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The nature of interchain electronic species in conjugated polymers has been the subject of much debate. In this paper, we exploit a novel near-field scanning optical microscopy (NSOM)-based solvatochromism method to spatially image the difference in dipole moment, and hence the difference in degree of charge separation, between the ground and electronic excited states of the emissive interchain species in films of poly(2-methoxy-5-(2‘-ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV). The method uses NSOM to collect emission from near the surface of solid samples that are placed into contact with liquids of varying polarity. The solvatochromic spectral shifts of the interfacial luminescence are measured as a function of solvent polarity; the results are analyzed with an interfacial dielectric continuum model to determine the dipole moment of emissive excited states. Experiments performed on films of the laser dye trans-4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran (DCM) in poly(methyl methacrylate) (PMMA) demonstrate that our interfacial NSOM solvatochromic method and analysis can successfully reproduce the known dipole change of DCM upon photoexcitation. With the method calibrated, we then apply it to the interchain luminescence from the surface of thermally annealed MEH-PPV films. The interfacial solvatochromic analysis reveals that the dominant interchain species in annealed MEH-PPV films is “excimer-like”, exhibiting an ~4−7 D decrease in dipole moment upon optical excitation. In a few highly localized regions of the film (ca. 1−2 μm in diameter), however, the interchain excited state exhibits a large (~9−13 D) increase in dipole moment upon excitation, indicative of minority interchain species with a large degree of charge separation, such as exciplexes or polaron pairs. The large variation in excited-state dipole moments observed throughout the film is suggestive of an entire family of interchain species, each characterized by a different degree of charge separation. The fact that the large-dipole interchain species are found in spatially segregated domains implies that interchain charge separation in conjugated polymer films is associated with the presence of defects. When the molecular weight of the polymer is lowered, the large excited-state dipole regions increase in spatial extent, suggesting that the defects that promote charge separation are intrinsic and may be associated with the chain ends.

We demonstrate the combination of coherent anti-Stoke Raman scattering (CARS) with near-field scanning optical microscopy (NSOM) for chemically selective imaging via intrinsic vibrational resonances with spatial resolution below the diffraction limit. Femtosecond, near-IR pulses are used to produce CARS signals from human hepatocytes, and by tuning the CARS signal to be resonant with C−H stretching frequencies, image contrast is observed with an optical spatial resolution of ~128 nm.

The emerging field of coherent nonlinear near-field scanning optical microscopy (NSOM) is reviewed. Second harmonic, third harmonic, and sum frequency generation (SHG, THG, and SFG) are explored as means of providing chemically and environmentally selective probes with nanometer spatial resolution provided by scanning probe microscopy. Chemical selectivity is generated via resonant enhancement of the nonlinear signals, whereas interface vs bulk contrast is achieved by the order (even vs odd) of the optical process. A method of producing higher spectral resolution, and thus increased chemical selectivity, is also demonstrated in the form of near-field detected multiplex (or “broad-bandwidth”) SFG (MSFG). Applications to biological and material samples are described.

The nonlinear optical response of semiconductor nanowires has potential application for frequency conversion in nanoscale optical circuitry. Here, second- and third-harmonic generation (SHG, THG) are imaged on single zinc oxide (ZnO) nanowires using near-field scanning optical microscopy (NSOM). The absolute magnitudes of the two independent χ⁽²⁾ elements of a single wire are determined, and the nanowire SHG and THG emission patterns as a function of incident polarization are attributed to the hexagonal nanowire geometry and χ⁽²⁾ tensor symmetry.

Ultraviolet lasing from single zinc oxide nanowires is demonstrated at room temperature. Near-field optical microscopy images quantify the localization and the divergence of the laser beam. The linewidths, wavelengths, and power dependence of the nanowire emission characterize the nanowire as an active optical cavity. These individual nanolasers could serve as miniaturized light sources for microanalysis, information storage, and optical computing.

It is becoming increasingly clear that the way in which a conjugated polymer film is cast affects the interactions between polymer chains and thus the optical and electrical properties of the film. Given that conjugated polymer films cast in different ways also show different nanometer-scale surface topographies, the question that arises is:  What is the correlation between surface topography, local chain packing, and the local electronic properties of a conjugated polymer film? In this paper, we address this question using fluorescence near-field scanning optical microscopy (NSOM) to examine films of poly(2-methoxy-5-(2‘-ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV) that were prepared in different ways. The spatially resolved photoluminescence (SRPL) spectra collected on top of the nanometer-scale topographic features (“bumps”) exhibited by spin-cast MEH-PPV films show an enhancement of the red portion of the emission relative to spectra collected from flat regions of the film. Moreover, photooxidative damage (signified by a red-shift and drop in quantum yield of the SRPL) occurs much more quickly in the flat regions of the MEH-PPV films than on the topographic bumps. Taken together, these observations suggest that the bumps on the films correspond to regions in which the chains are packed more tightly:  the red-shifted emission results from increased interchain interactions, while the decreased photooxidation rate results from the fact that oxygen cannot easily diffuse between the tightly packed polymer chains. We also find that the spatial homogeneity of MEH-PPV films can be greatly improved by annealing:  heating the films above the glass transition temperature removes the topographic features and produces a uniform but weak and red-shifted SRPL due to increased interchain interactions. In contrast to spin-cast films, the SRPL of annealed films undergoes a blue-shift upon photooxidation. This result can be explained by considering the differences between the local chain packing in annealed and nonannealed films, combined with the fact that excitations in the film tend to migrate to low-energy aggregated-chain “traps”. All of these results provide insight into how polymer film morphology can be controlled through film processing conditions to improve the optical properties and the performance of electroluminescent devices based on this class of materials.

We present the first results from studies of liquid water microjets by soft X-ray absorption spectroscopy. Near the oxygen K-edge (~530 eV) a fine-structure pattern very similar to that found for gaseous water monomers is observed when the surface-selective total ion yield (TIY) is measured, but a broadened and blue-shifted spectrum emerges when detecting the bulk-sensitive total electron yield (TEY). TIY EXAFS measurements produce a nearest neighbor O−O distance for surface molecules (3.00 Å) slightly longer than that of the isolated water dimer (2.98 Å), whereas the O−O distance extracted from TEY EXAFS corresponds to that accepted for bulk water (2.85 Å). Together, these results evidence an equilibrium liquid water surface dominated by water molecules interacting weakly at longer distances than in the bulk, thus supporting predictions from computer simulations.

Infrared sum-frequency generation (SFG) has been detected with near-field scanning optical microscopy (NSOM) for the first time. SFG NSOM images of a chemical vapor deposited (CVD) disk of zinc selenide were recorded as a function of infrared (IR) wavelengths ranging from 3.1 to 4.4 μm from an amplified femtosecond laser. Striation patterns, which are not observed in the topography, were detected in the optical images and exhibited increased SFG in comparison to homogeneous regions of the semiconductor. SFG images were demonstrated to have ~ λ/20 spatial resolution in comparison to the IR wavelengths used to produce the sum frequency.

Second Harmonic Generation (SHG) has been used in combination with a near-field scanning optical microscope (NSOM) to image biological samples for the first time. Near-field SHG images of azure B stained mouse natural killer (NK) cells were recorded at four different wavelengths using a tunable near-infrared femtosecond laser. The surface specificity of SHG allows membrane structures to be selectively imaged with subdiffraction spatial resolution under ambient conditions. Resonance enhancement of SHG is shown to provide a chemically specific contrast mechanism in NSOM imaging, revealing submicron features associated with the cytolytic granular membrane.