Most physics experiment techniques detect the physical properties of materials by linear response, while the newly developed nonlinear spectroscopy is to obtain more information about the system by detecting the nonlinear response of materials. One of the representatives of this type of new spectroscopy technology is two-dimensional coherent spectroscopy, which uses multiple light pulses to excite a system and then measures the nonlinear response of the system. In the infrared, visible, and ultraviolet bands, this powerful spectroscopy method is widely used in chemistry, biology and other fields to finely describe the electronic structure, chemical reactions, and even life processes of atomic and molecular systems [1].
recently emerged terahertz two-dimensional coherence spectroscopy extends this technology to the energy scale of semiconductor heterojunctions, superconductors, quantum magnets, electronic glass and other related electronic systems, opening up A brand new window of cognitive related electronic systems [2]. However, people still lack understanding of the nonlinear optical properties of related electronic systems. On the one hand, it is not clear whether the nonlinear optical response of this type of system is directly related to its physical properties. On the other hand, the existing theory uses the energy level transition language to describe the nonlinear optical process, which is suitable for atoms and molecules and other small-body systems, but it is not convenient to analyze many-body systems. Finding the direct connection between the nonlinear optical response of the correlated electronic system and its physical properties, and exploring a new language for describing the nonlinear optical response of the multi-body system constitutes two major theoretical challenges in the field.
Figure 1. (a) shows the specific settings of the model. The blue arrow represents the spin, and the red arrow represents the circularly polarized light pulse that causes the nonlinear response of the system. (b) is a schematic diagram of light echo. The red solid line is the light pulse, and the echo signal appears when t=tau (orange solid line).(c) is a schematic diagram of the "lens effect" image.
To meet these challenges, Institute of Physics, Chinese Academy of Sciences /Beijing National Research Center for Condensed Matter Physics Theoretical and Computational Laboratory Wan Yuan associate research team and collaborators took the lead in conducting related electronics Theoretical exploration of two-dimensional coherent spectroscopy and related ultrafast dynamic processes in the system [3]. Recently, this team has achieved new results in this direction. Doctoral student Li Zilong under the guidance of Associate Researcher Wan Yuan, and Professor Masaaki Oshikawa from the Institute of Physical Properties of the University of Tokyo studied the nonlinear optical response of a typical strongly correlated electronic system-Asainaga-La, which is widely present in one-dimensional quantum spin chains. Tinger liquid [Figure 1(a)], and discovered a unique ultrafast kinetic phenomenon [4].
Figure 2. Animation showing the attenuation of the echo signal as the pulse spacing increases.
Through analytical calculations, they found that there is a phenomenon of photon echo in this kind of gapless multi-body system-when the system is excited by light pulses with three time intervals of tau and tw, The photon echo is reflected by the sudden enhancement of the nonlinear signal when t=tau [Figure 1(b)]. This phenomenon is similar to the echo phenomenon-the time tau for the sound source to travel to the whispering wall is always equal to the time t for the sound to travel after reflection. The intensity of the echo decreases as the distance between the sound source and the whispering wall increases. Similarly, the intensity of the photon echo signal attenuates with the increase of the pulse interval tau [Figure 2].
They found that the echo signal comes from a unique ultrafast dynamic process excited by the topological fraction in the system.And named it the "lens effect" of fractional excitation: the first light pulse excites a fractional excitation that propagates to the left and right, respectively, and the second and third light pulses change the propagation direction and topological charge of the two fractional excitations. . Then the two scores stimulated to reunite at the same time and space point [Figure 1(c)]. This process is as if the world line excited by fractions is being reconverged by a "space-time lens". Dissipation and dispersion will hinder the free propagation of fractional excitation, thereby suppressing the "lens effect". Therefore, the dissipation and dispersion of the fractional excitation are directly reflected in the attenuation of the echo signal, which can be detected sensitively. Such information is often difficult to extract by conventional spectroscopy methods.
A similar photon echo phenomenon also exists in atomic and molecular systems, which comes from the quantum interference effect in the time dimension. The “lens effect” excited by the topological fraction in Chaoyong-Rattinger liquid indicates that the photon echo in a multi-body system can originate from a broader quantum interference phenomenon that belongs to the entire space-time category. This discovery conceptually expands the physical mechanism of photon echo. Their work provides a new theoretical basis for the application of nonlinear spectroscopy in strongly correlated systems, and also reveals the great potential of nonlinear spectroscopy.
related work was published in Physical ReviewX. This project was funded by the National Natural Science Foundation of China and Chinese Academy of Sciences strategic pilot technology special funding.
References
- S. Mukamel, Infra Principles of Nonlinear Optical and Copy of Methods, Ptros. Spectroscopy (Cambridge University Press, 2011); Weng Yuxiang, Chen Hailong, etc., "Ultrafast Laser Spectroscopy Principles and Technical Fundamentals" (Chemical Industry Press, 2013).
- M. Woerner al span_120spanspan em 120spanspan , New J. Phys 15 , 025039 (2013);... J. Lu et al , Phys Rev. Lett .. 118 , 207204 (2017); F. Mahmood span1sp an et al ., Nat. Phys. 17 , 627 (2021). 129 span_strong span. , 257401 (2019).
- Z. Li, M. Oshikawa, and Y. Wan, Phys. Rev. X strong 129spans 193_span 193_strong 193_span 193_span 193_strong_span 03 _span 03
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