No need for negative critics. It's not even a downvote ;. But if a post is even partially wrong in my opinion, then I as a member of this community feel the need to comment on this wrongness, so that the author could fix it or explain why it's not wrong.
I think it's better than a silent downvote. If you want exact mechanism operating - you need to study this field exactly with required math apparatus. No analogy will help at this point.
However a lot of people here comes without good physics background, so analogies to them must be made at a "very accessible" level. Sacrificing exact mechanism details. Your right to down-vote of course, but also your right to post a better analogy,- in this way community will get a lot more benefits. This way the readers are made aware of the limitations. This type of "resonance" is described mathematically in the link, and pictorially: Here is an answer for modeling stimulated emission in QED.
Gert Well, some of the links might, but then this would be a link-only answer. The question is about lasers. Show 1 more comment. Sign up or log in Sign up using Google. Sign up using Facebook. Sign up using Email and Password. Post as a guest Name. Email Required, but never shown. Featured on Meta. Now live: A fully responsive profile. Linked Related 3.
If spontaneous emission occurs in a living organism, such a firefly, the process is called bioluminescence. At temperatures above absolute zero, some electrons in atoms are thermally excited to energy levels above the ground state. These electrons decay and emit a photon by spontaneous emission. Any object at a temperature above absolute zero naturally emits photons by spontaneous emission, and this process is called blackbody radiation.
In , Max Planck derived a formula for the energy density per unit bandwidth of a blackbody radiator by making the assumption that only discrete energies are allowed [10, p. His work agreed with known experimental data, and it is one of the fundamental ideas of quantum mechanics. For a nice derivation, see [84, p. The first term represents the number of modes per unit frequency per unit volume while the second term represents the average energy per mode. Photons emitted by a blackbody radiator have a relatively wide range of wavelengths, and this bandwidth depends on temperature.
Room temperature corresponds to around K. From the figure, we can see that black body radiators at higher temperatures emit both more photons and have a larger fraction of photons emitted fall in the visible range.
Stimulated emission is the process in which an excited electron or molecule interacts with a photon, decays to an available lower energy level, and in the process gives o a photon.
As with the other processes, this process can occur in isolated atoms, ionic compounds, organic molecules, and other types of materials, and it can occur in solids, liquids, and gases. If an incoming photon, with energy equal to the difference between allowed energy levels, interacts with an electron in an excited state, stimulated emission can occur.
The energy of the excited electron will be converted to the energy of a photon. The stimulated photon will have the same frequency, direction, phase, and electromagnetic polarization as the incoming photon which initiated the process [10, p.
The processes of absorption, spontaneous emission, and stimulated emission are illustrated by energy level diagrams in Fig. Energy is on the vertical axis, and nothing is plotted on the horizontal axis. Only two energy levels are shown, so this diagram illustrates only a small fraction of possible energy levels of a material. The lower energy level is labeled 1. It may represent, for example, the highest occupied energy level of an electron in an isolated atom, or it may represent the valence band of a semiconductor.
The higher energy level is labeled 2, and it may represent the lowest unoccupied energy level of an electron in an isolated atom or the conduction band of a semiconductor. The dot represents an electron occupying the energy level at the start of the process. The squiggly arrows represent a photon absorbed or emitted by the process. The vertical arrow shows how the internal energy of the electron changes in the process.
During absorption, an electron takes energy from an incoming photon, and the internal energy of the electron increases. During spontaneous emission, the internal energy of an electron decreases, and a photon is emitted. Stimulated emission occurs when a photon, with energy equal to the energy gap of the levels, interacts with the electron. In the process, the electron decays to the lower energy level, and a photon is produced with the same frequency, direction, phase, and electromagnetic polarization as the original photon.
The figures do not illustrate a change in position of the electrons. Instead, they illustrate a change in energy and internal momentum. The descriptions of the processes above involve changes in energy levels of an electron. However, absorption, spontaneous emission, and stimulated emission can instead involve vibrational energy states of molecules.
For example, a photon may be absorbed by a molecule, and the energy may cause the molecule to go from one allowed vibrational state to another with higher internal energy. Similarly, this molecule may spontaneously decay from the higher energy state to a lower energy state emitting a photon by spontaneous emission or by stimulated emission.
An example involving molecular vibration states is a carbon dioxide laser. However, to simplify the discussion in this text, we will assume that electron energy levels are involved. That process is called spontaneous emission. It is also possible that the photon emission is stimulated provoked by incoming photons [1], if these have a suitable photon energy or optical frequency ; this is called stimulated emission see Figure 1. In that case, a photon is emitted into the mode of the incoming photon.
In effect, the power of the incoming radiation is amplified. This is the physical basis of light amplification in laser amplifiers and laser oscillators. The physics of stimulated emission can be described in the context of quantum optics. There are also semiclassical descriptions treating the interaction of an oscillating dipole with an electromagnetic field , and the original idea of stimulated emission was published by Einstein [1] before quantum mechanics were fully developed.
Note that the amplification effect of stimulated emission can be reduced or entirely suppressed in a medium where too many laser-active atoms are in the lower state of the laser transition , because these atoms absorb photons and thus attenuate light. In a simple two-level system, laser amplification requires a so-called population inversion.
The rate of stimulated emission processes for an excited atom can be calculated as the product of the so-called emission cross section and the photon flux density number of photons per unit area and time. Such terms are regularly used in rate equation modeling. The photon flux density can be calculated as the optical intensity divided by the photon energy. In a laser operated well above threshold , stimulated emission dominates over spontaneous emission , and the power efficiency can be high.
For that condition to be fulfilled, the incident optical intensity must be higher than the saturation intensity. How can a photon sent to an excited atom cause emission of another photon rather than causing the electron to get even further excited to the next energy state? And why are the two emitted photons coherent, have the same phase? Answer from the author :. If there is a suitable higher energy state, such excited state absorption can indeed happen. The relative probabilities are determined by transition cross sections.
Coherence cannot be defined between just two photons, even if many talk as if it could, using naive models of photons. Also, the phase of one photon is undefined, therefore also the relative phase between two of them. I am afraid there is no simple answer to your question. If the incident photon's energy is used to stimulate the emission, why is another photon of the same energy emitted again instead of having only one photon from the host being released?
0コメント