Fig. 1. Operation principle of photoconductive antenna emitter

Fig. 2. Related works of PCA based on metal nano-metamaterials. (a) Schematic of PCA integrated with metal nano-gratings and its THz spectra^[97]; (b) schematic of PCA integrated with Ag nano-islands and its THz spectra^[98]; (c) schematic of PCA integrated with hexagonal metal nano-array and its THz time-domain signals^[99]; (d) schematic of PCA integrated with metal nano-gratings pumped by 1570 nm femtosecond light and its THz time-domain signals^[102]

Fig. 3. Related works of PCA based on metal nano-electrodes. (a) Schematic of PCA with interlaced structure and its THz time-domain signals^[116]; (b) schematic and operation principle of bowtie PCA based on metal nano-electrodes^[117];(c) schematic and pump electric field distribution of 3D nano-electrodes of PCA, and optical-terahertz conversion efficiency of 3D PCA ^[118];(d) bias field distribution on nano-electrodes of H-type dipole antenna^[119]

Fig. 4. Related works of PCA array based on metal nano-metamaterials. (a) Schematic and average radiated power spectra of plasmonic PCA emitter array^[126]; (b) schematic of large-area plasmonic PCA detector array and its optical transmission/absorption characteristics ^[130]

Fig. 5. Related works of photomixers based on metal nano-metamaterials. (a) Schematic and output intensity of CW THz photomixer with interdigitated electrodes and tip-to-tip nano-gap electrodes^[135]; (b) schematic and radiated THz power of plasmonic photomixer^[137]

Fig. 6. Related works of PCA detectors based on dielectric nano-metamaterials. (a) Schematic and optical absorption spectra of all-dielectric metamaterial enhanced PCA detectors^[141]; (b) SEM photo of PCA detectors with GaAs resonators, simulated absorption, reflectivity, transmission of metasurfaces and absorption of 200 nm flat GaAs substrate for 800 nm incident light with polarization direction along GaAs resonator connection^[142]

Fig. 7. PCA emitters based on dielectric nano-metamaterials^[143]. (a) Schematic of metamaterials-assisted PCA and unit cells; (b) measured optical anti-reflectance effect of nanograting; (c) schematics of 4F THz time-domain spectroscopy system and added reflective microscopic imaging system; (d) THz time-domain signals of metamaterials-assisted PCA emitter and traditional PCA emitter; (e) THz power spectra of metamaterials-assisted PCA emitter and traditional PCA emitter as well as power enhancement; (f) simulated bias electric field distributions of flat GaAs (left) and nanograting (right)

Fig. 8. Related works of PCA integrated with metal micro-structures. (a) Micrographs of PCA transmitter with micron-SRR^[144]; (b) measured THz amplitude spectrum radiated by emitter^[144]; (c) measured amplitude spectrum of THz wave under both pumping and switching excitation and that under only pumping excitation with schematic of pump light and switch pulse shown in illustration and solid and dashed lines indicating measured results under electric field with polarization of x and y, respectively^[144]; (d) schematic of PCA detector with corrugated metal structure^[145]; (e) normalized amplitude spectra showing sensitivity enhancement relative to simple dipole antenna^[145]

Fig. 9. PCA integrated metal micro-SRR^[146]. (a) Schematic of meta-antenna with SRRs; (b) simulated electrical field distributions along coplanar lines of reference PCA (left) and meta-antenna (right) at 0.54 THz; (c) averaged THz amplitude spectra with root mean square error bars; (d) THz radiation process of meta-antenna; (e) THz amplitude spectra of meta-antennas with various distances d between SRRs and gap; (f) measured amplitude spectra of meta-antenna with SRRs inside/outside coplanar lines, respectively

Fig. 10. PCA integrated with dielectric meta-lens^[155]. (a) Front view of all-dielectric meta-lens designed for photoconductive antenna; (b) far-field radiation mode of meta-lens; (c) simulated electric field profile of THz wave (1 THz) propagating through meta-lens