Optical near-field distribution in an asymmetrically illuminated tip–sample system for laser/STM nanopatterning |
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Authors: | ZB Wang BS Luk’yanchuk L Li PL Crouse Z Liu G Dearden KG Watkins |
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Institution: | (1) Laser Processing Research Centre, School of Mechanical, Aerospace and Civil Engineering, University of Manchester, Sackville Street, Manchester, M60 1QD, UK;(2) Data Storage Institute, DSI Building, 5 Engineering Drive 1, Singapore, 117608, Singapore;(3) Corrosion and Protection Centre, School of Materials, University of Manchester, The Mill, Manchester, M60 1QD, UK;(4) Laser Group, Department of Engineering, University of Liverpool, Brownlow Street, Liverpool, L69 3GH, UK |
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Abstract: | In surface nano-patterning using an atomic force microscope (AFM) tip in scanning tunnelling microscopy (STM) mode and illuminated
by a laser, two controversial physical mechanisms exist in the literature: the field-enhancement (FE) model and the thermal-induced
mechanical contact (TMC) model. Due to the presence of evanescent waves in the optical near-field, the exact calculation of
the field distribution of the tip–sample system in micro/nano scales becomes complicated. There is a lack of understanding
of the asymmetrically illuminated tip–sample system. In this paper, full 3D finite-difference time-domain (FDTD) analysis
was carried out to investigate the field distribution in different tip–sample systems. The effects of different tip/sample
materials (either dielectric or plasmonic material), the gap distance, and laser incidence angles on the field distribution/enhancement
have been studied. For the first time, we have demonstrated two new effects which are helpful in distinguishing the controversial
mechanisms: (1) on the sample surface, the field peak position has a shift away from the tip-axis at large angles of incidence,
and (2) the field enhancement could depend strongly on the horizontal component (perpendicular to tip-axis) of the incident
wave instead of the vertical component (along tip-axis). The optimal incident angle is around 30° for the maximum field under
the tip. The existence of field-distribution nodes on the 3D tip surface that leads to the in-homogenous heating of the tip
is also predicted.
PACS 81.16.Mk; 61.80.Ba; 81.16.Rf; 81.65.Cf |
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