Exciton-polariton emission from organic semiconductor optical waveguides

We photo-excite slab polymer waveguides doped with J-aggregating dye molecules and measure the leaky emission from strongly coupled waveguide exciton polariton modes at room temperature. We show that the momentum of the waveguide exciton polaritons can be controlled by modifying the thickness of the excitonic waveguide. Non-resonantly pumped excitons in the slab excitonic waveguide decay into transverse electric and transverse magnetic strongly coupled exciton waveguide modes with radial symmetry. These leak to cones of light with radial and azimuthal polarizations

Transient LTRE analysis reveals the demographic and trait-mediated processes that buffer population growth.

Temporal variation in environmental conditions affects population growth directly via its impact on vital rates, and indirectly through induced variation in demographic structure and phenotypic trait distributions. We currently know very little about how these processes jointly mediate population responses to their environment. To address this gap, we develop a general transient life table response experiment (LTRE) which partitions the contributions to population growth arising from variation in (1) survival and reproduction, (2) demographic structure, (3) trait values and (4) climatic drivers. We apply the LTRE to a population of yellow-bellied marmots (Marmota flaviventer) to demonstrate the impact of demographic and trait-mediated processes. Our analysis provides a new perspective on demographic buffering, which may be a more subtle phenomena than is currently assumed. The new LTRE framework presents opportunities to improve our understanding of how trait variation influences population dynamics and adaptation in stochastic environments

High harmonic generation from Bloch electrons in solids

We study the generation of high harmonic radiation by Bloch electrons in a model transparent solid driven by a strong mid-infrared laser field. We solve the single-electron time-dependent Schr\"odinger equation (TDSE) using a velocity-gauge method [New J. Phys. 15, 013006 (2013)] that is numerically stable as the laser intensity and number of energy bands are increased. The resulting harmonic spectrum exhibits a primary plateau due to the coupling of the valence band to the first conduction band, with a cutoff energy that scales linearly with field strength and laser wavelength. We also find a weaker second plateau due to coupling to higher-lying conduction bands, with a cutoff that is also approximately linear in the field strength. To facilitate the analysis of the time-frequency characteristics of the emitted harmonics, we also solve the TDSE in a time-dependent basis set, the Houston states [Phys. Rev. B 33, 5494 (1986)], which allows us to separate inter-band and intra-band contributions to the time-dependent current. We find that the inter-band and intra-band contributions display very different time-frequency characteristics. We show that solutions in these two bases are equivalent under an unitary transformation but that, unlike the velocity gauge method, the Houston state treatment is numerically unstable when more than a few low lying energy bands are used

Direct, high resolution, four‐dimensional measurements of the fine scale structure of Sc≫1 molecular mixing in turbulent flows

Results from highly resolved, four‐dimensional measurements of the fine structure of the fully space‐ and time‐varying Sc≫1 conserved scalar field and the associated scalar energy dissipation rate field in a turbulent flow are presented. The resolution achieved in all three spatial dimensions and in time reaches down to the local strain‐limited molecular diffusion scale in the flow, allowing all three components of the instantaneous scalar gradient vector field ∇ζ(x,t) and their time evolution at every point in the data space to be directly evaluated. Results are presented in the form of fine structure maps of the instantaneous dissipation field loge ∇ζ⋅∇ζ(x,t) in several spatially adjacent data planes within an individual three‐dimensional spatial data volume, as well as in several temporally successive data planes from a sequence of such three‐dimensional data volumes. The degree of anisotopy in the underlying scalar gradient field is characterized in terms of the joint distribution β(ϑ,φ) of spherical orientation angles. The probability density of true scalar energy dissipation rates is presented and compared with the distributions that would result from lower‐dimensional measurements of the scalar gradient vector. From this the ‘‘spottiness’’ of the scalar dissipation field is directly quantified by determining the true fraction of the total dissipation that occurs in any given volume fraction of the flow.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70449/2/PFADEB-3-5-1115-1.pd

Recombination Ghosts in Littrow Configuration: Implications for Spectrographs Using Volume Phase Holographic Gratings

We report the discovery of optical ghosts generated when using Volume Phase Holographic (VPH) gratings in spectrographs employing the Littrow configuration. The ghost is caused by light reflected off the detector surface, recollimated by the camera, recombined by, and reflected from, the grating and reimaged by the camera onto the detector. This recombination can occur in two different ways. We observe this ghost in two spectrographs being developed by the University of Wisconsin - Madison: the Robert Stobie Spectrograph for the Southern African Large Telescope and the Bench Spectrograph for the WIYN 3.5m telescope. The typical ratio of the brightness of the ghost relative to the integrated flux of the spectrum is of order 10^-4, implying a recombination efficiency of the VPH gratings of order 10^-3 or higher, consistent with the output of rigorous coupled wave analysis. Any spectrograph employing VPH gratings, including grisms, in Littrow configuration will suffer from this ghost, though the general effect is not intrinsic to VPH gratings themselves and has been observed in systems with conventional gratings in non-Littrow configurations. We explain the geometric configurations that can result in the ghost as well as a more general prescription for predicting its position and brightness on the detector. We make recommendations for mitigating the ghost effects for spectrographs and gratings currently built. We further suggest design modifications for future VPH gratings to eliminate the problem entirely, including tilted fringes and/or prismatic substrates. We discuss the resultant implications on the spectrograph performance metrics.Comment: 13 pages, 8 figures, emulateapj style, accepted for publication in PAS

Flight investigation of the effect of tail boom strakes on helicopter directional control

A joint U.S. Army/NASA flight investigation was conducted utilizing a single-rotor helicopter to determine the effectiveness of horizontally mounted tail boom strakes on directional controllability and tail rotor power during low-speed, crosswind operating conditions. Three configurations were investigated: (1) baseline (strakes off), (2) single strake (strake at upper shoulder on port side of boom), and (3) double strake (upper strake plus a lower strake on same side of boom). The strakes were employed as a means to separate airflow over the tail boom and change fuselage yawing moments in a direction to improve the yaw control margin and reduce tail rotor power. Crosswind data were obtained in 5-knot increments of airspeed from 0 to 35 knots and in 30 deg increments of wind azimuth from 0 deg to 330 deg. At the most critical wind azimuth and airspeed in terms of tail rotor power, the strakes improved the pedal margin by 6 percent of total travel and reduced tail rotor power required by 17 percent. The increase in yaw control and reduction in tail rotor power offered by the strakes can expand the helicopter operating envelope in terms of gross weight and altitude capability. The strakes did not affect the flying qualities of the vehicle at airspeeds between 35 and 100 knots