全介质光栅在太赫兹波段的光调控特性

Optical modulation characteristics of all-dielectric grating at terahertz frequencies

采用激光直写技术在100μm厚的Si衬底上制备了全介质光栅结构,利用近红外光抽运-太赫兹探测(near infrared pump-Terahertz probe)技术对该全介质光栅在THz波段的光谱响应及其光调控特性进行了测试,最后结合电磁仿真结果,对米谐振(Mie resonance)的形成机理和光调控机理进行了解释并对调控光作用下全介质光栅的电导率数值进行了估算.研究结果表明:在光栅与THz偏振垂直的情况下,该全介质光栅在0—1.0 THz范围内有3个典型的米谐振峰且谐振模式各不相同;随着调控光功率的增加,3个谐振峰的谐振强度出现了不同程度的减弱,其中第一个谐振峰的光调控幅度达到50%以上,调控光作用下米谐振强度的减弱是由于光生载流子对入射THz波的吸收和散射导致了介质光栅内部感生电磁场减弱引起的.上述工作对全介质超材料在THz波段的共振特性研究和相关光调控器件的研制具有重要参考价值.

In recent years, metamaterials（MMs） have been widely investigated for their exotic electromagnetic characteristics which cannot be achieved in nature. However, one of the main limitations in traditional metallic-film MMs is a high level of radiation loss in metal and insertion loss of the high-permittivity or thick substrate. Fortunately, all-dielectric MMs with high refractive-index dielectric structures show significantly less material loss than their metallic counterparts. In this paper, an all-dielectric grating is fabricated on a 100-μm-thick silicon wafer by using direct-laser-writing technique,and the properties of its Mie resonances are investigated by THz time-domain spectroscopy. Then we measure the spectral response of the all-dielectric grating under the optical modulation by a near-infrared pump-THz probe method. The modulation light source is an 808 nm continuous semiconductor laser with a maximum power（10 W）. To give an insight into the underlying mechanisms of the Mie-type resonance effects on the arrayed, silicon pillars, the transmission of the all-dielectric grating is investigated numerically by the finite-element simulations through using CST Microwave Studio.In our experiment, the incident THz magnetic field is along the grating lines. The research results show that three typical Mie resonances are excited from 0 to 1 THz in the all-dielectric structure, and all the three resonant modes are different in the distributions of electric field and magnetic field. Furthermore, it is found that the resonance intensities of these three resonance peaks appear to be weakened variously with the increase of the optical power, and the first resonant peak modulation amplitude maximally reaches more than 50%. Combining the simulation results, we prove that the decrease of Mie resonance intensity under photo-excitation is caused by the absorption and the scattering of the incident THz wave by photo-generated carriers. Besides, we estimate the conductivity values of the all-dielectric grating under different optical excitations and find that the conductivity values reach 1000 S/m and 1500 S/m corresponding to 5 W and 10 W optical excitation, respectively. The estimated conductivity data will play an important role in the prospective optical modulation simulation. All the results mentioned above will provide an important reference for researches on the resonance properties of the all-dielectric metamaterials and the development of related functional devices.