Ultrashort pulse generation in the mid-IR

Recent developments in laser sources operating in the mid-IR (3–8μm) have been motivated by the numerous possibilities for both fundamental and applied research. One example is the ability to unambiguously detect pollutants and carcinogens due to the much larger oscillator strengths of their absorption features in the mid-IR spectral region compared with the visible. Broadband sources are of particular interest for spectroscopic applications since they remove the need for arduous scanning or several lasers and allow simultaneous use of multiple absorption features thus increasing the confidence level of detection. In addition, sources capable of producing ultrashort and intense mid-IR radiation are gaining relevance in attoscience and strong-field physics due to wavelength scaling of re-collision based processes. In this paper we review the state-of-the-art in sources of coherent, pulsed mid-IR radiation. First we discuss semi-conductor based sources which are compact and turnkey, but typically do not yield short pulse duration. Mid-IR laser gain material based approaches will be discussed, either for direct broadband mid-IR lasers or as narrowband pump lasers for parametric amplification in nonlinear crystals. The main part will focus on mid-IR generation and amplification based on parametric frequency conversion, enabling highest mid-IR peak power pulses. Lastly we close with an overview of nonlinear post-compression techniques, for decreasing pulse duration to the sub-2-optical-cycle regime.Peer ReviewedPostprint (author's final draft

Ultrafast nonlinear optical response of Dirac fermions in graphene

The speed of solid-state electronic devices, determined by the temporal dynamics of charge carriers, could potentially reach unprecedented petahertz frequencies through direct manipulation by optical fields, consisting in a million-fold increase from state-of-the-art technology. In graphene, charge carrier manipulation is facilitated by exceptionally strong coupling to optical fields, from which stems an important back-action of photoexcited carriers. Here we investigate the instantaneous response of graphene to ultrafast optical fields, elucidating the role of hot carriers on sub-100 fs timescales. The measured nonlinear response and its dependence on interaction time and field polarization reveal the back-action of hot carriers over timescales commensurate with the optical field. An intuitive picture is given for the carrier trajectories in response to the optical-field polarization state. We note that the peculiar interplay between optical fields and charge carriers in graphene may also apply to surface states in topological insulators with similar Dirac cone dispersion relations.Peer ReviewedPostprint (published version

Broadband mid-IR frequency comb with CSP and AGS from a Er,Tm:Ho fiber laser

We report on the generation of a 2500 nm bandwidth frequency comb at 6.5 μm central wavelength based on critically phase-matched parametric down-conversion in the nonlinear crystal CdSiP 2 CdSiP2 (CSP), driven by a compact Er,Tm:Ho fiber laser. The generated ultra-broadband pulses show a transform-limited duration of 2.3 optical cycles and carry up to 150 pJ of energy at a 100 MHz pulse repetition rate. For comparison, the spectrum generated in AgGaS 2 AgGaS2 (AGS) spans from 6.2 to 7.4 μm at full-width at half-maximum (FWHM) with a pulse energy of 3 pJ. A full 3D nonlinear wave propagation code is used for optimization of the noncollinear angle, propagation direction, and crystal thickness.Peer ReviewedPostprint (author's final draft

Local chromatic number of quadrangulations of surfaces

The local chromatic number of a graph G, as introduced in [4], is the minimum integer k such that G admits a proper coloring (with an arbitrary number of colors) in which the neighborhood of each vertex uses less than k colors. In [17] a connection of the local chromatic number to topological properties of (a box complex of) the graph was established and in [18] it was shown that a topological condition implying the usual chromatic number being at least four has the stronger consequence that the local chromatic number is also at least four. As a consequence one obtains a generalization of the following theorem of Youngs [19]: If a quadrangulation of the projective plane is not bipartite it has chromatic number four. The generalization states that in this case the local chromatic number is also four. Both papers [1] and [13] generalize Youngs’ result to arbitrary non-orientable surfaces replacing the condition of the graph being not bipartite by a more technical condition of an odd quadrangulation. This paper investigates when these general results are true for the local chromatic number instead of the chromatic number. Surprisingly, we find out that (unlike in the case of the chromatic number) this depends on the genus of the surface. For the non-orientable surfaces of genus at most four, the local chromatic number of any odd quadrangulation is at least four, but this is not true for non-orientable surfaces of genus 5 or higher. We also prove that face subdivisions of odd quadrangulations and Fisk triangulations of arbitrary surfaces exhibit the same behavior for the local chromatic number as they do for the usual chromatic number

Coevolution of relative brain size and life expectancy in parrots

Previous studies have demonstrated a correlation between longevity and brain size in a variety of taxa. Little research has been devoted to understanding this link in parrots; yet parrots are well-known for both their exceptionally long lives and cognitive complexity. We employed a large-scale comparative analysis that investigated the influence of brain size and life-history variables on longevity in parrots. Specifically, we addressed two hypotheses for evolutionary drivers of longevity: the cognitive buffer hypothesis, which proposes that increased cognitive abilities enable longer lifespans, and the expensive brain hypothesis, which holds that increases in lifespan are caused by prolonged developmental time of, and increased parental investment in, large-brained offspring. We estimated life expectancy from detailed zoo records for 133 818 individuals across 244 parrot species. Using a principled Bayesian approach that addresses data uncertainty and imputation of missing values, we found a consistent correlation between relative brain size and life expectancy in parrots. This correlation was best explained by a direct effect of relative brain size. Notably, we found no effects of developmental time, clutch size or age at first reproduction. Our results suggest that selection for enhanced cognitive abilities in parrots has in turn promoted longer lifespans

Influence of orbital symmetry on diffraction imaging with rescattering electron wave packets

Citation: Pullen, M. G., Wolter, B., Le, A. T., Baudisch, M., Sclafani, M., Pires, H., . . . Biegert, J. (2016). Influence of orbital symmetry on diffraction imaging with rescattering electron wave packets. Nature Communications, 7, 6. doi:10.1038/ncomms11922The ability to directly follow and time-resolve the rearrangement of the nuclei within molecules is a frontier of science that requires atomic spatial and few-femtosecond temporal resolutions. While laser-induced electron diffraction can meet these requirements, it was recently concluded that molecules with particular orbital symmetries (such as pi(g)) cannot be imaged using purely backscattering electron wave packets without molecular alignment. Here, we demonstrate, in direct contradiction to these findings, that the orientation and shape of molecular orbitals presents no impediment for retrieving molecular structure with adequate sampling of the momentum transfer space. We overcome previous issues by showcasing retrieval of the structure of randomly oriented O-2 and C2H2 molecules, with pi(g) and pi(u) symmetries, respectively, and where their ionization probabilities do not maximize along their molecular axes. While this removes a serious bottleneck for laser-induced diffraction imaging, we find unexpectedly strong backscattering contributions from low-Z atoms

Towards a verified compiler prototype for the synchronous language SIGNAL

International audienceSIGNAL belongs to the synchronous languages family which are widely used in the design of safety-critical real-time systems such as avionics, space systems, and nuclear power plants. This paper reports a compiler prototype for SIGNAL. Compared with the existing SIGNAL compiler, we propose a new intermediate representation (named S-CGA, a variant of clocked guarded actions), to integrate more synchronous programs into our compiler prototype in the future. The front-end of the compiler, i.e., the translation from SIGNAL to S-CGA, is presented. As well, the proof of semantics preservation is mechanized in the theorem prover Coq. Moreover, we present the back-end of the compiler, including sequential code generation and multithreaded code generation with time-predictable properties. With the rising importance of multi-core processors in safety-critical embedded systems or cyber-physical systems (CPS), there is a growing need for model-driven generation of multithreaded code and thus mapping on multi-core. We propose a time-predictable multi-core architecture model in architecture analysis and design language (AADL), and map the multi-threaded code to this model

Low-fidelity fabrication: Speeding up design iteration of 3D objects

Low-fidelity fabrication systems speed up rapid prototyping by printing intermediate versions of a prototype as fast, low-fidelity previews. Only the final version is fabricated as a full high-fidelity 3D print. This allows designers to iterate more quickly-achieving a better design in less time. Depending on what is currently being tested, low-fidelity fabrication is implemented in different ways: (1) faBrickator allows for a modular approach by substituting sub-volumes of the 3D model with building blocks. (2) WirePrint allows for quickly testing the shape of an object, such as the ergonomic fit, by printing wireframe structures. (3) Platener preserves the technical function by substituting 3D print with laser-cut plates of the same size and thickness. At our CHI'15 interactivity booth, we give a combined live demo of all three low-fidelity fabrication systems- putting special focus on our new low-fidelity fabrication system Platener (paper at CHI'15)