Design of a nonlinear metastructure for temperature detection and biosensing based on the second harmonic generation in theory

When the second harmonic generation (SHG) occurs in temperature-controlled ferroelectric crystals, it serves as an effective method for detecting temperature and changes in the refractive index (RI). A one-dimensional nonlinear metastructure (NM) composed of periodically poled lithium niobate crystals is designed precisely to exhibit significant SHG enhancement in the visible to near-infrared spectral range. When a tunable fundamental electromagnetic wave is incident at a frequency (<i>f</i>) near 241.067 THz, the NM achieves frequency conversion and enables wide-range temperature detection (25-105 °C) at the second harmonic wave (SHW) scale 2<i>f</i> with a sensitivity of <i>S</i> = 0.32% per °C. By altering the filling material of the sample layer, it is possible to distinguish between three types of healthy or cancerous liver tissues using RI biosensing with a fundamental wave around 273 THz. The sensing range is from 1.3463 to 1.3655, with an <i>S</i> value of 14.75 THz per RIU. The proposed NM integrates the SHW effect with optical detection and sensing, offering new insights for SHW applications and biomedical fields.