Abstract: The finite-difference time-domain(FDTD) method is employed to numerically examine the effect of various SiO2 trench sizes on the absorptance of silicon crystal wafers. Effective medium theory (EMT) is also used to help further explain absorptance predictions. The results show that EMT can readily predict spectral absorptance for sub-wavelength gratings. The cavity resonance effect may increase absorptance as trench width increases. The absorptance curves had strong oscillations at short wavelength when the period was comparable to the wavelength and the ratio of SiO2 trench width to the period was not too small.