Figure 8: Subsurface sensing: illumination of a Si wafer from below by μm. The incident light scatters through cracks in the wafer. (a) A subwavelength dimension crack in a Si wafer can be detected by a near-field scanning optical microscope (NSOM) only if the detector is brought very close to the crack (< /6). (b) If an AlInAs/InGaAs HMM is placed above the wafer, then the NSOM can detect the crack up to from the wafer. The HMM allows high spatial frequencies to propagate and enhances their intensity in the image.