Search Results (28 total)

We report experiments at Jefferson National Accelerator Facility (Jlab) and computer simulations performed at the Naval Postgraduate School (NPS) designed to probe the small Rayleigh length regime. We compare the gain, power, and sensitivity to mirror and electron beam misalignments as a function of decreasing Rayleigh length. The agreement is quite good, with experiments and simulations showing comparable trends as the Rayleigh length is decreased. In particular, we find that the gain and power do not decrease substantially at short Rayleigh length, contrary to a common Gaussian-mode filling factor argument. Within currently achievable alignment tolerances, the gain and power are still acceptable for FEL operation.

Conventional free electron laser (FEL) oscillators minimize the optical mode volume around the electron beam in the undulator by making the resonator Rayleigh length about one third to one half of the undulator length. This maximizes gain and beam-mode coupling. In compact configurations of high-power infrared FELs or moderate power UV FELs, the resulting optical intensity can damage the resonator mirrors. To increase the spot size and thereby reduce the optical intensity at the mirrors below the damage threshold, a shorter Rayleigh length can be used, but the FEL interaction is significantly altered. We model this interaction using a coordinate system that expands with the rapidly diffracting optical mode from the ends of the undulator to the mirrors. Simulations show that the interaction of the strongly focused optical mode with a narrow electron beam inside the undulator distorts the optical wave front so it is no longer in the fundamental Gaussian mode. The simulations are used to study how mode distortion affects the single-pass gain in weak fields, and the steady-state extraction in strong fields.

Motivated by the prospect of constructing a short Rayleigh length free-electron laser in a high-vibration environment, we demonstrate the use of a collection of rays to study the effect of mirror vibration and distortion on the behavior of the fundamental optical mode of a cold-cavity resonator. We find that the ray collection accurately describes both on-axis and off-axis optical beams. We show that a tilt or transverse shift of a mirror causes the optical mode to rock about the original resonator axis, while a longitudinal mirror shift or a change in the mirror’s radius of curvature causes the beam diameter at a mirror to successively dilate and contract on the mirror. Results are in excellent agreement with analytic calculations and wave front propagation simulations as long as the mirrors remain large with respect to the beam diameter.

We present experiments and simulations showing the behavior of a free-electron laser (FEL) with both positive and negative linear tapers along the wiggler. We show the power desynchronism curve widths, efficiency, exhaust electron energy spread, and wavelength dependence as a function of taper for 3- and 6-μm optical wavelengths and for resonators with 10% and 2% loss/pass. Simulations of the experiments, using a multimode analysis, are seen to be in general agreement with the experimental results, carried out at the IR Demo FEL at Thomas Jefferson National Accelerator Facility. We find that short-pulse effects are more effective than tapers in producing high efficiency with low exhaust energy spread, and the expected performance enhancement of FEL tapering is not achieved.

Chiral nematic droplets of various radii R and intrinsic pitch P with parallel boundary conditions have been observed using a polarizing microscope. The visual appearances of the droplets have been compared with computer simulations from model director fields. It is found that the director configuration depends strongly on the ratio R/P. For P>R, the droplets have a twisted bipolar structure. For P<R, the director field takes on the Frank-Pryce structure. For P≪R, the droplet appearance is similar to that of a radial nematic drop, but the director field is still described by the Frank-Pryce model. We present a phase stability diagram for the two structures, and show how the structural transition between them proceeds continuously through intermediate states.

The director field of a cholesteric liquid crystal confined to cylindrical cavities with parallel and perpendicular surface anchoring was studied by means of optical polarizing microscopy. The chirality of the liquid crystal could be varied continuously due to a temperature-induced twist inversion of the cholesteric structure. The results indicate the occurrence of a twisted escaped radial structure for perpendicular anchoring and an eccentric double twist configuration for parallel anchoring. © 1996 The American Physical Society.

We have identified the director configurations n(r) of nematic droplets subjected to electric fields under conditions of negative dielectric anistropy and parallel surface anchoring. Drops are examined using polarization microscopy, with electric fields applied parallel or perpendicular to the light direction. The behavior of the resulting textures and defects is obtained by comparing the droplet appearance with computer simulated transmission patterns using intuitively obtained model director configurations. In the field-off state, the director is essentially azimuthal (concentric texture) near the surface, ‘‘escaping’’ to the axis direction (bipolar texture) along the droplet axis. For low fields, the director configuration becomes either more bipolar or more concentric depending on whether the droplet axis is initially parallel or perpendicular to the electric field. For high fields, the bipolar structure evolves into one in which the directors form planar bipolar sheets escaped at the poles (escaped planar bipolar texture). The concentric structure, however, becomes unstable and a novel transition to the more stable escaped planar bipolar structure is described.

Using polarization microscopy, we have investigated the director configurations of droplets of a nematic liquid crystal with negative dielectric anisotropy and perpendicular boundary conditions subjected to electric fields. The droplets were suspended in liquid poly(dimethyl siloxane) and electric fields were applied both parallel and perpendicular to the viewing direction. The resulting director pattern is deduced by comparing the experimental transmission patterns with patterns calculated from various director models. For low fields, we observe a director field which is essentially radial, but with some azimuthal twist superimposed and a radial point (hedgehog) defect at the center. At intermediate fields, the hedgehog defect moves away from the center along the electric-field direction, while at the highest fields a line defect appears which lies along the diameter parallel to the field. Comparison of the transmission patterns with model calculations yields good agreement for the low- and intermediate-field cases, and reasonable agreement for the high-field case.

Measurements of the 1/f noise have been performed on YBa₂Cu₃O_7-δ single crystals for currents in the a-b plane and along the c axis. In the normal state, the normalized noise spectral density S_v/V² in the a-b plane is comparable with that along the c axis. In the superconducting transition region, a large enhancement of S_v/V² is observed along the c axis while no such enhancement is observed in the a-b plane. Our experimental data suggest that while the 1/f noise in the normal state may be due to structural defects, a completely different noise source exists in the superconductig transition region. We discuss the 1/f noise behavior in the superconducting transition region in terms of the anisotropic nature of YBa₂Cu₃O_7-δ and the motion of magnetic vortices.

We have measured the electric-field dependence of the reflectivity of the blue phase III (BP III) of cholesteric liquid crystals when the dielectric anisotropy ε_a<0. We propose a model which assumes that the boundary aligns the sample to a depth L and in which L is increased by the applied field. The free energy is proportional to Eⁿ, where n depends on the structure of BP III. The data are fitted well by a Lorentzian line shape, and the peak intensity and linewidth agree well with the theory for n=2.1. This value is consistent with the double-twist model of BP III, but not with cubic and quasicrystal models, since the energies of these latter structures depend on higher powers of E.

1/f noise measurements have been performed on YBa₂Cu₃O_7-δ single crystals with distinct resistivity characteristics. The noise spectral density is found to be several orders of magnitude larger than that of metals, with a temperature dependence that is independent of the resistivity. In the superconducting-transition region, we do not observe anomalously enhanced 1/f noise. We discuss the important role played by the crystal morphology and we present a tunneling model which explains the experimental data quantitatively.

Electric fields can cause structural changes of the blue phases (BP) that appears in chiral liquid crystals. In BP I and BP II, electrostrictive effects cause a shift of the Bragg reflection wavelengths with little intensity change. In BP III, for systems with negative dielectric anisotropy, there is a strong increase in the selective reflection intensity in addition to electrostriction. We have measured the time response of these effects to on-off modulated electric fields for a material with negative dielectric anisotropy. We observe that the response times for BP I and BP II electrostriction are ∼10 s, while the response times for BP III peak intensification are ∼10 ms. We discuss the results in terms of models for these mechanisms and the as yet unknown structure of BP III.

We have performed the first measurements on surface-aligned samples of the blue phase III of chiral liquid crystals and have tested proposed icosahedral, cubic-domain, and double-twist structures for blue phase III. Angular-reflection studies on surface-aligned samples show no evidence of expected icosahedral structure; uv reflection measurements on unaligned samples show no evidence of expected cubic-domain structure; and attempts to observe Cano fringes show no evidence for any spatial periodicity.

We have studied the effect of electric fields on the blue phase III (BPIII) of chiral liquid crystals. A temperature–electric-field phase diagram is presented and the behavior of the BPIII selective reflections over this diagram is shown. The selective reflection consists of a broad peak situated on a sloping background; we argue that the background may be a remnant of additional structure at lower wavelength. A comparison of our data with three models for BPIII structure is made.

We have measured the phase diagrams of mixtures of chiral and racemic CE4, CE5, and CE2 in order to study the effect of varying chirality on the existence of blue phases in chiral liquid crystals. We observe three blue phases—BPI, BPII, and BPIII—which progressively appear at the isotropic phase boundary as the chirality is increased. The temperature widths of both BPI and BPIII become wider with increasing chirality, while BPII becomes narrower and vanishes at high chirality. We argue that this may be a universal feature of blue-phase temperature-chirality phase diagrams and discuss our results in light of previous theory and experiment.

Polarizing microscopy, optical activity, reflection, and transmission measurements are used to determine the phase diagram for the blue phases of chiral 4’’-(2-methylbutylphenyl)-4’ -(2-methylbutyl)-4-biphenylcarboxylate (CE2) mixed with nonchiral 4-n-pentylbenzenethio-4’-n-heptyloxybenzoate (7¯S5). These mixtures exhibit a chirality higher than previously investigated. The phase diagram shows two new features: (1) blue phase II becomes unstable at high concentrations of the chiral component, and (2) blue phase III possesses a wider temperature range than blue phase I for the most chiral mixtures.

We have studied the effect of chiral strain on phase transitions in a chiral material by examining the cholesteric (Ch) blue-phase (BP) liquid-crystal system in the Cano-wedge configuration. We observe both Ch and BP Cano rings whose positions are explained by Ch elastic theory. In the phase-transition region, chiral strain causes the positions of the Ch-BP coexistence rings to be temperature dependent and to exhibit reentrancy with respect to chiral strain. A free-energy model is presented which explains the gross features of the positions of the coexistence rings.

We have measured the phase diagram for mixtures of chiral and racemic cholesteric S-(+)-4-(2-methylbutyl)phenyl-4-decyloxyben- zoate (CE6) in order to study the effect of changing chirality on the appearance of the cholesteric blue phases. We have also measured the latent heats at the isotropic transitions. Our phase diagram is compared with the Landau theory of Hornreich and Shtrikman and the data of Marcus and Goodby.

We have performed polarized Bragg-scattering measurements from the cholesteric blue phases (BP) of a mixture of cholesteryl oleyl carbonate (COC) and 4-cyano-4’-(n-hexyl)biphenyl (6CB). Our data, which are reported in the form of a temperature-dependent Mueller scattering matrix, confirm the previous results obtained in a more complicated mixture by Flack, Crooker, and Svoboda. The scattering is dominated, in both BP I and BP II, by the m=-2 order-parameter coefficient, and we observe no change in the Mueller matrix at the BP I–BP II transition despite a discontinuous step in the Bragg wavelength. The structural implications of the results are discussed within the context of the theory of Grebel, Hornreich, and Shtrikman.

Because of the high symmetry of a cubic lattice, the interpretation of Bragg-scattering results may be complicated by the presence of multiple-scattering effects. We calculate here the polarization properties of light that is multiply Bragg scattered from chiral cubic structures with a view to interpreting the (111) backscatter from blue-phase lattices. We find that doubly and triply scattered light from (100)- and (110)-like Bragg planes having m=2 symmetry (using the notation of Hornreich and Shtrikman) has the same polarization as singly backscattered light from the (111), m=0 plane. The results are used to interpret the scattering measurements of Tanimoto and Crooker.

Polarized Bragg-scattering measurements using visible light have been used to determine the structure of the blue phases of cholesteric liquid crystals. Observations of the third Bragg peak (in order of decreasing wavelength) in CB15-E9 mixtures [chiral 4-cyano-4′-(2-methyl)butylbiphenyl (CB15) and a proprietary mixture of nematic biphenyl compounds (E9) from British Drug House] reveal that, for these mixtures, blue phase II has a body-centered-cubic structure.

Using polarized light scattered at 45° from mixtures of chiral CB15 and nonchiral E9 liquid crystals, we have measured the Mueller matrices characterizing the Bragg reflections of blue-phase structures BPI, BPIIA, and BPIIB. Surprisingly, all of these structures possess essentially the same Mueller matrix, namely, that of an elliptical polarizer with axial ratio ≅0.7. The implications of the results on the allowed space groups of the blue phases is discussed in the context of the theory of Hornreich and Shtrikman.

A study of Bragg reflections in both blue phases (BPI, BPII) of two mixtures of chiral and nonchiral biphenyls indicates both phases probably have the same translational symmetry and are either body-centered or simple cubic. The temperature dependence of several Bragg reflections indicates that the BPI-BPII transition may be either continuous or discontinuous and that, given the bcc lattice, the lattice parameter is commensurate with the pitch. These results are discussed relative to existing theories.

The measurements of the anomalous effective viscosity η obtained by Keyes and coworkers are analyzed in terms of the two separate viscosity coefficients ν and μ.