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3Laser field enhancement near defects in close-packed colloidal monolayers of dielectric spherical microlenses
M. Sveshnikova, A. Pikulin*, N. Bityurin
Institute of Applied Physics, 46 Ulyanov Str., 603950 Nizhny Novgorod, Russia
Colloidal lithography (CL) is a cost-effective technique capable of producing surface structures of various kinds. This technique employs the particle monolayers that are deposited from the colloidal solution on a material surface. There almost perfect particle arrangement can be obtained due to the self-organization. Particularly, the near-field CL relies on the irradiation of the spherical particles by the laser beam [1]. The dielectric microspheres act as near-field microlenses that generate localized photonic jets, which provide local material modification.
Typically, each sphere provides its own photonic jet thus resulting in a hexagonal periodical pattern on the surface. However, more complex patterns may be required for applications. By combining the near-field colloidal lithography and the multiple-beam interference of the incident laser light one can obtain more complex patterns of the jets [2]. Another way is to produce patterns of spheres themselves. For instance, tri-block Janus particles self-organize in a Kagome lattice [3]. Alternatively, patterns can be produced from ordinary polystyrene (PS) microspheres on the substrate where the surface wettability is selectively modified by UV radiation [4].
Despite the physical approach that results in a non-regular placement of the spheres, calculation of the field enhancement near different types of defects in the periodical monolayer of spheres proves to be important. In this work, we model the field enhancement near the spheres at the edge of the close-packed domain. The calculation shows the greater field enhancement beneath the edge spheres than others.
Another often seen defect is the vacancy, i.e. the missing sphere. Calculation shows the field enhancement maximum under the vacancy. This maximum is of similar strength compared to those under the spheres (see Fig. 1).
a)
-1.5 -1 -0.5 0 0.5 1 1.5 x, Jim
b)
Fig. 1. a) Vacancy defect within the monolayer of dielectric spheres; b) enhancement of the electric field square under the vacancy and the
surrounding spheres.
Formation of the field maximum under the vacancy is explained by the superposition of the contributions from the spheres surrounding the vacancy. This is an important result that can explain the material modification under the vacancy.
This work was supported by the Russian Science Foundation under project No. 22-19-00322.
[1] L. Chen, Y. Zhou, Y. Li, M. Hong, Microsphere enhanced optical imaging and patterning: From physics to applications, Appl. Phys. Rev. 6, 021304, (2019).
[2] N. Mitin and A. Pikulin, Interference surface patterning using colloidal particle lens arrays, Opt. Lett. 45, 6134, (2020).
[3] Q. Chen, S.C. Bae, S. Granick, Directed self-assembly of a colloidal kagome lattice, Nature 469, 381, (2011).
[4] V. Bredikhin and N. Bityurin, 2D mesoscale colloidal crystal patterns on polymer substrates, Mater. Res. Express 5, 055306, (2018).
