A team of researchers from University of California and the University of Kassel in Germany has developed a way to halt the ultra-fast melting of silicon using well-timed lasers. This new technique could open possibilities for controlling material behavior under extreme conditions.
The breakthrough could also help improve the accuracy of experiments that study how energy moves through solids. This is important as scientists have long struggled to determine whether the changes caused in silicon by a laser beam come from simple heating (thermal effects) or from direct disruption of the atomic bonds (non-thermal effects).
The researchers investigated how powerful, ultra-fast laser pulses influence the atomic arrangement of silicon, a key material in electronics and solar technology. Through sophisticated computer modeling, they demonstrated that a single high-energy laser burst can melt silicon in mere fractions of a trillionth of a second.
Laser beams separated by 126 femtoseconds
Called non-thermal melting, this phenomenon occurs so rapidly that atoms collapse out of their orderly arrangement before any significant heating takes place.
However, by dividing the laser energy into two separate pulses and synchronizing them with precision, the scientists managed to halt the melting mid-process and lock the material into a new, metastable form.
This was done through a method called ‘ab initio molecular dynamics’, which simulates atomic and electronic behavior based on fundamental principles of physics. Using advanced computer simulations, the laser beam was split into two pulses, separated by 126 femtoseconds (that’s 0.000000000000126 seconds).
The experiment revealed that while the initial laser pulse initiates atomic motion, the second pulse disrupts this motion in a way that stops the atoms from losing their ordered arrangement. As a result, the material temporarily stays solid despite having absorbed sufficient energy to trigger melting.
Precise laser pluses ‘freeze’ atoms in place
Interestingly, the team found that this metastable form preserves most of the electronic characteristics of the original crystalline silicon, including a slightly smaller band gap, a key factor in determining how the material conducts electricity.
Furthermore, the scientists also observed that the atomic vibrations, or phonons, in this state were cooler and more stable than anticipated, indicating that the second pulse effectively “freezes” the atoms in place.
This new technique shows that it is possible to control ultra-fast atomic changes with precise laser timing. The study also implies the approach could be extended to other materials with comparable behavior, potentially facilitating the formation of new phases of matter or enhancing the accuracy of experiments that investigate how energy moves between electrons and atoms.
The study authors opine that future research could focus on refining this technique for various materials to gain deeper insights into the physics governing light–matter interactions.
“This mechanism can be generalized to other materials, potentially enabling structural and/or electronic transitions to metastable phases in the high-excitation regime. In addition, our approach could be used to switch off nonthermal contributions in experiments, allowing reliable electron-phonon coupling constants to be obtained more easily,” the authors note in their abstract.
The study has been published in the journal Communications Physics.