$L_{n}$ Slot Photonic Crystal Microcavity for Refractive Index Gas Sensing

We propose and experimentally demonstrate a series of
$L_{n}$
slot photonic crystal (PhC) microcavities, which operate as refractive index (RI) gas sensors. The cavities are simply composed of a silicon slab triangular photonic crystal with
$n$
holes replaced by a slot, which do not require sophisticated design or high fabrication resolution. With the increase in
$n$
, the quality factor of the cavity exponentially increases, which is explained by the envelope of electric field approaching a Gaussian profile. An
$L_{9}$
slot PhC microcavity with a quality factor exceeding 30 000, sensitivity of 421 nm per RI unit (RIU), and detection limit down to
$1\times 10^{-5}$
RIU was experimentally demonstrated. The performance of the device is comparable with other fine-tuned PhC microcavity structures. Due to its simple structure and high fabrication tolerance, it could have wide applications in optical sensors.