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Beamshaping Paper Submit JO Stokes, J. L. , Sarua, A., Pugh, J. R., Dorh, N., Munns, J. W., Bassindale, P. G., Ahmad, N., Orr-Ewing, A. J., & Cryan, M. J. (2015). Purcell enhancement and focusing effects in plasmonic nanoantenna arrays. Journal of the Optical Society of America B, 32(10), 2158- 2163. https://doi.org/10.1364/JOSAB.32.002158 Peer reviewed version Link to published version (if available): 10.1364/JOSAB.32.002158 Link to publication record in Explore Bristol Research PDF-document © 2015 Optical Society of America University of Bristol - Explore Bristol Research General rights This document is made available in accordance with publisher policies. Please cite only the published version using the reference above. Full terms of use are available: http://www.bristol.ac.uk/red/research-policy/pure/user-guides/ebr-terms/ Purcell Enhancement and Focusing Effects in Plasmonic Nanoantenna Arrays J. L. Stokes1, A. Sarua2, J. R. Pugh1, N. Dorh1, J. W. Munns2, P. G. Bassingdale3, N. Ahmad1, A.J.Orr-Ewing4 and M. J. Cryan1. 1Photonics Research Group, Department of Electrical and Electronic Engineering, 2School of Physics, 3Department of Mechanical Engineering, 4School of Chemistry University of Bristol, Bristol, BS8 1UB, UK. E-Mail: [email protected], [email protected] Abstract: This paper presents measured fluorescence results for PMMA-dye coated 5 x 5 gold plasmonic nanoantenna arrays. The paper uses numerical electromagnetic modelling to show how array size and element spacing can be used to control emitted beamshape and compares this with experimental data. The Friis formula from RF antenna theory is used to calculate the intensity enhancement produced by the array. A figure-of-merit is then developed which accounts for the very small mode volume from which the array emission is occurring. 1. Introduction Plasmonic nanoantennas are of considerable interest for use in single molecule detection and sensing applications where they resonantly enhance fluorescent emission from dye molecules, and very large enhancements have been measured in the literature [1], [2]. This is known as Purcell enhancement, first identified by E. Purcell who showed the that electromagnetic environment which surrounds an emitter strongly affects the emission of radiation [3]. Furthermore, nanoantennas can be used to beamshape or focus the radiation from the molecules which can increase the field intensity in the measurement plane and therefore improve detection single-to-noise ratio [4]. Nanoantennas are typically made from conductive material such as noble metals which can have significant optical losses which limits Purcell enhancement. 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. This paper measures fluorescent emission from a PMMA-dye coated 5 x 5 array of two–arm dipole antennas and shows beam shaping effects which are related to antenna element spacing. A scanning confocal microscope is used to measure the emission and we use 3D Finite Difference Time Domain (FDTD) simulations of 3 x 3 and 5 x 5 arrays to interpret the measured results in terms of array beamshaping effects. The paper then uses the Friis formula from RF antenna theory [7] combined with array gain estimated from FDTD modelling to calculate the power enhancement produced by the array. The Friis formula calculates the power transfer between two antennas of known gain separated by a distance, d at a wavelength,λ. In our case we have two sets of measurements one with and one without the array, since d, λ and the receiving optics are fixed, with an estimate of array gain we can calculate the increase in power produced by the array. This in effect separates Purcell enhancement which occurs at each individual antenna from the collective focusing effect of the array. The paper then introduces a figure-of-merit in order to allow fair comparison between the array and non-array measurements based on an estimate of the array mode volume. A number of recent publications have investigated radiation pattern manipulation but each only investigates the beamshape from a single element such as a nanocube or single antenna [8], [9]. This paper on the other hand, modifies the emission pattern of fluorescent molecules 1 using a 2D array of antennas and progresses further to separate Purcell effect from array beamshaping enhancement. Here we use dipole nanoantennas, which consist of two coupled arms that are separated by a small distance, typically 10’s of nanometers. This creates high field intensities in the gap region which results in an extremely small mode volume, well below the diffraction limit. This makes the dipole antenna an excellent choice for coupling to fluorescent molecules, as small mode volume is an essential requirement for Purcell enhancement. Furthermore, this type of antenna has been widely used in the RF domain to provide controllable beamshaping. Thus dipole antennas are ideal for combining Purcell and beamshaping effects, though it should be noted that in terms of fabrication, maintaining uniform gaps throughout the 25 elements of a 5 x 5 array remains a significant challenge. Section 2 describes the fabrication procedure using Focused Ion Beam etching and measurements using scanning confocal microscope. Section 3 shows FDTD modelling of 3 x 3 and 5 x 5 arrays. Section 4 uses FDTD to analyse array gain using the Friis formula which enables an estimate intensity enhancement to be made. Our figure-of-merit based on mode volume is then introduced and calculations are performed to compare the array and non-array results and Section 5 draws conclusions. 2. Fabrication and Measurement All nanoantennas in this paper are fabricated on gold coated glass substrates from Platypus Technologies [10]. The fabrication of the nanostructures is performed using the Focused Ion Beam (FIB) etching technique [11]. To achieve repeatable nanoantennas it was necessary to create a five step procedure. (i) A C-shape etch is performed with a relatively high ion beam current of 50 pA to create an isolated arm into which the array will be etched, (ii) An etch is performed at 4 pA to leave thin fingers of Au shown in Figure 1(a), (iii) The fingers are then polished to leave uniform edges and to remove re-deposition of Au around the antennas, shown in Figure 1(b), (iv) A vertical etch is performed to create the x-axis pitch and gaps between the antenna arms, shown in Figure 1(c). (v) The final etch removes the remaining arm of Au, leaving antennas isolated from the remaining Au on the sample as shown in Figure 1(d). The nominal dimensions of the fabricated antennas arms are 100 x 40 x 50nm (length x width x thickness) with a gap of 50 nm with a horizontal (x) and vertical (y) centre-to-centre element spacing of 450 nm and 230 nm respectively. 2 Figure 1 – Images of the array fabrication process (a) Fingers after second etch. (b) Fingers after polishing. (c) Antennas next to remaining Au arm with horizontal (x) and vertical (y) centre-to-centre element spacing of 450 nm and 230 nm respectively. (d) Arm removed and image of whole area. In order to introduce a fluorescent dye into the antenna array it is most straightforward to mix a dye with a polymer such as PMMA and then use a spin coating process to obtain a thin, flat layer of polymer which evenly coats the whole sample [12]. The dye molecule used is LDS 798 from Photonic Solutions [13]. When the dye is mixed with PMMA the emission band is blueshifted and this shift has been considered when designing the nanoantenna array in order to ensure that an overlap exists with the resonance of the antennas. Figure 2 shows the measured emission spectrum from the dye mixed with PMMA, the FDTD simulated resonance of a dipole antenna with two arms of length 100 nm, width of 40 nm and thickness of 50 nm with a gap of 50 nm and a 5 x 5 array of the same antennas with horizontal (x) and vertical (y) centre-to-centre element spacing of 450 nm and 230 nm respectively. In the FDTD model the antennas are excited using a plane wave from the air. The dye/PMMA layer is accounted for by covering the antennas with a layer of dielectric with a thickness of 250 nm and a refractive index of n = 1.485 [14]. As expected this layer redshifts the antenna resonance significantly, a redshift of 152 nm is found for a single antenna and 89 nm for a 5 x 5 array. When moving from a single antenna to an array, mutual coupling effects between the elements can be seen to shift the resonant frequency. This is dependent on element spacing and shows that knowledge of the single element response is not sufficient for array design. Figure 2 shows that there is significant overlap between the dye emission peak and the 5 x 5 nanoantenna array resonance peak which should enable emission enhancement to occur. It should be noted that this overlap is not ideal, so that further improvement in future results can be expected. 3 Figure 2 – Measured emission spectrum from LDS798 dye mixed with PMMA(solid line), and FDTD modelled resonances of a single (dashed line) and 5 x 5 array (dotted line) of dipole nanoantenna with dimensions matching those fabricated.
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