Enhanced optical squeezing from quasi-bound states in the continuum and Fano resonances without nonlinearity

To achieve a high degree of quantum noise squeezing, nonlinear optical interaction process is often employed. Here, we propose to utilize quasi-bound states in the continuum (quasi-BICs) and Fano resonances to enhance optical squeezing without nonlinearity. The theory of quantization for electromagnetic fields in the periodic nanostructure with dispersion and absorption has been developed by means of the Green's function technique with the plane wave expansion method. The quasi-BICs and Fano resonances of radiation modes are realized by designing the photonic crystal slab structure. Based on these quasi-BICs and Fano resonances, we demonstrate that strong squeezed states can be realized by using the balanced homodyne detection scheme. The squeezing degree can be improved by more than 14 times when a weak squeezed states passes through the structure with quasi-BICs and Fano resonances. The advantage of this method is that it is not only efficient but also easy to implement because the nonlinear optical processes are not employed, which is very beneficial for the quantum information processing and precision metrology.