Nicolas Hô

Direct femtosecond laser writing of waveguides in As2S3 thin films

Single-channel waveguides and Y couplers were fabricated in chalcogenide thin films by use of femtosecond laser pulses from a 25-MHz repetition rate Ti:sapphire laser. Refractive-index differentials (delta n > 10(-2)) were measured through interferometric microscopy and are higher than the typical values reported for oxide glasses. The dependence of the index differential on the peak intensity reveals the nonlinear nature of the photosensitivity in arsenic trisulfide below its bandgap energy, and the refractive-index change is correlated to the photoinduced structural changes inferred by Raman spectroscopy data. A free-electron model to predict the parametric dependence of delta n is proposed.

Photosensitivity of As2S3 chalcogenide thin films at 1.5 microm.

Chalcogenide glasses are promising candidates for all-optical switching and various nonlinear applications. However, we show that As2S3 thin films are photosensitive at wavelengths in the 1.5-microm telecommunication window. This sensitivity is evidenced by the formation of self-written waveguides in slabs, where channels as narrow as 1 microm are created. We also show the detrimental effects of such photosensitivity in ridge waveguides. This photosensitivity seems to occur only in thin-film form and not in bulk samples or fibers.

Synaptic background activity affects the dynamics of dendritic integration in model neocortical pyramidal neurons

Abstract Neocortical pyramidal neurons in vivo are subject to an intense synaptic background activity which may significantly impact on dendritic integration, but this aspect is largely unexplored. Here we use computational models of morphologically-reconstructed pyramidal neurons, in which synaptic background activity was simulated according to recent measurements in cat parietal cortex. We show that background activity markedly enhances voltage attenuation, which results in a relative electrotonic “isolation” of different dendritic segments. On the other hand, the active propagation of action potentials in dendrites is minimally affected. The consequence is that inputs are integrated locally and their impact on the soma is independent on their position in the dendritic tree. We conclude that background activity sets up a dynamics of dendritic integration which is radically different compared to quiescent states.

Ablation and optical property modification of transparent materials with femtosecond lasers

Because of the unique laser-matter interaction processes involved, femtosecond laser micro-machining and femtosecond laser materials processing techniques are developing rapidly to stages where they may be introduced into manufacturing. Yet in both these areas, some complex interaction phenomena are not fully understood. In this talk we describe two studies of fundamental processes that impact both of these areas. These studies were made in transparent media, but their findings will be applicable to many non-transparent materials. Micro-machining in confined regions can give rise to new physical mechanisms emerging to dominate the machining process. We show this occurs in deep hole drilling of glasses by femtosecond laser pulse, where self-focusing effects takes over in the ablating process. The conditions under which this occurs will be described, and other configurations discussed where these phenomena may be important. At intensities below that required for ablation, structural modification of materials may be effected by femtosecond laser pulses. This has opened pathways towards direct femtosecond laser writing of optical waveguides, micro-fluidic systems and other structures. We will describe the controlled variation of refractive index that can be created in certain types of glasses and there potential for optical waveguides, and active optical elements. The evolution of these techniques will lead to their eventual integration for the fabrication of multi-component systems on a single chip.

Creation of microchannels in a photosensitive As 2 S 3 slab waveguide

We report the creation of microchannels in a photosensitive material, the arsenic trisulfide As2S3. We show that microchannels are created through the process of self-writing and are highly sensitive to the photosensitivity of the material as well as the quality of the incident wave front. The very high photosensitivity of As2S3 allows the self-written waveguide to become much smaller than the incident beam. We present a numerical analysis based on the nonlinear Schrodinger equation that accounts well for the diversity of the microchannels that were experimentally observed and shows that they can actually guide light.

Consequences of correlated synaptic bombardment on the responsiveness of neocortical pyramidal neurons

Model neocortical pyramidal neurons were investigated to evaluate the impact of correlated synaptic bombardment on cellular responsiveness. The responses to simulated glutamatergic (AMPA-mediated) excitatory inputs were analyzed by comparing correlated and uncorrelated background activity. Pyramidal cells behaved stochastically, consistent with the high variability of responses typically observed in vivo. Interestingly, the responsiveness was enhanced in the presence of correlated background activity, allowing the neuron to respond to inputs that would normally be subthreshold. We suggest that during active states, neocortical neurons should be described by stochastic equations which may better reflect their behavior, compared to a deterministic description.

BioSense/SR-BioSpectra demonstrations of wide area/early warning for bioaerosol threats: program description and early test and evaluation results

Threats associated with bioaerosol weapons have been around for several decades and have been mostly associated with terrorist activities or rogue nations. Up to the turn of the millennium, defence concepts against such menaces relied mainly on point or in-situ detection technologies. Over the last 10 years, significant efforts have been deployed by multiple countries to supplement the limited spatial coverage of a network of one or more point bio-detectors using lidar technology. The addition of such technology makes it possible to detect within seconds suspect aerosol clouds over area of several tens of square kilometers and track their trajectories. These additional capabilities are paramount in directing presumptive ID missions, mapping hazardous areas, establishing efficient counter-measures and supporting subsequent forensic investigations. In order to develop such capabilities, Defence Research and Development Canada (DRDC) and the Chemical, Biological, Radiological-Nuclear, and Explosives Research and Technology Initiative (CRTI) have supported two major demonstrations based on spectrally resolved Laser Induced Fluorescence (LIF) lidar: BioSense, aimed at defence military missions in wide open spaces, and SR-BioSpectra, aimed at surveillance of enclosed or semienclosed wide spaces common to defence and public security missions. This article first reviews briefly the modeling behind these demonstration concepts. Second, the lidar-adapted and the benchtop bioaerosol LIF chambers (BSL1), developed to challenge the constructed detection systems and to accelerate the population of the library of spectral LIF properties of bioaerosols and interferents of interest, will be described. Next, the most recent test and evaluation (T&E) results obtained with SR-BioSpectra and BioSense are reported. Finally, a brief discussion stating the way ahead for a complete defence suite is provided.

Hyperspectral fluorescence lifetime lidar for geological exploration

We have developed a small, relatively lightweight and efficient lidar instrument for remotely detecting and classifying minerals. The system is based on a pulsed, eye-safe, diode pumped Nd:YAG laser, tripled (355nm) or quadrupled (266nm), for UV excitation of minerals, which then fluoresce with a typical spectrum and lifetime. Fluorescence is detected through a telescope / filter / fiber bundle / spectrograph / multi-channel detector system capable of photon counting. Transmission and detection efficiency have been optimized to reduce the need for high optical excitation energy. Detection electronics are based on gated charge integration using a multi-anode photomultiplier tube. Spectra shown are measured in the 420 to 720 nm visible range with 355 nm laser excitation. Results show that it is relatively easy to distinguish between vegetation and non-vegetation spectra using lifetime data. Lifetime of vegetation is relatively short when compared to the mineral samples investigated. Although results shown are measured in a controlled environment on the ground, the system is being developed for eventual use in a low altitude airborne application. System parameters are presented and upgrade paths are discussed.

Intensity and polarization dependences of the supercontinuum generation in birefringent and highly nonlinear microstructured fibers

We present experimental results highlighting the physical mechanism responsible for the initial spectral broadening of femtosecond Ti:Sapphire pulses in a highly birefringent and nonlinear microstructured fiber.

Creation of micro-channels in a photosensitive As2S3 slab waveguide

The creation of microchannels in a photosensitive material, the arsenic trisulphide As2S3, is reported. It is shown that microchannels are created through the process of self-writing and are very sensitive to the photosensitivity of the material, the quality of the incident wavefront and the light intensity. The very large photosensitivity of As2S3 allows for the self-written waveguide to become much smaller than the incident beam. It can indeed be as small as 1 micron wide. We present a numerical analysis based on the nonlinear Schroedinger equation that accounts well for the diversity of the microchannels experimentally observed. It is also shown that the microchannels can actually guide light efficiently.