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A grating coupled terahertz (THz) surface plasmon polariton (SPP) resonant biochemical sensing structure is designed and simulated, which is formed by etching submillimeter gratings on the surface of indium antimonide (InSb) substrate. The simulation results based on the phase matching equation show that when the TM polarized THz parallel beam is illuminated into the grating interval at a 30° incidence angle, the ± 1 order THz diffraction beam of the grating can excite low-frequency SPP and high-frequency SPP with opposite propagation directions, respectively. The commercial THz time-domain spectroscopy (TDS) devices can be used to accurately measure the low-frequency SPP easily. So, the dependence of the resonance characteristic and sensing characteristic of low-frequency SPP on grating structural parameters is systematically simulated and analyzed in this work. The simulation results indicate that the InSb grating bare sensing chip cannot provide a detectable response to the tyrosine monolayer adsorption layer. The reason for this is that the penetration depth of the low-frequency SPP evanescent field is much greater than the thickness of the tyrosine adsorption layer, resulting in insufficient interaction between the two. In order to detect biomolecules, the sensitization method of InSb grating covered with porous film is simulated and analyzed in this work. Porous film has a molecular enrichment effect, which can extend the interaction between THz surface wave and biological target from a single molecule scale to the entire film thickness, thereby improving the biological detection sensitivity of the sensor. The simulation results show that taking tyrosine for example, when the InSb grating surface is covered with a porous polymethyl methacrylate (PMMA) film with a thickness of 120 μm and a porosity of 0.4, its adsorption sensitivity is 0.39 THz/unit volume fraction.
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Keywords:
- terahertz surface plasmon resonance /
- InSb grating /
- biochemical sensing /
- porous materials
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图 1 使用Drude模型计算的InSb介电常数
Figure 1. InSb dielectric constant calculated with the Drude model.
图 2 光栅耦合原理示意图
Figure 2. Schematic diagram of grating coupling principle.
图 3 掺杂型InSb色散曲线
Figure 3. Dispersion curve of doped InSb.
图 4 低频SPP与高频SPP的共振频率随光栅周期的变化
Figure 4. Resonance frequency variation curves of low-frequency SPP and high-frequency SPP with grating period.
图 5 光栅高度为10 μm、占空比为0.4的条件下, THz反射谱随光栅周期的变化
Figure 5. Variation of THz reflection spectrum with grating period under the condition of a grating height of 10 μm and a duty cycle of 0.4.
图 6 (a) 光栅周期为120 μm时共振频率随介质折射率的变化, 及线性拟合得到的折射率灵敏度; (b) 折射率灵敏度随光栅周期的变化
Figure 6. 6. (a) Resonance frequency variation curve with medium refractive index when the grating period is 120 μm, and the refractive index sensitivity obtained by linear fitting; (b) variation curve of refractive index sensitivity with grating period.
图 7 光栅表面吸附10 nm酪氨酸与未吸附时的THz反射谱
Figure 7. THz reflection spectrum of 10 nm tyrosine adsorbed on grating surface and without adsorption.
图 8 InSb光栅表面覆盖多孔PMMA薄膜制备流程示意图
Figure 8. Schematic diagram of the preparation process of InSb grating covered with porous PMMA film.
图 9 多孔PMMA等效折射率n随孔隙率P的变化
Figure 9. Variation curve of equivalent refractive index n of porous PMMA with porosity P.
图 10 在厚度h = 25 μm情况下的仿真结果 (a) 折射率灵敏度随多孔PMMA孔隙率P的变化; (b) 介质层平均电场模随多孔PMMA孔隙率P的变化; (c) 不同孔隙率下的场分布示意图; (d) 不同孔隙率下介质层与多孔PMMA层界面处的电场强度; 其中图(b), (c), (d)均为介质层折射率$ {n}_{{\mathrm{d}}}=1 $的结果
Figure 10. Simulation results in the case of h = 25 μm: (a) Variation curve of refractive index sensitivity with porosity P of porous PMMA; (b) variation curve of average electric field mode of dielectric layer with porosity P of porous PMMA; (c) schematic diagram of field distribution under different porosity; (d) the electric field intensity curve at the interface between the dielectric layer and the porous PMMA layer under different porosity. Note: panels (b), (c), (d) are the results under the condition that the refractive index of the dielectric layer $ {n}_{{\mathrm{d}}}=1 $.
图 11 在孔隙率P = 0.4情况下的仿真结果 (a) 折射率灵敏度随多孔PMMA厚度h的变化; (b) 介质层平均电场模随多孔PMMA厚度h的变化; (c) 不同厚度下的场分布示意图; (d) 不同厚度下介质层与多孔PMMA层界面处的电场强度曲线; 其中图(b), (c), (d)均为介质层折射率$ {n}_{{\mathrm{d}}}=1 $的结果
Figure 11. Simulation results in the case of P = 0.4: (a) Variation curve of refractive index sensitivity with porous PMMA thickness h; (b) variation curve of average electric field mode of dielectric layer with thickness h of porous PMMA; (c) schematic diagram of field distribution under different thickness; (d) the electric field intensity curves at the interface between the dielectric layer and the porous PMMA layer under different thickness. Note: panels (b), (c), (d) are the results of the refractive index of the dielectric layer $ {n}_{{\mathrm{d}}}=1 $.
图 12 (a) 不同孔隙率下多孔PMMA吸附酪氨酸体积分数$ {\eta }_{3}= $0, 0.05, 0.1, 0.15, 0.2时的等效折射率n; (b) $ {\eta }_{3} $由0变为0.2时, 不同孔隙率多孔PMMA的等效折射率变化量$ \Delta n $
Figure 12. (a) Equivalent refractive index n when the volume fraction of tyrosine adsorbed by porous PMMA $ {\eta }_{3} $ is 0, 0.05, 0.1, 0.15 and 0.2 under different porosity; (b) the change of equivalent refractive index $ \Delta n $ of porous PMMA with different porosity when $ {\eta }_{3} $ changing from 0 to 0.2.
图 13 在孔隙率P = 0.4条件下的仿真结果 (a) 覆盖厚度为40 μm的多孔PMMA, 传感器吸附不同体积分数酪氨酸的反射光谱; (b) 覆盖厚度为40 μm的多孔PMMA, 传感器吸附不同体积分数酪氨酸的共振频率变化曲线, 及线性拟合得到的吸附灵敏度; (c) 覆盖厚度为120 μm的多孔PMMA, 传感器吸附不同体积分数酪氨酸的反射光谱; (d) 覆盖20—240 μm厚度的多孔PMMA, 传感器吸附灵敏度变化曲线
Figure 13. Simulation results in the case of porosity P = 0.4: (a) The reflection spectra of the sensor after adsorption of tyrosine with different volume fractions when the thickness of the covered porous PMMA is 40 μm; (b) the resonance frequency curve of the sensor after adsorption of tyrosine with different volume fractions and the adsorption sensitivity obtained by linear fitting when the thickness of the covered porous PMMA is 40 μm; (c) the reflection spectra of the sensor after adsorption of tyrosine with different volume fractions when the thickness of the covered porous PMMA is 40 μm; (d) variation curves of sensor adsorption sensitivity when the thickness of covered porous PMMA is 20–240 μm.
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