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We present a highly temporally and spatially resolved direct measurement of the propagation of ionization and recombination fronts in bulk ZnSe. The fronts were created by two-photon absorption (TPA) of 10 ps, 800 nm 'pump' pulses incident on both sides of a 1 mm thick crystal. The measurement was achieved by the spectral analysis of the frequency shift of a 100 fs 'probe' pulse transversely propagating through the crystal at the time of excitation and recombination. The probe was frequency shifted due to the temporal change in the refractive index of the medium. The delay between the probe and the pumps was finely scanned for time-resolved measurements, while imaging of the probe onto a spectrometer slit provided spatial resolution across the crystal. The spatially and temporally resolved shift agrees well with a model that includes TPA and recombination. Our set-up provides a general technique for measurements of rapid ionization and recombination processes.

We have measured the dielectric properties and thickness of thin semiconductor epitaxy layers by the reflection of THz radiation from the surface of a two-layered semiconductor wafer. When reflecting from two interfaces the electromagnetic pulse has a destructive interference at a specific wavelength dependent on the thickness of the outer layer and its dielectric function. Near that frequency the reflection coefficient has a significant drop. By extending the incident pulse spectrum to include this interference frequency, a measurement of the thickness can be obtained together with a direct measurement of the carrier number density. By this technique epitaxy layers of thickness down to 15 μm are characterized.

Periodic plasma density modulations have been observed in a laser spark created by focusing a 5–20 MW laser pulse into nitrogen at 1 to 2 atmospheres. The formation of such structures has been studied by measuring the period as a function of the wavelength λ and the incident angle γ of the laser pulse. The results show good agreement with a surface-instability model. The intensity of the scattered laser light has the same periodicity and is consistent with the measured plasma density profile.

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.

Experimental evidences of Raman amplification of ultrashort pulses in microcapillary plasmas are presented. The amplification of 100\char21{}500 fs pulses was investigated in microcapillaries with different lengths. The experimental data, together with simulation results, indicate that the resonance condition for Raman amplification in high-density plasma, ${n}_{e}\ensuremath{\sim}1\ensuremath{-}3\ifmmode\times\else\texttimes\fi{}{10}^{20}{\mathrm{cm}}^{\ensuremath{-}3},$ existed only in a very short plasma column. Such an assumption makes it possible to reconcile the experimental results and theoretical predictions. Investigations in very short microcapillaries (0.2\char21{}0.5 mm) with a broadband seed pulse further support this hypothesis and the amplification factor is in agreement with the linear growth rate.

Raman amplification of ultrashort pulses is demonstrated in microcapillary plasmas. Experiments in very short microcapillaries (0.2 – 0.5 mm) with a broadband seed pulse show that the amplification factor is in agreement with the linear growth rate.

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.

The optimum conditions for gain generation for the transition to the ground state at 13.5 nm in Liiii ions were investigated in discharge created pre‐plasma. The spatial and temporal evolution of plasma density was measured and a hollow density profile was observed. Waveguiding of high power laser pulses has been demonstrated in discharge created plasma channels, which can be helpful in increasing the interaction length, thus improving the gain‐length product.

We present guiding measurements of 0.8-μm, 100-fs pulses through LiF microcapillaries using discharge plasma. We inspect the transmission at conditions fit for soft-x-ray lasing. While up to 45% transmission is observed for input intensities of 1×1018 W/cm2, only ∼2% of the energy is contained in the small guided mode of 25 μm (FWHM). We investigate the dependence of the guiding on input intensity and focusing geometry. The transmission decreases for unmatched focal spots and as the intensity increases.

We experimentally demonstrated amplification of ultrashort laser pulses (200 fs pulses) in microcapillary plasmas by a counterpropagating pumping beam. Energy amplification as large as a factor of 5 was observed. Importantly, a nonlinear pump depletion regime must have been accessed, since the parameters support not only the amplification of the pulse, but an absolutely growing instability if the pump is not depleted. Further indication of accessing the nonlinear depletion is indicated by the relative insensitivity to the initial pulse energy.

The objective of this paper is to present the results of recent experiments on lasing to ground state in H-like LiIII ions in microcapillary plasma. The experiments were conducted at both Princeton University and NRL. Two methods were used to fill the microcapillaries with plasma: ablation of microcapillary walls when irradiated with a low power laser beam of a few ns duration, or fast electrical discharge. Powerful subpicosecond laser beams (with wavelength of 0.25 μm at Princeton University and 1.053 μm at NRL) were used for longitudinal pumping of microcapillary plasma following by the fast recombination and gain generation in the transition to ground state of hydrogen-like LiIII ions. In these experiments the dependence of the gain on the specific method of the preplasma creation, electric discharge versus low-power laser, pumping laser wavelength (0.248 μm versus 1.053 μm), and the geometry of the experiment (microcapillary dimensions and electrode design) were investigated. Results of these investigations are presented and discussed with the particular emphasis on the experiments with the 1μm subpicosecond laser pumping of discharge-produced microcapillary plasma.