rus
Issue #5/2025
G. I. Kropotov, D. A. Popov, D. I. Tsypishka, A. A. Shakhmin
Optics for Terahertz Applications
Optics for Terahertz Applications
DOI: 10.22184/1993-7296.FRos.2025.19.5.364.376
The article discusses the production features of the optical elements for application in the THz range devices and equipment, describes the properties and specifications of optical materials for its production, and provides the application examples. Special attention is paid to the elements used exclusively in the THz systems and devices.
The article discusses the production features of the optical elements for application in the THz range devices and equipment, describes the properties and specifications of optical materials for its production, and provides the application examples. Special attention is paid to the elements used exclusively in the THz systems and devices.
Теги: fresnel’s lenses f-theta lenses f-theta линзы optical elements for the terahertz range terahertz (thz) radiation линзы френеля оптические элементы для терагерцевого диапазона терагерцевое излучение
Optics for Terahertz Applications
G. I. Kropotov, D. A. Popov, D. I. Tsypishka, A. A. Shakhmin
TYDEX LLC, Saint Petersburg, Russia
The article discusses the production features of the optical elements for application in the THz range devices and equipment, describes the properties and specifications of optical materials for its production, and provides the application examples. Special attention is paid to the elements used exclusively in the THz systems and devices.
Keywords: terahertz (THz) radiation, optical elements for the terahertz range, Fresnel’s lenses, f-theta lenses.
Article received: June 26, 2025
Article accepted: July 21, 2025
Introduction
The terahertz (THz) radiation is electromagnetic radiation in the frequency range from 0.1 to 10 THz (wavelength of 3 mm – 30 μm). It penetrates well through many dielectric materials, such as plastic, fabric, paper, cardboard, leather, ceramics. It is absorbed by the Earth’s atmosphere (mainly by the water molecules) and is reflected from the metal surface. Radiation in this range is easily focused similar to the infrared (IR) radiation. Moreover, it is not ionizing, like X-rays. These properties are used in the production of pharmaceutical drugs and quality control of the food products. The application of THz radiation is of the utmost interest in such areas as safety monitoring, packaging inspection, characterization of semiconductor materials, chemical composition analysis, biochemical research and telecommunications.
The production of devices operating in the THz range is related to the development of passive optical elements made of materials that transmit the THz radiation rather well. These materials should have the low dispersion of THz waves, high thermal stability, mechanical strength, and chemical inertness. The reflective and transmitting properties of optical components can be changed, enhanced, or suppressed in a controlled manner by selection of the suitable materials and application of the special optical coatings to them.
Selection of materials
for the THz optics production
Conventionally, the high-resistance silicon (HRFZ-Si) grown by the floating-zone method [1] and having a specific resistance in the n-type from 10 kOhm×cm, or in the p-type from 25 kOhm×cm and higher is used for the production of passive components of THz photonics. HRFZ-Si is an isotropic crystalline material being transparent in a wide range of wavelengths, starting from the near IR, and having a constant transmission at the level of 50–54% from 40 micrometers to the microwaves (fig. 1). This allows the development of various optical elements for the THz optoelectronics on its basis.
In addition to Si, the crystals such as crystalline quartz, sapphire, germanium, and diamond play an important role in the THz applications.
Crystalline quartz is a birefringent material that should be considered if the radiation polarization is important. For work in the THz range, the material in the “x-cut” geometry is used in the production of λ/2 and λ/4 wave plates, when the plate surface is perpendicular to the x-axis of the crystal.
It is reasonable to use the fused quartz along with the crystalline quartz for the production of optical elements of devices operating in the THz range, when the radiation polarization is not important. Its cost is lower, and its processing does not require any orientation of the crystal and optical elements along the crystallographic axes. The transmission spectra of crystalline and fused quartz are shown in fig. 2.
Sapphire similar to the crystalline and fused quartz, is transparent in the THz region as well as in the visible one (fig. 3). Similar to HRFZ-Si, the hypo-hyper- and hemispherical lenses for the THz detectors and generators based on the photoconductive antennas are made from sapphire that has a refractive index close to the silicon’s values.
Germanium is used in the THz range to manufacture the active elements of acoustic and optic devices [2]. It is also applied in the multispectral thermal imaging devices of the IR + THz ranges [3].
The polycrystalline diamond that is transparent for ultraviolet radiation to the millimeter range, is used to produce the entry and exit gates in the powerful THz radiation sources. The polycrystalline diamond has high thermal conductivity and a low thermal-expansion coefficient that allows it to withstand high radiation loads [4].
A promising material for the production of optics in the long-wavelength region of the THz range is gallium arsenide (GaAs). This can be judged by comparing its transmission spectrum with the spectra of HRFZ-Si and Ge (fig. 5).
Another group of materials for THz photonics includes the polymers. Some of them have excellent transparency within the THz range with relatively low reflectivity. Polymethylpentene (TPX), Picarin (Tsurupica), cycloolefin polymer (Zeonex), cyclic olefin copolymer (COC or TOPAS), polyethylene (HDPE), high-molecular-weight polyethylene (UHMWPE), polytetrafluoroethylene (PTFE, fluoroplast or Teflon), and polypropylene (PP) find their practical application [5]. At the long wavelengths, the transmission of these polymers is structureless and flat. At the short wavelengths (typically less than 200 micrometers), there are some typical absorption bands (fig. 6) related to the natural vibrations, with the enhanced scattering by the material inhomogeneities. The polymers usually become less transparent as the wavelength decreases.
TPX is the lightest of all well-known polymers. It is transparent in the ultraviolet, visible and far infrared ranges that, for example, allows using a helium-neon laser beam to tune the THz optical systems. Its refractive index is ~1.46, and it is almost independent of wavelength in the long-wavelength range. TPX is a hard and durable material that can be mechanically converted into various optical components, such as lenses and windows. A certain specific application of TPX is its use as an exit gate in the gas molecular lasers optically pumped by a CO2 laser, due to its transparency in the entire THz range and absolute suppression of pump radiation in the vicinity of λ = 10 micrometers. Moreover, the TPX gates are also used in the cryostats as the “cold” gates.
The TPX transmission in the THz range is independent of temperature. The temperature coefficient of the refractive index is –3.0 · 10−4 K−1 (for the temperature range of 8–120 K).
Polyethylene is a light and elastic crystallized material. The heat resistance of some polyethylene grades reaches 110 °C, allowing cooling to –45 ÷ –120 °C, depending on the grade. Polyethylene has good dielectric characteristics, chemical and radiation resistance. The refractive index is ~ 1.54 and is changed little over a wide range of wavelengths. The high-density polyethylene (HDPE) or ultra-high-molecular-weight high-density polyethylene (UHMWPE) are usually used to produce the optical components. The thin HDPE films are applied in the THz polarizers. HDPE is also used to make the gates for optoacoustic receivers. It should be noted that the HDPE transmission in the THz range does not depend on temperature that allows it to be used in the cryostats. Its temperature coefficient of refractive index is –6.2·10–4 K‑1 for the temperature range of 8–120K. UHMWPE has a good combination of rigidity and mechanical damping capacity, and lends itself to machining with no difficulty. Based on the combination of properties, UHMWPE is easier to polish than HDPE that is important in the production of optical elements.
Polytetrafluoroethylene (PTFE) is a hard, heavy, white plastic with a density of about 2.2 g/cm3. Its melting point is 327 °C, and it retains its properties over a wide temperature range from –73 °C to 204 °C. Its refractive index is n ≈ 1.43 over a wide range of wavelengths.
ZEONEX has excellent mechanical properties, good chemical resistance and very low vacuum degassing in the conditions of up to 10–12 mmHg. The superior mechanical properties of ZEONEX compared to TPX make it possible to use this plastic for the production of THz prisms.
COC is attractive due to its high operating temperature, excellent transmittance, low birefringence, and low moisture absorption. An important advantage of COC is the possible production of various terahertz components using the 3D printing process.
In some cases it is more convenient to use polypropylene to manufacture various passive elements of the THz range. Polypropylene is obtained by polymerizing propylene in the presence of metal-complex catalysts. It is more resistant to the aggressive environments compared to the HDPE polyethylene and is better processed mechanically.
The efficient conversion of THz-range light beams requires the use of antireflective optical coatings, similar to the methods when such coatings are used in other (visible and IR) radiation regions. However, in the THz region, the special polymer coatings that are transparent in this wavelength range are required. These include poly-para-xylylene (PPX) (parylene) coatings.
The PPX coatings are specified by high homogeneity and high protective properties, inertness to most organic solvents and solutions of acids, alkalis and salts. The coating development does not require any high temperatures that eliminates overheating of parts. There are no internal stresses in the coating.
While using the refractive index of the PPX n = 1.63, the antireflection coating thickness can be determined by calculation for antireflection in the required wavelength range. Fig. 7 shows a section of the transmission spectrum of a crystalline quartz plate with a two-sided antireflection coating made of PPX with a thickness of 60 μm. The range of antireflection wavelengths is 200–600 μm. The antireflection coatings made of PPX can also be applied to the HRFZ-Si substrates.
THz gates, lenses and mirrors
In various THz devices and equipment, the gates made of HRFZ-Si, crystalline quartz (z-cut) and sapphire, as well as plastics, are widely used. The gates can be either plane-parallel or wedge-shaped.
The gates made of crystalline quartz have the following important properties:
For the THz applications, TYDEX manufactures the lenses of classical shape using HRFZ-Si and TPX [6]. A special category includes the hemispherical lenses of various shapes: hyperhemispheres, hemispheres, hypohemispheres and “bullet” lenses (fig. 8).
A hyperhemisphere is a small optical surface made in the shape of a hemisphere, but with a boundary bend exceeding 180°. The hyperhemispheres and hemispheres are used in the THz optical systems based on the solid-state immersion effect [7]. A hypohemisphere is a truncated form of a hemispherical lens. It is used in various THz optical systems and in the THz microscopy [8]. The “bullet” lens is a hemisphere combined with a cylinder, where the spherical surface radius is R, the cylinder diameter is 2R, and the total thickness is L. Such lenses are applied in the manufacture of integrated lens antennas, including those for the THz and microwave ranges. The HRFZ-Si hemispheres and hyperhemispheres are widely used to produce the matrix-type receiving devices in the THz scanning and visualization systems [9,10].
Si axicons. A conical lens or axicon (fig. 9) is used to transform a Gaussian beam into a Bessel beam. The axicon focuses the radiation into a line consisting of many points along the optical axis. The resulting line generates a ring of constant width with an increasing diameter as the distance to the element increases (fig. 10). The axicons are made of HRFZ-Si.
THz Fresnel lenses. The radiation control is necessary in many THz applications. Most often, it is achieved using the full-parabolic deflectors and refractive optics. However, the diffractive optics provide new, fundamentally different possibilities, since it allows for spatial transformation of the beam. In order to meet the needs for diffractive optics in the THz frequency range, a calculation method and production technology for the THz Fresnel lenses using the 3D printing process have been developed [9].
The Fresnel lenses have a multi-level profile that is close to the ideal form of the phase function of the diffraction focuser into a spot to a maximum level. The monochromatic and broadband Fresnel lenses have various thickness of the microrelief profile. The THz-monochromatic Fresnel lenses (1st order kinoforms) operate at the rated frequency, frequencies multiple of the rated frequency, and in a wide range of high frequencies. The THz-broadband Fresnel lenses (high-order kinoforms) have a wide operating frequency range that is “shifted” closer to the rated frequency. Thus, the THz-monochromatic Fresnel lenses should be used in the cases where efficient control of the THz-continuous radiation beam is required. The THz-broadband Fresnel lenses can be used to control the THz-pulsed radiation. Thermoplastic made of cycloolefin copolymer COC is used as a material for both types of THz Fresnel lenses. The main advantages of Fresnel lenses over the refractive lenses include the almost complete absence of spherical aberrations, smaller thickness and lighter weight when working with the large diameter beams and, as a result, better radiation resistance due to the less radiation absorption inside the lens body.
The THz monochromatic Fresnel lenses focus THz radiation better than the refractive lenses. A comparison of radiation intensity focused by a diffractive Fresnel’s lens with the radiation intensity focused by a TPX refractive lens is shown in Fig. 12.
The THz broadband Fresnel lenses focus THz radiation no worse than the refractive lenses. Figure 13 shows a comparison of the radiation intensity spectrum focused by a diffractive Fresnel lens (rated operating frequency 1 THz, focal length 90 mm, lens diameter 93 mm) with the intensity spectrum of radiation focused by a refractive lens made of TPX.
To focus radiation in the THz imaging and scanning systems, it is possible to use f-theta lenses [6] (Fig. 14) instead of spherical lenses. They are capable of providing a flat focusing surface and a nearly constant spot size over the entire scanning field. Such lenses are designed to provide a linear beam offset as a linear function of the deflection angle that results in a constant scanning speed on a flat surface. They are manufactured using the 3D printing process and COC. The f-theta lenses also provide a constant phase of broadband THz radiation in the scanning plane and high image resolution. The images obtained by the f-theta lenses have barrel distortion, and the spot position on the image plane is directly proportional to the scanning angle. This feature eliminates the need for comprehensive electronic adjustment and allows for the assembly of a fast, relatively inexpensive, and space-saving scanning system. The f-theta lenses also provide a constant phase of broadband THz radiation in the scanning plane and high image resolution.
Fig. 15 shows the dependence of the transverse coordinate of the THz radiation maximum at the focus of f-theta lens from the angle of deviation (angle of incidence of the THz radiation on the f-theta lens). As can be seen from the figure, in the range of angles from –25 to +25 degrees this dependence is linear that allows scanning a flat surface at a constant speed.
For the applications where it is necessary to completely reflect THz radiation, TYDEX suggests using the THz mirrors [6]. These mirrors are made of fused quartz coated with gold and protected by the films of yttrium dioxide (I2O3) and silicon dioxide (SiO2). The reflection spectrum of such a mirror is shown in fig. 16.
THz spectrum splitters
and beam splitters
For the applications where it is necessary to reflect the near infrared (NIR) or intermediate infrared (IIR) radiation while maintaining good transmission in the THz range, TYDEX offers the THz spectrum splitters [6].
The NIR-THz spectrum splitter is used to separate the THz radiation from the pump of a titanium-sapphire laser, and the IIR-THz spectrum splitter is used to separate from the CO2 laser pumping. The high-resistance substrates made of Si or crystalline quartz with a highly reflective dielectric coating (R>90%) in the range of 730–860 nm are applied to produce the NIR-THz spectrum splitters. When producing the IIR-THz spectrum splitters, the highly reflective coatings in the range of 9–11 μm are applied to the substrates made of the same materials. The transmission and reflection spectra of the spectrum splitters are shown in Figures 17 and 18.
The beam splitters are optical elements designed to split a light beam into two or more parts.
They are used in various optical schemes, as well as in the interferometers. TYDEX manufactures two types of the THz beam splitters [12]:
single-pass beam splitters (they are used in the optical circuits where the radiation passes through the splitter once);
multi-pass beam splitters for interferometers (such a splitter is intended for the multiple beam passes). In contrast to a single-pass one, a multi-pass beam splitter has very high plane-parallelism of surfaces.
The material for THz beam splitters is high-resistance Si. The transmission and reflection spectra are shown in Fig. 1.
Conclusion
The expanded application of THz radiation makes the development and production of efficient optical elements for the THz range relevant. The novelty in this area is the development and production of optical elements for controlling the THz radiation beam specifications in the THz imaging and scanning systems. Examples of such solutions include the Fresnel’s lenses and f-theta lenses.
Various materials are used to produce the THz optics, both those well-known due to the manufacture of classical optics and new ones that have not previously found application for this purpose. The spectra and other specifications of crystals and plastics given in the article allow us to select the necessary materials for the manufacture of optical components for use in the research THz systems, as well as in various THz devices.
AUTHORS
Kropotov Grigory, Cand. of Scien. (Phys&Math.), Senior Researcher, General Manager LLC «TYDEX», e-mail: grigorykropotov@tydex.ru; Saint Petersburg, Russia.
ORCID: 0000-0001-9041-6701
Popov Dmitry, Cand. of Scien. (Phys&Math.), Researcher LLC «TYDEX», e-mail: dmitriypopov@tydex.ru; Saint Petersburg, Russia.
ORCID: 0009-0007-2427-2341
Tsypishka Dmitry, Cand. of Scien. (Phys&Math.), associate professor, Deputy General Manager LLC «TYDEX», e-mail: dmitrytsypishka@tydex.ru; Saint Petersburg, Russia.
ORCID: 0000-0003-1914-5245
Shakhmin Alexander, Cand. of Scien. (Phys&Math.), Leading Research Engineer LLC «TYDEX», e-mail: AlexeyShakhmin@tydex.ru; Saint Petersburg, Russia.
ORCID: 0009-0003-9566-2823
CONTRIBUTION OF THE AUTHORS
The article is based on the work of all members of the author’s team.
CONFLICT OF INTEREST
The authors declare that there is no conflict of interest. All the authors participated in the writing of the manuscript and agree with its full text.
G. I. Kropotov, D. A. Popov, D. I. Tsypishka, A. A. Shakhmin
TYDEX LLC, Saint Petersburg, Russia
The article discusses the production features of the optical elements for application in the THz range devices and equipment, describes the properties and specifications of optical materials for its production, and provides the application examples. Special attention is paid to the elements used exclusively in the THz systems and devices.
Keywords: terahertz (THz) radiation, optical elements for the terahertz range, Fresnel’s lenses, f-theta lenses.
Article received: June 26, 2025
Article accepted: July 21, 2025
Introduction
The terahertz (THz) radiation is electromagnetic radiation in the frequency range from 0.1 to 10 THz (wavelength of 3 mm – 30 μm). It penetrates well through many dielectric materials, such as plastic, fabric, paper, cardboard, leather, ceramics. It is absorbed by the Earth’s atmosphere (mainly by the water molecules) and is reflected from the metal surface. Radiation in this range is easily focused similar to the infrared (IR) radiation. Moreover, it is not ionizing, like X-rays. These properties are used in the production of pharmaceutical drugs and quality control of the food products. The application of THz radiation is of the utmost interest in such areas as safety monitoring, packaging inspection, characterization of semiconductor materials, chemical composition analysis, biochemical research and telecommunications.
The production of devices operating in the THz range is related to the development of passive optical elements made of materials that transmit the THz radiation rather well. These materials should have the low dispersion of THz waves, high thermal stability, mechanical strength, and chemical inertness. The reflective and transmitting properties of optical components can be changed, enhanced, or suppressed in a controlled manner by selection of the suitable materials and application of the special optical coatings to them.
Selection of materials
for the THz optics production
Conventionally, the high-resistance silicon (HRFZ-Si) grown by the floating-zone method [1] and having a specific resistance in the n-type from 10 kOhm×cm, or in the p-type from 25 kOhm×cm and higher is used for the production of passive components of THz photonics. HRFZ-Si is an isotropic crystalline material being transparent in a wide range of wavelengths, starting from the near IR, and having a constant transmission at the level of 50–54% from 40 micrometers to the microwaves (fig. 1). This allows the development of various optical elements for the THz optoelectronics on its basis.
In addition to Si, the crystals such as crystalline quartz, sapphire, germanium, and diamond play an important role in the THz applications.
Crystalline quartz is a birefringent material that should be considered if the radiation polarization is important. For work in the THz range, the material in the “x-cut” geometry is used in the production of λ/2 and λ/4 wave plates, when the plate surface is perpendicular to the x-axis of the crystal.
It is reasonable to use the fused quartz along with the crystalline quartz for the production of optical elements of devices operating in the THz range, when the radiation polarization is not important. Its cost is lower, and its processing does not require any orientation of the crystal and optical elements along the crystallographic axes. The transmission spectra of crystalline and fused quartz are shown in fig. 2.
Sapphire similar to the crystalline and fused quartz, is transparent in the THz region as well as in the visible one (fig. 3). Similar to HRFZ-Si, the hypo-hyper- and hemispherical lenses for the THz detectors and generators based on the photoconductive antennas are made from sapphire that has a refractive index close to the silicon’s values.
Germanium is used in the THz range to manufacture the active elements of acoustic and optic devices [2]. It is also applied in the multispectral thermal imaging devices of the IR + THz ranges [3].
The polycrystalline diamond that is transparent for ultraviolet radiation to the millimeter range, is used to produce the entry and exit gates in the powerful THz radiation sources. The polycrystalline diamond has high thermal conductivity and a low thermal-expansion coefficient that allows it to withstand high radiation loads [4].
A promising material for the production of optics in the long-wavelength region of the THz range is gallium arsenide (GaAs). This can be judged by comparing its transmission spectrum with the spectra of HRFZ-Si and Ge (fig. 5).
Another group of materials for THz photonics includes the polymers. Some of them have excellent transparency within the THz range with relatively low reflectivity. Polymethylpentene (TPX), Picarin (Tsurupica), cycloolefin polymer (Zeonex), cyclic olefin copolymer (COC or TOPAS), polyethylene (HDPE), high-molecular-weight polyethylene (UHMWPE), polytetrafluoroethylene (PTFE, fluoroplast or Teflon), and polypropylene (PP) find their practical application [5]. At the long wavelengths, the transmission of these polymers is structureless and flat. At the short wavelengths (typically less than 200 micrometers), there are some typical absorption bands (fig. 6) related to the natural vibrations, with the enhanced scattering by the material inhomogeneities. The polymers usually become less transparent as the wavelength decreases.
TPX is the lightest of all well-known polymers. It is transparent in the ultraviolet, visible and far infrared ranges that, for example, allows using a helium-neon laser beam to tune the THz optical systems. Its refractive index is ~1.46, and it is almost independent of wavelength in the long-wavelength range. TPX is a hard and durable material that can be mechanically converted into various optical components, such as lenses and windows. A certain specific application of TPX is its use as an exit gate in the gas molecular lasers optically pumped by a CO2 laser, due to its transparency in the entire THz range and absolute suppression of pump radiation in the vicinity of λ = 10 micrometers. Moreover, the TPX gates are also used in the cryostats as the “cold” gates.
The TPX transmission in the THz range is independent of temperature. The temperature coefficient of the refractive index is –3.0 · 10−4 K−1 (for the temperature range of 8–120 K).
Polyethylene is a light and elastic crystallized material. The heat resistance of some polyethylene grades reaches 110 °C, allowing cooling to –45 ÷ –120 °C, depending on the grade. Polyethylene has good dielectric characteristics, chemical and radiation resistance. The refractive index is ~ 1.54 and is changed little over a wide range of wavelengths. The high-density polyethylene (HDPE) or ultra-high-molecular-weight high-density polyethylene (UHMWPE) are usually used to produce the optical components. The thin HDPE films are applied in the THz polarizers. HDPE is also used to make the gates for optoacoustic receivers. It should be noted that the HDPE transmission in the THz range does not depend on temperature that allows it to be used in the cryostats. Its temperature coefficient of refractive index is –6.2·10–4 K‑1 for the temperature range of 8–120K. UHMWPE has a good combination of rigidity and mechanical damping capacity, and lends itself to machining with no difficulty. Based on the combination of properties, UHMWPE is easier to polish than HDPE that is important in the production of optical elements.
Polytetrafluoroethylene (PTFE) is a hard, heavy, white plastic with a density of about 2.2 g/cm3. Its melting point is 327 °C, and it retains its properties over a wide temperature range from –73 °C to 204 °C. Its refractive index is n ≈ 1.43 over a wide range of wavelengths.
ZEONEX has excellent mechanical properties, good chemical resistance and very low vacuum degassing in the conditions of up to 10–12 mmHg. The superior mechanical properties of ZEONEX compared to TPX make it possible to use this plastic for the production of THz prisms.
COC is attractive due to its high operating temperature, excellent transmittance, low birefringence, and low moisture absorption. An important advantage of COC is the possible production of various terahertz components using the 3D printing process.
In some cases it is more convenient to use polypropylene to manufacture various passive elements of the THz range. Polypropylene is obtained by polymerizing propylene in the presence of metal-complex catalysts. It is more resistant to the aggressive environments compared to the HDPE polyethylene and is better processed mechanically.
The efficient conversion of THz-range light beams requires the use of antireflective optical coatings, similar to the methods when such coatings are used in other (visible and IR) radiation regions. However, in the THz region, the special polymer coatings that are transparent in this wavelength range are required. These include poly-para-xylylene (PPX) (parylene) coatings.
The PPX coatings are specified by high homogeneity and high protective properties, inertness to most organic solvents and solutions of acids, alkalis and salts. The coating development does not require any high temperatures that eliminates overheating of parts. There are no internal stresses in the coating.
While using the refractive index of the PPX n = 1.63, the antireflection coating thickness can be determined by calculation for antireflection in the required wavelength range. Fig. 7 shows a section of the transmission spectrum of a crystalline quartz plate with a two-sided antireflection coating made of PPX with a thickness of 60 μm. The range of antireflection wavelengths is 200–600 μm. The antireflection coatings made of PPX can also be applied to the HRFZ-Si substrates.
THz gates, lenses and mirrors
In various THz devices and equipment, the gates made of HRFZ-Si, crystalline quartz (z-cut) and sapphire, as well as plastics, are widely used. The gates can be either plane-parallel or wedge-shaped.
The gates made of crystalline quartz have the following important properties:
- they are transparent in the visible wavelength range that allows easy tuning of the optical system for a helium-neon laser;
- they do not change the state of linear polarization of the beam;
- they can be cooled below the λ-point of liquid helium.
For the THz applications, TYDEX manufactures the lenses of classical shape using HRFZ-Si and TPX [6]. A special category includes the hemispherical lenses of various shapes: hyperhemispheres, hemispheres, hypohemispheres and “bullet” lenses (fig. 8).
A hyperhemisphere is a small optical surface made in the shape of a hemisphere, but with a boundary bend exceeding 180°. The hyperhemispheres and hemispheres are used in the THz optical systems based on the solid-state immersion effect [7]. A hypohemisphere is a truncated form of a hemispherical lens. It is used in various THz optical systems and in the THz microscopy [8]. The “bullet” lens is a hemisphere combined with a cylinder, where the spherical surface radius is R, the cylinder diameter is 2R, and the total thickness is L. Such lenses are applied in the manufacture of integrated lens antennas, including those for the THz and microwave ranges. The HRFZ-Si hemispheres and hyperhemispheres are widely used to produce the matrix-type receiving devices in the THz scanning and visualization systems [9,10].
Si axicons. A conical lens or axicon (fig. 9) is used to transform a Gaussian beam into a Bessel beam. The axicon focuses the radiation into a line consisting of many points along the optical axis. The resulting line generates a ring of constant width with an increasing diameter as the distance to the element increases (fig. 10). The axicons are made of HRFZ-Si.
THz Fresnel lenses. The radiation control is necessary in many THz applications. Most often, it is achieved using the full-parabolic deflectors and refractive optics. However, the diffractive optics provide new, fundamentally different possibilities, since it allows for spatial transformation of the beam. In order to meet the needs for diffractive optics in the THz frequency range, a calculation method and production technology for the THz Fresnel lenses using the 3D printing process have been developed [9].
The Fresnel lenses have a multi-level profile that is close to the ideal form of the phase function of the diffraction focuser into a spot to a maximum level. The monochromatic and broadband Fresnel lenses have various thickness of the microrelief profile. The THz-monochromatic Fresnel lenses (1st order kinoforms) operate at the rated frequency, frequencies multiple of the rated frequency, and in a wide range of high frequencies. The THz-broadband Fresnel lenses (high-order kinoforms) have a wide operating frequency range that is “shifted” closer to the rated frequency. Thus, the THz-monochromatic Fresnel lenses should be used in the cases where efficient control of the THz-continuous radiation beam is required. The THz-broadband Fresnel lenses can be used to control the THz-pulsed radiation. Thermoplastic made of cycloolefin copolymer COC is used as a material for both types of THz Fresnel lenses. The main advantages of Fresnel lenses over the refractive lenses include the almost complete absence of spherical aberrations, smaller thickness and lighter weight when working with the large diameter beams and, as a result, better radiation resistance due to the less radiation absorption inside the lens body.
The THz monochromatic Fresnel lenses focus THz radiation better than the refractive lenses. A comparison of radiation intensity focused by a diffractive Fresnel’s lens with the radiation intensity focused by a TPX refractive lens is shown in Fig. 12.
The THz broadband Fresnel lenses focus THz radiation no worse than the refractive lenses. Figure 13 shows a comparison of the radiation intensity spectrum focused by a diffractive Fresnel lens (rated operating frequency 1 THz, focal length 90 mm, lens diameter 93 mm) with the intensity spectrum of radiation focused by a refractive lens made of TPX.
To focus radiation in the THz imaging and scanning systems, it is possible to use f-theta lenses [6] (Fig. 14) instead of spherical lenses. They are capable of providing a flat focusing surface and a nearly constant spot size over the entire scanning field. Such lenses are designed to provide a linear beam offset as a linear function of the deflection angle that results in a constant scanning speed on a flat surface. They are manufactured using the 3D printing process and COC. The f-theta lenses also provide a constant phase of broadband THz radiation in the scanning plane and high image resolution. The images obtained by the f-theta lenses have barrel distortion, and the spot position on the image plane is directly proportional to the scanning angle. This feature eliminates the need for comprehensive electronic adjustment and allows for the assembly of a fast, relatively inexpensive, and space-saving scanning system. The f-theta lenses also provide a constant phase of broadband THz radiation in the scanning plane and high image resolution.
Fig. 15 shows the dependence of the transverse coordinate of the THz radiation maximum at the focus of f-theta lens from the angle of deviation (angle of incidence of the THz radiation on the f-theta lens). As can be seen from the figure, in the range of angles from –25 to +25 degrees this dependence is linear that allows scanning a flat surface at a constant speed.
For the applications where it is necessary to completely reflect THz radiation, TYDEX suggests using the THz mirrors [6]. These mirrors are made of fused quartz coated with gold and protected by the films of yttrium dioxide (I2O3) and silicon dioxide (SiO2). The reflection spectrum of such a mirror is shown in fig. 16.
THz spectrum splitters
and beam splitters
For the applications where it is necessary to reflect the near infrared (NIR) or intermediate infrared (IIR) radiation while maintaining good transmission in the THz range, TYDEX offers the THz spectrum splitters [6].
The NIR-THz spectrum splitter is used to separate the THz radiation from the pump of a titanium-sapphire laser, and the IIR-THz spectrum splitter is used to separate from the CO2 laser pumping. The high-resistance substrates made of Si or crystalline quartz with a highly reflective dielectric coating (R>90%) in the range of 730–860 nm are applied to produce the NIR-THz spectrum splitters. When producing the IIR-THz spectrum splitters, the highly reflective coatings in the range of 9–11 μm are applied to the substrates made of the same materials. The transmission and reflection spectra of the spectrum splitters are shown in Figures 17 and 18.
The beam splitters are optical elements designed to split a light beam into two or more parts.
They are used in various optical schemes, as well as in the interferometers. TYDEX manufactures two types of the THz beam splitters [12]:
single-pass beam splitters (they are used in the optical circuits where the radiation passes through the splitter once);
multi-pass beam splitters for interferometers (such a splitter is intended for the multiple beam passes). In contrast to a single-pass one, a multi-pass beam splitter has very high plane-parallelism of surfaces.
The material for THz beam splitters is high-resistance Si. The transmission and reflection spectra are shown in Fig. 1.
Conclusion
The expanded application of THz radiation makes the development and production of efficient optical elements for the THz range relevant. The novelty in this area is the development and production of optical elements for controlling the THz radiation beam specifications in the THz imaging and scanning systems. Examples of such solutions include the Fresnel’s lenses and f-theta lenses.
Various materials are used to produce the THz optics, both those well-known due to the manufacture of classical optics and new ones that have not previously found application for this purpose. The spectra and other specifications of crystals and plastics given in the article allow us to select the necessary materials for the manufacture of optical components for use in the research THz systems, as well as in various THz devices.
AUTHORS
Kropotov Grigory, Cand. of Scien. (Phys&Math.), Senior Researcher, General Manager LLC «TYDEX», e-mail: grigorykropotov@tydex.ru; Saint Petersburg, Russia.
ORCID: 0000-0001-9041-6701
Popov Dmitry, Cand. of Scien. (Phys&Math.), Researcher LLC «TYDEX», e-mail: dmitriypopov@tydex.ru; Saint Petersburg, Russia.
ORCID: 0009-0007-2427-2341
Tsypishka Dmitry, Cand. of Scien. (Phys&Math.), associate professor, Deputy General Manager LLC «TYDEX», e-mail: dmitrytsypishka@tydex.ru; Saint Petersburg, Russia.
ORCID: 0000-0003-1914-5245
Shakhmin Alexander, Cand. of Scien. (Phys&Math.), Leading Research Engineer LLC «TYDEX», e-mail: AlexeyShakhmin@tydex.ru; Saint Petersburg, Russia.
ORCID: 0009-0003-9566-2823
CONTRIBUTION OF THE AUTHORS
The article is based on the work of all members of the author’s team.
CONFLICT OF INTEREST
The authors declare that there is no conflict of interest. All the authors participated in the writing of the manuscript and agree with its full text.
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