This study proposes a one-dimensional defective photonic crystal (1D-DPC) biosensor based on the phasechange material Ge2Sb2Te5 (GST) for label-free detection of thyroid nodules in the visible range. The
structure is designed as [(Diamond/Silica)3 GDG (Diamond/Silica)3
], where G represents GST layers and D
is an air-cavity defect. This design effectively distinguishes between benign and malignant thyroid
nodules. The main advantage of GST is its reversible phase transition between amorphous and crystalline
states, which changes the material's refractive index and absorption coefficient. By placing two GST
buffer layers around the defect, the biosensor's resonance can be actively tuned, leading to higher
sensitivity and better spectral selectivity. Transmission spectra were calculated using the transfer matrix
method (TMM) in MATLAB for both normal and oblique TE-polarized incidence. Performance metrics—
including sensitivity (S), quality factor (Q), and figure of merit (FOM)—were evaluated for both GST
phases. At normal incidence and 220 nm defect thickness in the crystalline state, the biosensor achieved
a sensitivity of 267 nm RIU−1 and a quality factor of 87.875, enabling clear differentiation between
malignant and benign cases. The key novelty is comparing THz spectra of liquid and lyophilized plasma,
where lyophilization enhances contrast and malignant-benign separation by reducing water masking
effects. The results were compared with previous photonic crystal biosensors, showing that the GSTbased design offers superior tunability and diagnostic accuracy due to its unique phase-change properties.