Monitoring photonic nanojets from microsphere arrays by femtosecond laser ablation of thin films

By comparing finite-difference time-domain near field simulations and femtosecond laser ablation of thin films, we characterize in three dimensional-space photonic nanojets from microsphere arrays. We demonstrate periodic drilling of transparent films with thickness up to 100 nm (onto absorbing substrates) is feasible with 1-microm diameter silica spheres. Working with larger polystyrene spheres, the apparent increase of the propagation length of the photonic nanojets makes possible to drill films as thick as 500 nm. Interestingly, the lateral width of the produced craters can be maintained below 400 nm evidencing the low divergence of the nanojets. For backside illumination of the arrays, the ablation features are located at the top of the microspheres. We reveal field enhancements in and out the spheres as well as laser energy confinement at the particle substrate interface. The wide variety of features observed in the experiments open routes to fabricating nanomaterials.     

Controlling the electric field enhancement factor of photonic nanojets by using the magneto-optical effect

In recent years, many researchers have studied photonic nanojets. These nanojets are created when an incident plane wave is focused into a narrow and high intensity emerging optical beam leaving a micro-object (e.g. microcylinder). These narrow beams may find applications in particle imaging and detection, optical sensors, enhanced Raman scattering, and particle manipulation. They also allow the projection of a particle to the far-field where it can be easily visualized. In this paper, it is shown that the electric field enhancement factor can be dynamically controlled by the application of an external intense magnetic flux density.     

Plasmon-enhanced sub-wavelength laser ablation: plasmonic nanojets

In response to the incident light's electric field, the electron density oscillates in the plasmonic hotspots producing an electric current. Associated Ohmic losses raise the temperature of the material within the plasmonic hotspot above the melting point. A nanojet and nanosphere ejection can then be observed precisely from the plasmonic hotspots.     

Geometrical Optics

A previous paper [3] demonstrated that, in principle, holography is a promising technique for use in the measurement of coherence. Its usefulness in practice for obtaining reasonable quantities of accurate data had still to be established. In this paper some of the considerations which lead to an effective design for a holographic arrangement for measuring coherence are discussed. Of principal importance among these considerations are the total amount and accuracy of the information that can be obtained from the hologram. Examples are given of suitable holographic arrangements, and procedures are described to enable the accumulation of reasonably accurate coherence data. Graphs which plot the measured distribution of coherence for two pulsed lasers are shown.