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We show that the different bond orders of individual carbon-carbon bonds in polycyclic aromatic hydrocarbons and fullerenes can be distinguished by noncontact atomic force microscopy (AFM) with a carbon monoxide (CO)-functionalized tip. We found two different contrast mechanisms, which were corroborated by density functional theory calculations: The greater electron density in bonds of higher bond order led to a stronger Pauli repulsion, which enhanced the brightness of these bonds in high-resolution AFM images. The apparent bond length in the AFM images decreased with increasing bond order because of tilting of the CO molecule at the tip apex.

Scanning tunnelling microscopy and atomic force microscopy can be used to study the electronic and structural properties of surfaces, as well as molecules and nanostructures adsorbed on surfaces, with atomic precision, but they cannot directly probe the distribution of charge in these systems. However, another form of scanning probe microscopy, Kelvin probe force microscopy, can be used to measure the local contact potential difference between the scanning probe tip and the surface, a quantity that is closely related to the charge distribution on the surface. Here, we use a combination of scanning tunnelling microscopy, atomic force microscopy and Kelvin probe force microscopy to examine naphthalocyanine molecules (which have been used as molecular switches) on a thin insulating layer of NaCl on Cu(111). We show that Kelvin probe force microscopy can map the local contact potential difference of this system with submolecular resolution, and we use density functional theory calculations to verify that these maps reflect the intramolecular distribution of charge. This approach could help to provide fundamental insights into single-molecule switching and bond formation, processes that are usually accompanied by the redistribution of charge within or between molecules.

Individual pentacene and naphthalocyanine molecules adsorbed on a bilayer of NaCl grown on Cu(111) were investigated by means of scanning tunneling microscopy using CO-functionalized tips. The images of the frontier molecular orbitals show an increased lateral resolution compared with those of the bare tip and reflect the modulus squared of the lateral gradient of the wave functions. The contrast is explained by tunneling through the p-wave orbitals of the CO molecule. Comparison with calculations using a Tersoff-Hamann approach, including s- and p-wave tip states, demonstrates the significant contribution of p-wave tip states., (© 2011 American Physical Society)

We demonstrate an all-optical switch based on a waveguide-embedded 1D photonic crystal cavity fabricated in silicon-on-insulator technology. Light at the telecom wavelength is modulated at high-speed by control pulses in the near infrared, harnessing the plasma dispersion effect. The actual absorbed switching power required for a 3 dB modulation depth is measured to be as low as 6 fJ. While the switch-on time is on the order of a few picoseconds, the relaxation time is almost 500 ps and limited by the lifetime of the charge carriers.

Nature offers a huge and only partially explored variety of small molecules with potential pharmaceutical applications. Commonly used characterization methods for natural products include spectroscopic techniques such as nuclear magnetic resonance spectroscopy and mass spectrometry. In some cases, however, these techniques do not succeed in the unambiguous determination of the chemical structure of unknown compounds. To validate the usefulness of scanning probe microscopy as an adjunct to the other tools available for organic structure analysis, we used the natural product cephalandole A, which had previously been misassigned, and later corrected. Our results, corroborated by density functional theory, demonstrate that direct imaging of an organic compound with atomic-resolution force microscopy facilitates the accurate determination of its chemical structure. We anticipate that our method may be developed further towards molecular imaging with chemical sensitivity, and will become generally useful in solving certain classes of natural product structures.

A planar nanophotonic Fabry-Perot-like resonator that can defy strong absorption of about 20 000 cm(-1) in the cavity material is demonstrated. Visible laser emission is observed from two silicon subwavelength-sized high index contrast gratings with embedded polymer gain material. The size of the laser is reduced by an order of magnitude compared to established designs based on photonic bandgap structures. As silicon constitutes the most common carrier for electronics, the cost-efficient integration of compact laser sources for visible wavelengths comes within reach.

We study circular grating resonators (CGRs) which are formed by a central defect surrounded by concentric rings composing a grating and which display perfect azimuthal modal-symmetry. Because of their radial symmetry they exhibit a complete band gap for a minimal index contrast. However, as is the case for all 2D resonators their quality factors are limited by vertical losses. To reduce the vertical losses we introduce a chirp of the grating period by reducing it towards the central defect. The chirped CGRs exhibit drastically improved quality factors of up to tens of millions with a modal volume of a few cubic wavelengths.

Resolving individual atoms has always been the ultimate goal of surface microscopy. The scanning tunneling microscope images atomic-scale features on surfaces, but resolving single atoms within an adsorbed molecule remains a great challenge because the tunneling current is primarily sensitive to the local electron density of states close to the Fermi level. We demonstrate imaging of molecules with unprecedented atomic resolution by probing the short-range chemical forces with use of noncontact atomic force microscopy. The key step is functionalizing the microscope's tip apex with suitable, atomically well-defined terminations, such as CO molecules. Our experimental findings are corroborated by ab initio density functional theory calculations. Comparison with theory shows that Pauli repulsion is the source of the atomic resolution, whereas van der Waals and electrostatic forces only add a diffuse attractive background.

We demonstrate the suitability of microcavities based on circular grating resonators (CGRs) as fast switches. This type of optical resonator is characterized by a high quality factor and very small mode volume. The waveguide-coupled CGRs are fabricated with silicon-on-insulator technology compatible with standard complementary metal-oxide semiconductor (CMOS) processing. The linear optical properties of the CGRs are investigated by transmission spectroscopy. From 3D finite-difference time-domain simulations of isolated CGRs, we identify the measured resonances. We probe the spatial distribution and the parasitic losses of a resonant optical mode with scanning near-field optical microscopy. We observe fast all-optical switching within a few picoseconds by optically generating free charge carriers within the cavity.

Objective: To characterize pathologic changes in the cerebral cortex of patients with multiple sclerosis (MS) and progressive multifocal leukoencephalopathy (PML)., Methods: Autopsy brain tissue was obtained from 13 patients with PML, 4 patients with MS, 2 patients with HIV encephalopathy, and 1 subject without neurologic pathology. Immunohistochemistry for myelin proteins, inflammatory cells, and neurofilaments was performed to evaluate the distribution of cortical lesions, their inflammatory activity, and neuritic pathology. Confocal microscopy was applied to examine pathologic changes in neurites in PML cortex., Results: Leukocortical, intracortical, and subpial patterns of cortical demyelination were represented in MS brain tissue. In PML brain tissue intracortical and leukocortical but not subpial lesions were observed. Cortical lesions in PML and MS contained fewer inflammatory cells than demyelinated areas in the white matter. Neuritic pathology in cortical PML lesions was represented by dystrophic and transected neurites. Pathologic modifications in neuritic processes in PML were more evident in highly inflamed white matter than in gray matter areas of demyelination, reminiscent of previous reports of neuritic pathology in MS. JC virus-infected cells were associated with PML white matter, leukocortical and intracortical lesions., Conclusions: Cortical pathology represents a distinct feature of progressive multifocal leukoencephalopathy. Similarities and differences with regard to multiple sclerosis cortical pathology were noted and may be informative regarding the pathogenesis of both disorders.

The efficiency of a monomolecular film of (3-glycidoxypropyl) trimethoxysilane (GPTS) on a shear horizontal guided (Love) acoustic wave immunosensor to detect whole Escherichia coli (E. coli) bacteria is demonstrated. Direct anti-E. coli antibodies grafting onto the sensor surface did not lead to a significant bacteria immobilisation, partially attributed to the SiO2 sensor surface roughness. An innovative method has been set up to get around this difficulty and to detect whole bacteria. It consists in grafting goat anti-mouse antibodies (GAM) onto the sensor surface in a first step and introducing E. coli bacteria mixed with anti-E. coli antibodies onto the sensor in a second step. We describe the characteristics of such a technique like sample preparation time (lower than 30 min) and temperature improvements. A 37 degrees C experimental temperature led to the fastest bacteria binding kinetic, reducing the total analysis time. This method enables to keep the specificity of the antibody/antigen interaction and provides significant results in less than 1h. This leads to a detection threshold of 10(6) bacteria/ml in a 500 microl chamber.

We present general optimization arguments for resonator-based all-optical switching. Several generic resonator geometries, namely Fabry-Perot resonators, circular gratings as well as micro-ring resonators, are discussed and their particular features highlighted. We establish analytical models which allow a direct comparison of the different all-optical switch geometries. For the parameter range investigated, we find a clear advantage of photonic band-gap resonators (based on Bragg-type reflection) over micro-ring resonators (based on total internal reflection).

The effect of lattice termination on the surface states in a two-dimensional truncated photonic crystal slab is experimentally studied in a high-index-contrast silicon-on-insulator system. A single-mode silicon strip waveguide that is separated from the photonic crystal by a trench of variable width is used to evanescently couple to surface states in the surrounding lattice. It is demonstrated that the dispersion of the surface states depends strongly on the specific termination of the lattice.

The temporal-coupled-mode theory is directly applied to the design of devices that feature a resonator with a high quality factor. For the temporal-coupled-mode theory we calculate the decay rate of the resonator to determine the transmission properties of the device. The analysis using the decay rates requires little computational effort, and therefore the optimum device properties can be determined quickly. Two examples, a wavelength filter and a resonator crossing, are presented to illustrate the use of the analysis.

We report the design and testing of an SOI-based photonic integrated circuit containing two-dimensional membrane-type photonic crystal waveguides. The circuit comprises spot-size converters to efficiently couple light from a fiber into single-mode strip waveguides and butt-couplers to couple from strip waveguides to photonic crystal waveguides. Each optical interface was optimized to minimize back-reflections and reduce the Fabry-Perot noise. The transmission characteristics of each component are measured and record low propagation losses in photonic crystal waveguides of 24dB/cm are reported. The combination of an efficient two-stage coupling scheme and utilization of ultra-long (up to 2mm) photonic crystal waveguides reduces the uncertainty in determining the loss figure to 3dB/cm.

The detailed theory is presented together with design considerations and recent experimental results for an electronically controllable directional coupler for multimode optical fibers. The device, based on plastic-clad silica fiber, is of exceptionally simple construction and compact size and is highly reliable. A theoretical model of the coupler is described and compared with actual device performance. The coupling can be varied from -4 to -28 dB with an excess loss of <0.2 dB over the entire coupling range, in agreement with theory. Improvements in the present device design are also discussed.