First steps towards CO2 gas micro-sensors operating at 4.26 µm

In a context where the control of gases becomes important in a wide range of applications - health care, industry, housing, transportation, environment, etc. - we have in sight the realization of infrared optical micro-sensors. In particular, we wish to develop an optical micro-sensor operating at the wavelength 4.26 µm, wavelength corresponding to an absorption band of carbon dioxide, the main greenhouse gas. The first step consists in manufacturing straight waveguides, but also circuits such as Y-junctions or interferometers, that are capable of operating at this wavelength. The straight waveguides and other guiding structures are obtained by stacking and etching of layers of the ternary system Ge-Se-Te, a chalcogenide system widely studied for its transparency properties in the infrared. Manufacturing objects are realized by: (i) depositing a first low refractive index Ge-Se-Te layer (buffer layer) on a Si substrate by thermal co-evaporation; (ii) depositing a second layer Ge-Se-Te characterized by a higher refractive index (guiding layer), again by thermal co-evaporation, and (iii) modifying the geometry of the second layer by laser lithography and ion beam etching. The waveguides opto-geometrical parameters such as refractive indices, thicknesses of the layers, etching depth and waveguide core width are set through a design process to obtain a single mode behavior at 4.26 µm. After fabrication, objects are optically characterized at λ = 4.26 µm on a bench dedicated to the study.