding spin and quantum state dynamics in nanoscale metal systems. [ABSTRACT FROM AUTHOR]

The use of plasmon amplification of nonlinear optical wave-mixing signals to generate optical images in which the position of the scattering point source can be determined with nanometer accuracy is described. Solid gold nanosphere dimers were used as a model system for the nonlinear medium, which converted the Ti:sapphire fundamental to its second harmonic frequency. Matching the fundamental wave energy to the localized surface plasmon resonance of the electromagnetically coupled nanospheres was critical for achieving the high localization accuracy. Our technique, named Nonlinear Optical Localization using Electromagnetic Surface fields (NOLES) imaging, routinely yielded nonlinear optical images with 1-nm localization accuracy at rates ≥2 fps and can also be used as a photo-switching localization contrast method. This high level of accuracy in pinpointing the signal point source position exceeded that made possible using conventional diffraction-limited far-field methods by 160×. The NOLES technique, with its high temporal resolution and spatial accuracy that far surpass the performance typical of fluorescence-based imaging, will be relevant for imaging dynamic chemical, biological, and material environments. [ABSTRACT FROM AUTHOR]

Steady-state and time-resolved spectroscopies were employed to study the photodissociation of both the neutral (HS-CH2-COOH) and doubly deprotonated (-S-CH2-COO-) forms of thioglycolic acid (TGA), a common surface-passivating ligand used in the aqueous synthesis and organization of semiconducting nanostructures. Room temperature UV-Vis absorption spectroscopy indicated strong absorption by the S1 and S2 excited states at 250 nm and 185 nm, respectively. The spectrum also contained a weaker absorption band that extended to approximately 550 nm, which was assigned to the πCO*←nO transition. Femtosecond time-resolved transient absorption spectroscopy was performed on TGA using 400 nm excitation and a white-light continuum probe to provide the temporally and spectrally resolved data. Both forms of TGA underwent a photoinduced dissociation from the excited state to form an α-thiol-substituted acyl radical (α-TAR, S-CH2-CO•). For the acidic form of TGA, radical formation occurred with an apparent time constant of 60 ± 5 fs; subsequent unimolecular decay took 400 ± 60 fs. Similar kinetics were observed for the deprotonated form of TGA (70 ± 10 fs radical formation; 420 ± 40 fs decay). The production of the α-TAR was corroborated by the observation of its characteristic optical absorption. Time-resolved data indicated that the photoinduced dissociation of TGA via cleavage of the C-OH bond occurred rapidly (≤100 fs). The prevalence of TGA in aqueous semiconducting nanoparticles makes its absorption in the visible spectral region and subsequent dissociation key to understanding the behavior of nanoscale systems. [ABSTRACT FROM AUTHOR]

We examined the electronic relaxation dynamics for mono and bimetallic Au144−xAgx(SC8H9)60 monolayer-protected clusters (MPCs) using femtosecond time-resolved transient extinction spectroscopy. MPCs provide compositionally well-defined model systems for structure-specific determination of nanoscale electronic properties. Based on pulse-energy-dependent transient extinction data, we quantified electron–phonon coupling constants for three distinct Au144−xAgx(SC8H9)60 MPC samples, where x = 0, 0 ＜ x ＜ 30, and x∼ 30, as Gx=0 = (1.61 ± 0.1) × 1016 W m−3 K−1, Gx＜30 = (1.74 ± 0.1) × 1016 W m−3 K−1 and Gx∼30 = (2.07 ± 0.15) × 1016 W m−3 K−1, respectively. These results reflect a trend of greater electron–phonon coupling efficiency with increasing silver content. Based on these data, we conclude that gold-atom replacement by silver occurs at surface sites of the 114-atom metal core of the MPC. Definitive determinations of functional response to nanoscale “alloy” formation and dopant inclusion are critical to establishing predictive models for the development of materials that feature nanoparticles as active components. [ABSTRACT FROM AUTHOR]

The participation of the valence orbitals of actinides in bonding has been debated for decades. Recent experimental and computational investigations demonstrated the involvement of 6p, 6d and/or 5f orbitals in bonding. However, structural and spectroscopic data, as well as theory, indicate a decrease in covalency across the actinide series, and the evidence points to highly ionic, lanthanide-like bonding for late actinides. Here we show that chemical differentiation between californium and lanthanides can be achieved by using ligands that are both highly polarizable and substantially rearrange on complexation. A ligand that suits both of these desired properties is polyborate. We demonstrate that the 5f, 6d and 7p orbitals are all involved in bonding in a Cf(III) borate, and that large crystal-field effects are present. Synthetic, structural and spectroscopic data are complemented by quantum mechanical calculations to support these observations. [ABSTRACT FROM AUTHOR]

A bifluorophoric molecule (1) capable of intramolecular Förster Resonance Energy Transfer (FRET) is reported. The emission intensity of the FRET acceptor in 1 depends on the molar absorptivity of the donor, which is a function of zinc(ii) complexation. The FRET dynamics of [Zn(1)](ClO4)2 is characterized by femtosecond time-resolved transient absorption spectroscopy. The solvent-mediated relaxation of the charge-transfer (CT) state of the isolated donor and the FRET process of the donor–acceptor conjugate are on similar time scales (40–50 ps in CH3CN), but distinguishable by the opposite solvent polarity dependency. As the solvent polarity increases, the efficiency of Columbic-based FRET is reduced, whereas CT relaxation is accelerated. In addition to revealing a method to distinguish CT and FRET dynamics, this work provides a photophysical foundation for developing indicators based on the FRET strategy. [ABSTRACT FROM AUTHOR]

The adsorption orientation of the iron(iii) tetrakis(1-methyl-4-pyridyl)porphine (FeTMPyP) μ-oxo-bridged dimer species located in the inter-particle gap of solid gold nanosphere (SGN) and hollow gold nanosphere (HGN) networks was studied using a combination of linear extinction and surface-enhanced Raman Scattering (SERS) measurements. Nanoparticle aggregation was accomplished by μM additions of iron porphyrin to the colloidal SGN and HGN solutions. Aggregation was monitored by measuring the UV-Visible-NIR extinction spectra of the nanoparticle systems; a broadened and red-shifted localized surface plasmon resonance (LSPR), relative to the LSPR of the isolated nanoparticles, indicated aggregate formation. In conjunction with the LSPR extinction measurements, wavelength-dependent SERS measurements were used to determine the orientation of the FeTMPyP with respect to the SGN and HGN surfaces. [ABSTRACT FROM AUTHOR]

Electronic energy relaxation in hollow gold nanospheres (HGNs) was studied using femtosecond time-resolved transient absorption spectroscopy. A range of HGNs having outer diameter-to-shell thickness aspect ratios of 3.5 to 9.5 were synthesized by a galvanic replacement method. The HGNs exhibited electron–phonon relaxation times that decreased from 1.18 ± 0.16 to 0.59 ± 0.08 ps as the aspect ratio increased over this range. The corresponding electron–phonon coupling constants, G, ranged from (1.67 ± 0.22) to (3.33 ± 0.45) × 1016 W m−3 K−1. Electron–phonon coupling was also determined for solid gold nanospheres (SGNs) with diameters spanning 20 nm to 83 nm; no size dependence was observed for these structures. The HGNs with high aspect ratios exhibited larger electron–phonon coupling constants than the SGNs, whose average G value was (1.9 ± 0.2) × 1016 W m−3 K−1. By comparison, low-aspect ratio HGNs exhibited values comparable to SGNs. The electron–phonon coupling of high-aspect ratio HGNs was also influenced by the surrounding fluid dielectric; slightly smaller G values were obtained when methanol was the solvent as opposed to water. This coupling enhancement observed for high-aspect ratio HGNs was attributed to the large surface to volume ratio of these structures, which results in non-negligible contributions from the environment. [ABSTRACT FROM AUTHOR]