Magneto-optical spectroscopy of rare earth doped fluoride glasses

Abstract

An optical spectrometer system was developed to measure magneto-optic rotation (MOR or Faraday effect) and magnetic circular dichroism (MCD) in solid, liquid or gaseous samples. The operation of the spectrometer system is based on the modulation of the polarisation of the light beam by a three-component birefringence modulator. This system could be used over the wavelength range 320 - 850 nm, field strengths from 0 - 0.6 T and at temperatures from 15K to room temperature. Under ideal conditions a minimum Faraday rotation of 0 01° and a minimum MCD of 0.006° could be measured, however the sensitivity of the system decreased if the light throughput was reduced. This system was used to measure the Faraday rotation and MCD spectra of a variety of rare earth (III) ion doped fluorozirconate glasses. The base glass used was the ZBLAN composition (zirconium, barium, lanthanum, aluminium and sodium fluorides). ZBLAN glasses doped with 2% Nd3+, 1% Eu3+ and 1%Ho3+ were grown in a vertical furnace. The Faraday rotation of these glasses (as well as 10% Ce3+, 8% Pr3+ and 6% Dy3+ doped ZBLAN glasses grown at Monash University) was measured at room temperature and ~0 5 T over the wavelength range 400-850 nm. In wavelength regions far from absorption lines the paramagnetic component of the Verdet constant followed the relationship V = K/λ2-λ20. The Faraday rotation per rare earth ion in ZBLAN glasses at 400 and 600nm and the effective transition wavelengths, λ0, were ~30-60% smaller than previously published values for the same ions in phosphate or oxide glasses The temperature dependence of the Faraday rotation was investigated for Ce3+, Pr3+ and Nd3+ doped samples. At low temperatures the Faraday rotation deviated from the Curie law type dependence as a result of thermal depopulation of the higher lying energy levels The MCD spectra of Pr3+ and Nd3+ doped samples at room temperature and low temperatures were compared. The MCD increased at low temperatures, although not by as much as the Curie law would indicate. Further evidence of thermal depopulation could be seen in some transition lines as a change in lineshape as the C term dominates at low temperature The temperature dependence of the Faraday rotation for the Ce3+ doped ZBLAN glass was consistent with the Ce3+ site being at the centre of a square antiprism of F- ions. The crystal field splitting of the 2F5/2 state was deduced from the least squares fit to the temperature dependence of the Faraday rotation. The MJ = ±5/2 states were 31±2 cm-1 above the MJ = ± 3/2 states and the MJ = ±1/2 states 39 ± 3 cm-1 above the MJ = ±3/2 states. This is about an order of magnitude smaller than the crystal field splittings for most crystalline fluorides.