Novel terahertz spectrum measuring instrument and method

Abstract

The invention provides a novel terahertz spectrometer. The device mainly comprises a terahertz complex color light source system, a collimation module, a long-wave-pass optical filter, a terahertz tunable filter, a voltage supply module, a chopper, a controller, a to-be-detected sample, a focusing module, a terahertz detector, a data collector and a measurement and control system. The terahertz complex-color light source system is used for generating complex-color terahertz light beams, the collimation system is used for generating parallel light, the long-wave-pass optical filter is used for filtering long-wave-band components in the light source, the terahertz tunable filter and the voltage supply module are used for converting the complex-color light beams into monochromatic light beams, and the chopper and the controller are used for reducing measurement noise. The focusing module, the terahertz detector and the data collector are used for measuring the intensity of terahertz light beams, and the measurement and control system is used for controlling the whole system. Compared with an existing common terahertz spectrometer, the novel terahertz spectrometer has the advantages of being low in cost, high in vertical coordinate measurement accuracy (about 0.5%), high in measurement speed (about second magnitude) and the like. The novel terahertz spectrometer can provide a new scheme for the terahertz spectrum measurement field, the new scheme and an existing terahertz spectrum measurement method can form an angle potential, and a better terahertz spectrum measurement selection can be provided for many application scenes; and meanwhile, a novel terahertz time-resolved spectrum technology, a novel terahertz spectrum pumping detection technology and a novel terahertz spectrum imaging technology are expected to be created.

Description

Technical Field

[0001] This invention relates to the field of terahertz spectroscopy measurement, and in particular to a novel terahertz spectrometer. Background Technology

[0002] Terahertz spectroscopy contains a wealth of physical and chemical information about matter, such as vibrational energy levels, rotational energy levels, hydrogen bonds, and van der Waals forces, which have many characteristic peaks in the terahertz band. Terahertz spectroscopy technology has therefore been widely used in defense, military, biomedicine, non-destructive testing and other fields [1-5]. For example, different explosives and different glucose have completely different terahertz absorption spectra [6, 7].

[0003] Terahertz spectroscopy measurement technology is a very important research topic in the field of terahertz spectroscopy (which mainly includes spectral principles, spectral measurement, and spectral analysis). The existing common terahertz spectroscopy measurement methods mainly include classical time-domain spectroscopy [8-10], asynchronous sampling time-domain spectroscopy [11-15], Fourier transform spectrometer measurement method [16-19], wavelength scanning method based on back-wave oscillator [20-22], wavelength scanning method based on frequency mixing [23, 24], and single-exposure measurement method based on array detector [25, 26].

[0004] The measurement principle of classical time-domain spectroscopy is as follows: a pulsed light from a femtosecond laser is split into two beams by a beam splitter. One beam is incident on a terahertz generator to produce a terahertz pulse. This terahertz pulse is collected by a terahertz detector after passing through a system containing the sample under test. The other beam is also collected by a terahertz detector after passing through a time-delay system. The transient electric field intensity of the terahertz pulse at the arrival of the femtosecond laser pulse can be measured by photoconductive sampling or free-space electro-optic sampling. By controlling the time delay system to adjust the time delay between the two pulses and measuring the transient electric field intensity of the terahertz pulse sequentially at different time delays, the time-domain waveform of the terahertz pulse can be obtained. After Fourier transforming this time-domain waveform, the spectrum of the terahertz pulse can be obtained. By comparing the terahertz spectra before and after placing the sample, the absorption spectrum of the sample under test can be obtained. The main advantages of classical time-domain spectroscopy are: (1) It adopts a coherent measurement method, which can obtain the amplitude and phase of the measured electric field, thereby extracting parameters such as the absorption coefficient, refractive index, and dielectric constant of the sample; (2) It has a wide measurement spectral range (about 0.1-10 THz); (3) It has transient characteristics, with the typical pulse width of terahertz pulses on the order of picoseconds. The main disadvantages are: (1) It relies on repeatable mechanical scanning, and the measurement time is relatively long (about minutes); (2) The spectral resolution is not high, about tens of GHz; (3) It is expensive (about 1 million); (4) The spectral calculation process requires Fourier transform, and the measurement accuracy of the spectral ordinate is not high (about 3%).

[0005] Compared with classical time-domain spectroscopy, the main feature of asynchronous sampling time-domain spectroscopy is the use of two femtosecond lasers. Their output pulse periods have slight differences, so the time difference between the terahertz pulse and the corresponding laser pulse arriving at the detector increases sequentially. This allows the time-domain waveform of the terahertz pulse to be obtained without relying on a time delay system (avoiding repetitive mechanical scanning). In addition to the advantages of classical time-domain spectroscopy, asynchronous sampling time-domain spectroscopy also features fast measurement speed (on the order of seconds) and high spectral resolution (on the order of tens of MHz). The main drawbacks of asynchronous sampling time-domain spectroscopy are: (1) it requires two femtosecond lasers, making the entire system more expensive (around 1.5 million); (2) the spectral calculation process still requires Fourier transform, resulting in low accuracy of the spectral ordinate measurement (around 3%).

[0006] The principle of Fourier transform spectrometry for measuring terahertz spectra is as follows: After passing through a beam splitter, the terahertz beam is divided into two beams. After being reflected by a moving mirror and a stationary mirror respectively, the two beams are combined and interfere at the detector position. By mechanically scanning the moving mirror and using the detector to record the interference intensity at different scanning positions in sequence, an interference intensity sequence is obtained. Then, Fourier transform is performed on the interference intensity sequence to obtain the terahertz spectral information. The main advantages of Fourier transform spectrometry are a wide spectral range (about 0.1-10 THz) and a high signal-to-noise ratio. The main disadvantages are: (1) generally requiring repeatable mechanical scanning, the measurement time is relatively long (about minutes); (2) the spectral calculation process requires Fourier transform, and the accuracy of the spectral ordinate measurement is not high (about 3%); (3) expensive (about 2 million).

[0007] The working principle of the back-wave oscillator-based wavelength scanning method is as follows: the back-wave oscillator outputs monochromatic terahertz light. By changing the operating voltage, the wavelength of the output terahertz light can be adjusted, thus allowing the measurement of the terahertz spectrum of the sample using a very simple optical system. The main advantages of this method are: (1) the data processing is very simple and the measurement accuracy is high (around 0.5%); (2) no mechanical scanning is required and the measurement speed is fast (on the order of seconds); (3) the spectral resolution is high (around tens of MHz). The main disadvantages are: (1) the measurement spectral range is very narrow (around 0.1-1 THz), and (2) the price is high (around 1 million).

[0008] The working principle of the wavelength scanning method based on frequency mixing is as follows: beams from two lasers are first merged and then split. One beam radiates onto a mixer with a bias voltage to generate terahertz light. This light passes through the sample and reaches the mixer, which acts as a detector, where it merges with the other beam. After mixing, a detectable signal is generated. The main advantages of this method are: (1) the data processing is very simple and the measurement accuracy is high (around 0.5%); (2) no mechanical scanning is required, and the measurement speed is fast (on the order of seconds); (3) the spectral resolution is high (around tens of MHz). The main disadvantages are the narrow measurement spectral range (around 0.1-3 THz) and the high cost of the system (around 600,000 RMB).

[0009] The principle of the single-exposure measurement method based on array detectors is as follows: terahertz light is arranged according to wavelength on an array detector using dispersive elements, and the terahertz spectrum can be obtained through a single exposure. The main advantage of this method is its fast measurement speed (on the order of seconds), while its main disadvantages are low measurement accuracy (around 3%) and high cost (around 800,000 RMB).

[0010] The above analysis shows that each terahertz measurement method has its own advantages and disadvantages, and each method has its own application and scope. From the perspective of scientific instrument development trends, an ideal measuring instrument should simultaneously possess advantages such as low cost, miniaturization, high precision, high spectral resolution, and speed. The above analysis also shows that current measurement methods cannot meet the requirements of an ideal measuring instrument. Therefore, developing new terahertz spectroscopy measurement methods remains an important direction for development in this field.

[0011] References: 1. Singh, K., et al., Spectroscopic and imaging considerations of THz-TDS and ULF-Raman techniques towards practical security applications. Optics express, 2024. 32(2): p. 1314-1324. 2. Sebastiani, F., et al., Water Dynamics from THz Spectroscopy Reveal the Locus of a Liquid-Liquid Binodal Limit in Aqueous CaCO 3 Solutions. Angewandte Chemie-International Edition, 2017. 56(2): p. 490-495. 3. Markelz, AG, A. Roitberg, and EJ Heilweil, Pulsed terahertz spectroscopy of DNA, bovine serum albumin and collagen between 0.1 and 2.0 THz. Chemical Physics Letters, 2000. 320(1-2): p. 42-48. 4. Xie, L.J., Y. Yao, and Y.B. Ying, The Application of Terahertz Spectroscopy to Protein Detection: A Review. Applied Spectroscopy Reviews,2014. 49(6): p. 448-461. 5. Gente, R., et al., Scaled Bistatic Radar Cross Section Measurements of Aircraft With a Fiber-Coupled THz Time-Domain Spectrometer. Ieee Transactions on Terahertz Science And Technology, 2012. 2(4): p. 424-431. 6. Ho, L., M. Pepper, and P. Taday, TERAHERTZ SPECTROSCOPY Signatures and fingerprints. Nature Photonics, 2008. 2: p. 541-543. 7. Huang, S.T., et al., Progress in application of terahertz time- domain spectroscopy for pharmaceutical analyses. Frontiers In BioengineeringAnd Biotechnology, 2023. 11: p. 1219042. 8. Vanexter, M., C. Fattinger, and D. Grischkowsky, Terahertz Time- Domain Spectroscopy Of Water-Vapor. Optics Letters, 1989. 14(20): p. 1128-1130. 9. Grischkowsky, D., et al., Far-Infrared Time-Domain Spectroscopy with Terahertz Beams Of Dielectrics And Semiconductors. Journal Of theOptical Society Of America B-Optical Physics, 1990. 7(10): p. 2006-2015. 10. Jiang, Z.P. and X.C. Zhang, Electro-optic measurement of THz field pulses with a chirped optical beam. Applied Physics Letters, 1998. 72(16): p. 1945-1947. 11. Yasui, T., E. Saneyoshi, and T. Araki, Asynchronous optical sampling terahertz time-domain spectroscopy for ultrahigh spectral resolution and rapid data acquisition. Applied Physics Letters, 2005. 87(6). p. 061101 12. Bartels, A., et al., High-resolution THz spectrometer with kHz scan rates. Optics Express, 2006. 14(1): p. 430-437. 13. von Ribbeck, H.G., et al., Spectroscopic THz near-field microscope. Optics Express, 2008. 16(5): p. 3430-3438. 14. Yasui, T., et al., Fiber-based, hybrid terahertz spectrometer using dual fiber combs. Optics Letters, 2010. 35(10): p. 1689-1691. 15. Finneran, I.A., et al., Decade-Spanning High-Precision Terahertz Frequency Comb. Physical Review Letters, 2015. 114(16). p. 163902 16. Han, P.Y., et al., A direct comparison between terahertz time- domain spectroscopy and far-infrared Fourier transform spectroscopy. JournalOf Applied Physics, 2001. 89(4): p. 2357-2359. 17. Yasui, T., et al., Terahertz frequency comb by multifrequency- heterodyning photoconductive detection for high-accuracy, high-resolution terahertz spectroscopy. Applied Physics Letters, 2006. 88(24). p. 241104. 18. Yang, Y., et al., Terahertz multiheterodyne spectroscopy using laser frequency combs. Optica, 2016. 3(5): p. 499-502. 19. Nakajima, S., et al., Determination of starch crystallinity with the Fourier-transform terahertz spectrometer. Carbohydrate Polymers, 2021.262. p. 117928. 20. He, W., et al., High Power Wideband Gyrotron Backward Wave Oscillator Operating towards the Terahertz Region. Physical Review Letters, 2013. 110(16). p. 165101. 21. Dobroiu, A., et al., Terahertz imaging system based on a backward-wave oscillator. Applied Optics, 2004. 43(30): p. 5637-5646. 22. Mineo, M. and C. Paoloni, Corrugated Rectangular Waveguide Tunable Backward Wave Oscillator for Terahertz Applications. IEEE Transactions on Electron Devices, 2010. 57(6): p. 1481-1484. 23. Verghese, S., et al., Generation and detection of coherent terahertz waves using two photomixers. Applied Physics Letters, 1998. 73(26):p. 3824-3826. 24. Sartorius, B., et al., Continuous Wave Terahertz Systems Based on 1.5 μm Telecom Technologies. Journal Of Infrared Millimeter And TerahertzWaves, 2012. 33(4): p. 405-417. 25. Yang, T., et al., Compact terahertz spectrometer based on sequential modulation of disordered rough surfaces. Optics Letters, 2019. 44(24): p. 6061-6064. 26. Headland, D., et al., Terahertz Spectroscope Using CMOS Camera and Dispersive Optics. IEEE Transactions on Terahertz Science And Technology, 2020. 10(5): p. 513-523. Summary of the Invention

[0012] To address the problem that existing terahertz spectrometers cannot meet the requirements of ideal measurement instruments, this invention provides a novel terahertz spectrometer.

[0013] The technical solution of the present invention is as follows: a terahertz spectrometer, such as... Figure 1As shown, the spectrometer mainly includes a terahertz polychromatic light source system (1), a collimation module (2), a long-pass filter (3), a terahertz tunable filter (4), a chopper (5), a sample to be tested (6), a focusing module (7), a voltage supply module (8), a controller (9), a measurement and control system (10), a data acquisition unit (11), and a terahertz detector (12).

[0014] The terahertz polychromatic light source system (1) is used to generate polychromatic terahertz beams, the collimation system (2) is used to generate parallel light, the long-pass filter (3) is used to filter the long-wavelength components in the light source, the terahertz tunable filter (4) is used to convert the polychromatic beam into a monochromatic beam, the chopper (5) is used to reduce measurement noise, the focusing module (7) and the terahertz detector (12) are used to measure the intensity of the terahertz beam, the voltage supply module (8) is used to provide input voltage to the terahertz tunable filter (4), the controller (9) is used to operate the chopper (5), the data acquisition unit (11) is used to measure the output signal of the terahertz detector (12), and the measurement and control system (10) is used to automatically measure and control the entire system, realizing the functions of data acquisition and data processing.

[0015] The terahertz tunable filter (4) consists of 6 filtering modules, such as... Figure 2 As shown.

[0016] This invention also proposes a method for measuring terahertz spectra, comprising the following steps: ① Measure the dark field intensity value of the terahertz detector (12) with the terahertz polychromatic light source system (1) turned off; ② Turn on the terahertz polychromatic light source system (1), do not place a sample in the optical path, and let the test beam pass directly through the focusing module (7) and then illuminate the terahertz detector (12); ③ By using the voltage supply module (8), the output wavelength of the terahertz tunable filter (4) is set to λ1, and then the bright field intensity value of the terahertz detector (12) is measured and recorded as I1. r ; ④ By using the voltage supply module (8), the output wavelength of the terahertz tunable filter (4) is sequentially set to λ2, λ3... λ n Repeat step ③ to obtain the bright field intensity values ​​of the terahertz detector at each wavelength, denoted as I2. r I3 r ··· I n r ; ⑤ Install the sample to be tested in the optical path (6); ⑥ By using the voltage supply module (8), the output wavelength of the terahertz tunable filter (4) is set to λ1, and then the signal strength value of the terahertz detector (12) is measured and recorded as I1. s ; ⑦ By using the voltage supply module (8), the output wavelength of the terahertz tunable filter (4) is sequentially set to λ2, λ3... λ n Repeat step ⑥ to obtain the signal intensity values ​​of the terahertz detector at each wavelength, denoted as I2. s I3 s ··· I n s ; ⑧ Based on the dark field intensity value of the terahertz detector (12), and the bright field intensity value and signal intensity value at each wavelength, calculate the wavelengths λ1, λ2, ..., λ according to the following formula. n Transmission spectrum of the sample under test: ··· ⑨ Based on the transmission spectrum values ​​at each wavelength position, plot the transmission spectrum of the sample to be tested (6) and complete the measurement of the terahertz transmission spectrum. Advantages of the present invention

[0017] Compared with existing common terahertz spectral measurement methods, the novel spectral measurement system based on this invention has advantages such as low cost, high accuracy of ordinate measurement (around 0.5%), and fast measurement speed (on the order of seconds). Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the structure of the novel terahertz spectrometer proposed in this invention patent.

[0019] Figure 2 This is a schematic diagram of the structure of the tunable filter in the novel terahertz spectrometer proposed in this invention patent.

[0020] Figure 3 yes Figure 2 The transmission spectrum of filter module 1 is shown in the figure. The solid line represents the initial transmission spectrum, and the dashed line represents the transmission spectrum after the voltage is changed.

[0021] Figure 4 yes Figure 2 The transmission spectra of each of the six filtering modules.

[0022] Figure 5 yes Figure 2 The total transmission spectrum after the six modules are connected in series.

[0023] Figure 6 This is a schematic diagram of wavelength tuning for a terahertz tunable filter. Detailed Implementation

[0024] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments, but this should not be construed as limiting the scope of protection of the present invention.

[0025] according to Figure 2 The structure is designed to construct a terahertz tunable filter module. The terahertz polarizer is planned to be purchased from Shanghai Haoliang Optoelectronic Equipment Co., Ltd., and the terahertz liquid crystal is planned to be purchased from a relevant research team at Nanjing University. Based on the knowledge of polarization interference in parallel polarizer systems, when the fast axis of the liquid crystal is at 45° to the polarization direction, neglecting surface reflection and internal absorption, it can be known that... Figure 2 The transmission spectrum of filter module 1 is In the formula, Transmittance at different terahertz frequencies The thickness of the terahertz liquid crystal in the direction of light propagation is set to 0.6 mm. The birefringence of the terahertz liquid crystal is the difference in refractive index between the o-ray and e-ray, set to 0.3. The speed of light in a vacuum. The frequency is terahertz (the setting range is 0.4-2.9THz).

[0026] Figure 3 The solid line in the middle is Figure 2 The transmission spectrum of filter module 1. It should be noted that by changing the voltage applied to the terahertz liquid crystal, the birefringence of the terahertz liquid crystal can be adjusted, and the period and peak position of the transmission spectrum curve of the filter module will change accordingly, such as... Figure 3 As shown by the dashed line.

[0027] Six filtering modules were constructed sequentially, with the thickness of the liquid crystal in each module being as follows: , , , , and Their transmission spectral functions are shown in the following formula.

[0028] Figure 4 These are the transmission spectrum curves of the six individual filtering modules. When these six modules are connected in series, the overall transmission spectrum function is shown below.

[0029] The total transmission spectrum is as follows Figure 5 As shown, a high-performance terahertz filter has been formed, with a center frequency of 1.67 THz and a half-width at half-maximum of approximately 23 GHz.

[0030] By changing the voltage applied to the liquid crystal, the birefringence of the terahertz liquid crystal can be altered, thereby changing the output frequency of the filter. Figure 6 As shown, this achieves the purpose of tunable filtering.

[0031] Based on the terahertz tunable filter constructed above, combined with a terahertz polychromatic light source [the proposed high-pressure mercury lamp reported in the literature [H. Rubens and O. von Baeyer, On Extremely Long Waves, emitted by the Quartz Mercury Lamp, Philosophical Magazine, 21(125): 689-695,1911.], whose output power in the 0.1-2THz range is approximately 70μW], a collimation module, a long-pass filter, a chopper, a focusing module, and a terahertz detector [a Schottky diode detector is proposed, with a detection range of 0.01-1.8THz, a noise equivalent power of 10-10W·Hz-1 / 2, and a response time in the ps order of magnitude]. Alternatively, a more sensitive radiometric calorimeter with a detection range of 0.1-100 THz, a noise equivalent power of 10-14 W·Hz-1 / 2, and a response time in the order of nanoseconds, along with a voltage supply module, data acquisition unit, and measurement and control system, can be used to develop a new terahertz spectral measurement system.

[0032] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A terahertz spectral measuring instrument, comprising a terahertz polychromatic light source system (1), a collimation module (2), a long-pass filter (3), a terahertz tunable filter (4), a chopper (5), a sample to be tested (6), a focusing module (7), a voltage supply module (8), a controller (9), a measurement and control system (10), a data acquisition unit (11), and a terahertz detector (12). The collimation module (2) is placed at the beam emission direction of the terahertz polychromatic light source system (1), and the beam propagation direction of the beam emitted from the collimation module (2) is sequentially the long-pass filter (3), the terahertz tunable filter (4), the chopper (5), the sample to be tested (6), and the focusing module (7). The terahertz detector (12) is placed at the beam emission direction of the focusing module (7). The voltage supply module (8) is used to provide input voltage to the terahertz tunable filter (4). The controller (9) is used to operate the chopper (5). The data acquisition unit (11) is used to measure the output signal of the terahertz detector (12). The measurement and control system (10) is used for automated measurement and control of the entire system.

2. A terahertz tunable filter, comprising filter module 1, module 2, module 3, module 4, module 5, and module 6. Filter module 1 consists of two terahertz polarizers and one terahertz liquid crystal, while modules 2, 3, 4, 5, and 6 each consist of one terahertz polarizer and one terahertz liquid crystal.

3. A terahertz spectroscopy measurement method, comprising the following steps: ① Measure the dark field intensity value of the terahertz detector (12) with the terahertz polychromatic light source system (1) turned off; ② Turn on the terahertz polychromatic light source system (1), do not place a sample in the optical path, and let the test beam pass directly through the focusing module (7) and then illuminate the terahertz detector (12); ③ By using the voltage supply module (8), the output wavelength of the terahertz tunable filter (4) is set to λ1, and then the bright field intensity value of the terahertz detector (12) is measured and recorded as I1. r ; ④ By using the voltage supply module (8), the output wavelength of the terahertz tunable filter (4) is sequentially set to λ2, λ3... λ n Repeat step ③ to obtain the bright field intensity values ​​of the terahertz detector at each wavelength, denoted as I2. r I3 r ··· I n r ; ⑤ Install the sample to be tested in the optical path (6); ⑥ By using the voltage supply module (8), the output wavelength of the terahertz tunable filter (4) is set to λ1, and then the signal strength value of the terahertz detector (12) is measured and recorded as I1. s ; ⑦ By using the voltage supply module (8), the output wavelength of the terahertz tunable filter (4) is sequentially set to λ2, λ3... λ n Repeat step ⑥ to obtain the signal intensity values ​​of the terahertz detector at each wavelength, denoted as I2. s I3 s ··· I n s ; ⑧ Based on the dark field intensity value of the terahertz detector (12), and the bright field intensity value and signal intensity value at each wavelength, calculate the wavelengths λ1, λ2, ..., λ according to the following formula. n Transmission spectrum of the sample under test: ··· ⑨ Based on the transmission spectrum values ​​at each wavelength position, plot the transmission spectrum of the sample to be tested (6) and complete the measurement of the terahertz transmission spectrum.

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