Generation of structured coherent extreme ultraviolet beams from an MgO crystal.

Short wavelength high-harmonic sources are undergoing intense development for applications in spectroscopy and microscopy. Despite recent progress in peak and average power, spatial control over coherent extreme ultraviolet (XUV) beams remains a formidable challenge due to the lack of suitable optical elements for beam shaping and control. Here we demonstrate a robust and precise approach that structures XUV high-order harmonics in space as they are emitted from a nanostructured MgO crystal. Our demonstration paves the way for bridging the numerous applications of shaped light beams from the visible to the short wavelengths, with potential uses for applications in microscopy and nanoscale machining.

Observation of backward high-harmonic emission from solids.

We experimentally demonstrate backward emission of high-harmonics of a near-infrared laser from MgO and Si crystals in the direction of specular reflection. We show that the variation of the high-harmonic power with the angle of incidence can be predicted with nonlinear reflection coefficients derived originally for perturbative nonlinearities. A comparison of transmission and reflection geometries suggests that backward-propagating high-harmonics are an excellent reference to study nonlinear propagation of intense light in solids. Backward emission will enable phase matching of the high-harmonic beam and the integration of the functionalities of extended gas-phase high-harmonic beamlines into a single optical element. The potential to achieve phase matching paves the way to solid-state based high-harmonic sources with higher flux than the best transmission-based sources, where high-harmonics are strongly absorbed by the crystal itself.

Light amplification by seeded Kerr instability.

Amplification of femtosecond laser pulses typically requires a lasing medium or a nonlinear crystal. In either case, the chemical properties of the lasing medium or the momentum conservation in the nonlinear crystal constrain the frequency and the bandwidth of the amplified pulses. We demonstrate high gain amplification (greater than 1000) of widely tunable (0.5 to 2.2 micrometers) and short (less than 60 femtosecond) laser pulses, up to intensities of 1 terawatt per square centimeter, by seeding the modulation instability in an Y3Al5O12 crystal pumped by femtosecond near-infrared pulses. Our method avoids constraints related to doping and phase matching and therefore can occur in a wider pool of glasses and crystals even at far-infrared frequencies and for single-cycle pulses. Such amplified pulses are ideal to study strong-field processes in solids and highly excited states in gases.

Intense-Laser Solid State Physics: Unraveling the Difference between Semiconductors and Dielectrics.

Experiments on intense laser driven dielectrics have revealed population transfer to the conduction band to be oscillatory in time. This is in stark contrast to ionization in semiconductors and is currently unexplained. Current ionization theories neglect coupling between the valence and conduction band and therewith, the dynamic Stark shift. Our single-particle analysis identifies this as a potential reason for the different ionization behavior. The dynamic Stark shift increases the band gap with increasing laser intensities, thus suppressing ionization to an extent where virtual population oscillations become dominant. The dynamic Stark shift plays a role dominantly in dielectrics which, due to the larger band gap, can be exposed to significantly higher laser intensities.

Harmonic generation in solids with direct fiber laser pumping.

High harmonic generation in solids presents the possibility for bringing attosecond techniques to semiconductors and a simple source for frequency comb spectroscopy in the vacuum ultraviolet. We generate up to the seventh harmonic of a Tm fiber laser by focusing in silicon or zinc oxide. The harmonics are strong and stable, with no indication of material damage. Calculations show the potential for generating nineteenth harmonic photons at 12 eV photons of energy.

All-Optical Reconstruction of Crystal Band Structure.

The band structure of matter determines its properties. In solids, it is typically mapped with angle-resolved photoemission spectroscopy, in which the momentum and the energy of incoherent electrons are independently measured. Sometimes, however, photoelectrons are difficult or impossible to detect. Here we demonstrate an all-optical technique to reconstruct momentum-dependent band gaps by exploiting the coherent motion of electron-hole pairs driven by intense midinfrared femtosecond laser pulses. Applying the method to experimental data for a semiconductor ZnO crystal, we identify the split-off valence band as making the greatest contribution to tunneling to the conduction band. Our new band structure measurement technique is intrinsically bulk sensitive, does not require a vacuum, and has high temporal resolution, making it suitable to study reactions at ambient conditions, matter under extreme pressures, and ultrafast transient modifications to band structures.

Linking high harmonics from gases and solids.

When intense light interacts with an atomic gas, recollision between an ionizing electron and its parent ion creates high-order harmonics of the fundamental laser frequency. This sub-cycle effect generates coherent soft X-rays and attosecond pulses, and provides a means to image molecular orbitals. Recently, high harmonics have been generated from bulk crystals, but what mechanism dominates the emission remains uncertain. To resolve this issue, we adapt measurement methods from gas-phase research to solid zinc oxide driven by mid-infrared laser fields of 0.25 volts per ångström. We find that when we alter the generation process with a second-harmonic beam, the modified harmonic spectrum bears the signature of a generalized recollision between an electron and its associated hole. In addition, we find that solid-state high harmonics are perturbed by fields so weak that they are present in conventional electronic circuits, thus opening a route to integrate electronics with attosecond and high-harmonic technology. Future experiments will permit the band structure of a solid to be tomographically reconstructed.

Theoretical analysis of high-harmonic generation in solids.

We investigate theoretically high-harmonic generation (HHG) in bulk crystals exposed to intense midinfrared lasers with photon energies smaller than the band gap. The two main mechanisms, interband and intraband HHG, are explored. Our analysis indicates that the interband current neglected so far is the dominant mechanism for HHG. Saddle point analysis in the Keldysh limit yields an intuitive picture of interband HHG in solids similar to atomic HHG. Interband and intraband HHG exhibit a fundamentally different wavelength dependence. This signature can be used to experimentally distinguish between the two mechanisms in order to verify their importance.

Tunnel ionization dynamics of bound systems in laser fields: how long does it take for a bound electron to tunnel?

A numerical method is developed by which the tunnel ionization dynamics of bound systems in laser fields can be isolated from the total wave function, as given by the time-dependent Schrödinger equation. The analysis of the numerical data for a step function field reveals the following definition for the tunnel time. It is the time it takes the ground state to develop the underbarrier wave function components necessary for reaching the static field ionization rate. This definition is generalized to time varying laser fields. The tunnel time is found to scale with the Keldysh tunnel time. Our Letter establishes the physical meaning of the tunnel time, its relation to the Keldysh tunnel time, and suggests how it can be measured.

Synthesis and aldose reductase inhibitory activity of benzoyl-amino acid derivatives.

A series of N-(4-methoxy, 4-fluoro, 4-trifluoromethyl and 4-nitrobenzoyl)-L-amino acids was synthesized and their inhibitory activity towards bovine lens aldose reductase (ALR2) was tested.

Synthesis, antimicrobial and genotoxic properties of some benzoimidazole derivatives.

A number of 1H-benzoimidazol-2-ylamine and of 1-methyl-1H-benzoimidazol-2-ylamine derivatives were synthesized and the crystal and molecular structure of N-[4-(2-amino-benzoimidazole-1-sulfonyl)-phenyl] acetamide was determined by X-ray diffraction analysis. The compounds obtained were investigated for antimicrobial and genotoxic activities.

Glycyrrhizin and 18 beta-glycyrrhetinic acid: a comparative study of the pharmacological effects induced in the rat after prolonged oral treatment.

Recent clinical and toxicological studies have investigated the mineralcorticoid-like and hypertensive effects of liquorice, and we therefore set out to identify the active component responsible for these effects. We conducted a 30-day comparative analysis of glycyrrhizin and 18 beta-glycyrrhetinic acid and found that the latter causes significant variations both in systolic blood pressure and in the excretion in the urine of Ca++. The effects were fully reversible on suspension of treatment.

Inhibition of mutagenicity in Salmonella typhimurium by Glycyrrhiza glabra extract, glycyrrhizinic acid, 18 alpha- and 18 beta-glycyrrhetinic acids.

The effects of Glycyrrhiza glabra L. extract, glycyrrhizinic acid, 18 alpha- and 18 beta-glycyrrhetinic acids on the mutagenicity of the ethyl methanesulfonate, N-methyl-N'-nitro-N-nitrosoguanidine, and ribose-lysine Maillard model systems were investigated by using the Salmonella/microsome reversion assay. The protocol used allowed us to detect desmutagenic and antimutagenic activity and to avoid false positive results due to toxicity. For all the compounds tested, no desmutagenic activity was observed against ethyl methanesulfonate and N-methyl-N'-nitro-N-nitrosoguanidine; only Glycyrrhiza glabra extract showed antimutagenic activity against ethyl methanesulfonate. On using the ribose-lysine mutagenic browning mixture, the desmutagenic activities of the Glycyrrhiza glabra extract, glycyrrhizinic acid, 18 alpha- and 18 beta-glycyrrhetinic acids were observed. 18 beta-Glycyrrhetinic acid was the most active compound. Glycyrrhiza glabra extract also exhibited antimutagenic activity against ribose-lysine.

Determination of glycyrrhizin and 18 alpha-, 18 beta-glycyrrhetinic acid in rat plasma by high-performance thin-layer chromatography.

An HPTLC method is described for the simultaneous determination of glycyrrhizinic acid tripotassium salt (3K-G), 18 alpha- and 18 beta-glycyrrhetinic acid (18 alpha-GA, 18 beta-GA) in rat plasma, after oral administration of 3K-G (30 mg/kg once a day for 30 days). Systolic pressure and the volume of urine excreted in 24 hours were recorded during this period to observe any drug-induced effects.

Studies on the genotoxic properties of essential oils with Bacillus subtilis rec-assay and Salmonella/microsome reversion assay.

Genotoxic properties of essential oils from Anthemis nobilis L., Artemisia dracunculus L., Salvia officinalis L., Salvia sclarea L., Satureja hortensis L., Satureja montana L., Thymus capitatus L., Thymus citriodorus Schreb., Thymus vulgaris L., Citrus bergamia Risso, were studied with Bacillus subtilis rec-assay and Salmonella/microsome reversion assay. The essential oil of Artemisia dracunculus L. "Piemontese" turned out to be active in the rec-assay but not in the Salmonella test. DNA-damaging activity was demonstrated to be due to the estragol component of the oil. Advantages of the combined use of these two short-term microbial assays in genotoxic studies are discussed.

Sedative, anti-inflammatory and anti-diuretic effects induced in rats by essential oils of varieties of Anthemis nobilis: a comparative study.

The pharmacological properties of essential oils obtained from two varieties of Anthemis nobilis was studied. The two varieties, named "white-headed" or double flowered and "yellow-headed", present considerable morphological differences and yield essential oils with different composition. These essential oils proved to possess interesting anti-inflammatory and sedative properties, especially that derived from the "White-headed" variety.

Preparation and physicochemical properties of uracil derivatives with potential biological activity.

The characterization of mono- and dimethylated 5-substituted uracils was re-examined. Analysis of their physicochemical properties (pKa, U.V., delta 1H-N.M.R.) affords insight into the structural characteristics of 5-substituted uracil alkylated at the ring nitrogens.