rus
Issue #5/2023
G. I. Kropotov, A. A. Shakhmin, I. A. Kaplunov, V. E. Rogalin
Application of Spectral Devices in the Optical Engineering and Scientific Research
Application of Spectral Devices in the Optical Engineering and Scientific Research
DOI: 10.22184/1993-7296.FRos.2023.17.5.378.392
The up-to-date tools, instruments and measurement methods for the optically transparent materials in a wide spectral range (from ultraviolet to terahertz) are presented. Particular attention is paid to the terahertz range, a relatively new area in optics being at the interface with radiophysics.
The up-to-date tools, instruments and measurement methods for the optically transparent materials in a wide spectral range (from ultraviolet to terahertz) are presented. Particular attention is paid to the terahertz range, a relatively new area in optics being at the interface with radiophysics.
Теги: optical measurements optical specifications of transparent materials spectral control оптические измерения оптические характеристики прозрачных материалов спектральный контроль
Application of Spectral Devices in the Optical Engineering and Scientific Research
G. I. Kropotov 1, A. A. Shakhmin 1, I. A. Kaplunov 2, V. E. Rogalin 3
Tydex LLC, Saint Petersburg, Russia
Tver State University, Tver, Russia
Institute of Electrophysics and Power Engineering of the Russian Academy of Sciences, Saint Petersburg, Russia
The up-to-date tools, instruments and measurement methods for the optically transparent materials in a wide spectral range (from ultraviolet to terahertz) are presented. Particular attention is paid to the terahertz range, a relatively new area in optics being at the interface with radiophysics.
Keywords: optical measurements, spectral control, optical specifications of transparent materials
Article received: 03.08.2023
Article accepted: 31.08.2023
Introduction
During the production of optical devices (instruments, tools for various purposes), all produced optical elements must be certified in terms of their spectral specifications [1]. Most existent production facilities have the laboratories (structural subdivisions of a similar type) where the spectral measurements of optical products made of various materials are performed, including those with the comprehensive multilayer interference optical coatings. Depending on the business field of enterprises and the need for product control, it is possible to apply the special tools and devices providing measurements in various spectral ranges that can cover, in general, a wide spectral region, namely from ultraviolet to the millimeter (terahertz) wavelengths.
The main role of spectral instruments in the optical production is spectral control of the materials used and optical coatings applied to the products obtained. Prior to the final application of optical coatings, the structure and thickness of the coating layers are calculated to obtain the required spectral specifications, followed by the coating deposition on the test samples and measurement of the coating spectral properties. The final coating shall be applied only after its calibration and testing. As the standard samples for measuring the optical coatings properties of the products, the deposition witnesses made of the same material with a diameter of 20 to 45 mm shall be used in the form of a plane-parallel window with two polished surfaces or wedges with one polished surface and one matte surface.
The research materials science laboratories of the enterprises engaged in the synthesis of optical materials require their own tools and instruments or the ability to perform spectral measurements in a specialized laboratory. First of all, there is a need for such optical properties as transmission, reflection and refraction index in the spectral range designed for the operation of final products.
The purpose of this paper is to demonstrate the possibilities of spectral properties measurements in a wide spectral range on the basis of the optical spectroscopy laboratory of Tydex LLC (Saint Petersburg).
Devices and their specifications
For measurements in a wide range of electromagnetic spectrum (from the ultraviolet to the terahertz wavelength range) covering almost all tool applications related to the radiation generation and detection, with the image formation and transmission, an ability to control the direction of propagation and transformation of electromagnetic wave flows, and many more research and technological issues, the following devices can be efficiently used:
Wave spectrometer Photon RT by EssentOptics;
Fourier spectrometer Vertex 70 by Bruker;
Spatio-temporal THz spectrometer TERA K8 by Menlo Systems, the range of use of which is given in Table 1.
Spectral measurement procedure by Photon RT
The measurements in the shortest wavelength regions (ultraviolet, visible and near IR) can be made using the Photon RT wave spectrophotometer (EssentOpics) (Figure 1). The spectrophotometer Photon RT is designed to measure the spectral transmittance, reflection, optical density of flat optical elements with a coating in the polarized and non-polarized light in the spectral emission range from 185 to 1700 nm. The original optical circuit of the device with a reference channel and a radiation receiver rotating around the sample allows to perform the measurements at various incidence and reflection angles to the sample surface. The minimum sample dimensions are 12 × 10 mm, the maximum diameter with the device cover closed is up to 120 mm. The spectral resolution up to 0.3 nm and measurement accuracy up to 0.01% meet the current requirements for the similar research tools (Table 2).
Applied capabilities of the PHOTON RT spectrophoto-
meter:
Spectral range from 185 nm to 1 700 nm;
Measurements in the polarized light from 220 nm to 1700 nm;
Measurement of transmission T, Ts, Tp (for the angles 0–75°), calculation of T(s + p) / 2, for a given light incidence angle;
Measurement of absolute reflection R, Rs, Rp (for the angles 8–75°), calculation of R(s + p) / 2, for a given light incidence angle;
Measurement of the substrate material absorbance;
Automatic measurement and calculation of the complex refractive index and layer thickness for single-layer homogeneous coatings;
Measurement of the polarization and beam-splitting cubes;
Automatic beam shift compensation when measuring the thick sample transmission at various angles;
Calculation of the average R and T values in a given spectral range.
A deuterium discharge lamp, a halogen lamp, and a calibration mercury-argon lamp are used as the radiation sources. The dimensions of the light spot on the sample are 6 × 2 mm. The beam is collimated in the measurement channel, the beam divergence is ±1°. The spectral measurements of the product transmission values are performed at a given radiation incidence angle to the sample (from 0 to 75 degrees), and the reflection measurements are performed at an angle of incidence from 8 to 75 degrees. When switching from the transmission measurements to the reflection measurements for the light intensity registration, the photodetector is automatically moved around the sample holder on the reflected light beam axis, the sample holder is rotated by a set angle to the incident radiation. Such design features makes it possible to measure the transmission and reflection coefficients in the same local surface area of an optical element without an element removal from the measurement compartment. The background signal is measured as an empty channel without any sample, both for the transmission and reflection measurements.
Execution of measurements
The “PhotonSoft” software is used to control Photon RT, set the measurement parameters and display the measurement results on the computer screen. After initiation, the spectrometer needs to warm up for at least 30 minutes. Prior to the measurements, the necessary parameters shall be set, including the scanning range, scanning pitch, number of averagings, smoothing mode, polarization. Further, having determined that the optical channel is empty, the spectrometer shall be calibrated, according to the results of which a spectral curve will be observed on the screen, with a spectrum value of 100% in the entire set range. The next stage is to place the sample on the object table, then set the mode (reflection or transmission) and the table rotation angle, then the spectrum of sample under study shall be measured (Figure 2). The results in the form of a spectral curve of transmittance or reflectance values are displayed in the program, processed, stored as the numerical data and printed as the graphs (Figure 3).
Spectral measurement procedure by Bruker Vertex 70
If it is necessary to perform the measurements in the NIR, MIR and FIR spectral ranges, one can use the Bruker Vertex 70 Fourier spectrometer (Figure 4). The Vertex spectrometers stand out from the research FTIR spectrometers due to the versatility provided by the innovative optical system design. In this case, the appropriate combinations of radiation sources and receivers are used in varios parts of the infrared range, as well as the replaceable beam splitters in the Michelson interferometer (Table 3).
When performing the standard measurements by Vertex 70, the spectral resolution of measurements for the NIR, MIR ranges is set to 8 cm−1, for the FIR range – 4 cm−1, for the FFIR range – 2 cm−1; if the high-precision measurements are required, it is possible to set the spectral resolution up to 0.5 cm−1 in any operating range.
The basis of the Fourier spectrometer is a double-beam interferometer: when one of the mirrors is moved, the path difference between the interfering beams is changed. To reduce the external influences, the Vertex 70 interferometer is designed according to the circuit with mirrors in the form of corner reflectors. Vertex 70 is equipped with a 633 nm HeNe laser. The rated power output is 1 MW. The laser controls the position and speed of the interferometer’s moving mirror and is applied to determine the data sample positions. The source of NIR radiation (near infrared radiation) is a tungsten lamp in a halogen atmosphere. A globar lamp is used as a source of MIR and FIR radiation (medium and far infrared radiation) being a U-shaped silicon carbide arc that heats up and radiates when an electric current is passed. For the far FFIR wavelength range (up to 670 µm), a continuous spectrum of mercury arc lamp radiation is used. The lamp is located in a separate water-cooled housing; the lamp radiation enters the spectrometer through an external opening in the case. The illuminated area size on the sample, both during the transmission and reflection measurements, has the diameter of 1 cm.
The spectrometer is equipped with a DigiTect DLATGS detector with a built-in preamplifier. Such a detector assembly contains an A/D converter that converts the analog signal to the digital one directly in the detector. This digital signal is transmitted to the data processing circuit of the spectrometer’s electronics module. The standard detector for NIR and MIR bands is the pyroelectric RT-DL-TGS w/KBr detector that covers the spectral range from 12,000 to 250 cm−1, operates at the room temperature and has a sensitivity of D* > 2 × 108 (cm Hz1/2 W−1). An RT-DTGS w/PE detector is applied as a receiver for the FIR and FFIR bands, covering the 700–10 cm−1 spectral region with a sensitivity of D* > 4 × 108 (cm Hz1/2 W−1) and operating without cooling at the room temperature.
The Vertex 70 sources, detectors and beamsplitters are electronically coded to be recognized by the devices, and the appropriate parameters are loaded automatically. The technical condition of all spectrometer’s modules is constantly controlled by the self-diagnostics and information system through the control program that simplifies operation of the spectrometer and allows diagnosing any malfunctions.
All control functions of the Vertex 70 Fourier spectrometer are performed in the OPUS program by Bruker that provides the following: registration of interferograms; Fourier transform of interferograms into the spectrum using the phase correction and apodization; mathematical spectra processing; data presentation in the graphical and digital form on the display with saving as a file. Moreover, it is possible configure and test the device condition and receive reports for the Fourier spectrometer certification in the program.
Applied capabilities of the VERTEX 70 Fourier spectrometer:
Spectral range from 0.670 µm to 670 µm;
Measurement of transmission T in a sample-focused or collimated beam;
Measurement of reflection R for the angles 11–85°;
Measurement in the polarized light in all bands;
Calculation of the average R and T values in a given spectral range;
Mathematical transformations of the R and T spectral curves;
Determination of the polarization and extinction degree for the polarizers;
Determination of ellipticity of the wave plates made of crystalline quartz;
Spectra consolidation from all spectral ranges of Vertex 70 and other spectral tools.
The device is equipped with a set of additional attachments to measure transmission in a focused beam, transmission in a collimated beam, specular reflection at a minimum fixed beam incidence angle of 11 degrees, specular reflection at a variable beam incidence angle from 13 to 85 degrees (Figure 5). Each attachment can be supplemented with a holder for polarizer rotation by controlled motor with 0.5° increments. Moreover, it is possible to install a radiation polarizer before and/or after the measured sample for measurements in the polarized light over the entire operating wavelength band.
Vertex 70 is equipped with a dry air purging system; the sample compartment and internal part of the spectrometer case are purged separately. The chamber purging with dry air and drying with silica gel can significantly reduce the negative impact of carbon dioxide and water vapor in the air on the spectrum measurement results.
Execution of measurements
The necessary radiation source, beam splitter and windows in the flanges on the inner walls of the sample compartment are installed on Vertex 70 for each spectral measurement range, and a radiation receiver is selected. After that, the required measurement parameters for a specific band are selected in the OPUS software.
When measuring the sample transmission, it is necessary to measure the empty channel interferogram in the absence of a sample. It is taken as a background or reference signal, then the interferogram of sample under study is measured. As a result of the inverse Fourier transform of these interferograms, the spectra of sample and background signal are restored. The apodization and phase correction procedures are automatically applied to eliminate the transformation artifacts. By dividing the first spectrum by the second, the transmission spectrum of the sample is determined. The software automatically excludes the spectral dependence of the radiation source intensity, spectral properties of the beam splitter and other optical system elements, and sensitivity spectrum of the detector. The software is able to exclude the atmospheric absorption of water vapor and CO2 from the final spectrum.
When measuring reflection, the interferogram of reflected signal from the gold mirror installed in place of the sample, is measured as a background, and then interferogram of the reflected signal from the sample is measured. As a result of the inverse Fourier transform of these interferograms, the spectra of sample and background signal are restored. The result of dividing the sample spectrum by the background signal is then multiplied by the well-known reflectance spectrum of the reference mirror with specular gold coating (Figure 6). In the wavelength band exceeding 10 µm, the mirror reflectance is assumed to be constant (RAu = 98.5%). The number of scanning cycles to measure the background signal and the sample is set equal to 32. Upon completion of measurements, all cycles are averaged into one spectrum.
Table 3 shows that the NIR, MIR, FIR and FFIR spectral bands are overlapped. The merging of individual spectra into a single spectrum covering the range from 0.670 to 670 µm is performed using the Merge Spectra function of the OPUS software by Bruker (Figure 7). The final measurement results in the form of a spectral curve of transmittance or reflectance values are stored as the numerical data and printed as the graphs.
Spectral measurement procedure by TERA K8
In order to work in a longer wavelength band (up to λ = 1 500 μm), the THz time-domain spectroscopy method (THz-TDS) is applied. The measurements are performed using a TERA K8 THz-TDS system (Menlo Systems) setup (Figure 8). The TERA K8 THz spectrometer is a turnkey solution for the spatio-temporal THz spectroscopy. The open design allows this spectrometer to be used in a variety of scientific fields. TERA K8 includes a tunable femtosecond pump laser with a wavelength of 780 nm or 1 560 nm, an optical delay line, a THz emitter and two detectors, the THz optical elements, and a computer with the specialized software for the obtained data processing.
The time-domain spectroscopy method is based on the coherent detection of terahertz radiation pulses transmitted or reflected from the sample under study using the same laser pulse. An important distinctive feature of this method is the ability to measure the electric field of a terahertz pulse with high resolution. Such pulse provides information not only in relation to the amplitude, as in the case of Fourier spectroscopy, but also in relation to the signal phase.
The ultrashort laser pulse is divided into a pump pulse and an probe pulse. The pump pulse is used to generate a terahertz pulse being formed as a result of the laser radiation impact on a photoconductive antenna. The probe laser pulse interacts with a certain section of the terahertz pulse in the detector. By using the mechanical delay line, the arrival time of the probe pulse at the detector is changed in relation to the pump pulse; as a result of scanning the time delay interval between the probe pulse and the pump pulse, various pulse sections are measured with a time resolution corresponding to the probe pulse duration. Thus, the electric field of terahertz pulse is recorded in the detector as a time delay function of the probe pulse, and the entire temporal shape of the terahertz pulse is measured.
To generate and detect the broadband THz radiation, it is possible to apply the TERA8-1 LT-GaAs semiconductor antennas excited by a femtosecond laser. The THz emitter and detector have a microstrip photoresistor antenna grown on a GaAs (LT-GaAs) substrate at low temperature. The terahertz (THz) radiation is obtained and collimated by a built-in hemispherical silicon lens mounted on the XY platform. The antenna photoresistance geometry, silicon lens parameters, and properties of the LT-GaAs epitaxial layers are optimized for the maximum terahertz (THz) output efficiency while maintaining the optimal bandwidth. The specifications of TERA K8 Menlo Systems are given in Table 4.
Applied capabilities of the TERA K8 THz-TDS spectrometer:
Spectral range from 150 µm to 1500 µm;
Measurement of transmission T in a sample-focused or collimated beam;
Measurement of reflection R at an angle of 45°;
Measurement in the polarized light;
Full amplitude and phase detection;
Determination of the polarization and extinction degree for the polarizers;
Determination of ellipticity of the wave plates made of crystalline quartz;
Determination of the refractive index of materials;
THz image acquisition.
The spacious sample area allows easy integration of additional equipment such as the polarizers, rotators, special sample holders with the heating or cooling elements, mechanical sample advancers, or synchronized radiation receivers. To obtain a THz image, this spectrometer applies a special TERAImage unit including a two-dimensional motorized slider with a sample holder and software for the image acquisition and reconstruction.
Execution of measurements
In the case of measuring the transmission spectrum of a sample, the radiating and receiving antennas are located on the same line and at the same height; four TPX lenses with a focal length of 54 mm are installed in the path of the THz radiation pulse. The configuration of these lenses and their position on the measuring rail is arranged in such a way that the radiation is focused into the minimum aperture at the sample location and transmits the maximum power from the radiating antenna to the receiving antenna. The minimum aperture into which radiation can be focused is 5 mm. In the case of measuring the reflection spectrum from a sample, configuration of the radiating and receiving antennas is changed, they are installed at an angle of 90 degrees to each other. The sample is placed at the focal point at an angle of 45 degrees to the incident radiation. Thus, the reflection measurement from the sample is performed only at an angle of 45 degrees relative to the normal line to the sample surface. The device circuit includes two receiving antennas: one for the transmission measurement, and one for the reflection measurement that allows not to rebuild the device when changing the measurement mode (Figure 9).
The time dependences of the receiving antenna photocurrent are measured using the K8 TeraScan software supplied with the spectrometer (Figure 10). The search for the pulse temporal shape, setting the best position of lenses along the optical line, as well as fine-tuning the silicon lens position of the radiating and receiving antennas are performed using the fast delay line scanning function. Next, the scanning parameters are set in the TeraScan program, and the time dependence of the THz pulse amplitude is measured. To obtain the spectral specifications of sample under study during the transmission signal measurements, the waveform of the radiation pulse transmitted through the empty channel is measured as the background, then the waveform of the pulse transmitted through the sample placed at the focus point of the focusing lens is measured. In the case of the reflection signal measurements, the pulse waveform reflection from a mirror with a gold coating is measured as a background, the mirror is set in the same position as the measured sample.
The inverse Fourier transform of the measured background and sample temporal waveforms is performed using the TeraMat program, the results are saved to a file in the digital form. Moreover, this program allows to calculate the refractive index of the sample material. Further processing of the measurement results consists in dividing the sample signal by the background and squaring to convert the spectral dependence of the electric field amplitude of the electromagnetic wave into the radiation intensity spectrum. To obtain a full spectrum from 185 nm to 1 500 µm, all spectra from three devices shall be prepared and imported into the OPUS program and combined using the special Merge Spectra function.
Conclusion
The presented devices and spectral measurement methods allow completing the research tasks to study the optical properties of materials that are promising for use in the equipment operating in various spectral ranges (from ultraviolet to terahertz) [3–21].
The optical elements, components of optical systems and devices for the field of science and industry, during the production of which the considered devices are used to obtain spectral specifications, are given on the website of Tydex LLC (Saint Petersburg) [2].
The authors declare that they have no conflict of interest. All authors took part in the article preparation and supplemented the manuscript in terms of their work.
Contribution of composite authors: The article was prepared on the basis of long-term work experience of all composite authors.
ABOUT AUTHORS
Kaplunov Ivan A. – Dr. of Sc. (Eng.)Tver State University;
e-mail: kaplunov.ia@tversu.ru, Tver, Russia.
ORCID 0000-0002-1726-3451
Kropotov Grigory I. – Cand. of Sc. (Phys.&Math), General Manager TYDEX, LLC.,
St. Petersburg, RUSSIA.
ORCID 0000-0001-9041-6701
Rogalin Vladimir E. – Dr. of Sc. (Phys.&Math), Institute of Electrophysics and Electric Power, Russian Academy of Sciences, St. Petersburg, Russia.
ORCID 0000-0002-2980-5385
Shakhmin Alexey A. – Cand. of Sc. (Phys.&Math), lead engineer TYDEX, LLC., St. Petersburg, Russia.
ORCID 0009-0003-9566-2823
G. I. Kropotov 1, A. A. Shakhmin 1, I. A. Kaplunov 2, V. E. Rogalin 3
Tydex LLC, Saint Petersburg, Russia
Tver State University, Tver, Russia
Institute of Electrophysics and Power Engineering of the Russian Academy of Sciences, Saint Petersburg, Russia
The up-to-date tools, instruments and measurement methods for the optically transparent materials in a wide spectral range (from ultraviolet to terahertz) are presented. Particular attention is paid to the terahertz range, a relatively new area in optics being at the interface with radiophysics.
Keywords: optical measurements, spectral control, optical specifications of transparent materials
Article received: 03.08.2023
Article accepted: 31.08.2023
Introduction
During the production of optical devices (instruments, tools for various purposes), all produced optical elements must be certified in terms of their spectral specifications [1]. Most existent production facilities have the laboratories (structural subdivisions of a similar type) where the spectral measurements of optical products made of various materials are performed, including those with the comprehensive multilayer interference optical coatings. Depending on the business field of enterprises and the need for product control, it is possible to apply the special tools and devices providing measurements in various spectral ranges that can cover, in general, a wide spectral region, namely from ultraviolet to the millimeter (terahertz) wavelengths.
The main role of spectral instruments in the optical production is spectral control of the materials used and optical coatings applied to the products obtained. Prior to the final application of optical coatings, the structure and thickness of the coating layers are calculated to obtain the required spectral specifications, followed by the coating deposition on the test samples and measurement of the coating spectral properties. The final coating shall be applied only after its calibration and testing. As the standard samples for measuring the optical coatings properties of the products, the deposition witnesses made of the same material with a diameter of 20 to 45 mm shall be used in the form of a plane-parallel window with two polished surfaces or wedges with one polished surface and one matte surface.
The research materials science laboratories of the enterprises engaged in the synthesis of optical materials require their own tools and instruments or the ability to perform spectral measurements in a specialized laboratory. First of all, there is a need for such optical properties as transmission, reflection and refraction index in the spectral range designed for the operation of final products.
The purpose of this paper is to demonstrate the possibilities of spectral properties measurements in a wide spectral range on the basis of the optical spectroscopy laboratory of Tydex LLC (Saint Petersburg).
Devices and their specifications
For measurements in a wide range of electromagnetic spectrum (from the ultraviolet to the terahertz wavelength range) covering almost all tool applications related to the radiation generation and detection, with the image formation and transmission, an ability to control the direction of propagation and transformation of electromagnetic wave flows, and many more research and technological issues, the following devices can be efficiently used:
Wave spectrometer Photon RT by EssentOptics;
Fourier spectrometer Vertex 70 by Bruker;
Spatio-temporal THz spectrometer TERA K8 by Menlo Systems, the range of use of which is given in Table 1.
Spectral measurement procedure by Photon RT
The measurements in the shortest wavelength regions (ultraviolet, visible and near IR) can be made using the Photon RT wave spectrophotometer (EssentOpics) (Figure 1). The spectrophotometer Photon RT is designed to measure the spectral transmittance, reflection, optical density of flat optical elements with a coating in the polarized and non-polarized light in the spectral emission range from 185 to 1700 nm. The original optical circuit of the device with a reference channel and a radiation receiver rotating around the sample allows to perform the measurements at various incidence and reflection angles to the sample surface. The minimum sample dimensions are 12 × 10 mm, the maximum diameter with the device cover closed is up to 120 mm. The spectral resolution up to 0.3 nm and measurement accuracy up to 0.01% meet the current requirements for the similar research tools (Table 2).
Applied capabilities of the PHOTON RT spectrophoto-
meter:
Spectral range from 185 nm to 1 700 nm;
Measurements in the polarized light from 220 nm to 1700 nm;
Measurement of transmission T, Ts, Tp (for the angles 0–75°), calculation of T(s + p) / 2, for a given light incidence angle;
Measurement of absolute reflection R, Rs, Rp (for the angles 8–75°), calculation of R(s + p) / 2, for a given light incidence angle;
Measurement of the substrate material absorbance;
Automatic measurement and calculation of the complex refractive index and layer thickness for single-layer homogeneous coatings;
Measurement of the polarization and beam-splitting cubes;
Automatic beam shift compensation when measuring the thick sample transmission at various angles;
Calculation of the average R and T values in a given spectral range.
A deuterium discharge lamp, a halogen lamp, and a calibration mercury-argon lamp are used as the radiation sources. The dimensions of the light spot on the sample are 6 × 2 mm. The beam is collimated in the measurement channel, the beam divergence is ±1°. The spectral measurements of the product transmission values are performed at a given radiation incidence angle to the sample (from 0 to 75 degrees), and the reflection measurements are performed at an angle of incidence from 8 to 75 degrees. When switching from the transmission measurements to the reflection measurements for the light intensity registration, the photodetector is automatically moved around the sample holder on the reflected light beam axis, the sample holder is rotated by a set angle to the incident radiation. Such design features makes it possible to measure the transmission and reflection coefficients in the same local surface area of an optical element without an element removal from the measurement compartment. The background signal is measured as an empty channel without any sample, both for the transmission and reflection measurements.
Execution of measurements
The “PhotonSoft” software is used to control Photon RT, set the measurement parameters and display the measurement results on the computer screen. After initiation, the spectrometer needs to warm up for at least 30 minutes. Prior to the measurements, the necessary parameters shall be set, including the scanning range, scanning pitch, number of averagings, smoothing mode, polarization. Further, having determined that the optical channel is empty, the spectrometer shall be calibrated, according to the results of which a spectral curve will be observed on the screen, with a spectrum value of 100% in the entire set range. The next stage is to place the sample on the object table, then set the mode (reflection or transmission) and the table rotation angle, then the spectrum of sample under study shall be measured (Figure 2). The results in the form of a spectral curve of transmittance or reflectance values are displayed in the program, processed, stored as the numerical data and printed as the graphs (Figure 3).
Spectral measurement procedure by Bruker Vertex 70
If it is necessary to perform the measurements in the NIR, MIR and FIR spectral ranges, one can use the Bruker Vertex 70 Fourier spectrometer (Figure 4). The Vertex spectrometers stand out from the research FTIR spectrometers due to the versatility provided by the innovative optical system design. In this case, the appropriate combinations of radiation sources and receivers are used in varios parts of the infrared range, as well as the replaceable beam splitters in the Michelson interferometer (Table 3).
When performing the standard measurements by Vertex 70, the spectral resolution of measurements for the NIR, MIR ranges is set to 8 cm−1, for the FIR range – 4 cm−1, for the FFIR range – 2 cm−1; if the high-precision measurements are required, it is possible to set the spectral resolution up to 0.5 cm−1 in any operating range.
The basis of the Fourier spectrometer is a double-beam interferometer: when one of the mirrors is moved, the path difference between the interfering beams is changed. To reduce the external influences, the Vertex 70 interferometer is designed according to the circuit with mirrors in the form of corner reflectors. Vertex 70 is equipped with a 633 nm HeNe laser. The rated power output is 1 MW. The laser controls the position and speed of the interferometer’s moving mirror and is applied to determine the data sample positions. The source of NIR radiation (near infrared radiation) is a tungsten lamp in a halogen atmosphere. A globar lamp is used as a source of MIR and FIR radiation (medium and far infrared radiation) being a U-shaped silicon carbide arc that heats up and radiates when an electric current is passed. For the far FFIR wavelength range (up to 670 µm), a continuous spectrum of mercury arc lamp radiation is used. The lamp is located in a separate water-cooled housing; the lamp radiation enters the spectrometer through an external opening in the case. The illuminated area size on the sample, both during the transmission and reflection measurements, has the diameter of 1 cm.
The spectrometer is equipped with a DigiTect DLATGS detector with a built-in preamplifier. Such a detector assembly contains an A/D converter that converts the analog signal to the digital one directly in the detector. This digital signal is transmitted to the data processing circuit of the spectrometer’s electronics module. The standard detector for NIR and MIR bands is the pyroelectric RT-DL-TGS w/KBr detector that covers the spectral range from 12,000 to 250 cm−1, operates at the room temperature and has a sensitivity of D* > 2 × 108 (cm Hz1/2 W−1). An RT-DTGS w/PE detector is applied as a receiver for the FIR and FFIR bands, covering the 700–10 cm−1 spectral region with a sensitivity of D* > 4 × 108 (cm Hz1/2 W−1) and operating without cooling at the room temperature.
The Vertex 70 sources, detectors and beamsplitters are electronically coded to be recognized by the devices, and the appropriate parameters are loaded automatically. The technical condition of all spectrometer’s modules is constantly controlled by the self-diagnostics and information system through the control program that simplifies operation of the spectrometer and allows diagnosing any malfunctions.
All control functions of the Vertex 70 Fourier spectrometer are performed in the OPUS program by Bruker that provides the following: registration of interferograms; Fourier transform of interferograms into the spectrum using the phase correction and apodization; mathematical spectra processing; data presentation in the graphical and digital form on the display with saving as a file. Moreover, it is possible configure and test the device condition and receive reports for the Fourier spectrometer certification in the program.
Applied capabilities of the VERTEX 70 Fourier spectrometer:
Spectral range from 0.670 µm to 670 µm;
Measurement of transmission T in a sample-focused or collimated beam;
Measurement of reflection R for the angles 11–85°;
Measurement in the polarized light in all bands;
Calculation of the average R and T values in a given spectral range;
Mathematical transformations of the R and T spectral curves;
Determination of the polarization and extinction degree for the polarizers;
Determination of ellipticity of the wave plates made of crystalline quartz;
Spectra consolidation from all spectral ranges of Vertex 70 and other spectral tools.
The device is equipped with a set of additional attachments to measure transmission in a focused beam, transmission in a collimated beam, specular reflection at a minimum fixed beam incidence angle of 11 degrees, specular reflection at a variable beam incidence angle from 13 to 85 degrees (Figure 5). Each attachment can be supplemented with a holder for polarizer rotation by controlled motor with 0.5° increments. Moreover, it is possible to install a radiation polarizer before and/or after the measured sample for measurements in the polarized light over the entire operating wavelength band.
Vertex 70 is equipped with a dry air purging system; the sample compartment and internal part of the spectrometer case are purged separately. The chamber purging with dry air and drying with silica gel can significantly reduce the negative impact of carbon dioxide and water vapor in the air on the spectrum measurement results.
Execution of measurements
The necessary radiation source, beam splitter and windows in the flanges on the inner walls of the sample compartment are installed on Vertex 70 for each spectral measurement range, and a radiation receiver is selected. After that, the required measurement parameters for a specific band are selected in the OPUS software.
When measuring the sample transmission, it is necessary to measure the empty channel interferogram in the absence of a sample. It is taken as a background or reference signal, then the interferogram of sample under study is measured. As a result of the inverse Fourier transform of these interferograms, the spectra of sample and background signal are restored. The apodization and phase correction procedures are automatically applied to eliminate the transformation artifacts. By dividing the first spectrum by the second, the transmission spectrum of the sample is determined. The software automatically excludes the spectral dependence of the radiation source intensity, spectral properties of the beam splitter and other optical system elements, and sensitivity spectrum of the detector. The software is able to exclude the atmospheric absorption of water vapor and CO2 from the final spectrum.
When measuring reflection, the interferogram of reflected signal from the gold mirror installed in place of the sample, is measured as a background, and then interferogram of the reflected signal from the sample is measured. As a result of the inverse Fourier transform of these interferograms, the spectra of sample and background signal are restored. The result of dividing the sample spectrum by the background signal is then multiplied by the well-known reflectance spectrum of the reference mirror with specular gold coating (Figure 6). In the wavelength band exceeding 10 µm, the mirror reflectance is assumed to be constant (RAu = 98.5%). The number of scanning cycles to measure the background signal and the sample is set equal to 32. Upon completion of measurements, all cycles are averaged into one spectrum.
Table 3 shows that the NIR, MIR, FIR and FFIR spectral bands are overlapped. The merging of individual spectra into a single spectrum covering the range from 0.670 to 670 µm is performed using the Merge Spectra function of the OPUS software by Bruker (Figure 7). The final measurement results in the form of a spectral curve of transmittance or reflectance values are stored as the numerical data and printed as the graphs.
Spectral measurement procedure by TERA K8
In order to work in a longer wavelength band (up to λ = 1 500 μm), the THz time-domain spectroscopy method (THz-TDS) is applied. The measurements are performed using a TERA K8 THz-TDS system (Menlo Systems) setup (Figure 8). The TERA K8 THz spectrometer is a turnkey solution for the spatio-temporal THz spectroscopy. The open design allows this spectrometer to be used in a variety of scientific fields. TERA K8 includes a tunable femtosecond pump laser with a wavelength of 780 nm or 1 560 nm, an optical delay line, a THz emitter and two detectors, the THz optical elements, and a computer with the specialized software for the obtained data processing.
The time-domain spectroscopy method is based on the coherent detection of terahertz radiation pulses transmitted or reflected from the sample under study using the same laser pulse. An important distinctive feature of this method is the ability to measure the electric field of a terahertz pulse with high resolution. Such pulse provides information not only in relation to the amplitude, as in the case of Fourier spectroscopy, but also in relation to the signal phase.
The ultrashort laser pulse is divided into a pump pulse and an probe pulse. The pump pulse is used to generate a terahertz pulse being formed as a result of the laser radiation impact on a photoconductive antenna. The probe laser pulse interacts with a certain section of the terahertz pulse in the detector. By using the mechanical delay line, the arrival time of the probe pulse at the detector is changed in relation to the pump pulse; as a result of scanning the time delay interval between the probe pulse and the pump pulse, various pulse sections are measured with a time resolution corresponding to the probe pulse duration. Thus, the electric field of terahertz pulse is recorded in the detector as a time delay function of the probe pulse, and the entire temporal shape of the terahertz pulse is measured.
To generate and detect the broadband THz radiation, it is possible to apply the TERA8-1 LT-GaAs semiconductor antennas excited by a femtosecond laser. The THz emitter and detector have a microstrip photoresistor antenna grown on a GaAs (LT-GaAs) substrate at low temperature. The terahertz (THz) radiation is obtained and collimated by a built-in hemispherical silicon lens mounted on the XY platform. The antenna photoresistance geometry, silicon lens parameters, and properties of the LT-GaAs epitaxial layers are optimized for the maximum terahertz (THz) output efficiency while maintaining the optimal bandwidth. The specifications of TERA K8 Menlo Systems are given in Table 4.
Applied capabilities of the TERA K8 THz-TDS spectrometer:
Spectral range from 150 µm to 1500 µm;
Measurement of transmission T in a sample-focused or collimated beam;
Measurement of reflection R at an angle of 45°;
Measurement in the polarized light;
Full amplitude and phase detection;
Determination of the polarization and extinction degree for the polarizers;
Determination of ellipticity of the wave plates made of crystalline quartz;
Determination of the refractive index of materials;
THz image acquisition.
The spacious sample area allows easy integration of additional equipment such as the polarizers, rotators, special sample holders with the heating or cooling elements, mechanical sample advancers, or synchronized radiation receivers. To obtain a THz image, this spectrometer applies a special TERAImage unit including a two-dimensional motorized slider with a sample holder and software for the image acquisition and reconstruction.
Execution of measurements
In the case of measuring the transmission spectrum of a sample, the radiating and receiving antennas are located on the same line and at the same height; four TPX lenses with a focal length of 54 mm are installed in the path of the THz radiation pulse. The configuration of these lenses and their position on the measuring rail is arranged in such a way that the radiation is focused into the minimum aperture at the sample location and transmits the maximum power from the radiating antenna to the receiving antenna. The minimum aperture into which radiation can be focused is 5 mm. In the case of measuring the reflection spectrum from a sample, configuration of the radiating and receiving antennas is changed, they are installed at an angle of 90 degrees to each other. The sample is placed at the focal point at an angle of 45 degrees to the incident radiation. Thus, the reflection measurement from the sample is performed only at an angle of 45 degrees relative to the normal line to the sample surface. The device circuit includes two receiving antennas: one for the transmission measurement, and one for the reflection measurement that allows not to rebuild the device when changing the measurement mode (Figure 9).
The time dependences of the receiving antenna photocurrent are measured using the K8 TeraScan software supplied with the spectrometer (Figure 10). The search for the pulse temporal shape, setting the best position of lenses along the optical line, as well as fine-tuning the silicon lens position of the radiating and receiving antennas are performed using the fast delay line scanning function. Next, the scanning parameters are set in the TeraScan program, and the time dependence of the THz pulse amplitude is measured. To obtain the spectral specifications of sample under study during the transmission signal measurements, the waveform of the radiation pulse transmitted through the empty channel is measured as the background, then the waveform of the pulse transmitted through the sample placed at the focus point of the focusing lens is measured. In the case of the reflection signal measurements, the pulse waveform reflection from a mirror with a gold coating is measured as a background, the mirror is set in the same position as the measured sample.
The inverse Fourier transform of the measured background and sample temporal waveforms is performed using the TeraMat program, the results are saved to a file in the digital form. Moreover, this program allows to calculate the refractive index of the sample material. Further processing of the measurement results consists in dividing the sample signal by the background and squaring to convert the spectral dependence of the electric field amplitude of the electromagnetic wave into the radiation intensity spectrum. To obtain a full spectrum from 185 nm to 1 500 µm, all spectra from three devices shall be prepared and imported into the OPUS program and combined using the special Merge Spectra function.
Conclusion
The presented devices and spectral measurement methods allow completing the research tasks to study the optical properties of materials that are promising for use in the equipment operating in various spectral ranges (from ultraviolet to terahertz) [3–21].
The optical elements, components of optical systems and devices for the field of science and industry, during the production of which the considered devices are used to obtain spectral specifications, are given on the website of Tydex LLC (Saint Petersburg) [2].
The authors declare that they have no conflict of interest. All authors took part in the article preparation and supplemented the manuscript in terms of their work.
Contribution of composite authors: The article was prepared on the basis of long-term work experience of all composite authors.
ABOUT AUTHORS
Kaplunov Ivan A. – Dr. of Sc. (Eng.)Tver State University;
e-mail: kaplunov.ia@tversu.ru, Tver, Russia.
ORCID 0000-0002-1726-3451
Kropotov Grigory I. – Cand. of Sc. (Phys.&Math), General Manager TYDEX, LLC.,
St. Petersburg, RUSSIA.
ORCID 0000-0001-9041-6701
Rogalin Vladimir E. – Dr. of Sc. (Phys.&Math), Institute of Electrophysics and Electric Power, Russian Academy of Sciences, St. Petersburg, Russia.
ORCID 0000-0002-2980-5385
Shakhmin Alexey A. – Cand. of Sc. (Phys.&Math), lead engineer TYDEX, LLC., St. Petersburg, Russia.
ORCID 0009-0003-9566-2823
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