Fabrication of solid immersion lens applied to infrared microscopy to improve the spatial resolution over its diffraction limit

Infra-Red (IR) objective achieves a few micrometers of spatial resolution with high Numerical Aperture (NA) of about 0.75, for example, in mid-IR. However, submicron resolution is hard to achieve in Mid-IR because of the long wavelength compared to the visible range. To overcome the limitation, a solid immersion lens (SIL) is incorporated into the conventional objective so that the high refractive index of SIL contributes to obtain the high spatial resolution image of sample immersed in SIL. Germanium is a typical material of SIL in the infrared wavelengths because of the high refractive index and the high transmittance. In our study, we fabricated a Germanium-SIL using the quantified parameters of the ultra precision machining. The parameters are tool rake angle, cutting speed, feed rate, and depth of cut. The surface shape of the fabricated SIL was measured with the accuracy of 0.0376 μm in RMS and 0.3159 μm in PV. We applied the fabricated SIL to a custom IR objective to investigate the improvement of its spatial resolution. Optical performance of the IR objective was evaluated with and without SIL. As results, the IR objective with SIL achieved 1.23 μm of the spatial resolution from the 3.9 μm of IR objective without SIL