Shahab F Khan

Explosive supersaturated amplification on 3d→2p Xe(L) hollow atom transitions at λ ∼ 2.7-2.9 Å

The Xe(L) system at ? ~ 2.9 ? uniformly exhibits all of the canonical attributes of a strongly saturated amplifier on the full ensemble of single-vacancy Xeq+ transition arrays (q = 31, 32, 34, 35, 36) that exhibit gain. The key observables are (1) sharp spectral narrowing, (2) the detection of a narrow directed beam (??x200 ?rad), (3) an increase in the amplitude of the emission and the development of an intense output (?106 enhancement) and (4) the observation of deep spectral hole-burning on the inhomogeneously broadened spontaneous emission profile. Experimentally determined by two methods, (a) line narrowing and (b) signal enhancement, the observations for several single-vacancy 3d?2p transitions indicate a range of values for the effective small signal (linear) gain constant given by go25?100 cm?1. Quantitative analysis shows that this result stands in clear conflict with the corresponding upper bound go40?80 cm?1 that is based on available spectroscopic data and estimated with conventional theory. Overall, the observed values deviate substantially from expectations scaled to the spectral density of the measured Xe(L) spontaneous emission profile; they are systematically too high. The most extreme example is the heavily saturated Xe32+ transition at ? = 2.71 ?, a case that fails to reconcile the lower bound of the measured signal strength with the corresponding theoretically predicted maximal value; the former falls above the latter by a factor exceeding 400 giving an enormous gap. Moreover, although saturation is a prominent characteristic of the amplification at ?2.71 ?, as demonstrated by spectral hole-burning, the theoretical upper bound of go given for this transition is far too small for saturation to be reached. The Xe31+ transition at ?2.93 ? exhibits comparably pronounced anomalous behaviour. This double paradox is resolved with the Ansatz that the amplification is governed principally by the saturated gain gs, not the conventionally described small signal value go. This interpretation is further supported by the observation of deep spectral hole-burning, the signature of strong saturation, that occurs uniformly across the spectrum of the spontaneous emission profile. The effective amplification exhibits an anomalously weak dependence on the spectral density; saturation is the rule, not the exception. A lucid manifestation of the saturation is the recording of spectrally resolved x-ray yields on the Xe31+ array that are sufficiently high to produce gross structural damage to the material in the film plane of the spectrograph. The behaviour of the amplifier can be best described as an explosive supersaturated amplification. The source of this exceptionally strong amplification can be traced to the dynamically enhanced radiative response of the excited Xe hollow atom states located in the clusters that are mode coupled to the plasma waveguide forming the amplifying channel.

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.

Xe(L) Coherent X‐Ray Source at λ ∼ 2.9 Å for Biological Nanoimaging

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.

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.