Sankar Poopalasingam

Realization of the conceptual ideal for x-ray amplification

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 gs, 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.

Double optimization of Xe(L) amplifier power scaling at λ ~ 2.9 Å

The spectral and spatial characteristics of the Xe(L) amplifier at ? ~ 2.9 ? determine an optimum for the scaling of the peak power with channel length. The Xe31+ and Xe32+ (3d ? 2p) transition arrays represent two identical spectral optima for amplification, a property stemming from the extremum of spectral components (3245) characteristic of their electron configurations. Adroit matching of the spatial distribution of the intensity characteristic of the propagating 248 nm pulse dynamically generating the self-trapped plasma channel with the intensity required to excite selectively and efficiently the Xe31+ and Xe32+ arrays can also simultaneously maximize the spatial volume of the excitation. The net outcome of this double maximization is an amplifying channel for the optimal transitions that possesses high gain (~100 cm?1), low losses (<10?1cm?1) and a diameter of 15?20 ?m, a size sufficient to produce an x-ray pulse energy of ~50?100 mJ from a channel having an average xenon density of ~1020 cm?3 and a length of 1 cm. Since previous studies have experimentally demonstrated the ability to produce a saturated bandwidth of ~60 eV, a magnitude sufficient to support a pulse duration of ~30 as, peak powers Px 1 PW are clearly within the scaling limits of the Xe(L) system. The corresponding peak brightness scaling limit is accordingly bounded from below by Px/?2 1030 W cm?2 sr?1.

Single-pulse characteristics of the Xe(L) amplifier on the Xe35+ (3d→2p) transition array at λ ≅ 2.86 Å

The triple comparison of (1) single-pulse spectral data, recorded with a CCD-equipped von H?mos spectrometer both axially and transversely; (2) axially measured time-integrated spectra registered on a film and (3) single-pulse x-ray images of the morphology of the self-trapped plasma channel, recorded simultaneously with the single-pulse spectra, establishes several leading characteristics of the saturated amplification observed on the Xe35+ transition array at ? 2.86 ?. The chief findings are (?) absolute positive correlation of amplification with the formation of a plasma channel, (?) a perfect spectral match of the amplified transitions in the comparison of axially recorded single-pulse and time-integrated film data and (?) exact spectral correspondence of both the axially registered single-pulse and time-integrated film data with single-pulse transversely measured spectra exhibiting deep spectral hole burning at the position of the Xe35+ array.

Temperature enhancement of Xe(L) x-ray amplifier (λ ∼ 2.9 Å) emission

Cooling of the xenon nozzle flow to T = 230 K produces three leading effects. They are (1) a ~2.5-fold enhancement of the Xe(L) hollow atom emission on the single-vacancy 3d ? 2p charge state arrays, (2) the production of amplifying self-trapped plasma channels with significantly enhanced lengths and (3) very sharply augmented emission on () Xe(L) double-vacancy transitions in the ? 2.80 ? region.

Power scaling of the Xe(L) amplifier at λ~ 2.8 Å into the petawatt regime

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 Px ~ 6.0 PW, an unfocused peak intensity Ix ~ 3.4 × 1021 W cm−2, peak brightness figures corresponding to B ~ 4.1 × 1034 photons s−1 mm−2 mrad−2 (0.1% bandwidth)−1 and Px/λ2 ~ 7.6 × 1030 W cm−2 sr−1, and an x-ray pulse length τx ~ 5–10 as.

Spatially resolved observation of the spectral hole burning in the Xe(L) amplifier on single and double vacancy 3d → 2p transitions in the 2.62 Å < λ < 2.94 Å range

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 and double vacancy 3d → 2p transition arrays, estimated herein at ~30%, and (c) the development of a very short x-ray pulse width τx. 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 τx ~5–10 as, a value substantially less than the canonical atomic time ao/αc 24 as.

The Nuclear Epoch of Laser Interactions

The history of power compression is a series of developmental epochs that are (1) characteristically marked by a technological breakthrough and (2) generally separated by a factor of ∼1010 in power density. Based on new advances in high‐power coherent x‐ray technology, the transition to a new nuclear epoch of laser interactions is presently commencing. Chief outcomes foreseen are (1) the generation of power densities in the 1028–1030 W/cm3 realm, (2) the controlled induction of nuclear interactions, and (3) the production of new states and forms of nuclear matter.

Rewriting the rules governing high intensity interactions of light with matter

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(16) 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(10)5s(2)5p(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.

Observation of Nonlinear Optical Coupling in the Kiloelectronvolt X-Ray Regime

Experimental findings with Xe(M) radiation in the ~1 keV X-ray region have confirmed the presence of a predicted zone of anomalously strengthened radiative coupling operative at sufficiently high intensity (I >; 1015 W/cm2) and frequency (hω >; 5 eV). These new results herald the general existence of a strongly enhanced modality of radiative interaction that is based on ordered-driven electron motions in the attosecond regime.

Enhanced nonlinear coupling in the keV x-ray range: Xe(L) hollow atom excitation with Xe(M) radiation at ℏω ≅ 1 keV

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 ? in which ?? ??Z2?, 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 at ?1 keV. The nonlinear cross section experimentally determined for the ?+?Xe???[Xeq+(L)]*?+?qe? amplitude is ???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 5? 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 Z2??>?1, a fundamentally new region of strong coupling is entered. The experimental value of 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.