Figure 1 : Device image and quality factors.

From: Photonic quasi-crystal terahertz lasers

Figure 1

(a) Scanning electron microscope image of a prototype device. Holes of radius r=6 μm and a lattice spatial length scale a=23 μm have been lithographically designed at each vertex of a Penrose pattern and imprinted into the top Cr/Au metallization of the THz QCL (see inset). To implement strong absorbing boundary conditions, a thin (35 μm) Cr (7 nm) border was preliminary evaporated around the mesa pattern. The 75-nm-thick n+ contact layer was then removed in the holes to reduce the cavity losses, and decagonal mesa structures were etched down to the bottom metal layer to avoid lateral current spreading. (b) Form factor S(k) in reciprocal space of the Penrose quasi crystal in a. (c) Quality factor Q of the computed optical modes as a function of the radiation frequency for a device having r=6 μm and a=23 μm. The dashed vertical lines indicate the QCL gain bandwidth; the circle identifies the optical modes having the highest Q factors, labelled with P, L and H.