In this paper, we report the light-matter interaction properties of crystalline and amorphous Ge ₂ Sb ₂ Tes (GST) through design and simulation on different substrates based on finite difference time domain method (FDTD). The absorption, reflectance and transmission coefficients of GST have been investigated for various thicknesses of GST on different type substrates. We used silicon as semiconductor, silicon dioxide as insulator, and metal as conductor substrates over GST material. The iterations are performed for the optical radiations of wavelength ranging from 650 nm to 2500 nm (the near-infrared range and a portion of visible radiation). Gold metal of 22 nm thickness is taken due to its high reflectance. Absorbance, reflectance and transmittance have been observed for all the combinations/arrangements and a suitable application range for the photo-detection application is proposed at various platforms which ranges from the CMOS compatible detecting device to SOI based to stand-alone detecting devices. The shift in the peaks of properties is observed in the two stable phases of GST for the tunable photo-detection. A comparison with other simulated and fabricated works is also presented here to substantiate the approach.