Shaking the Fundamentals of Physics: At the Limits of the Photoelectric Effect

By way of the classical photoeffect, Einstein proved in 1905 that light also has particle character. However, with extremely high light intensities, remarkable things happen in the process. Scientists of the Physikalisch-Technische Bundesanstalt (Germany) have found this out with colleagues at FLASH in Hamburg, the first free-electron laser (FEL) for soft X-rays worldwide.

The current models based on Einstein's idea are simply described in such a way: A photon knocks an external electron out of an atom, provided that the photon energy is high enough. However, with wavelengths of only 13 nanometers and high radiation intensities of several petawatt per square centimeter something else - at least with some atoms - happens: With xenon, a whole light-wave packet immediately seems to knock out a huge number of internal electrons. This effect is strongly dependent on the material and not only on the characteristics of the exciting radiation, as accepted before. The work, which is currently published in the journal Physical Review Letters, has significance for future experiments of materials research at the new large X-ray laser facilities of the world.

The scientists actually wanted to develop methods for the radiometric characterization of X-ray lasers. They irradiated different gases to derive the laser strength from the ionization effect. The aim: with the laser well characterized was, for example, the testing of EUV lithography mirrors. The EUV lithography (EUV stands for extreme ultraviolet) at wavelengths in the range of 13 nanometers is considered as the future technology for the production of ever smaller computer chips.

However, during their experiments at FLASH, the new free-electron laser (FEL) in Hamburg, which currently allows the generation of EUV radiation and soft X-rays of the highest intensity in the world, they unexpectedly discovered things which concern the fundamentals of physics.

With the classical photoelectric effect (a), a single light particle (photon) of sufficient energy interacts with a single electron of the material. The process is energetically described by the Einstein equation (1905) and demonstrates the quantum structure of light. Only at very high intensities, does the multiphoton ionization occur, a process which is described in the extreme case of highly intensive ultra-short light flashes as emitted by long-wave femtosecond lasers, again, in the wave picture of light (b).

Nevertheless, the suitable theoretical models fail in the short-wave X-ray regime as shown by the experiments in Hamburg in which, for the first time, soft X-ray irradiance levels of several petawatts per square centimeter were achieved by strong beam focusing. The comparative quantitative studies prove that the degree of light-matter interaction and, thereby, the nature of the X-ray light are decisively determined by the structure of the atom and correlations in, above all, inner electron shells.

In the extreme case (xenon), a whole wave packet of photons seems to lead to the simultaneous emission of several inner electrons (c).

More information: Extreme ultraviolet laser excites atomic giant resonance. M. Richter et al., Phys. Rev. Lett. (2009) - online publication expected: April 27, 2009.

Photoelectric effect at ultra-high intensities. A. A. Sorokin et al., Phys. Rev. Lett. 99, 213002 (2007)

Source: Physikalisch-Technische Bundesanstalt

Curved light bends the rules

Everyone knows that light travels in a straight line — right? A couple of years back, however, physicists discovered something very different for certain laser pulses that have one intense peak next to a series of smaller peaks. The brightest part of these lopsided "Airy" pulses, they found, appear to follow a curved trajectory.

Researchers in the US have now found that sufficiently intense Airy pulses can ionize the surrounding air molecules and create curved filaments of plasma. What's more, Airy pulses interact with air such that the pulses are continually focused and so can travel long distances without being dispersed.

The bright white light given off by the plasma filaments could be used make remote spectroscopic measurements of the atmosphere — and the bending effect itself could be exploited in new kinds of waveguide.

The bendy behaviour of Airy pulses was first discovered in 2007 by Demetrios Christodoulides and colleagues at the University of Florida. Interference between the peaks causes the intense peak to veer off in one direction, while the other peaks move in the opposite direction. Although the total momentum of the pulse travels in a straight line, its brightest part appears to follow a curved path.

Christodoulides and his colleagues have now teamed up with Pavel Polynkin and others at the University of Arizona to create curved “filaments” of plasma using Airy pulses. The key to their success, according to Jerome Kasparian of the University of Geneva who was not part of the group, is their ability to — for the first time — create Airy pulses of extremely high intensity.

The team began with an intense infrared laser pulse that is about 35 fs in duration. The initially pancake-shaped pulse, which is symmetric around its direction of propagation, is then passed through a “phase mask” and then a lens, giving it a chevron shape with an intense peak at the vertex (see figure). This Airy pulse then travels about 1 m through air to a fluorescent screen where the light is detected.

As well as confirming that extremely intense Airy pulses appear to curve, the pulses also produced curved filaments of plasma by ionizing nearby molecules in the air.

Although physicists have long known that symmetric laser pulses can create such filaments, the process has proved very difficult to study. This is because symmetric laser pulses travel in the same direction as the white light given off by the plasmas they create, which means that any device that attempts to detect this light is dazzled or even destroyed by the pulse.

With Airy pulses, however, Polynkin, Christodoulides and colleagues discovered that the plasma light travels in straight lines tangentially to the curvature of the bright peak. The plasma light can therefore be detected — and perhaps even be used as a source of white light for spectroscopy.

Firing intense and long-range pulses into the air, for example, could allow researchers to make remote spectroscopic measurements of the atmosphere.

Polynkin also speculates that intense pulses could be fired into thunderclouds to create filaments that "guide" lightning to safe locations on the ground.

Studying the plasma light itself could even help physicists gain a better understanding of the complicated non-linear optics that define how intense laser beams travel through air. These include a “self-healing” effect whereby the beam is continually refocused by the plasma — rather than being dispersed — allowing intense pulses to travel very long distances.

The team are now studying the creation of curved filaments in water rather than air.

Soliton laser offers broad tunability

A femtosecond soliton source with fast and broad spectral tunability has been developed by researchers in Argentina. The source, which comprises a Ti:sapphire laser and a highly nonlinear photonic-crystal fibre, can be tuned from 850 nm to 1000 nm with nearly constant pulse width and average power (Optics Letters 34 842).

The key to the laser's tuning performance is the use of solitons generated in the photonic-crystal fibre. At the low-power coupling regime, solitons can be tuned over a broad range of wavelengths from 850 to 1000 nm. The solitons generated in the fibre maintain almost constant pulse and spectral widths regardless of input power.

In the set-up, a photonic-crystal fibre measuring 75 cm in length is pumped with a Ti:sapphire laser that provides 37 fs pulses at a repetition rate of 94 MHz and a wavelength of 830 nm. Average power ranging from 1 to 10 mW is pumped into the fibre, controlled by an acousto-optic modulator (AOM).