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DOI: 10.22184/1993-7296.FRos.2024.18.8.610.620
This review discusses various ways to create single-photon sources (SPS). The task of
generating single photons can be solved in various ways, and at the moment there is no one among them that would be significantly preferable. The first part of the review discussed the
requirements for single-photon sources and criteria for characterizing sources. The first part
of the review included single photons sources based on single ions and based on single atoms.
The second part reviews SPS based on quantum dots and color centers in crystals.
This review discusses various ways to create single-photon sources (SPS). The task of
generating single photons can be solved in various ways, and at the moment there is no one among them that would be significantly preferable. The first part of the review discussed the
requirements for single-photon sources and criteria for characterizing sources. The first part
of the review included single photons sources based on single ions and based on single atoms.
The second part reviews SPS based on quantum dots and color centers in crystals.
Single-Photon Sources. Review. Part 2
V. G. Krishtop
Institute of Microelectronics Technology and High Purity Materials RAS, Chernogolovka, Moscow region, Russia.
JSC “InfoTeСS”, Moscow.
Moscow Institute of Physics and Technology, Dolgoprudny, Moscow Region, Russia.
This review discusses various ways to create single-photon sources (SPS). The task of generating single photons can be solved in various ways, and at the moment there is no one among them that would be significantly preferable.
The first part of the review discussed the requirements for single-photon sources and criteria for characterizing sources. The first part of the review included single photons sources based on single ions and based on single atoms.
The second part reviews SPS based on quantum dots and color centers in crystals.
Keywords: quantum dots, microcavity for quantum dots, tunable microresonators, color centers in diamond, NV center radiation collection, nanodiamonds, CC centers and G centers in silicon, silicon vacancies in silicon carbide.
The article received on: 01.08.2024
The article accepted on: 16.08.2024
Quantum dots [65, 66]
A quantum dot (QD) is an artificially created nanostructure with quantum properties. In a semiconductor quantum dot, the three-dimensional motion of electrons and holes is limited inside a low-size nanostructure in three dimensions. This results in formation of dimensional quantization levels similar to atomic levels. Electron transitions between these levels can occur with the emission or absorption of photons, which makes it possible to implement single-photon sources with both optical and electric pumping on quantum dots.
Quantum dots typically range in size from a few nanometers to several tens of nanometers. This makes it possible to set their optical and electronic properties by changing their sizes.
In the case of optical excitation, an electron-hole pair is created due to the absorption of photons. The quantum dot is excited by a laser and after excitation, the electron in the quantum dot moves to a higher energy level with the formation of a nonequilibrium electron-hole pair. After that, the nonequilibrium electron-hole pair anihilates and emits a photon. This process is organized in such a way that only one electron-hole pair is involved in the process each time, and only one photon is emitted.
In the case of electrical pumping, the excitation is created by the drift of an electron and a hole from the doped regions of the device to a quantum dot. Due to the Coulomb blockade, it is possible to implement a mode of operation in which charges can pass through the quantum dot in a controlled manner and only one at a time. This mode is called an electronic or photonic turnstile [67].
Quantum dots have found wide application in various fields such as optics, photonics, quantum computing, cryptography, medicine and solar energy industry. Quantum dots, in which exactly one electron-hole pair is excited and annihilated and exactly one photon is emitted, are sometimes called “artificial atoms” (just like other structures with atom-like levels), such quantum dots are well suited for true single-photon sources.
Currently, the most efficient single-photon sources based on quantum dots are made using III–V semiconductors. Quantum dots on elements III–V have a high value of the optical dipole moment, leading to a large coupling with limited or controlled optical modes, is a key parameter for quantum dot based single-photon sources. At the same time, systems based on II–VI materials are actively developing.
Standard microelectronic technologies are usually used for growing quantum dots: Molecular Beam Epitaxy (MBE) or Chemical Vapor Deposition (CVD).
Molecular Beam Epitaxy is an expensive and time-consuming technology that requires high-precision and expensive equipment (MBE machine), but it is possible to grow very high-quality structures with good repeatability. Chemical Vapor Deposition is much more technologically advanced for industrial production and significantly cheaper, but it is impossible to achieve the same quality as with MBE. In addition, quantum dots can be created using laser ablation or self-organization in “smart” materials.
Quantum dots can operate at different temperatures, depending on the material and the specific design. Most often, quantum dots operate at a helium temperature of 4 K or lower. This is necessary to fix the wavelength of the radiation, since the energy splitting of the electron-hole pair at the quantum dot is small, which is associated with a weak limiting potential due to the small band gap of semiconductors. For example, semiconductor quantum dots based on III–V compounds, such as cadmium indium selenide (CdSe) or gallium indium selenide (GaAs), usually require cooling to temperatures of about 10–100 K (–263 °C to –173 °C).
However, there are also quantum dots that can work without cooling. Single-photon electroluminescence has been demonstrated on quantum dots under electric pumping at room temperature [68, 69]. A number of research and development in this field aims to create quantum dots that retain their quantum properties at higher temperatures to facilitate integration into various devices and systems.
The basic principles of the technology and application of quantum dots are described in the review [70]. More details about the current state of the technology of single-photon sources based on epitaxial quantum dots can be found in relatively recent reviews [65, 71, 72]. In conclusion of this paragraph, we present a summary table (Table 2) from the review [71] on the single-photon properties of quantum dots depending on the material, with references to relevant works [73–99].
Microcavity-enhanced quantum dot single-photon sources
Placing a quantum dot in a resonant cavity significantly increases the efficiency of single photon generation. Due to the Purcell effect, the probability of photon emission by a quantum dot in a resonant cavity tuned to the appropriate frequency is significantly higher than in free space. Also, by controlling the сavity frequency, it is possible to implement an on-demand source. When the cavity frequency exactly corresponds to the energy of the radiative transition of the source located inside this сavity, the probability of photon emission by the source increases sharply. Similarly, the probability of photon emission decreases just as sharply when the cavity mode tunes out from the frequency of the source.
There are many ways to implement a quantum dot-resonant cavity system to operate as a source of single photons. The difficulty in using optical resonators is that single photons interacting with linear optical elements can group together in a multiphoton state in the resonator. Therefore, the use of resonant cavities imposes additional requirements on the control of such source. In particular, rapid excitation of such sources is necessary to prevent the accumulation effect. In addition, a post-pulse delay is necessary to minimize probability of participation of the photon accumulated in the cavity in the process of emitting the next one.
Typical structures of resonators are micropillars [101–103], tunable microresonators [104, 105], photonic crystal waveguides [106–108] containing various types of quantum dots [99, 100].
Color centers in diamond
Luminescence centers in diamond are a combination of crystal lattice defects and impurity atoms or molecules, mainly a combination of impurity atoms and vacancies. It is possible to build bright single-photon sources for quantum information technologies that are operable at room temperature at the “impurity-vacancy” type color centers (SiV, NV, where the letter “V” stands for “vacancy” – a defect in the crystal lattice in the form of the absence of an atom in its node).
A single nitrogen-vacancy (NV) center in diamond has been well studied, namely, a nitrogen-substituted vacancy with a negative charge NV-. The NV center has a structure in the form of a vacancy and a nitrogen atom in the nearest nodes [114] (see Fig. 9). Diamonds with such centers are found in nature and have a pink color, but they are quite rare and due to this they have a high price in the jewelry market. Synthesized nanodiamonds containing NV centers are used to create single-photon sources.
At the moment, there are more than 500 different color centers in diamond with emission wavelengths from the middle NIR to the visible range [115]. At the moment, color centers associated with the following chemical elements have been discovered: H, He, Li, C, B, N, O, Ne, Si, P, Ti, Cr, Co, Ni, Zn, Ge, As, Zr, Ag, Xe, Ta, W, Tl, Er, Eu, etc. [116]. The configuration and spectrum of the color center depend on the vacancy environment of the embedded atom (the possibility of formation of vacancy centers such as A-V, AVAV, A-V2, etc.). Some of the known color centers in diamond have unique properties for quantum and photonic applications: highly stable fluorescence of single centers at room temperature with a small phononless line width and high coherence of the spin states of these centers [117].
An active search for color centers with luminescence in the longer wavelength region of the spectrum (>800 nm) is underway, since devices with these centers need to be integrated into the existing telecommunications fiber-optic infrastructure. At the moment, the NE8 (793 nm) color center associated with Ni-N complexes remains the longest wavelength [118]. This wavelength is suitable for open space optical communication.
The development of diamond synthesis methods leads to the availability of technologies for creating luminescent centers (mainly NV and SiV) integrated into substrates with a scale size at the level of plates for microelectronics [119]. It is also possible to control the creation of single NV centers in bulk diamond with high structural perfection grown by synthesis at high pressures and temperatures [120]. The review [121] provides a practical understanding of the requirements and conditions for creating a diamond with the necessary chemical purity and crystalline perfection. These advances in the manufacture of materials have contributed to the fact that diamond has become one of the most promising solid-state quantum systems.
NV center radiation collection
A significant technical problem in the development and manufacture of single-photon sources based on single crystals of diamond is the low efficiency of radiation output from the NV center. For a flat surface of the diamond, the angle of total internal reflection is 24°50’. Although the NV center is a fairly bright center of luminescence capable of generating about 100 million photons per second, less than 5% of all radiation from the point center located below the surface will escape through the flat surface. Such low photon collection efficiency is due to the diamond-air interface. Diamond has a high refractive index ndiamond = 2.4 (for comparison, the refractive index of quartz is only 1.4), therefore, at the boundary with air, starting from a certain angle of incidence, a complete internal reflection occurs, as shown schematically in the figure.
The combination with the lens increases the efficiency of transmitting single-photon radiation into fiber, since it collects the emitted photons from a solid angle close to 2π, and in practice, the efficiency is limited by the fiber aperture only. Theoretically, this efficiency can be doubled by placing a reflector on the opposite side of the fiber, for example, a silver mirror or a Bragg grating at the NV center frequency [122].
The creation of immersion lenses over emitting NV centers using etching with a focused ion beam makes it possible to increase the collection of radiation to at least 10% and achieve a single photon frequency of 1 MHz [123]. This method was developed to simplify the technology of lens creation: methods of plasma anisotropic etching of lenses through multilayer lithographic masks [124] and methods of natural masking using oxide microballs [125] were tested.
In [126], the process of developing and creating a monolithic immersion metalens based on Fresnel optics with a nanostructured surface is described.
An effective solution to the problem of radiation output is the synthesis of nanodiamonds containing color centers, since the size of an individual nanodiamond is less than the wavelength of the emitted photon. Also, in some works, it was proposed to place nanodiamonds in specially etched gold-plated pits or embed them inside the resonator between Bragg mirrors applied directly to the end of the optical fiber [127–129].
For a single-photon fiber source, it is desirable to arrange the transmission of a single-photon signal to an optical fiber or an integrated waveguide. This can be done by placing a crystal with NV centers on the end of the fiber, or by applying a hemispherical lens (such as a collimator) to the surface of the diamond, directing radiation into the end of the fiber. A more complex method involves the integration of a nanodiamond containing a color center into an optical resonator, which implements optical coupling with a nearby waveguide.
It should also be noted that single-photon sources with electric pumping are being developed based on the color centers in diamond [130–132]. So far, the efficiency of electric pumping is about an order of magnitude inferior to optical pumping, but further development of this topic is very promising.
In addition, color centers exist not only in diamonds, and of course, work is underway to study the luminescence of color centers in other materials, and the possibilities of their application for single-photon detectors. As an example, we mention C–C centers and G centers in silicon [133, 134] and single silicon vacancies in high-purity silicon carbide [135–139].
In the following chapters we will consider single-photon sources based on carbon nanotubes and defects in them (engineering of defects in nanotubes), on nanocrystals and layered nanocrystals, on single molecules, in low-size structures, and metal ions in a polymer matrix, as well as ensemble systems, and sources based on nonlinear crystals.
ABOUT AUTHOR
V. G. Krishtop: e-mail: vladimir.krishtop@infotecs.ru.
ORCID: 0000-0001-6063-2657
V. G. Krishtop
Institute of Microelectronics Technology and High Purity Materials RAS, Chernogolovka, Moscow region, Russia.
JSC “InfoTeСS”, Moscow.
Moscow Institute of Physics and Technology, Dolgoprudny, Moscow Region, Russia.
This review discusses various ways to create single-photon sources (SPS). The task of generating single photons can be solved in various ways, and at the moment there is no one among them that would be significantly preferable.
The first part of the review discussed the requirements for single-photon sources and criteria for characterizing sources. The first part of the review included single photons sources based on single ions and based on single atoms.
The second part reviews SPS based on quantum dots and color centers in crystals.
Keywords: quantum dots, microcavity for quantum dots, tunable microresonators, color centers in diamond, NV center radiation collection, nanodiamonds, CC centers and G centers in silicon, silicon vacancies in silicon carbide.
The article received on: 01.08.2024
The article accepted on: 16.08.2024
Quantum dots [65, 66]
A quantum dot (QD) is an artificially created nanostructure with quantum properties. In a semiconductor quantum dot, the three-dimensional motion of electrons and holes is limited inside a low-size nanostructure in three dimensions. This results in formation of dimensional quantization levels similar to atomic levels. Electron transitions between these levels can occur with the emission or absorption of photons, which makes it possible to implement single-photon sources with both optical and electric pumping on quantum dots.
Quantum dots typically range in size from a few nanometers to several tens of nanometers. This makes it possible to set their optical and electronic properties by changing their sizes.
In the case of optical excitation, an electron-hole pair is created due to the absorption of photons. The quantum dot is excited by a laser and after excitation, the electron in the quantum dot moves to a higher energy level with the formation of a nonequilibrium electron-hole pair. After that, the nonequilibrium electron-hole pair anihilates and emits a photon. This process is organized in such a way that only one electron-hole pair is involved in the process each time, and only one photon is emitted.
In the case of electrical pumping, the excitation is created by the drift of an electron and a hole from the doped regions of the device to a quantum dot. Due to the Coulomb blockade, it is possible to implement a mode of operation in which charges can pass through the quantum dot in a controlled manner and only one at a time. This mode is called an electronic or photonic turnstile [67].
Quantum dots have found wide application in various fields such as optics, photonics, quantum computing, cryptography, medicine and solar energy industry. Quantum dots, in which exactly one electron-hole pair is excited and annihilated and exactly one photon is emitted, are sometimes called “artificial atoms” (just like other structures with atom-like levels), such quantum dots are well suited for true single-photon sources.
Currently, the most efficient single-photon sources based on quantum dots are made using III–V semiconductors. Quantum dots on elements III–V have a high value of the optical dipole moment, leading to a large coupling with limited or controlled optical modes, is a key parameter for quantum dot based single-photon sources. At the same time, systems based on II–VI materials are actively developing.
Standard microelectronic technologies are usually used for growing quantum dots: Molecular Beam Epitaxy (MBE) or Chemical Vapor Deposition (CVD).
Molecular Beam Epitaxy is an expensive and time-consuming technology that requires high-precision and expensive equipment (MBE machine), but it is possible to grow very high-quality structures with good repeatability. Chemical Vapor Deposition is much more technologically advanced for industrial production and significantly cheaper, but it is impossible to achieve the same quality as with MBE. In addition, quantum dots can be created using laser ablation or self-organization in “smart” materials.
Quantum dots can operate at different temperatures, depending on the material and the specific design. Most often, quantum dots operate at a helium temperature of 4 K or lower. This is necessary to fix the wavelength of the radiation, since the energy splitting of the electron-hole pair at the quantum dot is small, which is associated with a weak limiting potential due to the small band gap of semiconductors. For example, semiconductor quantum dots based on III–V compounds, such as cadmium indium selenide (CdSe) or gallium indium selenide (GaAs), usually require cooling to temperatures of about 10–100 K (–263 °C to –173 °C).
However, there are also quantum dots that can work without cooling. Single-photon electroluminescence has been demonstrated on quantum dots under electric pumping at room temperature [68, 69]. A number of research and development in this field aims to create quantum dots that retain their quantum properties at higher temperatures to facilitate integration into various devices and systems.
The basic principles of the technology and application of quantum dots are described in the review [70]. More details about the current state of the technology of single-photon sources based on epitaxial quantum dots can be found in relatively recent reviews [65, 71, 72]. In conclusion of this paragraph, we present a summary table (Table 2) from the review [71] on the single-photon properties of quantum dots depending on the material, with references to relevant works [73–99].
Microcavity-enhanced quantum dot single-photon sources
Placing a quantum dot in a resonant cavity significantly increases the efficiency of single photon generation. Due to the Purcell effect, the probability of photon emission by a quantum dot in a resonant cavity tuned to the appropriate frequency is significantly higher than in free space. Also, by controlling the сavity frequency, it is possible to implement an on-demand source. When the cavity frequency exactly corresponds to the energy of the radiative transition of the source located inside this сavity, the probability of photon emission by the source increases sharply. Similarly, the probability of photon emission decreases just as sharply when the cavity mode tunes out from the frequency of the source.
There are many ways to implement a quantum dot-resonant cavity system to operate as a source of single photons. The difficulty in using optical resonators is that single photons interacting with linear optical elements can group together in a multiphoton state in the resonator. Therefore, the use of resonant cavities imposes additional requirements on the control of such source. In particular, rapid excitation of such sources is necessary to prevent the accumulation effect. In addition, a post-pulse delay is necessary to minimize probability of participation of the photon accumulated in the cavity in the process of emitting the next one.
Typical structures of resonators are micropillars [101–103], tunable microresonators [104, 105], photonic crystal waveguides [106–108] containing various types of quantum dots [99, 100].
Color centers in diamond
Luminescence centers in diamond are a combination of crystal lattice defects and impurity atoms or molecules, mainly a combination of impurity atoms and vacancies. It is possible to build bright single-photon sources for quantum information technologies that are operable at room temperature at the “impurity-vacancy” type color centers (SiV, NV, where the letter “V” stands for “vacancy” – a defect in the crystal lattice in the form of the absence of an atom in its node).
A single nitrogen-vacancy (NV) center in diamond has been well studied, namely, a nitrogen-substituted vacancy with a negative charge NV-. The NV center has a structure in the form of a vacancy and a nitrogen atom in the nearest nodes [114] (see Fig. 9). Diamonds with such centers are found in nature and have a pink color, but they are quite rare and due to this they have a high price in the jewelry market. Synthesized nanodiamonds containing NV centers are used to create single-photon sources.
At the moment, there are more than 500 different color centers in diamond with emission wavelengths from the middle NIR to the visible range [115]. At the moment, color centers associated with the following chemical elements have been discovered: H, He, Li, C, B, N, O, Ne, Si, P, Ti, Cr, Co, Ni, Zn, Ge, As, Zr, Ag, Xe, Ta, W, Tl, Er, Eu, etc. [116]. The configuration and spectrum of the color center depend on the vacancy environment of the embedded atom (the possibility of formation of vacancy centers such as A-V, AVAV, A-V2, etc.). Some of the known color centers in diamond have unique properties for quantum and photonic applications: highly stable fluorescence of single centers at room temperature with a small phononless line width and high coherence of the spin states of these centers [117].
An active search for color centers with luminescence in the longer wavelength region of the spectrum (>800 nm) is underway, since devices with these centers need to be integrated into the existing telecommunications fiber-optic infrastructure. At the moment, the NE8 (793 nm) color center associated with Ni-N complexes remains the longest wavelength [118]. This wavelength is suitable for open space optical communication.
The development of diamond synthesis methods leads to the availability of technologies for creating luminescent centers (mainly NV and SiV) integrated into substrates with a scale size at the level of plates for microelectronics [119]. It is also possible to control the creation of single NV centers in bulk diamond with high structural perfection grown by synthesis at high pressures and temperatures [120]. The review [121] provides a practical understanding of the requirements and conditions for creating a diamond with the necessary chemical purity and crystalline perfection. These advances in the manufacture of materials have contributed to the fact that diamond has become one of the most promising solid-state quantum systems.
NV center radiation collection
A significant technical problem in the development and manufacture of single-photon sources based on single crystals of diamond is the low efficiency of radiation output from the NV center. For a flat surface of the diamond, the angle of total internal reflection is 24°50’. Although the NV center is a fairly bright center of luminescence capable of generating about 100 million photons per second, less than 5% of all radiation from the point center located below the surface will escape through the flat surface. Such low photon collection efficiency is due to the diamond-air interface. Diamond has a high refractive index ndiamond = 2.4 (for comparison, the refractive index of quartz is only 1.4), therefore, at the boundary with air, starting from a certain angle of incidence, a complete internal reflection occurs, as shown schematically in the figure.
The combination with the lens increases the efficiency of transmitting single-photon radiation into fiber, since it collects the emitted photons from a solid angle close to 2π, and in practice, the efficiency is limited by the fiber aperture only. Theoretically, this efficiency can be doubled by placing a reflector on the opposite side of the fiber, for example, a silver mirror or a Bragg grating at the NV center frequency [122].
The creation of immersion lenses over emitting NV centers using etching with a focused ion beam makes it possible to increase the collection of radiation to at least 10% and achieve a single photon frequency of 1 MHz [123]. This method was developed to simplify the technology of lens creation: methods of plasma anisotropic etching of lenses through multilayer lithographic masks [124] and methods of natural masking using oxide microballs [125] were tested.
In [126], the process of developing and creating a monolithic immersion metalens based on Fresnel optics with a nanostructured surface is described.
An effective solution to the problem of radiation output is the synthesis of nanodiamonds containing color centers, since the size of an individual nanodiamond is less than the wavelength of the emitted photon. Also, in some works, it was proposed to place nanodiamonds in specially etched gold-plated pits or embed them inside the resonator between Bragg mirrors applied directly to the end of the optical fiber [127–129].
For a single-photon fiber source, it is desirable to arrange the transmission of a single-photon signal to an optical fiber or an integrated waveguide. This can be done by placing a crystal with NV centers on the end of the fiber, or by applying a hemispherical lens (such as a collimator) to the surface of the diamond, directing radiation into the end of the fiber. A more complex method involves the integration of a nanodiamond containing a color center into an optical resonator, which implements optical coupling with a nearby waveguide.
It should also be noted that single-photon sources with electric pumping are being developed based on the color centers in diamond [130–132]. So far, the efficiency of electric pumping is about an order of magnitude inferior to optical pumping, but further development of this topic is very promising.
In addition, color centers exist not only in diamonds, and of course, work is underway to study the luminescence of color centers in other materials, and the possibilities of their application for single-photon detectors. As an example, we mention C–C centers and G centers in silicon [133, 134] and single silicon vacancies in high-purity silicon carbide [135–139].
In the following chapters we will consider single-photon sources based on carbon nanotubes and defects in them (engineering of defects in nanotubes), on nanocrystals and layered nanocrystals, on single molecules, in low-size structures, and metal ions in a polymer matrix, as well as ensemble systems, and sources based on nonlinear crystals.
ABOUT AUTHOR
V. G. Krishtop: e-mail: vladimir.krishtop@infotecs.ru.
ORCID: 0000-0001-6063-2657
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