R. Yen

Picosecond laser‐induced melting and resolidification morphology on Si

Ultrafast melting and resolidification on the surface of a silicon crystal has been induced by picosecond laser pulses at 532 and 266 nm. Optical microscopy and electron diffraction revealed the formation of amorphous silicon. Details of surface morphology are sensitive functions of pulse intensity, energy, wavelength, and crystallographic orientation.

Phase transformation on and charged particle emission from a silicon crystal surface, induced by picosecond laser pulses

Ultrafast melting and resolidification on a (111) sufarce of a silicon crystal wafer, induced by 20‐ps pulses at 532‐nm wavelength, is accompanied by the emission of charged particles. This emission is studied as a function of pulse energy, in combination with time‐resolved reflectivity changes and post‐annealing morphology. The data provide evidence that thermalization time for a dense carrier plasma and the lattice is shorter than 10−11 s.

Thermally assisted multiphoton photoelectric emission from tungsten

Abstract The dependence of photocurrent on laser intensity has been studied up to the damage threshold of tungsten using 30 ps pulses from a Nd:YAG laser. For intensities below ∼1 GW/cm 2 , four-photon photoelectric emission is observed. For higher intensities three-photon assisted thermionic emission dominates. A generalized Fowler-DuBridge theory accounts quantitatively for our observations. A polycrystal surface cleaned by picosecond laser pulses has a work function different from surfaces cleaned by ordinary heating methods.

Space-time resolved reflectivity measurements of picosecond laser-pulse induced phase transitions in (111) silicon surface layers

The changes in reflectivity of a silicon surface, irradiated by a green picosecond pulse, are probed during and following that pulse with a spatial resolution of 10 μm. The data indicate the development of a liquid phase, and a resolidification either into a single crystal or an amorphous phase. The latter has a characteristic ring-type pattern, and occurs only at locations where the incident picosecond laser fluence lies between 0.2 and 0.26 J/cm2. The reflectivity data appear to be in good quantitative agreement with a “simple heating” model, in which the electrons and phonons maintain a local thermodynamic equilibrium on a picosecond time scale.

Two-photon absorption coefficients in uv window and coating materials

The two-photon absorption coefficients of some uv transmitting window and coating materials are measured at 355 nm and 266 nm. In materials with band gaps larger than 2homega, the nonlinear absorption is below the detection limit of 3 x 10(-6) cm/MW. CdF(2) has a two-photon absorption coefficient 1.6 x 10(-3) cm/MW at 266 nm, which is comparable with that of the alkali-halides. The newly developed window material LiYF(4) has no detectable nonlinear loss at 266 nm.

Picosecond laser interaction with metallic zirconium

UV laser pulses of two picosecond duration have been used to observe photoelectric emission and thermally enhanced photoelectric emission from a zirconium metal surface. The data show that the electron and lattice temperature remain approximately equal and the electron‐phonon energy relaxation time is less than 1 ps.

Lack of importance of ambient gases on picosecond laser‐induced phase transitions of silicon

A 10‐nsec pulsed ruby laser was used to prepare atomically clean silicon surfaces in UHV. With picosecond Nd:YAG laser pulses at 532 amd 266 nm, the amorphous patterns formed on the atomically clean silicon surfaces in UHV were compared with those formed in air and other ambient conditions. Results show that the picosecond laser‐induced phase transition of silicon does not depend on the ambient conditions or on the native surface oxide layer of silicon.

Incident angle and polarization dependence of four-photon photoemission from tungsten☆

Abstract The importance of classical optics in analyzing photoemission data and the power of two-channel comparative technique in multiphoton experiments have revealed that in tungsten both surface and volume effect contribute to the photocurrent. Their relative influence depends on the angle of incident and the polarization. The ratio of surface to volume effect is obtained for the first time.

PICOSECOND LASER PULSE-INDUCED MELTING AND RESOLIDIFCATION MORPHOLOGY ON SILICON

Picosecond Nd:YAG laser pulses at 1.06 μ m and its harmonics are used to induce fast heating and quenching on the surface of crystal and ion-implanted amorphous silicon. A transformation between crystal and amorphous silicon is revealed by optical microscopy and transmission electron microscopy. Details of the morphology are closely related to the pulse energy, wavelength, and crystallographic orientation of the silicon.