Integrated Germanium-on-Silicon Ring Resonator with High Q-Factor in the Mid-Infrared

The mid-infrared wavelength region (MIR, 3–15 μm) has a large potential for sensing applications, as many molecules have strong fundamental absorption lines in this range. However, the high-cost and bulky MIR technology has prevented a real breakthrough so far. The development of MIR photonic integrated circuits in group IV platforms can provide a crucial step towards large scale applications, with ring resonators expected to play an important role. Several resonators operating in the MIR have been demonstrated in silicon-based platforms. In particular, high Q-factors have been reported in silicon-on-insulator [1] and silicon-on-sapphire [2] which, however, have high losses beyond ∽3.5 and
$\sim 6.5 \ \mu \mathrm{m}$
, respectively. On the contrary, the transparency window of germanium-based chips can go up to 15
$\mu \mathrm{m}[3,4]$
. Until recently, however, only Q-factors in the order of 10
³
–10
⁴
were reported in germanium-based integrated resonators [5]–[8]. Last year our group reported a silicon-germanium ring exhibiting a loaded Q-factor of 90,000 at 4.18
$\mu \mathrm{m}$
[9]. Here we demonstrate a pure germanium ring resonator which, for the first time in any integrated germanium-based platform, reaches a Q-factor beyond 10
⁵
.