Near-field characterization of photonic nanodevices

The increasing density of data transmission, speed of all- optical signal processing, and demand for higher resolution microscopy and spectroscopy stimulate the development of the nanophotonics. Near-field microscopy is not limited by light diffraction and thus it can achieve sufficiently subwavelength resolution. Therefore this approach is perfect for nanophotonic device characterization. Heterodyne detection allows resolution of the optical phase and improves signal-to-noise performance in near-field microscopy. In this thesis we describe a Heterodyne Near-field Scanning Optical Microscope (HNSOM) and apply this approach to characterization of several classes of the photonic nanodevices. First, possible effects of the microscope probe are analyzed and experimentally studied. We show that a metal-coated NSOM probe can introduce loss to the waveguides and change the quality factor and resonant wavelength of the microring resonators. These effects should be taken into account especially for characterization of highly resonant photonic structures. Then various Photonic Crystal (PhC) devices are studied using the HNSOM. The modal structure of the single line defect PhC waveguide is found and the losses between this component and the channel waveguide are estimated. Using near-field characterization the performance of the self- collimating PhC lattice and the PhC polarization beam splitter are demonstrated. Another approach in nanophotonic device design is to directly transfer free- space functionality to a chip using metamaterials with refractive index variation on a deeply subwavelength scale. Such materials can be described using effective medium theory and have an index of refraction which depends on the structure period, filling factor and light polarization. Several photonic nanodevices utilizing this approach including a planar graded index lens are created and characterized using the HNSOM technique. The viability of the concept is confirmed in these measurements, some fabrication imperfections are found as well. The HNSOM setup is further enhanced by adding the low-coherence measurement capability which allows local study of the dispersive properties of the photonic nanodevices. The application of the technique to the characterization of group indices of refraction of silicon waveguides is shown

Nanoscale Imaging of Photocurrent and Efficiency in CdTe Solar Cells

The local collection characteristics of grain interiors and grain boundaries in thin-film CdTe polycrystalline solar cells are investigated using scanning photocurrent microscopy. The carriers are locally generated by light injected through a small aperture (50–300 nm) of a near-field scanning optical microscope in an illumination mode. Possible influence of rough surface topography on light coupling is examined and eliminated by sculpting smooth wedges on the granular CdTe surface. By varying the wavelength of light, nanoscale spatial variations in external quantum efficiency are mapped. We find that the grain boundaries (GBs) are better current collectors than the grain interiors (GIs). The increased collection efficiency is caused by two distinct effects associated with the material composition of GBs. First, GBs are charged, and the corresponding built-in field facilitates the separation and the extraction of the photogenerated carriers. Second, the GB regions generate more photocurrent at long wavelength corresponding to the band edge, which can be caused by a smaller local band gap. Resolving carrier collection with nanoscale resolution in solar cell materials is crucial for optimizing the polycrystalline device performance through appropriate thermal processing and passivation of defects and surfaces

Cell type specificity of neurovascular coupling in cerebral cortex.

Identification of the cellular players and molecular messengers that communicate neuronal activity to the vasculature driving cerebral hemodynamics is important for (1) the basic understanding of cerebrovascular regulation and (2) interpretation of functional Magnetic Resonance Imaging (fMRI) signals. Using a combination of optogenetic stimulation and 2-photon imaging in mice, we demonstrate that selective activation of cortical excitation and inhibition elicits distinct vascular responses and identify the vasoconstrictive mechanism as Neuropeptide Y (NPY) acting on Y1 receptors. The latter implies that task-related negative Blood Oxygenation Level Dependent (BOLD) fMRI signals in the cerebral cortex under normal physiological conditions may be mainly driven by the NPY-positive inhibitory neurons. Further, the NPY-Y1 pathway may offer a potential therapeutic target in cerebrovascular disease

Cell type specificity of neurovascular coupling in cerebral cortex

Identification of the cellular players and molecular messengers that communicate neuronal activity to the vasculature driving cerebral hemodynamics is important for (1) the basic understanding of cerebrovascular regulation and (2) interpretation of functional Magnetic Resonance Imaging (fMRI) signals. Using a combination of optogenetic stimulation and 2-photon imaging in mice, we demonstrate that selective activation of cortical excitation and inhibition elicits distinct vascular responses and identify the vasoconstrictive mechanism as Neuropeptide Y (NPY) acting on Y1 receptors. The latter implies that task-related negative Blood Oxygenation Level Dependent (BOLD) fMRI signals in the cerebral cortex under normal physiological conditions may be mainly driven by the NPY-positive inhibitory neurons. Further, the NPY-Y1 pathway may offer a potential therapeutic target in cerebrovascular disease. DOI: http://dx.doi.org/10.7554/eLife.14315.00