Femtosecond Enhancement Cavities in the Nonlinear Regime.

We combine high-finesse optical resonators and spatial-spectral interferometry to a highly phase-sensitive investigation technique for nonlinear light-matter interactions. We experimentally validate an ab initio model for the nonlinear response of a resonator housing a gas target, permitting the global optimization of intracavity conversion processes like high-order harmonic generation. We predict the feasibility of driving intracavity high-order harmonic generation far beyond intensity limitations observed in state-of-the-art systems by exploiting the intracavity nonlinearity to compress the pulses in time.

Dual frequency comb spectroscopy with a single laser.

We demonstrate a simple scheme for dual frequency comb spectroscopy in which the second frequency comb is generated by propagating the primary pulse train through a dazzler. The two frequency combs are combined behind a Mach-Zehnder interferometer, and the optical spectrum is read out by an rf-spectrum analyzer. The method is applied to record the overtone absorption spectrum of C2H2 (acetylene) in the wavelength region around 1.03 µm. A spectrum with a resolution of 4 cm(-1) is obtained, which compares well with that from the HITRAN database. A simple method for improving the spectral resolution is demonstrated.

Megawatt-scale average-power ultrashort pulses in an enhancement cavity.

We investigate power scaling of ultrashort-pulse enhancement cavities. We propose a model for the sensitivity of a cavity design to thermal deformations of the mirrors due to the high circulating powers. Using this model and optimized cavity mirrors, we demonstrate 400 kW of average power with 250 fs pulses and 670 kW with 10 ps pulses at a central wavelength of 1040 nm and a repetition rate of 250 MHz. These results represent an average power improvement of one order of magnitude compared to state-of-the-art systems with similar pulse durations and will thus benefit numerous applications such as the further scaling of tabletop sources of hard x rays (via Thomson scattering of relativistic electrons) and of soft x rays (via high harmonic generation).

Cavity-enhanced high-harmonic generation with spatially tailored driving fields.

We theoretically and experimentally investigate high-harmonic generation in a 78-MHz enhancement cavity with a transverse mode having on-axis intensity maxima at the focus and minima at an opening in the following mirror. We find that the conversion efficiency is comparable to that achievable with a Gaussian mode, whereas the output coupling efficiency can be significantly improved over any other demonstrated technique. This approach offers additional power scaling advantages and additional degrees of freedom in shaping the harmonic emission, paving the way to high-power extreme-ultraviolet frequency combs and the generation of multi-MHz repetition-rate-isolated attosecond pulses.

Towards reliable diagnostics of prostate cancer via breath.

Early detection of cancer is a key ingredient for saving many lives. Unfortunately, cancers of the urogenital system are difficult to detect at early stage. The existing noninvasive diagnostics of prostate cancer (PCa) suffer from low accuracy (< 70%) even at advanced stages. In an attempt to improve the accuracy, a small breath study of 63 volunteers representing three groups: (1) of 19 healthy, (2) 28 with PCa, (3) with 8 kidney cancer (KC) and 8 bladder cancer (BC) was performed. Ultrabroadband mid-infrared Fourier absorption spectroscopy revealed eight spectral ranges (SRs) that differentiate the groups. The resulting accuracies of supervised analyses exceeded 95% for four SRs in distinguishing (1) vs (2), three for (1) vs (3) and four SRs for (1) vs (2) + (3). The SRs were then attributed to volatile metabolites. Their origin and involvement in urogenital carcinogenesis are discussed.

Picosecond imaging of low-density plasmas by electron deflectometry.

We have imaged optical-field ionized plasmas with electron densities as low as 10(13) cm(-3) on a picosecond timescale using ultrashort electron pulses. Electric fields generated by the separation of charges are imprinted on a 20 keV probe electron pulse and reveal a cloud of electrons expanding away from a positively charged plasma core. Our method allows for a direct measurement of the electron energy required to escape the plasma and the total charge. Simulations reproduce the main features of the experiment and allow determination of the energy of the electrons.

XeI((2)pi((1/2))-(2)Sigma((1/2))) excimer emission at 1.3 microm.

The collisional enhancing of the magnetic dipole transition I(5(2)P((1/2))-5(2)P(3/2)) at 1.3152 microm by Xe is shown to proceed over an exciplex channel: I(5(2)P((1/2))) + Xe ? XeI((2)pi((1/2))) ? I(5(2)P(3/2)) + Xe + hnu(1.3 microm). The pseudo-first-order rate coefficient for this process is (1.6+/-0.2) x 10(-18) molecule(-1) cm(3) sec(-1)). The termolecular rate coefficient for XeI((2)pi((1/2))) excimer production is (2 +/- 1) x 10(-35) molecule(-2) cm(6) sec(-1). The radiative lifetime of the XeI excimer state is of the order of 100 nsec. Similar effects are briefly reported for C(2)F(6), SF(6), Kr, Ar, and i-C(3)F(7)I.

Effective traveling-wave excitation below the speed of light.

We demonstrate that effective traveling-wave excitation of high-gain amplifiers requires velocities that are remarkably slower than the velocity of light. Experiments with a femtosecond-laser-pumped molecular hydrogen laser exhibit pronounced enhancement of the intensity if an excitation velocity that is slower than the velocity of light is employed. These results are directly scalable to shorter wavelengths, paving the way for a more effective pump setup for x-ray lasers.

X rays from optical-field ionized plasmas at low density.

We present results of an experiment in which x rays from an optical-field ionized plasma are generated under well-controlled conditions in a low-pressure gas cell. In this way high-density effects such as electron heating, collisional ionization, and ionization defocusing are avoided. Using N(2) as the medium, we show that many features of the soft-x-ray emission follow theoretical predictions. In particular, higher x-ray intensity is observed on most lines for circularly than for linearly polarized light. However, several Li-like lines show anomalously strong emission for linear polarization. Mechanisms that may be responsible for this effect are discussed.

Hot-electron generation in copper and photopumping of cobalt

Hot electrons generated upon interaction of p-polarized 130 fs laser pulses with copper and penetrating into the target material are characterized with respect to their energy distribution and directionality. "Experimental" data are obtained by comparing the rear-side x-ray emission from layered targets with Monte Carlo electron-photon transport simulations. Theoretical electron energy distributions are derived by means of a one and a half-dimensional particle-in-cell code. Both sets of data consist of a two-temperature distribution of electrons propagating in a direction almost perpendicular to the target surface. The "experimental" data contain a considerably higher population of the lower temperature electrons. The discrepancy is explained by the intensity distribution of the laser spot. The results are used to design an experiment for demonstrating photopumping of cobalt with copper Kalpha radiation. A 10 &mgr;m copper foil is backed with 1 mm of polyethylene (PE) followed by 10 &mgr;m of cobalt, the rear-side Kalpha emission of which is measured. The PE layer prevents fast electrons from reaching the cobalt. Comparing the cobalt Kalpha emission with that of nickel, which is not photopumped by copper Kalpha shows enhancement by almost a factor of 2.

Self-photopumped neonlike x-ray laser.

We propose a new x-ray laser mechanism that uses radiation from the strongest 3d ? 2p Ne-like resonance line in an optically thick plasma to radiatively drive population from the Ne-like ground state to the 3d state, which then lases to two 3p states. Collisional mixing of the 3p states with nearby 3s and 3d states depopulates the lower laser states. Modeling is presented for this mechanism in Ne-like Ar, and in experiments we observe one potential 3d ? 3p lasing transition at 45.1 nm in Ne-like Ar.

Two-dimensional near-field images of the neonlike germanium soft-x-ray laser.

We present what are believed to be the first two-dimensional near-field images of the neonlike germanium x-ray laser obtained with multilayer imaging mirrors. The Asterix iodine laser, with a low-intensity prepulse 5.23 ns before the main pulse, was used to irradiate germanium slab targets. We observe a large difference in the spatial dependence of the J = 0-1 and J = 2-1 lines of germanium, with the J = 2-1 emission peaking farther away from the target surface. The prepulse level is also observed to have a significant effect on the spatial dependence of the germanium laser lines. A great deal of structure is observed in the near-field images, particularly in the J = 0-1 emission.

High-gain x-ray lasing at 11.1 nm in sodiumlike copper driven by a 20-J, 2-ps Nd:glass laser.

Evidence of high gain pumped by recombination has been observed in the 5g-4f transition at 11.1 nm in sodiumlike copper ions with use of a 20-J 2-ps Nd:glass laser system. The time- and space-integrated gain coefficient was 8.8 +/- 1.4 cm(-1), indicating a single-transit amplification of ~60 times. This experiment has shown that 2 ps is the optimum pulse duration to drive the sodiumlike copper recombination x-ray lasing at 11.1 nm.