Ultra-fast flash light revealing mystery of atoms, into the world of attoseconds (10-18 second)
Beyond the femtosecond (10-15 second) barrier

As momentary flash lights capture the motion of fast moving objects such as a bullet or explosion splinter, ultra-fast laser lights can be utilized to take pictures of ultra-fast natural phenomena following their dynamical change. The last decade has observed a rapid progress in femtosecond laser technology. This development allows us to carry out new kinds of experiments to open up a new area of nature that has never been explored before.
Is there no phenomenon that occurs in a faster time scale than a femtosecond? It is easy to note that there are phenomena taking place faster than in a femtosecond if we look into an atom. In a classical picture, an atom consists of a nucleus and electrons. Valance electrons circle the nucleus in a time scale of several hundred femtoseconds to picoseconds; on the other hand, an inner shell electron completes a circle in several hundred attoseconds to a few femtoseconds. For example, an electron in a ground state in a hydrogen atom takes about 150 attoseconds to circle the nucleus.
The dynamics related to neutrons and protons inside a nucleus of an atom take place in a time scale of zeptosecond (10-21 sec). An attosecond pulse makes possible the real-time observation and control of an electron’s motion in an atom. This is an area of science that no one has ever explored before. One attosecond corresponds to one millionth of one millionth of one millionth of one second. One second is still longer than as many attoseconds as seconds in the age of universe. It is too a short instant to imagine.

How to generate attosecond pulses

To generate attosecond pulses, two methods have been suggested. Both methods utilize the femtosecond laser technology. One method uses the interaction of a high power femtosecond laser with an electron bunch (a group of fast moving electrons) and the other uses high-harmonics generated from atoms. The method using high-harmonics from atoms has been successful up to now; however, the method using electron bunch has not yet tried. Professor Dong Eon Kim and research team have used high-harmonics from atoms generated in interaction with a femtosecond high power laser. When a femtosecond laser pulse is focused into one of inert gas such as He, Ne, Ar etc., one of electrons in an atom is freed through a tunneling process across Coulomb potential barrier induced to the laser field. This free electron moves under the influence of the laser field. Due to the change of the direction of the laser field, the electron turns around and moves back toward the atom from which it is originated. When the electron approaches the atom, it experiences Coulomb force from the atom and emits light in short wavelength region called extreme ultraviolet (XUV). The emission of this light is repeated every half cycle of the laser field. This phenomenon is manifested as peaks separated by an equal spacing of 2_ in the emission spectrum, where _ is the frequency of the laser.
To generate a single isolated attosecond pulse, the number of the emission must be reduced. For this purpose, a single-cycle laser would be an ideal laser, which does not exist yet. The team, in collaboration with Prof. F. Krausz’ group at Max Planck Institute for Quantum optics, a 5-fs laser of 750 nm was used. In this case, there were two or three half cycles of the laser field within 5-fs pulse width which lead to the emission of XUV light. For the successful generation of a single attosecond pulse, the phase of the laser field is to be controlled so that the peak of the laser cycle coincides with the peak of the pulse envelope and the continuum part in the spectrum of emitted XUV light has to be selected. In our study, a 5-fs laser f 750nm was focused onto He gas jet and Zr filter and Mo/Si multilayer mirror selected the continuum spectrum around 12.7 nm.

How to measure a single attosecond pulse The measurement of an attosecond pulse is as important as the generation of attosecond pulse, and it is not easy at all. Along with generation of an attosecond pulse, the measurement technique itself has rapidly advanced. Among various measurement methods suggested, two-color singlephoton ionization has been employed in this study. The two-color single-photon ionization is where photoelectrons are generated by XUV light under the influence of laser field. If the XUV light pulse width is much shorter than the half-period of laser field and the laser field itself is too weak to make an effect, final magnitude of momentum obtained in this ionization process depends on when it is born. Hence the measurement of photoelectron’s energy yields information about the pulse width of the XUV attosecond pulse. The photoelectron’s energy is readily measured by an electron spectrometer. This principle is similar to that of a streak camera. In a streak camera, the photoelectrons are streaked spatially on a phosphor screen by applying varying electric field to a deflection plate. This streak contains information about dynamical change of the phenomenon under interest. Length of the streak corresponds to the duration of the phenomenon under study. In the attosecond pulse measurement, photoelectrons created by the attosecond pulse are streaked by external weak laser field on the energy plane. The width of the streak on the energy plane contains information about the characteristics of the pulse, such as pulse duration and chirp. By changing the delay between the XUV attosecond pulse and the laser field, the photoelectron’s spectrum is changed.

The analysis of this change revealed that pulse duration in our study is about 170 attoseconds, shorter than the previous record (250 attosecond). Other measurement indicated that intensity of the current 170 attosecond pulse is about 10 times stronger than the previous 250 attosecond pulse. Application and outlook to future
As mentioned earlier, inside atoms and molecules, electrons move in a timescale of a few hundred attoseconds to a few ten femtoseconds; the attosecond pulse can be used to follow these motions of electrons and control them. Since the current attosecond pulse is shorter and stronger than the previous attosecond pulse, measurement
of ultrafast phenomena can be carried out more easily. The attosecond pulse is also utilized to characterize a light pulse. The period of the electric field of 800nm light is 1.7 femtoseconds, 10 times longer than the current attosecond pulse. Figure 1 shows the electric field of a 750 nm laser measured by the current attosecond pulse. This data was obtained by changing the delay between the XUV attosecond pulse and the laser. The oscillation
of the electric field is conspicuous and this measurement show that the pulse duration is 4.7 fs. Note that there is a phase difference of in the electric field between Figure 1 (a) and (b). This confirms that the current laser used in our study is phase-stabilized as it should be. This demonstrates that the attosecond pulse can be utilized to characterize a light pulse. As optical experiments become sophisticated, they demand a light pulse with a particular shape or characteristic. The attosecond pulse is well suited to characterize such light pulses.

Attoscience is at dawn due to the recent success in the generation, measurement and applications of single isolated attosecond pulses. We have now an eye with which we see phenomena that we were not able to see before and are able to study previously-known phenomena in a new direction. As one good look is better than to hear 100 times, attoscience will provide us with ways to investigate natural phenomena that we think know in a vivid manner and to control them. As femtochemistry has been opened along with the development of femtosecond laser technology and made enormous contribution to our understanding of nature, attoscience is expected to follow a similar path along with new development associated with attosecond technique.

We are now to newly define what can not be measured.

Professor Dong Eon Kim
Department of Physics
Tel: +82-54-279-2089
Fax: +82-54-279-5564
Email: kimd@postech.ac.kr