Tsinghua team rewrites material “genes” with femtosecond laser

Recently, Professor Zhou Shuyun’s research group and collaborators of the Department of Physics of Tsinghua University realized for the first time the pulsed laser-induced Flokai instantaneous band regulation in the semiconductor material black phosphorus, and found that it has a unique coupling effect and optical selection rules with the pseudospin of black phosphorus, and the related paper was published in Nature on February 2.

It is understood that the interaction between light and matter is an important detection method to explore the microscopic physical mechanism of low-dimensional quantum materials, and ultra-short and ultra-strong pulsed lasers can also be used as an effective means of regulating electronic structures and physical states to achieve new states and new effects that the equilibrium state does not have.

Schematic diagram of the semiconductor material Floquet energy band regulation. Photo courtesy of Tsinghua University

Low-dimensional quantum materials, including carbon nanotubes, graphene, transition metal chalcogenides, etc., have attracted wide attention for their novel physical properties and new device applications. For example, compared with the three-dimensional structure of graphite, graphene with its single-atom-level thickness can be regarded as a low-dimensional material such as “two-dimensional”, and the electronic structure will also change drastically due to the reduction of dimension. “The electronic band structure we studied can be popularly understood as the DNA of these materials, which determines the various properties of the materials,” explains Bao Changhua, a “Mizuki scholar” at Tsinghua University, “and what we have done is to use femtosecond lasers to manipulate the DNA of these materials to obtain some of the properties we want.” ”

At present, the research mainly focuses on the equilibrium state properties of materials, while the research on non-equilibrium physics and ultrafast dynamics is still in the development stage.

Zhou Shuyun’s team used pulsed lasers to control the time accuracy to trillionths of a second, taking a solid step to achieve instantaneous control of material properties. The measurement and regulation of electronic structures and physical properties on ultrafast time scales (picoseconds or even femtoseconds) can not only expand the frontier of non-equilibrium physics knowledge, but also lay an important scientific foundation for the development and application of new and high-speed devices in the future.

In the study of non-equilibrium ultrafast dynamics and transient state regulation, an important research direction that has attracted much attention is to induce changes in quantum matter states through periodic oscillation potential fields, and then realize the regulation of its electronic structure, which is called the Floquet project. Starting from the lattice structure of the material, electrons are affected by the periodically changing lattice in space, forming a periodic band structure in the momentum space, resulting in the possibility of the whole material showing a variety of properties of metals, insulators, semiconductors and even superconductors.

By analogy, the added periodic oscillating potential field will cause the electrons to periodically replicate the band structure in the energy space, thus forming a Frokai state. Furthermore, through the instantaneous regulation of the band structure, symmetry and topological properties of low-dimensional quantum materials through the interaction between electrons and periodic potential fields, new states of matter that the equilibrium state does not have can be realized, for example, the topological trivial material is transformed into a topological material, and the topological superconducting state far from the equilibrium state is realized.

“At present, international research in this area has just begun. On the one hand, we hope that Floquet’s band engineering can be realized in a wider range of material systems, thereby providing a new way to control the properties of materials more freely,” Zhou Shaohua, a 2017 doctoral student in the Department of Physics of Tsinghua University, introduced the development prospects and possible applications of this research field, “On the other hand, it is the application of femtosecond lasers in the future in the regulation of material properties, such as realizing the non-trivial topology and superconducting topology of materials on ultrafast time scales. ”

The concept of the Flokai state has attracted widespread attention from physicists since it was proposed at the beginning of the last century, and has been applied to the fields of condensed matter physics, cold atomic physics and optical lattices. In the past decade, Floquet’s instantaneous band and physical property regulation have developed into an important scientific frontier in condensed matter physics and materials science. However, despite the abundance of theoretical predictions, in stark contrast to very little experimental progress in condensed matter systems. Many key scientific questions still need to be confirmed experimentally.


Ultrafast time-resolved angle-resolved photoelectron spectroscopy was used to realize the instantaneous band regulation of Floquet in black phosphorus. Photo courtesy of Tsinghua University

Zhou Shuyun’s research group has been committed to the study of electronic energy spectrum and non-equilibrium ultrafast dynamics of low-dimensional quantum materials for many years, especially the experimental research of Floquet energy bands and state regulation. In studying the instantaneous band regulation of Floquet, the research team used a method similar to “filming electrons” – recording its key moments in the dynamic process under the excitation of light, from before the arrival of light, just when it arrived, and after it left, to observe how it evolved. On this basis, they confirmed that the observed instantaneous energy gap was caused by Floquet band engineering by systematically exploring the response of the instantaneous energy gap to variables such as time, light intensity and electron doping.

More interestingly, the research team found that the Floquet band engineering in black phosphorus has a strong selectivity for the polarization of the excitation light source: only when the pump light polarization is along the armchair-shaped direction of black phosphorus, the instantaneous energy gap will appear, revealing that the Floquet band engineering regulation has specific optical selection rules. Combined with theoretical analysis, the research team pointed out that this peculiar polarization selection effect comes from the pseudospin degrees of freedom of black phosphorus (black phosphorus cells contain two daughter lattices, and the corresponding two-level system can be analogous to spin). These results not only provide important ideas for the regulation of Frogai bands, but also lay an important foundation for further exploring the instantaneous regulation of topological states of matter and related states (magnetism, superconductivity, etc.). In addition, this unique polarization selection effect is expected to be applied to optical polarization-related optoelectronic device applications in the future. (Source: Chen Bin, China Science News)

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