| Terahertz (THz) technology, which is one of the most rapid growing fields with extensive applications in time-domain spectroscopy, biological imaging, high sensitive sensing, and high-speed communication, is in great demands for high performance of devices and new materials to manipulate THz wave efficiently. Recently, increasing evidences have suggested that the unique optical properties of graphene in THz region make graphene desirable for manipulating THz wave. Many new proof-of-concept THz devices based on graphene, ranging from electro-optic, magneto-optic, to all-optic modulation, have been proposed based on the intrinsic THz properties of graphene as well as the new concept of plasmons and metamaterials in THz region. This paper studied the THz properties of graphene and the THz applications based on graphene for manipulating THz wave efficiently. It includes the study of THz conductivity of stacked multilayer graphene, graphene based broadband THz antireflection coatings, broadband tunable terahertz response of solution-processable reduced graphene oxide films, graphene based THz antireflection coatings and attenuators controlled by electric field, tunable magnetoplasmons for efficient THz modulator and isolator by gated monolayer graphene, and tunable magneto-optical Kerr effect in gated monolayer graphene in THz region. More details as follows:(1) In order to further extend the modulation depth of graphene based THz devices, THz time-domain spectroscopy has been employed to investigate stacked multilayer graphene on quartz. Experimental results combined with theoretical analysis proved that randomly stacked multilayer graphene, which has extended variation range of the THz conductivity, can be treated as multiple electronically decoupled monolayer graphene in the THz region.(2) This paper examined the potential of stacked multilayer graphene as broadband THz antireflection coating based on the impedance matching effect in experiment and theory. The reflected pulses from the quartz and silicon substrates were observed to change with the layer number and chemical doping degree of the graphene coating. Remarkable broadband impedance matching was achieved and analysed based on Drude model and thin film Fresnel coefficients.(3) This paper presented the fabrication of solution-processable and thickness-scalable uniform reduced graphene oxide (rGO) films that show tunable THz transmission response in a broad spectral bandwidth. By tuning the film thickness, remarkable broadband impedance matching is achieved by rGO film with suitable THz sheet conductivity.(4) Graphene based THz antireflection coatings and attenuators controlled by electric fields are provided. Said devices have several layers structure, which comprise at least two self-gated graphene layers and the medium layers, which separate the neighbouring graphene completely. Gate voltage and layer number can be changed, thus these devices show an exellent performance.(5) This paper proposed a device based on tunable magnetoplasmons in gated monolayer graphene for THz wave modulation and isolation. A superior modulation depth and giant Faraday rotation due to the cyclotron effect in the classical regime by intraband transitions in graphene offer an effective, uniform, and flexible tunability for THz wave. And the modulating and isolating manipulations by graphene are broadband.(6) This paper studied the voltage-dependent magneto-optical Kerr effect (MOKE) for THz applications in graphene. The results suggest that in quantum regime the conductivity information can be changed with the interference effects of the substrate originating from the phase change, especially the spectral line shape for the interband transitions can be changed. In classic regime, the giant magnetoplasmons effect of graphene and the synergetic effect of magnetic and electric modulation can result in broad-band manipulation.
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