Optically controlled solid-density transient plasma gratings.

A general approach for optically controlled spatial structuring of overdense plasmas generated at the surface of initially plain solid targets is presented. We demonstrate it experimentally by creating sinusoidal plasma gratings of adjustable spatial periodicity and depth, and study the interaction of these transient structures with an ultraintense laser pulse to establish their usability at relativistically high intensities. We then show how these gratings can be used as a "spatial ruler" to determine the source size of the high-order harmonic beams produced at the surface of an overdense plasma. These results open new directions both for the metrology of laser-plasma interactions and the emerging field of ultrahigh intensity plasmonics.

Spectral characterization of laser-driven solid-based high harmonics in the coherent wake emission regime.

Laser-produced surface high-harmonic generation is an attractive source of extreme ultraviolet radiation due to its coherent properties and high peak power. By operating at subrelativistic laser intensities in the coherent wake emission regime, the harmonic spectrum was experimentally studied versus laser properties. At higher laser intensities (>10(17) W/cm(2)) a higher spectral cutoff was observed, with accompanying blueshifting and spectral broadening of the individual orders. A model based on an expanding critical surface provides qualitative agreement with the observations.

Direct observation of density-gradient effects in harmonic generation from plasma mirrors.

High-order harmonics and attosecond pulses of light can be generated when ultraintense, ultrashort laser pulses reflect off a solid-density plasma with a sharp vacuum interface, i.e., a plasma mirror. We demonstrate experimentally the key influence of the steepness of the plasma-vacuum interface on the interaction, by measuring the spectral and spatial properties of harmonics generated on a plasma mirror whose initial density gradient scale length L is continuously varied. Time-resolved interferometry is used to separately measure this scale length.

Tracking propagation of ultrashort intense laser pulses in gases via probing of ionization.

We use optical interferometry to study the propagation of femtosecond laser pulses in gases. We show the measurements of propagation in a nitrogen gas jet and we compare the results with propagation in He under the same irradiation conditions. We find that in the case of nitrogen, the detailed temporal structure of the laser pulse can be tracked and visualized by measuring the phase and the resulting electron-density map. A dramatically different behavior occurs in He gas jets, where no details of the temporal structure of the laser pulse are visible. These observations are explained in terms of the ionization dynamics of nitrogen compared to helium. These circumstances make N2 gas sensitive to variations in the electric field and, therefore, allow the laser-pulse temporal and spatial structures to be visualized in detail.

Microscopic theory of the Casimir force at thermal equilibrium: large-separation asymptotics.

We present an entirely microscopic calculation of the Casimir force f(d) between two metallic plates in the limit of large separation d . The models of metals consist of mobile quantum charges in thermal equilibrium with the photon field at positive temperature T . Fluctuations of all degrees of freedom, matter and field, are treated according to the principles of quantum electrodynamics and statistical physics without recourse to approximations or intermediate assumptions. Our main result is the correctness of the asymptotic universal formula f(d) approximately -zeta(3)kBT/8pid3, d-->infinity. This supports the fact that, in the framework of the Lifshitz theory of electromagnetic fluctuations, transverse electric modes do not contribute in this regime. Moreover, the microscopic origin of universality is seen to rely on perfect screening sum rules that hold in great generality for conducting media.

A rearranging ligand enables allosteric control of catalytic activity in copper-containing nitrite reductase.

In Cu-containing nitrite reductase from Alcaligenes faecalis S-6 the axial methionine ligand of the type-1 site was replaced (M150G) to make the copper ion accessible to external ligands that might affect the enzyme's catalytic activity. The type-1 site optical spectrum of M150G (A(460)/A(600)=0.71) differs significantly from that of the native nitrite reductase (A(460)/A(600)=1.3). The midpoint potential of the type-1 site of nitrite reductase M150G (E(M)=312(+/-5)mV versus hydrogen) is higher than that of the native enzyme (E(M)=213(+/-5)mV). M150G has a lower catalytic activity (k(cat)=133(+/-6)s(-1)) than the wild-type nitrite reductase (k(cat)=416(+/-10)s(-1)). The binding of external ligands to M150G restores spectral properties, midpoint potential (E(M)<225mV), and catalytic activity (k(cat)=374(+/-28)s(-1)). Also the M150H (A(460)/A(600)=7.7, E(M)=104(+/-5)mV, k(cat)=0.099(+/-0.006)s(-1)) and M150T (A(460)/A(600)=0.085, E(M)=340(+/-5)mV, k(cat)=126(+/-2)s(-1)) variants were characterized. Crystal structures show that the ligands act as allosteric effectors by displacing Met62, which moves to bind to the Cu in the position emptied by the M150G mutation. The reconstituted type-1 site has an otherwise unaltered geometry. The observation that removal of an endogenous ligand can introduce allosteric control in a redox enzyme suggests potential for structural and functional flexibility of copper-containing redox sites.

Calorimetric and spectroscopic investigations of the thermal denaturation of wild type nitrite reductase.

Nitrite reductase (NiR) is a multicopper protein, with a trimeric structure containing two types of copper site: type 1 is present in each subunit whereas type 2 is localized at the subunits interface. The paper reports on the thermal behaviour of wild type NiR from Alcaligenes faecalis S-6. The temperature-induced changes of the copper centres are characterized by optical spectroscopy and electron paramagnetic resonance spectroscopy, and by establishing the thermal stability by differential scanning calorimetry. The calorimetric profile of the enzyme shows a single endothermic peak with maximum heat absorption at T(m) approximately 100 degrees C, revealing an exceptional thermal stability. The thermal transition is irreversible and the scan rate dependence of the calorimetric trace indicates that the denaturation of NiR is kinetically controlled. The divergence of the activation energy values determined by different methods is used as a criterion for the inapplicability of the one-step irreversible model. The best fit of the DSC profiles is obtained when the classical Lumry-Eyring model, N<-->U-->F, is considered. The simulation results indicate that the irreversible step prevails on the reversible one. Moreover, it is found that the conformational changes within the type-1 copper environments precede the denaturation of the whole protein. No evidence of protein dissociation within the temperature range investigated was observed.

Femtosecond interferometry of propagation of a laminar ionization front in a gas.

We use optical interferometry to investigate ultrafast ionization induced by an intense, ultrashort laser pulse propagating in a helium gas. Besides standard phase shift information, our interferograms show a localized region of fringe visibility depletion (FVD) that moves along the laser propagation axis at luminal velocity. We find that such a loss of visibility can be quantitatively explained by the ultrafast change of refractive index due to the field ionization of the gas in the laser pulse width. We demonstrate that by combining the post facto phase shift distribution with the probe pulse transit effect in the ionizing region, the analysis of the observed FVD yields significant information on the ultrafast dynamics of propagation of the ionization front in the gas.

Thermal stability of wild type and disulfide bridge containing mutant of poplar plastocyanin.

A comparative study of the thermal stability of wild type poplar plastocyanin and of a mutant form containing a disulfide bridge between residues 21 and 25 was performed using differential scanning calorimetry and optical spectroscopic techniques. For wild type plastocyanin the transition temperature, determined from the calorimetric profiles, is 62.7 degrees C at the scan rate of 60 degrees C/h, whereas for the mutant it is reduced to 58.0 degrees C. In both cases, the endothermic peak is followed by an exothermic one at higher temperatures. The unfolding process monitored by optical absorption at 596 nm also reveals a reduced thermal stability of the mutated plastocyanin compared to the wild type protein, with transition temperatures of 54.8 and 58.0 degrees C, respectively. For both proteins, the denaturation process was found to be irreversible and dependent on the scan rate preventing the thermodynamic analysis of the unfolding process. In parallel, small conformational changes between wild type and mutant plastocyanin emerge from fluorescence spectroscopy measurements. Here, a difference in the interaction of the two proteins between the microenvironment surrounding the fluorophores and the solvent was proposed. The destabilization observed in the disulfide containing mutant of plastocyanin suggests that the double mutation, Ile21Cys and Glu25Cys, introduces strain into the protein which offsets the stabilizing effect expected from the formation of a covalent crosslink.

Complete characterization of a plasma mirror for the production of high-contrast ultraintense laser pulses.

Improving the temporal contrast of ultrashort and ultraintense laser pulses is a major technical issue for high-field experiments. This can be achieved using a so-called "plasma mirror." We present a detailed experimental and theoretical study of the plasma mirror that allows us to quantitatively assess the performances of this system. Our experimental results include time-resolved measurements of the plasma mirror reflectivity, and of the phase distortions it induces on the reflected beam. Using an antireflection coated plate as a target, an improvement of the contrast ratio by more than two orders of magnitude can be achieved with a single plasma mirror. We demonstrate that this system is very robust against changes in the pulse fluence and imperfections of the beam spatial profile, which is essential for applications.

Optical properties of relativistic plasma mirrors.

The advent of ultrahigh-power femtosecond lasers creates a need for an entirely new class of optical components based on plasmas. The most promising of these are known as plasma mirrors, formed when an intense femtosecond laser ionizes a solid surface. These mirrors specularly reflect the main part of a laser pulse and can be used as active optical elements to manipulate its temporal and spatial properties. Unfortunately, the considerable pressures exerted by the laser can deform the mirror surface, unfavourably affecting the reflected beam and complicating, or even preventing, the use of plasma mirrors at ultrahigh intensities. Here we derive a simple analytical model of the basic physics involved in laser-induced deformation of a plasma mirror. We validate this model numerically and experimentally, and use it to show how such deformation might be mitigated by appropriate control of the laser phase.

Internal frequency conversion extreme ultraviolet interferometer using mutual coherence properties of two high-order-harmonic sources.

We report on an innovative two-dimensional imaging extreme ultraviolet (XUV) interferometer operating at 32 nm based on the mutual coherence of two laser high order harmonics (HOH) sources, separately generated in gas. We give the first evidence that the two mutually coherent HOH sources can be produced in two independent spatially separated gas jets, allowing for probing centimeter-sized objects. A magnification factor of 10 leads to a micron resolution associated with a subpicosecond temporal resolution. Single shot interferograms with a fringe visibility better than 30% are routinely produced. As a test of the XUV interferometer, we measure a maximum electronic density of 3x10(20) cm(-3) 1.1 ns after the creation of a plasma on aluminum target.

Phase properties of laser high-order harmonics generated on plasma mirrors.

As a high-intensity laser-pulse reflects on a plasma mirror, high-order harmonics of the incident frequency can be generated in the reflected beam. We present a numerical study of the phase properties of these individual harmonics, and demonstrate experimentally that they can be coherently controlled through the phase of the driving laser field. The harmonic intrinsic phase, resulting from the generation process, is directly related to the coherent sub-laser-cycle dynamics of plasma electrons, and thus constitutes a new experimental probe of these dynamics.

Fast-ion energy-flux enhancement from ultrathin foils irradiated by intense and high-contrast short laser pulses.

Recent significant improvements of the contrast ratio of chirped pulse amplified pulses allows us to extend the applicability domain of laser accelerated protons to very thin targets. In this framework, we propose an analytical model particularly suitable to reproducing ion laser acceleration experiments using high intensity and ultrahigh contrast pulses. The model is based on a self-consistent solution of the Poisson equation using an adiabatic approximation for laser generated fast electrons which allows one to find the target thickness maximizing the maximum proton (and ion) energies and population as a function of the laser parameters. Model furnished values show a good agreement with experimental data and 2D particle-in-cell simulation results.

van der Waals forces in presence of free charges: an exact derivation from equilibrium quantum correlations.

We study interatomic forces in a fluid consisting of a mixture of free charges and neutral atoms in the framework of the quantum many-body problem at nonzero temperature and nonzero density. Of central interest is the interplay between van der Waals forces and screening effects due to free charges. The analysis is carried out in a partially recombined hydrogen plasma in the Saha regime. The effective potentials in the medium between two atoms, or an atom and a charge, or two charges, are determined from the large-distance behavior of equilibrium proton-proton correlations. We show, in a proper low-temperature and low-density scaling limit, that those potentials all decay as r(-6) at large distance r, while the corresponding amplitudes are calculated exactly. In particular, the presence of free charges only causes a partial (nonexponential) screening of the atomic potential, and it does not modify its typical r(-6) decay. That potential reduces to the standard van der Waals form for two atoms in vacuum when the temperature is driven to zero. The analysis is based on first principles: it does not assume preformed atoms and takes into account in a coherent way all effects, quantum mechanical binding, ionization, and collective screening, which originate from the Coulomb potential. Our method relies on the path integral representation of the quantum Coulomb gas.

Proton acceleration with high-intensity ultrahigh-contrast laser pulses.

We report on simultaneous measurements of backward- and forward-accelerated protons spectra when an ultrahigh intensity (approximately 5 x 10(18) W/cm(20), ultrahigh contrast (>10(10)) laser pulse interacts with foils of thickness ranging from 0.08 to 105 microm. Under such conditions, free of preplasma originating from ionization of the laser-irradiated surface, we show that the maximum proton energies are proportional to the p component of the laser electric field only and not to the ponderomotive force and that the characteristics of the proton beams originating from both target sides are almost identical. All these points have been corroborated by extensive 1D and 2D particle-in-cell simulations showing a very good agreement with the experimental data.

Protein film voltammetry of copper-containing nitrite reductase reveals reversible inactivation.

The Cu-containing nitrite reductase from Alcaligenes faecalis S-6 catalyzes the one-electron reduction of nitrite to nitric oxide (NO). Electrons enter the enzyme at the so-called type-1 Cu site and are then transferred internally to the catalytic type-2 Cu site. Protein film voltammetry experiments were carried out to obtain detailed information about the catalytic cycle. The homotrimeric structure of the enzyme is reflected in a distribution of the heterogeneous electron-transfer rates around three main values. Otherwise, the properties and the mode of operation of the enzyme when it is adsorbed as a film on a pyrolytic graphite electrode are essentially unchanged compared to those of the free enzyme in solution. It was established that the reduced type-2 site exists in either an active or an inactive conformation with an interconversion rate of approximately 0.1 s(-1). The random sequential mechanism comprises two routes, one in which the type-2 site is reduced first and subsequently binds nitrite, which is then converted into NO, and another in which the oxidized type-2 site binds nitrite and then accepts an electron to produce NO. At high nitrite concentration, the second route prevails and internal electron transfer is rate-limiting. The midpoint potentials of both sites could be established under catalytic conditions. Binding of nitrite to the type-2 site does not affect the midpoint potential of the type-1 site, thereby excluding cooperativity between the two sites.

Thermal stability effects of removing the type-2 copper ligand His306 at the interface of nitrite reductase subunits.

Nitrite reductase (NiR) is a highly stable trimeric protein, which denatures via an intermediate, N(3)<--(k)-->U(3)--(k)-->F (N-native, U-unfolded and F-final). To understand the role of interfacial residues on protein stability, a type-2 copper site ligand, His306, has been mutated to an alanine. The characterization of the native state of the mutated protein highlights that this mutation prevents copper ions from binding to the type-2 site and eliminates catalytic activity. No significant alteration of the geometry of the type-1 site is observed. Study of the thermal denaturation of this His306Ala NiR variant by differential scanning calorimetry shows an endothermic irreversible profile, with maximum heat absorption at T (max) approximately equal to 85 degrees C, i.e., 15 degrees C lower than the corresponding value found for wild-type protein. The reduction of the protein thermal stability induced by the His306Ala replacement was also shown by optical spectroscopy. The denaturation pathway of the variant is compatible with the kinetic model N(3)--(k)-->F(3), where the protein irreversibly passes from the native to the final state. No evidence of subunits' dissociation has been found within the unfolding process. The results show that the type-2 copper sites, situated at the interface of two monomers, significantly contribute to both the stability and the denaturation mechanism of NiR.

Effect of the methionine ligand on the reorganization energy of the type-1 copper site of nitrite reductase.

Copper-containing nitrite reductase harbors a type-1 and a type-2 Cu site. The former acts as the electron acceptor site of the enzyme, and the latter is the site of catalytic action. The effect of the methionine ligand on the reorganization energy of the type-1 site was explored by studying the electron-transfer kinetics between NiR (wild type (wt) and the variants Met150Gly and Met150Thr) with Fe(II)EDTA and Fe(II)HEDTA. The mutations increased the reorganization energy by 0.3 eV (30 kJ mol-1). A similar increase was found from pulse radiolysis experiments on the wt NIR and three variants (Met150Gly, Met150His, and Met150Thr). Binding of the nearby Met62 to the type-1 Cu site in Met150Gly (under influence of an allosteric effector) lowered the reorganization energy back to approximately the wt value. According to XRD data the structure of the reduced type-1 site in Met150Gly NiR in the presence of an allosteric effector is similar to that in the reduced wt NiR (solved to 1.85 A), compatible with the similarity in reorganization energy.

Protonation of a histidine copper ligand in fern plastocyanin.

Plastocyanin is a small blue copper protein that shuttles electrons as part of the photosynthetic redox chain. Its redox behavior is changed at low pH as a result of protonation of the solvent-exposed copper-coordinating histidine. Protonation and subsequent redox inactivation could have a role in the down regulation of photosynthesis. As opposed to plastocyanin from other sources, in fern plastocyanin His90 protonation at low pH has been reported not to occur. Two possible reasons for that have been proposed: pi-pi stacking between Phe12 and His90 and lack of a hydrogen bond with the backbone oxygen of Gly36. We have produced this fern plastocyanin recombinantly and examined the properties of wild-type protein and mutants Phe12Leu, Gly36Pro, and the double mutant with NMR spectroscopy, X-ray crystallography, and cyclic voltammetry. The results demonstrate that, contrary to earlier reports, protonation of His90 in the wild-type protein does occur in solution with a pKa of 4.4 (+/-0.1). Neither the single mutants nor the double mutant exhibit a change in protonation behavior, indicating that the suggested interactions have no influence. The crystal structure at low pH of the Gly36Pro variant does not show His90 protonation, similar to what was found for the wild-type protein. The structure suggests that movement of the imidazole ring is hindered by crystal contacts. This study illustrates a significant difference between results obtained in solution by NMR and by crystallography.