Since the invention of the laser more than 50 years ago, scientists have striven to achieve amplification on atomic transitions of increasingly shorter wavelength. The introduction of X-ray free-electron lasers makes it possible to pump new atomic X-ray lasers with ultrashort pulse duration, extreme spectral brightness and full temporal coherence. Here we describe the implementation of an X-ray laser in the kiloelectronvolt energy regime, based on atomic population inversion and driven by rapid K-shell photo-ionization using pulses from an X-ray free-electron laser. We established a population inversion of the Kα transition in singly ionized neon at 1.46 nanometres (corresponding to a photon energy of 849 electronvolts) in an elongated plasma column created by irradiation of a gas medium. We observed strong amplified spontaneous emission from the end of the excited plasma. This resulted in femtosecond-duration, high-intensity X-ray pulses of much shorter wavelength and greater brilliance than achieved with previous atomic X-ray lasers. Moreover, this scheme provides greatly increased wavelength stability, monochromaticity and improved temporal coherence by comparison with present-day X-ray free-electron lasers. The atomic X-ray lasers realized here may be useful for high-resolution spectroscopy and nonlinear X-ray studies.

https://www.engr.colostate.edu/ece//faculty/rocca/pdf/journals/rohringer_etal_2012.pdf

Atomic inner-shell X-ray laser at 1.46 nanometres pumped by an X-ray free-electron laser

We have observed and characterized 501.6 nm collective spontaneous emission (superfluorescence) following 1s(2) → 1s3p excitation of helium atoms by 53.7 nm free-electron laser radiation. Emitted pulse energies of up to 100 nJ are observed, corresponding to a photon number conversion efficiency of up to 10%. We observe the peak intensity to scale as ρ(2) and the emitted pulse width and delay to scale as ρ(-1), where ρ is the atom number density. Emitted pulses as short as 1 ps are observed, which corresponds to a rate around 75,000 times faster than the spontaneous 1s3p → 1s2s decay rate. To our knowledge, this is the first observation of superfluorescence following pumping in the extreme ultraviolet wavelength region, and extension of the technique to the generation of extreme ultraviolet and x-ray superfluorescence pulses should be straightforward by using suitable atomic systems and pump wavelengths.

Observation of free-electron-laser-induced collective spontaneous emission (superfluorescence).

We discuss possibilities of pumping an atomic inner-shell x-ray laser with an x-ray free-electron laser (XFEL). Self-consistent gain calculations show that with the first available XFEL, the Linac Coherent Light Source at Stanford, it will become possible to produce subfemtosecond x-ray pulses at intensities reaching $6\ifmmode\times\else\texttimes\fi{}{10}^{16}\text{ }\text{W}/{\text{cm}}^{2}$. Small-signal gain calculations indicate that saturation of more than one lasing line is possible, resulting in temporally separated femtosecond x-ray pulses of different wavelengths. The presented lasing scheme creates broad capability for advancing the field of high-intensity ultrashort x-ray physics.

Atomic inner-shell x-ray laser pumped by an x-ray free-electron laser

The document from the EAPC Ethics Task Force has much to commend it. It takes a reasonable stance on the way that requests for euthanasia are to be dealt with, supporting the patient with pallia tive care but not expecting palliative medicine specialists to administer euthanasia drugs. The document recognizes the different cultural and politica l settings in which medicine is practised, such as the disastrous effect on British medicine of Shipman’s murders. The recent World Health Organization modification of the definition of palliative care, as used in the document, has focused palliative care beyond routine good clinical practice. Educators in particular will welcome the clarification around withholding and withdrawing treatment, and the encouragement of open dialogue should ensure a heightened awareness of and focus on the patient’s needs. Yet it is precisely those needs that are poorly understood. The patient needs control over a situation that is by and large quite beyond controlling, i.e., when disease has shown itself non-responsive to all the interventions that have been attempted. Perhaps this fundamental need for control warrants further exploring in the patient’s terms rather than the healthcare professional’s parlance of treatments and interventions. Perhaps there are some important issues in this whole debate that have been argued from the wrong starting point. Should we be talking of euthanasia and physicianassisted suicide as two separate entities, or should we be talking of physician-assisted death only? In other words, is there any difference in ethical terms or in intent if the patient takes the lethal dose of medicine by mouth or proffers a vein for it to be injected intravenously? I think not. I would suggest that it is more accurate to talk of intravenous euthanasia or oral euthanasia; in both instances the doctor has undertaken the same discussion with the patient and in both instances has also supplied the drug to induce death. In one instance the patient lifts the glass to his or her lips, in the other the patient lifts an arm for the substance to be injected. In the former, failed induction of death orally will require the physician to intervene and to induce death with an injected dose of barbiturate and muscle-paralysing agent. One must ask oneself why so few patients commit suicide and anecdotally they do not seem to be the same cohort of patients as those who discuss euthanasia. When the patient expresses the wish of j̀ust wanting to die’ is the patient asking for something other than death? Is he or she asking to have some control, any control over fate; and the only thing that can be asked for reasonably is control over the timing of death. It may be worth for a moment considering why patients whose disease has put them metaphorically `on death row’ do not commit suicide. Many are waiting, indeed hoping, for a r̀eprieve’ through medical intervention. Yet there are many people, especially young men in Britain’s jails, who do kill themselves every year and who join the swelling ranks of suicides in the Registrar General’s tables of causes of death, listed with others whose despair was so great that they could not live any longer. Perhaps suicide is so fundamentally different because it is the ultimate in self-harm through despair and selfloathing, whereas the request for euthanasia represents a need to regain some control over one’s destiny while still having capacity; interestingly, the Dutch experience suggests that many requests for euthanasia do not result in a lethal dose of drug being administered, but in the patient feeling they could have it ìf things get too bad to bear’. All through our lives we live as if we have control over destiny; we feel that we must live with the consequences of our actions and that our decisions are therefore major self-determinants. Those who commit suicide are diametrically opposite / they feel totally worthless and valueless to the extent that life has no value and so the anger, self-loathing and low self-esteem erupt into suicide. Perhaps the EAPC ethics group should reconsider their nomenclature. They should be clear that suicide and euthanasia are driven by fundamentally different needs and should not be confused. Euthanasia is always physician-complicit even when not directly physicianadministered, so oral or intravenous euthanasia is more accurate and more honest than talking of physicianassisted suicide. Suicide is the ultimate act of self-harm; euthanasia is a desperate way to gain the last vestige of control over a situation. Euthanasia has as its final outcome the destruction of self with help. Palliative Medicine 2003; 17: 137

From the UK

Within the next few years X-ray free-electron lasers (XFELs) now under construction are expected to generate highly collimated XUV pulses with 1013 photons and a duration of 100 fs. Focusing this radiation to a spot some 10 μm in diameter generates intensities of up to 1016 W/cm2. Such pump intensities make feasible the investigation of photopumped XUV lasers using this radiation. We present simulations taking into account two different mechanisms generating the gain: (1) photoionization with subsequent three-body recombination, which takes advantage of the monochromaticity of the pump radiation to generate very cold electrons; (2) inner-shell ionization in which transient inversion is obtained by generating a hole in an otherwise completely filled shell. The simulations show that under appropriate conditions both mechanisms generate very high gain. However, a number of further issues must be considered, such as the propagation of the pump pulse in the medium to be pumped.

Photopumping of XUV lasers by XFEL radiation

The high brilliance expected from X-ray Free-Electron Lasers (XFELs) now under construction makes the proposal of a photo-pumped soft X-ray laser again feasible. We present simulations of a photo-pumped three-body recombination laser at He-α (λ = 58.4 nm) and Ly-α (λ = 30.4 nm) in helium, pumped by the XFEL at DESY/Hamburg. A low electron temperature, and thus a high recombination rate are provided by adjusting the pump photon energy closely above the ionization potential of the lasing ion. Our simulations show that pump parameters obtainable by the DESY XFEL are sufficient to generate He-α laser gain of ≈ 100 cm−1 and similar gain for the Ly-α laser. The gain duration depends critically on density and pump intensity and is between 100 and 300 fs for both lasers. A saturated photo-pumped soft X-ray laser seems to be possible by irradiating a He gas puff with XFEL pulses in a travelling-wave arrangement. The realization of such a laser could provide a better understanding of the physics of photo-pumped lasers and thus help developing table top X-ray lasers. In addition, the beam parameters of the photo-pumped X-ray laser could be superior to those of the pumping laser.

Ke Lan

Papers

Photopumping of XUV lasers by XFEL radiation

Simulation of He-α and Ly-α soft X-ray lasers in helium pumped by DESY/XFEL-radiation