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From theory to experiment: How did Maria Goeppert-Mayer predict two-photon absorption?
Two-photon absorption (TPA) is a fascinating phenomenon in atomic physics, a concept that has its roots in scientific exploration in the early 20th century. Maria Goeppert Mayer first predicted this process in 1931 in her doctoral thesis and showed how photons can influence the excited states of atoms or molecules under different conditions. With the development of science and technology, especially the invention of laser, this theory was soon verified by experiments and attracted widespread attention from the scientific community.
Two-photon absorption is defined as the simultaneous absorption of two photons in a virtual energy state, which excites an atom or molecule from one state to a higher energy state.
Two-photon absorption is not only an important theory in atomic physics, but also represents a nonlinear optical process, where the absorption probability is proportional to the square of the light intensity. With the development of lasers and other high-intensity light sources, scientists can observe two-photon absorption in certain materials, which provides a new way to explore the interaction between light and matter.
It is worth noting that the process of two-photon absorption can be divided into degenerate absorption occurring on photons of the same frequency and non-degenerate absorption occurring on photons of different frequencies. Mayer's predictions laid the foundation for studying this complex phenomenon, but her theory did not receive widespread attention at the time, and people did not begin to take her work seriously until decades later.
Mayer's prediction of two-photon absorption was first proposed in her doctoral thesis, and the formation of this theory is closely related to the early optical model.
Furthermore, the two-photon absorption process predicted by Mayer involves quantum mechanical thinking. In this framework, light is viewed as photons and it is stated that two-photon absorption requires that the energy of the photons be able to bridge the energy gap within the atom. This means that scientists studying this phenomenon must use corresponding optical techniques, such as tunable lasers, to observe clear absorption features.
The possibility of two-photon absorption depends not only on the intensity of the light, but also on the degree of light matching and precise control of the light source.
Subsequent experimental verification, such as the observation of two-photon excited fluorescence in barium-doped crystals, marked the successful application of Mayer's theory. These early findings paved the way for subsequent observations of two-photon absorption phenomena in other materials such as germanium vapor and cadmium sulfide.
With the deepening of our understanding of the two-photon absorption process, the study of selection rules has gradually become a focus. The selection rules for two-photon absorption are different from those for single-photon absorption, which enables certain molecules to undergo efficient photon conversion under specific optical conditions, further strengthening the importance of two-photon absorption in modern materials science.
Two-photon absorption can be measured using a variety of techniques, including two-photon fluorescence, Z-scanning, self-diffraction, and nonlinear transmission.
Through these techniques, researchers can obtain changes in the two-photon absorption cross-section at different wavelengths, which is crucial for the development of new optical materials and applications. At the same time, these studies also highlight the potential of nonlinear optical materials in optoelectronic devices.
Although the phenomenon of two-photon absorption has been extensively studied and validated, many scientists and engineers remain aware that there are many physical processes that have not yet been fully understood or explored. With the advancement of science and technology, new materials and methods are constantly being designed, which means we still have a long way to go in the study of two-photon absorption. How will future explorations affect our understanding and application of optical phenomena?
