Photonic nanojets for laser surgery

Abstract

Focusing of light is widely used in optical microprobes where a stable and well-confined beam of photons is scanned or directed over some area of a biological sample or photonic structure. Such focusing microprobes may be useful for laser surgery, optical endoscopy and spectroscopy, high-density optical-data storage, and photo-induced patterning of thin films. A fundamental principle of diffraction-limited optics is that the spatial resolution of focusing devices is limited by the wavelength of the incident light and by the aperture of the objective-lens system. Spatial resolution beyond the classical optical diffraction limit can be obtained using near-field optical phenomena1 or special material properties such as fluorescent, nonlinear,2 or negative refractive-index3 effects. However, low-transmission of nearfield microprobes and difficulties in realizing desirable material properties limit the usefulness of these approaches for many biomedical and photonic applications. A few years ago, it was demonstrated that a small wavelength-scale microsphere with a refractive index of approximately 1.6 produces a narrow focused beam, termed a ‘nanoscale photonic jet.’4 Such a photonic nanojet propagates with little divergence for several wavelengths into the surrounding medium, while maintaining a subwavelength transverse beam width. The concept of nanojets is attractive for designing focusing microprobes with high optical-transmission properties. Note, however, that photonic nanojets from single spheres require strictly plane-wave illumination, which is not readily available in devices using flexible optical-delivery systems. More recently, we observed periodic focusing in chains of polystyrene microspheres assembled on substrates.6, 7 In these chains, the photonic nanojets were quasi-periodically reproduced along the chain, giving rise to novel ‘nanojet-induced modes’ (NIMs). We saw that the coupled nanojets reduced Figure 1. (a) Packing of 125 m spheres inside a microcapillary tube. (b) Ray tracing of paraxial (red) and skew (blue and green) beams for microspheres with refractive index n=1.9 performed by ZEMAX-EE software.5 Only transmitted rays are shown. (c) Microprobe inserted in a gel. (d) Focusing at a wavelength D 0:63 m with microcapillary tube in contact with tissue.