Lithium niobate photonic crystal cavity for chemical detection

We present a single hole defect (SHD) cavity based on lithium niobate substrate (LiNbO3) for sensing applications. The physicochemical reaction between the gas and a sensitive absorbent layer induces not only refractive index and thickness layer variations, but also a variation in the sensitive layer's absorption. Plane wave expansion (PWE) and finite difference time domain (FDTD) simulations were performed to study the effect of the absorption on the transmission spectrum of the cavity. The proposed structure consists of a biperiodic triangular lattice of circular air holes on a lithium niobate substrate, in which a defect is introduced by reducing the center pore radius. A third medium, corresponding to the porphyrin sensitive layer is introduced as a ring-shaped intermediate layer around the holes. The optical properties (refractive index, absorption) of the Porphyrin layer are modified in presence of benzene molecules, which induces intensity variation of the transmitted light through the cavity. In view of enhancing the intensity variation, the parameters of the cavity are determined to make the resonance peak of the cavity coincides with the absorption peak of the sensitive layer at l=420 nm. The sensitivity of the structure to changes in the thickness of the porphyrin was evaluated by the PWE-Supercell method. The results show that a variation in thickness of 1 nm of the sensitive layer implies a shift of 0.5 nm of the resonance peak, at the operating wavelength of 419.5 nm 2D-FDTD calculations based on a homemade code were performed to analyze the resonant mode. From this study we can conclude that the resonance peak encounters a variation in transmittivity of 23% when the Porphyrin layer is exposed to 50ppm of benzene, which corresponds to a variation of the refractive index of 10- 7 and a variation of 6×10-4 for the extinction coefficient. Consequently the sensitivity of the device is estimated to be about 3 ppm. In order to validate the feasibility of the device, we have developed and fabricated different cavities on a lithium niobate substrate at l=1.55nm operating wavelength, which is a preliminary step before the development of cavities operating at 419.5nm wavelength. The cavities were patterned on a proton exchanged waveguide by FIB milling. The resulting experimental transmission spectrum through the PhC cavity were performed and compared to the FDTD-2D theoretical transmission through the same structure. A good agreement is obtained between theoretical and experimental results, confirming the existence of two gaps lying in the wavelength ranges [720nm-830nm] and [1280nm-1900nm] respectively. Complementary experimental results will be presented, which show the sensitivity of the transmitted spectrum to geometrical variations of the PhC structure.