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The mystery of quantum physics: How does two-photon excitation allow us to explore the interior of molecules?
In the field of quantum physics, two-photon absorption (TPA) is a fascinating phenomenon that allows scientists to observe and study the internal structure and behavior of molecules in a completely new way. Simply put, two-photon absorption is the process when two photons (either of the same or different frequencies) are absorbed simultaneously to excite an atom or molecule, thereby raising its energy state to a higher electronically excited state. This process allows us to explore the properties of molecules in a non-destructive way without causing harm to them.
The wonderful thing about two-photon absorption is that its probability is proportional to the square of the intensity of the light, which necessitates the use of high-intensity lasers when studying this phenomenon.
Two-photon absorption was first predicted by Maria Goeppert-Meier in 1931 and was experimentally verified 30 years later with the advent of laser technology. Scientists first observed two-photon excited fluorescence in a crystal doped with European platinum, and subsequently in sodium vapor and cadmium sulfide semiconductors. These preliminary findings laid the foundation for the development of two-photon technology and led to many applications, including biomedical imaging and materials science.
Basic principles of two-photon absorption
In two-photon absorption, the light molecule transfers energy through a virtual energy level, which means it does not need to rely on an intermediate electronic state to absorb the photon. In this process, the energy of the photons needs to sum up enough to push the molecule from the ground state to the excited state, and this process is considered nonlinear because it requires two photons to arrive at the same molecular position at the same time for each absorption to interact.
Through the process of two-photon absorption, we can explore the structure of molecules and even image them under a microscope, which has great potential in biological and chemical research.
Application in experiments
Two-photon excitation technology is widely used in the fields of biological imaging and materials science and is known for its ability to observe cellular and molecular behavior with high resolution. At the same time, due to its low damage to samples, scientists can conduct dynamic experiments through long-term observation. The most common experimental setups for this technique utilize pulsed lasers, such as optical parametric oscillators for laser diode or frequency doubled Nd:YAG pumping.
Selection rules and measurement techniques
The selection rules for two-photon absorption are completely different from those of single-photon absorption, and its measurement methods are diverse, including two-photon excited fluorescence, self-focusing, z-scanning and other technologies. Central to these methods is the identification of how many photons are absorbed by a sample and how the structure of the molecule affects its absorption properties.
These experiments not only promote basic scientific research, but also bring innovations in engineering and technology, becoming an important tool for detecting the properties of materials.
Future challenges and prospects
Although research on two-photon absorption technology has made significant progress, it still faces many challenges. On the one hand, how to accurately control photon energy and intensity in different environments to obtain the required excitation effect is still a technology that needs to be deepened; on the other hand, the sensitivity to leakage and noise also needs to be further improved so that Reduce interference in more practical applications.
In summary, two-photon excitation technology is not only a scientific breakthrough, it also prompts us to rethink the nature of molecules and our understanding of the microscopic world. As this field continues to develop, what new discoveries and possibilities will be waiting for us to explore in the future?
