Proceedings Article | 6 April 2020
KEYWORDS: Photonic nanostructures, Laser processing, Dielectrics, Optical fibers, Fiber lasers, Single mode fibers, Optical spheres, Etching, Cladding, Wave propagation
The photonic nanojet (PNJ) was first reported by Chen et al.in 2004 through finite-difference time domain modeling of cylindrical structures under plan wave illumination. PNJ is a propagative beam with a full width at half maximum (FWHM) slightly smaller than a half wavelength. The power density can be more than 200 times higher than the one of the incident wave. This concentration can be achieved using a transparent dielectric microcylinder with a wavelength-scale radius. Gustav Mie obtained the exact solution of Maxwell’s equations for a dielectric microsphere in 1908. Applying this method, PNJ in the 3D case of a dielectric sphere was first reported by Li and Lecler. The photonic nanojet appears as a narrow and elongated spot with a high intensity electromagnetic field. The key parameters of PNJs, which are their FWHM, focal distance, decay length, and light intensity maximum, have been the subject of extensive theoretical and experimental studies. An in-depth understanding of photonic nanojet is nevertheless needed to fully exploit the potential of microspheres as optical super-lens. The original properties of PNJs make them ideal candidate in a wide range of applications, including nonlinear absorption enhancement, single-molecule fluorescence measurement, laser surgery, super-resolved microscopy, manipulation and detection of single sub-100 nm nanoparticles and biomolecules and laser sub-micro etching. Recently, the possibility to obtain a PNJ using an optical fiber with a shaped tip has opened new interests in the challenging field of direct laser subwavelength micromachining. To start with, the fiber is easier to move than the microsphere. Furthermore, the sphere is corrupted after the first irradiation due to their contact with the sample. On the opposite, the fiber tips have no contact with the processed surface and are not altered by the removed material. PNJs have already demonstrated the ability to reduce the laser etching size using shaped optical fiber tips. With a shaped optical fiber tips, the PNJ phenomenon is only due to the fundamental mode of the fiber. However, a standard single-mode fiber has three drawbacks for PNJ generation. (1) The small core diameter (<10 μm) makes it difficult to couple high power in the single mode fiber. (2) Given the small diameter of the core compared to the one of the optical cladding, the core does not see the curvature of the shaped tip; only the cladding is shaped. (3) The distance between the tip and the irradiated sample, in the magnitude order of the core diameter, is too short for industrial systems. We show how single mode fiber and especially large mode area (LMA) fiber can achieve the same process with 8 times less power, maintaining a reasonable working distance which may open new possibilities for direct laser micro-and nano-processing.
