Hybrid Electro-Optically Modulated Microcombs

Optical frequency combs based on mode-locked lasers have proven to be invaluable tools for a wide range of applications in precision spectroscopy and metrology. A novel principle of optical frequency comb generation in whispering-gallery mode microresonators ("microcombs") has been developed recently, which represents a promising route towards chip-level integration and out-of-the-lab use of these devices. Presently, two families of microcombs have been demonstrated: combs with electronically detectable mode spacing that can be directly stabilized, and broadband combs with up to octave-spanning spectra but mode spacings beyond electronic detection limits. However, it has not yet been possible to achieve these two key requirements simultaneously, as will be critical for most microcomb applications. Here we present a key step to overcome this problem by interleaving an electro-optic comb with the spectrum from a parametric microcomb. This allows, for the first time, direct control and stabilization of a microcomb spectrum with large mode spacing (>140 GHz) with no need for an additional mode-locked laser frequency comb. The attained residual 1-second-instability of the microcomb comb spacing is 10^-15, with a microwave reference limited absolute instability of 10^-12 at a 140 GHz mode spacing.Comment: 8 pages, 4 figures; accepted for publication in Physical Review Letter

The core-periphery model with three regions

We study a 3-region core-periphery model à la Krugman and compare our results with those of the standard 2-region model. The conditions for the stability of the dispersion and concentration configurations are established. Like in the 2-region model, dispersion and concentration can be simultaneously stable. We show that the 2- region (3-region) model favors the dispersion (concentration) of economic activity. Finally, we extend the core-periphery model to the case of n regions and show that stability of concentration with 2 regions implies stability of concentration with any even number of regions.new economic geography, core-periphery

Review of catalytic systems and thermodynamics for the Guerbet condensation reaction and challenges for biomass valorization

The Guerbet condensation reaction is an alcohol coupling reaction that has been known for more than a century. Because of the increasing availability of bio-based alcohol feedstock, this reaction is of growing importance and interest in terms of value chains of renewable chemical and biofuel production. Due to the specific branching pattern of the alcohol products, the Guerbet reaction has many interesting applications. In comparison to their linear isomers, branched-chain Guerbet alcohols have extremely low melting points and excellent fluidity. This review provides thermodynamic insights and unravels the various mechanistic steps involved. A comprehensive overview of the homogeneous, heterogeneous and combined homogeneous and heterogeneous catalytic systems described in published reports and patents is also given. Technological considerations, challenges and perspectives for the Guerbet chemistry are discussed

Parametric seeding of a microresonator optical frequency comb

We have investigated parametric seeding of a microresonator frequency comb (microcomb) by way of a pump laser with two electro-optic-modulation sidebands. We show that the pump-sideband spacing is precisely replicated throughout the microcomb's optical spectrum, and we demonstrate a record absolute line-spacing stability for microcombs of $1.6\times10^{-13}$ at 1 s. The spectrum of a parametric comb is complex, and often non-equidistant subcombs are observed. Our results demonstrate that parametric seeding can not only control the subcombs, but can lead to the generation of a strictly equidistant microcomb spectrum.Comment: 10 pages, 5 figure

Self-referencing a continuous-wave laser with electro-optic modulation

We phase-coherently measure the frequency of continuous-wave (CW) laser light by use of optical-phase modulation and f-2f nonlinear interferometry. Periodic electro-optic modulation (EOM) transforms the CW laser into a continuous train of picosecond optical pulses. Subsequent nonlinear-fiber broadening of this EOM frequency comb produces a supercontinuum with 160 THz of bandwidth. A critical intermediate step is optical filtering of the EOM comb to reduce electronic-noise-induced decoherence of the supercontinuum. Applying f-2f self-referencing with the supercontinuum yields the carrier-envelope offset frequency of the EOM comb, which is precisely the difference of the CW laser frequency and an exact integer multiple of the EOM pulse repetition rate. Here we demonstrate absolute optical frequency metrology and synthesis applications of the self-referenced CW laser with <5E-14 fractional accuracy and stability.Comment: 8 pages, 4 figure

Etching suspended superconducting hybrid junctions from a multilayer

A novel method to fabricate large-area superconducting hybrid tunnel junctions with a suspended central normal metal part is presented. The samples are fabricated by combining photo-lithography and chemical etch of a superconductor - insulator - normal metal multilayer. The process involves few fabrication steps, is reliable and produces extremely high-quality tunnel junctions. Under an appropriate voltage bias, a significant electronic cooling is demonstrated

SimKat: a virtual laboratory to explore the impact of rainfall variability associated with the A2 climate change scenario on the Western Australian wheat-belt [Abstract]

The wheat-belt of Western Australia is one of the most vulnerable regions to climate change in Australia (DAFWA, 2007). Over the last century, the landscape is Western Australian has changed drastically with almost 20 million hectares of native vegetation being converted into pastures and annual crops to create the so-called "Western Australian wheat-belt" (Turner and Ward, 2002) This significant landuse change has contributed to the modification of the regional water cycle. McFarlane and Williamson (2002) estimated that about 10% of cropping land in south-west Australia is affected by dry-land salinity which could increase to up to 30% in the coming decades. This loss is estimated to cost A$80 million per year in lost production, and A$500 million a year due to damaged infrastructure. Moreover, since the mid 1970's, winter rainfall has declined by more than 15% (Smith et al., 2000). Predicted changes in winter rainfall, for 2070, range from a 60% reduction up to an increase by 10% (Pittock, 2003). However, one of the more likely scenarios is a reduction in winter rainfall of about 15% by 2030 and 30% by 2070 (IOCI, 2002). In order to explore the long-term effects and consequences of rainfall uncertainty and climate change on these already threatened socio-ecological systems, we have developed SimKat, an agent-based model developed with the CORMAS platform. SimKat combines simplified biophysical processes of paddock cover with likely CO2 impact on potential yields, dry-land salinity changes, likely rainfall scenarios and farmers¿ decision making processes. Variations in temperature are not accounted for at this stage of model¿s development. Simulated agents - farmers make decisions about their future landuse pattern based on their land cover productivity and market returns. Agents are also attributed various risk-related attitudes towards market and mitigation signals. To account for rainfall variability, we use 50 rainfall series from 2005 to 2055, generated through a downscaling technique that relates changes in atmospheric predictors from a General Circulation Model. The model explores the impact of each rainfall series in association with the A2 climate change scenario on the viability of the simulated agricultural region based on the following simulated indicators: farm numbers, salinity extension, regional income, crop-pasture ratio. Yield potential and technological trends influencing farmers¿ ability to crop are also studied. Simulated scenarios discuss the impact of rainfall variability and atmospheric CO2 increase on individual and regional farm viability. The scenarios provide means to closely analyse the resilience of the simulated agricultural region to potential impacts of climate uncertainty. (Résumé d'auteur