Axial ratio

About: Axial ratio is a research topic. Over the lifetime, 2493 publications have been published within this topic receiving 27920 citations.

Evidence for room-temperature multiferroicity in a compound with a giant axial ratio.

Abstract: In the search for multiferroic materials magnetic compounds with a strongly elongated unit-cell (large axial ratio $c/a$) have been scrutinized intensely However, none was hitherto proven to have a switchable polarization, an essential feature of ferroelectrics Here, we provide evidence for the epitaxial stabilization of a monoclinic phase of ${\mathrm{BiFeO}}_{3}$ with a giant axial ratio ($c/a=123$) that is both ferroelectric and magnetic at room temperature Surprisingly, and in contrast with previous theoretical predictions, the polarization does not increase dramatically with $c/a$ We discuss our results in terms of the competition between polar and antiferrodistortive instabilities and give perspectives for engineering multiferroic phases

Millimeter-Wave Transmitarrays for Wavefront and Polarization Control

Abstract: Two separate transmitarrays that operate at 77 GHz are designed and fabricated. The first transmitarray acts as a quarter-wave plate that transforms a linearly polarized incident wave into a circularly polarized transmitted wave. The second transmitarray acts as both a quarter-wave plate and a beam refracting surface to provide polarization and wavefront control. When the second transmittarray is illuminated with a normally incident, linearly polarized beam, the transmitted field is efficiently refracted to 45 °, and the polarization is converted to circular. The half-power bandwidth was measured to be 17%, and the axial ratio of the transmitted field remained below 2.5 dB over the entire bandwidth. Both designs have a subwavelength thickness of 0.4 mm (λ°/9.7). The developed structures are fabricated with low-cost printed-circuit-board processes on flexible substrates. The transmitarrays are realized by cascading three patterned metallic surfaces (sheet admittances) to achieve complete phase control, while maintaining high transmission. Polarization conversion is accomplished with anisotropic sheets that independently control the field polarized along the two orthogonal axes. The structures are analyzed with both circuit- and fields-based approaches.

90 degree polarization rotator using a bilayered chiral metamaterial with giant optical activity

Abstract: A bilayered chiral metamaterial (CMM) is proposed to realize a 90 degree polarization rotator, whose giant optical activity is due to the transverse magnetic dipole coupling among the metallic wire pairs of enantiomeric patterns. By transmission through this thin bilayered structure of less than \lambda/30 thick, a linearly polarized wave is converted to its cross polarization with a resonant polarization conversion efficiency (PCE) of over 90%. Meanwhile, the axial ratio of the transmitted wave is better than 40 dB. It is demonstrated that the chirality in the propagation direction makes this efficient cross-polarization conversion possible. The transversely isotropic property of this polarization rotator is also experimentally verified. The optical activity of the present structure is about 2700 degree/\lambda, which is the largest optical activity that can be found in literature.

A reconfigurable microstrip antenna for switchable polarization

Abstract: A novel reconfigurable microstrip antenna with switchable polarization sense is proposed. The proposed antenna has a simple structure, consisting of a corner-truncated square radiating patch, four small triangular conductors, and a microstrip line feed. Using independently biased PIN diodes on the patch, it can produce linear polarization, or left- or right-hand circular polarization according to bias voltages. From the measured results, low cross-polarization levels when operated in the linear state and good axial ratios in the circular state are observed.

Bipolar Supernova Explosions

Abstract: We discuss the optical spectropolarimetry of several core-collapse supernovae, SN 1996cb (Type IIB), SN 1997X (Type Ic), and SN 1998S (Type IIn). The data show polarization evolution of several spectral features at levels from 0.5% to above 4%. The observed line polarization is intrinsic to the supernovae and not of interstellar origin. These data suggest that the distribution of ejected matter is highly aspherical. In the case of the Type IIn SN 1998S, the major-to-minor axis ratio must be larger than 2.5 if the polarization is 3% from an oblate spheroidal ejecta seen edge-on. A well-defined symmetry axis can be deduced from spectropolarimetry for SN 1998S, but the Type IIB events SN 1993J and SN 1996cb seem to possess much more complicated geometries with polarization position angles showing larger irregular variations across spectral features; the latter may be associated with large-scale clumpiness of the ejecta. The observed degree of polarization of the Type Ic SN 1997X is above 4%. The data reveal a trend that the degree of polarization increases with decreasing envelope mass and with the depth within the ejecta. The high axial ratio of the ejecta is difficult to explain in terms of the conventional neutrino-driven core-collapse models for Type II explosions. Highly asymmetric explosion mechanisms such as the formation of bipolar jets during core collapse may be a necessary ingredient for models of all core-collapse supernovae.