Terahertz-wave (THz-wave) sources have attracted a great deal of attention because the THz frequency region has a number of important potential applications in the medical and imaging fields [1,2]. Many coherent THz sources, based on photoconductive antenna, quantum cascade lasers, and non-linear optical effects, such as difference-frequency generation (DFG), optical parametric oscillation, and optical rectification, have been developed for these applications [3-17]. In particular, DFG can be used to produce tunable, narrow linewidth, high-power THz-waves, with ns-pulsed and continuous wave (CW) operation at room temperature [18,19]. This DFG process is suitable for wave sources used in THz telecommunications and in-situ security screening.

A GaP crystal is an attractive nonlinear optical material for coherent THz-wave generation via the DFG process, due to its transparent properties in the infrared and THz regions and relatively high conversion efficiency due to THz phonon-polariton excitation [7-10,12-14]. Our previous works have demonstrated THz-wave generation from bulk GaP crystals under small-angle, noncollinear χ(2) phase-matched DFG conditions [7-9]. Using a THz-wave source, a frequency-tunable THz spectrometer was constructed to study the THz spectra of biomolecules (e.g. sugars, nucleosides, nucleotides, and amino acids) [20-22]. When the cross-section size of the GaP rod-waveguide structure coincides with the wavelength of the THz wave, the THz wave is confined in the waveguide, thus, inducing a change in the refractive index. As a result, THz waves are produced under a collinear χ(2) phase-matching configuration. We have reported THz-wave generation from GaP-THz rod and rib-waveguide structures and confirmed the enhancement of the conversion efficiency in the DFG process, using the wave-guiding effect for THz waves [23,24].

Recently, photonic crystal (PC) waveguides have been investigated because they allow the optical properties of the material to be modified. PC components have been developed in the optical region [25-28]. PCs in the THz region are also of interest because they could contribute to the development of THz technology. In contrast to passive devices, such as filters and PC fibers, few active devices have been investigated [29-33]. PC-embedded waveguides in the THz region make it possible to enhance the nonlinear optical effect, such as DFG in GaP, due to the strong phonon-polariton confinement in the PC waveguides.

In this paper, we fabricated two-dimensional (2D) photonic structures in GaP waveguides using inductivelycoupled plasma reactive-ion etching (ICP-RIE). The THz-wave output characteristics of the PC waveguides were compared with those of GaP planar waveguides.

GaP-PC waveguides were fabricated using the ICP-RIE technique on an undoped semi-insulating (001) GaP wafer (Sumitomo Metal Mining, Co. Ltd.; resistivity: >10⁵ W cm; thickness: 300 mm). The fabrication process involved spin-coating an XP KMPR-1025 negative photoresist onto a 60-μm-thick GaP wafer, patterned using UV (i-line) exposure. Using a Cl₂/Ar plasma chemistry in the ICP-IRE process, etch rates as high as 2 mm·min⁻¹ were achieved to fabricate the PC waveguide structures. The waveguide samples were cleaved into 10 × 10 mm rectangles, along the <110> crystal direction. No antireflection coating was applied to the input or output surfaces of the waveguide. Figure 1 shows a scanning electron microscopy (SEM) image of one of the fabricated PC waveguides, with line defects in the G-K direction. Holes were arranged in a triangular lattice, with a lattice constant, a, of 200 mm; the radius of the air holes, r, was 82 mm. By removing rows of holes, we created 300- and 500-mm-wide line-defect waveguides along the G and K directions, respectively.

We calculated the photonic band diagram of the PC slab without line defects using the plane-wave expansion method (Figure 2(a)). The lowest stop band-gap region was estimated to exist in the frequency range of 0.4 to 0.55 THz for transverse electric (TE) polarization. We also calculated the THz transmission spectra for the THzPC slab and waveguide using three-dimensional (3-D) finite-difference time-domain (3D-FDTD) simulations, shown in Figure 2(b). The modeled structures were the same as the PC without line defects and the 300-mmwide PC waveguide. The THz-wave transmission spectra indicated high transmittance in the stop band-gap region for the PC waveguide. Thus, this suggests the existence of defect modes originating from the line defects in this frequency region.

Figure 3 illustrates a schematic of the experimental setup used for DFG of THz waves in GaP-PC waveguides, where Cr:Forsterite (Cr:F) lasers were used as the incident source. Both the pump and signal beams were delivered from two-channel Cr:F lasers (LOTIS TII) pumped with a double-pulse Q-switched Nd:YAG laser (repetition rate: 10 Hz, wavelength: 1064 nm). The wavelength of the pump source was fixed at 1216 nm; the signal source was variable from 1217 to 1221 nm, which corresponded to a frequency difference of 0.2 to 1.0 THz. The pulse energies of the pump and signal sources were adjusted to 0.5 mJ (pulse width: 22 ns; beam diameter: 1 mm). The pump and signal beam were collinearly combined using a beamsplitter and then directed to the input edge of the waveguide; the propagation direction of the two beams was parallel to the G-K direction. The generated THz wave was collected by a high-density polyethylene (HDPE) lens and detected by a liquid-helium-cooled Si-bolometer.