Vertical-cavity and randomly scattered lasing from different thicknesses of epitaxial ZnO films grown on Y₂O₃-buffered Si (111)

Two different types of lasing modes, vertical Fabry-Perot cavity and random lasing, were observed in ZnO epi-films of different thicknesses grown on Si (111) substrates. Under optical excitation at room temperature by a frequency tripled Nd:YVO₄ laser with wavelength of 355 nm, the lasing thresholds are low due to high crystalline quality of the ZnO epitaxial films, which act as microresonators. For the thick ZnO layer (1,200 nm), its lasing action is originated from the random scattering due to the high density of crack networks developed in the thick ZnO film. However, the low crack density of the thin film (555 nm) fails to provide feedback loops essential for random scattering. Nevertheless, even the lower threshold lasing is achieved by the Fabry-Perot cavity formed by two interfaces of the thin ZnO film. The associated lasing modes of the thin ZnO film can be characterized as the transverse Gaussian modes attributed to the smooth curved surfaces.

Single domain m-plane ZnO grown on m-plane sapphire by radio frequency magnetron sputtering

High-quality m-plane orientated ZnO films have been successfully grown on m-plane sapphire by using radio frequency magnetron sputtering deposition. The introduction of a nanometer-thick, low-temperature-grown ZnO buffer layer effectively eliminates inclusions of other undesirable orientations. The structure characteristics of the ZnO epi-layers were thoroughly studied by synchrotron X-ray scattering and transmission electron microscopy (TEM). The in-plane epitaxial relationship between ZnO and sapphire follows (0002)(ZnO) [parallel] (112[overline]0)(sapphire) and (112[overline]0)(ZnO) [parallel] (0006)(sapphire) and the ZnO/sapphire interface structure can be described by the domain matching epitaxy along the [112[overline]0](ZnO) direction. The vibrational properties of the films were investigated by polarization dependent micro-Raman spectroscopy. Both XRD and micro-Raman results reveal that the obtained m-ZnO layers are under an anisotropic biaxial strain but still retains a hexagonal lattice.

Threading dislocations in domain-matching epitaxial films of ZnO

The structures of high-quality ZnO epitaxial films grown by pulsed-laser deposition on sapphire (0001) without an oxygen gas flow were investigated by X-ray diffraction and transmission electron microscopy. The great disparity of X-ray diffraction line widths between the normal and in-plane reflections reveals the specific threading dislocation geometry of ZnO. Most threading dislocations are pure edge dislocations. From a combination of scattering and microscopic results, it is found that threading dislocations are not uniformly distributed in the ZnO films, but the films consist of columnar epitaxial cores surrounded by annular regions of edge threading dislocations in large density. The local surface morphology and capacitance signal obtained from atomic force and scanning capacitance microscopes indicate that the aggregation of threading dislocations leads to high interface traps at the annular regions.

Transient response of a thresholdless microdroplet dye laser

Lasing in a cavity formed by a dye-doped micrometer-sized liquid droplet showed a new thresholdless steady-state behavior and a time-dependent response. The thresholdless behavior is associated with enhancement and inhibition of the spontaneous emission. The time-dependent response is related to various quality factors of the cavity modes and was determined to be 8 x 10(8).

Charge-coupled device polarimetry and its measurement of the Stokes vector of light transmitted by a polymer plate

The Stokes vector of light transmitted by a polycarbonate plate is studied by a modified charge-coupled device polarimeter. A sheet polarizer is measured for comparison.

Transmission through plasma created by laser-induced breakdown of water droplets

The temporally and spatially resolved transmission is measured with a green probe beam through the plasma created by an intense near-IR beam that is incident upon a water droplet. We report the propagation speed of the expanding plasma and the evolution of the transmission decrease and recovery at various locations along a line in the direction of the IR beam.

Laser-induced breakdown in large transparent water droplets

Recent experiments on the laser-induced breakdown (LIB) of large transparent liquid droplets are reviewed. A physical model of LIB processes is presented with the aim of integrating the following recent results: (1) the internal and near-field distributions for large transparent spheres; (2) the location of LIB initiation based on spatially resolved plasma emission spectroscopic techniques; (3) spatially resolved but time-averaged density of the plasma plumes and temperature of the atomic species within the plasma; (4) the plasma front propagation velocities inside and outside the droplet; and (5) the fate of the remaining superheated droplet and the expelled material.

Time dependence of multiorder stimulated Raman scattering from single droplets

An optical multichannel detection technique was used to measure simultaneously the time profiles of the input laser pulse and the elastic scattering, as well as the time profiles of the spectrally resolved multiorder stimulated Raman scattering (SRS), from single droplets. The time delay between the multiorder SRS and the input laser pulse is consistent with the generalized four-wave mixing process for first-order stimulated Raman growth, starting from spontaneous noise or the parametric signal. The presence of an internal plasma associated with laser-induced breakdown within a droplet quenches the SRS and increases the elastic scattering.

Plasma spectroscopy of H, Li, and Na in plumes resulting from laser-induced droplet explosion

The plasma emission resulting from laser-induced breakdown of large transparent H(2)O droplets (with and without NaCl or LiCl) has been spectrally and spatially resolved along a strip which encompasses the droplet and two plasma plumes associated with materials streaming from the droplet. From the emission line shapes, relative emission intensity ratios, and absorption line reversals, estimates can be made of the electron density, plasma temperature, and spatial inhomogeneity of the plasma along a strip in the direction of the laser beam. Use of the emission lines of H, Li, and Na as atomic tracers for plasma diagnostics is discussed.

Propagation velocity of laser-induced plasma inside and outside a transparent droplet

The supersonic propagation velocity of the emission front of plasma produced by laser-induced breakdown of a micrometer-sized transparent droplet flowing in a gas was measured with a streak camera at three intensity levels. At low input intensity, the plasma velocities in the gas away from and toward the shadow face were determined. At medium input intensity, the plasma velocities in the gas outside the shadow face and within the liquid (traveling toward the illuminated face) were measured. At high input intensity, the plasma velocities in the gas outside the shadow face, within the liquid, and in the gas outside the illuminated face were deduced.

Spatial distribution of the internal and near-field intensities of large cylindrical and spherical scatterers

Spatial distributions of the near-field and internal electromagnetic intensities have been calculated and experimentally observed for dielectric cylinders and spheres which are large relative to the incident wavelength. Two prominent features of the calculated results are the high intensity peaks which exist in both the internal and near fields of these objects, even for nonresonant conditions, and the well-defined shadow behind the objects. Such intensity distributions were confirmed by using the fluorescence from iodine vapor to image the near-field intensity distribution and the fluorescence from ethanol droplets impregnated with rhodamine 590 to image the internal-intensity distribution.

Laser-induced explosion of H2O droplets: spatially resolved spectra

Photographs and spatially resolved spectra were obtained with radiation generated by an exploding water droplet. The emission within the droplet consists of stimulated Raman scattering and a continuum associated with the created plasma. The forward plume (outside the shadow face) contains plasma and atomic hydrogen ejected from the droplet. The backward plume (behind the illuminated face) contains plasma, H, and ionized O and N, resulting from ionized air. Mechanisms for laser-induced explosion of large transparent water droplets are briefly discussed.