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From electrons to photons: How does the amazing process of photoluminescence work?
Photoluminescence (PL) is a phenomenon in which a substance emits light after absorbing photons (electromagnetic radiation). This emission of light occurs after the material's electrons are excited by photons to a higher energy state, exhibiting the power of photoluminescence. The time difference between absorption and emission can vary from as little as femtoseconds to milliseconds, making photoluminescence an important tool for exploring and understanding the properties of matter.
"The process of photoluminescence reveals the energy conversion of electrons when they return from the excited state to the ground state."
Photoluminescence can be classified based on a variety of parameters, such as the relationship between the energy of the exciting photon and the energy of the photon produced. Among them, the notable mode is resonance excitation, which uses photons of a specific wavelength to rapidly emit photons of equal energy. This process is often called resonance fluorescence. In a gas or solution, this process primarily involves electrons but has no significant effect on the chemical's internal energy conversion.
In crystalline inorganic semiconductors, secondary emission processes can become quite complex, containing coherent contributions such as resonant Rayleigh scattering, and incoherent contributions such as Joule recombination. Among these complex phenomena, the radiative recombination of electron-hole pairs involved becomes more important, especially in solids.
"In chemistry-related disciplines, the fast process of photoluminescence is called fluorescence, while the slower process when the electrons pass through different quantum states is called phosphorescence."
Among them, fluorescence is usually a fast process. However, part of the energy in the emitted photons is consumed, so the re-emitted photons usually have lower energy. This phenomenon is called red shift. Phosphorescence, on the other hand, is a much slower process because the electrons, when excited, enter states with different spin multiplicity, usually a triplet state. In this case, the process of electrons returning to a low-energy state is prohibited by quantum mechanics, causing the emission process to last for minutes or even hours, which explains the "glow in the dark" phenomenon.
Photoluminescence is also an important tool for measuring the purity and crystal quality of semiconductors such as gallium nitride (GaN) and indium phosphide (InP). Through time-resolved photoluminescence (TRPL) technology, researchers can measure the decay of photoluminescence over time to evaluate the minority carrier lifetime of III-V semiconductors.
In a typical PL experiment, the semiconductor is excited by a light source, and the energy of the photons used is usually higher than the energy gap of the material. When photons are absorbed, electrons and holes are created. These carriers then relax in energy and momentum, and eventually recombine in the form of emitted photons.
"The photoluminescence process is characterized by being highly sensitive to the internal electric field and dielectric environment in the material. Such effects further increase the complexity of the process."
The ideal quantum well structure becomes a model system showing the basic process of photoluminescence, which can help scientists understand how different excitation conditions affect the production of light. Resonant excitation is one of the most representative phenomena of this process, indicating that coherent processes will significantly contribute to spontaneous emission after excitation. Through these studies, scientists continue to explore materials science and optoelectronic technology, hoping to discover new application potential and potential breakthroughs.
When exploring material properties, experimental methods such as photoluminescence spectroscopy are widely used. Its rapid and non-destructive method provides an optimal solution for studying the optical and electronic properties of materials of different sizes. Especially in the evaluation of solar cell absorber materials, photoluminescence technology can predict the maximum voltage generation of the material.
With the advancement of science and technology, the understanding of photoluminescence has also deepened. Researchers continue to make unremitting efforts to improve the performance and application efficiency of materials and explore their potential impact in future optoelectronic technology. In the future, with the advancement of science and technology, to what extent will this natural phenomenon continue to provide us with innovative solutions?
