Wen-Feng Hsieh

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.

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.

Two-photon-pumped lasing in microdroplets

Lasing is observed in laser-dye-doped ethanol droplets after two-photon absorption by the dye molecules. The two-photon-pumped lasing emission by the droplets is at a higher frequency than the input laser. Competitive nonlinear-optical effects that occur at high input-laser intensity are discussed.

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.

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.

Internal and external laser-induced avalanche breakdown of single droplets in an argon atmosphere

Laser-induced breakdown of a transparent micrometer-sized H2O droplet can be initiated only in the Ar gas in a strip outside the droplet shadow face. At a higher-input 0.532-μm Q-switched laser intensity, breakdown can also occur within the droplet. The resultant internal plasma blocks the laser from reaching the region outside the shadow face and absorbs more of the laser pulse to produce a shock wave and/or a laser-supported detonation wave. Various combinations of liquid and surrounding gas were investigated at different input intensities in order to provide information on the breakdown processes in a transparent droplet.

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.

Temporally and spatially resolved spectroscopy of laser-induced plasma from a droplet

A new diagnostic technique provides temporally and spatially resolved information at two settable wavelengths within the plasma-emission profile of a single transparent water droplet (40 μm in radius and containing 4 M NaCl) irradiated by a high-intensity laser beam. Data from this technique are compared with those from conventional techniques that separately provide time-averaged spatially resolved spectra or spatially averaged temporally resolved spectra of the plasma emission from droplets.

Laser-induced breakout and detonation waves in droplets. I. Experiments

Temporally and spatially resolved plasma emission and stimulated Raman-scattering profiles of water droplets are simultaneously recorded at two selectable wavelengths corresponding to the emission of the resonant and nonresonant sodium, the plasma continuum, the Balmer hydrogen, and the stimulated Raman scattering of water. The propagation speed of the breakout plasma from the droplet shadow face (in the form of a packet) and from the illuminated face is determined. The relative heating of the atoms in the plasma is extracted from the differences between the emission profiles of sodium and hydrogen, which have different electronic energy levels. The plasma response to the multipulse structure of the input laser radiation provides information on plasma shedding and heating for the subsequent pulses and an additional check of the validity of the one-dimensional electrohydrodynamic model, which includes a multipulse input.