Search

We present a search for anisotropic cosmic birefringence in 500 deg$^2$ of southern sky observed at 150 GHz with the SPTpol camera on the South Pole Telescope. We reconstruct a map of cosmic polarization rotation anisotropies using higher-order correlations between the observed cosmic microwave background (CMB) $E$ and $B$ fields. We then measure the angular power spectrum of this map, which is found to be consistent with zero. The non-detection is translated into an upper limit on the amplitude of the scale-invariant cosmic rotation power spectrum, $L(L+1)C_L^{\alpha\alpha}/2\pi < 0.10 \times 10^{-4}$ rad$^2$ (0.033 deg$^2$, 95% C.L.). This upper limit can be used to place constraints on the strength of primordial magnetic fields, $B_{1 \rm Mpc} < 17 {\rm nG} $ (95% C.L.), and on the coupling constant of the Chern-Simons electromagnetic term $g_{a\gamma} < 4.0 \times 10^{-2}/H_I $ (95% C.L.), where $H_I$ is the inflationary Hubble scale. For the first time, we also cross-correlate the CMB temperature fluctuations with the reconstructed rotation angle map, a signal expected to be non-vanishing in certain theoretical scenarios, and find no detectable signal. We perform a suite of systematics and consistency checks and find no evidence for contamination., Comment: 17 pages, 7 figures - new subsection on non-Gaussian foregrounds, conclusions unchanged - updated to match published version on PRD

We report a B-mode power spectrum measurement from the cosmic microwave background (CMB) polarization anisotropy observations made using the SPTpol instrument on the South Pole Telescope. This work uses 500 deg$^2$ of SPTpol data, a five-fold increase over the last SPTpol B-mode release. As a result, the bandpower uncertainties have been reduced by more than a factor of two, and the measurement extends to lower multipoles: $52 < \ell < 2301$. Data from both 95 and 150 GHz are used, allowing for three cross-spectra: 95 GHz x 95 GHz, 95 GHz x 150 GHz, and 150 GHz x 150 GHz. B-mode power is detected at very high significance; we find $P(BB < 0) = 5.8 \times 10^{-71}$, corresponding to a $18.1 ��$ detection of power. An upper limit is set on the tensor-to-scalar ratio, $r < 0.44$ at 95% confidence (the expected $1 ��$ constraint on $r$ given the measurement uncertainties is 0.22). We find the measured B-mode power is consistent with the Planck best-fit $��$CDM model predictions. Scaling the predicted lensing B-mode power in this model by a factor Alens, the data prefer Alens = $1.17 \pm 0.13$. These data are currently the most precise measurements of B-mode power at $\ell > 320$., 16 pages, 4 figures, Submitted to PRD

Using a new double pulse technique, we have observed for the first time that charge asymmetric dissociation in diatomic molecules leaves one of the fragments in an electronically excited state. For example, we observed the reaction ${\mathrm{I}}_{2}+(\mathrm{pulse}1)\ensuremath{\rightarrow}({\mathrm{I}}_{2}^{2+}{)}^{**}\ensuremath{\rightarrow}{\mathrm{I}}^{0+}+({\mathrm{I}}^{2+}{)}^{*}+(\mathrm{pulse}2)\ensuremath{\rightarrow}{\mathrm{I}}^{0+}+{\mathrm{I}}^{3+}$, demonstrating that the ${\mathrm{I}}^{2+}$ fragment must have been in an excited state. More generally, just as asymmetric dissociation implies that the initial molecular ion is in an excited electronic state, the observation of asymmetric channels in the postdissociation ionization shows that the ionic fragments are themselves electronically excited.

We measured the optical breakdown threshold (OBT) in dielectrics with different band gaps for single and double 25-fs 800-nm transform-limited laser pulses. Our pump-probe double pulse measurements indicate that the plasma energy in dielectrics experiences ultrafast decay which lasts only $\ensuremath{\sim}100\mathrm{fs}$ and does not follow an exponential decay curve. Therefore, a decay term must be included in the electron density rate equation. Our double pulse measurements also demonstrate that the OBT is temperature dependent. The OBT in dielectrics was determined using a novel technique, which eliminates the ambiguity in its definition and also allows real-time data acquisition.

Strong-field single ionization and double ionization of two diatomic molecules, ${\mathrm{O}}_{2}$ and ${\mathrm{N}}_{2},$ are studied and compared to Xe and Ar, using an intense ultrashort pulse Ti:sapphire laser in the $2\ifmmode\times\else\texttimes\fi{}{10}^{13}$ to $8\ifmmode\times\else\texttimes\fi{}{10}^{14}{\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}$ intensity range. ${\mathrm{N}}_{2}$ behaves like a structureless atom for both single and double ionization. The recently reported suppression of the ${\mathrm{O}}_{2}^{+}$ ion yield compared to ${\mathrm{Xe}}^{+}$ is confirmed in our experiment, but we show that the suppression is not due to dissociative recombination. Rather, we conclude that the ionization rate of ${\mathrm{O}}_{2}$ is below that predicted by tunneling ionization. We extend the study to the double ionization of ${\mathrm{O}}_{2}$ and find a distinctly reduced nonsequential double-ionization rate. We find evidence that electronic structure influences strong-field tunneling ionization in molecules.

We present ion time-of-flight data showing strong-field ionization of iodine molecules up to ${\mathrm{I}}_{2}^{13+}$ using 33-fsec laser pulses. Every even molecular charge state shows an charge asymmetric dissociation channel $({\mathrm{I}}_{2}^{2n+}\ensuremath{\rightarrow}{\mathrm{I}}^{(n+1)+}+{\mathrm{I}}^{(n\ensuremath{-}1)+})$ with a significant branching ratio (15--30 %) in addition to the symmetric dissociation channel $({\mathrm{I}}_{2}^{2n+}\ensuremath{\rightarrow}{\mathrm{I}}^{n+}+{\mathrm{I}}^{n+})$ showing the general nature of this process. Furthermore, all charge states are formed at internuclear separations less than the critical separation for electron localization and enhanced ionization ${(R}_{c}).$ One channel, in particular, ${\mathrm{I}}_{2}^{10+}\ensuremath{\rightarrow}{\mathrm{I}}^{6+}+{\mathrm{I}}^{4+},$ represents a direct electronic excitation by nonresonant strong-field interaction to a state 20.9 eV above the ground state $({\mathrm{I}}_{2}^{10+}\ensuremath{\rightarrow}{\mathrm{I}}^{5+}+{\mathrm{I}}^{5+}).$ We show that the interaction of the dipole moment of the asymmetric charge distribution with the external field can account for this energy and the excitation mostly likely involves charge-transfer states. These results establish strong-field ionization of molecules as an efficient method for populating highly excited electronic states.

Gravitational lensing of the cosmic microwave background generates a curl pattern in the observed polarization. This "B-mode" signal provides a measure of the projected mass distribution over the entire observable Universe and also acts as a contaminant for the measurement of primordial gravity-wave signals. In this Letter we present the first detection of gravitational lensing B modes, using first-season data from the polarization-sensitive receiver on the South Pole Telescope (SPTpol). We construct a template for the lensing B-mode signal by combining E-mode polarization measured by SPTpol with estimates of the lensing potential from a Herschel-SPIRE map of the cosmic infrared background. We compare this template to the B modes measured directly by SPTpol, finding a non-zero correlation at 7.7 sigma significance. The correlation has an amplitude and scale-dependence consistent with theoretical expectations, is robust with respect to analysis choices, and constitutes the first measurement of a powerful cosmological observable., Comment: Two additional null tests, matches version published in PRL