Photonic nanojets for laser surgery

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 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 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.

Figure 1. (a) Packing of 125μm 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 λ=0.63μm with microcapillary tube in contact with tissue.

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 in size along the chain, reaching wavelength-scale dimensions even for noncollimated input beams. We studied the periodicity, 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 advantage of chains of microspheres over single lenses in light-focusing applications is illustrated in Figure 1(b) for high-index (n = 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 structures can focus light into tissue in ‘contact mode’ that provides laser-scalpel interaction with tissue in close proximity to the end sphere. We used a gel-like medium to mimic the optical properties of a biological tissue, as illustrated in Figure 1(c). Such structures permit focused beams with spot sizes of several wavelengths and with a treatment depth of approximately 10–20μm in tissue, as shown in Figure 1(d). We investigated the light-attenuation properties of the chains of 50μm polystyrene beads inside capillary tubing using local light sources (dye-doped spheres) and imaging through the sidewall, as illustrated in Figure 2(b). The measured attenuation data we found was in agreement with the results obtained by ray-tracing software⁵ (ZEMAX-EE), as shown in Figure 2(c).

Figure 2. (a) Packing of 50μm polystyrene spheres inside a microcapillary. (b) Scattering image illustrating optical transport. (c) Experimental (Exp., circles) and calculated (squares) attenuation of light (in arbitrary units, a.u.).

Simple integration of microsphere arrays with flexible fibers and hollow waveguides permits sharp focusing of a light beam and operation in contact mode with tissue. Thus, microsphere arrays can be used in a variety of biomedical and photonics applications as a compact focusing tool. Potential applications include ultraprecise laser procedures on the eye and brain or piercing a cell, and the coupling of light into photonic nanostructures. We are pursuing this research to develop a novel optical scalpel for ultraprecise ophthalmic laser surgery.

We gratefully acknowledge support for our work from the US Army Research Office under contract W911NF-09-1-0450 (J. T. Prater) and from the National Science Foundation under grant ECCS-0824067. This work was also partially supported by funds provided by the University of North Carolina at Charlotte.