optical characterisation of rare-earth doped fluoride and phosphate ...

3example, in the case of a typical doped microsphere with a Q factor of 10 6 , for 1 mWlight coupled in, the cavity mode power can be amplied by a factor of 10 4 to 10W. In passive cavities, with much higher Q factors, the mode can reach up to kW'sof power. Early work on nonlinear processes in liquid microdroplets by Lin et. al[8] demonstrated stimulated Raman scattering. Stimulated Brilluoin scattering wasshown by Zhang and Chang [9].Others had also shown very low threshold powersfor dispersive bistability in microspheres, which had the eect of overwhelmingany observable Kerr nonlinearity at room temperature. Later, Treussart et al. [10]produced measurements for Kerr nonlinearity in a silica microsphere at 2 K. Followingthe development of ultra-high Q resonators, along with bre taper couplingtechniques, several studies realised low threshold, high eciency stimulated Ramanemission [11], parametric oscillations [12], and cascaded Raman lasing [13] in silicamicrospheres and microtoroids, as well as a measurement of the Kerr nonlinearity atroom temperature [14].Optical bistability (OB) in a sodium vapour was rst reported by Gibbs in 1976[15] and, since then, numerous other materials exhibiting the phenomenon have beenstudied, including Yb 3+ doped glasses and crystals, and semiconductors [16]-[22]. Adevice with this attribute must have a nonlinear process and a feedback mechanismthat causes the output signal to operate in either of two stable output states thatare dependent on the history of the input signal. The mechanisms responsible fornonlinearity in doped glasses and crystals are varied, with many requiring cryogenictemperatures (typically < 40 K) to maintain the necessary low atomic decay ratesin Yb 3+dimer and monomer systems [17]. Alternative and more easily achievablemechanisms include photon avalanche and thermal avalanche [16, 19], suggesting that