Historical research guide · 0960 · 2012
Extremely high-intensity laser interactions with fundamental quantum systems
Antonino Di Piazza · Carsten Müller · Karen Z. Hatsagortsyan · Christoph H. Keitel
In one minute
Antonino Di Piazza, Carsten Müller, Karen Hatsagortsyan and Christoph Keitel, all then at the Max Planck Institute for Nuclear Physics in Heidelberg, wrote the standard survey of what happens when light is made strong enough to act on more than matter. Ordinary laser physics asks how atoms respond to a light wave. Above roughly 10 to the 22 watts per square centimetre — an intensity first reached at Michigan in 2008 — the questions change. The light begins to act on the electron’s own recoil from its radiation, on the vacuum itself, and on the mass of the particles it can conjure out of empty space. The review walks through each. It explains why an intense field gives empty space a refractive index and makes it birefringent, as stress does to glass; how two beams of light can scatter off one another with no matter anywhere; how a strong enough field turns photons into electron-positron pairs; and how, past some intensity, those pairs breed more pairs in an avalanche that may cap how bright a laser can ever be.
Connection
Chapter 2 treats the vacuum as a real medium you can push on, and this is the review that says exactly how hard you have to push before it answers, in watts, and with which instrument you would see the answer. Chapter 4 needs the vacuum’s optical properties to be adjustable, because a field-dependent refractive index is what the polarizable-vacuum picture of metric engineering asks for — and here that adjustment is written down, in standard quantum electrodynamics, and costed.
From the source
Past the relativistic threshold, a laser stops being something you do to matter and becomes an instrument for probing the vacuum. The authors set their subject at optical intensities above 10 to the 21 watts per square centimetre, well beyond the 10 to the 17 or 18 mark at which an electron already reaches relativistic speed within a single laser period and the magnetic part of the Lorentz force grows as large as the electric part. The record achieved intensity they report is 2 times 10 to the 22 watts per square centimetre, reached in 2008 when the HERCULES laser at the University of Michigan was upgraded to 300 terawatts and focused to a spot about 1.3 micrometres across. At those fields the recoil an electron feels from its own emitted radiation stops being a small correction and starts dominating its motion, which is why the review devotes a whole section to the century-old radiation-reaction problem.Section I, Introduction, paragraphs 3 to 5; Section II.A, first paragraphs; Section VI, opening
An electromagnetic field gives the vacuum a refractive index, and the index depends on which way the light is polarised. Because photons interact with one another through virtual electron-positron pairs, the vacuum carries an effective Lagrangian — the Euler-Heisenberg Lagrangian of 1936 — from which a polarisation and a magnetisation of empty space follow directly. The consequence the authors draw is stated plainly: the presence of an electromagnetic field in the vacuum alters the vacuum’s refractive index, and because the background field picks out a direction, the polarised vacuum behaves as a birefringent medium. Two independent probe polarisations travel at two slightly different speeds, in the ratio four to seven, both a little slower than the field-free speed of light. This is the laboratory statement of a vacuum whose optical properties are set by the field it contains.Section VII.A.2, equations 35 to 38; the Lagrangian itself is equation 32 in Section VII.A
Light scattering off light, with no matter present anywhere, has not yet been observed — and the review collects the laser experiments designed to catch it. The cross section is minute and climbs as the sixth power of the photon energy, which is why the process has escaped detection. Three schemes are costed. A three-beam laser-assisted geometry on the Astra Gemini system at the Central Laser Facility in the United Kingdom is predicted to yield about 0.07 scattered photons per shot, roughly one every fifteen shots. A double slit built only out of light — a probe pulse colliding head-on with two parallel strong beams — is predicted to put about four photons per shot into a genuine interference pattern at a strong-field intensity of 5 times 10 to the 24 watts per square centimetre. And a Bragg stack of ten beams at 2.3 times 10 to the 23 watts per square centimetre is predicted to give about five scattered photons per shot.Section VII.A.1, equations 30, 33 and 34; Figures 13, 14 and 15
The vacuum-birefringence measurement is within a factor of a hundred of being routine. Send a linearly polarised x-ray probe along a strong optical standing wave and it comes out elliptically polarised. For an ELI-class beam at 10 to the 25 watts per square centimetre and a probe wavelength of 0.1 nanometres, the predicted ellipticity is of order 10 to the minus 7 — while x-ray polarimetry demonstrated in 2011 already measures ellipticities of order 10 to the minus 9 at 0.2 nanometres. The authors also warn that tight focusing shrinks the interaction region enough that diffraction matters: in some geometries diffraction cuts the predicted ellipticity by an order of magnitude and rotates the axis of the polarisation ellipse, so the simple refractive-index calculation is not the whole answer.Section VII.A.2, equation 39 and the following four paragraphs
Turning light into matter has been done once, and the numbers say why only once. The E-144 experiment at SLAC remains the only measurement of laser-driven multiphoton electron-positron pair production: about 100 positrons detected in 22,000 shots, each shot colliding roughly 10 million electrons of 46.6 GeV with a laser at 1.3 times 10 to the 18 watts per square centimetre. Energy and momentum conservation forbid a single plane wave from making pairs out of vacuum however intense it is, because all its photons travel the same way, so a second source of energy is always needed — a high-energy photon, a nucleus, or a counterpropagating beam. The critical field at which a constant electric field would supply a pair its whole rest energy across one Compton wavelength is about 1.3 times 10 to the 16 volts per centimetre, corresponding to an intensity of about 4.6 times 10 to the 29 watts per square centimetre.Section VIII, opening and Section VIII.A; Section IX, first paragraph; List of frequently-used symbols
What to watch: the avalanche that may put a ceiling on laser intensity, and the single laboratory that could test any of this. Bell and Kirk showed in 2008 that one electron initially at rest in the standing wave of two counterpropagating circularly polarised beams can prime a cascade — photons making pairs, pairs radiating photons — at around 10 to the 24 watts per square centimetre, five orders of magnitude below the critical intensity, whereas in a single plane wave the same electron would need the critical field itself. Because a cascade converts the field into particles, its onset is expected to set the maximum attainable laser intensity. The authors close by naming the real obstacle to the whole programme: most of these processes need a high-energy particle beam and an ultra-intense laser in the same building, and compact laser-plasma accelerators are the most promising route to putting them there.Section IX, first three paragraphs; Section XIV, Conclusion and outlook, paragraphs 4 and 7
The way in
https://doi.org/10.1103/revmodphys.84.1177How to cite it
Antonino Di Piazza, Carsten Müller, Karen Z. Hatsagortsyan, Christoph H. Keitel (2012) Extremely high-intensity laser interactions with fundamental quantum systems doi:10.1103/revmodphys.84.1177