High-Brightness Plasmon-Enhanced Nanostructured Gold Photoemitter

Y. Gong, Alan G. Joly, L. M. Kong, Patrick Z. El-Khoury, and Wayne P. Hess

Phys. Rev. Applied 2, 064012 – Published 30 December 2014

Abstract

We fabricate plasmonic nanohole arrays in gold thin films by focused-ion-beam lithography. Subsequent heat treatment of the lithographic patterns induces the growth of sub-100-nm structures including tips, rods, and flakes, all localized in the nanohole-array region. The coupled nanohole-array–nanostructure system comprises an efficient photoemitter. High-brightness photoemission is observed from this construct, following 780-nm femtosecond laser irradiation, using photoemission electron microscopy (PEEM). By comparing our PEEM observables to finite-difference time-domain simulations, we demonstrate that photoemission from the sub-100-nm structures is enhanced in the region of propagating surface plasmons launched from the nanohole arrays.

Received 29 January 2014

DOI:https://doi.org/10.1103/PhysRevApplied.2.064012

Authors & Affiliations

Y. Gong, Alan G. Joly, L. M. Kong, Patrick Z. El-Khoury, and Wayne P. Hess^*

Pacific Northwest National Laboratory, Physical Sciences Division, P. O. Box 999, Richland, Washington 99352, USA

^*Corresponding author. wayne.hess@pnnl.gov

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Issue

Vol. 2, Iss. 6 — December 2014

Subject Areas

Plasmonics

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Images

Figure 1
(a) 100-nm gold thin film is sputtered on a clean glass substrate with nanohole arrays ( $5 \times 5$ ) fabricated by FIB lithography. (b) The sample is then annealed at $200 ° C$ for 2 h in air. (c) The resulting sample is transferred into the PEEM for the photoemission measurement under fs laser irradiation with an incident angle $θ = 75 °$ . Schematically illustrated in (c) are $s$ and $p$ polarization.
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Figure 2
(a) SEM image of the nanostructured photoemitter which consists of a ( $5 \times 5$ ) nanohole array (1000-nm-diameter holes separated by 2500 nm) fabricated by FIB lithography and heat treated to form nanostructures in the vicinity of the nanoholes within the etched region. These nanostructures only form near the nanohole array, and not on the flat film. (b) Magnified SEM image of the metal nanostructures consisting of nanotips, nanorods, and nanoflakes grown from the gold thin film. (c) Magnified gold surface far from the nanohole array appears identical to the gold thin film prior to heat treatment. (d) PEEM image of the nanostructured photoemitters with 2500-nm nanohole separations and 1000-nm hole diameter illuminated by the UV lamp.
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Figure 3
(a) PEEM image of the nanostructured photoemitters with 2500-nm nanohole separations and 1000-nm hole diameter irradiated by a 780-nm $p$ -polarized laser at 75° from normal. The nanostructures produce strong photoemission with the brightest region always to the right side of the nanohole array, forming a stripe pattern aligned with the laser propagation. Dashed circles indicate hole positions while the arrow indicates the laser propagation direction. The exposure time is 0.15 s. (b) Under illumination of $s$ -polarized light, nanostructured photoemitters show very low photoemission from both the nanostructured and flat surface regions (color bars are in arbitrary units). (c) FDTD calculated electric-field-intensity map for a $5 \times 5$ nanohole array etched in a gold film irradiated by a 780-nm fs laser pulse. The plane wave is polarized with electric vector parallel to the out-of-plane axis ( $p$ polarization). (d) FDTD simulation of the nanohole array with $s$ -polarized laser pulse. The laser pulse propagates from left to right in both simulations.
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Figure 4
(a) Normalized photoemission intensity for nanostructured photoemitters plotted as a function of nanohole diameter and separation. The photoemission intensity increases modestly when the separation between nanoholes decreases from 3000 nm to 1500 nm. (b) PEEM image of a nanohole array (separation and hole diameter of 2500 and 1000 nm respectively) without self-assembled nanostructure (no heat treatment). (c) Nanohole-separation dependence of normalized photoemission intensity for nanohole arrays without self-assembled nanostructure. The trend apparent in (a) is consistent with the results in (c). (d) Photoemission current is plotted as a function of laser intensity for the self-assembled nanostructure photoemitter on a log-log scale. Three-photon absorption process is observed (green line) under low laser intensity; however, under high laser intensity (red line), the data yield a sixth-order photoemission scaling. The inset shows the polarization dependence of the photoemission intensity measured in the region of high laser intensity where 0° represents $p$ polarization. Dashed line shows power dependence of a gold nanograting photoemitter (Polyakov et al. [6]).
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