Fig. 1. Photoconductivity of graphene. (a) Illustration of graphene energy bands under different doping levels and carrier transitions with intraband transitions for hole-doped graphene as an example; (b) real part of frequency-dependent photoconductivity of single-layer graphene[29]

Fig. 2. Terahertz relative transmittance of graphene under different conditions in experiment. (a) Undoped graphene under different growth temperatures; (b) chemical doped graphene under different growth temperatures[27]; (c) graphene with different stacking layers[13]; (d) nitrogen doped (N-doped) graphene under different doping concentrations[38]; (e) graphene oxide films under different reduction temperatures; (f) reduced graphene oxide (rGO) under different filtration volumes (film thickness)[40]

Fig. 3. Optical pump -THz probe spectroscopy of graphene. (a) THz time-domain transmission signal of graphene without pump pulse; (b) pump scanning results of terahertz time-domain signal (solid lines indicate bi-exponential fitting results of relaxation processes)

Fig. 4. Terahertz emission properties of graphene. (a) Left: transient photo-induced electron and hole distributions at oblique incidence; top right: nonequilibrium population distribution of electrons in graphene without pump; bottom right: nonequilibrium electron population distribution at oblique incidence; (b) THz electric field emitted by graphene at time and frequency domains[18]; (c) SEM of vertically grown graphene (VGG) surface; (d) THz emission spectra of graphene and VGG[65]; (e) integral int

Fig. 5. Active terahertz devices based on graphene. (a) Structural diagram of graphene/Si modulator; (b) time-domain transmission of device under combined regulation of gate voltage and illumination[77]; (c) extinction ratio of graphene/SiO2/Si device under combined regulation of gate voltage and magnetic field; (d) Faraday rotation angle of graphene/SiO2/Si device under combined regulation of gate voltage and magnetic field[14]

Fig. 6. Graphene-based metamaterials for terahertz modulators. (a) Schematic of graphene/metal SRR; (b) relative THz transmissivity of graphene/SRR and N-doped graphene/SRR [17]; (c) top: schematic of graphene SRR dimer, bottom: transmissivity versus terahertz frequency and structural clearance; (d) transmissivity and three resonance modes vary with different Fermi energies[101]; (e) schematic of chiral graphene-metamaterial; (f) conversion transmissivity for left and right circular polarized THz waves[

Fig. 7. Impedance matching principle of terahertz wave at graphene/substrate interface. (a) Schematic of THz pulse propagating through graphene/substrate[13]; (b) reflection suppression within samples for THz time-domain spectra after impedance matching

Fig. 8. Impedance matching properties of graphene-based materials. (a) Relative amplitude transmissivity of first two transmission pulses of graphene (undoped)/quartz versus number of layers; (b) relative amplitude transmissivity of first two transmission pulses of graphene (chemically doped)/silicon versus number of layers[13]; (c) relative amplitude transmissivity of reflected pulse at quartz/graphene interface versus incident angle and number of layers; (d) interface reflection coefficient of quartz/

Fig. 9. Spectra and physical mechanism of terahertz radiation for gated graphene/SiO2/Si. (a) Terahertz radiation signal versus applied gate voltage; (b) terahertz radiation intensities at three different azimuthal angles versus applied gate voltage; (c) graphene/SiO2/Si interface in depletion case; (d) graphene/SiO2/Si interface in weak inversion case; (e) graphene/SiO2/Si interface in strong inversion case [19]