Off-axis parabolic mirror optics for polarized Raman spectroscopy at low temperature

We report the development of a detection optics for the integration of Raman scattering and scanning probe microscopy at low temperature based on a parabolic mirror. In our set-up half of the paraboloid mirror covers a solid angle of $\pi$ corresponding to a numerical aperture of N.A.\,$\approx 0.85$. The optical system can be used for far- and near-field spectroscopy. In the far field the polarizations can be maintained to within 80-90\%. In combination with a scanning microscope (AFM/STM), tunneling or near-field experiments are possible with less than 10\% loss of aperture. Our set-up provides ideal conditions for the future development of tip-enhanced Raman spectroscopy (TERS) at low temperature.