Interaction of laser-cooled 87^{87}Rb atoms with higher order modes of an optical nanofiber

Optical nanofibres are used to confine light to subwavelength regions and are very promising tools for the development of optical fibre-based quantum networks using cold, neutral atoms. To date, experimental studies on atoms near nanofibres have focussed on fundamental fibre mode interactions. In this work, we demonstrate the integration of a few-mode optical nanofibre into a magneto-optical trap for $^{87}$Rb atoms. The nanofibre, with a waist diameter of $\sim$700 nm, supports both the fundamental and first group of higher order modes and is used for atomic fluorescence and absorption studies. In general, light propagating in higher order fibre modes has a greater evanescent field extension around the waist in comparison with the fundamental mode. By exploiting this behaviour, we demonstrate that the detected signal of fluorescent photons emitted from a cloud of cold atoms centred at the nanofibre waist is larger ($\sim$6 times) when higher order guided modes are considered as compared to the fundamental mode. Absorption of on-resonance, higher order mode probe light by the laser-cooled atoms is also observed. These advances should facilitate the realisation of atom trapping schemes based on higher order mode interference.Comment: 11 pages, 8 figure

Optical binding of particles in the evanescent field of microfiber modes

We investigated the optical binding between dielectric microparticles in the evanescent fields of the first group of higher order microfiber modes. Particle groups consisting of up to five particles were propelled along the fiber and neighboring interactions were experimentally investigated and supported by numerical simulation

Light-induced rotation of dielectric microparticles around an optical nanofiber

Evanescent electromagnetic fields near a waveguide can exert a transverse radiation force on scattering objects. To prove this experimentally, we demonstrate light-induced orbiting of isotropic, dielectric microparticles around an optical nanofiber that guides elliptically polarized, fundamental modes. The orbit frequency is proportional to the helicity of the coupled light. Interestingly, the observed motion is opposite to the energy flow circulation around the fiber. This result verifies the theoretically predicted negative optical torque on a sufficiently large particle in the vicinity of a nanofiber

Nonlinear force dependence on optically bound micro-particle arrays in the evanescent fields of fundamental and higher order microfibre modes

Particles trapped in the evanescent field of an ultrathin optical fibre interact over very long distances via multiple scattering of the fibre-guided fields. In ultrathin fibres that support higher order modes, these interactions are stronger and exhibit qualitatively new behaviour due to the coupling of different fibre modes, which have different propagation wave-vectors, by the particles. Here, we study one dimensional longitudinal optical binding interactions of chains of 3 μm polystyrene spheres under the influence of the evanescent fields of a two-mode microfibre. The observation of long-range interactions, self-ordering and speed variation of particle chains reveals strong optical binding effects between the particles that can be modelled well by a tritter scattering-matrix approach. The optical forces, optical binding interactions and the velocity of bounded particle chains are calculated using this method. Results show good agreement with finite element numerical simulations. Experimental data and theoretical analysis show that higher order modes in a microfibre offer a promising method to not only obtain stable, multiple particle trapping or faster particle propulsion speeds, but that they also allow for better control over each individual trapped object in particle ensembles near the microfibre surface

Evaluation of the hepatotoxicity of Psoralea corylifolia L. based on a zebrafish model

Objective:Psoralea corylifolia L. (FP) has received increasing attention due to its potential hepatotoxicity.Methods: In this study, zebrafish were treated with different concentrations of an aqueous extract of FP (AEFP; 40, 50, or 60 μg/mL), and the hepatotoxic effects of tonicity were determined by the mortality rate, liver morphology, fluorescence area and intensity of the liver, biochemical indices, and pathological tissue staining. The mRNA expression of target genes in the bile acid metabolic signaling pathway and lipid metabolic pathway was detected by qPCR, and the mechanism of toxicity was initially investigated. AEFP (50 μg/mL) was administered in combination with FXR or a peroxisome proliferator-activated receptor α (PPARα) agonist/inhibitor to further define the target of toxicity.Results: Experiments on toxic effects showed that, compared with no treatment, AEFP administration resulted in liver atrophy, a smaller fluorescence area in the liver, and a lower fluorescence intensity (p < 0.05); alanine transaminase (ALT), aspartate transaminase (AST), and γ-GT levels were significantly elevated in zebrafish (p < 0.01), and TBA, TBIL, total cholesterol (TC), TG, low-density lipoprotein cholesterol (LDL-C), and high-density lipoprotein cholesterol (HDL-C) levels were elevated to different degrees (p < 0.05); and increased lipid droplets in the liver appeared as fatty deposits. Molecular biological validation revealed that AEFP inhibited the expression of the FXR gene, causing an increase in the expression of the downstream genes SHP, CYP7A1, CYP8B1, BSEP, MRP2, NTCP, peroxisome proliferator-activated receptor γ (PPARγ), ME-1, SCD-1, lipoprotein lipase (LPL), CPT-1, and CPT-2 and a decrease in the expression of PPARα (p < 0.05).Conclusion: This study demonstrated that tonic acid extracts are hepatotoxic to zebrafish through the inhibition of FXR and PPARα expression, thereby causing bile acid and lipid metabolism disorders

Particle manipulation using ultrathin optical fibres

Optical manipulation in the vicinity of ultrathin optical fibres has shown potential across several fields including control and delivery of dielectric, metallic and biological microscopic objects, and cold atom probing and trapping. The unique properties of ultrathin fibres, such as strong field confinement, large evanescent fields in the transverse plane, and interaction lengths greater than the Rayleigh range, are key factors needed for the study of fibre-based lightmatter interactions. Differences in the evanescent fields between the various tapered fibre modes give rise to differing interactions with particles. In this thesis, we studied the propulsion of polystyrene particles (diameters of 1 μm ~ 5 μm) in the evanescent field of higher order modes and the fundamental mode in aqueous solutions. For a power of 25 mW, particles were propelled eight times faster in the case of the higher order modes when compared to the fundamental mode. The dependency of particle speed on particle type was also investigated. We further explored the longitudinal optical binding interactions of chains of 3 μm polystyrene spheres in the evanescent fields of the fundamental and the higher order modes of a microfibre. Using a theoretical model based on a scattering-matrix approach, the optical forces, optical binding interactions, and the velocity of bounded particle chains were calculated and compared with experimental observations. Additionally, we examined the transfer of angular momentum of the fundamental mode and the higher order modes of a microfibre on polystyrene microparticles. For a fundamental mode with circular polarisation, the transfer of spin angular momentum was evident via the rotation of particles around the fibre. Furthermore, we investigated the angular momentum of selectively excited higher order modes and its transfer to trapped particles. The results are useful in understanding the evanescent behaviour of different tapered fibre modes