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Improvement of Sensing and Trapping Efficiency of Double Nanohole Apertures via Enhancing the Wedge Plasmon Polariton Modes with Tapered Cusps Mostafa Ghorbanzadeh, Steven Jones, Mohammad Kazem Moravvej-Farshi, and Reuven Gordon ACS Photonics, Just Accepted Manuscript • Publication Date (Web): 04 Apr 2017 Downloaded from http://pubs.acs.org on April 5, 2017
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Improvement of Sensing and Trapping Efficiency of Double Nanohole Apertures via Enhancing the Wedge Plasmon Polariton Modes with Tapered Cusps Mostafa Ghorbanzadeh1,2*, Steven Jones2, Mohammad Kazem Moravvej-Farshi1, and Reuven Gordon2 1
Faculty of Electrical and Computer Engineering, Tarbiat Modares University, P. O. Box 14115194, Tehran 1411713116, Iran 2
Department of Electrical and Computer Engineering, University of Victoria, Victoria, BC, Canada, V8P5C2 *
Email:
[email protected];
[email protected] ABSTRACT: In the past few years double nanohole (DNH) apertures in a gold film have been used extensively to trap and sense biological and artificial dielectric nanoparticles. Using numerical simulations we show that the conical shape of a DNH, milled by a focused ion beam into a thin gold layer, which is an inherent property of the fabrication process, plays a critical role in the sensitivity of the DNHs and is beneficial to the optical sensing and trapping
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applications. The slope of the metallic wedges in an appropriately designed DNH leads to 2D nanofocusing of gap surface plasmons (GSPs) and couples them to the wedge plasmon polaritons (WPPs), creating “hot spots” required for trapping. The transmission variations due to the trapping polystyrene nanoparticles of radii 11±1 nm by particularly designed DNHs, measured at the wavelength near the corresponding wedge mode resonance, are shown to be in good agreements with numerical results using conically modeled DNHs. This observation highlights the extreme sensitivity of aperture assisted trapping, specifically with regard to the DNH structure. These findings open up new routes toward the design and optimization of efficient aperture structures for trapping and sensing applications.
KEYWORDS: Double nanohole, optical trapping, nanoparticles, nanoplasmonics
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Since Ashkin1 has demonstrated the capability of laser beams in trapping of micro objects in the 1970s, optical forces have been exploited widely for the manipulation of micro- and nanoparticles. Due to the non-invasive nature of light, optical manipulation methods are of high interest for sensing on a single particle level, and have been extensively employed for applications in biological, environmental, and physical sciences. Extending the conventional farfield diffraction limited optical tweezers towards the nanoscale; near field trapping, based on confined and enhanced electromagnetic fields via plasmonic structures2,3 has revealed exciting possibilities for the integration of multifunctional lab-on-a-chip systems to trap, manipulate, sort, and sense micro, nano, and biological particles4–8. Nanoplasmonic apertures, which exhibit the self-induced back-action effect9–13, can be integrated into a laser tweezer microscopy systems that facilitates efficient trapping is an advantageous emergent method. Double nanohole (DNH) apertures (see Figure 1) that have been ion milled into a thin gold layer, have shown inspiring experimental results when used for: manipulation of silica nano-spheres14 and single proteins15, calculation of nanoparticle size and concentration16, and investigation of protein binding17,18, trap stiffness19, vibrations of Raman-active single isolated nanoparticles20, fluorescent molecules21, nanoparticle’s characteristic Stokes lines22, and superparamagnetic nanoparticles23. Chen et. al.24 have recently reported an experimentally observed additional wedge mode near the trapping wavelength of ~850 nm, analogues to the wedge plasmon polariton (WPP) mode of a single or double wedge25–28. They have also compared the measured and simulated transmission spectra of DNHs. In their simulations, they have estimated the DNHs’ geometries by considering the cusps 2D curvature (in x-y plane) and ignoring the effect of the wedge profile along the DNH depth (along z-direction). The effects of the wedge modes on the numerically obtained transmission spectra, however, are shown to be much weaker than that on the
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experimentally measured spectra (see Figure 5 of Ref
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). A reasonable explanation for this
difference can be attributed to the approximation of 3D DNH with conical profiles (Figure 1) by simplified 2D DNHs with cylindrical wedge profiles. Knowing the conical nature and testing a similar DNH but with different dimensions, whose scanning electron microscope (SEM) images as viewed from the top at two different angles (0° and 30°) is illustrated in Figure 1a and 1b; we first have numerically simulated the transmission spectra and the corresponding WPP mode by taking the effect of the wedge profile along the depth of the 3D DNH into account. The DNH structure of Figure 1 was fabricated by using a focused ion beam (FIB) to mill the apertures into an Au film of thickness tG=100 nm that had been deposited on top of a glass microscope slide, similar to those of the group’s earlier works14–24. As can be observed from the images shown in this figure, the tip separation at the most narrow region of the aperture is not uniform along the depth of the milled region in gold layer and tends to become narrower towards the bottom of the DNH. This slope in the aperture profile is an inherent result of the fabrication process. Here we will show that this gap variation, which has not previously been considered in theory, can enhance the trapping efficiency of DNHs and has a critical role in the sensitivity of the DNHs to nanometer scale objects. Here, as sketched schematically in Figure 1c, unlike the previously considered cylindrical DNHs, we model the DNH structure by considering two penetrated conical holes separated by distance L (conical DNH). In this model the tip separation varies from ∆t at the top to ∆b at the bottom (see Figure 1d) caused by the variation of the top and bottom holes radii from Rt to Rb (see Figure 1e). Also, to be more precise in the modeling of the DNH, we consider that the glass substrate can be milled up to 100nm during the fabrication process (see Supplementary Information for the effect of the milling depth and related
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discussions). For the example shown in Figure 1a and 1b, Rt≈130nm, Rb≈110nm, and L≈211nm that leads to the gap variation from ∆t=152 to ∆b=62nm. Using the three dimensional finite difference time domain (3D-FDTD) numerical method, we applied an incident wave with E-field along the x-axis (see Figure 1b) from the top of the DNH and calculated the transmitted power at the bottom of the DNH (see Supplementary Information for the details of the simulation technique). In order to see the effect of the cusps slope on the transmission, we have considered three DNH samples all with the same L=100nm and depth of etched substrate, but different wedge profiles: (i) Rt=70nm, Rb=56nm (conical DNH) (ii) Rt=Rb=70 nm (cylindrical DNH), and (iii) Rt=Rb=56 nm (cylindrical DNH). The calculated transmission spectra are compared in Figure 2. The solid curve with two distinct peaks represent the transmission spectrum for the 3D DNH with conical wedge profile (inset (i)), whereas the dashes and dots-dashes represent the spectra for two 2D DNHs with cylindrical wedge profiles (insets (ii) and (iii)). The peak appearing at longer wavelength (λ~1000 nm) for each of the three cases corresponds to the well-known Zeroth order Fabry-Perot (FP0) mode. Nevertheless, the peak appearing at the wavelength λ=678 nm in the solid curve corresponds to the cusps WPP mode. As can be observed from the dashes and the dots-dashes, when the cusps profiles is estimated by cylindrical geometries the corresponding WPP modes can hardly be detected via the resultant transmission spectra. To elaborate more on this significant difference and discuss about the nature of the emerged wedge mode in more detail, we have calculated the mode intensity (|E|2) profiles for all three cases, as compared in Figure 3. The enhancement of the mode intensities (|E|2) for both WPP (λWPP=678 nm) and FP0 (λ~1000nm), with respect to the incident mode intensities, through all three model DNHs estimated by profiles (i)-(iii) calculated in the x-z plane at y=0 are illustrated in Columns 1, 3 and
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4 of Figure 3a and b. The enhancements for WPP and FP0 modes by the conical DNH sampled in two x-y planes are illustrated in column 2 of the same figures. In both figures, the top and bottom profiles are taken at z=95 nm and z=5 nm. The 2D nanofocusing observed in columns 1 and 2 of Figure3a, is due to the conical geometry of the DNH, a similar phenomenon reported for the spherical lenses29. In fact, when gap surface plasmons (GSPs) propagate along –z direction, the nanofocusing occurs along the x-direction (column 1) because ∆t>∆b and along the ydirection (column2) because θt>θb. Gradual focusing of GSPs leads to gradual decrease in the GSPs wavevector, coupling the GSPs to WPPs and hence enhancing the emerged WPPs efficiently (Figure3c). The GSP to WPP conversion is similar to the conversion of the flat SPP to the WPP30. Figure 3c compares the profiles of the WPP and FP0 modes enhancements as functions of x, sampled along the line z=2nm, y=0, for all thee estimated cusps profiles (i)-(iii). As can be observed from this illustrative comparison, the efficient GSP-WPP coupling at λw causes the peak of WPP mode in the conical cusps of the conical DNH with profile (i) to enhance ~ 2.5 times more than the enhancement for the cylindrical DNH with profile (iii) and about an order of magnitude more than that for the cylindrical DNH with profile (ii). Also observed from this comparison, is the different positions at which the WPP and FP0 modes intensities peak. Unlike for the WPP modes that peak within the gold layer at the hot spots, peaks of the FP0 modes occur within the water, similar to the observation reported earlier24. All these observations reveal that the conical geometries of FIB milled DHNs in thin gold layers, which is an inherent property of the fabrication technique, is beneficial to trapping nanoparticles by optical forces, as discussed in Supplementary Information and also shown in Figure S1.
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Now, we present the results of our investigation on the effect of the slope of the conical DNH on transmission spectra through the DNH in absence and in presence of a trapped nanoparticle, named hereafter no-trap and trapped transmissions (Tno-trap and Ttrapped). Figure 4a and 4b show the calculated transmission spectra when the nanoholes (a) top radii are fixed at Rt=70nm and their bottom radii varies in the range of 50.75 nm≤ Rb≤70 nm and (b) bottom radii are fixed at Rb=54nm and the top radii are varied in the range of 54 nm≤ Rt≤ 100 nm. The solid curve, in each case, show the no-trap transmission spectrum and the corresponding dashed curve is attributed to the trapped transmission spectrum for which a polystyrene (PS) nanoparticle of radius r=20 nm is trapped by the DNH. As can be observed from both figures, for both no-trap and trapped cases, the steeper the slop of the conical DNH (i.e. S= (Rt− Rb )/tG) the stronger the corresponding WPP mode and the larger the red shifts observed in λWPP. Moreover, comparison of each solid curve with its corresponding dashed curve, in either figure, reveals that when the PS nanoparticle of refractive index n=1.59 is trapped the transmission spectrum exhibits a slight red shifted. Despite the dielectric loading effect, caused by the presence of a particle with refractive index greater than 1.33 within the aperture filled with water, the size of the nanoparticle is too small to cause a significant red shift. Also seen in these two figures is that as the slope becomes S ≥18% the first order Fabry-Perot mode also starts to appear at the wavelength λFP1
