THz Super-Resolution Imaging in Transmission Technology by Using Butterfly and Pattern Device Samples

Recently there has been an increasing demand for terahertz technology, especially in imaging. In the past few decades, the applications of terahertz (THz) imaging technology have seen significant developments in the fields of biology, medical diagnosis, food safety, and nondestructive testing. The medical and semiconductor industry has always attracted significant attention worldwide. In particular, the importance of real and perfect inspection technologies has been growing due to an increasing demand for improving the quality of life and developing industries. This paper presents the research of THz super-resolution imaging in transmission mode by using different samples. We have reported transmission measurement at different THz frequency of each sample. The butterfly sample used super-resolution THz imaging. The THz super-resolution is obtained excellent at 1.8 THz, it is near about 1 micrometer. Good resolution images have been obtained. This new THz super-resolution techniques can apply in medical and security purposes. Further applications will be reported in the coming papers.


Introduction
Terahertz reflection and transmission mode imaging are the latest in the middle of noninvasive sensing technologies and currently, massive attention is pointed towards their use in several applications [1]. Among possible applications, like jade stone and other biologi-cal and human body inspection represents one of the most prominent application of THz technology [2,3]. This paper deals with the exploitation of THz imaging technology for drawback in a semiconductor device, natural birds living styles, and food quality control and assessment. In particular, the report aims at reviewing the latest developments regarding THz imaging, both in terms of measurement systems and data processing methodologies and application. In a recent market, the available sensing techniques in industry and body sensing and food inspection includes X-ray imaging [4,5], thermal imaging [6], ultrasonic imaging [7], fluorescence imaging [8], and electromagnetic (EM) systems working at microwave frequencies [9]. However, each one of these THz imaging techniques has advantages and limitations. For example, X-rays systems, which are increasingly used in the food production industry and medical industry for quality control inspection, provide high resolution images but have difficulties in detecting low density objects such as plastic, glass, wood or insects. On the other hand, infrared (IR) and optical technologies [10][11][12] have the advantage of being fast and safe but they are limited by wavelength range and a poor penetration capability and strong absorption in water [13][14][15][16][17]. Instead, fluorescence imaging is effective only when objects with fluorescent compounds are investigated. Therefore, in the current framework, terahertz (THz) imaging appears as an emerging technology, which offers several advantages. THz are electromagnetic waves ranging from 0.1 to 30 THz (wavelength from 3 mm to 10 μm), which are currently exploited in various applications, including medical diagnosis [18], and pharmaceutical analysis [19]. As microwaves and infrared signals, THz waves are non ionizing radiations thus they allow a safe survey without requiring specific security protocols. Moreover, similar to microwave imaging and differently from X-ray technology, THz systems allow the detection and localization of changes in terms of electromagnetic properties with a high resolution (generally speacking, THz spatial resolution is in the order of some hundred microns while the microwave spatial resolution is typically about centimeters). On the other hand, the THz capability of penetrating inside a material is limited to a few millimeters. In addition, THz waves are strongly attenuated by water and suffer the environment humidity conditions. Currently, several studies regarding the detection of both defects in plastic packages, security purpose [20][21][22], and food contaminants [23] have shown the effectiveness of THz imaging technology in the field of food industry. This report, provides an overview of the THz imaging for a semiconductor pattern device and beauty of natural birds inspection by abbreviation the current state of the technological growth and recalling important case studies presented in literature. For the sake of completeness, the reports starts with a brief description of the main THz technologies, i.e. THz pulsed and time domain systems as well as continuous wave ones. Then, THz imaging in transmission mode. We have already reported THz imaging application [2,3] can compare with presented this report. One can also compare with reported results based on graphene [24][25][26]. Finally, the main open challenges and future perspectives are discussed.

Experimental Setup and Methods of Working Function
The proposed experimental setup can be seen in Fig. 1. The high frequency THz sources are available for about 3 THz. One of the curves plotted between frequency and dynamic range can seen in Fig. 1(b). The details of the optical THz setup with step by step separa-tion between instruments can be seen in Fig. 1. A complete working function of the THz optical system is depicted in Fig. 1. We just checked the original data about the examples, because they are all made of metal so we cannot test the curve no matter in time domain or frequency domain. We can only imagine the rest of the example which is not made of metal. So we have provided the THz images of all four samples, not the curve. We use a fiber femtosecond laser as the light source, and a fiber-coupled terahertz photoconductive antenna as the source and detector of the terahertz pulse. The femtosecond laser generated by the fiber femtosecond laser is divided into two beams by a fiber beam splitter, one beam is used to generate terahertz pulses, and the other is used to detect terahertz pulses. After the femtosecond laser used to generate the terahertz pulse passes through the fiber splitter, it is connected to the terahertz photoconductive antenna coupled with the fiber through a fiber jumper. After the femtosecond laser used to detect terahertz pulses passes through the fiber splitter, it is connected to the fixed time delay line and the electronically controlled step time delay line the fiber jumper, and then the fast scan time delay line through the fiber jumper finally, the optical fiber-coupled terahertz photoconductive antenna for detection is connected through an optical fiber jumper. The length of the optical fiber in the terahertz generation optical path is approximately the same as the length of the optical fiber in the terahertz detection optical path. The fiber-coupled terahertz photoconductive antenna used to generate and detect terahertz pulses can be installed in a transmissive measurements module and a reflective measurements module to perform transmissive and reflective measurements, respectively. In this experiment, we perform transmission imaging measurement on the sample. We fixed the sample under test on the scanning imaging table and located it near the focal position of the terahertz lens. The scanning imaging table is used to drive the sample to be tested for two-dimensional movement, and both the X-axis and Y-axis are driven by a stepping motor and a lead screw. Install the terahertz emission source module and the terahertz detector module into corresponding positions respectively, and fix them with positioning pins and screws. After the transmissive imaging module is installed, it will trigger the corresponding photoelectric switch to give feedback, indicating that the transmissive imaging module has been installed in place, and automatically adjust the stepping electronic control time delay line so that the terahertz time-domain spectrum is located at the fast scan time delay line within the time window, transmission-type terahertz imaging measurement can thus be performed.

Samples Details
In proposed paper, we have used four different samples. The detail size and other properties are given as: Sample 1: Size: 9.9*7.4 cm, It is too large, so measure only the red frame.
Step distance of X and Y axis of the sample plane: 0.25 mm. In addition to the image, can the terahertz intensity signal sequence of all sampling points of the sample be given.

Sample-1
Sample-2 Sample 2: Size: 9.5*8.4 cm, if it is too big, so, it can only measure the red frame. Sample 1 and Sample-2 boths are pattern sample. Sample 3: Size: 2.5*1.2 cm and Sample 4: size: 6.5*6.5 cm. Sample-3 look like pattern switch and Sample-4 is butterfly shape.

Results and Discussion
The main issues of THz technology, that what type of THz imaging are you trying to use? Using standard imaging techniques you will be bound by the diffraction limit, which is proportional to the wavelength. This will vary depending on whether you are using full field or monochromatic THz systems. In either case, 0.1 THz is 3 mm in wavelength and 10THz is 0.03 mm,The THz transmission imaging model is one of the best novel requirement THz model in recent industry. Figure 2, shows the schematic diagram of the THz imaging model, which is used in proposed report. In Fig. 2(b), is presented by using different types of mirrors, lenses, and antenna in different places according mention above in Fig. 2(a).
In THz transmission imaging system shows various properties of material composition, and structure, after analysis, we can easily differentiate between substantial part and other weaker portions of the butterfly shape structure and other pattern samples. Figures (1-2) shows the THz transmission model and its application in different ways. Total 8 mirrors are used (M1-M8). Four parabolic mirrors (PM1-PM4) are also used for imaging measure- ments. For better resolution, we need to use a beamsplitter and photoconductive antennas, which are already used, as can be seen in Fig. 2(b). Figure 1 (a) is the experimental setup and, Fig. 1(b-c) shows the curve between frequency and dynamic range, time, and amplitude. So one can be clear that we can use it in different applications for the proposed THz transmission technology. Figure 3, shows how the intensity varies by increasing the angle of polarization from 0 to 90 degrees the THz intensity got decreased from 0.7 THz to 0.05. We can also try with other high THz ranges with different angles. By graph, the measured intensity is shown. Figure 4 represents-THz an image of sample-1 with different THz ranges  (1.5 THz and 1.6 THz). In ordinary images can not see the inner line's structure due to the low resolution. In some way. we have taken THz images of the same sample with different frequency ranges (1.725THz, 1.8 THz, 1.9 THz, 2.03 THz, 2.1 THz, and 2.2 THz, which can see in Figs. (4-7) part, it represents various layers and pattern image with connecting to resolution in the sample structure is based on peak to low intensity in THz frequency. Different properties are being analyzed in a sample like layers gap, density, and structure, which is helpful for scientific research.
In the next step, we have done THz images of sample-2 can be seen in Fig. 7. Further, we have done THz images of sample-3 and sample-4 at 1.8 THz frequency, which can be seen in Figs. (8)(9). It is very clear, how the resolution effect if plotted between index of point and index of point in X,Y axis, further taken image between an index of point and amplitude. One can see that, a THz image in Figs. (8)(9) with the sample image of patter device and butterfly expressing enhances parameter to understand the deeper side of the sample. In the 2D image analysis in Figs. (4-10), different samples show different properties like layer, gap, and impurities of other material compositions have been seen.

Discussion
The advantage of THz transmission imaging over conventional gray scale imaging can be seen through the THz transmission imaging system. In Fig. 4, we have taken THz images in the transmission of sample-1 at 1.5THz and 1.6 THz spectrum. One can easily differentiate between both images. For the measurement, the mirrors and parabolic mirrors is an essential factor for detection in which different intensity was observed. Beamsplitter and Phtoconducting antenna is the main key point for getting a better resolution for transmission THz imaging system [24][25][26]. Here reported some references about THz imaging. Our proposed methods are better than reported before in terms of better resolution. Similarly, we have presented the measurement THz transmission images of the sample-1 at different THz spectrums (1.725 THz and 1.8 THz ) shown in Figs. 5, (1.9 THz and 2.03 THz ) shown in Fig. 6, and (2.1 THz and 2.2 THz) shown in Fig. 7. This analysis allows us to view more than just an image and observe beyond the limits of resolution of the camera by which our imaging was limited. The resolution of the obtained image from the THz camera in Fig. 10, used butterfly metal samples, the resolution is good as we expect. It is near about 1 micrometer.
After completing the results of sample-1, we have taken an image of sample-2, which can be seen in Fig. 8. There have very clear differences between images in terms of resolution.
In the next report, we have presented the THz transmission image of sample-3 at a 1.8 THz frequency spectrum. It is a very clear image of the metal pattern sample, also one can easily compare boths images.
Further, we have applied with butterfly metal sample, which can be seen in Fig. 10. The resolution of the above images is good as near about 1 micrometer which is the best in THz transmission imaging systems. Terahertz transmission imaging is a valuable technique in the fields of chemistry, physics, electrical engineering, materials science, and medicine. This report has presented the basic THz images at different THz frequency ranges by using metal samples. The core principle of this new technology is that the electric field is measured in the time domain. A significant advantage of THz technology is that the THz spec- trum is complex-valued, meaning it provides amplitude and phase information at every frequency point within the usable bandwidth.

Conclusion
In a recent report, we presented an overview of the THz transmission imaging and sensing technology for medical and industry application. This paper has starting by presenting the adopted main THz imaging technology by using pattern devices and butterfly samples. In recent article, we mainly focused on THz imaging in transmission mode. The THz superresolution is obtained excellent at 1.8THz, it is near about 1 micrometer. Good resolution images have been obtained. THz imaging results were presented at different frequencies of different four samples, and imaging resolution is very excellent. Recently terahertz products for better imaging applications are limited to single or few pixels only with raster scanning techniques to produce a single terahertz image frame. Finally, we summarise the obstacles in the way of the application of THz imaging application technology in clinical, industrial, and other major detection, which need to be investigated and overcome in the future. Co., Ltd. The director THz production 2020.06-now The co-founder of VY Photonics (Nanjing) Co., Ltd. He has done a lot of experience in THz devices. He is also working with Hubei Polytechnic University as Visiting Researchers. He has 4 Chinese patent.