10.1117/2.1201002.002578 Photonic nanojets for laser surgery Vasily N. Astratov, Arash Darafsheh, Matthew D. Kerr, Kenneth W. Allen, Nathaniel M. Fried, Andrew N. Antoszyk, and Howard S. Ying Microspheres inside capillaries or hollow waveguides permit compact focusing of light from multimodal sources. 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 prin- ciple 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 phenomena 1 or special material properties such as fluorescent, nonlinear, 2 or negative refractive-index 3 effects. However, low-transmission of near- field 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 wave- length-scale microsphere with a refractive index of approxi- mately 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 focu- sing microprobes with high optical-transmission properties. Note, however, that photonic nanojets from single spheres re- quire 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 repro- duced 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. in size along the chain, reaching wavelength-scale dimensions even for noncollimated input beams. We studied the periodici- ty, spectral-transmission properties, and losses of NIMs for such chains. 6, 7 We found that beams incident along a chain of microspheres have minimal propagation losses, which results in preferential filtering of modes with the best focusing properties. This ad- vantage of chains of microspheres over single lenses in light- focusing applications is illustrated in Figure 1(b) for high-index (n D 1:9) spheres. It allows the use of multimodal light sources coupled to chains of microspheres that can be integrated with fibers or hollow waveguides. Figures 1(a) and 2(a) show integrated structures assembled inside microcapillaries. Using high-index spheres, such struc- tures can focus light into tissue in ‘contact mode’ that provides laser-scalpel interaction with tissue in close proximity to the Continued on next page