Demonstration of 3-port grating phase relations

We experimentally demonstrate the phase relations of 3-port gratings by investigating 3-port coupled Fabry-Perot cavities. Two different gratings which have the same 1st order diffraction efficiency but differ substantially in their 2nd order diffraction efficiency have been designed and manufactured. Using the gratings as couplers to Fabry-Perot cavities we could validate the results of an earlier theoretical description of the phases at a three port grating

On the observability of bow shocks of Galactic runaway OB stars

Massive stars that have been ejected from their parent cluster and supersonically sailing away through the interstellar medium (ISM) are classified as exiled. They generate circumstellar bow shock nebulae that can be observed. We present two-dimensional, axisymmetric hydrodynamical simulations of a representative sample of stellar wind bow shocks from Galactic OB stars in an ambient medium of densities ranging from n_ISM=0.01 up to 10.0/cm3. Independently of their location in the Galaxy, we confirm that the infrared is the most appropriated waveband to search for bow shocks from massive stars. Their spectral energy distribution is the convenient tool to analyze them since their emission does not depend on the temporary effects which could affect unstable, thin-shelled bow shocks. Our numerical models of Galactic bow shocks generated by high-mass (~40 Mo) runaway stars yield H$\alpha$ fluxes which could be observed by facilities such as the SuperCOSMOS H-Alpha Survey. The brightest bow shock nebulae are produced in the denser regions of the ISM. We predict that bow shocks in the field observed at Ha by means of Rayleigh-sensitive facilities are formed around stars of initial mass larger than about 20 Mo. Our models of bow shocks from OB stars have the emission maximum in the wavelength range 3 <= lambda <= 50 micrometer which can be up to several orders of magnitude brighter than the runaway stars themselves, particularly for stars of initial mass larger than 20 Mo.Comment: 13 pages, 12 figures. Accepted to MNRAS (2016

Diffractive Optics for Gravitational Wave Detectors

All-reflective interferometry based on nano-structured diffraction gratings offers new possibilities for gravitational wave detection. We investigate an all-reflective Fabry-Perot interferometer concept in 2nd order Littrow mount. The input-output relations for such a resonator are derived treating the grating coupler by means of a scattering matrix formalism. A low loss dielectric reflection grating has been designed and manufactured to test the properties of such a grating cavity

Constraining the near-surface response to lithospheric reorientation: Structural thermochronology along AlpArray geophysical transects

The shape of the present-day European Alps results from a complex tectonic and climatic history since the onset of convergence between the African and Eurasian plates. Low-temperature thermochronology data are a unique archive that can trace the cooling history of rocks back in time during exhumation from upper to middle crustal levels to Earth's surface. However, the precise mechanisms that led to cooling and exhumation are still debated. In this study, we investigated the potential for mantle processes, such as potential subducting slab break-off or slab reversal, to leave a fingerprint in the rock cooling record of the present-day surface along three key, north-south oriented geophysical transects: NFP-20E, TRANSALP and EASI. Along all transects, our zircon and apatite (U-Th)/He data reveal reset Neogene (and younger) cooling ages centred around core complexes such as the Lepontine Dome and the Tauern Window indicative of late exhumation during the Cenozoic Alpine orogeny. North and south of these complexes, the cooling ages become older, forming U-shaped age distributions around the reset centres. Thermal history reconstructions along TRANSALP confirm a conspicuous southward shift of cooling towards the Southern Alps approximately at the time of deep-seated exhumation of the Tauern Window driven by motion along the mid-crustal Tauern Ramp in the Mid-Miocene. Thermo-kinematic models along the transect confirm this southward shift of deformation and (i) reproduce the distribution of cooling ages and thermal history reconstructions, (ii) are consistent with the present-day structural geometry along the transect, (iii) and the observed surface heat flux. It is possible that rock cooling is primarily driven by rock displacement along active faults and less by climatic and/or mantle buoyancy forces, which are both not included in the applied modelling approach. Our comprehensive thermochronological analyses allow two interpretations concerning mantle processes: (i) Assuming a strong coupling between the subducting and overriding plate, hence, the applicability of doubly-vergent orogen kinematics, then the thermochronological data are most consistent with an ongoing reversal in continental subduction polarity. (ii) A high degree of decoupling would negate the possibility that mantle processes are archived in the thermochronological record

3D-MHD simulations of an accretion disk with star-disk boundary layer

We present global 3D MHD simulations of geometrically thin but unstratified accretion disks in which a near Keplerian disk rotates between two bounding regions with initial rotation profiles that are stable to the MRI. The inner region models the boundary layer between the disk and an assumed more slowly rotating central, non magnetic star. We investigate the dynamical evolution of this system in response to initial vertical and toroidal fields imposed in a variety of domains contained within the near Keplerian disk. Cases with both non zero and zero net magnetic flux are considered and sustained dynamo activity found in runs for up to fifty orbital periods at the outer boundary of the near Keplerian disk. Simulations starting from fields with small radial scale and with zero net flux lead to the lowest levels of turbulence and smoothest variation of disk mean state variables. For our computational set up, average values of the Shakura & Sunyaev (1973) $\alpha$ parameter in the Keplerian disk are typically $0.004\pm 0.002.$ Magnetic field eventually always diffuses into the boundary layer resulting in the build up of toroidal field inward angular momentum transport and the accretion of disk material. The mean radial velocity, while exhibiting large temporal fluctuations is always subsonic. Simulations starting with net toroidal flux may yield an average $\alpha \sim 0.04.$ While being characterized by one order of magnitude larger average $\alpha$, simulations starting from vertical fields with large radial scale and net flux may lead to the formation of persistent non-homogeneous, non-axisymmetric magnetically dominated regions of very low density.Comment: Accepted for publication in Ap

Experimental demonstration of a suspended diffractively coupled optical cavity

All-reflective optical systems are under consideration for future gravitational wave detector topologies. One approach in proposed designs is to use diffraction gratings as input couplers for Fabry–Perot cavities. We present an experimental demonstration of a fully suspended diffractively coupled cavity and investigate the use of conventional Pound–Drever–Hall length sensing and control techniques to maintain the required operating condition

Diffractive beam splitter characterization via a power-recycled interferometer

We used the high-precision laser interferometer technique of power recycling to characterize the optical loss of an all-reflective grating beam splitter. This beam splitter was used to set up a Michelson interferometer with a power-recycling resonator with a finesse of 883. Analyzing the results obtained, we determined the beam splitter's total optical loss to be (0.193+/-0.019)%. Low loss all-reflective beam splitters might find application in future high-power laser interferometers for the detection of gravitational waves

Monolithic dielectric surfaces as new low-loss light-matter interfaces

We propose a new mirror architecture, which is solely based upon structuring of the surface of a monolithic, possibly monocrystalline, bulk material. We found that a structure of T-shaped ridges of a subwavelength grating can theoretically provide 100% reflectivity. Since no material needs to be added to the mirror device, lowest mechanical loss can also be expected. Our approach might have compelling applications as a new light-matter interface