High refractive index immersion liquid for superresolution 3D imaging using sapphire-based aplanatic numerical aperture increasing lens optics.

Optically transparent immersion liquids with refractive index (n∼1.77) to match the sapphire-based aplanatic numerical aperture increasing lens (aNAIL) are necessary for achieving deep 3D imaging with high spatial resolution. We report that antimony tribromide (SbBr3) salt dissolved in liquid diiodomethane (CH2I2) provides a new high refractive index immersion liquid for optics applications. The refractive index is tunable from n=1.74 (pure) to n=1.873 (saturated), by adjusting either salt concentration or temperature; this allows it to match (or even exceed) the refractive index of sapphire. Importantly, the solution gives excellent light transmittance in the ultraviolet to near-infrared range, an improvement over commercially available immersion liquids. This refractive-index-matched immersion liquid formulation has enabled us to develop a sapphire-based aNAIL objective that has both high numerical aperture (NA=1.17) and long working distance (WD=12 mm). This opens up new possibilities for deep 3D imaging with high spatial resolution.

Enhanced transmission with tunable Fano-like profile in magnetic nanofluids.

We observe a Fano-like resonance in a magnetically polarizable nanofluid. Under an external magnetic field, the transmittance spectrum of a ferrofluid emulsion containing droplet size of ~220 nm shows an enhanced peak with a Fano-like profile, which is attributed to a localized waveguide resonance from random array of tubes with charged inner surface that are formed by the alignment of the droplets. Furthermore, by varying the magnetic field, the Fano profile is tuned and an opaque emulsion is turned into a transparent one. This finding may have interesting applications in tunable photonic devices.

Experimental investigation of magnetic-field-induced aggregation kinetics in nonaqueous ferrofluids.

We investigate the influence of field ramp rate on the kinetics of magnetic dipole-dipole induced chainlike structure formation in a nonaqueous nanoparticle dispersion using light scattering studies. With increase in magnetic field, at a constant ramp rate, the transmitted light intensity diminishes and the transmitted light spot is transformed to a diffused ring due to scattering from the self-assembled linear aggregates. The decay rate of transmitted intensity increases up to an optimum ramp rate, above which the trend becomes reverse. At an optimum ramp rate, the minimum time for initial aggregation coincides with the exposure time where the intensity decay is fastest. The variation of transmitted intensity at different ramp rate is explained on the basis of initial aggregation time that depends on Brownian motion, dipolar magnetic attraction and multibody hydrodynamic interactions. The slope of the transmitted light intensity after the removal of magnetic field depends on the time required for dissociation of ordered linear structures. Disappearance of the ring pattern and the reappearance of original light spot, upon removal of the magnetic field, confirm the perfect reversibility of the linear aggregates. The observed concentration dependant decay rates are in good agreement with aggregation theory.

Experimental evidence for reversible zippering of chains in magnetic nanofluids under external magnetic fields.

We study the time-dependent transmitted intensity and the scattered pattern from magnetic nanofluids at constant ramping of uniform external magnetic field. The nanofluid used is the dispersion of magnetite particles with an average diameter of 6.5 nm with a protective surfactant coating. We observe several critical fields at which the transmitted light intensity decreases drastically followed by the formation of a ringlike pattern on a screen placed perpendicular to the field direction. Interestingly, the critical fields occur at a regular interval of 20 G. The observed critical fields are attributed to zippering transitions of the chains due to attractive energy well when the chains are of different lengths or shifted with respect to one another. Interaction energy calculations show a decrease in the energy of the system due to dipolar interactions at different critical fields confirming the lowering of the system energy through lateral coalescence. The observed zippering phenomenon is perfectly reversible.

Light scattering in a magnetically polarizable nanoparticle suspension.

We investigate magnetic-field-induced changes on transmitted light intensity in a magnetic disordered phase of iron oxide nanoparticle suspension. We observe a dramatic decrease in the transmitted light intensity at a critical magnetic field. The critical magnetic field follows power-law dependence with the volume fraction of the nanoparticles suggesting a disorder-order structural transition. The light intensity recovers fully when the magnetic field is switched off. We discuss the possible reasons for the reduction in the light intensity under the influence of magnetic field. Among the various mechanisms such as Kerker's condition for zero forward scattering, Faraday effect, Christiansen effect, photoinduced refractive index mismatch between the two components of the dispersion, etc., the resonances within the magnetic scatterers appear to be the plausible cause for the extinction of light. The circular pattern observed on a screen placed perpendicular to the incident beam confirms the formation of rodlike structures along the direction of propagation of the light.