| Terahertz wave(THz)technology is widely used in various fields,for example the terahertz imaging technology? nondestructive testing?safety checking and medical diagnosis etc,that all due to its low photon energy and transparency to the most non-polar materials such as: clothing? paper?plastic.And the qualitative identification of biochemical samples or gases as well as their quantitative monitoring such as thickness?concentration have become the research hotspot in the terahertz field.However,terahertz fingerprint detectors based on the characteristic absorption peaks of substance have a low detection sensitivity resulting in the requirement of a greater amount of samples.As described above,this will limit the use of terahertz fingerprint detection in medical diagnosis,industrial production and other aspects.So in this thesis,we put forward the research point based on ultra-sensitive terahertz fingerprint sensors designs to achieve high selectivity and trace samples detection.The main work of this thesis is described as follows:(1)We theoretically design and study a coupled comb-shaped spoof surface plasmon waveguide for terahertz fingerprint detection.The sensor is composed of two identical?opposite-oriented single waveguides,that located on a quartz substrate.The dependence of waveguide sensitivity on structural size,such as the coupling gap between the two single waveguides,the period of a unit cell as well as the waveguide length,is numerically calculated using the finite element and finite difference time domain methods.We show that reducing the coupling gap and increasing the waveguide length can effectively improve the sensitivity of this waveguide.For example,when the lactose thickness is 2.3 m,the absorption rate at its fingerprint of 529 GHz can approach 30%,which is two orders of magnitude stronger than that through the bare lactose.This ultra-thin and highly sensitive fingerprint absorption magnification sensor outperforms the previous reported-the required sample thickness of tens of microns or even thicker.(2)We propose a novel THz fingerprint detection method and verify it by a one-dimensional photonic crystal micro-cavity.Using a high-Q resonant micro-cavity or micro-ring structure,we can design the resonant mode frequency of the device to match the characteristic frequency of target sample and then the transmittance at resonant peak will drastically decline with a slight resonant frequency shift due to the fact that the high-Q resonant mode is sensitive to the material loss and THz radiation at resonance will bounce back and forth for multiple times interacting with the sample which loaded in this mocro-cavity or micro-ring.For the case the resonant mode frequency is not equal to the fingerprint frequency of sample then the resonant peak will hardly be weakened and just shows a resonant frequency shift.Thus,substance specific identification will be achieved with a high sensitivity and accuracy characteristics.In order to verify the feasibility of this detection method,we use a one-dimensional photonic crystal cavity which can form high Q resonances to calculate the sensing effect for the lactose.The results show that the attenuation of the resonant peak caused by 10 nm of lactose at its fingerprint of 529 GHz is 5%,that promotes the minimum detection thickness of lactose in terahertz band to the nanometer level.Besides,we also discuss the effect of sample location?resonant frequency and quality factor(Q value)on sensing performance.In order to better serve as practical guidance,we also simulate the sensing effect of photonic crystal microcavity with lossy silicon wafers for lactose detection and obtain the lowest detection thickness being 13 nm within 5% detection limit.The sensing effect is still significant.Finally,it is worth mentioning that the absorption rate at lactose fingerprint 529 GHz can be magnified more than 300 times through this micro-cavity.(3)We also has a try on the low concentration gas detection using this photonic crystal micro-cavity with high Q resonance.We take Hydrogen cyanide(HCN)as an example and take the frequency of 1239.89 GHz as its spectral fingerprint for the specific identification.When the volume of HCN per cubic meter of atmosphere is 2 cubic centimeters(the volume concentration of HCN is 2ppm),the peak attenuation of micro-cavity resonant mode can reach 1% under a standard pressure(1atm).When the gas concentration increases to 10 ppm,the peak attenuation is 5%,which is 200 times more than that through the gas cell for the same air pressure and the same gas concentration.More importantly,the structure length of this micro-cavity is only 5.56 mm,which is beneficial to device integration and form multi-channel gas detection. |
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