Scientists used terahertz radiation for measurements of strongly excited material
In-depth understanding of the electrical conductivity of matter is the key to many cutting-edge research and applications, ranging from phase-change memory in microelectronics to magnetospheres rooted in planetary interiors due to the motion of the conductive fluid. Unique states of material created by ultrafast table-top lasers or free-electron lasers (FEL) allow us to gain insight into atomic levels. However, it also requires sub-picosecond resolution to capture the details on the timescale of atomic motion. Therefore, in conductivity measurements it prevents the use of contact diagnostics such as multimeter and four-point-probe. Although ultrafast optical or X-ray measurements can provide information on high frequency electrical conductivity, they require complex models to extrapolate the intrinsic direct current (DC) conductivity of material.
The terahertz radiation (1 THz= 1012 Hz (cycles per second)) offers a unique solution to tackle this dilemma. The THz electromagnetic wave behaves like DC electric-field to the sample because the oscillation of its electric field is slow compared to the electron momentum relaxation frequencies in solid and liquid materials (typically 1013Hz or larger), and the width of each THz cycle is short enough to resolve sub-picosecond dynamics. Nevertheless, to measure the conductivity of strongly excited materials in the irreversible regime still requires high brightness THz radiation in order to penetrate the dense electron cloud as well as high sensitivity to detect the THz temporal profile in a single shot.
An international research team, led by scientists from the SLAC National Accelerator Laboratory and DESY, have recently measured the electrical conductivity of strongly heated material using the THz FEL radiation at FLASH. In this study, gold nano-foil samples were heated by the FLASH extreme ultraviolet (XUV) FEL pulses to electron temperatures up to 16,000 °C. As the thermal energy transfers from the electrons to the ions, the sample transits from cold to superheated solid and eventually melts into warm dense liquid. The researchers have determined the DC electrical conductivity by measuring the transmitted THz electric field through the heated samples. The multi-cycle THz pulses from FLASH provide continuous measurements with temporal resolutions better than 500 femtoseconds.
Read more on the DESY website
Image: Artist’s impression: origins of the electrical conductivity in superheated solids measured with THZ radiation at FLASH at DESY
Credit: Z. Chen, SLAC