Terahertz (THz) technology has become popular worldwide as a new approach to detecting biomolecules because the vibrational and rotational energy levels of many biomolecules fall in the THz band and because the THz wave has the characteristics of low electronic energy, which will not damage the samples to be measured. Many biomolecules need to maintain their biological activity in liquid environment. However, as a polar molecule, water has a strong absorption of THz wave, which is mainly because the vibration frequency of hydrogen bond in aqueous solution is within the THz frequency range. Therefore, the best solution is to reduce the action distance between the aqueous solution and THz wave and control it within 100 μm. Microfluidic chips can meet such requirements. Therefore, the combination of THz technology and microfluidic technology can study the dynamic characteristics of biomolecules in an aqueous solution. The microfluidic chip was fabricated using ZEONOR 1420Rs. The THz transmittance of the material can exceed 95%. The depth of the microchannel in the microfluidic chip is 50 μm. In addition, the chip has the characteristics of good airtightness, portability, convenient disassembling, and reusability. Seventeen kinds of electrolytes were tested with the chip. The results show that the THz spectral intensity of electrolyte composed of different anions and cations, so the spectral characteristics of other electrolyte solutions can be obtained according to the spectral information of these detected ions.
Terahertz spectroscopic characteristics are related to the large amplitude vibration of molecules and their interaction, so it can be used for molecular detection. Sodium nitrate and sodium nitrite are two very important salts in people's lives. Sodium nitrate is usually used as quick-acting fertilizer in acidic soil, but excessive use will cause certain pollution to the environment. If sodium nitrate is heated, it becomes very stable sodium nitrite, which is usually added to food as a preservative and colorant, but excessive consumption of nitrite can lead to food poisoning and even death. Therefore, it is necessary to detect sodium nitrate and sodium nitrite. Based on THz-TDS system, the spectral characteristics of sodium nitrate and sodium nitrite in 0.1-1 THz band were studied. Polyethylene, sodium nitrate and sodium nitrite were mixed in proportion of 1:1, 1:2, 1:3, 1:4 and 1:5, respectively. The mixture was evenly grinded and stirred, and then pressed. The prepared samples were tested in THz-TDS system, and the time-domain spectra of the samples with different proportions were obtained. The terahertz frequency spectrum and absorption spectra of the two samples at different ratios were obtained by Fourier transform. After comparing spectra of the two samples, it is found that they have their own absorption peaks, and their peak positions are obviously different. Therefore, it is very important to study these two substances by terahertz technology.
Compared with traditional imaging, optical field imaging records the four-dimensional position information and direction information of the object. It has two degrees of freedom more than the two-dimensional traditional image, so it has more abundant image information in the later processing. According to Levoy's theory of light field rendering, light field can be parameterized by two parallel planes. Compared with visible light, terahertz has the characteristics of long wavelength, strong penetration and low energy. Therefore, terahertz optical field imaging technology will have the advantages of wide dynamic range and deep field. This paper summarizes the acquisition methods of light field and describes the post-processing methods of light field, including digital refocusing, super-resolution, depth estimation, synthetic aperture imaging and microscopic imaging technology. Finally, the application of terahertz optical field imaging and the possible problems are summarized.
Terahertz on-chip system integrates the generation of terahertz, detection devices and transmission lines on the same substrate. It has the characteristics of small size, high measurement stability and easy operation. This paper is devoted to the study of the electrode of the photoconductive antenna necessary for the generation and detection of terahertz. HFSS software is used to simulate seven types of antennas with two types, large aperture and small aperture. The S11 images and VSWR images of each structure are obtained, and the emission efficiency of the optimal antenna electrode structure is obtained by comparison. For small aperture antenna, rectangular antenna has the highest transmitting efficiency; for large aperture antenna, dipole antenna and dish antenna have higher efficiency. But in the butterfly antenna, when the antenna spacing and the antenna length are both 50μm, the reflection power is the smallest and the performance is the best. These simulation results provide a parameter basis for the subsequent fabrication of chips.
Proc. SPIE. 11196, Infrared, Millimeter-Wave, and Terahertz Technologies VI
KEYWORDS: Signal to noise ratio, Transmitters, Oscillators, Information and communication technologies, Denoising, Interference (communication), Telecommunications, Terahertz radiation, Antennas, Signal generators
With the rapid development of 5G communication technology, terahertz wave has attracted extensive attention due to its large capacity, good directivity and strong anti-interference ability. However, in the terahertz communication system based on the subharmonic mixing method, in the process of generating the terahertz signal, due to the high frequency multiplication, the floor and phase noise of the original frequency source pass through the frequency doubling and amplification links, and the spectrum is realized. The shifting and amplification are degraded, while the circuitry of the link itself also produces an in-band noise superposition. In order to reduce noise, this paper studied on the basis of the principle of harmonic mixing terahertz communication system of the high harmonics and the vibration noise impact on the quality of communication, this paper proposes a method of harmonic interference suppression method this method by selecting the required harmonic band-pass filtering, set the appropriate frequency range can make other harmonic component to fall outside the frequency band, so as to achieve effect of reducing the noise of a terahertz communication system.
Terahertz light field imaging is also known as computational imaging, and its acquisition is limited to terahertz camera scanning. Camera scanning light field imaging systems are limited by the performance of the device, and there are problems such as large image noise and small field of view. This paper proposes a single-camera terahertz light field imaging, using Matlab software for system simulation and image processing, reducing output image noise by adjusting simulation parameters, improving image quality, and browning the image to make the outline clearer. Further analysis of the improvement in imaging quality lays the foundation for the development of terahertz light field imaging in refocusing and depth estimation. Finally, it is pointed out that the terahertz band has strong penetrating power in imaging and can play a unique advantage in biometrics, military detection and security.
Light field microscopic imaging technology is achieved by inserting a micro-lens array capable of capturing light field information on the relay image plane of a conventional optical microscope. The multi-focus surface can be extracted by the refocusing technique to increase the geometric depth of field. The terahertz wave has many advantages, such as strong penetrating power, low photon energy , good coherence and so on. Therefore, It can be used for microscopic imaging to obtain more comprehensive information, and can also improve the depth of field and resolution of imaging, and is also suitable for the detection of living organisms. Based on the principle of light field microscopic imaging and terahertz microscopic imaging technology, this paper analyzed the depth of field is related to the wavelength of the light source and the size of the micro-lens array. It has been found that the depth of terahertz light field microscopy technology can be increased by at least one to two orders of magnitude when the wavelength range is extended to the terahertz band.
With the advent of the 5G era, the demand for wireless communication rates is increasing, making high-speed wireless communication become a hot spot and focus. Due to the wider frequency bandwidth and good confidentiality, the terahertz band has attracted wide attention in the field of high-speed wireless communication above Gbps. However, terahertz wave is easily absorbed by water and attenuates seriously when it is transmitted in the atmosphere, which seriously restricts the development of communication in the terahertz band. However, the spread spectrum communication has attracted much attention in the field of terahertz communication because of its strong anti-fading, anti-interference and good confidentiality. In this paper, the simulation model of direct sequence spread spectrum (DSSS) communication system is established by using MATLAB, and the baseband signal and m-sequence spreading code of different rates are set, and 310GHz is used as the carrier frequency to transmit in additive white Gaussian noise channels with signal-to-noise ratio (SNR) of 17dB to -1dB. Medium transmission, by checking the bit error rate (BER), to verify the anti-interference of the DSSS technology applied in the terahertz band. The simulation results show that when the spread spectrum gain is large, the DSSS can be used in terahertz communication under the same BER to transmit in channels with a SNR of 10dB worse. Therefore, the application of spread spectrum communication technology to terahertz communication has a good development scenario and will effectively promote the development of terahertz communication.
The terahertz monolithic integrated device is to integrate the pumping area of the terahertz generation, the detection area of the terahertz receiving and the metal waveguide of terahertz transmission on the same substrate. The terahertz generation and detection device use a photoconductive antenna structure，the metal waveguide use a microstrip line structure. The evanescent terahertz-bandwidth electric field extending above the terahertz transmission line interacts with, and is modified by, overlaid dielectric samples, thus enabling the characteristic vibrational absorption resonances in the sample to be probed. In this device structure, since the semiconductor substrate of the photoconductive antenna is located between the strip conductor and the dielectric layer of the microstrip line, and the semiconductor substrate cannot grow on the dielectric layer directly. So how to prepare the semiconductor substrate of the photoconductive antenna and how to bond the semiconductor substrate to the dielectric layer of the microstrip line is a key step in the terahertz monolithic integrated device. In order to solve this critical problem, the epitaxial wafer structure of the two semiconductor substrates is given and transferred to the desired substrate by two methods, respectively.
A simple system based on two cascaded Mach-Zehnder modulators, which can generate 160GHz optical terahertz waves from 40GHz microwave sources, is simulated and tested in this paper. Fiber grating filter is used in the system to filter out optical carrier. By properly adjusting the modulator DC bias voltages and the signal voltages and phases, 4-tupling optical terahertz wave can be generated with fiber grating. This notch fiber grating filter is greatly suitable for terahertz over fiber (TOF) communication system. This scheme greatly reduces the cost of long-distance terahertz communication. Furthermore, 10Gbps digital signal is modulated in the 160GHz optical terahertz wave.
Vanadium dioxide is a functional material with thermotropic properties. The phase transition temperature is close to room temperature, and the metal-insulator transition at the phase transition temperature is accompanied by the abrupt change of light and electrical properties. It has important research value and Potential application prospects. In this paper, the research status and improvement methods of the preparation of vanadium dioxide thin films by hydrothermal method are reviewed, and the research progress of the modulation characteristics of vanadium dioxide thin terahertz in recent years is reviewed, and the future development trend is put forward.
The light field includes the direction information and location information. Light field imaging can capture the whole light field by single exposure. The four-dimensional light field function model represented by two-plane parameter, which is proposed by Levoy, is adopted in the light field. Acquisition of light field is based on the microlens array, camera array and the mask. We calculate the dates of light-field to synthetize light field image. The processing techniques of light field data include technology of refocusing rendering, technology of synthetic aperture and technology of microscopic imaging. Introducing the technology of light field imaging into THz, the efficiency of 3D imaging is higher than that of conventional THz 3D imaging technology. The advantages compared with visible light field imaging include large depth of field, wide dynamic range and true three-dimensional. It has broad application prospects.
Since many vibrations and rotational levels of biomolecules fall within the THz band, THz spectroscopy can be used to identify biological samples. In addition, most biomolecules need to maintain their biological activity in a liquid environment, but water as polar substance has strong absorption to the THz wave. Thus, it is difficult to detect the sample information in aqueous solution using THz wave. In order to prevent the information of biological samples were masked in the solution, many research methods were used to explore how to reduce the water absorption of terahertz. In this paper, we have developed a real-time chemical methodology through transmission Terahertz time-domain spectroscopy (THz-TDS) system. The material of Zeonor 1020r is used as substrate and cover plate, and PDMS as channel interlayer. The transmission of the empty microfluidic chip is more than 80% in the range of 0.2-2.6 THz by THz-TDS system. Then, experiments were carried out using chips, which were filled with different volumes of 1, 2- propanediol, and it has been proved that the microfluidic chip could reduce the water absorption of terahertz. Finally, in order to further explore the reduction of terahertz to water absorption, we inject different concentrations of electrolyte to the chip. The results show that with the addition of different electrolytes, terahertz transmission line has evident changes. It can be taken into account that the electrolyte has different effects about the hydrogen bonds in the aqueous solution. Some of them can promote water molecules clusters, while others destroy them. Based on the basis of microfluidic chip, the discovery of this phenomenon can provide a way that reduces water absorption of terahertz. This work has laid a solid foundation for the subsequent study in reducing water absorption of terahertz.
Terahertz time domain spectroscopy system (THz-TDS) is the most commonly means of measuring terahertz time-domain spectroscopy. The time delay between the pump and probe laser is an important technology to realize THz time domain spectrum measurement. The translation platform with two mirrors and the mechanical structure is the popular means to adjust the optical path difference between the pump and probe laser to get the time delay of femtosecond pulse. Because of the limit of the mechanical structure and the phase-locked amplifier, this technique can’t scan spectrum fast. In order to obtain high quality signal, a long time will be taken to scan spectrum. So a more rapid and convenient time delay technology is required to Instead of the machine translation platform and accomplish the Rapid spectral measurement. Asynchronous optical sampling technique is a way to get the time delay by producing a very small frequency difference between the repetition frequency of two femtosecond lasers. The scanner time will be reduced, because of there is no waste of time, due to mechanical inertia, not only by using the asynchronous optical sampling method to replace the mechanical structure without the influence of vibration. It will greatly increase the degree of integration by using the fiber femtosecond laser and highly integrated circuit to realize optical asynchronous sampling.
To solve the problem above, a terahertz time-domain spectroscopy system based on asynchronous sampling is designed in this thesis. The system is based of two femtosecond laser whose repetition frequency is 100MHz.In order to realize asynchronous sampling, the control circuit of the two lasers is the most important. This thesis focuses on the researching, designing and experiment of this circuit. Firstly, the circuit is designed overall. Then the selection of the key device and the designing of the circuit principle is done by myself. Secondly, the test of the circuit to phase locked the master and slave fiber femtosecond lasers has been done. As a result, the two lasers can phase locked on two repetition frequencies with a small frequency difference calculated by the circuit.
This paper proposed a polarization independent terahertz modulator based on gold-Si metamaterials with symmetric structure, and aimed to modulate terahertz wave in communication systems. The transmission properties have been investigated in terahertz regime. We find that the resonance frequency of this device can be actively controlled by pump laser. The numerical simulations and experiments with OPTP system show that this device acts as a modulator with intensity modulation depth of 70% by gold structure.
The fundamental parameters of THz modulators based on the SRR structure metamaterial have been researched and found that the SRR size and opening width have a significant effect on the resonant frequency, meanwhile the thickness and width of this structure are very weak influence on the resonant frequency. The resonant frequency and response time have been calculated theoretically. The resonant frequency fits the simulation results well, and response time is mainly determined by the carrier lifetime which is in nanosecond order. We also proposed a program of experimental measurements of the SRR modulation speed. Finally we simulated a special SRR structure with metal strips on the back of wafer to increase modulation depth significantly.
Terahertz modulation techniques are an indispensable key technique to achieve the future wireless terahertz wave high-speed communication systems. Metamaterials is artificial materials that geometric structure units are embedded in the traditional medium materials. In this letter we will introduce metamaterials terahertz modulation techniques and analyze the advantage and disadvantage of recent research progress in the world, including electrically controlled THz metamaterials modulators, optical controlled THz metamaterials modulators and some new modulators based on material-graphene
In this paper, the kinetics of photosensitive initiating polymerization and photopolymer holographic recording materials were studied. Four sensitizers that could be sensitive to He-Ne (632.8 nm) laser were synthesized and chosen for this study: (1) NK-529, (2) NK-3960, (3) (MCD), (4) ECD. The long-wavelength sensitive photoinitiating system are composed of one of the four compounds above, 2- chlorohexaarrylbiimidazole(o-cl-HABI) and 3-mercapto-4- methyl-4H-1,2,4-triazole (MTA), which acted as sensitizer, initiator and hydrogen-donor respectively. The kinetic study was carried out by using dilatometer, we found the relationships between the rate of polymerization and the concentration of each component. We believe that the photopolymerization was initiated by free radicals which were produced by the electron transfer between the sensitizer and the initiator in the excited state. Comparing the monomer conversion of these four systems, we found: MCD >ECD >NK529 >NK3960. We chose the system (MCD- HABI-MTA) as a photoinitiating system of photopolymer holographic materials. The holographic material was composed of the above photoinitiating system, a binder, a mono- or multi-functional monomer, and other additives. Adding the third beam to expose the photopolymer plate simultaneously during the initial holographic exposure can increase the effective exposure sensitivity of the photopolymer plate. Mechanisms of photoinitiating polymerization and hologram formation are discussed. More than 80% of reflection grating diffraction efficiency can be obtained. The holographic gratings have a good physical and chemical stability under ambient conditions.