Nanoscience and Nano Technology

Biography

Biography: Zhaoxin Geng

Abstract

Terahertz (THz) modulator plays an important role in THz communication system. There are different modulators which have been developed to meet different application. Especially, THz modulator based on metamaterials need complex fabrication processes and high cost, therefore, the simpler and cheaper devices are in greatly demand for THz communication in near future. Two-dimension (2D) materials, such as graphene, MoS₂, WS2 and other beyond graphene materials, were introduced to modulator to balance between modulation characters and cost. Therefore, different THz modulators based on different 2D materials were developed and compare the difference among them. The results illustrate that graphene and MoS₂ did not present ideal results due to their relatively narrow band gap. WS₂, as another kind of 2D materials, which has wider band gap and some merits such as better chemical and thermal stability, could be used for photoelectric devices in THz regime. An all-optical pumped THz modulator based on WS₂-silicon, MoS₂-silicon and graphene-silicon heterostructure were demonstrated. The p-type WS₂ was formed by an annealing treatment in air and the silicon substrate was slightly n-type doped with a high-resistivity. Both the modulator and bare silicon (as reference) were measured by THz time-domain spectroscope (THz-TDS) system. The results compared with bare silicon, THz transmissivity of the modulator significantly reduced when the power of pumping laser increased. A relative low normalized transmission of this device was around 5% with a radiation light power of 4 W, while the bare silicon was 45.6% under the same condition. The working mechanism of the modulator lies in that carriers transfer between the silicon and WS₂ when the pump light illuminates the chip. We compared the difference between results of the bare silicon, the WS₂-Si sample before annealed and after annealed and found that the WS₂-Si sample could separate electrons and holes more effectively. THz transmissivity decreased when conductivity of the modulator increased due to separation of electrons and holes. Meanwhile, the MoS₂-based device even exhibited much higher modulation efficiency compared with the graphene-based device. The mechanism of the convincing modulation enhancement originated from MoS₂ annealed as a p-doping, which is different from that of graphene-based modulator. Therefore, under a guidance of the working mechanism, THz modulators with higher modulation depth could be developed based WS₂-silicon heterostructure through changing the annealing time, layers of WS₂, bias voltage, pumping light power and so on. The unique optical modulating properties of the device based on 2D materials exhibit tremendous promise for applications in terahertz communication based graphene device, we also developed other optoelectronic devices such as frequency tripler, thermo-optic modulator and mixer.