Graphene is considered to be a promising candidate to replace indium tin oxide (ITO) as transparent conductive electrodes in optoelectronics applications. We use graphene films as transparent electrodes for THz applications such as phase shifters, reflector, and intensity modulators, leading to a low voltage operation and very small insertion loss.
First, we experimentally demonstrate the excellent performance of THz modulators based on graphene/ionic liquid/graphene sandwich structures. The modulation covers a broadband frequency range from 0.1 to 2.5 THz with the modulation depth of up to 99% by applying a small gate voltage of 3 V. To our knowledge, this is the highest modulation ratio from graphene based THz devices to date. We also report a highly efficient tunable THz reflector in graphene. By applying a small gate voltage (up to ± 3 V), the reflectance of graphene is modulated from a minimum of 0.79% to a maximum of 33.4% using graphene/ionic liquid structures at room temperature, and the reflection tuning is uniform within a wide spectral range (0.1 – 1.5 THz). In addition, we design and fabricate terahertz phase shifters based on thin liquid crystal cells sandwiched by two graphene layers. A maximum 10.8 degree phase shift is obtained with 5 V voltage. The proposed phase shifters are fully electrical controllable, continuous tunable, and require very low DC voltages for operation.
Finally, we show a high performance THz emitter based on ferromagnetic/nonmagnetic heterostructures. Our THz emitter based on nonmagnetic (NM) and ferromagnetic (FM) heterostructures has a peak intensity exceeding 500 μm thick ZnTe crystals (standard THz emitters). We have also fabricated the devices on flexible substrates with a great performance, and demonstrated that the devices can be driven by low power lasers.