Research On Metamaterial Based Terahertz Bandstop Filter And Its Application In Sensing

The Terahertz wave has many characteristics,such as the wide bandwidth,the high penetration,the fingerprint spectrums of material recognition and so on.The terahertz technology has many applications in the fields of communications,security checking,bio-sensing and become a research hotspot.However,the immaturity of efficient terahertz sources,sensitive detectors and functional devices limits the development and application of terahertz technology.The emergence of metamaterials promotes the development of terahertz detections,sensing,functional devices and other related fields.A metamaterials is a kind of artificial homogeneous medium composed of sub-wavelength structural units,and has exotic characteristics,such as the negative refractive index,the negative permeability,which are not available in natural materials.The functional devices with large modulation depth,and high sensitivity has become a key issue in the field of terahertz scientific research.The multi-band tunable modulator can broaden the application range of metamaterials and has certain research value.In this paper,two mechanically tunable filters are designed based on the near-field coupling among the element structures of metamaterials,and the coupling mechanism is analyzed with the equivalent circuit model.The sensing characteristics of multiband stopband filter are also discussed in this paper.The main results of work and research are as follows:1.Research on the main theories and analysis methods of metamaterials.Firstly,the Maxwell's equation is used to analyze the propagation characteristics of electromagnetic wave in a homogeneous medium metamaterials.Then the equation model of the electronic collective motion is used to analyze the polarization of the electric field and magnetization of the magnetic field on the metamaterials.While,the relationships between the permittivity and permeability of the metamaterials and the frequency of the electromagnetic wave are obtained,respectively.2.The idea of metamaterials structure designed in this paper is based on the near field coupling effect of dipole.The formation mechanisms of electric and magnetic dipole are introduced in detail,and the coupling effects of electric dipole and magnetic dipole on the resonance frequency of metamaterials are analyzed,which is complex.The equivalent RLC circuit model of the metamaterials is easy to understand the mechanism of the coupling effect,which is convenient for the design and research of metamaterials.The hybridization theory of dipoles and LC equivalent circuit model are used to explain the resonance mechanism of the two structures.3.Study of metamaterials terahertz bandstop filter based on the electromagnetic induced transparent.A bilayer dual"H" orthogonal metamaterials,terahertz multiband stopband filter is designed based on the electromagnetic induced transparent.When the electromagnetic wave incident angle varies in the range from 0° to 10°,the change from single-stopband filter to double-stopband filter is realized at 0.92 THz.As the incident angle increases,the center frequency of low frequency stopband appears a red shift,while that of high frequency stopband appears a blue shift.Numerical analysis shows that the stop-band filter realizes the adjustable function of the center frequency.In this paper,the concept and analysis method of EIT are described,and the surface currents of resonance and transparent peaks in EIT effect are analyzed by finite element method.According to the hybrid theory of electric dipoles,the coupling effect of multiple dipoles is analyzed,and the EIT effect is explained from the mode of trapped mode.According to the distribution of electric dipoles,the equivalent LC circuit model of the structure is obtained,and the principles of realizing EIT effect and stopband filtering are analyzed from the resonance condition of the circuit model.On this basis,the influence of structure size on the transmission spectrum of stopband filter is discussed in this paper.It is found that the changes of geometric size have little effect on the transmission amplitude,but mainly affect the central frequency and bandwidth of stopband.4.Research on metamaterials terahertz multiband filter.A mechanical tunable terahertz multiband bandstop filter based on near-field coupling effect is designed and fabricated,which is composed of a double split ring resonator fabricated on a silicon substrate in a micromachining process.Initially,the device has four resonance absorption peaks on transmission spectra.When the azimuth increases from 0° to 90°.the transmission amplitude at these four absorption peaks can be simultaneously modulated,while the fourth resonance peak has a blue shift.The transmission and tuning characteristics of the single loop of the filter are analyzed by finite element method.It is found that although the single split ring has the tuning function with the change of azimuth angle,the resonance peaks of double rings do not coincide completely with that of two single rings.By analyzing the surface current distributions at the four resonant frequencies combining the dipole hybridization theory,the equivalent circuit of each resonance is obtained,that not only explains the difference between the above two single-ring resonance peaks and the double-ring resonance peaks,but also interprets the modulation mechanism of the transmission amplitude.By analyzing the effects of geometric parameters on the central frequency of each stopband,it is found that the shape size has a great influence on the bandstop filter.The outer ring size affects the position of the first three resonance peaks,and the ring size affects the position of the last three resonance peaks.The sample is characterized by using a THz time domain spectroscopy(THz-TDS)system.The experiment is in good agreement with the simulation,which shows that the filter has the function of mechanical tunable four-band bandstop filter in the terahertz reginon.5.Research on the sensing application of the multi-stopband terahertz filter.It is found that the sensitivities of the four stopbands are respectively analyzed by simulation,which is revealed by the sensing theory of metamaterials.Then,two sensing experiments are carried out by using the four-band bandstop filter.The first experiment is an identification method of water-soluble Vitamins B(B1,B3,B5andB6),which is difficult to distinguish by using THz-TDS.The experimental results are as follows:(1)The four kind of aqueous solutions of vitamin B in the same concentration(0.5 mg/?l)are respectively dripped on the surface of the sensor,and the different vitamin B lead to different frequency offsets of each resonance peak.And the frequency shift law of each resonance peak produced by the different vitamins is the same,and the order is B3,B5,B6 and B1 according to the order of frequency shift from large to small,which indicates that each stopband of the four stopband has the sensing function of material.(2)For any kind of aqueous solution of vitamin B in the same concentration(0.5 mg/?l),the sensitivity of the sensor formed by the coupling of inner and outer rings are higher than that of the sensor formed by the electric dipoles of the single outer or inner ring.The second experiment is an identification of the aqueous solution concentration of vitamin B6,and its performance is evaluated.The experiment result shows that the concentration range of the B6 measured by the sensor is approximately 0.25 mg/?l?1.25 mg/?l,and the resolution is about 0.25 mg/?l.The result indicates that the device is of the excellent sensing properties and can be used in bio-medicine and other fields.The innovations of the paper are as follows:(1)A terahertz multiband stopband filter is designed based on the electromagnetic induced transparent to obtain the change from single-stopband filter to double stopband filter at 0.92THz.(2)A four-band stop-band filter is experimentally demonstrated to realize the modulation of amplitude by tuning the azimuth in the range of 0.2 THz? 1.0 THz.(3)An effective and efficient method is proposed for the identification of water-soluble vitamin B group.The deficiencies of the paper are as follows:(1)The stopband filter based on the EIT effect is only verified by simulation instead of experiment.(2)The hybrid theory of dipole and the equivalent circuit model are used to explain the electromagnetic characteristics of the device,but the changes of external signals and the effects of structure parameters are not discussed.