Maxim Abashin

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.14315.001

Near-field characterization of propagating optical modes in photonic crystal waveguides

We analyze the propagating optical modes in a Silicon membrane photonic crystal waveguide, based on subwavelength-resolution amplitude and phase measurements of the optical fields using a heterodyne near-field scanning optical microscope (H-NSOM). Fourier analysis of the experimentally obtained optical amplitude and phase data permits identification of the propagating waveguide modes, including the direction of propagation (in contrast to intensity-only measurement techniques). This analysis reveals the presence of two superposed propagating modes in the waveguide. The characteristics of each mode are determined and found to be consistent with theoretical predictions within the limits of fabrication tolerances. An analysis of the relative amplitudes of these two modes as a function of wavelength show periodic oscillation with a period of approximately 3.3 nm. The coupling efficiency between the ridge waveguide and the photonic crystal waveguide is also estimated and found to be consistent with the internal propagating mode characteristics. The combination of high-sensitivity amplitude and phase measurements, subwavelength spatial resolution, and appropriate interpretive techniques permits the in-situ observation of the optical properties of the device with an unprecedented level of detail, and facilitates the characterization and optimization of nanostructure-based photonic devices and systems.

Wave front evolution of negatively refracted waves in a photonic crystal

Using a heterodyne near-field scanning microscope, the authors analyze the phase and amplitude of the electric field of an optical wave across the boundary of positive to negative refraction media. The photonic crystal acts as an extremely anisotropic material with a negative curvature of its dispersion surface whose shape is resolved experimentally. This extreme anisotropy results in the beam having a peculiar phase evolution through propagation that does not occur in isotropic media. A focusing wave is produced by negative refraction, which has diverging wave fronts before the internal focus and converging wave fronts after the focus.

Observation of amplitude and phase in ridge and photonic crystal waveguides operating at 1.55 μm by use of heterodyne scanning near-field optical microscopy

We apply heterodyne scanning near-field optical microscopy (SNOM) to observe with subwavelength resolution the amplitude and phase of optical fields propagating in several microfabricated waveguide devices operating around the 1.55 microm wavelength. Good agreement between the SNOM measurements and predicted optical mode propagation characteristics in standard ridge waveguides demonstrates the validity of the method. In situ observation of the subwavelength-scale distribution and propagation of optical fields in straight and 90 degrees bend photonic crystal waveguides facilitates a more detailed understanding of the optical performance characteristics of these devices and illustrates the usefulness of the technique for investigating nanostructured photonic devices.

Composite dielectric metasurfaces for phase control of vector field

We designed, fabricated, and characterized a dielectric metamaterial lens created by varying the density of subwavelength low refractive index nanoholes in a high refractive index substrate, resulting in a locally variable effective refraction index. It is shown that a constructed graded index lens can overcome diffraction effects even when the aperture/wavelength (D/λ) ratio is smaller than 40. In addition to the conventional design of a polarization insensitive lens, we also show that a polarization diversity lens (f(o)≠f(e)) can be realized by arranging nanoholes in patterns with variable density in different transverse directions. Such a anisotropic microlens demonstrates polarization dependent focal lengths of 32 and 22 μm for linearly x- and y-polarized light, respectively, operating at a wavelength of λ=1550  nm. We also show numerically and demonstrate experimentally achromatic performance of the devices operating in the wavelength range of 1500-1900 nm with full width at half-maximum (FWHM) of the focal spots of about 4 μm.

Effects produced by metal-coated near-field probes on the performance of silicon waveguides and resonators

We study the effects of metal-coated fiber near-field probes on the performance of nanophotonic devices. Employing a heterodyne near-field scanning optical microscope and analyzing transmission characteristics, we find that a metal-coated probe can typically introduce a 3 dB intensity loss and a 0.2 rad phase shift during characterization of a straight waveguide made in a silicon-on-insulator system. In resonant nanophotonic structures such as a 5 mum radius microring resonator, we demonstrate that the probe induces a 1 nm shift in resonant wavelength and decreases the resonator quality factor, Q, from 1100 to 480.

Near-field measurement of amplitude and phase in silicon waveguides with liquid cladding

In recent years Near-Field Scanning Optical Microscopy (NSOM) has emerged as an important characterization tool for guided-wave photonic devices. The NSOM uses a subwavelength aperture probe to couple evanescent waves to the far-field, allowing subdiffraction-limited imaging and measurement of local properties of photonic structures. By integrating the NSOM into one of the arms of a heterodyne interferometer, we may image near-field phase as well as amplitude. However, on-chip guided-wave devices are typically coated with a solid overcladding. Heterodyne NSOM (H-NSOM) characterization of these devices has typically been limited to similar devices without cladding since the probe cannot penetrate the solid cladding layer to access the evanescent fields contained within. Here we demonstrate a technique that allows optical near-field characterization of devices while preserving their optical properties. To do so, a liquid overcladding is introduced to emulate the actual overcladding of the final operational device while allowing the probe to sample the evanescent field at the core-cladding interface for analysis by the NSOM. This technique enables metrology on the actual rather than duplicate device and preserves the dispersion of the optical structures to replicate the designed structure. To our best knowledge this is the only H-NSOM technique allowing characterization of photonic circuits in their final form.

Heterodyne near-field scanning optical microscopy with spectrally broad sources

We propose to use low-coherence-length cw optical sources with a broad spectrum in heterodyne near-field scanning microscopy in order to imitate optical pulse propagation and to obtain information about spectrally variant properties of nanophotonic components. The dispersion difference in the interferometer arms for a symmetric acousto-optic modulator arrangement is shown to be negligible over appreciable bandwidths. Demonstration of the principle of operation and viability of this approach is provided by measurement of the group refractive index of a silicon channel waveguide.

Total internal reflection photonic crystal prism.

An integrated total internal reflection prism is demonstrated that generates a transversely localized evanescent wave along the boundary between a photonic crystal and an etched out trench. The reflection can be described by either the odd symmetry of the Bloch wave or a tangential momentum matching condition. In addition, the Bloch wave propagates through the photonic crystal in a negative refraction regime, which manages diffraction within the prism. A device with three input channels has been fabricated and tested that illuminates different regions of the reflection interface. The reflected wave is then sampled by a photonic wire array, where the individual channels are resolved. Heterodyne near field scanning optical microscopy is used to characterize the spatial phase variation of the evanescent wave and its decay constant.

2-D array wavelength demultiplexing by hybrid waveguide and free-space optics

We demonstrate wavelength demultiplexing into a raster-scanned 8/spl times/9 array by combining a 1/spl times/8 arrayed waveguide grating with a free-space grating multiplexer. 9 diffraction orders from each AWG output are separated onto a 2D array on an InGaAs camera or coupled into a scanned output fiber. This first proof-of-principle device had 0.2 nm channel -20 dB width, >35 dB spectral extinction and 15 to 25 dB insertion loss over the 70 nm operating range.