Optically controlled magnetic-field etching on the nano-scale

Abstract

Electric and magnetic fields play an important role in both chemical and physical reactions. However, since the coupling efficiency between magnetic fields and electrons is low in comparison with that between electric fields and electrons in the visible wavelength region, the magnetic field is negligible in photo-induced reactions. Here, we performed photo-etching of ZrO₂ nano-stripe structures, and identified an etching-property polarisation dependence. Specifically, the etching rate and etched profiles depend on the structure width. To evaluate this polarisation-dependent etching, we performed numerical calculations using a finite-difference time-domain method. Remarkably, the numerical results revealed that the polarisation-dependent etching properties were determined by the magnetic field distributions, rather than the electric field distributions. As nano-scale structures induce a localised magnetic field, the discovery of this etching dependence on the magnetic field is expected to introduce a new perspective on advanced nano-scale structure fabrication.

Keywords: nano-scale; near-field etching; optically controlled magnetic-field interaction.

Figure 1

Polarisation-dependent etching. (a) and (c) are AFM images taken before etching. (b) and (d) are AFM images after 3 h. Perpendicular (x-polarised) and parallel (y-polarised) samples are shown in (b) and (d), respectively. Images (a) and (b), and (c) and (d), were obtained at the same respective positions. (e–g) Cross-sectional profiles of x-polarised samples. Red dashed line: pre-etched sample in (a), red solid line: etched sample in (b), black solid line: difference in height before and after etching. The pre-etched widths, w_b, were (e) 411.17, (f) 432.81, and (g) 454.45 nm. (h–j) Cross-sectional profiles of y-polarised samples. Blue dashed line: pre-etched sample in (c), blue solid line: etched sample in (d), black solid line: difference in height before and after etching. w_b were (h) 414.06, (i) 424.41, and (j) 445.11 nm. (k) Etching height as a function of w.

Figure 2

Size and polarisation dependence of electric- and magnetic-field intensity distributions. (a) Schematic of calculation model. (b–e) w = 400 nm. (b) |E|² with x-polarisation. (c) |E|² with y-polarisation. (d) |H|² with x-polarisation. (e) |H|² with y-polarisation. (f–i) w = 450 nm. (f) |E|² with x-polarisation. (g) |E|² with y-polarisation. (h) |H|² with x-polarisation. (i) |H|² with y-polarisation. The red dashed line corresponds to the ZrO₂ land profiles.

Figure 3

Comparison between pre- and post-etching heights. In (a–f), the red/blue and black solid lines correspond to the calculated |H|² and the change in etching height, respectively. Further, the red and blue solid lines correspond to x- and y-polarisation, respectively. The calculated w and w_b with x-polarisation are (a) 410 and 411.17, (b) 430 and 432.81, and (c) 450 and 454.45 nm, respectively. The calculated w and the pre-etching widths, w_b, with y-polarisation are (d) 410 and 414.06, (e) 430 and 424.41, and (f) 450 and 445.11 nm, respectively. (g) Peak values of calculated magnetic-field intensity as a function of w.

Figure 4

Magnetic field distributions depending on incident light polarisations. (a) x-polarisation, (b) y-polarisation. |H₀| indicates the magnetic field intensity in the absence of nano-stripe structures. Schematic of magnetic-field generation for (c) x- and (d) y-polarisation. The calculated w is 400 nm.