Search

We investigated the variations in reflectivity during the phase transition between amorphous and crystalline states of a Bi-doped GeTe-Sb2Te3 pseudobinary compound film with subnanosecond laser pulses, using a pump-and-probe technique. We also used a two-laser static tester to estimate the onset time of crystallization under 2.0-micros pulse excitation. Experimental results indicate that the formation of a melt-quenched amorphous mark is completed in approximately 1 ns, but that crystalline mark formation on an as-deposited amorphous region requires several hundred nanoseconds. Simple arguments based on heat diffusion are used to explain the time scale of amorphization and the threshold for creation of a burned-out hole in the phase-change film.

A new intensity-noise-subtraction scheme for an interferometric fiber-optic gyroscope is demonstrated. Together with the light circulating through the gyro coil, an appropriately attenuated light beam from the source coupler dead end of the gyro is directed to the gyro photodetector. When the gyro is unmodulated, or modulated with a square wave at the proper frequency, the intensity noises of the two beams bear opposite phases and interfere destructively on the detector. For an unmodulated gyro, we demonstrate an intensity noise-reduction factor of ~35, and for the same gyro modulated with a square wave at the proper frequency, we demonstrate a reduction factor of ~15.

Studies on the generation of optical gain in the constituents of air are motivated by potential applications in air lasing [1], a concept, the realization of which would enable efficient single-ended remote sensing in the atmosphere. Various schemes of turning air into an active laser medium are being investigated [2-4]. One of the most promising approaches is based on pumping atmospheric nitrogen by ultra-intense femtosecond laser pulses propagating in the filamentation regime in air [5]. It has been suggested that air filamentation may result in the optical gain on the electronic transitions of N2+ at 391 nm and 428 nm emission wavelengths [6]. However, the gain mechanism responsible for those emissions has been controversial.

Studies of femtosecond laser ablation are motivated both by its applications in micromachining and by the need to understand optical damage by intense femtosecond laser pulses. Virtually all of the prior studies have been conducted for the cases in which the ablating laser pulse strikes the surface of the material at normal angle of incidence (AOI). However, in practice, many if not most optical elements are used at oblique AOIs (most commonly at 45O and at Brewster's angle). In spite of the widespread use of optics with oblique incidence of the laser beam, there has been no systematic study reported on the dependence of ablation (or damage) threshold on the AOI for different materials.

Summary form only given. The realization of remotely pumped laser-like sources in the common air is an enabling step towards the development of new standoff sensing schemes in the atmosphere [1]. Although laser sources generating light in the opposite direction relative to that of the pumping radiation are particularly desirable, forward-emitting sources may also be useful. Pulsed backward-propagating laser emission at 845nm wavelength from atmospheric oxygen has been recently demonstrated [2]. The lasing scheme involved pumping by nanosecond deep-UV laser pulses at 226nm. An intense research effort is currently under way that targets the development of viable lasing schemes in another major constituent of air, nitrogen. Remote lasing in the atmospheric nitrogen under external laser pumping has been analyzed numerically and found feasible [3,4]. In 2003, it has been suggested that dilute plasma filaments that are produced through self-focusing of ultra-intense femtosecond infrared laser pulses in air emit weak directional laser-like radiation at the 357nm and 391nm transitions of neutral and singly-ionized nitrogen molecules, respectively [5]. Since 2003, those findings have not been independently confirmed. More recently, the observation of forward-propagating emission on various UV lines of singly ionized nitrogen molecules has been reported, under the conditions of pumping by tightly focused, tunable mid-IR femtosecond laser pulses [6]. The emission was found to be seeded by the spectral tail of the fifth harmonic of the pump.In this contribution, we report the demonstration of transient UV nitrogen lasing at a standoff distance of 2 meters. Our lasing scheme is based on pumping by the Comet laser system at the Jupiter Laser Facility at Lawrence Livermore National Laboratory. The pump laser beam at 1,053nm wavelength with up to 12 Joules of energy per pulse and pulse duration tunable from 0.5ps to ~20ps was weakly focused into the ambient air. Forward-propagating emission at 337nm wavelength was detected by a spectrometer and its energy was characterized by a pyroelectric energy meter. Our data is summarized in the Fig. 1. The emitted radiation at 337nm wavelength was polarized parallel to the polarization of the pump beam, suggesting that the emission was seeded by the spectral tail of the third harmonic of the pump. Optical gain at the 337nm wavelength was estimated as 1.5cm-1. The measured maximum pulse energy at that wavelength was 0.4mJ.