Factors affecting the shape of MBE-grown laterally aligned Fe nanowires.

Various microstructural and chemical analysis techniques were applied to study two types (type-A and B) of self-assembled laterally aligned Fe nanowires (NWs) fabricated by molecular beam epitaxy on a ZnS buffer layer. The formation of the three-dimensional shapes of these NWs was found to be driven by the principle of surface energy minimization. We have provided phenomenological models to address the factors affecting the observed topological shape of these NWs, including the role of the lattice relationship between the Fe NWs and the underlying buffer layer, growth temperature, Fe nominal coverage and substrate orientation. Magnetic hysteresis measurements were performed at different temperature, demonstrating the Fe NWs possess a coercivity about 30 times larger than that of a Fe thin film. The observed gradual magnetization reversal indicates the magnetization process is accomplished by the rotation of magnetic moments within a single domain.

Impurity effect on weak antilocalization in the topological insulator Bi2Te3.

We study the weak antilocalization (WAL) effect in topological insulator Bi(2)Te(3) thin films at low temperatures. The two-dimensional WAL effect associated with surface carriers is revealed in the tilted magnetic field dependence of magnetoconductance. Our data demonstrate that the observed WAL is robust against deposition of nonmagnetic Au impurities on the surface of the thin films, but it is quenched by the deposition of magnetic Fe impurities which destroy the π Berry phase of the topological surface states. The magnetoconductance data of a 5 nm Bi(2)Te(3) film suggests that a crossover from symplectic to unitary classes is observed with the deposition of Fe impurities.

Lasing from dye-doped icosahedral quasicrystals in dichromate gelatin emulsions.

Lasing requires an active gain medium and a feedback mechanism. In conventional lasers the feedback is provided externally, e.g. by mirrors. An alternate approach is through Bloch waves in photonic crystals composed of periodic dielectric materials in which propagation of light in certain frequency ranges, known as photonic bandgaps, is forbidden. Compared to periodic crystals, quasicrystals have higher symmetry and are more favorable for the formation of photonic bandgaps. Hence quasicrystals are more efficient in providing the feedback mechanism for lasing. Here we report observation of lasing at visible wavelengths from dye-doped three-dimensional icosahedral quasicrystals fabricated in dichromate gelatin emulsions using a novel seven-beam optical interference holographic method. Multi-directional lasing exhibiting the icosahedral symmetry was observed. The lasing modes and pattern were explained by using the lasing condition expressed in the reciprocal lattice space of the icosahedral quasicrystal.

Simultaneous perfect phase matching for second and third harmonic generations in ZnS/YF(3) photonic crystal for visible emissions.

Theoretically designed and experimentally realized simultaneous perfect phase matching of second and third harmonic generations were demonstrated in a one-dimensional ZnS/YF(3) photonic crystal (PC) structure. Dramatic enhancement of second harmonic generation (SHG) and third harmonic generation (THG) in forward and backward directions near the photonic band edge were observed. This enhancement came from a combination of large ZnS nonlinear susceptibility coefficients, high density of optical modes and perfect phase matching of the fundamental and the harmonic waves near the photonic band edge due to modification of the dispersion curve by the PC structure. Total SHG and THG conversion efficiency over 4% is measured in only six micrometers length of photonic crystal. Theoretical calculations show good agreement with experimental measurements.

Growth, reaction and nanowire formation of Fe on the ZnS(1 0 0) surface.

The growth and reaction of Fe on a ZnS(1 0 0) substrate are studied in situ and with high lateral resolution using low energy electron microscopy (LEEM), micro low energy electron diffraction ( µLEED), x-ray photoemission electron microscopy (XPEEM), microprobe x-ray photoelectron spectroscopy ( µXPS) and x-ray magnetic circular dichroism PEEM (XMCDPEEM) for complementary structural, chemical, and magnetic characterization. Initially, a two-dimensional (Fe, Zn)S reaction layer forms with thickness that depends on growth temperature. Further growth results in the formation of a variety of three-dimensional crystals, most of them strongly elongated in the form of 'nanowires' of two distinct types, labeled as A and B. Type A nanowires are oriented near the ZnS[1 1 0] direction and are composed of Fe. Type B nanowires are oriented predominantly along directions a few degrees off the ZnS[0 0 1] direction and are identified as Greigite (Fe3S4). Both types of nanowires are magnetic with Curie temperatures above 450 °C. The understanding of the reactive growth mechanism in this system that is provided by these investigations may help to develop growth methods for other elemental and transition metal chalcogenide nanostructures on ZnS and possibly on other II-VI semiconductor surfaces.

Optical nonlinearity of nanocrystalline Au/ZnO composite films.

The third-order nonlinear optical susceptibilities, chi(3), of composite films consisting of nanocrystalline Au and ZnO particles were investigated by use of a degenerate four-wave mixing scheme. The maximum value of chi(3), measured at a laser wavelength of 532 nm and a pulse duration of 70 ps, was approximately 2 x 10(-6) esu. Also, this chi(3) value was achieved with small absorption (the surface-plasmon peak was at the 615-nm wavelength). Our composite materials showed no discernible degradation after they were subjected to irradiation for a total of 3 x 10(7) high-intensity pulses (24 Mw/cm2, 70-ps pulse duration at 500 Hz) during 16 h of testing.