Selective Absorption of Circularly Polarized Light in Broad Bands by the Zeeman Components of Divalent Thulium in Calcium Fluoride

The paramagnetic circular dichroism of the $4f-5d$ absorption bands of Ca${\mathrm{F}}_2$:${\mathrm{Tm}}^{2+}$ was investigated at 2.0°K and 9.0kG. Using group theory it is shown that the bands can be separated into $E_{\frac{5}{2}}$ and $G_{\frac{3}{2}}$ character. Experimentally it was found that in this case the bands are primarily $G_{\frac{3}{2}}$ in character. Optical pumping experiments were performed where changes as large as ±20% were easily observed in the electron-paramagnetic-resonance absorption signal at $X$-band frequencies and 1.5°K by changing the polarization of the pump light from right to left circular. A method for optically monitoring the difference in the population of the two Zeeman components of the ground state is also reported on.