Fabrication of sub-10 nm gap arrays over large areas for plasmonic sensors

Siegfried, Thomas; Ekinci, Yasin; Solak, Harun H.; Martin, Olivier J. F.; Sigg, H.

doi:10.1063/1.3672045

Cited by 78 publications

(84 citation statements)

References 26 publications

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“…For the 80 nm thick Au layers and gap sizes below 15 nm, the SERS intensity increases strongly due to stronger coupling between the crescents forming a nanogap channel. 29 …”

Section: Resultsmentioning

confidence: 99%

Engineering Metal Adhesion Layers That Do Not Deteriorate Plasmon Resonances

et al. 2013

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G reat amounts of numerical and experimental investigations have been devoted to boosting the signal of plasmonic sensors fabricated from noble metal nanoantennas with sharp edges or with gaps separated by a few nanometers. 1À8 The field enhancement depends mainly on the ability to couple incident photons to localized surface plasmon modes. Such resonant modes are prone to damping by scattering and absorption in the metal and its surrounding materials, thus, limiting the achievable enhancement. 9 By interference with subradiant modes, as in Fano resonant systems, radiative losses can be reduced, but not suppressed completely. 10,11 In any case, the field enhancement is often severely reduced by the presence of adhesion layers. These layers are required to pin the noble metals to the substrate, particularly when the fabrication requires lift-off and sonication or when the applications demand high structural robustness. 12 Thanks to their broad availability and process compatibility with the evaporation of the noble metals, Cr and Ti are the most commonly used adhesion materials, with typical thicknesses ranging between 1 and 10 nm. 13À15 Although the existence of the adhesion layer is an important part of the plasmonic structure, its influence on the near and far field of the plasmonic modes is often neglected. 9,16 The perturbation of the plasmon resonance can be derived from the real and imaginary parts of the adhesion materials dielectric function which affects the refractive index locally and introduces absorption. Under such circumstances, surface and localized surface plasmon resonances (SPR and LSPR) are red-shifted and broadened, leading to reduced signal sensitivities, caused by the reduced quality factor Q of the resonant mode. 17À21 As a consequence, processes where high near-field amplitudes are needed, such as surface enhanced Raman scattering (SERS) and fluorescence, 16 ABSTRACT Adhesion layers, required to stabilize metallic nanostructures, dramatically deteriorate the performances of plasmonic sensors, by severely damping the plasmon modes. In this article, we show that these detrimental effects critically depend on the overlap of the electromagnetic near-field of the resonant plasmon mode with the adhesion layer and can be minimized by careful engineering of the latter. We study the dependence of the geometrical parameters such as layer thickness and shape on the near-field of localized plasmon resonances for traditional adhesion layers such as Cr, Ti, and TiO 2 . Our experiments and simulations reveal a strong dependence of the damping on the layer thickness, in agreement with the exponential decay of the plasmon near-field. We developed a method to minimize the damping by selective deposition of thin adhesion layers (<1 nm) in a manner that prevents the layer to overlap with the hotspots of the plasmonic structure. Such a designed structure enables the use of standard Cr and Ti adhesion materials to fabricate robust plasmonic sensors without deteriorating their sensitivity.

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“…For the 80 nm thick Au layers and gap sizes below 15 nm, the SERS intensity increases strongly due to stronger coupling between the crescents forming a nanogap channel. 29 …”

Section: Resultsmentioning

confidence: 99%

Engineering Metal Adhesion Layers That Do Not Deteriorate Plasmon Resonances

et al. 2013

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“…Similarly to the reflectance, the SERS signal strongly depends on the polarization. 39 A maximum is observed for a gap of 55 ± 1.5 nm, and the corresponding value of the modulation damping parameter is 0.24. This value is in very good agreement with our universal theoretical prediction of maximal energy storage for 1/4, confirming experimentally the validity of the existence of a critical coupling for optimal nearfield enhancement.…”

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confidence: 89%

Plasmonic Radiance: Probing Structure at the Ångström Scale with Visible Light

et al. 2013

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Plasmonic modes with long radiative lifetimes combine strong nanoscale light confinement with a narrow spectral line width carrying the signature of Fano resonances, making them very promising for nanophotonic applications such as sensing, lasing, and switching. Their coupling to incident radiation, also known as radiance, determines their optical properties and optimal use in applications. In this work, we theoretically and experimentally demonstrate that the radiance of a plasmonic mode can be classified into three different regimes. In the weak coupling regime, the line shape exhibits remarkable sensitivity to the dielectric environment. We show that geometrical displacements and deformations at the Ångstrom scale can be detected optically by measuring the radiance. In the intermediate regime, the electromagnetic energy stored in the mode is maximal, with large electric field enhancements that can be exploited in surface enhanced spectroscopy applications. In the strong coupling regime, the interaction can result in hybridized modes with tunable energies. KEYWORDS: Fano resonances, electromagnetically induced transparency, plasmonic nanosensors, surface enhanced Raman scattering (SERS), extraordinary optical transmission W ith their ability to concentrate light at a deepsubwavelength scale by excitation of surface plasmons, metallic nanostructures play a major role in current nanoscience. 1 In particular, optical tweezers, 2 antennas, 3,4 lasers, 5 photodetectors, 6 or biochemical sensing platforms 7,8 have been scaled down to the nanometer range. However, their performance are limited by the short lifetimes of surface plasmon resonances. 9 Recently, it has been shown that the use of plasmonic modes with long radiative life times (subradiant modes) can drastically enhance the performance of nanophotonic devices. 10−12 Their spectral response carry an asymmetric line shape with sharp spectral features, characteristic of Fano resonances. 13−22 The coupling of subradiant modes to radiation, their radiance, is directly controlled by the geometrical configuration of the nanostructures, 19 and also governs their optical response and the location and amplitude of light confinement. 20 Despite the fact that the use of subradiant modes for nanophotonic applications critically depends on the coupling strength, the choice of an optimal regime has so far never been addressed.In this work, we use a universal model for interacting radiative and localized channels to investigate the radiance of a plasmonic mode. We demonstrate that the radiance of a plasmonic mode can be classified into three different regimes. In the weak coupling regime, the radiance of the mode is small and extremely sensitive to perturbations in the coupling. We introduce a novel sensing concept, radiance sensing, which is based on the sensitivity of the radiance to the environment, rather than conventional plasmon sensing relying on the wavelength shift of plasmon modes. 8 In the intermediate regime, the radiative damping is equal to the int...

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“…27 Precise control of gap sizes even below 10 nm was obtained for both 1D and 2D periodic arrays of different thicknesses, without the need of further fabrication steps such as lift-off and etching which would jeopardize sub-10 nm accuracy. 21 The Au is evaporated at grazing angles onto a grating pattern of transparent HSQ photoresist, such that by shadowing the opposite grating sidewall an elongated vertical channel with nanometric width is obtained (schematic and details in Figure S1 of the Supporting Information). 21 The gap size is primarily defined by the resist mask dimensions, the angle of incidence, and the deposition thickness.…”

Section: −24mentioning

confidence: 99%

“…21 The Au is evaporated at grazing angles onto a grating pattern of transparent HSQ photoresist, such that by shadowing the opposite grating sidewall an elongated vertical channel with nanometric width is obtained (schematic and details in Figure S1 of the Supporting Information). 21 The gap size is primarily defined by the resist mask dimensions, the angle of incidence, and the deposition thickness. For a given metal thickness, the gap size is varied by changing the duty cycle of the resist pattern with the exposure and development time.…”

Section: −24mentioning

confidence: 99%

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Gap Plasmons and Near-Field Enhancement in Closely Packed Sub-10 nm Gap Resonators

et al. 2013

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Pairs of metal nanoparticles with a sub-10 nm gap are an efficient way to achieve extreme near-field enhancement for sensing applications. We demonstrate an attractive alternative based on Fabry− Perot type nanogap resonators, where the resonance is defined by the gap width and vertical elongation instead of the particle geometry. We discuss the crucial design parameters for such gap plasmons to produce maximum near-field enhancement for surface-enhanced Raman scattering and show compatibility of the pattern processing with low-cost and low-resolution lithography. We find a minimum critical metal thickness of 80 nm and observe that the mode coupling from the far field increases by tapering the gap opening. We also show the saturation of the Raman signal for nanogap periodicities below 1 μm, demonstrating efficient funneling of light into such nanogap arrays.

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Fabrication of sub-10 nm gap arrays over large areas for plasmonic sensors

Cited by 78 publications

References 26 publications

Engineering Metal Adhesion Layers That Do Not Deteriorate Plasmon Resonances

Engineering Metal Adhesion Layers That Do Not Deteriorate Plasmon Resonances

Plasmonic Radiance: Probing Structure at the Ångström Scale with Visible Light

Gap Plasmons and Near-Field Enhancement in Closely Packed Sub-10 nm Gap Resonators

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