Fig. 1. (a) Schematic of the SLM-assisted 2PP system and process flow of composite advanced manufacturing strategy. HWP, half-wave plate; BE, beam expander; SLM, spatial light modulator; Iris, iris diaphragm; ND filter, neutral density filter; BS, beam splitter; Shutter, mechanical shutter; MO, microscope objective; Camera, CCD camera. The monitor is synchronized with the liquid crystal surface of the SLM and used to display the phase mask loaded on the SLM in real time; inset on the monitor presents the phase pattern to generate the Bessel beams; the corresponding 2D light fields at the focus area in the silicon surface are presented in (b). (b) Relative position between Bessel beams and the photoresist; the yellow dashed box identifies the area where laser direct writing occurs. (c) The hologram that is projected onto the SLM is composed of the axicon phase and blazed grating. (d) Iris filters out diffraction orders other than the −1st order.

Fig. 2. Design and matching manufacturing process of SPP waveguide devices. (a) and (b) Detailed geometric characteristics of SPP waveguide devices. (c) Manufacturing process flow of spoof SPP waveguide; the whole process is divided into two parts: additive manufacturing and subtractive manufacturing, with 2PP and FLA as the core technologies, respectively.

Fig. 3. Simulation and experimental results of Bessel beams: (a) and (c) longitudinal intensity profiles along propagation; the solid white line represents the intensity of the bright main lobe, and the inset shows the Fourier spectrum profiles of the Bessel beams. (b) and (d) Intensity profiles in the transverse plane with the maximum intensity [white dashed line in (a) and (c)]. (e) and (f) SEM images of the crisscrossed microstructures directly written by the femtosecond Bessel beams with a feature size of ∼3.3  μm.

Fig. 4. Basic structure forming of SPP waveguide devices. (a) SEM photos of the waveguide structure from top view and 45° view. (b) Surface profile curve scanned along the dotted arrow in the inset of (a); the slowly rising or falling edge of the curve is not the shape of the structure itself, which is caused by the slow rising (falling) of the probe during displacement.

Fig. 5. Optical microscope and SEM photos of waveguide structure and excitation region. (a) Optical microscope photo of the entire device. (b)–(d) SEM photos from top and 45° views. Inset shows the zoomed-in region in the frame of (d).

Fig. 6. THz near-field scanning results of on-chip SPP waveguide devices. (a) Optical micrograph of THz near-field probe. (b) Near-field intensity distribution of the waveguide devices. (c) Spectral distribution of excitation region and straight waveguide region. (d) Attenuation curve of SPPs wave signal intensity from excitation region to straight waveguide region. (e) Time-domain signal of SPPs wave at the beginning (X=0, Y=0) and end (X=4, Y=0) of the waveguide. (f) Intensity as a function of increasing Z; the red dots are experimental results, and the solid line is the exponential fit; the sub-graph shows time-domain signals of SPPs at different heights (Z=50  μm and 500 μm).

Fig. 7. Functional tests of the spoof SPPs waveguide devices with the excitation region fabricated by FIB: (a) SEM images of the excitation array, (b) SPP spectra (0.5–0.7 THz) in the excitation regions fabricated by Bessel beams FLA (red marks and lines) and FIB (blue marks and lines), (c) SPP wave transmissions along the waveguides, and (d) near-field intensity profile of the device with the excitation region fabricated by FIB.

Fig. 8. Dispersion relation curves of the SPPs mode of metal pillars.

Fig. 9. Numerical simulations of SPPs waveguide devices. (a) Near-field intensity profile of SPPs at 0.6 THz. (b) Normalized electric component (EZ) distributions in the Y−Z cross section at the white dashed line in (a).

Fig. 10. Schematic of the scanning near-field terahertz microscopy system.

Fig. 11. Terahertz waves focused on the excitation region. (a) Intensity profile. (b) Phase profile. (c) Time-domain signal at the position of maximum intensity in (a).