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TECHNICAL PAPERS

# Obtaining Subwavelength Optical Spots Using Nanoscale Ridge Apertures

[+] Author and Article Information
E. X. Jin

School of Mechanical Engineering, Purdue University, West Lafayette, IN 47907xjin@purdue.edu

X. Xu1

School of Mechanical Engineering, Purdue University, West Lafayette, IN 47907xxu@ecn.purdue.edu

1

Corresponding author.

J. Heat Transfer 129(1), 37-43 (Jun 16, 2006) (7 pages) doi:10.1115/1.2401196 History: Received January 23, 2006; Revised June 16, 2006

## Abstract

Concentrating light into a nanometer domain is needed for optically based materials processing at the nanoscale. Conventional nanometer-sized apertures suffer from low light transmission, therefore poor near-field radiation. It has been suggested that ridge apertures in various shapes can provide enhanced transmission while maintaining the subwavelength optical resolution. In this work, the near-field radiation from an $H$-shaped ridge nanoaperture fabricated in an aluminum thin film is experimentally characterized using near-field scanning optical microscopy. With the incident light polarized along the direction across the gap in the $H$ aperture, the $H$ aperture is capable of providing an optical spot of about $106nm$ by $80nm$ in full-width half-maximum size, which is comparable to its gap size and substantially smaller than those obtained from the square and rectangular apertures of the same opening area. Finite different time domain simulations are used to explain the experimental results. Variations between the spot sizes obtained from a $3×3$ array of $H$ apertures are about 4–6%. The consistency and reliability of the near-field radiation from the $H$ apertures show their potential as an efficient near-field light source for materials processing at the nanoscale.

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## Figures

Figure 1

SEM images of: (a)H aperture (a=210nm, b=170nm, s=100nm, d=60nm); (b) square aperture (160nm by 160nm); (c) small rectangular aperture (100nm by 60nm); and (d) large rectangular aperture (400nm by 60nm) fabricated in a 75nm aluminum film on a quartz substrate. The scale bars are 200nm.

Figure 2

Schematic view of the specially designed NSOM system using a cantilever aperture probe

Figure 3

(a) SEM image of cantilever aperture probe made by FIB milling; (b) NSOM image of a pair of nanoholes used to characterize the optical resolution the aperture probe; and (c) line scan on the NSOM image shows the 10–90% edge resolution of 78nm

Figure 4

(a) NSOM image of the H aperture (210nm×170nm outline with a 100nm×60nm gap) and line scans in (b) A-A horizontal direction and (c) B-B vertical direction show the FWHM spot size is 106nm by 80nm. The illumination light is polarized along the y direction across the gap the H aperture as indicated in the inset of (a). The scale bar in (a) is 200nm.

Figure 5

NSOM images of: (a) the 160nm×160nm square aperture and (b) the 400nm×60nm rectangular aperture. The FWHM sizes of the near-field optical spots shown in the images are 174nm×136nm and 212nm×120nm for the square and rectangular apertures, respectively. The insets show the SEM pictures of the imaged apertures. The illumination light is polarized along the y direction as indicated in the insets. The scale bars are 200nm.

Figure 6

NSOM images of: (a) the H aperture and (b) the 160nm×160nm square aperture. The FWHM sizes of the near-field optical spots shown in the images are 170nm×188nm and 167nm×132nm for the H and square apertures, respectively. The insets show the SEM pictures of the imaged apertures. The illumination light is polarized along the x direction as indicated in the insets. The scale bars are 200nm.

Figure 7

(a) SEM image of an array of H apertures fabricated in an aluminum film and (b) three-dimensional NSOM image of the array. The illumination light is polarized along the y direction across the gap of the H apertures as indicated by the arrow in the inset of (a). The scale bar in (a) is 500nm.

Figure 8

Left column: the computed near-field intensity distribution (normalized by incident intensity) on the xy plane at 10nm below (a) the H aperture; (c) the square aperture; and (e) the rectangular aperture in a 75-nm-thick aluminum film on a quartz substrate. Right column: light propagation on the xz plane through the middle of (b) the H aperture; (d) the square aperture; and (f) the rectangular aperture. The y polarized plane wave at 458nm wavelength is incident from the quartz side. The arrows indicate the polarization direction of the incident light.

Figure 9

Left column: the computed near-field intensity distribution (normalized by incident intensity) on the xy plane at 10nm below (a); the H aperture, and (c) the square aperture in a 75-nm-thick aluminum film on a quartz substrate. Right column: light propagation on the xz plane through the middle of (b) the H aperture, and (d) the rectangular aperture. The x polarized plane wave at 458nm wavelength is incident from the quartz side. The arrows indicate the polarization direction of the incident light.

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