Study shows how light can transform an insulating material into a semimetal

May 14, 2024

Overview of the electronic band structure of TiSe2 in equilibrium and after excitation.

The elements in the periodic table are divided into metals, semimetals and non-metals. The distinction is based on their chemical and physical properties and is determined, in particular, by the movement of electrons and the materials' ability to conduct electrical energy: metals are excellent conductors, semimetals have limited conductivity, non-metals are insulating materials, they do not conduct electricity.

These states, however, are not immutable. We know that an insulating material can be transformed into a metal: with chemistry, by introducing atoms with a different number of electrons into the material; or with very high pressures, a condition that can only be created in dedicated laboratories and which is difficult to transfer to other technological applications.

Science Advances has published the results of a study conducted by the Department of Physics of the University of Trento, the Department of Physics of the University of California Berkeley and the Materials Science Division of the Lawrence Berkeley National Laboratory that proposes a third way for the transformation of an insulating material into a semimetal.

"We have observed that, by exposing an insulating material to ultrafast laser pulses (10 femtoseconds, or 10 million billionth of a second), it is possible to alter the movement of electrons," says Alessandra Lanzara, professor of Physics from UC Berkeley and corresponding author of the study, together with Ph.D. student Maxi Huber, leading author of the paper.

This result can only be achieved through photoexcitation above a threshold fluence and with the appropriate material. "We used titanium diselenide (1T-TiSe2), a material that I had the opportunity to study in great depth during my career," says Professor Matteo Calandra from UniTrento and researcher Giovanni Marini, co-authors of the study.

"Titanium diselenide has very special characteristics: it is an insulating material, but it looks like a metal. For example, it is bright, while non-conductors are usually opaque and do not reflect light."

Based on the experimental results and calculations of the two research teams, exposing this material to ultrafast laser pulses alters its energy states and the movement of the electrons and, above a threshold fluence, transforms it into a semimetal for a short period of time (just under 500 femtoseconds).

There is a difference compared to the chemical transformation: the material's transformation is not permanent and as soon as the exposition to the laser is interrupted, it returns to its original state. This process multiplies the possible applications.

"For example," explains Calandra, "we can imagine devices with properties that change from insulating to semi-metallic in a very short time, a necessary feature to develop much more powerful computers, capable of processing complex calculations in a very short time. Today, the computing capacity is based on the use of electric fields, but the possibility of using light opens up new horizons in this field of application."

UniTrento worked on the theoretical and simulation part of the paper. The University of Berkley focused on the experimental part of the research.