Double‐etch geometry for millimeter‐wave photonic band‐gap crystals

We have designed and developed a new double‐etch technique for fabricating three‐dimensional millimeter‐wave photonic band‐gap crystals. This technique doubles the band‐gap frequency obtainable from silicon wafers. By introducing overetching, the double‐etch geometry allows one‐way tuning of the midgap frequency. We have experimentally demonstrated this property by fabricating and testing structures with different overetch ratios. Terahertz spectroscopy techniques were used to measure photonic band‐gap crystals with midgap frequencies ranging from 340 to 375 GHz.