Lasers' Ultrafast Acoustics Create Nanoearthquakes

Technische Universitat Dortumund Professor Manfred Bayer, an experimental physicist, has recently been awarded a 1.5-million-euro grant through the Deutsche Forschungsgesellschaft's (DFG) “Reinhard Koselleck-Program,” which has enabled him to conduct groundbreaking research in the field of ultrafast acoustics. The line of investigations has revealed that laser bombardments can trigger nanoearthquakes on areas several nanometers away from the place where they strike, creating shock waves that spread circularly from the original point.

The research of ultrafast acoustics began on thin metal films. They were bombarded with extremely energetic laser pulses, very short in duration. The reaction that is incited in the material is known as a “breathing movement.” During this stage, the metal film expands and contracts as the laser beam hits it. The entire stage lasts only a few tens of picoseconds, AlphaGalileo reports, and this is where things get really interesting, according to the team.

When they placed the thin metal film next to another substance, the experts noted that the breathing movement caused distortion waves in the material, which apparently propagated in very much the same way earthquake waves do when tremors occur in nature. These phenomena travel several nanometers before disappearing. At the macro-scale, when an earthquake occurs, the shock waves travel for hundreds of miles before disappearing.

Professor Bayer's team has also been able to determine that the form and intensity of the waves themselves can be entirely controlled using the laser pulse. Various intensities, durations and angles significantly modified the path and structure of the breathing movement-induced waves. The research could have applications in fields such as light-emitting-diode (LED) and laser technologies, where the emission of light is tightly related to the modulations of electrons inside the emitters.

Before the nanoearthquakes are formed, during the breathing movement, electrons in the atoms making up the thin metal film experience drastic modifications in the amounts of energies on their electrons. When they expand, more energy is lost, while, during the contraction, energy is added. Additionally, when the waves get transmitted to another substance, they cause pecometer-sized variations in the position of their constituent atoms, which is a large difference by nanoscale standards.