Abstract

Mid-IR nanoscopy has proved its ability to recognize material at nanometer scale through spectroscopic or even single wavelength imaging [1,2]. A high optical contrast can indeed be obtained between different materials as the permittivity of solid materials exhibit strong variations near phononic resonances, usually below 1000cm⁻¹. One limitation of the technique is that a weak contrast is obtained if the permittivities of two materials are too close, which can be the case for example far from phonon resonances. As another example, doping imaging in silicon is difficult still difficult at 10.6μm, although the free electrons or holes can modify notably the permittivities as shown in figure 1. It has been shown previously that even longer wavelengths seem to be necessary to image unambiguously different doped zones [3,4] at least with common near-field probes. However, in a recent paper we have calculated that the phonon confinement occurring in decananometric scale objects such as near-field probes can lead to a drastic modification of the probe’s permittivity [5]. This effect can be very useful as for a range of probe permittivity, much stronger contrast can be expected. To experimentally demonstrate these theoretical expectations, we have built a mid-IR near field microscope and imaged doped Si gratings with a period of 2 μm (fabricated in CEA-LETI) with carefully prepared tungsten tip stuck on a tuning fork. The detection has been done using tips with different radius (30 nm to 100 nm) and a very strong contrast between P doped and P+ doped can be observed for a variety of small-radius probes. An example of highly contrasted profile obtained on the doped silicon grating is shown in figure2.

© 2011 Optical Society of America