Quantum electronics

Quantum electronics is a field of physics that studies the methods of amplification and generation of electromagnetic radiation , based on the use of the phenomenon of stimulated emission in nonequilibrium quantum systems , as well as the properties of amplifiers and generators obtained in this way and their use in electronic devices .

Content

Physical Foundations of Quantum Electronics

From the point of view of classical electronics, electromagnetic radiation is generated due to the kinetic energy of free electrons moving harmoniously in an oscillatory circuit . In accordance with the concepts of quantum electronics, the radiation energy is taken from the internal energy of quantum systems ( atoms , molecules , ions ), released during radiative transitions between its energy levels . Radiative transitions are of two types - spontaneous emission and stimulated emission . During spontaneous emission, an excited system spontaneously, without external influences, emits a photon whose characteristics ( frequency , polarization , direction of propagation) are in no way connected with the characteristics of photons emitted by other particles. A fundamentally different situation is observed in the case of stimulated emission of a photon under the influence of external radiation of the same frequency. In this case, a photon is formed with exactly the same properties as the photons that caused its appearance, that is, coherent radiation is formed. Finally, there is a process of absorption of photons from external radiation, opposite to stimulated emission.

Usually absorption prevails over stimulated emission. If it were possible to achieve the opposite situation, the matter would amplify the initial external (forcing) wave. Consider the transitions between energy levels $E_{i}$ ${\ displaystyle E_ {i}}$ and $E_{k}$ ${\ displaystyle E_ {k}}$ characterized by frequency $\nu$ ${\ displaystyle \ nu}$ , so that $h\nu =E_{i}-E_{k}^{}$ ${\ displaystyle h \ nu = E_ {i} -E_ {k} ^ {}}$ ( $h$ ${\ displaystyle h}$ - Planck constant ). The transition probabilities are determined through the so-called. Einstein's coefficients $A$ ${\ displaystyle A}$ and $B$ ${\ displaystyle B}$ :

for spontaneous transitions $w_{ik}^{s}=A_{ik}$ ${\ displaystyle w_ {ik} ^ {s} = A_ {ik}}$ ,
for absorption $w_{ki}=B_{ki}\rho _{\nu }^{}$ ${\ displaystyle w_ {ki} = B_ {ki} \ rho _ {\ nu} ^ {}}$ ,
for stimulated emission $w_{ik}=B_{ik}\rho _{\nu }^{}$ ${\ displaystyle w_ {ik} = B_ {ik} \ rho _ {\ nu} ^ {}}$ ( $\rho _{\nu }^{}$ ${\ displaystyle \ rho _ {\ nu} ^ {}}$ - spectral volumetric energy density).

Wherein $A_{ik}={\frac {8\pi h\nu ^{3}}{c^{3}}}B_{ik}$ ${\ displaystyle A_ {ik} = {\ frac {8 \ pi h \ nu ^ {3}} {c ^ {3}}} B_ {ik}}$ , $B_{ki}=B_{ik}={\frac {32\pi ^{3}}{3}}{\frac {d_{ik}^{2}}{h^{2}}}$ ${\ displaystyle B_ {ki} = B_ {ik} = {\ frac {32 \ pi ^ {3}} {3}} {\ frac {d_ {ik} ^ {2}} {h ^ {2}}} }$ (levels are considered non-degenerate ). The change in the energy density of the electromagnetic wave is equal to the difference in the energy emitted and absorbed in the forced processes and is proportional to the difference in the level populations:

(n_{i}-n_{k})h\nu B_{ik}\rho _{\nu }^{}

{\ displaystyle (n_ {i} -n_ {k}) h \ nu B_ {ik} \ rho _ {\ nu} ^ {}}

.

In a state of thermodynamic equilibrium, populations obey the Boltzmann distribution, so that

n_{i}=n_{k}\exp {(-h\nu /kT)}<n_{k}^{}

{\ displaystyle n_ {i} = n_ {k} \ exp {(-h \ nu / kT)} <n_ {k} ^ {}}

,

therefore, energy is absorbed by the system and the wave is attenuated. For the wave to amplify, it is necessary that the condition $n_{i}>n_{k}^{}$ ${\ displaystyle n_ {i}> n_ {k} ^ {}}$ , that is, the system was in a nonequilibrium state. This situation, when the population of the upper level is greater than the lower, is called the population inversion , or a system with a negative temperature . This state of the system is characterized by a negative value of the absorption index , that is, an amplification of the electromagnetic wave occurs.

It is only possible to create population inversions by spending energy - the so-called pump energy. An environment with population inversion is called active. Thus, in an active medium, coherent radiation amplification can be obtained. To turn an amplifier into a generator , it is necessary to place the medium in a positive feedback system that returns part of the radiation back to the medium. Volume feedback and open resonators are used to create feedback. Finally, in order to create stable generation, it is necessary to exceed the energy of stimulated emission over energy losses ( scattering , heating of the medium, useful radiation), which leads to the requirement that the pump power exceed a certain threshold value.

It should be noted that Einstein’s Femenological theory was constructed for the case when the emitter is in free space and which emits into an infinite number of space modes. When placing the emitter in a space with a limited number of modes, the Einstein coefficients $A_{ik},B_{ik}$ ${\ displaystyle A_ {ik}, B_ {ik}}$ change, see the Purcell Factor article

From the History of Quantum Electronics

Background

The concept of stimulated emission was introduced by A. Einstein in 1917 on the basis of thermodynamic considerations and was used to obtain the Planck formula . In 1940, V.A. Fabrikant proposed using stimulated emission to amplify light, but at that time this idea was not appreciated. An important direct precursor to quantum electronics was radio spectroscopy , which provided many experimental methods for working with molecular and atomic beams ( I. Rabi , 1937 ) and set the task of creating quantum standards for frequency and time . It should also be noted that the discovery in 1944 by E. K. Zavoisky of electron paramagnetic resonance became an important stage.

Masers

The birth date of quantum electronics can be considered 1954 , when N. G. Basov and A. M. Prokhorov in the USSR and independently J. Gordon, H. Zeiger and H. Townes (CH Townes) in the United States created the first quantum generator ( maser ) on ammonia molecules. Generation in it is carried out at a wavelength of 1.25 cm, corresponding to transitions between states of molecules with a mirror-symmetric structure. The population inversion is achieved due to the spatial separation of excited and unexcited molecules in a strongly inhomogeneous electric field (see the Stark effect ). The sorted molecular beam is passed through a cavity resonator , which serves to provide feedback. Subsequently, other molecular generators were created, for example, a maser on a beam of hydrogen molecules . Modern masers allow frequency stability to be achieved. $\Delta \nu /\nu \approx 10^{-11}-10^{-13}$ ${\ displaystyle \ Delta \ nu / \ nu \ approx 10 ^ {- 11} -10 ^ {- 13}}$ That allows you to create ultra-precise watches .

The next important step in the development of quantum electronics was the three-level method proposed in 1955 by N. G. Basov and A. M. Prokhorov , which made it possible to significantly simplify the achievement of inversion and use optical pumping for this purpose. On this basis, in 1957-1958 , G.E. D. Scovil (HED Scovil) and others created quantum amplifiers based on paramagnetic crystals (for example, ruby ) operating in the radio range.

Lasers

To advance quantum generators into the optical frequency region, A.M. Prokhorov 's idea of using open resonators (a system of parallel mirrors, as in the Fabry-Perot resonator ), which are extremely convenient for pumping, turned out to be important. The first ruby crystal laser to emit radiation at a wavelength of 0.6934 microns was created by Th. Maiman in 1960 . Optical pumping in it is realized using pulsed discharge lamps . The ruby laser was the first solid-state laser , and neodymium glass and garnet crystals with neodymium (wavelength 1.06 μm) are also distinguished. Solid-state lasers allowed the generation of high-power short ( $10^{-9}$ ${\ displaystyle 10 ^ {- 9}}$ c) and ultrashort ( $10^{-12}$ ${\ displaystyle 10 ^ {- 12}}$ c) light pulses in Q-switching and cavity mode locking circuits.

Soon A. Javan created the first gas laser using a mixture of helium and neon atoms (wavelength 0.6328 μm). Pumping in it is carried out by electron impact in a gas discharge and the resonant transfer of energy from the auxiliary gas (in this case, helium ) to the main ( neon ). Among other types of gas lasers , powerful carbon dioxide lasers (wavelength 10.6 μm, auxiliary gases - nitrogen and helium ), argon lasers (0.4880 and 0.5145 μm), cadmium laser (0.4416 and 0.3250 μm), a copper vapor laser , excimer lasers (pumping due to decay of molecules in the ground state), chemical lasers (pumping due to chemical reactions , for example, a chain reaction of a fluorine compound with hydrogen ).

In 1958, N. G. Basov , B. M. Vul, and Yu. M. Popov laid the foundations of the theory of semiconductor lasers , and already in 1962 the first injection laser was created [R. Hall (RN Hall), U. Dumke (WL Dumke, etc.). Interest in them is due to the simplicity of manufacture, high efficiency and the possibility of smooth frequency tuning in a wide range (the radiation wavelength is determined by the band gap ). A significant result is the creation in 1968 of semiconductor heterostructure lasers .

In the late 1960s , lasers based on organic dye molecules with an extremely wide gain band were developed and created, which makes it possible to smoothly tune the generation frequency using dispersion elements ( prisms , diffraction grating ). A set of several dyes allows you to cover the entire optical range.