Direct picosecond time resolution of unimolecular reactions initiated by local mode excitation

The concept of local mode (LM) states [1] in large molecules raises the possibilty of inducing chemical reactions from a well-defined initial state (bond-selective chemistry). The results of linewidth and energy measurements in gases, [2(a)] and low temperature solids, [2(b)] however, indicate that the relaxation times for such high energy (> 15000 cm^-1) states can be extremely short, < 1ps. Because of the lack of direct time-resolved measurements, the following fundamental questions have not been unequivocally answered: What are the homogeneous linewidths of LM states and what are the rates of energy relaxation or reaction out of these states? Over the past five years we have made several attempts to observe the picosecond dynamics of LM states. Due to the inherent difficulties associated with making these measurements, such as the very small oscillator strength (σ < 10^-23 cm^2), an extremely sensitive probing technique becomes imperative

Antenna-coupled silicon-organic hybrid integrated photonic crystal modulator for broadband electromagnetic wave detection

In this work, we design, fabricate and characterize a compact, broadband and highly sensitive integrated photonic electromagnetic field sensor based on a silicon-organic hybrid modulator driven by a bowtie antenna. The large electro-optic (EO) coefficient of organic polymer, the slow-light effects in the silicon slot photonic crystal waveguide (PCW), and the broadband field enhancement provided by the bowtie antenna, are all combined to enhance the interaction of microwaves and optical waves, enabling a high EO modulation efficiency and thus a high sensitivity. The modulator is experimentally demonstrated with a record-high effective in-device EO modulation efficiency of r33=1230pm/V. Modulation response up to 40GHz is measured, with a 3-dB bandwidth of 11GHz. The slot PCW has an interaction length of 300um, and the bowtie antenna has an area smaller than 1cm2. The bowtie antenna in the device is experimentally demonstrated to have a broadband characteristics with a central resonance frequency of 10GHz, as well as a large beam width which enables the detection of electromagnetic waves from a large range of incident angles. The sensor is experimentally demonstrated with a minimum detectable electromagnetic power density of 8.4mW/m2 at 8.4GHz, corresponding to a minimum detectable electric field of 2.5V/m and an ultra-high sensitivity of 0.000027V/m Hz^-1/2 ever demonstrated. To the best of our knowledge, this is the first silicon-organic hybrid device and also the first PCW device used for the photonic detection of electromagnetic waves. Finally, we propose some future work, including a Teraherz wave sensor based on antenna-coupled electro-optic polymer filled plasmonic slot waveguide, as well as a fully packaged and tailgated device.Comment: 20 pages, 16 figure

Economic insecurity during the Great Recession and metabolic, inflammatory and liver function biomarkers: analysis of the UK Household Longitudinal Study

Background: Economic insecurity correlates with adverse health outcomes, but the biological pathways involved are not well understood. We examine how changes in economic insecurity relate to metabolic, inflammatory and liver function biomarkers. Methods: Blood analyte data were taken from 6520 individuals (aged 25–59 years) participating in Understanding Society. Economic insecurity was measured using an indicator of subjective financial strain and by asking participants whether they had missed any bill, council tax, rent or mortgage payments in the past year. We investigated longitudinal changes in economic insecurity (remained secure, increase in economic insecurity, decrease in economic insecurity, remained insecure) and the accumulation of economic insecurity. Linear regression models were calculated for nine (logged) biomarker outcomes related to metabolic, inflammatory, liver and kidney function (as falsification tests), adjusting for potential confounders. Results: Compared with those who remained economically stable, people who experienced consistent economic insecurity (using both measures) had worsened levels of high-density lipoprotein (HDL)-cholesterol, triglycerides, C reactive protein (CRP), fibrinogen and glycated haemoglobin. Increased economic insecurity was associated with adverse levels of HDL-cholesterol (0.955, 95% CI 0.929 to 0.982), triglycerides (1.077, 95% CI 1.018 to 1.139) and CRP (1.114, 95% CI 1.012 to 1.227), using the measure of financial strain. Results for the other measure were generally consistent, apart from the higher levels of gamma-glutamyl transferase observed among those experiencing persistent insecurity (1.200, 95% CI 1.110 to 1.297). Conclusion: Economic insecurity is associated with adverse metabolic and inflammatory biomarkers (particularly HDL-cholesterol, triglycerides and CRP), heightening risk for a range of health conditions

Thermally stable hybrid cavity laser based on silicon nitride gratings

Funding: Science Foundation Ireland (SFI) (16/ERCS/3838, SFI12/RC/2276); Engineering and Physical Sciences Research Council (EPSRC) (EP/L017008/1, EP/L505079/1); H2020 LEIT Information and Communication Technologies (ICT) (COSMICC nr. 688516, H2020-ICT27-2015); H2020 European Research Council (ERC) (337508)In this paper, we show the experimental results of a thermally stable Si3N4 external cavity (SiN EC) laser with high power output and the lowest SiN EC laser threshold to our knowledge. The device consists of a 250 μm sized reflective semiconductor optical amplifier butt-coupled to a passive chip based on a series of Si3N4 Bragg gratings acting as narrow reflectors. A threshold of 12 mA has been achieved, with a typical side-mode suppression ratio of 45 dB and measured power output higher than 3 mW. Furthermore, we achieved a mode-hop free-lasing regime in the range of 15–62 mA and wavelength thermal stability up to 80°C. This solves the challenges related to cavity resonances’ thermal shift and shows the possibility for this device to be integrated in dense wavelength-division multiplexing (WDM) and heat-intensive optical interconnects technologies.PostprintPeer reviewe

Electromagnetic shock waves from transmission lines

We have observed subpicosecond electrical pulses to propagate on 5-pm coplanar transmission lines at velocities faster than the phase velocity in the underlying dielectric. This situation produces an electromagnetic shock wave in a manner similar to Cherenkov radiation and electro-optic Cherenkov radiation. Using time-domain spectroscopy, we have measured the strong frequency-dependent loss of energy in the propagating electrical pulse due to this radiation.Peer reviewedElectrical and Computer Engineerin

Carrier dynamics of electrons and holes in moderately doped silicon

A time-domain spectroscopic technique, based on the generation and detection of a collimated beam of subpicosecond broadband terahertz pulses, is used to measure the absorption and dispersion of n- and p-type silicon, with resistivities of 0.1, 1, and 10 Ohm-cm in the submillimeter range of 0.1-2 THz. From the transmission measurements performed at room temperature and at 80 K, the absorption and dispersion, and concomitantly the full complex conductivity, of the doped silicon could be obtained. The results provide an accurate view on the dynamics of the electrons and the holes. Although the simple Drude model, with an energy-independent relaxation time, gives a surprisingly accurate description of the observed carrier dynamics, the measurements do show that some refinements are needed. An extended model, with an energy-dependent carrier-relaxation rate, can explain most of the observed deviations from the simple Drude model.Peer reviewedElectrical and Computer Engineerin