Optical detection of ethanol vapor using a porous silicon Bragg reflector in the SWIR region

In this study, a porous silicon Bragg reflector was designed and evaluated as an optical sensor for ethanol vapor detection in the short-wave infrared (SWIR) region. The multilayer structure, fabricated through electrochemical etching on p-type silicon substrates, consisted of alternating high- and low-porosity layers to form a periodic refractive index profile. A central microcavity was incorporated to the enhance optical resonance sensitivity. Ethanol vapor exposure experiments were conducted at room temperature using a controlled bubbler-based delivery system, and the reflectance spectra were monitored in real time. The results showed that ethanol infiltration into the porous matrix caused a redshift in the resonance wavelength due to increased effective refractive indices. A maximum shift of 155 nm was recorded at an ethanol concentration of 11.5%, whereas a reversible 10 nm shift was observed at a concentration of 1.2%. Upon nitrogen purging, the resonance wavelength returned to its original position, confirming the reversable nature of the sensing mechanism. The sensor also demonstrated rapid response and recovery times of 2 s and 4 s, respectively, under 1.6% ethanol, although both times increased at higher concentrations. These results highlight the potential of porous silicon Bragg reflectors as fast, reversible, and highly sensitive optical sensors for volatile organic compound (VOC) detection at room temperature, particularly in the SWIR spectral range.