Terahertz emission spectroscopy explores emerging symmetry-breaking materials

Figure 1: Terahertz pulse emission in symmetry-breaking materials


It is no exaggeration to say that symmetry is perhaps the most important property of the laws of the natural world. In particular, symmetry seems to be everywhere in many disciplines and research directions represented by mathematics and physics. In fact, since primary and secondary school, we have dealt extensively with the concept of symmetry, such as coordinate axis symmetry, translation symmetry and rotational symmetry. In physics , symmetry is when a system is mathematically or physically invariant under certain transformations, and each satisfies the corresponding conservation law. In mathematics , these laws satisfying symmetries can be described by group theory , such as Lie groups and finite groups.

In condensed matter material systems, when these symmetries are broken, many novel properties and phenomena often appear, such as magnetism, ferroelectricity, and superconductivity. In order to study symmetry in these material systems, techniques such as X-rays, neutrons, and electron scattering are often used to probe the properties of crystal materials related to lattice, magnetism, and charge sequence. In addition, nonlinear optical means are also one of the effective ways to detect symmetry. The nonlinear properties mentioned here refer to the second, third, or higher optical response of the material to the incident electromagnetic field. Using these nonlinear properties, novel physical properties that cannot be observed by conventional linear means can be described. The terahertz (1 THz = 1012 s-1) emission spectroscopy technique mentioned below is a typical example of such methods.

Based on the above research background, Hou-Tong Chen’s research group from Los Alamos National Laboratory recently published a review article on Light:Science & Applications entitled “Ultrafast Terahertz Emission from Emerging Symmetry-Broken Materials”.

This paper systematically sorts out the relevant research on the use of terahertz emission spectroscopy to reveal the basic properties and complex dynamic behavior of new materials in recent years. These include quantum materials (such as superconductors and magnetic materials, etc.), as well as low-dimensional materials (such as graphene and metal nanostructures, etc.), in addition to highlighting the importance of symmetry in related research, and the use of intrinsic and external (such as artificial microstructure) properties of material systems to design their related properties.

As an emerging research method, terahertz emission spectroscopy can be used to study the steady-state and ultrafast kinetic properties of novel material systems, which can give many information that cannot be revealed by other means. Among them, the most critical concept is the rectification effect, which shows how to convert the operating frequency to a low-frequency electromagnetic field under the high-frequency electromagnetic field of the optical frequency. This behavior is similar to the conversion of AC signals to DC signals in circuits, which can power electronic devices and batteries. The appearance of the rectification process is closely related to symmetry breaking, of which the relatively common is generally spatial inversion symmetry, in addition, time inversion symmetry in magnetic systems will also be broken.

In simple terms, generating terahertz emission requires a certain directionality of the material in space and/or time. Therefore, if terahertz radiation can be generated, information about symmetry in the material system under study can be obtained. By measuring the dependence of terahertz emission signals on parameters such as polarization, frequency, and amplitude of incident light, it is possible to obtain detailed information about the structural, electrical and magnetic properties of materials, as well as the interaction of light and matter in them.

Figure 2: Basic principle of terahertz emission spectroscopy based on symmetry-breaking materials.

Another topic in this paper is about the interaction of intrinsic (e.g., atomic lattice, etc.) and external (e.g., artificial structure, etc.) properties in materials. Among them, the artificial structure may introduce new symmetry and further enhance the relatively weak terahertz response of the original material itself, achieving strong terahertz emission.

Chen Houtong said that this review article is dedicated to summarizing the important material systems and basic physical mechanisms in the current research on terahertz emission. At the same time, it also tries to highlight new opportunities for designing materials and symmetry in the interaction between light and matter in artificial structural systems, such as plasmon metasurfaces.

This paper suggests that this interaction based on the intrinsic, external and composite structures of emerging materials may inspire further exploration of more complex and diverse physical properties and phenomena, which is expected to go beyond the paradigm of existing materials research. (Source: China Optics)

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