Research

Quantum many-body interaction is an important problem in the study of atomic and molecular physics, and the related quantum multi-body dynamics is one of the most challenging basic physics issues. In the dynamics of many-electron systems, the relative motion due to the mutual repulsion of electrons will cause dynamic changes in ionic potential, thus affecting the final results of the whole dynamics process. From the birth of quantum mechanics to the beginning of this century, scientists have made continuous efforts to accurately solve the most basic quantum three-body dynamics problems. However, under the action of femtosecond laser field, such many-body systems become more complicated, and the calculation will involve more excited and intermediate state processes, which still cannot be solved accurately at present, hindering the understanding of atomic and molecular quantum systems in femtosecond laser field. The two-electron ionization process of atoms in a strong laser field is a typical quantum three-body problem, which involves the interaction between electrons and nuclei and the correlation between electrons, and its characteristic time is on the order of attosecond. With the aid of attosecond-resolved measurement technology, it is expected to fundamentally understand the three-body quantum interaction in a complex laser field, and provide a new idea for this basic physics issue that has long plagued the scientific community.

Today, a major challenge in condensed matter physics is how to understand the physical mechanism that plays a decisive role in the important physical properties of complex materials, and how to use the known physical properties of these materials to provide important guidance in finding new materials. Among them, quantum materials, that is, materials that cannot be described by single electron energy band structure, such as strongly correlated systems and low-dimensional nanomaterials, are of particular interest. These materials have such novel physical properties as HTS and colossal magnetoresistance (CMR), so they have broad application prospects and potential. However, since the degrees of freedom such as lattice, charge, spin, and orbit are closely related in these systems, how to separate the role of each degree of freedom and find out the decisive physical laws becomes a major challenge in condensed matter physics and materials science today.

Although HTS and iron-based superconductivity have been discovered for more than three decades and more than a decade respectively, the mechanism leading to such superconductivity is still unclear and has become one of the most important physics issues in condensed matter physics research. Because of the close correlation of elementary excitations in quantum material systems, how to separate the key factors that play a decisive role in physical properties is still a severe challenge in the research of HTS mechanism. In combination of traditional electron momentum and energy measurements with time-resolved measurements, it is expected to separate excitation processes on different time scales, providing a brand-new window for the study of these quantum materials and allowing for direct study of the formation of superconducting electron Cooper pairs. This will provide important information for revealing the "insulator-superconductor phase transition" in superconducting materials and provide a direct experimental basis for people to understand the HTS mechanism finally.

Traditional light sources used for photoelectron spectroscopy include ultraviolet lamps, synchrotron radiation, and free electron lasers, but none of them can achieve time-resolved measurements. New UV laser emerging in recent years has made time-resolved measurements possible, but its highest photon energy is only 6-7 eV. Attosecond laser generated by higher harmonics has excellent time-resolved characteristics, but more importantly, its photon energy is adjustable from 20 to 100 eV. Therefore, it becomes one of the best tools for time-resolved photoelectron spectroscopy, can provide an ideal light source for explaining the HTS mechanism.

Photovoltaic power is considered to be a major choice for the next generation clean energy, and photoinduced charge transfer on the surface is its main physical process. This is also a common concern for many physical and chemical processes, such as photocatalysis, photoelectrochemistry, solar energy conversion, and nano-optoelectronics. A central issue in solar photovoltaic technology is how photoinduced excitons separate the generated free electrons and holes to generate photocurrents. Studies on charge-transfer exciton dissociation dynamics may drive people to use new solar energy conversion strategies so as to greatly improve the light conversion efficiency of solar cells. The surface photoinduced charge transfer process on the nanometer scale often occurs on the order of femtosecond or even attosecond, and in study on it, it is required to track the dynamics of electrons on the surface or interface on the corresponding time scale. Attosecond laser greatly improves the spatial and temporal resolution in studies, not only contributing to understanding this dynamical process in essence, but also greatly impacting the studies on green energy.

On the attosecond time scale, charge transfer processes between biological molecules can be revealed. This will help people to understand the nature of life processes from a microscopic level and how these molecules are related to each other and form such a complex network. In studies on dynamics of small molecules and biomacromolecules, attosecond pulses will also help people understand the action mechanism of small molecule drugs. For example, the anti-cancer drug Cisplatin is known to treat tumors by forming adducts with DNA to lead to apoptosis of cancer cells, but its microscopic mechanisms such as the formation of chemical bonds between Cisplatin and DNA is still unknown. If attosecond pulses can be used to study the electronic dynamics in Cisplatin, they will help people understand the action mechanism of this signature anticancer drug, provide information from a microphysical perspective, and serve to improve existing drugs and develop new drugs.

An important characteristic of soft X-ray attosecond laser is that its frequency reaches the "water window" band (about 2.3-4.4 nm wavelength), in which photons cannot be absorbed by water, but can be strongly absorbed by carbon and nitrogen atoms, which are the main elements of biomolecules. Therefore, attosecond pulses are ideal light sources for microscopic imaging of living bodies with great contrast. It is well known that single-molecule super-resolution fluorescence microscopy, which breaks the diffraction limit, was awarded the 2014 Nobel Prize (Eric Betzig, Stefan W. Hell, William E. Moerner), and that it is widely used in biological systems. However, on the one hand, fluorescent labeling is required in this technology, and on the other hand, the time resolution is limited, preventing high-speed imaging and limiting observation of dynamics. Unmarked, high-speed, super-resolution imaging of biological systems is a coveted dream in science, while attosecond pulses are expected to make this dream true. In addition, since attosecond pulses are able to control some biochemical reactions and chemical bonds of biological macromolecules and even regulate the interaction dynamics between molecules so as to realize the directional manipulation of biological systems and obtain expected effects, they can also help to develop some new treatment methods in medicine and enable precise treatment.

The basic unit of modern electronic technology is semiconductor transistor. Because there is a certain but not large energy gap between the conduction band and the valence band, the transition of electrons between them can be controlled by only a small electric field, so as to control the movement of electrons in materials and realize the switching circuit. However, the bandwidth is narrow and the response time is long as the energy gap is not large. Theoretically, current semiconductor transistors have a maximum response speed of 100 GHz (10¹¹ Hz), and are also included in microwave electronics field as their frequencies are within the microwave band. As affected by thermal effects and other factors, the maximum response speed of the actual product is only 3-5 GHz, which is the maximum processing frequency of current computer CPU. To further improve the speed of computers, materials with larger energy band gap and higher response frequency are required, generally insulators. But in such materials, stronger electric fields are required to drive the electrons to move, while strong electric fields will break down and damage the insulators. This is a bottleneck that cannot be broken through by current electronic technology.

Recently, a series of frontier research results of attosecond physics are promising to help people to break through this bottleneck. In measurement of attosecond pulse, it is found that the strong laser can be driven in glass to generate instantaneous current, which has a response time less than 1 femtosecond, completely synchronizing with the oscillation of light wave, and a response frequency up to PHz (10¹⁵ Hz). Measurement of such transient process by attosecond pulse reveals that, although the light wave field is very strong, the energy of the whole pulse is not high because the pulse width of the driving laser is only a few femtoseconds. In such a short time, the material will not be broken down, and the influence of current can be completely recovered. This indicates that the electronic switching of PHz can be realized by utilizing light waves to drive the motion of electrons in the insulator to improve the response speed of existing circuits by 10,000 times. Especially during driving of transient current by light wave, the energy loss is basically negligible. The calorific value of integrated circuits made by using this process will be far less than that of the current electronic products, so as to help further integration of circuits and improvement of response frequency. Therefore, the latest research on attosecond physics lays a foundation for the development of new-generation high-speed computers, and the electronics will also enter a new stage - lightwave electronics.

In a word, the application of attosecond laser is not only expected to help make breakthroughs in basic research such as quantum many-body problems and HTS generation mechanisms, but also possible to make key progress in such important issues as lightwave electronics, clean energy and biomedicine that are expected to accomplish industrial upgrading and have a bearing on human health.