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An attosecond is one quintillionth (10-18) of a second and is a term used in photon research. For comparison, a millisecond (ms or msec) is one thousandth of a second and is commonly used in measuring the time to read to or write from a hard disk or a CD-ROM player or to measure packet travel time on the Internet. A microsecond (us or Greek letter mu plus s) is one millionth (10-6) of a second. A nanosecond (ns or nsec) is one billionth (10-9) of a second and is a common measurement of read or write access time to random access memory (RAM). A picosecond is one trillionth (10-12) of a second, or one millionth of a microsecond. A femtosecond is one millionth of a nanosecond or 10-15 of a second and is a measurement sometimes used in laser technology Another Defination is- It's 10-18 second or just a billionth of a billionth of a second. This article from DISCOVER says that researchers reached this new limit in time using high-energy laser pulses. Here are some excerpts. Recently, an international team of physicists finally succeeded in breaking the so-called femtosecond barrier. With a complex, high-energy laser, they generated a pulse of light little more than half a femtosecond long -- 650 attoseconds, to be precise.
"This is the real timescale of matter," says Paul Corkum, a physicist with the Steacie Institute for Molecular Sciences in Ottawa and one of the principal investigators in the study. "We're gaining the ability to look at the microworld of atoms and molecules on its own terms."
What are the potential usages of this new unit? The femtosecond is great for handling whole atoms and molecules. But for the physicist interested in electrons, which are far smaller, lighter, and faster than the atomic nuclei they swarm around, that timescale is just too slow. "We're interested in taking this a step further," says Ferenc Krausz, a principal investigator of the study and a physicist at the Photonics Institute at the Vienna University of Technology.
No sooner had the physicists caught an attosecond pulse than they demonstrated its usefulness. They aimed an attosecond pulse and a longer pulse of red light into a gas of krypton atoms. The attosecond pulse excited the krypton atoms, kicking electrons free; then the red-light pulse hit the electrons and took a reading of their energy. By adjusting the time delay between the two pulses, the scientists gained a very precise measurement-within a matter of attoseconds-of how long it takes the electron to decay. Never before had electron dynamics been studied on so short a timescale. Research Group- Generation of Attosecond Electron Bunches Research Overview:
Pioneered at CUOS, the program of Relativistic Nonlinear Optics in the λ3 Regime (where optical pulse energy is focused to a volume of a few cubic wavelengths) has initiated research into new possibilities and applications of light at its extreme limit of concentration. A very natural scheme for generation of isolated attosecond pulses was discovered in 2003. Based only on relativistic coherent motion of electrons, which deflect the incident laser pulse at each half-cycle into different directions and compress the radiation, this scheme gives the expectation of obtaining 10 % efficiency for attosecond pulse formation. In general, the short pulse formation occurs whenever relativistically strong laser pulses interact with near-critical or overcritical plasmas, but, more responsive plasmas act more efficiently. Further studying the details of laser-plasma interaction in the λ3 regime, we have also discovered that attosecond electron bunches as well can be generated under similar conditions. These dense electron bunches can provide a path to near-unity conversion efficiency of the counterpropagating radiation to coherent x-rays. Useful Links: Resolving Physical Processes on the Attosecond Time Scale Laser pulses of femtosecond duration allow many molecular and atomic processes to be investigated. Now physicists are reaching for the next frontier. In his Perspective, Lewenstein charts recent progress that has led to the generation of attosecond pulses. He highlights the report by Kienberger et al., who have used such pulses to "steer" an electron wave-packet like a classical particle. Attosecond pulse generation and detection The conjecture of producing a train of attosecond pulses using high-order harmonic generation (HHG) has been put forward almost 10 years ago. More recently, detailed theoretical investigations have confirmed this possibility and shown that it is indeed conceivable to generate a train or even a single attosecond pulse. The method utilizes the coherent properties of the high harmonics produced in the interaction of laser light with atoms in a manner analogous to the short pulse production in mode-locked lasers. Nonetheless, it is only now that experimental evidence has started accumulating, indicating that the femtosecond barrier towards attosecond pulses might have fallen. Provided that they are well characterized, they may serve as the fastest camera, able to record frozen snapshots of ultra fast electron dynamics. Consequently, they are of great interest to a variety of scientific areas including atomic, molecular, solid state and plasma physics, as well as to material science. The challenging problem is to find a measuring technique that unequivocally verifies the existence of attosecond pulses and it provides a precise temporal characterization. 
In principle, one should be able to apply the well known techniques from  The idea of utilizing a grating as a beam splitter is based on the natural splitting of an incident beam through dispersion into different orders. This is depicted in the figure above where a monochromatic beam incident on the grating is diffracted into one zeroth order and two first orders (for simplicity, only one is shown). Two mirrors reflect the zeroth order and one of the first orders straight back through the grating. In the second passage through the grating, the first order of the primary zeroth order automatically propagates in the same direction as the zeroth order of the primary first order. Because of the equal number of dispersions into the first and zeroth order involved, the splitting of the original beam in the two arms of the interferometer is exactly 50% independently of the efficiency of the grating. The transmission grating interferometer exhibits an additional feature, namely it spectrally analyzes the incident radiation. This property is especially desirable in case of gas harmonics produced by relatively long laser pulses where the spectrum is discrete. Then, the isolation of a single harmonic or a group of harmonics can be easily implemented by simple geometrical obstacles like apertures or knife-edges. 
The first experimental utilization of this grating based interferometer was the characterization with respect to its duration and spatial coherence of the third harmonic of a 1kHz repetition rate Ti:sapphire laser system delivering ~50fs pulses . The harmonic was generated in an Ar gas cell and its temporal characterization was performed by measuring the second order interferometric as well as the intensity AC traces in a second cell containing gaseous Toluene (C7H8). The results are depicted in the figure below along with the Fourier spectrum of the autocorrelation trace. To assess the overall performance of the grating Michelson interferometer, a measurement was undertaken of the TH pulse duration using a conventional Michelson interferometer with a 1mm thick fused silica beam splitter. The total optical path from source to detector through optical components and air was nearly the same for both interferometers. Within the accuracy of the measurement, both interferometers gave the same value for the TH pulse duration. This demonstrate the applicability of this new type of interferometer in the XUV spectral region and therefore its appropriateness for higher harmonics autocorrelation measurements The observation of a non-linear process, e.g. multiphoton ionization or inner-shell transition, induced by short-pulse coherent XUV radiation, such as the high-order harmonics of fs-laser radiation from rare gases, has been a challenging problem for a long time now. This is not only because of the interesting new physics inherent to the process, but also because it opens up the possibility of applying well established approaches in fs-laser pulse metrology to short pulses in the XUV wavelength region by providing an appropriate non-linear detector. Thus, second or higher order auto-correlation techniques can be appropriately modified for XUV  In an experiment performed with the first module of the 10 Hz Ti:sapphire laser system at Max-Planck-Institut für Quantenoptik, ATLAS, delivering 130 fs long pulses, two-photon ionization of He induced by a broadband XUV spectrum consisting of the 7th harmonic (~113 nm) to the 13th harmonic (~61 nm) of a Ti:sapphire laser was observed. In terms of the perturbation theory, this is a second order processes induced by the shortest wavelength and broadest bandwidth ever reported so far. A schematic of the experimental set-up used is shown in the previous figure. The laser beam is focused with a 1.5 m lens into a Xenon gas-jet in which the harmonics are generated. A Kirckpatrick-Baez focusing system is introduced in the propagation axis of the XUV radiation. This comprises two gold-coated spherical mirrors of 5 m radius of curvature one fixed in the x-y plane and the other in the x-z plane. The fundamental is filtered out by a 0.16 µm thick Indium filter, which also selects the 7th to 13th harmonic. At focus of the Kirckpatrick-Baez system, the XUV light interacts with a second pulsed He gas-jet. The ions resulting from two-photon ionization by the harmonic beam are detected by a time of flight energy analyzer as shown in the figure of the setup. The harmonics are monitored simultaneously with the ionization signal by means of an XUV monochromator coupled to the exit of the ionization chamber. The He+ ion signal was thus measured as a function of the total XUV intensity and the results of this measurement in a log-log scale are shown in figure to the left. The slope of a fitted straight line is 2.3±0.3. Further, solving the Time Dependent Schrödinger Equation for He in a coherent polychromatic field, the He+ ion yield was calculated as a function of the total XUV intensity and found to be in reasonable agreement with its measured value. These results establish the feasibility of a second order auto-correlation measurement of superposition of harmonics and thus open up the prospect of conducting second order auto-correlation measurements of coherent superposition of higher harmonics aiming at the unambiguous quantitative temporal characterization of attosecond pulse trains. From Femtoseconds to Attoseconds If you were asked to draw an accurate representation of a pulse of laser light that lasts 4.5/1,000,000,000,000,000 of a second (4.5 femtoseconds), how would you do it? Where would you start?
Can't even imagine such a timeframe? Considering it's the shortest laser light pulse that has ever been produced, who could blame you. Maybe it will help to think of it this way: the duration of this light pulse "is to a minute as a minute is to the age of the universe." Very short.
Still having trouble? Then consider the illustration below, which represents a 4.5-femtosecond pulse. It shows two aspects of light. In the first, that light is a wave (the red lines). In the second, a short pulse is the total of waves of different wavelengths (the blue line). The ticks on the horizontal axis are 3 femtoseconds apart. Attosecond in news- Follow the links below for reports on past events related to the Attosecond Technology project, as well as details and information on upcoming events. Upcoming events: Past events:
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