Fluid detection based on liquid crystals-controlled tunable layered hyperstructures for second harmonic generation

A detection device composed of multi-layer nonlinear hyperstructures and liquid crystals is proposed in this paper to achieve second harmonic generation. Introducing defect layers leads to sharp transmission peaks within the electromagnetic band gap. In the initial state and full bias states of the liquid crystals, the detection ranges of the transmission peaks of second harmonic resonance (exceeding 0.9) for refractive index are 1.27∼1.30 refractive index unit (RIU) and 1.30∼1.33 RIU, respectively, with sensitivities of 40.71 THz/RIU and 38.44 THz/RIU. The generation of defect results in localized electromagnetic wave energy, enhancing the second harmonic generation conversion efficiency. At this point, in the initial state and full bias states of the liquid crystals, the second harmonic generation conversion efficiency can be detected with sensitivities of 39 THz/RIU and 44.72 THz/RIU, respectively, within the range of 1.4491∼1.4758 RIU and 1.3337∼1.3395 RIU, enabling fluid detection based on second harmonic mode for various types of oils and model aqueous solutions, including their constituent components. With its non-contact, label-free, and high-sensitivity detection capabilities, this method holds significant value in electromagnetic detection and component recognition.