Hydrogen Detection Using a Functionalized Photonic Crystal Vertical Cavity Laser

A thin layer of palladium (Pd) was deposited on the surface of an etched air hole photonic crystal (PhC) vertical cavity surface emitting laser (VCSEL) to form a hydrogen gas (H₂) sensor. H₂ reacts with Pd, forming PdH_x, which induces a shift in the complex refractive index. With Pd on its surface, the reflectivity of the top distributed Bragg reflector depends on the H₂ concentration. We present device operation theory and experimental results showing a 60% output power increase and a 52 pm redshift of the lasing wavelength at 4% H₂. Since the PhC VCSEL laser is single mode and the wavelength shift is comparable to the laser linewidth, the sensor can provide an accurate quantification of the H₂ concentration over the range of 0-4% H₂.