Vladislav V. Yakovlev

REGENERATIVE PULSE SHAPING AND AMPLIFICATION OF ULTRABROADBAND OPTICAL PULSES

Regenerative pulse shaping is used to alleviate gain narrowing during ultrashort-pulse amplification. Amplification bandwidths of ~ 100 nm, or nearly three times wider than the traditional gain-narrowing limit, are produced with a modified Ti:sapphire regenerative amplifier. This novel regenerative amplifier has been used to amplify pulses to the 5-mJ level with a bandwidth sufficient to support ~ 10-fs pulses.

Generation of 18-fs, multiterawatt pulses by regenerative pulse shaping and chirped-pulse amplification

Transform-limited, 18-fs pulses of 4.4-TW peak power are produced in a Ti:sapphire-based chirped-pulsed amplification system at a repetition rate of 50 Hz. Regenerative pulse shaping is used to control gain narrowing during amplification, and an optimized, quintic-phase-limited dispersion compensation scheme is used to control higher-order phase distortions over a bandwidth of ~100 nm. Seed pulses are temporally stretched >100,000 times before amplification.

Raman microscopy analysis of phase transformation mechanisms in vanadium dioxide

Raman microscopy is used to study the evolution of vibrational modes of vanadium dioxide single crystals and thin films in the vicinity of the phase transition. The results support the electron correlation model of phase transformation.

Quantum control of I2 in the gas phase and in condensed phase solid Kr matrix

We present experimental results and theoretical simulations for an example of quantum control in both gas and condensed phase environments. Specifically, we show that the natural spreading of vibrational wavepackets in anharmonic potentials can be counteracted when the wavepackets are prepared with properly tailored ultrafast light pulses, both for gas phase I2 and for I2 embedded in a cold Kr matrix. We use laser induced fluorescence to probe the evolution of the shaped wavepacket. In the gas phase, at 313 K, we show that molecular rotations play an important role in determining the localization of the prepared superposition. In the simulations, the role of rotations is taken into account using both exact quantum dynamics and nearly classical theory. For the condensed phase, since the dimensionality of the system precludes exact quantum simulations, nearly classical theory is used to model the process and to interpret the data. Both numerical simulations and experimental results indicate that a properly ...

Comparison of coherent and spontaneous Raman microspectroscopies for noninvasive detection of single bacterial endospores

Single bacterial spores were analyzed by using nonlinear Raman microspectroscopy based on coherent anti-Stokes Raman scattering (CARS). The Raman spectra were retrieved from CARS spectra and found to be in excellent agreement with conventionally collected Raman spectra. The phase retrieval method based on maximum entropy model revealed significant robustness to external noise. The direct comparison of signal amplitudes exhibited a factor of 100 stronger CARS signal, as compared with the Raman signal.

Inhomogeneous deformation of silicon surface layers probed by second-harmonic generation in reflection

Semiconductor crystals possessing inversion symmetry (Si, Ge) are known to have a rather weak second-order nonlinearity of the quadrupole type [ Phys. Rev. Lett.51, 1983 ( 1983)], since the electric-dipole contribution is forbidden by symmetry. We report the experimental observation of anomalously highly efficient second-harmonic generation (SGH) in reflection from the surface of Si under inhomogeneous deformation. This effect is believed to be due to an electric-dipole contribution to the second-order susceptibility induced in the near-surface layer by inhomogeneous mechanical stress. This fact is consistent with theoretical calculations based on the molecular sp3-orbital model. Experimentally we observed an increase in the second-harmonic intensity by more than 2 orders of magnitude and a modification of the second-harmonic intensity dependence on crystal orientation with respect to the surface normal in the case of ion-implanted, pulsed-laser-annealed Si (111) samples. A similar effect was observed with thermally oxidized Si wafers and silicide-on-cSi structures. These results demonstrate the sensitivity of SHG in reflection to the presence of inhomogeneous stress in Si surface layers, which enables one to use SHG for nondestructive monitoring of stress in semiconductor structures.

Broadly tunable 30-fs pulses produced by optical parametric amplification

Broadly tunable 30-fs pulses have been generated by parametric amplification of a white-light continuum at kilohertz repetition rates.

VO2 films with strong semiconductor to metal phase transition prepared by the precursor oxidation process

We describe a relatively simple, reliable, and reproducible preparation technique, the precursor oxidation process, for making VO2 films with strong semiconductor-to-metal phase transition. Sputter-deposited metal precursor V films were oxidized in situ in the deposition chamber for 2.5–7h at 370–415°C in 0.2Torr O2 to form 22–220nm VO2. The strength [resistivity ratio, RR=ρS∕ρM] and sharpness (hysteresis width ΔTC) of T-dependent semiconductor-to-metal hysteretic phase transition in VO2 were our most immediate and relevant quality indicators. In 200-nm-range films, the process was optimized to yield RR=(1–2)×103, ΔTC∼11°C and absolute resistivity in a semiconducting phase ρS=0.4±0.2Ωm, close to resistivity in bulk single crystals of VO2. Films were characterized by scanning electron microscopy, atomic force microscopy, grazing-incidence x-ray diffraction, and Raman spectroscopy, and found to be polycrystalline single-phase VO2. We also measured optical reflectivity RT(λ) from 200to1100nm, and Rλ(T) from ...

Ultrafast rainbow: tunable ultrashort pulses from a solid-state kilohertz system

Two stages of type II optical parametric amplification of a white-light continuum are used for efficient generation of ultrashort (30–50-fs) pulses at kilohertz repetition rates. Various nonlinear techniques can be used to cover the range from 12 µm to 280 nm. The amplitude and the phase of the generated UV pulses are characterized by use of frequency-resolved optical gating.

Efficient third-harmonic generation in a thin nanocrystalline film of ZnO

Nonlinear optical conversion is studied in thin films of wide-bandgap materials. Very high conversion efficiency to the third-harmonic radiation is achieved for an unamplified femtosecond Cr4+:forsterite laser in a submicron-thick film of a nanocrystalline ZnO pulsed-laser-deposited on a fused silica substrate.