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The use of angularly selective mirrors (ASMs) for the expansion of the mode area and modal discrimination of microchip lasers is presented. Results suggest the reasonable achievement of an increase of one order of magnitude of the mode area without causing overheating and thermal fracture.

The finite recovery time [T.sub.s] of the bleached absorber is presented as one of the possible mechanisms accounting for the increase-maximum-decrease in pulse energy E with the pumping rate [W.sub.P] in cw-pumped passively Q-switched solid-state lasers, by analytically evaluating the sign of the derivative [delta]E/[delta][W.sub.P]. The results show that, in the low pump regime (T > [T.sub.s], T is the interpulse period), the initial population density [n.sub.i] remains constant, the final population density [n.sub.f] decreases with [W.sub.P], and this results in a monotonic increase of E with [W.sub.P]. In the high pump regime (T < [T.sub.s]), [n.sub.i] decreases but [n.sub.f] remains the same with [W.sub.P]; this results in a monotonic decrease of E with [W.sub.P]. At the critical region (T [approximately equal to] [T.sub.s]), E reaches its maximum value. A cw-pumped Yb:YAG laser passively Q switched by a [Cr.sup.4+]:YAG absorber is demonstrated to confirm this model. The theoretical model is also applied to the analysis of three previously reported passive Q switching solid-state [Nd:GdV[O.sub.4], [Nd.sup.+]:La[Sc.sub.3][(B[O.sub.3]).sub.4] ([Nd.sup.+]:LSB), and Nd:YAG] lasers experiments. OCIS codes: 140.3540, 140.3480, 140.3580.

The synthesis, spectroscopic studies and laser oscillation experiments conducted on N[d.sup.3+]-doped [Y.sub.2][O.sub.3] ceramic prepared by the vacuum sintering technique, is reported. Results show that the quality of the kind of Nd:[Y.sub.2][O.sub.3] ceramic is sufficient for use as a highly efficient laser media.

A differential-absorption lidar system that uses a long-life transmitter for monitoring of atomic-mercury concentrations in the atmosphere has been developed. The third harmonic of a tunable dye laser with LDS 765 dye pumped by the second harmonic of a Nd:YAG laser was used as the emitted beam from the transmitter. By use of this system, atmospheric concentrations of atomic mercury of less than 0.4 part in [10.sup.12] were measured. The time trend of the measured concentration agreed with that obtained by a conventional gold amalgamation method combined with atomic absorption spectroscopy on the ground. [c] 2004 Optical Society of America OCIS codes: 010.7030, 140.2050, 140.3600, 190.7220, 280.1120, 280.1910.

Passively Q-switched ceramic [Nd.sup.3+]:YAG lasers with ceramic [Cr.sup.4+]:YAG saturable absorbers are demonstrated. When the lasers are pumped by a I-W cw laser diode, optical-optical efficiency as great as 22% is obtained with [Cr.sup.4+] :YAG of initial transmission ranging from 94% to 79%. The results are similar to those in their crystalline counterparts. The operation of Brewster's angle and the polarization state of the laser output are also investigated. [c] 2004 Optical Society of America OCIS codes: 140.3380, 140.3540, 140.3480.

A photoconductive switch-arrayed antenna with a chemical vapor-deposited diamond film was developed to generate high-power terahertz (THz) radiation. With this device, an electric field stress of 2 x [10.sup.6] V/cm can be applied to photoconductive gaps because of the high breakdown threshold of diamond and the overcoated gap structure for the prevention of surface flashover. This level of field stress can alleviate the current problem of saturation in THz emission by use of a photoconductive antenna. The device consists of more than two thousand 20 [micro]m x 2.8 mm emitters. In an experiment using an ultrashort pulse Kr*F laser, we obtained an energy density of 10 [micro]J/[cm.sup.2] on the emitter surface at E = [10.sup.5] V/cm. This density was larger than that of the current large-aperture antenna. There was no severe saturation in photoconductive current up to E = [10.sup.6] V/cm, and a focused intensity of 200 MW/[cm.sup.2] can be expected. OCIS codes: 320.7160, 320.7080, 230.0250, 320.2250, 230.6080, 320.7130.