The identification of a universal biosignature that could be sensed remotely is critical to the prospects for success in the search for life elsewhere in the universe. A candidate universal biosignature is homochirality, which is likely to be a generic property of all biochemical life. Because of the optical activity of chiral molecules, it has been hypothesized that this unique characteristic may provide a suitable remote sensing probe using circular polarization spectroscopy. Here, we report the detection of circular polarization in light scattered by photosynthetic microbes. We show that the circular polarization appears to arise from circular dichroism of the strong electronic transitions of photosynthetic absorption bands. We conclude that circular polarization spectroscopy could provide a powerful remote sensing technique for generic life searches.

The response of the molecular stretch mode of CO/Cu(100) near 2086 cm-I (VI) to resonant infrared, and nonresonant visible and ultraviolet pumping is measured on a picosecond time scale. Fourier transform infrared measurements establish that VI is anharmonic ally coupled to the frustrated translation near 32 cm-I (V4), so that transient shifts in VI indicate population changes in V4' The VI response to visible and ultraviolet pumping is characterized by a spectral shift near zero delay time, which decays with a =2 ps time constant to an intermediate value, which then decays on a =200 ps time scale. The data agree well with a model whereby V4 couples to both the photogenerated hot electrons and to the heated phonons. The characteristic coupling times to these two heat baths are found to both be a few picoseconds.

By combining surface wrinkling and nanopatterned polymer films, we create anisotropic, hierarchical surfaces whose larger length-scale (wrinkling wavelength) depends intimately on the geometry and orientation of the smaller length-scale (nanopattern). We systematically vary the pattern pitch, pattern height, and residual layer thickness to ascertain the dependence of the wrinkling wavelength on the nanopattern geometry. We apply a composite mechanics model to gain a quantitative understanding of the relationship between the geometric parameters and the anisotropy in wrinkling wavelength. Additionally, these results shed light on the effect of surface roughness, as represented by the nanopattern, on the metrology of thin films via surface wrinkling.

We study the decay of imprinted polystyrene (PS) patterns under thermal annealing using light diffraction. The first-order diffraction intensity from the imprinted gratings was measured as a function of annealing time. A local intensity maximum is observed as a function of annealing time. This "abnormal" intensity variation can be qualitatively understood, using rigorous coupled wave approximation calculations, as a characteristic diffraction from patterns with specific shape/height. We demonstrate that this diffraction anomaly can be used to characterize the temperature dependence of the pattern decay rate. The activation energies of the pattern decay are found to be similar to those of the segmental and chain relaxations. Comparisons between PS samples of different molecular mass reveal that the patterns decay through different mechanisms. For unentangled PS, the decay of the imprinted pattern follows the surface-tension-driven viscous flow, with a viscosity similar to the steady-state viscosity. However, for highly entangled PS, large residual stresses introduced from the imprinting process cause the pattern to decay much faster than expected from surface-tension-driven viscous flow.

The transient infrared response of CO/Pt(111) following picosecond visible excitation is reported. A spectrally broad decrease in reflectivity correlates with heating of the Pt lattice, and an observed shift in the CO(v=0→1) transition is interpreted as heating of the 60 cm−1 in-plane frustrated translational mode. A phenomenological three temperature model that assumes the adsorbate vibrational temperature Tads exclusively couples to either the electronic temperature Te (with a time constant τe) or to the lattice temperature Tlat (with a time constant τlat) describes the temporal response of the adsorbate vibrations. The lattice phonon temperature Tlat(z,t) and measured temperature dependence of the optical constants predict the observed spectrally broad reflectivity change. Density matrix methods model the infrared response of the transiently heated molecule. Limits of τe=2±1 ps or τlat<1 ps are established by comparison of predicted spectra and the data.

Original article can be found at : http://www.sciencedirect.com/ Copyright ElsevierBiological molecules exhibit homochirality and are optically active. Therefore, it is possible that the scattering of light by biological molecules might result in a macroscopic signature in the form of circular polarization. If this is the case, then circular polarization spectroscopy, which may be utilized in remote sensing, can offer a powerful indicator of the presence of a universal biosignature, namely homochirality. Here, we describe laboratory experiments designed to investigate this idea. We focus on photosynthetic microorganisms, and also show results from macroscopic vegetation and control minerals. In the microorganisms, we find unambiguous circular polarization associated with electronic absorption bands of the photosynthetic apparatus. Macroscopic vegetation yields a stronger and more complex signature while the control minerals produce low-levels of circular polarization unrelated to their spectra. We propose a heuristic explanation of our results, which is that the polarization is produced by circular dichroism in the material after the light has undergone its last scattering event. The results are encouraging for the use of circular polarization spectroscopy in remote sensing of a generic biomarker from space or the ground

A goniometric optical scatter instrument has been developed at the National Institute of Standards and Technology which can readily perform measurements of optical scatter and its associated polarization in directions out of the plane of incidence. In this article the coordinate transformations that are required to operate such a goniometer with respect to sample-specific coordinates are described. We present new methods for measuring the 3×3 nonhanded Mueller matrix elements using dual rotating half-wave retarders, and present a subset of the Mueller matrix, referred to as the bidirectional ellipsometric parameters which have been shown to simplify the interpretation of the data. The results of out-of-plane Mueller matrix and bidirectional ellipsometric measurements from a titanium nitride layer on silicon are presented.

Second-order nonlinear mixing is evaluated as a probe of the depletion electric field in the near-surface region of GaAs͑001͒. A phenomenological model is presented whereby the nonlinear susceptibility is expanded in the depletion electric field. For GaAs, three terms contribute to the observed nonlinear mixing: the dipole-allowed bulk contribution and first-and second-order contributions in the depletion electric field. All three contributions can be isolated by a combination of rotational anisotropy and photomodulation studies. The second-harmonic signal from oxidized GaAs͑001͒ surfaces was measured as a function of azimuthal sample orientation, photomodulation, and dopant density for fundamental photon energies in the region 1.17-1.51 eV. The first-order depletion-electric-field contribution dominates the second-order contribution, and can be comparable to the intrinsic bulk contribution for high surface fields. The results rule out significant contributions arising from either bulk electric quadrupole or surface dipole effects. ͓S0163-1829͑97͒00616-4͔