A terahertz spectrum measurement system

Abstract

The present invention relates to a terahertz spectrum measurement system, which relates to the field of terahertz spectrum, and includes: a laser, a polarization beam splitter, a first crystal, a second crystal, a near-infrared spectrometer, an aperture and a pump light adjustment module; the pump light emitted by the laser is input into the polarization beam splitter to generate reflected pump light and transmitted pump light; the reflected pump light passes through the first crystal to generate a terahertz wave; the terahertz wave passes through the sample and is transmitted to the second crystal; the transmitted pump light passes through the pump light adjustment module and is transmitted to the second crystal; the second crystal converts the terahertz wave and the pump light into an up-conversion signal; the aperture is used to filter out the pump light transmitted by the second crystal; and the near-infrared spectrometer is used to measure the terahertz spectrum of the up-conversion signal. The present invention can realize convenient measurement of terahertz spectrum.

Description

Technical Field

[0001] The invention relates to the field of terahertz spectroscopy, and in particular to a terahertz spectroscopy measurement system. Background Art

[0002] In recent years, terahertz spectroscopy technology has developed rapidly. Due to its advantages in material identification, material composition quantification, spectral imaging, etc., femtosecond laser-based terahertz time-domain spectroscopy (THz-TDS) technology has been widely used in scientific research, production and other fields. Femtosecond lasers are used to generate terahertz radiation, and then femtosecond lasers are used to measure terahertz in the time domain, and then the frequency domain spectrum is obtained through Fourier transform. Scanning is required to obtain the waveform of the terahertz time domain signal, which makes the measurement process of the terahertz time domain signal more complicated. Therefore, a more convenient measurement system is needed. Summary of the invention

[0003] The purpose of the present invention is to provide a terahertz spectrum measurement system to achieve convenient measurement of the terahertz spectrum.

[0004] To achieve the above object, the present invention provides the following solutions:

[0005] A terahertz spectrum measurement system includes: a laser, a polarization beam splitter, a first crystal, a second crystal, a near-infrared spectrometer, an aperture, and a pump light adjustment module;

[0006] The polarization beam splitter is arranged on the output optical path of the laser; the pump light emitted by the laser is input into the polarization beam splitter to generate reflected pump light and transmitted pump light;

[0007] The first crystal and the sample are sequentially arranged on the output optical path of the reflected pump light; the reflected pump light passes through the first crystal to generate a terahertz wave; the second crystal is arranged on the output optical path of the terahertz wave; the terahertz wave passes through the sample and is transmitted to the second crystal;

[0008] The pump light adjustment module is arranged on the output optical path of the transmission pump light; the transmission pump light passes through the pump light adjustment module and is transmitted to the second crystal; the second crystal converts the terahertz wave and the pump light into an up-conversion signal; the infrared spectrometer is arranged on the output path of the up-conversion signal; the aperture is arranged between the second crystal and the near-infrared spectrometer; the aperture is used to filter out the pump light transmitted by the second crystal; the near-infrared spectrometer is used to measure the terahertz spectrum of the up-conversion signal.

[0009] Optionally, the measurement system further includes a first silicon prism and a second silicon prism; the first silicon prism is attached to a side of the first crystal close to the sample; the second silicon prism is attached to a side of the second crystal close to the sample; the second silicon prism is used to couple the terahertz wave to the second crystal.

[0010] Optionally, the measurement system also includes a collimating and focusing module; the collimating and focusing module is arranged between the first crystal and the second crystal; the collimating lens focusing module includes a cylindrical lens and a convex lens; the cylindrical lens is arranged between the first crystal and the sample; the cylindrical lens collimates the terahertz wave; the convex lens is arranged between the sample and the second crystal; the convex lens focuses the terahertz wave passing through the sample on the second crystal.

[0011] Optionally, the measurement system further includes a first half-wave plate and a first reflector; the first half-wave plate is arranged between the laser and the polarization beam splitter; the first half-wave plate is used to adjust the ratio of horizontal polarization and vertical polarization of the pump light; the first reflector is arranged between the polarization beam splitter and the first crystal; the first reflector receives the reflected pump light and reflects the reflected pump light onto the first crystal.

[0012] Optionally, the pump light adjustment module includes a second reflector, a third reflector, a translation stage and a fourth reflector; the second reflector is arranged between the polarization beam splitter and the third reflector; the third reflector and the fourth reflector are both arranged on the translation stage; the transmitted pump light is reflected to the third reflector through the second reflector; the fourth reflector is arranged on the reflection light path of the third reflector; the fourth reflector is used to reflect the transmitted pump light passing through the third reflector to the second crystal.

[0013] Optionally, the measurement system further includes a second half-wave plate; the second half-wave plate is arranged between the second reflector and the third reflector.

[0014] Optionally, the measurement system further includes a fifth reflector; the fifth reflector is arranged on the reflection light path of the fourth reflector; the fifth reflector is used to reflect the transmission pump light reflected by the fourth reflector to the second crystal.

[0015] Optionally, the laser is a Nd:YAG laser with a pulse width of 100 picoseconds; the first crystal and the second crystal are both MgO:LiNbO3 crystals.

[0016] Optionally, the first silicon prism and the second silicon prism are both right-angle trapezoidal silicon prisms; the lower base surface of the first silicon prism is bonded to the surface of the first crystal; the lower base surface of the second silicon prism is bonded to the surface of the second crystal; and the emission direction of the terahertz wave is perpendicular to the inclined surfaces of the first silicon prism and the second silicon prism.

[0017] Optionally, the measurement system further includes an isolation cover; the laser, the polarization beam splitter, the first crystal, the second crystal, the near-infrared spectrometer, the aperture and the pump light adjustment module are all arranged in the isolation cover.

[0018] According to the specific embodiments provided by the present invention, the present invention discloses the following technical effects:

[0019] The polarization beam splitter of the present invention is arranged on the output optical path of the laser; the pump light emitted by the laser is input into the polarization beam splitter to generate reflected pump light and transmitted pump light; the first crystal and the sample are arranged in sequence on the output optical path of the reflected pump light; the reflected pump light passes through the first crystal to generate a terahertz wave; the second crystal is arranged on the output optical path of the terahertz wave; the terahertz wave passes through the sample and is transmitted to the second crystal; the pump light adjustment module is arranged on the output optical path of the transmitted pump light; the transmitted pump light passes through the pump light adjustment module and is transmitted to the second crystal; the second crystal converts the terahertz wave and the pump light into an up-conversion signal; the infrared spectrometer is arranged on the output path of the up-conversion signal; the aperture is arranged between the second crystal and the near-infrared spectrometer; the aperture is used to filter out the pump light transmitted by the second crystal; the near-infrared spectrometer is used to measure the terahertz spectrum of the up-conversion signal. The broadband terahertz radiation generated by the laser incident on the first crystal penetrates the sample and then is up-converted in the second crystal. The up-conversion signal spectrum is measured by using a near-infrared spectrometer, thereby indirectly performing portable measurement of the terahertz spectrum of the sample. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0021] Figure 1 A schematic diagram of the structure of a terahertz spectrum measurement system provided by the present invention;

[0022] Figure 2 is the terahertz spectrum without passing through the sample;

[0023] Figure 3 This is the terahertz spectrum after passing through the sample (assuming that the sample has 30dB absorption at 1.8THz);

[0024] Figure 4 This is the spectrum of the up-conversion signal when no sample is placed;

[0025] Figure 5 This is the up-conversion signal spectrum after placing the sample.

[0026] Explanation of symbols:

[0027] 1-subnanosecond Nd:YAG laser, 2-first half-wave plate, 3-polarization beam splitter, 4-first reflector, 5-first crystal, 6-first silicon prism, 7-cylindrical lens, 8-sample, 9-convex lens, 10-second silicon prism, 11-second crystal, 12-second reflector, 13-second half-wave plate, 14-third reflector, 15-fourth reflector, 16-translation stage, 17-fifth reflector, 18-aperture, 19-near-infrared spectrometer, 20-isolation cover. DETAILED DESCRIPTION

[0028] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0029] The purpose of the present invention is to provide a terahertz spectrum measurement system to achieve convenient measurement of the terahertz spectrum.

[0030] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0031] like Figure 1 As shown, a terahertz spectrum measurement system provided by the present invention includes: a laser, a polarization beam splitter 3, a first crystal 5, a second crystal 11, a near-infrared spectrometer 19, an aperture 18 and a pump light adjustment module.

[0032] The polarization beam splitter is arranged on the output optical path of the laser; the pump light emitted by the laser is input into the polarization beam splitter 3 to generate reflected pump light and transmitted pump light.

[0033] The first crystal 5 and the sample 8 are sequentially arranged on the output optical path of the reflected pump light; the reflected pump light passes through the first crystal 5 to generate a terahertz wave; the second crystal 11 is arranged on the output optical path of the terahertz wave; the terahertz wave passes through the sample 8 and is transmitted to the second crystal 11.

[0034] The pump light adjustment module is arranged on the output optical path of the transmission pump light; the transmission pump light passes through the pump light adjustment module and is transmitted to the second crystal 11; the second crystal 11 converts the terahertz wave and the pump light into an up-conversion signal; the near-infrared spectrometer 19 is connected to a computer. The infrared spectrometer is arranged on the output path of the up-conversion signal; the aperture 18 is arranged between the second crystal 11 and the near-infrared spectrometer 19; the aperture 18 is used to filter out the pump light transmitted by the second crystal 11; the near-infrared spectrometer 19 is used to measure the terahertz spectrum of the up-conversion signal.

[0035] As an optional implementation, the measurement system also includes a first silicon prism 6 and a second silicon prism 10; the first silicon prism 6 is attached to the side of the first crystal 5 close to the sample 8; the second silicon prism 10 is attached to the side of the second crystal 11 close to the sample 8; the second silicon prism 10 is used to couple the terahertz wave to the second crystal 11.

[0036] As an optional implementation, the measurement system further includes a collimating and focusing module; the collimating and focusing module is arranged between the first crystal 5 and the second crystal 11; the collimating lens focusing module includes a cylindrical lens 7 and a convex lens 9; the cylindrical lens 7 is arranged between the first crystal 5 and the sample 8; the cylindrical lens 7 collimates the terahertz wave; the convex lens 9 is arranged between the sample 8 and the second crystal 11; the convex lens 9 focuses the terahertz wave passing through the sample 8 on the second crystal 11. In the measurement system, measurement is also achieved without the collimating and focusing module, but the terahertz collection efficiency will be reduced, making the signal-to-noise ratio of the measurement worse.

[0037] As an optional implementation, the measurement system also includes a first half-wave plate 2 and a first reflector 4; the first half-wave plate 2 is arranged between the laser and the polarization beam splitter 3; the first half-wave plate 2 is used to adjust the ratio of horizontal polarization and vertical polarization of the pump light; the first reflector 4 is arranged between the polarization beam splitter 3 and the first crystal 5; the first reflector 4 receives the reflected pump light and reflects the reflected pump light onto the first crystal 5.

[0038] As an optional implementation, the pump light adjustment module includes a second reflector 12, a third reflector 14, a translation stage 16 and a fourth reflector 15; the second reflector 12 is arranged between the polarization beam splitter 3 and the third reflector 14; the third reflector 14 and the fourth reflector 15 are both arranged on the translation stage 16; the transmission pump light is reflected to the third reflector 14 through the second reflector 12; the fourth reflector 15 is arranged on the reflection light path of the third reflector 14; the fourth reflector 15 is used to reflect the transmission pump light passing through the third reflector 14 to the second crystal 11. The translation stage 16 in the present invention is a one-dimensional translation stage.

[0039] As an optional implementation, the measurement system further includes a second half-wave plate 13 ; the second half-wave plate 13 is disposed between the second reflector 12 and the third reflector 14 .

[0040] As an optional implementation, the measurement system further includes a fifth reflector 17 ; the fifth reflector 17 is disposed on the reflection light path of the fourth reflector 15 ; the fifth reflector 17 is used to reflect the transmission pump light reflected by the fourth reflector 15 to the second crystal 11 .

[0041] As an optional implementation, the laser is a Nd:YAG laser 1 with a pulse width of 100 picoseconds; the first crystal 5 and the second crystal 11 are both MgO:LiNbO3 crystals. Alternatively, the first crystal 5 and the second crystal 11 are both LiNbO3 crystals, and doping with MgO can increase the damage threshold of LiNbO3. The pulse width of the sub-nanosecond Nd:YAG laser 1 is 400ps, the energy is 9mJ, and the spot diameter is 1.5mm, of which 6mJ is used to pump the first MgO:LiNbO3 crystal to generate broadband terahertz radiation, and 3mJ is used for up-conversion detection. The length of both crystals is 65mm. The broadband terahertz radiation generated after the sub-nanosecond Nd:YAG laser 1 is incident on the LiNbO3 crystal is used as the terahertz source in terahertz spectroscopy measurement.

[0042] As an optional implementation, the first silicon prism 6 and the second silicon prism 10 are both right-angle trapezoidal silicon prisms; the lower bottom surface of the first silicon prism 6 is in contact with the surface of the first crystal 5; the lower bottom surface of the second silicon prism 10 is in contact with the surface of the second crystal 11; the emission direction of the terahertz wave is perpendicular to the inclined surfaces of the first silicon prism 6 and the second silicon prism 10. The second silicon prism 10 is in contact with the second crystal 11, with the purpose of coupling the terahertz to the second crystal 11. The first silicon prism 6 is in contact with the first crystal 5. Since the refractive index of the first crystal 5 is very large in the terahertz band (the refractive index of LiNbO3 in the terahertz band is around 5), if the first silicon prism 6 is not placed, the terahertz will be totally reflected at the interface between the first crystal 5 and the air. The refractive index of silicon is about 3.4. After the first silicon prism 6 is placed, the terahertz will not be totally reflected at the interface between the first crystal 5 and the first silicon prism 6. The terahertz can enter the first silicon prism 6 and then output to the air from the other side of the first silicon prism 6.

[0043] As an optional implementation, the measurement system further includes an isolation cover 20; the laser, the polarization beam splitter 3, the first crystal 5, the second crystal 11, the near-infrared spectrometer 19, the aperture 18 and the pump light adjustment module are all arranged in the isolation cover 20. The isolation cover 20 is filled with dry air.

[0044] The broadband terahertz radiation generated by the Nd:YAG laser incident on LiNbO3 penetrates the sample 8 and then is up-converted in another piece of LiNbO3. The terahertz spectrum of the sample 8 is indirectly measured by measuring the up-conversion signal spectrum using a near-infrared spectrometer 19. If the sample 8 is at a frequency v T There is an absorption peak at ν. P -ν T Place (ν P The present invention can perform single pulse measurement and directly obtain frequency domain signals without Fourier transform.

[0045] The working principle of the system provided by the present invention is as follows:

[0046] The pump light is emitted by a sub-nanosecond Nd:YAG laser 1 and passes through the first half-wave plate 2. The function of the first half-wave plate 2 is to adjust the ratio of horizontal polarization and vertical polarization in the pump light, so as to adjust the ratio of reflection and transmission of the polarization beam splitter 3. The function of the polarization beam splitter 3 is to split the pump light into two beams according to the polarization direction, with vertical polarization reflection and horizontal polarization transmission, which are used for terahertz generation and detection respectively. After passing through the first half-wave plate 2, the pump light is divided into two beams by the polarization beam splitter 3, and the energy ratio of the two beams can be adjusted by rotating the half-wave plate. Reflection part: The pump light (vertical polarization) reflected by the polarization beam splitter 3 is projected into the first through the first reflector 4, and the direction of the crystal optical axis is along the vertical direction. At this time, broadband terahertz radiation will be generated, and the propagation direction is as follows: Figure 1 As shown by the double lines in the middle. Due to spontaneous down conversion (due to the spontaneous down conversion (SPDC) effect, the vertically polarized pump light will generate broadband terahertz radiation after entering the first crystal 5, and its propagation direction is at an angle of ~64 degrees with the pump light), broadband terahertz radiation with a frequency range of about 1.5-3THz will be generated. In order to avoid total reflection of terahertz at the interface between the crystal and the air. The first silicon prism 6 is placed close to the first crystal 5, and the terahertz is coupled from the surface of the first silicon prism 6 to the air. The terahertz wave is coupled out by the right-angle trapezoidal silicon prism (bottom angle 41°), and the terahertz emission direction is perpendicular to the inclined surface of the silicon prism. The terahertz wave is collimated by the cylindrical lens 7 and passes through the sample 8, and then focused to the surface of the second silicon prism 10 by the convex lens 9. The terahertz wave absorbed by the sample 8 is coupled into another 65mm MgO:LiNbO3 crystal through the second silicon prism 10. The pump light transmitted by the polarization beam splitter 3 is used for terahertz detection. Since the pump light required for detection is vertically polarized, the second half-wave plate 13 is added to change the polarization direction of the light. The second reflector 12, the third reflector 14, the fourth reflector 15, and the translation stage 16 are used to adjust the time when the pump light for detection reaches the crystal, so that it reaches the crystal at the same time as the terahertz.

[0047] The pump light transmitted by the polarization beam splitter 3 is also injected into the second MgO:LiNbO3 crystal after the polarization direction is changed to vertical by the second half-wave plate 13. The terahertz wave and the pump light are simultaneously entered into the second MgO:LiNbO3 crystal by adjusting the translation stage 16. At this time, due to the effect of the difference frequency parameter, a broadband up-conversion signal will be generated. After filtering out the remaining pump light with the aperture 18, the spectrum of the up-conversion signal is measured using a near-infrared spectrometer 19. The up-conversion signal is generated by the second crystal 11. Its spectral components are related to the terahertz components injected by the second silicon prism 10. The frequency of the pump light is 282THz (corresponding to a wavelength of 1064nm), and the up-conversion signal is generated by the pump light and the terahertz, and its frequency is the difference between the pump light and the terahertz. For example, the frequency component of 280.5THz in the up-conversion signal (1069.5nm) corresponds to the component of 1.5THz in the terahertz, and the intensity is proportional to the signal of 1.5THz in the terahertz signal. If the 1.5 THz component is absorbed more by sample 8, a pit will appear at the beginning of 1069.5 nm in the up-conversion spectrum. ) Different terahertz frequency components correspond to the frequency components in the up-conversion spectrum whose frequency is the difference between the pump frequency and the terahertz frequency.

[0048] If a certain terahertz frequency component is more severely absorbed by the sample 8 than other frequency components, a depression will appear at the corresponding frequency in the up-conversion spectrum. In order to reduce the influence of moisture in the air on the experiment, the system can be placed in an isolation cover 20 filled with dry air.

[0049] Figure 2 The middle is the terahertz spectrum without sample. Figure 3 is the terahertz spectrum after passing through the sample (assuming the sample has 30dB absorption at 1.8THz), Figure 4 is the up-conversion signal spectrum without placing the sample, Figure 5 is the up-conversion signal spectrum after placing the sample. The device of the present invention is numerically simulated. Figure 2-Figure 5 It can be seen that the dip at 1071 nm in the upconversion spectrum corresponds to the absorption peak of the sample at 1.8 THz.

[0050] The device provided by the present invention only needs to use a sub-nanosecond Nd:YAG laser and some optical elements, and does not need a femtosecond laser and a time domain or frequency domain scanning system. The measurement process is a single pulse measurement, which is relatively convenient.

[0051] The various embodiments in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the various embodiments can be referenced to each other.

[0052] This article uses specific examples to illustrate the principles and implementation methods of the present invention. The above examples are only used to help understand the device and its core ideas of the present invention. At the same time, for those skilled in the art, according to the ideas of the present invention, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as limiting the present invention.

Claims

1. A terahertz spectrum measurement system, characterized in that: include: A laser, a polarization beam splitter, a first crystal, a second crystal, a near-infrared spectrometer, an aperture, and a pump light adjustment module; The polarization beam splitter is arranged on the output optical path of the laser; the pump light emitted by the laser is input into the polarization beam splitter to generate reflected pump light and transmitted pump light; The first crystal and the sample are sequentially arranged on the output optical path of the reflected pump light; the reflected pump light passes through the first crystal to generate a terahertz wave; the second crystal is arranged on the output optical path of the terahertz wave; the terahertz wave passes through the sample and is transmitted to the second crystal; The pump light regulating module is arranged on the output optical path of the transmission pump light; the transmission pump light passes through the pump light regulating module and is transmitted to the second crystal; the second crystal converts the terahertz wave and the pump light into an up-conversion signal; the infrared spectrometer is arranged on the output path of the up-conversion signal; the aperture is arranged between the second crystal and the near-infrared spectrometer; the aperture is used to filter out the pump light transmitted by the second crystal; the near-infrared spectrometer is used to measure the terahertz spectrum of the up-conversion signal; The pump light adjustment module comprises a second reflector, a third reflector, a translation stage and a fourth reflector; the second reflector is arranged between the polarization beam splitter and the third reflector; the third reflector and the fourth reflector are both arranged on the translation stage; The transmission pump light is reflected to the third reflector through the second reflector; the fourth reflector is arranged on the reflection light path of the third reflector; the fourth reflector is used to reflect the transmission pump light passing through the third reflector to the second crystal; The measurement system further comprises a second half-wave plate; the second half-wave plate is arranged between the second reflector and the third reflector; the second half-wave plate is used to change the polarization direction of the transmitted pump light; The laser is a Nd:YAG laser with a pulse width of 100 picoseconds; the first crystal and the second crystal are both MgO:LiNbO3 crystals.

2. The terahertz spectrum measurement system according to claim 1, characterized in that: The measurement system also includes a first silicon prism and a second silicon prism; the first silicon prism is attached to a side of the first crystal close to the sample; the second silicon prism is attached to a side of the second crystal close to the sample; the second silicon prism is used to couple the terahertz wave to the second crystal.

3. The terahertz spectrum measurement system according to claim 1, characterized in that: The measurement system also includes a collimating and focusing module; the collimating and focusing module is arranged between the first crystal and the second crystal; the collimating lens focusing module includes a cylindrical lens and a convex lens; the cylindrical lens is arranged between the first crystal and the sample; the cylindrical lens collimates the terahertz wave; the convex lens is arranged between the sample and the second crystal; the convex lens focuses the terahertz wave passing through the sample on the second crystal.

4. The terahertz spectrum measurement system according to claim 1, characterized in that: The measurement system further comprises a first half-wave plate and a first reflector; the first half-wave plate is arranged between the laser and the polarization beam splitter; the first half-wave plate is used to adjust the ratio of the horizontal polarization and the vertical polarization of the pump light; The first reflector is disposed between the polarization beam splitter and the first crystal; the first reflector receives the reflected pump light and reflects the reflected pump light onto the first crystal.

5. The terahertz spectrum measurement system according to claim 1, characterized in that: The measurement system further includes a fifth reflector; the fifth reflector is arranged on the reflection light path of the fourth reflector; the fifth reflector is used to reflect the transmission pump light reflected by the fourth reflector to the second crystal.

6. The terahertz spectrum measurement system according to claim 2, characterized in that: The first silicon prism and the second silicon prism are both right-angle trapezoidal silicon prisms; the lower bottom surface of the first silicon prism is bonded to the surface of the first crystal; the lower bottom surface of the second silicon prism is bonded to the surface of the second crystal; the emission direction of the terahertz wave is perpendicular to the inclined surfaces of the first silicon prism and the second silicon prism.

7. The terahertz spectrum measurement system according to claim 1, characterized in that: The measurement system further includes an isolation cover; the laser, the polarization beam splitter, the first crystal, the second crystal, the near-infrared spectrometer, the aperture and the pump light adjustment module are all arranged in the isolation cover.

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