In principle, a microspherical resonator pendulum can be trapped using the optical forces produced by two
simultaneously excited whispering gallery modes of a photonic molecule, comprising two microspherical cavities. The
cavity-enhanced optical force generated in the photonic molecule creates an optomechanical potential that can trap the
pendulum at an equilibrium position by carefully choosing the excitation laser frequencies. In this paper, we study the
mechanical characteristics of microsphere pendulums by evanescently detecting the pendulum motion. Nonlinear effects
like electromagnetically induced transparency and absorption, and Fano resonances have also been observed using a
microsphere-fiber taper system and the results are presented here.
Spherical whispering gallery mode (WGM) resonators can be used in a broad range of applications from bio-sensing to
laser engineering. Beyond the interest for applied studies, such resonators are also of interest for more fundamental
studies, e.g. cavity QED. A key requirement for many applications is the ability to tune the resonator to an energy
transition of the atomic species (or material) under investigation. Heretofore, heating the cavity with an external heater,
or deforming the cavity mechanically, have been the two main approaches used to tune the cavity size. We demonstrate
thermo-optical methods of tuning the WGM resonance frequencies of doped glass microspheres over a very large
dynamic range. Er:Yb phosphate glass (IOG2) microspheres are pumped at 978 nm via a tapered optical fibre. This
causes internal heating of the microsphere and the temperature of the mode volume can reach temperatures higher than
800°C. With the heat concentrated in the optical mode volume, the resonance frequency has been tuned by ~700 GHz
nonlinearly. Alternatively, we show that large linear tuning up to ~488 GHz is achievable if the microsphere is
separately heated by coupling laser light into its support stem.
We demonstrate upconversion lasing and fluorescence from active microspheres fabricated from a novel fluorozirconate,
Er3+-doped glass, ZBNA, when pumped around 978 nm through a tapered optical fibre. An ultralow, green lasing
threshold of ~3 μW for 550 nm emissions is measured. This is one order of magnitude lower than that previously
obtained for ZBLAN microspheres. Optical bistability effects observed within the microspheres indicate that this
material is suitable for low-frequency, all-optical switching. The bistable mechanism is discussed and attributed to shifts
of the resonances due to thermal expansion of the sphere, where the heat is generated by phonon transitions excited after
optical pumping around 978 nm. We also report multiple bistability loops within the microspheres. In a separate
experiment, the latching behaviour of the microspheres is illustrated.
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