RU2523744C2 - Active material for optically pumped maser and optically pumped maser - Google Patents

Abstract

FIELD: physics, optics.

SUBSTANCE: invention relates to quantum electronics. The active material for an optically pumped maser includes a silicon carbide crystal with paramagnetic vacancy defects. The optically pumped maser includes a microwave generator (1), a circulator (2), a magnet (3) between the poles of which there is a resonator (4) with a translucent window (5), an active material (6) in form of a silicon carbide crystal with paramagnetic vacancy defects, placed inside the resonator (4), and a pulsed or continuous light source (7), optically connected through the translucent window (5) of the resonator (4) to the active material (6).

EFFECT: enabling operation of the maser at room temperature.

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Description

The invention relates to the field of quantum electronics, and more particularly to solid-state optically pumped masers and active materials for optically pumped masers.

As you know, a maser is a quantum generator emitting coherent electromagnetic waves of superhigh frequency (microwave), its name is an abbreviation of the phrase “Microwave Amplification by Stimulated Emission of Radiation”, the authors of the discovery Maser: AM Prokhorov, N.G. Basov and C. Townes). Masers are used in technology (in particular, in space communications), in physical research, and also as standard-frequency quantum generators.

By type of active material, in which quantum amplification and generation of electromagnetic waves occurs, masers are divided into two types. Masers using molecular beams and masers, in which condensed media (for example, crystals) with paramagnetic defects — paramagnetic generators and amplifiers — are used as active material.

Under ordinary conditions, paramagnetic defects in a magnetic field are in thermodynamic equilibrium with each other and with the environment, and according to the Boltzmann distribution, this means that the number of electrons at the spin sublevels in a magnetic field (Zeeman sublevels) in excited states is the smaller, the higher the energy of the excited condition. In order to make it possible to use the unique properties of stimulated emission, it is necessary to artificially create a thermodynamically nonequilibrium medium in which the number of electrons on the Zeeman sublevels of the excited state is greater than on the unexcited sublevel. Solid state masers are most often used as amplifiers of electromagnetic radiation in the millimeter and centimeter ranges (microwave range) of wavelengths. The active material in these masers is a crystal (the most common are masers on ruby crystals) placed in a resonator having two resonant frequencies: the pump frequency and the gain frequency. The inverse population of levels in the crystal is achieved as a result of absorption by the crystal of the pump electromagnetic radiation, the frequency of which is greater than the gain frequency. The main feature of solid-state masers is that the inverse population is carried out at artificially created levels: by placing the crystal together with the resonator in a constant magnetic field. The distance between the levels is determined by the magnitude of the external magnetic field. Masers operate at low temperatures (1.4–4.2 K) due to the need to reduce the relaxation rate between spin sublevels to create an inverse population of spin sublevels. The main property of paramagnetic quantum amplifiers is an extremely low noise level, which is several orders of magnitude lower than in the best amplifiers operating on other principles. The most widely used paramagnetic amplifiers are used in radio astronomy and radar to amplify weak radio signals. However, the widespread use of masers is impeded by the need to create low temperatures for their operation.

Active materials for optically pumped masers and devices based on them (optically pumped masers) were described in early works immediately after the discovery of the maser effect (see E. Sabisky, and C. Anderson, Solid-state optically pumped microwave masers, IEEE Journal of Quantum Electronics, Volume: 3, Issue: 7, 287-295, Jul 1967). As a result of optical pumping, an inverse population of the spin sublevels of defects in the solid at room temperature is created, which can be used to amplify microwave (microwave) radiation (maser effect), i.e., to create a maser. Active materials and devices based on them are used to amplify microwave radiation, microwave phase shift and microwave restriction functions. Active materials carry out electronic (spin) polarization of various systems under the influence of optical radiation.

Known active maser material with optical pumping (see patent US 3454885, IPC H01S 1/00, published 08/07/1969) in the form of a fluorite crystal containing paramagnetic defects in the form of doubly charged thulium ^ions Tm ²⁺ . The inverse population of the Zeeman sublevels of the ground state of thulium ions by optical pumping by circularly polarized light of a given direction relative to the crystalline fluorite axes into wide absorption bands of Tm ²⁺ , which selectively pumps certain spin sublevels of the ground state of thulium ions Tm ²⁺ .

This active material can be used for masers operating continuously up to a frequency of 30 GHz. The temperature required for the maser to work is 1.4-4.2 K. Under optimal conditions of optical pumping, the temperature can be raised to 6 K. The performance of this maser is comparable to the corresponding characteristics of a standard ruby-pumped microwave maser at 4.2 K operating at frequencies up to 10 GHz, but better for higher frequencies.

A disadvantage of the known optically pumped active maser material is the necessity of using low temperatures (not higher than 6 K) and also irradiation with circularly polarized light of a certain direction relative to the crystal axes.

Known maser with optical pumping (see patent US 3454885, IPC H01S 1/00, published 07.08.1969), including a microwave generator, circulator, magnet, between the poles of which there is a cryostat with a resonator placed in it with a translucent window, the active material is placed inside the resonator and the light source, through a system of a lens, filter, reflector and circular polarizer, optically coupled to the active material in the form of a fluorite crystal containing paramagnetic defects in the form of doubly charged thulium ^ions Tm ²⁺ .

A disadvantage of the known optical pumped maser is the need to use a cryostat to place the active material in it, which makes it difficult to use the maser, as well as the need to use a special optical system to provide circularly polarized light to the active material, and the light direction must be consistent with the orientation of the active material and magnetic field.

Known active maser material with optical pumping (see US patent 3678400, IPC H01S 1/02, published July 18, 1972) in the form of a transparent crystal of a cubic polytype, for example, Th ₂ , KCl, CaF ₂ , CsZrCl ₆ , ZnS, K ₂ PtCl ₆ , SnO ₂ containing defects in the form of ions in the ² Si _1/2 ground state, for example, Na ⁰ , Cu ⁰ , Ag ⁰ , Au ⁰ , Ca ⁺ , Sr ⁺ , Ba ⁺ , Ga ²⁺ , In ²⁺ , Tl ^{2 +} , Si ³⁺ , Sn ³⁺ , Pb ³⁺ or Bi ⁴⁺ (having one unpaired s-electron on the outer shell), possessing a nuclear magnetic moment. The optical pumping of such a crystal by unpolarized light creates an inverse polarization of electrons at the spin sublevels.

A disadvantage of the known active material is the need to use low temperatures (not higher than 12 K).

Known maser with optical pumping (see patent US 3678400, IPC H01S 1/02, published July 18, 1972), including a microwave generator, a circulator, a permanent magnet, between the poles of which there is a cryostat with a resonator placed in it with a translucent window, active material, positioned inside the resonator and the light source, through a system of focusing lenses and a filter optically coupled with the active material in the form of a crystal, such as zinc sulfide containing paramagnetic defects in ² Si _1/2 ground state as a doubly charged thallium Tl ²⁺ or gal I Ga ^2+.

A disadvantage of the known maser is the need to use a cryostat to place the active material in it, which complicates the use of the maser.

Known active material for a maser with optical pumping (see patent US 3736518, IPC H01S 1/02, published 05/29/1973) in the form of an alkali halide crystal, for example NaCl, NaBr, KCl, KBr, KI, RbBr, RbCl, CsBr, CsCl containing defects in the form of color centers (F centers). In such crystals, the phenomenon of dichroism manifests itself upon the absorption of circularly polarized light of a certain wavelength, while the inverse population of the spin energy sublevels in the magnetic field is realized, which allows one to obtain a maser effect. Alkaline halide crystals containing a sufficient concentration of defects in the form of F centers are promising active materials for the development of optically pumped masers. At the same time, the operating temperature can be raised to 8 K and masers can operate up to frequencies up to 40 GHz.

The disadvantages of the known active material are the need to use low temperatures and circularly polarized light of a certain direction, as well as the temporary instability of color centers.

Known maser with optical pumping (see patent US 3678400, IPC H01S 1/02, published July 18, 1972), including a microwave generator, a permanent magnet, between the poles of which there is a cryostat with a resonator placed in it with a translucent window, the active material placed inside resonator, and a light source in the form of a lattice of laser diodes, through a system of a focusing cylindrical lens and a polarizer, optically coupled to the active material in the form of an alkali halide crystal containing defects in the form of F centers.

A disadvantage of the known maser is the need to use a cryostat to place the active material in it, which complicates the use of the maser, as well as the need to create a device to ensure the supply of circularly polarized light to the active material, and the light direction must be consistent with the orientation of the magnetic field.

The active material for an optical pumped maser is known (see US patent 6515539, IPC H01S 1/00, published 04.02.2003) in the form of a multicomponent optically transparent matrix, for example, of dichloromethane and paraffin oil, including a chromophore (H ₂ tetraphenyl-porphyrin and etioporphyrin I) and stable free radicals. In the active material, electronic spin polarization of stable free radicals occurs, induced by the processes of intermolecular energy transfer acting through photoinduced radical triplet pairs as a result of optical pumping. During optical pumping of a multicomponent chemical system, a high spin polarization of electrons occurs at room temperature.

The disadvantages of the known active material are its liquid form, the need to use pulsed optical excitation, the lack of a constant chemical composition with strictly defined properties.

Known maser with optical pumping (see patent US 6515539, IPC H01S 1/00, published 04.02.2003), including a microwave generator, circulator, permanent magnet and / or electromagnet, between the poles of which there is a quartz resonator, an active material placed inside the resonator and a light source optically coupled to the active material in the form of a multicomponent chemical system consisting of a chromophore and stable radicals in an optically transparent matrix.

An advantage of the known optical pumped maser is the possibility of its operation at room temperature. However, its serious drawback is the liquid form of the active material and the variability of its chemical composition with strictly defined properties.

Known active material for a maser with optical pumping (see patent US 5291145, IPC H01S 1/02, published 01.03.1994), coinciding with the claimed solution for the largest number of essential features and adopted for the prototype. The active material is a silicon crystal containing defects in the form of four neutral vacancies located in the same {110} plane. These vacancies can be created by irradiation with high-energy electrons, fast neutrons, or ion beams.

The known active prototype material can operate at temperatures up to 20 K. The disadvantage of the active prototype material is the need to use a cryostat to place the active material in it, which makes it difficult to use a maser.

Known maser with optical pumping (see patent US 5291145, IPC H01S 1/02, published 01.03.1994 patent US 3678400, IPC H01S 1/02, published July 18, 1972), including a microwave generator, circulator, permanent magnet and / or electromagnet , between the poles of which there is a cryostat with a resonator placed in it with a translucent window, an active material placed inside the resonator, and a light source optically connected to the active material in the form of a silicon crystal containing defects in the form of four neutral vacancies located in the same {110 plane }.

The objective of the invention is to obtain an active material with highly stable paramagnetic defects for a maser with optical pumping, which would effectively create an inverse population of spin sublevels in the ground state at room temperature, as well as a solid-state maser with optical pumping based on the developed active material operating at room temperature .

The problem is solved by a group of inventions, united by a single inventive concept.

In terms of the active material, the problem is solved in that the active material for an optical pumped maser consists of a silicon carbide crystal containing paramagnetic vacancy defects.

The silicon carbide crystal may be a nanocrystal.

A silicon carbide crystal may contain a reduced content of C13 and / or Si29 isotopes with a nuclear magnetic moment.

Silicon carbide of cubic, hexagonal and rhombic polytypes can be used, containing vacancy defects, which include a silicon vacancy occupying different crystalline positions in the selected polytype. These defects have the property of alignment of electronic spins in the ground state of high spin under optical pumping, which leads to the creation of an inverse population of spin sublevels up to room temperature, while optical pumping is carried out in the absorption bands of these defects in these polytypes.

Silicon carbide nanocrystals can be used containing a countable number of defects with a maser effect up to a single defect.

The use of crystals and silicon carbide nanocrystals with a reduced content of C13 and Si29 isotopes with magnetic moments leads to a decrease in the EPR line width (ODMR) and an increase in relaxation times, and hence to an improvement in the performance of the maser, since an optically induced inverse level population will be created at lower powers of optical excitation and persist for a longer time. This is due to the fact that the EPR line widths are mainly determined by hyperfine interactions with the nuclei of the surrounding carbon and silicon atoms carrying nuclear magnetic moments; relaxation times are also associated with interactions with these nuclei (see PG Baranov, IV Ilyin, AA Soltamova, and EN Mokhov, Identification of the carbon antisite in SiC: EPR of C13 enriched crystals, Phys. Rev. B 77, 085120, 2008) .

As an active material for a maser with a fixed frequency determined by the splitting of the fine structure, based on paramagnetic defects in a solid in a zero magnetic field, in which the inverse population between the spin sublevels (maser effect) is created at room temperature by optical pumping, carbide can be used silicon polytype 6H-SiC, containing vacancy defects, which include a silicon vacancy occupying various crystalline positions in the selected polytype. These defects have the property of creating an inverse population of spin sublevels in the ground state under optical pumping, in particular in low magnetic fields at room temperature.

In the maser part, the problem is solved in that the maser with optical pumping includes an ultra-high frequency (microwave) generator, a circulator, a magnet, between the poles of which there is a resonator with a translucent window, active material placed inside the resonator, and a source of pulsed or continuous light optically coupled through translucent resonator window with active material. The microwave generator output is connected by a coaxial cable or waveguide to the input of the circulator, the input / output of which is connected by a coaxial cable or waveguide to the resonator input. The active material is a silicon carbide crystal containing paramagnetic vacancy defects.

This optical pumped maser is based on the polarization effect of spin sublevels under the influence of optical radiation and the creation of an inverse population between a part of these sublevels at room temperature in an active material based on silicon carbide with paramagnetic vacancy defects. In this case, amplification of the microwave signal of low power with very low additional noise occurs. Due to the fact that the populations of the spin sublevels are inverted, with optical pumping, the active material increases the intensity of microwave radiation in the cavity due to stimulated emission. Thus, the radiation reflected from the resonator can be amplified. Microwave radiation from the generator enters through the circulator into the resonator with a sample in the form of active material tuned to the frequency of the generator, the signal reflected from the resonator through the same circulator enters the detector. The frequency of microwave radiation, which should increase in the cavity due to interaction with the active material of the maser, is selected by applying an external magnetic field, leading to the splitting of the spin sublevels.

The claimed invention is illustrated by drawings, where:

figure 1 shows the spectrum of the pulsed EPR of defects associated with silicon vacancy in silicon carbide polytype 41-1-SiC, recorded at a frequency of 9.3 GHz in the orientations of the magnetic field parallel (upper spectrum) and perpendicular (lower spectrum) to the hexagonal axis of the crystal at room temperature with the inversion of spin sublevels (maser effect) caused by optical pumping by laser light with a wavelength of 800 nm (the inset shows two possible versions of the level system with an inverse population for the electron spin S = 1 and electron the spin spin S = 3/2, while the high-field transition is inverted, that is, in this experiment microwave radiation with a frequency of 9.3 GHz is observed; M _s is the projection of the electron spin on the direction of the magnetic field; B is the magnetic field; radiative transitions are indicated by thick lines );

figure 2 shows the spectrum of the pulsed EPR of defects associated with a silicon vacancy in silicon carbide of the 6H-SiC polytype, recorded at a frequency of 9.3 GHz in the orientation of the magnetic field perpendicular to the hexagonal axis of the crystal at room temperature with inversion of the spin sublevels (maser effect) caused by optical pumping laser light with a wavelength of 800 nm; below is a standard EPR spectrum recorded continuously with optical excitation; the inset shows two possible variants of a system of levels with an inverse population for the electron spin S = 1 and electron spin S = 3/2, while the low-field transition is inverted, that is, in this experiment, microwave radiation with a frequency of 9.3 GHz is observed; M _s - projection of the electron spin on the direction of the magnetic field; B - magnetic field (radiative transitions are indicated by thick lines);

figure 3. an example of a maser with optical injection in accordance with the present invention.

Paramagnetic defects associated with silicon vacancies are created in silicon carbide during growth. Their concentration can be significantly increased by quenching the crystal, that is, rapid cooling from temperatures of the order of 2000 ° C. Also, the concentration of these defects can be significantly increased by irradiation with ionizing radiation: high-energy electrons, fast neutrons, protons, fast ions.

The present maser with optical injection (see Fig. 3) contains a microwave generator 1, a circulator 2, a magnet 3, between the poles of which there is a resonator 4 with a translucent window 5, active material 6 placed inside the resonator 4, and a source of 7 pulsed or continuous light optically connected through the translucent window 5 of the resonator 4 with the active material 6. The output of the microwave generator 1 is connected by a coaxial cable 8 (or a waveguide) to the input of the circulator 2. The input / output of the circulator 2 is connected by a coaxial cable 9 (or a waveguide) to the input of the resonator 4, and at The radiation output from the circulator 2 is through a coaxial cable 10 (or waveguide).

The active material 6 is a silicon carbide crystal containing paramagnetic vacancy defects.

Optical injection maser operates as follows.

Microwave radiation from a generator 1 or from another radiation source, which needs to be amplified (for example, cosmic radiation), enters through a circulator 2 into a resonator 4, in which the active material 6 is placed. A magnetic field on the active material 6 is created by a magnet 3. On the active material 6 through the translucent window 5 in the resonator 4, light from the light source 7 is incident. As a light source 7, various types of lasers can be selected, the radiation energy of which falls into the absorption bands of defects in the selected active material 6 to create an inverse population of spin sublevels. As an example, for defects in silicon carbide, we used lasers emitting in the wavelength range 800–850 nm. For more efficient excitation of the active material 6, lasers in the form of a comb with a simultaneous set of different frequencies can be used, as well as tube emitters with a continuous spectrum. Microwave radiation entering the resonator 4 with the active material 6 as a result of interaction with the active material is amplified and then goes through the circulator 2 to the output. The frequency of microwave radiation, which should be amplified in the resonator 4 due to interaction with the active material 6, is selected by applying an external magnetic field created by the magnet 3 and leading to the splitting of the spin sublevels of defects in the active material.

Example. Silicon carbide (SiC) crystals of various polytypes containing vacancy defects, which include a silicon vacancy occupying different crystalline positions in the selected polytype, were placed in the resonator. These defects had the property of polarization of electron spins in the ground high-spin state (for two realized spin possibilities S = 1 and S = 3/2) under optical pumping, which led to the creation of an inverse population of spin sublevels up to room temperature, and optical pumping was carried out in absorption bands specified defects in these polytypes. The effect of microwave radiation at a frequency of 9.3 GHz is demonstrated for the polytype of silicon carbide 4H-SiC in figure 1 and for the polytype of silicon carbide 6H-SiC in figure 2. In figure 1. The spectrum of pulsed EPR of defects associated with a silicon vacancy in silicon carbide of the 4H-SiC polytype recorded at a frequency of 9.3 GHz in the orientations of the magnetic field parallel (upper spectrum) and perpendicular (lower spectrum) hexagonal axis of the crystal at room temperature with inversion of spin sublevels is shown (maser effect) caused by optical pumping by laser light with a wavelength of 800 nm. The inset shows two possible variants of a system of levels with an inverse population for the electron spin S = 1 and electron spin S = 3/2, while the high-field transition is inverted, i.e., microwave radiation with a frequency of 9.3 GHz is observed in this experiment. Figure 2 shows the spectrum of the pulsed EPR of defects associated with a silicon vacancy in silicon carbide of the 6H-SiC polytype, recorded at a frequency of 9.3 GHz in the orientation of the magnetic field perpendicular to the hexagonal axis of the crystal at room temperature with inversion of the spin sublevels (maser effect), caused by optical pumping of laser light with a wavelength of 800 nm. Below is a standard EPR spectrum recorded in a continuous mode with optical excitation. The inset shows two possible variants of a system of levels with an inverse population for the electron spin S = 1 and electron spin S = 3/2, while the low-field transition is inverted, i.e., microwave radiation with a frequency of 9.3 GHz is observed in this experiment.

Figure 2 shows (the energy level diagram in the inset) that the inverse population of spin sublevels is created at room temperature under the influence of optical irradiation even in zero or near zero (terrestrial magnetic field) magnetic fields. Thus, this active material can operate at fixed frequencies determined by the physical parameters of a stable defect, that is, it is possible to measure small magnetic fields in the same way as in modern gas quantum magnetometers. In this case, the use of 6H-SiC nanocrystals will make it possible to carry out such measurements locally.

This maser is based on the polarization effect of spin sublevels under the influence of optical radiation and, as a result, creates an inverse population between part of these sublevels at room temperature in an active material based on silicon carbide with vacancy defects. The presence of an inverse population leads to the amplification of a microwave signal of low power with very low additional noise. Due to the fact that the populations of the spin sublevels are inverted, with optical pumping, the active material increases the intensity of microwave radiation in the cavity due to stimulated emission.

Claims (6)

1. The active material for a maser with optical pumping, consisting of a silicon carbide crystal containing paramagnetic vacancy defects. 2. The active material according to claim 1, in which the silicon carbide crystal is a nanocrystal. 3. The active material according to claim 1, in which the silicon carbide crystal contains a reduced content of isotopes C13 and / or Si29 with a nuclear magnetic moment. 4. Optically pumped maser, including an ultra-high frequency (microwave) generator, a circulator, a magnet, between the poles of which there is a resonator with a translucent window, active material placed inside the resonator, and a source of pulsed or continuous light optically coupled through the translucent window of the resonator with an active material, the output of the microwave generator is connected by a coaxial cable or waveguide to the input of the circulator, the input / output of which is connected by a coaxial cable or waveguide to the resonator input, and the active the material is a silicon carbide crystal containing paramagnetic vacancy defects. 5. The maser according to claim 4, in which the silicon carbide crystal is a nanocrystal. 6. The maser according to claim 4, in which the silicon carbide crystal contains a reduced content of isotopes C13 and / or Si29 with a nuclear magnetic moment.

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