Yoav Avitzour

Sensitive femtosecond coherent anti-Stokes Raman spectroscopy discrimination between dipicolinic acid and dinicotinic acid

We demonstrate that femtosecond ultraviolet and visible coherent anti-Stokes Raman spectroscopy provides the sensitivity and specificity needed to distinguish between two similar molecules of pyridinedicarboxylic acid. The Fourier transforms of the temporal measurements provide the energy difference between the ground state vibrational modes. Quantum chemical calculations provide theoretical predictions that agree well with the measurements. The present technique allows us to distinguish 10 cm(-1) frequency shifts by using pulses ten times broader than the shifts.

Picosecond pulse frequency upshifting by rapid free-carrier creation in ZnSe

The frequency upshifting of 0.8 μm picosecond laser pulses was demonstrated using the temporal change of the free carrier density in a ZnSe semiconductor crystal. The crystal was ionized by transverse propagating ps pulses. Shifts of up to 1.6 nm were observed, which agree within 25% with the theory.

Discrimination of dipicolinic acid and its interferents by femtosecond coherent Raman spectroscopy

Measurements of the beat frequencies between vibrational modes of dipicolinic acid (DPA) and a series of other molecules (interferents) are presented. The results were obtained from femtosecond time-resolved coherent Raman scattering, and the vibrational level spacings were determined from a Fourier transform of the signal versus probe pulse delay. The entire spectrum of the generated signal is recorded in order to demonstrate multimode excitation and to explain the variety of qualitatively different traces that can be obtained for the same molecule. Since the spectral signature of DPA is unique enough to be used for identification purposes, this technique has the potential to detect hazardous bacterial species, such as anthrax spores.

Feasibility of achieving gain in transition to the ground state of C VI at 3.4 nm

We present numerical studies of recombination gain in the transition to the ground state of H-like C (2→1 transition at λ=3.4 nm). It is shown that high gain (up to about 180 cm−1) can be achieved using currently available, relatively compact, laser technology. The model includes the ionization of the plasma by an ultraintense, ultrashort laser pulse, followed by plasma expansion, cooling, and recombination. Transient population inversion is generated during the recombination process. We investigate the behavior of the gain with respect to different plasma parameters and pump pulse parameters and explain how the different properties of the plasma and the pump pulse affect the gain.

Numerical simulation of the effect of hydrogen on recombination gain in the transition to ground state of Li III

Numerical simulations of recombination gain in the Li III transition to ground state (2→1 at 13.5 nm) are presented. The plasma simulated is a mixture of Li and H ions, and the space-time-dependent gain coefficient is calculated for different mixing ratios and different pumping beam parameters. The numerical model includes the initial optical field ionization of the plasma by an intense 100 fs laser pulse, taking into account residual heating, particle collisions, and spatial effects. Gain is then calculated during the process of recombination as the plasma expands and cools. We show that the addition of hydrogen to the plasma can lead to higher gain with a less restrictive range of experimental parameters. We analyze the effects of the addition of hydrogen on the gain and point to the optimal plasma and pump parameters to produce gain.

Experimental and Theoretical Simulations for Conditions for Lasing at 13.5 nm in LiIII

We present results related to the search for optimum conditions for lasing to ground state of H‐like LiIII ions at 13.5 nm. These conditions are being considered from the point of view of the development of a prototype of a very compact 13.5 nm laser for metrology of soft x‐ray (EUV) lithography. Theoretical simulations are discussed in relation to experimental data. Experiments on channeling of ultrashort high intensity pumping laser beam in microcapillary plasma are presented for conditions appropriate for lasing at 13.5 nm. We are also discussing Raman amplification of ultrashort pulses in microcapillary plasma as a possibility for future use in X‐ray lasers.

Toward Ultraintense Compact RBS Pump for Recombination 3.4 nm Laser via OFI

In our presentation we overview progress we made in developing a new ultrashort and ultraintensive laser system based on Raman backscattering (RBS) amplifier /compressor from time of 10th XRL Conference in Berlin to present time of 11th XRL Conference in Belfast. One of the main objectives of RBS laser system development is to use it for pumping of recombination X-ray laser in transition to ground state of CVI ions at 3.4 nm. Using elaborate computer code the processes of Optical Field Ionization, electron energy distribution, and recombination were calculated. It was shown that in very earlier stage of recombination, when electron energy distribution is strongly non-Maxwellian, high gain in transition from the first excited level n=2 to ground level m=1 can be generated. Adding large amount of hydrogen gas into initial gas containing carbon atoms (e.g. methane, CH4) the calculated gain has reached values up to 150–200 cm−2 Taking into account this very encouraging result, we have proceed with arrangement of experimental setup. We will present the observation of plasma channels and measurements of electron density distribution required for generation of gain at 3.4 nm.

Feasibility of 3.4 nm Laser Pumped by Ultraintense RBS Laser

Our presentation consisted of two parts. In the first part we presented main theoretical results on the plasma and pumping conditions required to generate gain at 3.4 nm in H-like CVI ions in transition from the first excited state to ground state. Transient population inversion is generated during the recombination process. It was shown that high gain (up to G~200 cm-1) can be achieved using currently available compact lasers. In the second part we presented a new type of compact laser generating ultra-short and ultra-intensive pulses via Raman Backscattering (RBS) amplification and compression in plasma. We achieved large (up to 1000) “seed” amplification and its compression from~1 psec down to 150 fsec in only 2 mm long plasma pumped with~1014 W/cm2 pulses. We also presented very recent experiments on amplification in 2 passes setup. Such RBS amplifier and compressor is expected to provide in not too far future intensities~1020 W/cm2 at high repetition rate in compact (university type) system, which would be ideal pump not only for 3.4 nm laser but even for shorter wavelength lasers.

Theoretical Modeling of Recombination Gain in LiIII Transition to Ground State

We present numerical calculation of recombination gain in LiIII transition to ground state (2 → 1). The model includes the initial ionization of the plasma by an intense fs laser pulse, and continues through the expansion and cooling of the plasma simultaneously with the recombination process. We show that although initial estimations of the energy absorption by the plasma from the ionization laser does not allow for recombination gain, the expansion and cooling processes that take place immediately after ionization, in addition to the non‐Maxwellian distribution of electrons in the plasma, give rise to high gain under feasible experimental conditions.