Long-range optical trapping and binding of microparticles in hollow-core photonic crystal fibre

Optically levitated micro-and nanoparticles offer an ideal playground for investigating photon–phonon interactions over macroscopic distances.Here we report the observation of long-range optical binding of multiple levitated microparticles,mediated by intermodal scattering and interference inside the evacuated core of a hollow-core photonic crystal fibre(HC-PCF).Three polystyrene particles with a diameter of 1μm are stably bound together with an inter-particle distance of~40μm,or 50 times longer than the wavelength of the trapping laser.The levitated boundparticle array can be translated to-and-fro over centimetre distances along the fibre.When evacuated to a gas pressure of 6 mbar,the collective mechanical modes of the bound-particle array are able to be observed.The measured interparticle distance at equilibrium and mechanical eigenfrequencies are supported by a novel analytical formalism modelling the dynamics of the binding process.The HC-PCF system offers a unique platform for investigating the rich optomechanical dynamics of arrays of levitated particles in a well-isolated and protected environment.

Broadband mid-infrared supercontinuum generation in dispersion-engineered As_(2)S_(3)-silica nanospike waveguides pumped by 2.8 μm femtosecond laser

Broadband mid-infrared(IR)supercontinuum laser sources are essential for spectroscopy in the molecular fingerprint region.Here,we report generation of octave-spanning and coherent mid-IR supercontinua in As_(2)S_(3)-silica nanospike hybrid waveguides pumped by a custom-built 2.8μm femtosecond fiber laser.The waveguides are formed by pressure-assisted melt-filling of molten As_(2)S_(3) into silica capillaries,allowing the dispersion and nonlinearity to be precisely tailored.Continuous coherent spectra spanning from 1.1μm to 4.8μm(30 dB level)are observed when the waveguide is designed so that 2.8μm lies in the anomalous dispersion regime.Moreover,linearly tapered millimeter-scale As_(2)S_(3)-silica waveguides are fabricated and investigated for the first time,to the best of our knowledge,showing much broader supercontinua than uniform waveguides,with improved spectral coherence.The waveguides are demonstrated to be long-term stable and water-resistant due to the shielding of the As_(2)S_(3) by the fused silica sheath.They offer an alternative route to generating broadband mid-IR supercontinua,with applications in frequency metrology and molecular spectroscopy,especially in humid and aqueous environments.

Optomechanical cooling and self-stabilization of a waveguide coupled to a whispering-gallery-mode resonator

Laser cooling of mechanical degrees of freedom is one of the most significant achievements in the field of opto-mechanics.Here,we report,for the first time to the best of our knowledge,efficient passive optomechanical cooling of the motion of a freestanding waveguide coupled to a whispering gallery-mode(WGM)resonator.The waveguide is an 8 mm long glass-fiber nanospike,which has a fundamental flexural resonance atΩ/2π=2.5 kHz and a Q-factor of 1.2×10^5.Upon launching^250μW laser power at an optical frequency close to the WGM resonant frequency,we observed cooling of the nanospike resonance from room temperature down to 1.8 K.Simultaneous cooling of the first higher-order mechanical mode is also observed.The strong suppression of the overall Brownian motion of the nanospike,observed as an 11.6 dB reduction in its mean square displacement,indicates strong optomechanical stabilization of linear coupling between the nanospike and the cavity mode.The cooling is caused predominantly by a combination of photothermal effects and optical forces between nanospike and WGM resonator.The results are of direct relevance in the many applications of WGM resonators,including atom physics,optomechanics,and sensing.