Birefringent modifications induced by femtosecond filaments in optical glass

We present a study of the refractive index modifications spontaneously induced by infrared femtosecond filaments propagating in the bulk of transparent solids. It was found that extended modified refractive index channels up to several mm in length can be formed under loose focusing conditions in fused silica. The observed birefringence zone at the beginning of these channels is attributed to the anisotropic stress induced via nonlinear losses in the high intensity region of the filament.     

Interaction of derivatives of 7-amino-1,5-benzo-diazepin-2-ones with α,β-unsaturated ketones

Derivatives of new condensed heterosystems, the tetracyclic [1,4]diazepino[3,2,1-hi]pyrido-[4,3,2-cd]indoles and tricyclic [1,4]diazepino[2,3-g]- and -[2,3-f]quinolines have been synthesized by the interaction of 7-amino-4-phenyl(or methyl)-1,3,4,5-tetrahydro(or 1,3-dihydro)-2H-1,5-benzodiazepin- 2-ones with dimethyl 2-oxoglutaconate and methyl 4-oxo-2-pentenoate. The corresponding carboxylic acids of the new derivatives were synthesized by alkaline hydrolysis of the methyl esters. The direction of the cyclocondensation reaction was determined by both the structure of the diazepine ring of the initial amines and also by the structure of the α,β-unsaturated ketones. Quantum-chemical calculations have been carried out of the minimal values of the local energy of ionization and of the resonance stabilization energy on specific atoms of the initial amines.     

Reactions of 5-(6-Methyl-2,4-dioxo-1,2,3,4-tetrahydro-3-pyrimidinyl)-methyl-1,3,4-oxadiazole-2-thione with Electrophiles

Summary.  Treatment of 5-(6-methyl-2,4-dioxo-1,2,3,4-tetrahydro-3-pyrimidinyl)-methyl-1,3,4-oxadiazole-2-thione with haloalkanes yielded oxadiazole S-alkyl derivatives, whereas its reaction with formaldehyde and amines resulted in formation of oxadiazole N(3)-aminomethyl derivatives. The alkylation of 2-alkylsulfanyl-5-(6-methyl-2,4-dioxo-1,2,3,4-tetrahydro-3-pyrimidinyl)-methyl-1,3,4-oxadiazoles with methyl bromoacetate proceeded at the N(1)-position of pyrimidine to give 2-alkylsulfanyl-5-(1-methoxycarbonylmethyl-6-methyl-2,4-dioxo-1,2,3,4-tetrahydro-3-pyrimidinyl)-methyl-1,3,4-oxadiazoles, whereas aminomethylation, bromination, or nitration took place at position 5 of pyrimidine ring and afforded the corresponding 5-pyrimidine substituted derivatives. Received May 9, 2001. Accepted (revised) August 17, 2001     

Stripe patterns in degenerate optical parametric oscillators

We experimentally demonstrate the stripe (or roll) patterns in a broad-aperture degenerate optical parametric oscillator in a plane-mirror minicavity. The stabilization of stripes is achieved by seed injection at a subharmonic frequency. We measure the temporal spectra of the stripe pattern and obtain the 1/f-like noise spectra.     

Energy extraction improvement in picosecond amplifiers by pulse tilting

We report a novel tilted pulse amplification technique that, in contrast with the chirped-pulse amplification method, allows stretching of ultrashort pulses independently upon the initial pulse bandwidth. A gain factor of 4.5 and good reconstruction of a 15 ps pulse after stretching to 200 ps in a double-pass end-pumped Nd:YAG amplifier have been achieved.     

Application of time-resolved digital holographic microscopy in studies of early femtosecond laser ablation

We studied evolution of femtosecond laser ablation by employing novel method of time-resolved off-axis digital holographic microscopy. Phase and amplitude profiles of early shock front and ablation plume dynamics of irradiated tempered steel were reconstructed from the digital holograms. In order to gain additional information, digital holographic microscopy was combined with plasma emission imaging. By using both techniques simultaneously we studied material response to multi-pulse irradiation, shock wave propagation, ablation plume formation and plasma emission. The significant changes in ablation performance were observed when using multi-pulse irradiation if compared to widely investigated single-shot regime.