CN109632095B - Visual terahertz power meter based on cholesteric liquid crystal and testing method thereof - Google Patents
Visual terahertz power meter based on cholesteric liquid crystal and testing method thereof Download PDFInfo
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- G01—MEASURING; TESTING
- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
- G01J1/00—Photometry, e.g. photographic exposure meter
- G01J1/48—Photometry, e.g. photographic exposure meter using chemical effects
- G01J1/50—Photometry, e.g. photographic exposure meter using chemical effects using change in colour of an indicator, e.g. actinometer
Abstract
The invention discloses a visual terahertz dynamometer based on cholesteric liquid crystal and a test method thereof, wherein the visual terahertz dynamometer comprises a capsule type cholesteric liquid crystal film which generates visible color area change along with the change of THz intensity, the capsule type cholesteric liquid crystal film comprises a three-layer structure, and an acrylic resin film, cholesteric liquid crystal and carbon black are sequentially arranged from top to bottom. The equipment and the testing method are simple and efficient, can especially detect the power of the strong THz wave, and quantize the increase of the size of a color change area caused by THz absorption by utilizing the thermochromatic effect and the thermal diffusion effect of the cholesteric liquid crystal. The device is not limited by saturation of color changes and is robust and does not require any additional components to measure temperature.
Description
Technical Field
The invention relates to a visual terahertz power meter based on cholesteric liquid crystal and a test method thereof, which can be used in the technical field.
Background
Terahertz (THz) waves are at frequencies of 0.1-10THz (1 THz-10)12Hz, corresponding to a wavelength of 3000-30 μm). Since THz waves have unique advantages: the low photon energy is suitable for biopsy of biological tissues; the framework vibration and rotation energy levels of a plurality of condensed substances and biological macromolecules and weak interaction (hydrogen bonds and the like) among a plurality of molecules are all in the THz frequency band; many non-metallic, non-polar materials absorb the THz wave less and have high permeability; compared with visible light and infrared light, the THz wave has extremely high directivity and strong cloud penetration capacity, can realize the wireless transmission rate of more than Gbit/s, is small in interference of environmental noise and the like, so that the THz scientific technology at the leading edge of the current world interdisciplinary disciplines has important application prospects in the fields of safety inspection, biomedicine, nondestructive detection and high-speed communication. As a key core device THz detector of the THz system, the THz detector is of great importance to the improvement of the performance and the research and development of a novel THz detector.
In the current various THz detectors, incoherent detection can only detect the THz intensity, and coherent detection needs a local oscillation signal source; the other time domain measurement of the THz ultrashort pulse is usually carried out by adopting a photoconductive antenna sampling method or an electro-optical detection method by utilizing a nonlinear crystal; THz near-field detection generally requires a probe. The above methods mainly use electrical detection, the required circuit system has a complex structure, high cost and limited application range, and many performance indexes are close to the theoretical limit. The Golay Cell (kohleri detector) based on thermal effect can indirectly measure THz radiation by an optical method, has wide response wave band and can work at room temperature, but the position is relatively fixed in order to ensure the stability of gas in a gas chamber when the Golay Cell (kohleri detector) is used, the maximum detection power is also lower, for example, the current Golay-based THz power meter of the TYDEX company only reaches 10 muW, the reaction time is long, and the kohleri detector is generally only used in the occasions where the radiation changes slowly. At present, a novel THz detector with low cost, simple structure and wide market is urgently needed. The THz radiation can be converted into visible light by benefiting from a high-sensitivity detection technology of a visible light wave band, and the multi-parameter related characteristics of the THz wave, such as frequency, amplitude, phase, polarization and the like can be obtained by analyzing the change characteristics of the visible light. Thus, there is still a large space to exploit by optical methods for measuring THz radiation. In recent years, alternative thermal detection techniques using Cholesteric Liquid Crystals (CLC) have emerged. kellmann and Renk first demonstrated a CLC imager for 337 μm radiation. Woolard et al reported a CLC imager for a terahertz quantum cascade laser, but both were single frequency THz radiation and had low utility.
Disclosure of Invention
The invention aims to solve the problems in the prior art and provides a visual terahertz power meter based on cholesteric liquid crystal and a test method thereof.
The purpose of the invention is realized by the following technical scheme: the visual terahertz power meter based on the cholesteric liquid crystal comprises a capsule type cholesteric liquid crystal film which generates visible color area change along with THz intensity change, wherein the capsule type cholesteric liquid crystal film comprises a three-layer structure and is sequentially provided with an acrylic resin film, cholesteric liquid crystal and carbon black from top to bottom.
Preferably, the cholesteric liquid crystal is dispersed in an organic solvent and directly coated on the acrylic resin layer.
Preferably, the carbon black adheres to the back of the cholesteric liquid crystal.
Preferably, the diameter of the cholesteric liquid crystal is 10-20 mu m.
The invention also discloses a test method of the visual terahertz power meter based on the cholesteric liquid crystal, which comprises the following steps:
s1: calibrating the radiation power of the terahertz meter by adopting a pyroelectric detector;
s2: adopting an intelligent device to shoot images under different THz powers;
s3: and analyzing the CCLCF image based on the hue obtained in the step S2 by using ImageJ software, extracting the hue value of the obtained image, carrying out quantitative analysis on the hue value to obtain an image with quantitative color change, and quantizing the THz wave intensity through the area with the CCLCF color change to obtain the corresponding relation with the THz power.
Preferably, in the S2 step, the smart device is a smart device with bluetooth.
Preferably, in the step of S3, the software is ImageJ software.
Preferably, in the step S3, the THz field threshold is about 0.07mW, and when the threshold is exceeded, the color change visible to the naked eye can be generated, and the color change diameter has an approximately linear relationship with the THz power.
Compared with the prior art, the invention adopting the technical scheme has the following technical effects:
the THz wave intensity is quantified through the color change area of the CCLCF by utilizing the thermochromic effect and the thermal diffusion effect of the temperature-sensitive CLC, and the change is visible to the naked eye, so that any additional electronic equipment, power supply or connecting cable is not needed, and the equipment is simple in structure and convenient to prepare and carry.
At high THz power densities, both the thermochromic effect and the thermal diffusion effect are always present in the CLC. Thus, the device is not limited by saturation of the color change. The thermal detection technology has the characteristics of stability, practicability and easiness in research.
The prepared CCLCF film has flexibility and insensitivity to pressure, so the preparation method is easy to expand and operate, simple, convenient and efficient, convenient and effective to use at room temperature, and stable in device performance, and all indexes of the prepared CCLCF film meet the practical requirements of terahertz photonic devices.
The terahertz wave power meter can measure very high THz power density, can detect THz strength in a wider frequency range, and has wide application prospects in the fields of THz imaging, biosensing, detection and the like.
Drawings
Fig. 1 is a schematic diagram of a visual terahertz power meter CCLCF based on cholesteric liquid crystal according to the present invention.
FIG. 2 is a graph of the effect of temperature of the present invention on the THz absorption spectrum of CCLCF.
Fig. 3 is a diagram of a test system for testing the power of a terahertz source based on a CCLCF according to the present invention.
Fig. 4 is a schematic diagram of the power test of a terahertz source based on the CCLCF of the present invention, where the THz beam is focused on the CCLCF by an off-axis Parabolic Mirror (PM).
Fig. 5 is a schematic diagram of the power test of a terahertz source based on the CCLCF of the present invention, and the measured THz transient waveform and its fourier spectrum characteristics.
Fig. 6 is a schematic diagram of shooting at different terahertz intensities by using a smartphone camera with bluetooth according to the present invention.
Fig. 7 shows the color change of different terahertz wave irradiation times according to the present invention.
Fig. 8 shows the color change of different terahertz wave irradiation times according to the present invention.
FIG. 9 is a graph of THz power versus transient color change area for 1 second of terahertz radiation in accordance with the present invention.
Detailed Description
Objects, advantages and features of the present invention will be illustrated and explained by the following non-limiting description of preferred embodiments. The embodiments are merely exemplary for applying the technical solutions of the present invention, and any technical solution formed by replacing or converting the equivalent thereof falls within the scope of the present invention claimed.
The invention discloses a visual terahertz power meter based on cholesteric liquid crystal, which comprises a capsule type cholesteric liquid crystal film generating visible color area change along with the change of THz intensity, wherein the capsule type cholesteric liquid crystal film 10 comprises a three-layer structure and sequentially comprises an acrylic resin film 1, cholesteric liquid crystal 2 and carbon black 3 from top to bottom. The capsule type Cholesteric Liquid crystal film is abbreviated as CCLCF (Capsule Cholesteric Liquid Crystal film).
The cholesteric liquid crystal is dispersed in an organic solvent and directly coated on an acrylic resin layer, the carbon black is adhered to the back of the cholesteric liquid crystal, and the diameter of the cholesteric liquid crystal is 10-20 mu m.
The invention also discloses a test method of the visual terahertz power meter based on the cholesteric liquid crystal, which comprises the following steps:
s1: calibrating the radiation power of the terahertz meter by adopting a pyroelectric detector;
s2: adopting an intelligent device to shoot images under different THz powers; in the step S2, the smart device is a smart device with bluetooth.
S3: and analyzing the CCLCF image based on the hue obtained in the step S2 by using ImageJ software, extracting the hue value of the obtained image, carrying out quantitative analysis on the hue value to obtain an image with quantitative color change, and quantizing the THz wave intensity through the area with the CCLCF color change to obtain the corresponding relation with the THz power. In step S3, the software is ImageJ software, the THz field threshold is about 0.07mW, and when the threshold is exceeded, a macroscopic color change can be produced, and the color change diameter has an approximately linear relationship with THz power.
CLC has an inherent self-organizing ability of a helical structure, with the pitch depending on temperature, while the selective reflection wavelength depends on the pitch. Terahertz radiation causes an increase in the CLC temperature, thereby changing the pitch. Thus, the temperature of the CCLCF is increased by the heat generated by the local absorption of terahertz radiation. At this point, the pitch of the CCLC is shortened, resulting in a color change and an increase in area visible to the naked eye. Particularly when terahertz radiation is focused, the diffusion phenomenon of color change is clearly shown at high THz power density. THz radiation is then measured by analyzing the image. Cholesteric liquid crystals are abbreviated as CLC, having the english name: cholesteric Liquid Crystal.
The invention realizes the quantitative visualization of the THz power by utilizing the characteristic that the CCLCF has different responses under different THz powers. The specific implementation technical scheme is as follows:
the cholesteric liquid crystal film CCLCF is designed and prepared, the structure consists of three layers, the middle layer is packaged into CLCs liquid drops with the diameter of 10-20 mu m, the CLCs liquid drops are dispersed in an organic solvent and directly coated on an acrylic resin layer, and in order to conveniently observe the change of color, a carbon black layer is adhered to the middle layer from the back side to form a complete CCLCF structure, and the size of the CCLCF structure is designed to be 1.5cm multiplied by 2.5 cm.
In order to realize the quantitative visualization of THz power, a cholesteric liquid crystal CLC with ultra-sensitive temperature must be used. As shown in fig. 2, fig. 2 is a graph showing the effect of temperature on the THz absorption spectrum of the CCLCF, and the ordinate in fig. 2 represents the absorbance and the abscissa represents the frequency. The CCLCF thermochromic liquid crystal is super sensitive to temperature, the curve in figure 2 shows THz absorption spectra at different temperatures, and the whole THz absorption area is 0-3 THz. As shown in fig. 2, the reflected light is green at room temperature of 23.8 c, and the color changes to blue, i.e., saturated, when the temperature reaches 25.5 c, where the temperature increases by only 1.7 c. The absorption of THz by CCLCF at 25.5 c was slightly greater than at 23.8 c, indicating that the absorption of THz by CCLCF is almost independent of temperature. The oscillation of the THz absorption spectrum is an interference effect caused by multiple reflections of the CCLCF, and the terahertz radiation causes an increase in the CLC temperature, thereby changing the pitch. Thus, THz power can be detected by measuring the reflected wavelength. While the color change of the CLC caused by the THz wave can be directly observed by the naked eye. Moreover, under high THz power density, the thermal diffusion effect in the CLC can be utilized without being limited by color saturation, and the method is particularly suitable for detecting high-power THz waves.
The specific test system is shown in figure 3The test system comprises a grating 30, a lens L1, a lens L2, an off-axis parabolic mirror PM1, a PM2, a PM3 and LiNbO3Crystal 50, filter 60 and THz polarizer 70. The system is used to generate a THz source and the capsule cholesteric liquid crystal film is used to measure the power of the system.
THz source is pumped LiNbO by femtosecond laser3Crystals produced using the tilted wavefront method. A femtosecond laser LegendElite (not shown) with a center wavelength of 800nm, an optical power of 4W, a pulse width of 100fs, and a repetition frequency of 1 kHz. Femtosecond laser pumping pulses pass through the grating, the lenses L1-L2 are incident to the lithium niobate crystal, and THz generated from the lithium niobate crystal is focused through the off-axis parabolic mirrors PM1, PM2 and PM 3. The capsule type cholesteric liquid crystal film was placed at the focus for testing. A black polypropylene filter was used to block unwanted pump laser and other radiation associated with THz. The source intensity of THz can be varied by rotating the THz polarizer. The intelligent mobile phone camera with the wireless Bluetooth technology is adopted to shoot images, so that the intelligent mobile phone camera is not only convenient, but also can realize mobile detection. Because of the temperature sensitive nature of CCLCF, it is desirable to control the temperature at a certain room temperature, where the initial temperature of CCLCF is 23.8 ℃.
Fig. 4 is a THz transient waveform diagram, and the ordinate of fig. 4 represents THz electric field intensity and the abscissa represents time. Fig. 4 shows the THz beam focused onto the CCLCF by an off-axis Parabolic Mirror (PM). The THz beam spot obtained with the THz camera (IRV-T0831, NEC) was about 300 μm. An industrial pyroelectric detector (THz-5B-MT, Gentec-EO) is adopted to calibrate and measure the terahertz radiation power, and the maximum average THz power (Pmax) in the system is 2.6 mW. Thus, the maximum THz power density can be as high as 4.0 × 103mW/cm2. Fig. 5 shows the measured THz transient waveform and its fourier spectral characteristics, from which fig. 5 it can be seen that the spectral range is mainly centered between 0-1THz, the ordinate of fig. 5 representing THz intensity and the abscissa representing frequency.
Example 1:
this embodiment is the relationship between THz power magnitude and response time and the color change diameter of the CCLCF under thermal equilibrium. FIG. 6 shows direct visualization at different THz powers, image displayThe color change diameter increases due to significant temperature increase and thermal diffusion that occurs as the THz intensity increases. For image analysis, the hue-based CCLCF images were processed using ImageJ software. Fig. 7 shows that the diameter of the color change increases with increasing THz power in the thermal equilibrium state, and fig. 7 shows the relationship between the color change diameter and THz power in the steady state. The ordinate of fig. 7 represents the diameter of the color change region, and the abscissa represents the THz power. The threshold detectable THz field is about 0.07mW, at which a macroscopic color change can be produced. Above the threshold, the diameter has an approximately linear relationship to the THz power. Even if the THz power density exceeds 4.0 x 103mW/cm2The film still works well. The relationship between response time and THz power is shown in fig. 8, where the ordinate of fig. 8 represents the diameter of the color change region and the abscissa represents time. Fig. 8 shows that the color-changing diameter increases with increasing response time under 1.3mW and 2.6mW terahertz radiation, and it can be seen from fig. 8 that the CCLCF has different responses at different THz powers under thermal equilibrium conditions. Thermal diffusion is faster at high THz power than at low THz power, but the stable diameter is much larger, and therefore the response time is slower. At THz power of 2.6mW maximum, about 30 seconds are required to reach equilibrium, whereas at 1.3mW the settling time is about 15 seconds. The inset shows images taken by the camera at 2, 5, 28 and 30 seconds of exposure time, below 2.6mW, visually indicating an increase in diameter change with response time.
Example 2:
in this embodiment, in a transient state, when terahertz radiation is performed for 1 second, the THz power is related to the transient color change area. The specific structure design is shown in figure 1, and the preparation method is the same as that of example 1. Transient detection can quickly obtain THz power by measurement at a point in time before thermal equilibrium. The THz power is varied by rotating the wire grid and the color change of the area is detected. When the wire grid polarizer θ is 0 ° (parallel to the polarization of the THz wave), no THz is transmitted. Then, the polarizer angles were changed to 40 °, 60 °, and 80 °, and radiation images were obtained from the insets photographed with the smartphone. The ordinate of fig. 9 represents THz power, and the abscissa represents the area of the color change region. Fig. 9 is a graph of the relationship between THz power and transient color change area when terahertz radiation is applied for 1 second, and it can be seen from fig. 9 that the change in CCLCF area is significantly different for different terahertz powers. Here, image analysis is performed based on the tone of the digital value 122. Based on the rapid measurement method, the proportional relation between the THz intensity and the color change area is obtained, and the quantitative evaluation of the approximate parabolic relation between the THz power and the color change area is also obtained.
The device and the test method are simple and efficient, can especially detect strong THz wave power, and quantize the increase of the size of a color change area caused by THz absorption by using the thermochromatic effect and the thermal diffusion effect of the CLC. The device is not limited by saturation of color changes and is robust and does not require any additional components to measure temperature.
According to the technical scheme, the THz wave intensity is detected by quantifying the macroscopic CCLCF color change area under THz wave irradiation with different powers by utilizing the thermochromic effect and the thermal diffusion effect of the capsule type cholesteric liquid crystal film (CCLCF), so that the visual THz wave power meter is realized. The novel THz power meter has the characteristics of flexibility, low cost, portability and the like, can be used in the fields of THz imaging, THz biosensing, THz detection and the like, and has wide application prospect.
The invention has various embodiments, and all technical solutions formed by adopting equivalent transformation or equivalent transformation are within the protection scope of the invention.
Claims (7)
1. The utility model provides a visual terahertz is power meter now based on cholesteric liquid crystal which characterized in that: the visual terahertz power meter comprises a capsule type cholesteric liquid crystal film which generates visible color area change along with the change of THz intensity, wherein the capsule type cholesteric liquid crystal film comprises a three-layer structure and is sequentially provided with an acrylic resin film (1), cholesteric liquid crystals (2) and carbon black (3) from top to bottom; the cholesteric liquid crystal (2) is coated on the acrylic resin layer (1), the carbon black (3) is adhered to the back surface of the cholesteric liquid crystal (2) as a THz absorption and heat conduction layer, and the cholesteric liquid crystal (2) is a droplet capsule with the diameter of 10-20 mu m.
2. The test method of the visual terahertz power meter based on the cholesteric liquid crystal is characterized in that: the method comprises the following steps:
s1: focusing a terahertz light beam on the capsule type cholesteric liquid crystal film through the off-axis parabolic mirror, and calibrating the radiation power of terahertz by adopting a pyroelectric detector;
s2: the radiation intensity of the terahertz waves is controlled through the rotation of the terahertz polaroid;
s3: adopting an intelligent device to shoot images under different THz powers;
s4: analyzing the capsule type cholesteric liquid crystal film Image based on the color tone obtained in the step S3 by using Image J software, extracting the color tone value of the obtained Image, carrying out quantitative analysis on the color tone value to obtain an Image with quantitative color change, and quantizing the THz wave intensity through the CCLCF color change area to obtain the corresponding relation with the THz power.
3. The test method of the visual terahertz power meter based on the cholesteric liquid crystal is characterized in that: in the step S1, the terahertz light beam is pumped by a femtosecond laser to LiNbO3Crystals produced using the tilted wavefront method.
4. The test method of the visual terahertz power meter based on the cholesteric liquid crystal is characterized in that: in the step S3, the smart device is a smart phone with bluetooth.
5. The test method of the visual terahertz power meter based on the cholesteric liquid crystal is characterized in that: the test method comprises two test methods, namely a steady state test and a transient state test:
s11: the steady state test is that under the thermal equilibrium state, the THz power and the response time are in relation with the color change diameter of the capsule type cholesteric liquid crystal film, and the capsule type cholesteric liquid crystal film image based on the color tone is processed by utilizing image processing software, so that the relation that the color change diameter is approximately linear with the THz power when the color change diameter is larger than a certain threshold value is obtained;
s12: the transient state test is a measurement method for rapidly obtaining THz power by measurement at a time point before thermal equilibrium, changes the THz power by rotating a linear grid polarizer and controls the radiation time of the THz, detects the color change of an area, and obtains quantitative evaluation of approximate parabolic relation between the THz power and the transient color change area by image analysis and data processing based on tone.
6. The test method of the visual terahertz power meter based on the cholesteric liquid crystal is characterized in that: in the steady state test method of S11, the threshold detectable THz field is around 0.07mW, at which a macroscopic color change can be produced.
7. The test method of the visual terahertz power meter based on the cholesteric liquid crystal is characterized in that: in the transient test method of S12, the terahertz radiation time is about 1 second.
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| CN104049426A (en) * | 2014-07-11 | 2014-09-17 | 南京大学 | Bandwidth adjustable liquid crystal terahertz wave plate based on porous graphene transparent electrode |
| CN107703652A (en) * | 2017-09-25 | 2018-02-16 | 南京邮电大学 | A kind of electrically-controlled liquid crystal based on graphene/Meta Materials coordinated drive is adjustable THz wave absorber and preparation method thereof |
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