No abstract

Whispering gallery mode lasers are of interest for a wide range of applications and especially biological sensing, exploiting the dependence of the resonance wavelengths on the surrounding refractive index. Upon lasing, the Q factors of the resonances are greatly improved, enabling measurements of wavelength shifts with increased accuracy. A way forward to improve the performance of the refractive index sensing mechanism is to reduce the size of the optical resonator, as the refractive index sensitivity is inversely proportional to the resonator dimensions. However, as the lasing threshold is believed to depend on the Q factor among other parameters, and the reduction of the microresonator size results in lower Q, this poses additional challenges for reaching the lasing threshold. In this letter, we demonstrate lasing in 10 μm diameter dye doped polystyrene microspheres in aqueous solution, the smallest polystyrene microsphere lasers ever reported in these conditions. We also investigate the dependence of the lasing threshold on the Q factor by changing the refractive index surrounding the sphere, highlighting a much stronger dependency than initially reported.

Tellurite glass fibers with embedded nanodiamond are attractive materials for quantum photonic applications. Reducing the loss of these fibers in the 600-800 nm wavelength range of nanodiamond fluorescence is essential to exploit the unique properties of nanodiamond in the new hybrid material. In the first part of this study, we report the effect of interaction of the tellurite glass melt with the embedded nanodiamond on the loss of the glasses. The glass fabrication conditions such as melting temperature and concentration of NDs added to the melt were found to have critical influence on the interaction. Based on this understanding, we identified promising fabrication conditions for decreasing the loss to levels required for practical applications.

Tellurite glass fibers with embedded nanodiamond are attractive materials for quantum photonics applications. Reducing the loss of these fibers in the 600-800 nm wavelength range of nanodiamond fluorescence is essential to exploit the unique properties of nanodiamond in the new hybrid material. The first part of this study reported the origin of loss in nanodiamond-doped glass and impact of glass fabrication conditions. Here, we report the fabrication of nanodiamond-doped tellurite fibers with significantly reduced loss in the visible through further understanding of the impact of glass fabrication conditions on the interaction of the glass melt with the embedded nanodiamond. We fabricated tellurite fibers containing nanodiamond in concentrations up to 0.7 ppm-weight, while reducing the loss by more than an order of magnitude down to 10 dB/m at 600-800 nm.

Traditional optical fibers are insensitive to magnetic fields, however many applications would benefit from fiber-based magnetometry devices. In this work, we demonstrate a magnetically sensitive optical fiber by doping nanodiamonds containing nitrogen vacancy centers into tellurite glass fibers. The fabrication process provides a robust and isolated sensing platform as the magnetic sensors are fixed in the tellurite glass matrix. Using optically detected magnetic resonance from the doped nanodiamonds, we demonstrate detection of local magnetic fields via side excitation and longitudinal collection. This is a first step towards intrinsically magneto-sensitive fiber devices with future applications in medical magneto-endoscopy and remote mineral exploration sensing.The sensing of magnetic fields is important for applications as diverse as mining exploration 1 and aircraft navigation 2 . Within medical fields, applications such as magneto-encephalography 3 and magneto-cardiology 4 are important methodologies for sensing spatially-resolved activity in the brain and heart, respectively. Although existing magnetometers such as superconducting quantum interference devices (SQUIDs) and optical atomic magnetometers are ultrasensitive to magnetic fields 5 , they are constrained in terms of their size, cost and temperature of operation. Optical fibers provide an ambient, robust, cheap and alternate approach for remote magnetic sensing. Optical fiber Bragg gratings have shown sensitivity to magnetic fields with thick coatings of magnetostrictive materials such as Terfenol-D applied in a polymeric matrix 6,7 . Multimode interference effects in spliced square no-core optical fiber surrounded by magnetic fluid also exhibit magnetic sensitivity 8,9 . However, for both of these solutions, the magnetic sensitive materials are external to the optical fibers. Solid state magnetometers based on the negatively-charged nitrogen-vacancy (NV) defect in diamond provide an additional avenue for sensitive magnetic field detection under ambient conditions 5,10 . The size of such systems can be of order nanometers opening up new opportunities for robust, miniature and remote magnetic sensors. Recent work has demonstrated magnetic sensitivities to oscillating fields of ~1 pT/ Hz from sensing volumes of order 100 µm 2 in bulk single crystal diamond 11 and ~290 nT/ Hz for nanodiamond (ND) crystals 12 .The magnetic field sensitivity for NV-based systems is dependent on the fluorescence collection efficiency. Conventional detection approaches utilize high numerical aperture objectives to collect isotropic emission from diamond defect centers. This approach yields a typical collection efficiency of emitted photons around 2% 13 but is limited by the size of the objective and mechanical instabilities. One attractive solution to this problem is to couple the NV fluorescence emission into guided modes of a waveguide, thereby reducing the overall size of the collection optics and enabling integration with mature photonic technologies. Several approach...