A dense sheet of electrons accelerated to close to the speed of light can act as a tunable mirror that generates bursts of laserlike radiation in the extreme ultraviolet range via reflection. The findings could pave the way for new methods of intense, attosecond pulse generation. Reflections of light are typically observed from surfaces that are at rest, such as the reflection from a piece of glass or a smooth surface of water. But, what happens to reflections from a mirror moving at an incredibly fast rate close to the speed of light? This question was answered by Albert Einstein’s theory of special relativity more than a century ago, and now an international team of researchers has investigated that query in an experiment. Using a laser pulse, physicists from Max-Planck Institute of Quantum Optics (MPQ), Ludwig Maximilians University (LMU) of Munich in Germany, Queens University Belfast in Northern Ireland and Rutherford Appleton Laboratory (RAL) near Oxford, England, created a “relativistic mirror” to carry out a Gedankenexperiment (thought experiment) formulated in 1905 by Einstein stating that the reflection from a mirror moving close to the speed of light could in principle result in bright light pulses in the short wavelength range. An international team of scientists has generated flashes of extreme ultraviolet radiation via the reflection from a mirror that moves close to the speed of light. A laser pulse (red, bottom), liberates electrons (green) from the carbon atoms of a nanometer-thin foil and accelerates them to close to the speed of light. An infrared light pulse impinges on the electron layer from the opposite direction and reflects off the electron mirror as a light burst in the EUV with a duration of only a few hundred attoseconds. Courtesy of Thorsten Naeser. In the experiment, conducted at RAL, the investigators irradiated a nanometer-thin, freestanding foil with a 50-fs ultraintense laser pulse. The impinging laser pulse liberated electrons from the carbon atoms of the foil and rapidly accelerated to close to the speed of light in less than 1 μm, forming a dense sheet of electrons capable of acting as a mirror. “This mirror structure is stable for only a few femtoseconds,” said Daniel Kiefer of MPQ and LMU, who wrote his dissertation on the topic. Within this extremely short lifetime, the scientists shot a secondary laser pulse with a wavelength in the near-infrared and a pulse duration of several femtoseconds from the opposite direction on the generated relativistic mirror structure. In contrast to a mirror at rest, light reflected from a moving mirror changes color as the reflected photons gain momentum from the mirror. This process is similar to a ball bouncing off a racket and accelerating to higher speeds. However, instead of moving faster, the reflected light shifts in its frequency — a phenomenon similar to the Doppler effect observed from an ambulance siren, whose volume depends on whether the ambulance is moving toward or away from the observer. The high velocity of the electron mirror in the experiment gave rise to a change in frequency upon reflection from the near-infrared to the extreme ultraviolet up to a wavelength of 60 to 80 nm, and the reflected pulses’ time duration was on the order of only a few hundred attoseconds. The findings could offer a new technique for generating intense, attosecond flashes of light, which would enable the electron motion of atoms to be resolved and provide insight into elementary processes in nature, which have been left unexplored. “Our laser systems will advance in the future, delivering even more powerful pulses with higher repetition rate and shorter pulse duration,” said Dr. Jörg Schreiber, a professor at LMU. “This scheme will benefit strongly from those developments in laser technology and, thus, may enable the generation of laserlike radiation with even higher intensity and shorter wavelengths — ideal to explore the microcosm.” Results of the study appeared in Nature Communications (doi: 10.1038/ncomms2775). For more information, visit: www.mpq.mpg.de