Fig. 1. Schematic illustration of THz time-domain spectroscopy (THz-TDS) system to evaluate the fabricated PCDs by using different fs-lasers with wavelengths of 800 nm and 1560 nm.

Fig. 2. Representative I-V characteristics of Si wafer deposited with gold ultrathin film. RF sputtering of gold was carried out for 30, 60, and 120 s at an RF power of 20 W.

Fig. 3. Optical microscope image of basic LT-GaAs PCD utilized in the present study. It has the structural parameters of antenna length L=28  μm, width of the transmission line l=4  μm, and width of dipole electrode w=10  μm.

Fig. 4. AFM images of the dipole gap region of LT-GaAs PCDs. Here (a) PCD-N, (b) PCD-A, and (c) PCD-B correspond to the PCD with RF sputtering time of gold for 0, 30, and 60 s, respectively.

Fig. 5. Ratios of increasing current IA/IN and IB/IN observed in the I-V characteristics at 800 nm fs-laser irradiation onto the dipole gap region of PCD at a power of 10 mW.

Fig. 6. THz time-domain waveforms detected by (a) PCD-A and (b) PCD-B compared with that of PCD-N at 800 nm fs-laser excitation. Here THz pulse was emitted from the p-InAs excited by 800 nm fs-laser.

Fig. 7. Ratios of increasing current IB/IN in the I-V characteristics under 800 nm and 1560 nm fs-laser excitations. In the measurements fs-lasers were illuminated onto the dipole gap regions of PCD-B at a power of 10 mW for 800 nm fs-laser and 30 mW for 1560 nm fs-laser.

Fig. 8. THz time-domain waveforms detected by PCD-B and PCD-N at 1560 nm fs-laser excitation. Here THz pulse was emitted from the p-InAs excited by a 1560 nm fs-laser.

Fig. 9. FFT spectra of THz time-domain waveforms detected by PCD-B at (a) 800 nm and (b) 1560 nm fs-laser excitation corresponding to the waveforms in Fig. 6(b) and Fig. 8, respectively. The inserted spectra in the bottom figures show those observed by PCD-N.

Fig. 10. Power dependence of IB and IN at 1560 nm fs-laser excitation.

Fig. 11. Schematic illustration of excitations of electrons in Au nano-islands to the GaAs conduction band by induced electric field modulation between Au nano-islands by LSPR and change in Fermi level with electron transfer at DC bias V=0.

Fig. 12. Relationship between resonance wavelength λL and aspect ratio R obtained by the calculation using Gan’s model [44,45].

Fig. 13. Rectangular fitting to gold nanoparticles for aspect ratio estimation.