Attoscience and Strong Field Physics


Attoscience deals with study of dynamical system on the timescale of less than a femtosecond (1\,fs = 10$^{-15}$\,s). It derives its name from the SI unit attosecond (1\,as = 10$^{-18}$\,s). Since these numbers are so mind-bogglingly small, it is usually necessary to put them into context. The age of the universe is approximately 5\times10^{17}\,s. Thus the ratio of 1 second to the age of the universe is approximately the same as the ratio of 1 attosecond to 1 second. Or to put it in an even more modern context, if the national debt of the USA was a second, then 1 pence would be equivilent to a femtosecond, and thus a single attosecond is virtually worthless (or is it).

What can one measure on an attosecond timescale

There are many physical processes that require such a short timescale to be observed. The simplest argument is to consider the orbital period of an electron in the lowest energy state in the Bohr model of hydrogen, which can easily be calculated to be approximately 150as. Therefore, if one would like to observer electronic motion, then it is necessary to study it on this timescale.

An important phenomenon that occurs on a sub-femtosecond timescale is Auger decay, which is when multiple electrons are ionized when matter is irradiated with light with a photon energy much higer than the atomic binding energy. Since this is a multi-electron correlated event, it is a hard task to simulate and thus the theory is not fully defined or tested.

Reflection from metals is dominated by the plasma frequency, which is the natural frequency of oscillations of the conduction electrons when the metal is considered as a plasma. Light with frequencies below the plasma frequency are reflected, whereas light at higher frequencies are transmitted (a result known as the ultraviolet transparency). The period of oscillation is typically on the order of several hundred attoseconds. Thus attoscience could probe the macro and mesoscopic collective oscillations of electrons in conducting materials.

In dielectric (insulating) materials, the electrons are fixed to the atomic cores and therefore the concept of a plasma is invalid. However, when irradiated with strong laser fields with a field strength comparable to the binding field strength, electrons can be pulled away from the core, resulting in excitation to the conduction band. Attosecond pulses could be used to study both the ionization process, and the resulting electron dynamics.

An even shorter timescale is reached when one considers the timescale in which electrons can respond to applied electric fields, which is related to the conductivity (or equivilently the resistivity) of the material. For conductors, this timescale is on the order of several attoseconds.

Strong-Field Physics

The generation of attosecond pulses is strongly linked with the topic of strong-field Physics (SFP). SFP can loosely be understood as the interaction of matter with strong laser fields, such that the stength of the laser field is comparable to the strength of the electron binding field. This equates to field intensities of 10^{13} - 10^{18}\,W/cm^2. At higher intensities, electrons can be accelarated to relativistic energies, such that the magnetic field of the pulse plays an important role, resulting in nonlinear propagation - this is the subject of relativistic or ultrastrong-field Physics.

Current Research

We are currently actively researching several areas in the field of attoscience and strong field physics. In the former case, we are developing experimental methods to enable the time-resolution of dynamical processes via such techniques as pump-probe spectroscopy and molecular orbital tomography. Since the electric field is a fundamental entity, the most amount of information one can obtain from an optical experiment can only be obtained through complete knowledge of the electric field interacting and subsequently scattered from the system. The dominant approach to obtaining this information is via photoionization of atomic / molecular gases in the presence of an intense low-frequency field which acts as a temporal phase modulator. Such a technique is inherently low-signal. We are taking a different approach in which the electric field is measured directly. This has the advantage of providing very high signal strengths as well as potential single-shot acquisition.

In the area of strong-field Physics, we are interested in the study and ultimate control of strong-field light-matter interactions, specifically for the development of novel laser sources, such as intense few-cycle ultraviolet pulses for use in biological pump-probe experiments, and relativistic single-cycle pulses.