Photonic crystals with a complete bandgap for TM-modes used as resonators for terahertz quantum-cascade lasers

The integration of photonic crystals (PC) and quantum-cascade lasers (QCL) can lead to a powerful set of new devices for various applications, such as multi-wavelength laser, detector arrays, on-chip integration and spectroscopy. The PCs are artificial crystals that have allowed and forbidden bands for the propagation of light. Typically PCs are built up by a regular pattern of air holes inside the active region of a QCL. Such structures have a complete bandgap for TE-modes only but not for TM-modes, which is the polarization of the inter-subband transitions in QCLs. Here we present the design and fabrication of a PC with a full bandgap for TM-modes to provide positive feedback for the active region. The PC is built up by a triangular lattice of rods surrounded by air. The ration r/a is 0.3 for all periods, where r is the radius of the rods and a the period of the crystal. The period has been varied from 22.18 can to 35.49 can. This structure has the first bandgap for TM-modes from 0.2 to 0.28 and the second one from 0.37 to 0.48 in terms of the normalized frequency f-a/c, where f is the frequency and c the speed of light. For the calculation of the bandstructure a 2D model has been used.