Takuji Nakagawa

Study of Multilayer Ceramic Photonic Crystals in THz Region

We have fabricated one-dimensional photonic crystals with a conventional ceramic layering process and investigated their terahertz wave transmission properties by terahertz time-domain spectroscopy (THz-TDS). In one-dimensional photonic crystals with a structural defect, a narrow transmission band (i.e., a defect mode) appears within the band gap. However, the dielectric losses of the ceramic materials significantly suppress the transmission intensity of the defect mode. The observed low transmission is mainly associated with the absorption loss of the defect layer material. Since the internal multiple reflections enhance the absorption loss, tuning the periodicity of the photonic crystal from a high reflectivity condition is effective for improving the transmission intensity of the defect mode. The measured highest peak transmission is -4.5 dB when periodicity is changed from a quarter-wavelength stack.

Terahertz Band-Pass Filter Fabricated by Multilayer Ceramic Technology

We fabricated a terahertz (THz) band-pass filter using multilayer ceramic technology. The filter design is based on an interference multilayer filter composed of three resonant cavities. The filter exhibited insertion loss as low as 1.5 dB at a center frequency of 0.15 THz. The bandwidth of the passband was modified by changing the optical length of the cavity. A demonstrated tuning of the full width at half maximum (FWHM) ranged from 12.5 to 6.9%. No significant change in transmission intensity was observed when the optical length of the cavity was changed. At a frequency of approximately 0.8 THz, filter insertion loss increased from 1.5 to 6 dB. The influence of dielectric losses of the ceramic materials on the filter insertion loss is also discussed.

Diamond-Structure Photonic Crystals Composed of Ceramic Spheres in Resin and Their Microwave Properties

Three-dimensional (3D) photonic crystals with a diamond structure composed of TiO2 ceramic spheres in a resin matrix were fabricated by stereolithography. The lattice constant was 12 mm. The diameter of the spheres was about 5 mm. These photonic crystals composed of ceramic spheres showed four complete photonic band gaps at around 5.7 GHz between the 2nd and 3rd bands, 7.8 GHz between the 8th and 9th bands, 10.4 GHz between the 24th and 25th bands, and 13.1 GHz between the 54th and 55th bands. The propagation characteristics of microwaves propagating through the photonic crystals agreed well with the calculated results using the plane-wave expansion method.