Stimulated emission is the process by which an incoming photon of a certain frequency can interact with an excited atomic electron (or other excited molecular state), causing it to drop to a lower energy level. The released energy is transferred to the electromagnetic field, creating a new photon with a phase, frequency, polarization and direction of motion that are identical to the photons of the incident wave. And this happens in contrast to spontaneous radiation, which works at random intervals, without taking into account the surrounding electromagnetic field.
Conditions for obtaining stimulated emission
The process is identical in form to atomic absorption, in which the energy of the absorbed photon causes an identical but opposite atomic transition: from lower tohigher energy level. In normal environments in thermal equilibrium, absorption exceeds stimulated emission because there are more electrons in lower energy states than in higher energy states.
However, when population inversion is present, the rate of stimulated emission exceeds the rate of absorption and pure optical amplification can be achieved. Such an amplifying medium, along with an optical resonator, forms the basis of a laser or a maser. Lacking a feedback mechanism, laser amplifiers and superluminescent sources also operate on the basis of stimulated emission.
What is the main condition for obtaining stimulated emission?
Electrons and their interactions with electromagnetic fields are important in our understanding of chemistry and physics. In the classical view, the energy of an electron revolving around an atomic nucleus is greater for orbits far from the atomic nucleus.
When an electron absorbs light energy (photons) or heat energy (phonons), it receives this incident quantum of energy. But transitions are only allowed between discrete energy levels, such as the two shown below. This results in emission and absorption lines.
Energy aspect
Next, we will talk about the main condition for obtaining induced radiation. When an electron is excited from a lower to a higher energy level, it is unlikely to stay that way forever. An electron in an excited state can decay to a lowerenergy state that is not occupied, in accordance with a certain time constant characterizing this transition.
When such an electron decays without external influence, emitting a photon, this is called spontaneous emission. The phase and direction associated with an emitted photon is random. Thus, a material with many atoms in such an excited state may result in radiation that has a narrow spectrum (centered around a single wavelength of light), but the individual photons will not have common phase relationships and will also be emitted in random directions. This is the mechanism of fluorescence and heat generation.
External electromagnetic field at the frequency associated with the transition can affect the quantum mechanical state of the atom without absorption. When an electron in an atom makes a transition between two stationary states (neither of which show a dipole field), it enters a transition state that has a dipole field and acts like a small electric dipole that oscillates at a characteristic frequency.
In response to an external electric field at this frequency, the probability of an electron transition to such a state increases significantly. Thus, the rate of transitions between two stationary states exceeds the magnitude of spontaneous emission. The transition from a higher to a lower energy state creates an additional photon with the same phase and direction as the incident photon. This is the forced emission process.
Opening
Stimulated emission was Einstein's theoretical discovery under the old quantum theory, in which radiation is described in terms of photons, which are quanta of the electromagnetic field. Such radiation can also occur in classical models without reference to photons or quantum mechanics.
Stimulated emission can be modeled mathematically given an atom that can be in one of two electronic energy states, a lower level state (possibly a ground state) and an excited state, with energies E1 and E2 respectively.
If an atom is in an excited state, it can decay into a lower state through a process of spontaneous emission, releasing the energy difference between the two states as a photon.
Alternatively, if an excited state atom is perturbed by an electric field of frequency ν0, it can emit an additional photon of the same frequency and in phase, thereby increasing the external field, leaving the atom in a lower energy state. This process is known as stimulated emission.
Proportionality
The constant of proportionality B21 used in the equations for determining spontaneous and induced emission is known as the Einstein coefficient B for that particular transition, and ρ(ν) is the radiation density of the incident field at frequency ν. Thus, the emission rate is proportional to the number of atoms in the excited state N2 and the density of incident photons. Such is the essencephenomena of stimulated emission.
At the same time, the process of atomic absorption will take place, which removes energy from the field, raising electrons from the lower state to the upper one. Its speed is determined by an essentially identical equation.
Thus, net power is released into an electric field equal to the energy of a photon h times this net transition rate. For this to be a positive number, indicating the total spontaneous and induced emission, there must be more atoms in the excited state than in the lower level.
Differences
The properties of stimulated emission compared to conventional light sources (which depend on spontaneous emission) is that the emitted photons have the same frequency, phase, polarization and direction of propagation as the incident photons. Thus, the involved photons are mutually coherent. Therefore, during inversion, optical amplification of the incident radiation occurs.
Energy Change
Although the energy generated by stimulated emission is always at the exact frequency of the field that stimulated it, the above description of the speed calculation only applies to excitation at a specific optical frequency, the strength of stimulated (or spontaneous) emission will decrease according to called the line shape. Considering only uniform broadening affecting atomic or molecular resonance, the spectral line shape function is described as a Lorentz distribution.
Thus, the stimulated emission is reduced by thiscoefficient. In practice, lineshape broadening due to inhomogeneous broadening can also take place, primarily due to the Doppler effect resulting from the distribution of velocities in the gas at a certain temperature. This has a Gaussian shape and reduces the peak strength of the line shape function. In a practical problem, the complete line shape function can be computed by convolving the individual line shape functions involved.
Stimulated emission can provide a physical mechanism for optical amplification. If an external source of energy stimulates more than 50% of the atoms in the ground state to transition to an excited state, then what is called a population inversion is created.
When light of the appropriate frequency passes through an inverted medium, photons are either absorbed by atoms that remain in the ground state or stimulate the excited atoms to emit additional photons of the same frequency, phase and direction. Since there are more atoms in the excited state than in the ground state, the result is an increase in the input intensity.
Radiation absorption
In physics, the absorption of electromagnetic radiation is the way in which the energy of a photon is absorbed by matter, usually the electrons of an atom. Thus, the electromagnetic energy is converted into the internal energy of the absorber, such as heat. The decrease in the intensity of a light wave propagating in a medium due to the absorption of some of its photons is often called attenuation.
Normally wave absorptiondoes not depend on their intensity (linear absorption), although under certain conditions (usually in optics) the medium changes transparency depending on the intensity of transmitted waves and saturable absorption.
There are several ways to quantify how quickly and efficiently radiation is absorbed in a given environment, such as the absorption coefficient and some closely related derivative quantities.
Attenuation factor
Several attenuation factor features:
- Attenuation factor, which is sometimes, but not always, synonymous with absorption factor.
- Molar absorption capacity is called the molar extinction coefficient. It is the absorbance divided by the molarity.
- The mass attenuation factor is the absorption factor divided by the density.
- The absorption and scattering cross sections are closely related to the coefficients (absorption and attenuation, respectively).
- Extinction in astronomy is equivalent to the damping factor.
Constant for equations
Other measures of radiation absorption are penetration depth and skin effect, propagation constant, attenuation constant, phase constant and complex wave number, complex refractive index and extinction coefficient, complex permittivity, electrical resistivity and conductivity.
Absorption
Absorption (also called optical density) and opticaldepth (also called optical thickness) are two interrelated measures.
All these quantities measure, at least to some extent, how much a medium absorbs radiation. However, practitioners of different fields and methods usually use different values taken from the list above.
The absorption of an object quantifies how much incident light is absorbed by it (instead of reflection or refraction). This may be related to other properties of the object through the Beer–Lambert law.
Precise absorbance measurements at many wavelengths make it possible to identify a substance using absorption spectroscopy, where the sample is illuminated from one side. A few examples of absorption are ultraviolet-visible spectroscopy, infrared spectroscopy, and X-ray absorption spectroscopy.
Application
Understanding and measuring the absorption of electromagnetic and induced radiation has many applications.
When distributed, for example, by radio, it is presented out of line of sight.
The stimulated emission of lasers is also well known.
In meteorology and climatology, global and local temperatures depend in part on the absorption of radiation by atmospheric gases (for example, the greenhouse effect), as well as land and ocean surfaces.
In medicine, X-rays are absorbed to varying degrees by different tissues (in particular, bone), which is the basis for radiography.
Also used in chemistry and materials science, as differentmaterials and molecules will absorb radiation to different degrees at different frequencies, allowing the material to be identified.
In optics, sunglasses, color filters, dyes and other similar materials are specially designed to take into account what visible wavelengths they absorb and in what proportions. The structure of glasses depends on the conditions under which stimulated emission appears.
In biology, photosynthetic organisms require light of the appropriate wavelength to be absorbed in the active region of chloroplasts. This is necessary so that light energy can be converted into chemical energy within sugars and other molecules.
It is known in physics that the D-region of the Earth's ionosphere significantly absorbs radio signals that fall into the high-frequency electromagnetic spectrum and are associated with induced radiation.
In nuclear physics, the absorption of nuclear radiation can be used to measure liquid levels, densitometry, or thickness measurements.
The main applications of induced radiation are quantum generators, lasers, optical devices.
