[en] Comparative single-pulse studies of self-trapped plasma channel formation in Xe and Kr cluster targets produced with 1–2 TW femtosecond 248 nm pulses reveal energy efficient channel formation (>90%) and highly robust stability for the channeled propagation in both materials. Images of the channel morphology produced by Thomson scattering from the electron density and direct visualization of the Xe(M) and Kr(L) x-ray emission from radiating ions illustrate the (1) channel formation, (2) the narrow region of confined trapped propagation, (3) the abrupt termination of the channel that occurs at the point the power falls below the critical power P_cr, and, in the case of Xe channels, (4) the presence of saturated absorption of Xe(M) radiation that generates an extended peripheral zone of ionization. The measured rates for energy deposition per unit length are ∼ 1.46 J cm⁻¹ and ∼ 0.82 J cm⁻¹ for Xe and Kr targets, respectively, and the single pulse Xe(M) energy yield is estimated to be > 50 mJ, a value indicating an efficiency >20% for ∼ 1 keV x-ray production from the incident 248 nm pulse. (paper)

[en] Experimental evidence demonstrating amplification on the Kr²⁶⁺ 3s→2p transition at λ ≅ 7.5 Å (∼1652 eV) generated from a (Kr)_n cluster medium in a self-trapped plasma channel produced with 248 nm femtosecond pulses is presented. The x-ray beam produced had a spectral width of ∼3 eV and a corresponding beam diameter of ∼150 µm, properties that were simultaneously determined by a two-dimensional x-ray spectral image formed with an axially placed von Hámos spectrometer and a matching Thomson image of the spatial electron density generated by the x-ray propagation. (paper)

[en] Anomalously enhanced nonlinear electromagnetic coupling can arise from ordered driven collective motions in many electron systems. The augmented strength of the interaction can be expressed as an effective increase in the fine structure constant α in which α  → Z²α, where Z specifies the number of electrons involved in the ordered response to the external field. The present work illustrates this phenomenon in the x-ray range with the observation of the 5-photon nonlinear excitation of Xe(L)* hollow atom states that are generated by intense (∼7 × 10¹⁵ W cm⁻²) Xe(M) radiation ${\mathrm{\gamma <!--\; ??\; -->}}_{\mathrm{M}}$ at ∼1 keV. The nonlinear cross section experimentally determined for the $5{\mathrm{\gamma <!--\; ??\; -->}}_{\mathrm{M}}$ + Xe → [Xe^q+(L)]* + qe⁻ amplitude is ${\mathrm{\sigma <!--\; ??\; -->}}_5$ ∼ 2 × 10⁻²¹ cm². The matching theoretical cross section corresponds to Z = 18, an outcome indicating the participation of the full Xe(4d¹⁰5s²5p⁶) supershell, a dynamic feature of Xe that also plays a significant role in the linear photoionization of neutral Xe atoms in the kilovolt region. For the high-intensity 5γ nonlinear coupling, the outcome for the Xe(L)* hollow atom excitation is an enhancement of the strength of the interaction by a factor of ∼10¹² and, with Z²α > 1, a fundamentally new region of strong coupling is entered. The experimental value of ${\mathrm{\sigma <!--\; ??\; -->}}_5$ is likewise shown to be in very good accord with an earlier analysis that estimated the upper bound of cross sections for high-order multi-photon cross sections in the combined high-Z and high-intensity limit. These results forecast the general presence of comparably enhanced coupling strengths in the interaction of sufficiently intense (I ≥ 7 × 10¹⁵ W cm⁻²) x-rays with high-Z atoms and molecules. (paper)

[en] The achievement of controlled saturation with high spectral resolution (∼2–3 eV) provides a refined diagnostic for the quantitative study of the propagation of intense beams of x-rays. This work demonstrates the characteristics of saturation spectroscopy with an analysis of Kr(L) autoionizing transitions at ħω = 1652 eV. The data include single-pulse x-ray spatial morphologies, corresponding Thomson images of the electron density, and spatially resolved transversely observed x-ray spectra that are recorded longitudinally along the direction of propagation of a Kr²⁶⁺ (λ₂₆ = 7.504 Å) beam in a Kr _n cluster medium. The results reveal the complex interactions associated with the propagation of the 7.504 Å x-ray beam and, based on sets of quantitative criteria, attribute the observed behavior to the saturation of 2p → 3d transitions to autoionizing states of Kr¹⁵⁺ and Kr¹⁶⁺ ionic species. (paper)

[en] The Xe(L) system is an amplifier with fundamentally different dynamic characteristics from all previously developed laser amplifiers; it represents the conceptual ideal through full utilization of the Kramers-Kronig relations that fundamentally couple the dispersive and absorptive components. The dispersive response of the system, through optimal governance of the power compression, rules the amplification and establishes a minimum gain for the amplifier. Accordingly, the amplification requires a minimum value of the dispersion to be surpassed; the corresponding gain follows automatically. As a leading consequence, since this minimum gain is sufficiently high, the key experimental observation is the uniform presence of saturated amplification signaled by strong spectral hole burning on all transitions exhibiting amplification, including double-vacancy lines. This cardinal signature demonstrates that the amplification is legislated by the saturated gain g_s, not the corresponding small signal value g₀. The chief outcome is that explosive dispersion yields perforce explosive amplification and the efficient generation of maximally bright coherent power

[en] The Xe(L) system at λ ∼ 2.9 A has demonstrated a peak brightness sufficient for high resolution imaging of living matter at the molecular scale

[en] Single-pulse and time-integrated spectral measurements of the characteristics of the Xe(L) amplifier at λ ∼ 2.8 A indicate an efficiency of energy extraction of ∼30% over a bandwidth of ∼500 eV. These observations, together with data from prior studies, provide a basis for estimating a corresponding set of scaling limits for a laboratory sized ∼4.5 keV Xe(L) system. Specifically, they are a peak power P_x ∼ 6.0 PW, an unfocused peak intensity I_x ∼ 3.4 x 10²¹ W cm^-2, peak brightness figures corresponding to B ∼ 4.1 x 10³⁴ photons s^-1 mm^-2 mrad^-2 (0.1% bandwidth)^-1 and P_x/λ² ∼ 7.6 x 10³⁰ W cm^-2 sr^-1, and an x-ray pulse length τ_x ∼ 5-10 as.

[en] The optimization of relativistic and ponderomotive self-channeling of ultra-powerful 248 nm laser pulses launched in underdense plasmas with an appropriate longitudinal gradient in the electron density profile located at the initial stage of the self-channeling leads to (1) stable channel formation and (2) highly efficient power compression producing power densities in the 10¹⁹-10²⁰ W/cm³ range. The comparison of theoretical studies with experimental results involving the correlation of (a) Thomson images of the electron density with (b) x-ray images of the channel morphology demonstrates that more than 90% of the incident 248 nm power can be trapped in stable channels and that this stable propagation can be extended to power levels significantly exceeding the critical power of the self-channeling process.

[en] The analysis of spatially resolved Xe(L) spectra obtained with Z-λ imaging reveals two prominent findings concerning the characteristics of the x-ray amplification occurring in self-trapped plasma channels formed by the focusing of multi-TW subpicosecond 248 nm laser pulses into a high-density gaseous Xe cluster target. They are (1) strongly saturated amplification across both lobes of the Xe(L) hollow atom 3d → 2p emission profile, a breadth that spans a spectral width of ∼600 eV, and (2) new evidence for the formation of x-ray spatial modes based on the signature of the transversely observed emission from the narrow trapped zone of the channel. The global characteristics of the spectral measurements, in concert with prior analyses of the strength of the amplification, indicate that the enhancement of the x-ray emission rate by intra-cluster superradiant dynamics plays a leading role in the amplification. This radiative interaction simultaneously promotes (a) a sharp boost in the effective gain, (b) the directly consequent efficient production of coherent Xe(L) x-rays from both single (2p-bar) and double (2s-bar2p-bar) vacancy 3d → 2p transition arrays, estimated herein at ∼30%, and (c) the development of a very short x-ray pulse width τ_x. In the limit of sufficiently strong superradiant coupling in the cluster, the system assumes a dynamically collective character and acts as a single homogeneously broadened transition whose effective radiative width approaches the full Xe(L) bandwidth, a breadth that establishes a potential lower limit of τ_x ∼5-10 as, a value substantially less than the canonical atomic time a_o/αc ≅ 24 as.

[en] The trajectory of discovery associated with the study of high-intensity nonlinear radiative interactions with matter and corresponding nonlinear modes of electromagnetic propagation through material that have been conducted over the last 50 years can be presented as a landscape in the intensity/quantum energy [I-ħω] plane. Based on an extensive series of experimental and theoretical findings, a universal zone of anomalous enhanced electromagnetic coupling, designated as the fundamental nonlinear domain, can be defined. Since the lower boundaries of this region for all atomic matter correspond to ħω ∼ 10"3 eV and I  ≈  10"1"6 W cm"−"2, it heralds a future dominated by x-ray and γ-ray studies of all phases of matter including nuclear states. The augmented strength of the interaction with materials can be generally expressed as an increase in the basic electromagnetic coupling constant in which the fine structure constant α  →  Z "2 α, where Z denotes the number of electrons participating in an ordered response to the driving field. Since radiative conditions strongly favoring the development of this enhanced electromagnetic coupling are readily produced in self-trapped plasma channels, the processes associated with the generation of nonlinear interactions with materials stand in natural alliance with the nonlinear mechanisms that induce confined propagation. An experimental example involving the Xe (4d"1"05s"25p"6) supershell for which Z  ≅  18 that falls in the specified anomalous nonlinear domain is described. This yields an effective coupling constant of Z "2 α  ≅  2.4  >  1, a magnitude comparable to the strong interaction and a value rendering as useless conventional perturbative analyses founded on an expansion in powers of α. This enhancement can be quantitatively understood as a direct consequence of the dominant role played by coherently driven multiply-excited states in the dynamics of the coupling. It is also conclusively demonstrated by an abundance of data that the utterly peerless champion of the experimental campaign leading to the definition of the fundamental nonlinear domain was excimer laser technology. The basis of this unique role was the ability to satisfy simultaneously a triplet (ω, I, P) of conditions stating the minimal values of the frequency ω, intensity I, and the power P necessary to enable the key physical processes to be experimentally observed and controllably combined. The historical confluence of these developments creates a solid foundation for the prediction of future advances in the fundamental understanding of ultra-high power density states of matter. The atomic findings graciously generalize to the composition of a nuclear stanza expressing the accessibility of the nuclear domain. With this basis serving as the launch platform, a cadenza of three grand challenge problems representing both new materials and new interactions is presented for future solution; they are (1) the performance of an experimental probe of the properties of the vacuum state associated with the dark energy at an intensity approaching the Schwinger/Heisenberg limit, (2) the attainment of amplification in the γ-ray region (∼1 MeV) and the discovery of a nuclear excimer, and (3) the determination of a path to the projected super-heavy nuclear island of stability. (review)