Research Of Silicon Composite Structure And Terahertz Control Characteristics

With continuous development of the type and properties of terahertz radiation sources and detection devices, people gain more in-depth and comprehensive knowledge of terahertz wave. Related applications of terahertz technology also spread to all areas.Because of its unique characteristics in electromagnetic spectrum, applications of terahertz wave in medical testing, wireless communications, imaging and detection and other areas are expected to exceed other spectrum bands. But the traditional function devices such as modulator / switch, filter, absorber and polarizer are incompatible with terahertz, the demands for functional devices in various fields become increasingly urgent. However, devices need different performances in different fields. For example, in terahertz communication, we demands modulator is fast enough to shorten the communication time of transmission, but its modulation depth is less demanding. Meanwhile terahertz imaging requires modulator has a sufficiently large modulation depth to guarantee the clarity and accuracy of the imaging system, on the contrary the requirements of its modulation rate can be very low, generally kHz magnitude is enough..This paper proposes two types of silicon composite structure to improve the modulation depth. Firstly, we combine gold doped silicon with graphene film and form the silicon / graphene composite structure. Experiment results show that the composite structure under the effect of laser can effectively improve the modulation depth of the device. When there is laser-free, graphene and silicon doped with gold are both highly transparent to terahertz waves, the transmittance is 65%.When irradiating gold-doped silicon surface with laser, the optical carriers diffuse to graphene layer due to the concentration difference, forming the electrically conductive layer in the "graphene-doped silicon gold" interface because of extremely high electron mobility of graphene. Therefore, the electrically conductive layer will greatly absorb and reflect terahertz wave with the result that the device’s transmission rate drop to 45% and modulation depth increase 50%, and compared to gold doped silicon, its modulation rate can rise to 2.2MHz. Secondly, we produce one kind of two-dimensional photonic crystal on the high-resistivity silicon substrate, form the photonic crystal / high-resistivity silicon composite structure. The composite structure can not only increase device’s modulation depth, but also reduce its insertion loss. We illustrate the effect of four different incident ways to modulation characteristics and give a reasonable explanation. When the laser is acting on silicon layer, the formed electrically conductive layer not only absorbs the incident terahertz wave from the front, but also reflects part of the terahertz wave to the photonic crystal in the form of diffuse reflection, and is localized incident in photonic crystals terahertz wave interference. thereby reducing the amplitude of the transmitted terahertz waves, improve debugging depth. The maximum modulation depth is up to 97%, but the modulation rate is small, only 10 kHz. Finally, we utilize the Si-PC composite structure in THz imaging, further illustrate increase of modulation depth owing to the composite structure.This paper presents two silicon composite structures which can effectively increase the modulation depth of modulation devices. And they have advantages of simple fabrication, low cost and good performances, can be used in terahertz imaging system.