High-efficiency generation of pulsed Lyman-α radiation by resonant laser wave mixing in low pressure Kr-Ar mixture.

We report an experimental generation of ns pulsed 121.568 nm Lyman-α radiation by the resonant nonlinear four-wave mixing of 212.556 nm and 845.015 nm radiation pulses providing a high conversion efficiency 1.7x10-3 with the output pulse energy 3.6 µJ achieved using a low pressure Kr-Ar mixture. Theoretical analysis shows that this efficiency is achieved due to the advantage of using (i) the high input laser intensities in combination with (ii) the low gas pressure allowing us to avoid the onset of full-scale discharge in the laser focus. In particular, under our experimental conditions the main mechanism of photoionization caused by the resonant 2-photon 212.556 nm radiation excitation of Kr atoms followed by the 1-photon ionization leads to ≈17% loss of Kr atoms and efficiency loss only by the end of the pulse. The energy of free electrons, generated by 212.556 nm radiation via (2 + 1)-photon ionization and accelerated mainly by 845.015 nm radiation, remains during the pulse below the level sufficient for the onset of full-scale discharge by the electron avalanche. Our analysis also suggests that ≈30-fold increase of 845.015 nm pulse energy can allow one to scale up the L-α radiation pulse energy towards the level of ≈100 µJ.

Temperature-controlled picosecond-pulsed high frequency second-harmonic generation by a periodically poled stoichiometric LiTaO(3).

We report experimental results of second-harmonic (SH) generation (SHG) by a quasi-phase-matched periodically poled Mg-doped stoichiometric LiTaO(3) crystal for 1030 nm input radiation of 18 ps pulse duration, within the range of peak input laser intensity I = 0.1-9.5 GW/cm(2) and under repetition rate 10-20 kHz. For I>3 GW/cm(2) SHG efficiency achieves the saturation level of η≈0.35 which can be maintained within a wide range of I = 3-9.5 GW/cm(2). The loss of SHG efficiency observed for I>5 GW/cm(2) can be recovered to the level of η≈0.35 by using temperature-controlled operation. By applying our experimental data we find the value of two-photon absorption (TPA) coefficient for 515 nm radiation, ß≈1.1-2.7 cm/GW, agreeing well with the theoretical estimate ß≈2.6 cm/GW. Our analysis suggests that the inhibition of SHG efficiency, its saturation and stabilization are due to a combined mechanism including: (i) non-steady-state ps effect scaled by ≈ζ(-2)[1-exp(-ζ)](2) as compared with the efficiency for ns pulsed operation (ζ = L/V2τP , L is the crystal length, τP is the pulse duration and V(2) is the group velocity of SH); (ii) dephasing caused by the spectral bandwidth of the input radiation (≈300 GHz); (iii) thermal dephasing caused by TPA of SH; and (iv) strong SH attenuation by TPA of order ≈I(2) (-1)dI2/dz≈-(0.8-8) cm(-1) for I = 1-9.5 GW/cm(2).

Coupled laser molecular trapping, cluster assembly, and deposition fed by laser-induced Marangoni convection.

A coupled mechanism for molecular aggregation in a thin water solution film by laser-tweezers is suggested based on (i) simulation of light intensity distribution and (ii) order of magnitude analysis of heat and mass transport induced by Marangoni convection. The analysis suggests that the laser induced temperature distribution develops within 1 ms and Marangoni convection flow commences within 0.01-1 s, which increases by 1-2 orders of magnitude the mass transfer of dissolved molecules into the laser focus where they are trapped and aggregate by attractive van der Waals forces. This mechanism, considered for the particular case of polymer assembly, suggests that it can also be successfully applied for assembling other types of clusters and molecular aggregates from solutions.

Numerical simulation and optimization of Q-switched 2 mum Tm,Ho:YLF laser.

Numerical simulation suggests that for obtaining a giant (G) pulse from a 2.06 mum solid state Tm,Ho:YLF laser by the active Q-switching technique, the optimal Ho concentration will be higher than that used in normal operation. In simulations of 500 ns G-pulse generation maximal efficiency occurred at 6 % Tm and 1.0 % Ho, in contrast with 0.4% Ho found to be optimal for the normal pulse generation. Maximal energy output from Tm,Ho:YLF lasers can be achieved by incorporating a delay of about 0.7 ms between 0.5 ms 780 nm LD pulsed pumping and the start of Q-switched G-pulse operation.

Computational model for operation of 2 mum co-doped Tm,Ho solid state lasers.

A computational model for operation of co-doped Tm,Ho solid-state lasers is developed coupling (i) 8-level rate equations with (ii) TEM00 laser beam distribution, and (iii) complex heat dissipation model. Simulations done for Q-switched approximately 0.1 J giant pulse generation by Tm,Ho:YLF laser show that approximately 43% of the 785 nm light diode side-pumped energy is directly transformed into the heat inside the crystal, whereas approximately 45% is the spontaneously emitted radiation from (3)F(4), (5)I(7) , (3)H(4) and (3)H(5) levels. In water-cooled operation this radiation is absorbed inside the thermal boundary layer where the heat transfer is dominated by heat conduction. In high-power operation the resulting temperature increase is shown to lead to (i) significant decrease in giant pulse energy and (ii) thermal lensing.

Thermal physics in carbon nanotube growth kinetics.

The growth of single wall carbon nanotubes (SWNTs) mediated by metal nanoparticles is considered within (i) the surface diffusion growth kinetics model coupled with (ii) a thermal model taking into account heat release of carbon adsorption-desorption on nanotube surface and carbon incorporation into the nanotube wall and (iii) carbon nanotube-inert gas collisional heat exchange. Numerical simulations performed together with analytical estimates reveal various temperature regimes occurring during SWNT growth. During the initial stage, which is characterized by SWNT lengths that are shorter than the surface diffusion length of carbon atoms adsorbed on the SWNT wall, the SWNT temperature remains constant and is significantly higher than that of the ambient gas. After this stage the SWNT temperature decreases towards that of gas and becomes nonuniformly distributed over the length of the SWNT. The rate of SWNT cooling depends on the SWNT-gas collisional energy transfer that, from molecular dynamics simulations, is seen to be efficient only in the SWNT radial direction. The decreasing SWNT temperature may lead to solidification of the catalytic metal nanoparticle terminating SWNT growth or triggering nucleation of a new carbon layer and growth of multiwall carbon nanotubes.

Morphological stabilization, destabilization, and open-end closure during carbon nanotube growth mediated by surface diffusion.

In this paper, the growth stability of open-ended carbon nanotubes mediated by surface diffusion on the lateral surface of the nanotube is considered in detail. Nanotube growth and destabilization is viewed as a competition of two processes at the open growth edge: (i) hexagon formation sustaining the continuous growth of the regular hexagonal network, and (ii) thermally activated pentagon formation, which causes inward bending of the nanotube wall resulting in end closure, i.e., growth termination. The edge of the open-ended nanotube, if it is fed by a sufficiently large surface diffusion flux, may remain stable even without extrinsic stabilizing effects. The closure of the open end of the growing nanotube is shown to happen whenever a change in the growth conditions (temperature, carbon vapor pressure, or surface area from which the open end is fed) decreases the surface diffusion flux, and the characteristic time for new atom arrival on the edge becomes larger than the characteristic time for pentagon defect formation. These kinetic effects are also shown to define the transition from single wall to multiwall nanotube growth. Additionally, the effect of surface diffusion feeding nanotube growth from behind the growth interface is shown to stabilize open edge morphology, effectively smoothing the growth perturbations which may be caused by diffusion-limited aggregation at the edge.