rus
Issue #8/2021
N. A. Kulchitsky, A. V. Naumov, V. V. Startsev, M. A. Demyanenko
Detection in the Terahertz Range. Part II
Detection in the Terahertz Range. Part II
DOI: 10.22184/1993-7296.FRos.2021.15.8.642.655
The article discusses the problems associated with the development of technology of THz radiation detectors. The article continues to review the state of affairs in the field of uncooled matrix bolometric receivers, recent progress in various methods of detecting terahertz radiation – direct detection and heterodyne detection. Attention is focused on the advantages and disadvantages of direct detection sensors and sensors with heterodyne detection.
The article discusses the problems associated with the development of technology of THz radiation detectors. The article continues to review the state of affairs in the field of uncooled matrix bolometric receivers, recent progress in various methods of detecting terahertz radiation – direct detection and heterodyne detection. Attention is focused on the advantages and disadvantages of direct detection sensors and sensors with heterodyne detection.
Теги: direct terahertz radiation sensitivity frequency band heterodyne detection гетеродинное детектирование полоса частот прямое терагерцевое излучение чувствительность
Detection in the Terahertz Range. Part II
N. A. Kulchitsky 1, 2, A. V. Naumov 3, V. V. Startsev 3, M. A. Demyanenko 4
The State Scientific Center of the Russian Federation, Joint Stock Company “Scientific and Production Association “Orion”, Moscow, Russia
MIREA- Russian Technological University, (RTU MIREA), Moscow, Russia
Joint Stock Company “Optical and Mechanical Design Bureau Astron”, Lytkarino, Moscow region, Russia
Rzhanov Institute of Semiconductor Physics of the SB RAS, Novosibirsk, Russia
The article discusses the problems associated with the development of technology of THz radiation detectors. The article continues to review the state of affairs in the field of uncooled matrix bolometric receivers, recent progress in various methods of detecting terahertz radiation – direct detection and heterodyne detection. Attention is focused on the advantages and disadvantages of direct detection sensors and sensors with heterodyne detection.
Keywords: heterodyne detection, frequency band, direct terahertz radiation sensitivity
The article was received: 01.10.2021
The article was accepted: 24.10.2021
Uncooled matrix microbolometric receivers. Part II
Currently, uncooled microbolometric receivers are the most suitable for creating inexpensive large-format matrix THz radiation receivers. 320 × 240 and 640 × 480 terahertz cameras that operate in real time are commercially available [1]. The minimum detectable power (MDP) of radiation at wavelengths of the order of 100 microns is 20–40 pW, which, taking into account the frequency band equal to about 10 kHz, corresponds to NEP = 2–4 ∙ 10–13 W / Hz1 / 2. Such small values of MDP and NEP, close to the corresponding values achieved in the IR region, are due to the fact that constructive solutions have been found that can significantly increase the absorption coefficient of THz radiation and, in some cases, make it close to unity. For effective absorption of THz radiation, the following methods are used: 1) antennas loaded with a resistive load [2,3] and 2) thin metal absorbers (several tens of nanometers thick) with a surface resistance from 40 to 377 ohms, depending on the distance between the absorber and the reflector [4,5], 3) metamaterials or frequency-selective surfaces [6], 4) black gold [7] and 5) carbon materials, primarily vertically aligned carbon nanotubes (VACNTs) [8,9]. In all cases, the possibility of achieving almost complete absorption of THz radiation is shown. At the same time, the former three types of absorbers are characterized to varying degrees by selective frequency dependence, and the latter two types allow creating broadband receivers.
The use of antennas connected in a capacitive or direct way with a resistive load located on the bolometer membrane was considered in the first part of the review. Only one trend of their development should be noted in this regard, namely, the -replacement of butterfly-shaped antennas with V-shaped antennas with a small metal area and, consequently, low mass and heat capacity, which makes it possible to increase the speed of the receivers [10].
In the case of thin metal absorbers, the absorption coefficient turns out to be close to unity if the absorber has a layer resistance equal to the vacuum impedance (377 ohms / square), and the gap between the absorber and the reflector d is equal to a quarter of the wavelength of the λ detected radiation. For λ = 100 microns, the optimal gap d is 25 microns. However, hanging microbolometers at such a height above the substrate is a difficult technological task, therefore, the Japanese company NEC [11,12] used a design in which a thick (7 microns) layer of silicon nitride was applied to the reflector, above which a bolometer was suspended at a height of (≈4 microns). The resulting optical thickness of the resonator was 7 · 1.9 + 4 ≈ 17 microns, which is sufficient to provide 90% absorption at a wavelength of 100 microns. The minimum detectable power of a 640 × 480 receiver with a pixel size of 23.5 microns was 20 pW / pixel at a frequency of 2.5 THz, slightly decreasing towards high frequencies (up to 10 pW at 4.3 THz) and significantly increasing towards low frequencies (up to 600 × 800 pW / pixel at 0.6 THz).
Another way to increase the sensitivity of the bolometer at long waves is the use of additional optical resonators formed by a gap equal to or multiple of λ / 2 between the microbolometer and the externally illuminated input silicon window (Fig. 1). These resonators can significantly increase the absorption coefficient due to constructive interference of electromagnetic radiation incident and reflected from the bolometer and then from the inside of the input window, or lower it as destructive interference occurs [13]. This causes the receiver to become narrow-band. The simplicity of these design solutions allowed NEC (Japan) to begin manufacturing high-sensitivity miniature THz cameras [1]. The main ways to increase the absorption coefficient of THz radiation for bolometers with a thin metal absorber are described in the paper [14], which provides analytical relations for calculating the spectral dependences of the absorption coefficient in bolometric structures of various types.
Another version of the THz-band absorber is being developed using metamaterials (or frequency selective-FSS surfaces) [15–17]. Such absorbers comprise two thin layers of metal separated by a dielectric layer, d thick. One of them is a solid layer of metal that does not pass radiation through the absorber, and the second–FSS layer is–selected with a topological pattern so that it can at some frequencies provide the impedance of the receiver ZD equal to the impedance of the vacuum Zv. In this case, no reflection of the incident radiation will occur, and, consequently, the absorption coefficient will be equal to unity. The impedance of the FSS layer can be represented as a serial LCR circuit [18]
,
where ω is the circular frequency, R is the resistance, C and L are the capacitance and inductance, the values of which are determined by the shape of the frequency-selective surface. The impedance of the dielectric gap between two layers of metal is equal to
,
where Zm is the impedance of the dielectric material between two layers of metal. Eventually, the total impedance of the structure (detector) is equal to
.
In resonance, when , for wavelengths that are much greater than the thickness of the dielectric layer d, we obtain that the supposed component of the impedance ZD is close to zero, and the value of the resistance R, at which the real component of the impedance of the structure will be equal to the impedance of the vacuum Zv, should meet the condition
.
Figure 2 shows a fragment of a matrix of two-level microbolometers with a THz absorber based on a metamaterial. The matrix format is 384 × 288, the pixel size is 35 microns. The first level of the bolometer, raised 3 microns above the fully reflective mirror, contains both a thermosensitive element (VOx) and a terahertz absorber. The second level, suspended 2.3 microns above the first level, is an infrared filter consisting of an array of 6 × 6 squares reflecting IR radiation and transmitting terahertz radiation. Figure 3 shows the spectral dependence of the absorber based on the metamaterial. It can be seen that a sufficiently high absorption coefficient (more than 90%) is realized at frequencies corresponding to wavelengths of about 70 microns.
Broadband bolometric receivers of the THz band are successfully developed using absorbers based on black gold and vertically aligned carbon nanotubes (VACNTs). Gold deposited in nitrogen is of a very loose structure consisting of random chains of nano-particles of gold. It has a very low density (up to 65 mg / cm3), which makes it possible to make broadband absorbers from thick (30–50 microns) layers of black gold, without having to increase the mass and heat capacity of the sensitive bolometer membrane significantly [19]. A layer of black gold with a thickness of 30 microns is equivalent in weight and heat capacity to a solid layer of gold with a thickness of 100 nm. The layers of black gold are characterized by a resistivity ρ from 0.1 to 25 ohms · cm, while ρ = 0.5 ohms · cm is optimal for the manufacture of absorbers [20]. Based on this technology, INO (Canada) produces wide-band terahertz matrix bolometric receivers. The image of the pixels of such a matrix is shown in Fig. 4 [19]. This technology, in addition to the process of applying black gold, contains another non-standard stage of production – namely, laser cutting of layers of black gold into pixels. Nevertheless, in Canada, the production of terahertz cameras in the format of 384 × 288 pixels, with a pixel size of 35 microns, has been introduced. The minimum detectable power of the camera is 11–34 pW / pixel in the range from 4.25 to 0.198 THz, which, taking into account the signal integration time equal to 40 microseconds, when converted to a frequency band equal to 1 Hz, gives a power equivalent to noise (NEP) from 0.11 to 0.32 pW / Hz1 / 2 [21]. This means that microbolometers bear the palm among uncooled terahertz radiation receivers.
The layers of multi-walled VACNTs, as well as the layers of black gold, have a low density (20–30 mg / cm3 [22]) and can be grown with a thickness of 50–100 microns or more, which provides almost complete absorption of the its radiation incident from 0.3 to 500 microns [23,24]. Currently, the growth temperature of VACNTs is 750–850 °С, which does not allow them to be grown on bolometers that are manufactured using silicon multiplexers.
However, lines of microbolometers with a membrane size of 100 × 100 microns were demonstrated, on which a 22 microns thick layer of VONT was grown (see Fig. 5). In so doing, vanadium oxide was used as a heat-sensitive layer, applied to the membrane before the nanotubes were grown. During the growth of the VACNTs layer, the temperature coefficient of resistance VOx decreased from the initial value of –3 % / K to –1.2 % / K, which is still significantly higher than the temperature coefficient of resistance of metals (≈0.3 % / K).
Detectors with heterodyne detection
In heterodyne detectors, signals with THz or sub-THz frequencies are converted into signals with lower intermediate frequencies (IF), providing information about the amplitude and phase of the input radiation. For several decades, such detectors have been used for high-resolution spectroscopic studies, space remote sensing, and relatively recently they have been used to form images in the millimeter and sub-millimeter range.
The diagram of heterodyne detection is shown in Fig. 6. In addition to the Ws signal and the background radiation power WB, the radiation power WLO is added from a local oscillator (for example, a laser or any other type of narrow-band radiation source). A local oscillator is required to ensure the optical mixing process. The main elements of millimeter or sub-millimeter heterodyne detectors are a mixer, which is necessary for mixing Ws and WLO and for generating a signal at an intermediate frequency . The key component of the mixer is a nonlinear mixing element (detector), in which the signal power and the LO radiation power interact when using some kind of diplexer (a filter designed to combine signals of different frequency ranges, which serves to combine two ports into one). When using a millimeter or sub-millimeter matrix, the choice of mixer is determined by the availability of a LO power source in a given spectral range, the operating temperature of the mixer, and the desired sensitivity.
Calculations assume that the LO pulse power required for Schottky mixers is 1 MW, for superconductor-insulator-superconductor (SIS) mixers – 40 µW, and it is 2 µW for mixers based on hot-electron bolometers (HEB). The coupling losses of the local oscillator were assumed to be equal to 3 dB [8]. A serious problem that limits the application of the heterodyne matrices in the sub-millimeter (THz) spectral region (i. e., for applications of high-resolution spectroscopy (ν / Δν ≈ 106 where ν is the frequency, Δν is the frequency interval), or photometry (ν / Δν ≈ 3–10) and for imaging lies in the technological limitations of the power of solid-state local oscillator (LO) or heterodyne oscillator. Due to the significant attenuation, THz waves are not very useful for long-distance communication, but due to the strong absorption of most materials, THz radiation provides information about the physical properties of materials.
The main advantage of heterodyne detection systems is that the information about the frequency and phase of the signal with frequency νs is converted into the frequency νIF, which is in a much lower frequency range corresponding to the response time of the electronics. This transformation is called a heterodyne transition (conversion). If the frequencies of the signal and the local oscillator are equal to each other, then (i. e. degenerates into a constant signal), then such a detection process is identified as a homodyne transition.
For efficient conversion and ensuring low noise in the millimeter and sub-millimeter spectral ranges, only a few types of detectors can be used as mixers. Devices with strong electrical quadratic nonlinearity are often used. Examples are directly biased diodes (SBD), superconductor-insulator-superconductor (SIS) with tunnel junction, semiconductor and superconducting HEBs, and superlattices (SL). Schematic volt-ampere characteristics of such devices are shown in Figure 7.
Together with reasonable conversion efficiency and low noise, these nonlinear devices should have a high operational conversion rate so that they can provide a wide bandwidth for subsequent signal amplification at much lower frequencies (f ≈ 1–30).
With a large LO power WLO, a relatively small Ws signal power can be detected. When this condition is met (at ), the quantum noise in the signal stream can be the dominant noise, as well as for internal signal amplification G = 1 for non-photoconductive detectors with a signal-to-noise ratio S / N = 1, it follows that
,
for the minimum detectable energy, we have . For communication efficiency η = 1, this means the quantum limit of signal detection. Since, in this case, the energy of one photon received by a non-photoconductive detector is transformed into the kinetic energy of one electron, which then crosses the barrier.
For heterodyne detection, it can be shown that NEP is equal to (BLIP mode):
.
It should be noted that for heterodyne detection, the units of measurement of NEP are W / Hz instead of W / Hz0.5, as for direct detection.
The sensitivity of heterodyne detectors is often given in terms of the mixer noise temperature Tmix, which correlates with the equivalent mixer noise power:
.
For the spectral zone of λ ≈ 3 mm (ν ≈ 100 GHz), where there is an atmospheric transparency window, the value is the fundamental limit of the noise temperature introduced by the uncertainty principle of any simultaneous measurements of the amplitude and phase of an electromagnetic wave.
The limit values of the noise temperature of heterodyne THz detectors are often compared using values . Since heterodyne detectors measure both amplitude and phase, they are regulated by the uncertainty principle concurrently and therefore they are limited by quantum noise at an absolute noise level of 48 K / THz.
Comparison of heterodyne detection and direct detection
Heterodyne detection offers high spectral resolution ν / Δν ≈ 105–106. Very high spectral resolution is possible as long as . But for heterodyne systems, especially for SBD receivers in the THz region, the local oscillator is a critical component.
At the same time, detectors with direct detection, as a rule, operate in a wide spectral range, and when the photon background is low, they can provide a sufficiently high resolution. They are preferable for moderate spectral resolution ν / Δν ≈ 103–104 or lower, as well as for image formation. Direct detection detectors can be used in applications where sensitivity is more important than spectral resolution.
Having a background-limited detector array is important from the point of view of excluding, for example, background sky noise, given that any spatially correlated component of this noise detected in all sensors in the matrix can be significantly suppressed. Among detectors with direct detection, low-temperature bolometers usually provide the highest sensitivity from the far-infrared to the millimeter range of the electromagnetic spectrum, providing background-limited characteristics with NEP ≈ (0.4–3) · 10–19 W / Hz0.5 at an operating temperature of 100–300 mK. Coherent detector systems and incoherent bolometric systems were used in the cosmic microwave background (CMB) conditions. Both types of detectors can be applied for ground-based space experiments.
Compared to direct detection, heterodyne detection has its advantages and disadvantages. The advantages of heterodyne detection are as follows:
The disadvantages of heterodyne detection are:
Coherent detection systems (with SIS or SBD mixers), as a rule, are limited in detecting signals with frequencies that exceed 1 THz. Heterodyne HEB mixers and direct detection detectors based on sensors that operate near the edge of the transition to the superconducting state (TES) have almost no practical limitations in use in the short-wave sub-millimetre range [27].
Conclusion
The real progress in the technology of THz detectors is provided by the solution of technological problems, new physical concepts and phenomena, as well as promising applications. The characteristics of several types of discrete detectors and small-format matrices operating at low or sub-Kelvin temperatures (for example, SIS, HEB, TES and cold electron bolometers (CEB)) are close to the limiting characteristics at a low background level. They cover the entire THz range. However, future sensitivity improvement will be provided by the use of large-format matrices with reading in the focal plane to provide high-resolution spectroscopy (ν / Δν ≈ 107) and recording at frequencies that exceed 1 THz. Superconducting HEB detectors are characterized by good background response characteristics, high counting speed and they are also promising as counters of individual photons in a wide IR spectral range. It is doubtful that superconducting HEBs operating at high temperatures will achieve the sensitivity of low-temperature superconducting HEBs due to excessive noise, but due to the short electron-phonon relaxation time, these materials are promising for broadband devices.
One of the important components of THz technology are uncooled or slightly cooled THz sensors, which require further sensitivity improvement that will make systems less complex and cumbersome. Most millimeter and submillimeter spectrometers with medium resolution often employ uncooled detectors operating in a wide frequency band. The advantages of uncooled detectors are the relative simplicity of the circuit, as well as their ability to operate at room temperature in a wide frequency band. Their NEP is in the range of 10–9–10–11 W / Hz0.5. Of interest are also studies aimed at creating new developments of terahertz detectors, for example, based on low-dimensional structures from HgCdTe, based on PbSnTe: In, based on quantum rings and Ge / Si quantum dots [27].
Uncooled or slightly cooled sensors based, for example, on the plasmonic resonance of 2D electrons in HEMTs are promising for use in large-format matrices in low-cost systems. Other uncooled THz thermal direct detection detectors with NEP that are developed or under development ~10–10–10–11 W / Hz0.5 can be used in many low-resolution spectroscopic applications and active surveillance systems.
ABOUT AUTHORS
Naumov A. V., Head of the Research Area, Joint Stock Company “Optical and Mechanical Design Bureau Astron”, Lytkarino, Moscow region, Russia.
ORCID: 0000-0001-6081-8304
Startsev V. V., Cand. of Sc.(Eng.), Chief Designer, Joint Stock Company “Optical and Mechanical Design Bureau Astron”, Lytkarino, Moscow region, Russia.
ORCID: 0000-0002-2800-544X
Kulchitsky N. A., Dr. of Sc. (Eng.), Deputy Head of the Department, The State Scientific Center of the Russian Federation, Joint Stock Company “Scientific and Production Association “Orion”, Prof. MIREA- Russian Technological University, (RTU MIREA), Moscow, Russia.
ORCID: 0000-0003-4664-4891
Demyanenko M. A., Cand. of Sc.(Phys&Math), Senior Researcher, Rzhanov Institute of Semiconductor Physics of the SB RAS, Novosibirsk, Russia.
ORCID: 0000-0002-8840-9446
N. A. Kulchitsky 1, 2, A. V. Naumov 3, V. V. Startsev 3, M. A. Demyanenko 4
The State Scientific Center of the Russian Federation, Joint Stock Company “Scientific and Production Association “Orion”, Moscow, Russia
MIREA- Russian Technological University, (RTU MIREA), Moscow, Russia
Joint Stock Company “Optical and Mechanical Design Bureau Astron”, Lytkarino, Moscow region, Russia
Rzhanov Institute of Semiconductor Physics of the SB RAS, Novosibirsk, Russia
The article discusses the problems associated with the development of technology of THz radiation detectors. The article continues to review the state of affairs in the field of uncooled matrix bolometric receivers, recent progress in various methods of detecting terahertz radiation – direct detection and heterodyne detection. Attention is focused on the advantages and disadvantages of direct detection sensors and sensors with heterodyne detection.
Keywords: heterodyne detection, frequency band, direct terahertz radiation sensitivity
The article was received: 01.10.2021
The article was accepted: 24.10.2021
Uncooled matrix microbolometric receivers. Part II
Currently, uncooled microbolometric receivers are the most suitable for creating inexpensive large-format matrix THz radiation receivers. 320 × 240 and 640 × 480 terahertz cameras that operate in real time are commercially available [1]. The minimum detectable power (MDP) of radiation at wavelengths of the order of 100 microns is 20–40 pW, which, taking into account the frequency band equal to about 10 kHz, corresponds to NEP = 2–4 ∙ 10–13 W / Hz1 / 2. Such small values of MDP and NEP, close to the corresponding values achieved in the IR region, are due to the fact that constructive solutions have been found that can significantly increase the absorption coefficient of THz radiation and, in some cases, make it close to unity. For effective absorption of THz radiation, the following methods are used: 1) antennas loaded with a resistive load [2,3] and 2) thin metal absorbers (several tens of nanometers thick) with a surface resistance from 40 to 377 ohms, depending on the distance between the absorber and the reflector [4,5], 3) metamaterials or frequency-selective surfaces [6], 4) black gold [7] and 5) carbon materials, primarily vertically aligned carbon nanotubes (VACNTs) [8,9]. In all cases, the possibility of achieving almost complete absorption of THz radiation is shown. At the same time, the former three types of absorbers are characterized to varying degrees by selective frequency dependence, and the latter two types allow creating broadband receivers.
The use of antennas connected in a capacitive or direct way with a resistive load located on the bolometer membrane was considered in the first part of the review. Only one trend of their development should be noted in this regard, namely, the -replacement of butterfly-shaped antennas with V-shaped antennas with a small metal area and, consequently, low mass and heat capacity, which makes it possible to increase the speed of the receivers [10].
In the case of thin metal absorbers, the absorption coefficient turns out to be close to unity if the absorber has a layer resistance equal to the vacuum impedance (377 ohms / square), and the gap between the absorber and the reflector d is equal to a quarter of the wavelength of the λ detected radiation. For λ = 100 microns, the optimal gap d is 25 microns. However, hanging microbolometers at such a height above the substrate is a difficult technological task, therefore, the Japanese company NEC [11,12] used a design in which a thick (7 microns) layer of silicon nitride was applied to the reflector, above which a bolometer was suspended at a height of (≈4 microns). The resulting optical thickness of the resonator was 7 · 1.9 + 4 ≈ 17 microns, which is sufficient to provide 90% absorption at a wavelength of 100 microns. The minimum detectable power of a 640 × 480 receiver with a pixel size of 23.5 microns was 20 pW / pixel at a frequency of 2.5 THz, slightly decreasing towards high frequencies (up to 10 pW at 4.3 THz) and significantly increasing towards low frequencies (up to 600 × 800 pW / pixel at 0.6 THz).
Another way to increase the sensitivity of the bolometer at long waves is the use of additional optical resonators formed by a gap equal to or multiple of λ / 2 between the microbolometer and the externally illuminated input silicon window (Fig. 1). These resonators can significantly increase the absorption coefficient due to constructive interference of electromagnetic radiation incident and reflected from the bolometer and then from the inside of the input window, or lower it as destructive interference occurs [13]. This causes the receiver to become narrow-band. The simplicity of these design solutions allowed NEC (Japan) to begin manufacturing high-sensitivity miniature THz cameras [1]. The main ways to increase the absorption coefficient of THz radiation for bolometers with a thin metal absorber are described in the paper [14], which provides analytical relations for calculating the spectral dependences of the absorption coefficient in bolometric structures of various types.
Another version of the THz-band absorber is being developed using metamaterials (or frequency selective-FSS surfaces) [15–17]. Such absorbers comprise two thin layers of metal separated by a dielectric layer, d thick. One of them is a solid layer of metal that does not pass radiation through the absorber, and the second–FSS layer is–selected with a topological pattern so that it can at some frequencies provide the impedance of the receiver ZD equal to the impedance of the vacuum Zv. In this case, no reflection of the incident radiation will occur, and, consequently, the absorption coefficient will be equal to unity. The impedance of the FSS layer can be represented as a serial LCR circuit [18]
,
where ω is the circular frequency, R is the resistance, C and L are the capacitance and inductance, the values of which are determined by the shape of the frequency-selective surface. The impedance of the dielectric gap between two layers of metal is equal to
,
where Zm is the impedance of the dielectric material between two layers of metal. Eventually, the total impedance of the structure (detector) is equal to
.
In resonance, when , for wavelengths that are much greater than the thickness of the dielectric layer d, we obtain that the supposed component of the impedance ZD is close to zero, and the value of the resistance R, at which the real component of the impedance of the structure will be equal to the impedance of the vacuum Zv, should meet the condition
.
Figure 2 shows a fragment of a matrix of two-level microbolometers with a THz absorber based on a metamaterial. The matrix format is 384 × 288, the pixel size is 35 microns. The first level of the bolometer, raised 3 microns above the fully reflective mirror, contains both a thermosensitive element (VOx) and a terahertz absorber. The second level, suspended 2.3 microns above the first level, is an infrared filter consisting of an array of 6 × 6 squares reflecting IR radiation and transmitting terahertz radiation. Figure 3 shows the spectral dependence of the absorber based on the metamaterial. It can be seen that a sufficiently high absorption coefficient (more than 90%) is realized at frequencies corresponding to wavelengths of about 70 microns.
Broadband bolometric receivers of the THz band are successfully developed using absorbers based on black gold and vertically aligned carbon nanotubes (VACNTs). Gold deposited in nitrogen is of a very loose structure consisting of random chains of nano-particles of gold. It has a very low density (up to 65 mg / cm3), which makes it possible to make broadband absorbers from thick (30–50 microns) layers of black gold, without having to increase the mass and heat capacity of the sensitive bolometer membrane significantly [19]. A layer of black gold with a thickness of 30 microns is equivalent in weight and heat capacity to a solid layer of gold with a thickness of 100 nm. The layers of black gold are characterized by a resistivity ρ from 0.1 to 25 ohms · cm, while ρ = 0.5 ohms · cm is optimal for the manufacture of absorbers [20]. Based on this technology, INO (Canada) produces wide-band terahertz matrix bolometric receivers. The image of the pixels of such a matrix is shown in Fig. 4 [19]. This technology, in addition to the process of applying black gold, contains another non-standard stage of production – namely, laser cutting of layers of black gold into pixels. Nevertheless, in Canada, the production of terahertz cameras in the format of 384 × 288 pixels, with a pixel size of 35 microns, has been introduced. The minimum detectable power of the camera is 11–34 pW / pixel in the range from 4.25 to 0.198 THz, which, taking into account the signal integration time equal to 40 microseconds, when converted to a frequency band equal to 1 Hz, gives a power equivalent to noise (NEP) from 0.11 to 0.32 pW / Hz1 / 2 [21]. This means that microbolometers bear the palm among uncooled terahertz radiation receivers.
The layers of multi-walled VACNTs, as well as the layers of black gold, have a low density (20–30 mg / cm3 [22]) and can be grown with a thickness of 50–100 microns or more, which provides almost complete absorption of the its radiation incident from 0.3 to 500 microns [23,24]. Currently, the growth temperature of VACNTs is 750–850 °С, which does not allow them to be grown on bolometers that are manufactured using silicon multiplexers.
However, lines of microbolometers with a membrane size of 100 × 100 microns were demonstrated, on which a 22 microns thick layer of VONT was grown (see Fig. 5). In so doing, vanadium oxide was used as a heat-sensitive layer, applied to the membrane before the nanotubes were grown. During the growth of the VACNTs layer, the temperature coefficient of resistance VOx decreased from the initial value of –3 % / K to –1.2 % / K, which is still significantly higher than the temperature coefficient of resistance of metals (≈0.3 % / K).
Detectors with heterodyne detection
In heterodyne detectors, signals with THz or sub-THz frequencies are converted into signals with lower intermediate frequencies (IF), providing information about the amplitude and phase of the input radiation. For several decades, such detectors have been used for high-resolution spectroscopic studies, space remote sensing, and relatively recently they have been used to form images in the millimeter and sub-millimeter range.
The diagram of heterodyne detection is shown in Fig. 6. In addition to the Ws signal and the background radiation power WB, the radiation power WLO is added from a local oscillator (for example, a laser or any other type of narrow-band radiation source). A local oscillator is required to ensure the optical mixing process. The main elements of millimeter or sub-millimeter heterodyne detectors are a mixer, which is necessary for mixing Ws and WLO and for generating a signal at an intermediate frequency . The key component of the mixer is a nonlinear mixing element (detector), in which the signal power and the LO radiation power interact when using some kind of diplexer (a filter designed to combine signals of different frequency ranges, which serves to combine two ports into one). When using a millimeter or sub-millimeter matrix, the choice of mixer is determined by the availability of a LO power source in a given spectral range, the operating temperature of the mixer, and the desired sensitivity.
Calculations assume that the LO pulse power required for Schottky mixers is 1 MW, for superconductor-insulator-superconductor (SIS) mixers – 40 µW, and it is 2 µW for mixers based on hot-electron bolometers (HEB). The coupling losses of the local oscillator were assumed to be equal to 3 dB [8]. A serious problem that limits the application of the heterodyne matrices in the sub-millimeter (THz) spectral region (i. e., for applications of high-resolution spectroscopy (ν / Δν ≈ 106 where ν is the frequency, Δν is the frequency interval), or photometry (ν / Δν ≈ 3–10) and for imaging lies in the technological limitations of the power of solid-state local oscillator (LO) or heterodyne oscillator. Due to the significant attenuation, THz waves are not very useful for long-distance communication, but due to the strong absorption of most materials, THz radiation provides information about the physical properties of materials.
The main advantage of heterodyne detection systems is that the information about the frequency and phase of the signal with frequency νs is converted into the frequency νIF, which is in a much lower frequency range corresponding to the response time of the electronics. This transformation is called a heterodyne transition (conversion). If the frequencies of the signal and the local oscillator are equal to each other, then (i. e. degenerates into a constant signal), then such a detection process is identified as a homodyne transition.
For efficient conversion and ensuring low noise in the millimeter and sub-millimeter spectral ranges, only a few types of detectors can be used as mixers. Devices with strong electrical quadratic nonlinearity are often used. Examples are directly biased diodes (SBD), superconductor-insulator-superconductor (SIS) with tunnel junction, semiconductor and superconducting HEBs, and superlattices (SL). Schematic volt-ampere characteristics of such devices are shown in Figure 7.
Together with reasonable conversion efficiency and low noise, these nonlinear devices should have a high operational conversion rate so that they can provide a wide bandwidth for subsequent signal amplification at much lower frequencies (f ≈ 1–30).
With a large LO power WLO, a relatively small Ws signal power can be detected. When this condition is met (at ), the quantum noise in the signal stream can be the dominant noise, as well as for internal signal amplification G = 1 for non-photoconductive detectors with a signal-to-noise ratio S / N = 1, it follows that
,
for the minimum detectable energy, we have . For communication efficiency η = 1, this means the quantum limit of signal detection. Since, in this case, the energy of one photon received by a non-photoconductive detector is transformed into the kinetic energy of one electron, which then crosses the barrier.
For heterodyne detection, it can be shown that NEP is equal to (BLIP mode):
.
It should be noted that for heterodyne detection, the units of measurement of NEP are W / Hz instead of W / Hz0.5, as for direct detection.
The sensitivity of heterodyne detectors is often given in terms of the mixer noise temperature Tmix, which correlates with the equivalent mixer noise power:
.
For the spectral zone of λ ≈ 3 mm (ν ≈ 100 GHz), where there is an atmospheric transparency window, the value is the fundamental limit of the noise temperature introduced by the uncertainty principle of any simultaneous measurements of the amplitude and phase of an electromagnetic wave.
The limit values of the noise temperature of heterodyne THz detectors are often compared using values . Since heterodyne detectors measure both amplitude and phase, they are regulated by the uncertainty principle concurrently and therefore they are limited by quantum noise at an absolute noise level of 48 K / THz.
Comparison of heterodyne detection and direct detection
Heterodyne detection offers high spectral resolution ν / Δν ≈ 105–106. Very high spectral resolution is possible as long as . But for heterodyne systems, especially for SBD receivers in the THz region, the local oscillator is a critical component.
At the same time, detectors with direct detection, as a rule, operate in a wide spectral range, and when the photon background is low, they can provide a sufficiently high resolution. They are preferable for moderate spectral resolution ν / Δν ≈ 103–104 or lower, as well as for image formation. Direct detection detectors can be used in applications where sensitivity is more important than spectral resolution.
Having a background-limited detector array is important from the point of view of excluding, for example, background sky noise, given that any spatially correlated component of this noise detected in all sensors in the matrix can be significantly suppressed. Among detectors with direct detection, low-temperature bolometers usually provide the highest sensitivity from the far-infrared to the millimeter range of the electromagnetic spectrum, providing background-limited characteristics with NEP ≈ (0.4–3) · 10–19 W / Hz0.5 at an operating temperature of 100–300 mK. Coherent detector systems and incoherent bolometric systems were used in the cosmic microwave background (CMB) conditions. Both types of detectors can be applied for ground-based space experiments.
Compared to direct detection, heterodyne detection has its advantages and disadvantages. The advantages of heterodyne detection are as follows:
- it can detect frequency modulation and phase modulation;
- the dominant noise follows from fluctuations in the power of the heterodyne WLO and it is more likely than background radiation noise, thus selectivity is ensured, for example, against background flux, etc.;
- the IF frequency conversion process provides amplification so that the output signal of the IF detector can be made greater than, for example, thermal and generation-recombination noise;
- the gain of the conversion is proportional to WLO / Ws and thus a much lower power of the radiation signal can be detected compared to direct detection.
The disadvantages of heterodyne detection are:
- both beams should coincide and be equal in diameter, and also their Pointing vectors should coincide;
- the wavefronts of both beams should have the same radii of curvature and have similar structures of transverse spatial modes, therefore they should be polarized in the same direction;
- difficulties in the production of large format matrices.
Coherent detection systems (with SIS or SBD mixers), as a rule, are limited in detecting signals with frequencies that exceed 1 THz. Heterodyne HEB mixers and direct detection detectors based on sensors that operate near the edge of the transition to the superconducting state (TES) have almost no practical limitations in use in the short-wave sub-millimetre range [27].
Conclusion
The real progress in the technology of THz detectors is provided by the solution of technological problems, new physical concepts and phenomena, as well as promising applications. The characteristics of several types of discrete detectors and small-format matrices operating at low or sub-Kelvin temperatures (for example, SIS, HEB, TES and cold electron bolometers (CEB)) are close to the limiting characteristics at a low background level. They cover the entire THz range. However, future sensitivity improvement will be provided by the use of large-format matrices with reading in the focal plane to provide high-resolution spectroscopy (ν / Δν ≈ 107) and recording at frequencies that exceed 1 THz. Superconducting HEB detectors are characterized by good background response characteristics, high counting speed and they are also promising as counters of individual photons in a wide IR spectral range. It is doubtful that superconducting HEBs operating at high temperatures will achieve the sensitivity of low-temperature superconducting HEBs due to excessive noise, but due to the short electron-phonon relaxation time, these materials are promising for broadband devices.
One of the important components of THz technology are uncooled or slightly cooled THz sensors, which require further sensitivity improvement that will make systems less complex and cumbersome. Most millimeter and submillimeter spectrometers with medium resolution often employ uncooled detectors operating in a wide frequency band. The advantages of uncooled detectors are the relative simplicity of the circuit, as well as their ability to operate at room temperature in a wide frequency band. Their NEP is in the range of 10–9–10–11 W / Hz0.5. Of interest are also studies aimed at creating new developments of terahertz detectors, for example, based on low-dimensional structures from HgCdTe, based on PbSnTe: In, based on quantum rings and Ge / Si quantum dots [27].
Uncooled or slightly cooled sensors based, for example, on the plasmonic resonance of 2D electrons in HEMTs are promising for use in large-format matrices in low-cost systems. Other uncooled THz thermal direct detection detectors with NEP that are developed or under development ~10–10–10–11 W / Hz0.5 can be used in many low-resolution spectroscopic applications and active surveillance systems.
ABOUT AUTHORS
Naumov A. V., Head of the Research Area, Joint Stock Company “Optical and Mechanical Design Bureau Astron”, Lytkarino, Moscow region, Russia.
ORCID: 0000-0001-6081-8304
Startsev V. V., Cand. of Sc.(Eng.), Chief Designer, Joint Stock Company “Optical and Mechanical Design Bureau Astron”, Lytkarino, Moscow region, Russia.
ORCID: 0000-0002-2800-544X
Kulchitsky N. A., Dr. of Sc. (Eng.), Deputy Head of the Department, The State Scientific Center of the Russian Federation, Joint Stock Company “Scientific and Production Association “Orion”, Prof. MIREA- Russian Technological University, (RTU MIREA), Moscow, Russia.
ORCID: 0000-0003-4664-4891
Demyanenko M. A., Cand. of Sc.(Phys&Math), Senior Researcher, Rzhanov Institute of Semiconductor Physics of the SB RAS, Novosibirsk, Russia.
ORCID: 0000-0002-8840-9446
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