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Broadband Aluminum Surface Plasmon Nano-hole Arrays

By Matthew George · App and Tech Notes · 21/03/14

2D aluminum plasmonic nano-hole arrays were fabricated and characterized with potential applications in surface plasmon resonance (SPR) sensing, surface-enhanced Raman scattering (SERS), and surface-enhanced fluorescence spectroscopy (SEFS). Potential markets include micro-arrays for bio-related assays and SERS substrates for trace level chemical detection.

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Figure 1a Full field 2D periodic aluminum nanostructures cut from 200mm diameter wafers. 3” microscope slide with 650nm pitch nano-hole array for visible and near-IR applications.

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Figure 1b  SEM cross section of Moxtek nano-hole array.

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Figure 1c  144nm pitch nano-post arrays for Vacuum and Deep UV spectroscopic applications.

Periodic micro- and nano-structures are slowly gaining traction in diverse areas such as bio-sensing, surface enhanced Raman spectroscopy (SERS), solar cell concentration and antireflection strategies, solid state lighting, as well as in integrated micro-optics, lasers, optical filters, cloaking structures, and even in electrochemical devices. Unfortunately, obtaining a reliable, cost effective, yet flexible manufacturing source for such structures has often been a challenge. Moxtek has addressed some of these challenges by leveraging existing capabilities in wafer-scale patterning of sub-wavelength nanowire grid polarizers into the fabrication of two-dimensionally periodic aluminum plasmonic nano-hole and nano-post arrays, as depicted in Figure 1. Aluminum has broadband surface plasmonic activity spanning from the vacuum ultraviolet into the infrared and is a better plasmonic material than gold or silver for applications in the blue and ultraviolet wavelengths.1 The thin passivating oxide layer of aluminum ensures minimal performance variation under most ambient conditions and Aluminum’s triple valence state provides for a high free electron density, which extends aluminum’s excellent optical properties into the vacuum UV and reduces the skin depth, allowing for thinner films and smaller aspect ratio nanostructures.2

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Figure 2 Normal incidence Transmittance of Moxtek aluminum nano-hole array with polarization parallel to the <10> and <11> crystal directions. Solid vertical lines indicate predicted locations of SPR Bloch modes while dashed lines indicate surface grating modes associated with the interface between metal and glass substrate (gray lines) and metal and air superstrate (white lines). The mode numbers increase with decreasing wavelength.

Experimental Conditions

650nm pitch 2D nano-hole array samples were fabricated on display grade glass using wafer-scale deposition, lithography, and etching techniques commonly used in semiconductor fabrication. Transmittance and reflectance measurements were performed using an Agilent CARY 5000 spectrophotometer. Rotation of the samples with respect to a fixed pre-analyzer allowed for measurements to be taken with the polarization parallel to the various crystallographic directions of the 2D nano-hole arrays. The pre-analyzers consisted of a stack of three Moxtek UVT240 aluminum nanowire grid polarizers (100nm pitch) on fused silica. An M-2000 variable angle spectroscopic ellipsometer (VASE) was used to characterize the optical properties of solid aluminum films (before nano-patterning) and the display grade glass substrates.

Results

The aluminum nano-hole arrays showed extraordinary optical transmission from the Deep UV through the near-IR with complex spectral features near the predicted resonance wavelengths for surface grating modes (Wood’s anomalies) and SPP Bloch modes. The resonance locations for the various Wood’s anomalies were calculated by using equation 1 below, where the normal incidence 2D grating equation is solved for mode order (m, n) with the diffracted order propagating parallel to the metal surface, where ns is the index of refraction of the surrounding medium (air superstrate or glass substrate in this case), and ax and ay are the lattice periodicities in x- and y-directions (both equal to 655nm). The Bloch mode locations were calculated using formula 2, the approximate condition for resonant excitation of bound modes for TM polarization, where eAl is the aluminum permittivity.3

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(1)

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(2)

Conclusions

Moxtek has leveraged expertise in patterning aluminum nanowires to fabricate 2D nano-hole arrays with potential applications in surface plasmon resonance (SPR) sensing, surface-enhanced Raman scattering (SERS), and surface-enhanced fluorescence spectroscopy (SEFS). The beneficial optical properties of Aluminum should allow for tuning of geometric array parameters to target applications ranging from the vacuum UV through the near-IR, with optimal performance in the ultraviolet regime.

References
  1. P.R. West et Al., Searching for Better Plasmonic Materials, Laser & Photonics Reviews, 2010, 4, pp. 795-808.
  2. M.W. Knight et Al., Aluminum Plasmonic Nanoantennas, Nano Letters, 2012, 12, pp. 6000-6004.
  3. M.E. Stewart et Al., Nanostructured Plasmonic Sensors, Chem. Rev., 2008, 108, pp. 494-521.

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