By taking advantage of the tensor nature of surface-enhanced Raman scattering (SERS), we track trajectories of the linker molecule and a CO molecule chemisorbed at the hot spot of a nano-dumbbell consisting of dibenzyldithio-linked silver nanospheres. The linear Stark shift of CO serves as an absolute gauge of the local field, while the polyatomic spectra characterize the vector components of the local field. We identify surface-enhanced Raman optical activity due to a transient asperity in the nanojunction in an otherwise uneventful SERS trajectory. During fusion of the spheres, we observe sequential evolution of the enhanced spectra from dipole-coupled Raman to quadrupole- and magnetic dipole-coupled Raman, followed by a transition from line spectra to band spectra, and the full reversal of the sequence. From the spectrum of CO, the sequence can be understood to track the evolution of the junction plasmon resonance from dipolar to quadrupolar to charge transfer as a function of intersphere separation, which evolves at a speed of ∼1 Å/min. The crossover to the conduction limit is marked by the transition of line spectra to Stark-broadened and shifted band spectra. As the junction closes on CO, the local field reaches 1 V/Å, limited to a current of 1 electron per vibrational cycle passing through the molecule, with associated Raman enhancement factor via the charge transfer plasmon resonance of 10(12). The local field identifies that a sharp protrusion is responsible for room-temperature chemisorption of CO on silver. The asymmetric phototunneling junction, Ag-CO-Ag, driven by the frequency-tunable charge transfer plasmon of the dumbbell antenna, combines the design elements of an ideal rectifying photocollector.

A Schiff base ligand has been synthesized by condensing 1,2-diaminobenzene with 4-(dimethylamino)cinnamaldehyde to give a donor-pi-acceptor-pi-donor system which does not show any two-photon absorption cross section but which does, upon complexation with Zn(II) or Cu(I), exhibit very high two-photon absorption cross sections.

For a series of rhodamine dyes, two-photon absorption (TPA) and two-photon fluorescence (TPF) have been performed in different solvents. Solvent-dependent TPA spectra of these dyes were measured with open aperture z-scan method and compared to their respective single-photon spectra at equivalent energies. In the TPA spectra, relative peak intensities and positions are highly solvent dependent, which could be a result of vibronic couplings that depend on solvent environment. Measured TPA cross-sections of rhodamine dyes are consistently higher in nonpolar solvents. Certain complementary and similarity between TPA and TPF are also elucidated. Finally, a two-photon figure-of-merit is presented for these dyes in different solvents as a function of wavelength.

We describe combined AFM/Raman measurements on single nanodumbbells, consisting of silver nanospheres linked with dibenzyl-4,4′dithiol (DBDT). The measured surface-enhanced Raman scattering (SERS) enhancement factor, EF exp = 3 × 10 7 at 532 nm, corresponds to the observed signal strength of a single DBDT molecule, the Raman cross section of which was determined to be dσ/dΩ = 6 × 10 −28 cm 2 /sr. We show that the product of the local field enhancement, EF P = (E i /E 0 ) 2 (E s /E 0 ) 2 = 3 × 10 6 , and the chemical contribution due to reduced detuning, EF C = (Δ 0 /Δ) 2 = 12, account for the observed effect. The chemical contribution is assessed by exploring model structures Ag n −S−DB−S−Ag m (n, m = 0, 3, 7, 20). The π−π* transition at 287 nm, which determines the polarizability of the bare molecule, acquires a DBDT-to-silver charge-transfer character upon binding to silver. The CT transition near 400 nm reduces the detuning but remains nonresonant at 532 nm. We observe a soft polarization dependence, which suggests optical activity, which in part is ascribed to coupling between plasmons and conjugated electrons of DBDT. Modest enhancement factors are sufficient to detect single molecules through nonresonant SERS.

An effective z-scan setup with a minimum thermal effect is shown for intensity-dependent measure of two-photon absorption (TPA) with high-repetition rate lasers. Use of an additional intensity modulation with an optical chopper provides enough blanking time for a high-repetition rate laser to yield equally accurate results in TPA measurements compared to a low repetition laser. Extension of this method of thermal effect management with an optical chopper to emission studies also results in good correspondence for two-photon cross-section measurements from either z-scan or two-photon fluorescence techniques. The method also significantly enhances two-photon fluorescence, which could be promising for multiphoton microscopy.