Ultrafast optical switching of three-dimensional Si inverse opal photonic band gap crystals

We present ultrafast optical switching experiments on 3D photonic band gap crystals. Switching the Si inverse opal is achieved by optically exciting free carriers by a two-photon process. We probe reflectivity in the frequency range of second order Bragg diffraction where the photonic band gap is predicted. We find good experimental switching conditions for free-carrier plasma frequencies between 0.3 and 0.7 times the optical frequency: we thus observe a large frequency shift of up to D omega/omega= 1.5% of all spectral features including the peak that corresponds to the photonic band gap. We deduce a corresponding large refractive index change of Dn'_Si/n'_Si= 2.0% and an induced absorption length that is longer than the sample thickness. We observe a fast decay time of 21 ps, which implies that switching could potentially be repeated at GHz rates. Such a high switching rate is relevant to future switching and modulation applications

Erbium-implanted high-Q silica toroidal microcavity laser on a silicon chip

Lasing from an erbium-doped high-Q silica toroidal microcavity coupled to a tapered optical fiber is demonstrated and analyzed. Average erbium ion concentrations were in the range 0.009–0.09 at. %, and a threshold power as low as 4.5 µW and an output lasing power as high as 39.4 µW are obtained from toroidal cavities with major diameters in the range 25–80 µm. Controlling lasing wavelength in a discrete way at each whispering-gallery mode was possible by changing the cavity loading, i.e., the distance between the tapered optical fiber and the microcavity. Analytic formulas predicting threshold power and differential slope efficiency are derived and their dependence on cavity loading, erbium ion concentration, and Q factor is analyzed. It is shown that the experimental results are in good agreement with the derived formulas

Simultaneous measurement of localized diffusion and flow using optical coherence tomography

We report on the simultaneous and localized measurements of the diffusion coefficient and flow velocity based on the normalized autocorrelation function using optical coherence tomography (OCT). Our results on a flowing solution of polystyrene spheres show that the flow velocity and the diffusion coefficient can be reliably estimated in a regime determined by the sample diffusivity, the local flow velocity, and the Gaussian beam waist. We experimentally show that a smaller beam waist results in an improvement of the velocity sensitivity at cost of the precision and accuracy of the estimation of the diffusion coefficient. Further, we show that the decay of the OCT autocorrelation due to flow depends only on the Gaussian beam waist irrespective of the sample position with respect to the focus position

Spectral domain optical coherence tomography imaging with an integrated optics spectrometer

We designed and fabricated an arrayed-waveguide grating (AWG) in silicon oxynitride as a spectrometer for spectral domain optical coherence tomography (SD-OCT). The AWG has a footprint of only 3.0 cm x 2.5 cm, operates at a center wavelength of 1300 nm, and has 78 nm free spectral range. OCT measurements are performed that demonstrate imaging up to a maximum depth of 1 mm with an axial resolution of 19 mu m, both in agreement with the AWG design parameters. Using the AWG spectrometer combined with a fiber-based SD-OCT system, we demonstrate cross-sectional OCT imaging of a multilayered scattering phantom. (C) 2011 Optical Society of Americ

Evaluation of multi-exponential curve fitting analysis of oxygen-quenched phosphorescence decay traces for recovering microvascular oxygen tension histograms

Although it is generally accepted that oxygen-quenched phosphorescence decay traces can be analyzed using the exponential series method (ESM), its application until now has been limited to a few (patho)physiological studies, probably because the reliability of the recovered oxygen tension (pO2) histograms has never been extensively evaluated and lacks documentation. The aim of this study was, therefore, to evaluate the use of the ESM to adequately determine pO2 histograms from phosphorescence decay traces. For this purpose we simulated decay traces corresponding to uni- and bimodal pO2 distributions and recovered the pO2 histograms at different signal-to-noise ratios (SNRs). Ultimately, we recovered microvascular pO2 histograms measured in the rat kidney in a model of endotoxemic shock and fluid resuscitation and showed that the mean microvascular oxygen tension, 〈pO2〉, decreased after induction of endotoxemia and that after 2 h of fluid resuscitation, 〈pO2〉 remained low, but the hypoxic peak that had arisen during endotoxemia was reduced. This finding illustrates the importance of recovering pO2 histograms under (patho)physiological conditions. In conclusion, this study has characterized how noise affects the recovery of pO2 histograms using the ESM and documented the reliability of the ESM for recovering both low- and high-pO2 distributions for SNRs typically found in experiments. This study might therefore serve as a frame of reference for investigations focused on oxygen (re)distribution during health and disease and encourage researchers to (re-)analyze data obtained in (earlier) studies possibly revealing new insights into complex disease states and treatment strategies

Pitch Comparisons between Electrical Stimulation of a Cochlear Implant and Acoustic Stimuli Presented to a Normal-hearing Contralateral Ear

Four cochlear implant users, having normal hearing in the unimplanted ear, compared the pitches of electrical and acoustic stimuli presented to the two ears. Comparisons were between 1,031-pps pulse trains and pure tones or between 12 and 25-pps electric pulse trains and bandpass-filtered acoustic pulse trains of the same rate. Three methods—pitch adjustment, constant stimuli, and interleaved adaptive procedures—were used. For all methods, we showed that the results can be strongly influenced by non-sensory biases arising from the range of acoustic stimuli presented, and proposed a series of checks that should be made to alert the experimenter to those biases. We then showed that the results of comparisons that survived these checks do not deviate consistently from the predictions of a widely-used cochlear frequency-to-place formula or of a computational cochlear model. We also demonstrate that substantial range effects occur with other widely used experimental methods, even for normal-hearing listeners

25th annual computational neuroscience meeting: CNS-2016

The same neuron may play different functional roles in the neural circuits to which it belongs. For example, neurons in the Tritonia pedal ganglia may participate in variable phases of the swim motor rhythms [1]. While such neuronal functional variability is likely to play a major role the delivery of the functionality of neural systems, it is difficult to study it in most nervous systems. We work on the pyloric rhythm network of the crustacean stomatogastric ganglion (STG) [2]. Typically network models of the STG treat neurons of the same functional type as a single model neuron (e.g. PD neurons), assuming the same conductance parameters for these neurons and implying their synchronous firing [3, 4]. However, simultaneous recording of PD neurons shows differences between the timings of spikes of these neurons. This may indicate functional variability of these neurons. Here we modelled separately the two PD neurons of the STG in a multi-neuron model of the pyloric network. Our neuron models comply with known correlations between conductance parameters of ionic currents. Our results reproduce the experimental finding of increasing spike time distance between spikes originating from the two model PD neurons during their synchronised burst phase. The PD neuron with the larger calcium conductance generates its spikes before the other PD neuron. Larger potassium conductance values in the follower neuron imply longer delays between spikes, see Fig. 17.Neuromodulators change the conductance parameters of neurons and maintain the ratios of these parameters [5]. Our results show that such changes may shift the individual contribution of two PD neurons to the PD-phase of the pyloric rhythm altering their functionality within this rhythm. Our work paves the way towards an accessible experimental and computational framework for the analysis of the mechanisms and impact of functional variability of neurons within the neural circuits to which they belong