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From electrons to excitons: How does terahertz change the way semiconductors work?
Terahertz technology, an electromagnetic field in the frequency range of hundreds of gigahertz to several terahertz, has opened up new horizons for materials science. Terahertz spectroscopy technology is quickly becoming a popular tool in scientific research and industry, especially in the research and development of semiconductor performance and applications. Through terahertz spectroscopy technology, researchers can analyze and control the state of many systems in detail, which exactly matches the energy of terahertz photons, igniting people's new understanding of quantum dynamics and reshaping the vision of future technology.
"Through terahertz spectroscopy, we can capture and study the dynamic behavior of electrons and excitons in semiconductors in the time domain."
In semiconductors, different electronic excitations have been widely used in lasers, electronic components, computers and other fields. These systems also constitute an interesting many-body system whose quantum properties can be changed through nanostructure design. Terahertz spectroscopy not only reveals the new technological potential of nanostructures but also condenses them down to the microscopic level as the fundamental properties of many-body systems are deeply explored.
Background and applications of terahertz
There are a variety of technologies for generating terahertz radiation, including antennas, quantum cascade lasers, free electron lasers, and optical rectification. Current experimental equipment can generate terahertz fields with strengths reaching MV/cm. Such strong fields have a huge impact on electronic systems. For example, through calculations, the amount of energy change is encountered at a microscopic distance of one nanometer, which allows the terahertz field to effectively promote the transformation of electrons.
"Such a strong field has a huge impact on electronic systems because it can induce electron transformations on a microscopic scale."
More importantly, the photon energy of the terahertz field is crucial for the study of semiconductors. It resonates with some striking many-body transitions, further arousing interest in holes and excitons. Excitons are formed by the combination of electrons and their corresponding holes due to Coulomb interaction. They appear to be in a state similar to that of hydrogen atoms. Their combination energy can vary from a few meV to hundreds of meV, which matches the energy of terahertz photons. Allowing us to uniquely detect their presence.
Terahertz time domain spectroscopy technology
Compared with optical spectroscopy, terahertz time-domain spectroscopy (THz-TDS) utilizes short terahertz pulses to directly measure the instantaneous evolution of the terahertz field. This technique provides a comprehensive analysis of the excitation dynamics of semiconductor samples. After a series of terahertz pulse experiments, researchers have been able to measure the material's composite dielectric constant, absorption coefficient, refractive index, etc.
"The characteristics of terahertz pulses passing through non-metallic materials make it a means of inspection and transmission of packaged items."
The intensity and duration of the terahertz light field are closely related to the energy of the photon. These factors together change the electronic dynamic behavior of the semiconductor. Therefore, this technology provides key support for a detailed understanding of the plasma behavior of the semiconductor. In addition, they couple well to the quantum state of excitons.
Terahertz-induced plasma and exciton transitions
Using the terahertz field, we can accelerate electrons out of equilibrium and observe the shielding process of Coulomb interaction. This phenomenon has been confirmed experimentally, showing that the shielding process is completed in an extremely short time, within tens of femtoseconds. These findings not only enrich the understanding of the behavior of excitons and electron plasma, but also provide new research ideas on quantum phenomena observable in experiments.
“As the terahertz field strength increases, we observe that internal transitions of excitons that are not nominally forbidden by the electric dipole become partially allowed.”
In extreme nonlinear optics, researchers have found that when terahertz pulses are synchronized with optical pulses, they can dynamically observe Rabi oscillations over time, and even the Autler-Townes splitting associated with these oscillations. As the terahertz field intensity increases, the platform further makes the process of multiphoton absorption and emission more dramatic.
Theory and Prospects
Systematically integrating the terahertz transition theory with the semiconductor Bloch equation enables us to comprehensively analyze the impact of the terahertz field. This provides a theoretical basis for understanding many-body quantum dynamics, thereby helping us predict the time-varying behavior of electron and hole distributions.
As terahertz technology continues to develop, its application potential and influence in semiconductor research have also expanded. This raises the question: How can terahertz technology further expand its role and impact in basic science and engineering applications in the future?
