Optical Properties of Cr Thin Films as Substrates for Microalgae Detection Using Spectroscopic Ellipsometry based Sensor

Developing sensitive and reliable sensors for detecting microalgae is vital for various environmental and industrial applications. This study investigates the potential of chromium (Cr) thin films as substrates in spectroscopic ellipsometry (SE)-based sensors. Optical properties of Cr thin films (20–75 nm) were characterized using SE, showing a strong thickness dependence that directly impacts their suitability for sensing technologies. For 75 nm Cr thin films, the optical constants n and k closely align with those of bulk Cr, resulting in high reflectance, low absorption, and stable performance across a broad light spectrum, making 75 nm film particularly well-suited for multi-wavelength sensing applications. In contrast, as the thickness decreases, quantum confinement effects become increasingly pronounced, particularly at 30 nm and 20 nm. This leads to modified optical responses, such as increased n and k particularly at specific photon energy ranges, due to enhanced electron-photon interactions. The tunable optical properties of Cr thin films make them adaptable for applications requiring customized responses. Optimal conditions for microalgae detection were identified with a Cr thin film thickness of 75 nm, and photon energy range of 2.65‒3.30 eV. This was confirmed by a significant change in the ellipsometry parameter Δ (40.63°), indicating high sensitivity under these parameters. These findings pave the way for the development of highly sensitive and specific SE-based sensors using Cr thin films as substrates, offering promising advancements in the detection of microalgae for environmental and industrial applications.