Toward Plasmonics with Nanometer Precision: Nonlinear Optics of Helium-Ion Milled Gold Nanoantennas

Abstract

Plasmonic nanoantennas are versatile tools for coherently controlling and directing light on the nanoscale. For these antennas, current fabrication techniques such as electron beam lithography (EBL) or focused ion beam (FIB) milling with Ga(+)-ions routinely achieve feature sizes in the 10 nm range. However, they suffer increasingly from inherent limitations when a precision of single nanometers down to atomic length scales is required, where exciting quantum mechanical effects are expected to affect the nanoantenna optics. Here, we demonstrate that a combined approach of Ga(+)-FIB and milling-based He(+)-ion lithography (HIL) for the fabrication of nanoantennas offers to readily overcome some of these limitations. Gold bowtie antennas with 6 nm gap size were fabricated with single-nanometer accuracy and high reproducibility. Using third harmonic (TH) spectroscopy, we find a substantial enhancement of the nonlinear emission intensity of single HIL-antennas compared to those produced by state-of-the-art gallium-based milling. Moreover, HIL-antennas show a vastly improved polarization contrast. This superior nonlinear performance of HIL-derived plasmonic structures is an excellent testimonial to the application of He(+)-ion beam milling for ultrahigh precision nanofabrication, which in turn can be viewed as a stepping stone to mastering quantum optical investigations in the near-field.

Plasmonic nanopore-based platforms for single-molecule Raman scattering

“…The enhancement of electric field results from stronger plasmonic coupling with narrower bowtie feed gap. It is worth mentioning that sub-10 nm solid-state nanopores and feed gaps (gap size should not be smaller than pore diameter) are hard to obtain on substrates with today's fabrication techniques [1,21,22]. However, extremely narrow bowtie gap (5 nm here, limited by techniques and pore diameter) does lead to giant electric field enhancement (120 times stronger as shown in Fig.…”

Squeezing Photons into a Point-Like Space

“…In 2014, Kollmann et al developed gold bowtie antennas with 6 nm gaps and single-nanometer accuracy by employing Ga + -FIB and milling-based He + -ion lithography (HIL) techniques. 29 They achieved photon squeezing in an area approximately 6 × 10 nm 2 and observed 3-fold enhancement of nonlinear intensity from a 6 nm-gap antenna compared to that from a 20 nm-gap antenna. In 2012, photoresist-based EBL was also used to fabricate single and array antennas with gap sizes below 10 nm under special fabrication conditions of high-energy electron beams (>100 keV) and thin SiN membrane substrates.…”

Purcell enhancement and focusing effects in plasmonic nanoantenna arrays

“…It may be possible to use focusing effects to overcome these loss limitations and this paper studies both Purcell enhancement and focusing effects in an array of gold nanoantennas [5]. Previous work has shown that in order to achieve high field enhancements extremely small gaps are required which are very difficult and therefore expensive to reliably fabricate [6]. This work shows that by utilising array effects larger gaps can be used whilst maintaining large enhancement factors.…”