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Abstract
This paper reports a systematic study of the plasmonic properties of periodic arrays of gold nanoparticles in contact with a novel transparent conducting oxide (TCO) material - Hafnium-doped zinc oxide (HZO). Resonant coupling of the grating diffraction orders with the propagating surface plasmons along the waveguide film, which can generate large field enhancements, are investigated by FDTD simulations. By tuning the ratio of Hf dopants to ZnO film, the plasmon resonances can be tuned and narrower resonance with FWHM of 2-5 nm are observed. Such plasmonic substrates are shown in this study to have high figure of merit in refractive index sensing applications.
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1. Introduction
The resonant coupling of electromagnetic (EM) waves to collective oscillations of free electrons in metals, known as surface plasmon resonances (SPRs), has been widely investigated because of its possible control of light properties at the nanometer scale [1,2]. The simplest types of plasmonic nanostructure that can support localized surface plasmon polaritons (LSPPs) are metal nanoparticles. However, because of radiative damping and dynamic depolarization (the effect of retardation within the particle), the spectral width of localized SPRs (LSPRs) in single metallic nanoparticles is usually broad [3]. But this can be mostly overcome in an array of nanoparticles by exploiting the diffractive coupling between neighboring particles that suppresses the radiative loss and produces higher intense electric fields compared to a single nanoparticle [4]. Another promising method to engineer the quality of SPR systems is to mediate the interactions between SPPs and cavity or dielectric waveguide modes, which results in a waveguide-plasmon hybridization with distinct properties for transmission [5].
Several transparent conductive oxides (TCO) are found to be promising plasmonic waveguide materials for many applications ranging from sensing, information processing, sub-wavelength imaging, etc. A commonly used TCO, Zinc Oxide (ZnO), is a n-type material owing it to its oxygen vacancies and Zn interstitials [6], with a relatively large direct band gap of value around 3.3 eV. Doping can induce modification in ZnO electrical, optical, and structural properties [7]. It has been demonstrated that Hafnium (Hf) doped ZnO shows a decrease in resistivity and increase in carrier concentration [8]. In this work, a detailed plasmonic study is performed on gold (Au) nanoparticles deposited on HZO film by changing the array period and the waveguide thickness. When the nanostructures are arranged in the form of periodic arrays, new wave vectors are introduced that are directly related to the grating parameters and to the appearance of the diffraction orders of the grating [9]. These wave vectors can be used to excite the plasmon modes supported by the grating structure, the Bragg modes [10], corresponding to the resonant coupling of the Bragg diffraction waves and the propagative surface plasmons of the HZO. Such a configuration generates an intense electric field, localized at the top and at the edge of nanostructures, resulting in a high figure of merit (FOM) that can be optimized by tuning the doping ratio of Hf to ZnO. This opens up a wide range of applications in plasmonics based sensors [11], enhanced spectroscopies like SERS (surface enhanced Raman spectroscopy) [12], surface plasmon enhanced infrared photodetection [13], and others. It has to be noted that the current state-of-the-art in nanolithography [14–17] enables the fabrication of nano structures on HZO film that are described in this paper, thereby making our proposed plasmonic devices viable and practical.
2. Numerical simulation methods
The plasmonic system under investigation consists of a periodic array of Au nanoparticles on the top of a thin Hf-doped ZnO film sandwiched between a glass substrate (ns = 1.46) and cover (air, na = 1), as demonstrated in Fig. 1, where t is the thickness of the waveguide layer, d and h are the diameter and height of the Au nanoparticle, and p is the period of the square lattice. The material properties of Hf-doped ZnO film for different doping ratios are taken from Ref. [18]. A three-dimensional finite difference time domain (3D-FDTD) method implemented in a commercial software from Lumerical Inc. (Canada) is used to calculate the transmission spectra and electric field distributions in the vicinity of the Au nanoparticles on HZO film.
Fig. 1. Schematic geometry of Au nanoparticle arrays on top of Hafnium doped ZnO layer sandwiched between the glass substrate and air.
The geometry of nanoparticle arrays is described by using Cartesian coordinates with the x and y axis in the plane and with z axis perpendicular to the plane of the arrays. The mesh size is 2 nm × 2 nm × 2 nm in all the simulations. Convergence checks lead to the conclusion that this mesh size is sufficient for accurate results. Perfectly matched layers (PML) are used as absorbing boundary condition for z-axis to prevent counterfeit reflections and periodic Bloch boundary conditions in x and y axes. Electromagnetic fields in the proximity of the nanoparticles were calculated assuming plane wave illumination, with wavelengths varying between 500 nm and 900 nm. The FDTD calculations were carried out for TE (transverse electric) polarization of the electric field, i.e. the incident E-field being polarized along the axis of the nanoparticle gratings. Optical constants of gold are taken from literature [19]. The periodic arrangement of the metallic nanoparticles can provide the necessary momentum of coupling between the SPPs and the waveguide modes, leading to the generation of Bragg modes and the corresponding resonances and it occurs when the following phase matching condition is fulfilled, as shown in Eq. (1),
The transmission spectra are measured at plane 200 nm below the glass/HZO film interface. A few FDTD simulations were run to find the optimized dimensions for the nanoparticles and the array periodicity so as to have a better figure of merit for the plasmon resonances. Figure 2(a) shows the comparison of transmission spectra for plasmonic structures with and without HZO film. Three transmission dips, related to different plasmonic modes, are observed for the nanostructure on top of HZO film.
Fig. 2. (a) Transmission spectra for the geometry with p = 400 nm, d = 100 nm and h = 50 nm and HZO film of thickness t = 200 nm depicted in Fig. 1. Electric field distributions of Exz components at three different wavelengths of 630 nm, 670 nm and 750 nm, corresponding to three transmission dips in (a): (b)-(d) with HZO film and (e)-(g) without HZO film. Scale bar is 50 nm. (In the current calculation the ratio of Hf to ZnO doping is taken as 1:1).
In general, a dielectric thin film supports one guided TM0 mode that in the limit of large waveguide thickness corresponds to the propagating plasmon mode at the glass substrate/HZO interface. In addition, a TM1 propagating plasmon mode travels at the opposite interface (air/nanoparticle interface). The two narrow dips are due to the interaction between the plasmon polaritons and these quasi guided TE/TM dielectric waveguide modes. Only when the hybrid modes confirm the period of pillar arrays can the Bragg resonances be formed in the x–y plane [21]. The wide resonance dip (observed in both the geometries) is assigned to the LSPR from the nanoparticles.
To better understand the nature of the resonances, the field distributions around the nanoparticles for the three resonant wavelengths are also assessed. Figure 2(b)-(d) shows the cross section of the electric field distributions, Exz at the three resonant dips with HZO film. LSPR around the nanostructure is clearly distinguished at 750 nm (Fig. 2(d)). For the lower wavelengths, significant field enhancement is observed at 670 nm (due to the Bragg mode at the Au/HZO interface, BMg) and at 630 nm (due to the Bragg mode at the Au/air interface, BMa), which represent the formation of SPPs.
3. Resonance shift with doping concentration
It has been reported in Ref. [18] that Hf doping of ZnO films allows engineering both electrical and optical properties and are demonstrated to have a better conducting performance than pure ZnO films. Hf is embedded in the ZnO matrix as HfO2 phases [18]. The concentration is obtained from the ratio of layer numbers of ZnO to HfO2 by changing the number of ZnO layers to one layer of HfO2. As the Hf doping increases, the carrier concentration is improved. A high negative electric permittivity associated with this improves the metallic property, with an increased band gap [22] and lesser inter band transitions. The bandgap of the HZO for different doping concentration can be found using Eqs. (2) and (3) [18],
where α is the absorption coefficient, h is Planck’s constant, ν is the photon frequency, Eg is the optical band gap and k is the extinction coefficient.It has been observed that the optical band gap increases with doping owing to the Burstein [23]-Moss [24] effect, where in the bandgap widens due to the filling of the lowest states of the conduction band that causes the Fermi level to move above the conduction band minimum. This in turn results in the blue shift of the plasmon resonances owing to a higher restoring force. This is observable in Fig. 3(a) and (b). And the higher value of permittivity with the increase in carrier concentration helps with the increase in group velocity. The increase in bandgap with doping allows higher energetic photons to be transmitted and thus improving the plasmonic response. As the doping concentration is increased, the FWHM (full width half maximum) of the plasmon resonance due to Bragg resonant modes improves to 2–5 nm from 5-8 nm for pure ZnO.
Fig. 3. (a) Transmission spectra of the geometry in Fig. 2 for different doping concentrations. The downward arrows indicate the blue shift of plasmon resonances with doping. In the panel, each curve is shifted upwards with respect to the previous for better visualization. (b) Shift in spectral position for the three plasmon resonances for different compositions.
4. Parameter study
In this section, the behavior of the plasmonic resonances for different thickness of HZO film and the periodicity of the structure are discussed. As it is seen in Fig. 4(a), there is a profound shift and appearance of resonances when the thickness, t, is varied due to the dependence of dielectric waveguide mode on the former. Only the wide resonance dip, that is attributed to the LSPs, is observed when the waveguide thickness is 50 nm since the thin waveguide film does not support guided modes [25]. As the thickness is increased from 100 to 200 nm, it generates the Bragg modes resulting in the appearance of the narrow dips in the transmission spectra owing to the interaction between plasmon polaritons and the dielectric waveguide modes. Figure 4(b) shows the shift in resonances as the lattice period is changed.
Fig. 4. Simulated transmission and absorption spectra with varying waveguide layer thicknesses. The transmission spectra are represented by the solid lines and dotted lines for the absorption spectra, respectively. In the panel, each curve is shifted upwards with respect to the previous for better visualization. (b) Calculated transmission spectra in dependence on the period of square lattice for a 200-nm-thick HZO film and Au nanoparticles of height 50 nm and diameter of 100 nm. The ratio of Hf to ZnO doping is 1:1.
The wide resonance from the localized SPPs exhibits a redshift of around 80 nm and a reduction in FWHM from 180 nm to 40 nm as the period is increased from 350 nm to 500 nm. The electromagnetic interactions between the neighboring particles in the periodic array cause these occurrences. Compared to the resonance from the LSPs, Bragg resonances are more influenced by the period changes due to its dependence on the waveguide modes in the dielectric film.
5. Nanoantenna array on top of HZO film
The majority of the designs of plasmonic substrates for sensing applications have been focused on the optimization of plasmonic resonances [26]. By changing the design geometry, the properties of the plasmonic resonances, as well as their interaction strength with the environment can be engineered. In such a scenario, nanoantennas have already demonstrated to have outstanding performance in plasmonic applications due to the high field intensity in the gap and the lightning-rod effect [27]. Nanoantennas are made of two gold nanotriangles separated by an ultra-small gap in a tip-to-tip configuration, the so-called bow tie, as shown in Fig. 5(a) [28]. In this section, we compare the electric field enhancements when a nanoantenna array is placed on top of HZO film instead of the nanoparticle array.
Fig. 5. (a) Schematic of plasmonic nanoantenna array placed on HZO film (b) Comparison of the field enhancement, abs(E/E0), spectra of arrays of nanoparticles and nanoantennas on HZO film. Electric field distributions of Exz components around nanoantenna for (c) Bragg modes at the nanoantenna/air interface (at 630 nm) (d) Bragg modes at the nanoantenna/HZO interface (at 680 nm) (e) LSPR position (at 760 nm) and (f) xy profile of the nanoantenna at 760 nm. Dashed lines represent the nanoantenna and HZO film, scale bar is 50 nm. The ratio of Hf to ZnO doping is 1:1.
Numerical simulations are performed to calculate the spectra and near-field electric distribution of the nanoantennas. The structure was excited by normally incident light with its polarization along the direction of the antennas. The side-length of the bowtie is 140 nm. The gap size between the nanotriangles is 20 nm. The array periods in x- and y-directions are p = 400 nm. When the nanoantenna is placed on HZO film, there are localized as well as propagating modes as explained before. Optimization of parameters of the bow tie antenna are done by changing the period and the gap between the nano triangles. By changing the array period, strong interactions are generated between the LSPRs and the non-localized SPRs when their resonant frequency gets closer to each other. Significantly strong electric field enhancements, abs(E/E0), around 10-fold, (Fig. 5(b)) are achieved in the given geometry by using these strong interactions. Here, E0 is the incident electric field and E is the electric field at the surface geometry. Figure 5(c)-(e) shows the cross section of the electric field distributions, Exz at the three resonant peaks and Fig. 5(f) represents the xy profile of the nanoantenna at LSPR resonance.
6. Application for refractive index sensing
A major advantage of the proposed system is its narrow plasmonic resonances of FWHM of around 2-5 nm. This feature can be adopted for one of the much-employed applications of plasmonics, refractive index sensing. The principle of plasmonic refractive index sensors is based on the strong dependence of the resonance frequency on the surrounding environment [29]. Even small changes of the material composition of the surrounding medium lead to resonance shifts that can be detected via optical readout. This mechanism can be utilized to detect a broad range of different analytes, such as gases [30], liquids [31], or biomolecules [32]. In this work, the resonance shifts of the Bragg modes, particularly those at the nanoantenna/glass interface are investigated, as they generate plasmon resonances with narrower line width and high field enhancement.
The transmission spectra of the proposed plasmonic sensor with nanoantennas for different analyte refractive indices ranging from 1.00 to 1.50 are presented in Fig. 6(a). The resonance wavelengths appear red shift with the increasing value of analyte refractive index. The wavelength sensitivity, S, is calculated from the dip position shift per refractive index unit (RIU) and can be defined by the following Eq. (4) [21],
where Δλdip is the dip shift, and Δna is the variation of analyte refractive index.Fig. 6. (a) Transmission spectra shift in different environmental media. The geometry parameters of the system are the same as that used in Fig. 5. (b) Calculated refractive index range sensitivity and FOM as a function of the refractive index.
Figure 6(b) shows sensitivity and figure of merit (FOM) values as a function of the analyte refractive indices. The transmission spectra exhibit sharper resonances with profound wavelength shift as the analyte refractive indices increase in the calculated sensing range of 1.00 to 1.50. Figure of merit of the system is calculated from the FWHM by using Eq. (5),
Change in refractive index leads to a notable FOM value up to 40 RIU-1 for nanoantenna arrays, compared to a 30 RIU-1 for nanoparticle arrays. It is thus demonstrated that our proposed plasmonic system may serve as high-performance efficient localized plasmonic sensor. It is noted that the sensitivity can be further improved by optimizing the design and geometry of the system to obtain sharper plasmonic resonances with narrower line width.
7. Conclusions
In this work, we discuss on the systematic study of the optical properties of plasmonic substrates composed of Au nanoparticles deposited on a thin HZO film. We observed several plasmon modes due to the coupling of the grating diffraction orders with the propagative plasmons inside the film. Several FDTD calculations were done to demonstrate the shift in the plasmonic spectral positions for several sets of Hafnium-doped ZnO samples. For a Hf:ZnO cycle ratio of 1:1, it is shown to have a better plasmonic response and higher field enhancement. The location and bandwidth of the three resonances can be tuned by the thickness of the waveguide layer and the period of the nanoparticle arrays. A FOM of 40 RIU-1 can be achieved for the proposed plasmonic system with nanoantennas that could have large range of applications in refractive index sensing and enhanced spectroscopies.
Acknowledgments
The authors cordially thank Mr. Juan Villegas at the New York University Abu Dhabi for the initial discussions on the material properties of Hf doped ZnO film.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. W. L. Barnes, A. Dereux, and T. W. Ebbesen, “Surface plasmon subwavelength optics,” Nature 424(6950), 824–830 (2003). [CrossRef]
2. Y. Jin, L. Zhou, J. Liang, and J. Zhu, “Electrochemically driven dynamic plasmonics,” Adv. Photonics 3(04), 044002 (2021). [CrossRef]
3. F. Wang and Y. R. Shen, “General Properties of Local Plasmons in Metal Nanostructures,” Phys. Rev. Lett. 97(20), 206806 (2006). [CrossRef]
4. S. Zou, N. Janel, and G. C. Schatz, “Silver nanoparticle array structures that produce remarkably narrow plasmon lineshapes,” The J. Chem. Phys. 120(23), 10871–10875 (2004). [CrossRef]
5. T. Zentgraf, S. Zhang, R. F. Oulton, and X. Zhang, “Ultranarrow coupling-induced transparency bands in hybrid plasmonic systems,” Phys. Rev. B 80(19), 195415 (2009). [CrossRef]
6. H. Morkoç and Ü Özgür, Zinc Oxide: Fundamentals, Materials and Device Technology (John Wiley & Sons, 2008).
7. S.-B. Zhu, Y. Geng, H.-L. Lu, Y. Zhang, Q.-Q. Sun, S.-J. Ding, and D. W. Zhang, “Effects of rapid thermal annealing on Hf-doped ZnO films grown by atomic layer deposition,” J. Alloys Compd. 577, 340–344 (2013). [CrossRef]
8. Y. Geng, Z.-Y. Xie, W. Yang, S.-S. Xu, Q.-Q. Sun, S.-J. Ding, H.-L. Lu, and D. W. Zhang, “Structural, optical, and electrical properties of Hf-doped ZnO films deposited by atomic layer deposition,” Surf. Coat. Technol. 232, 41–45 (2013). [CrossRef]
9. H. T. M. C. M. Baltar, K. Drozdowicz-Tomsia, and E. M. Goldys, “Plasmonic properties of periodic arrays of Ag nanocylinders and dimers, and the effects of an underlying Ag layer,” J. Phys. Chem. C 122(38), 22083–22093 (2018). [CrossRef]
10. E. J. Smythe, E. Cubukcu, and F. Capasso, “Optical properties of surface plasmon resonances of coupled metallic nanorods,” Opt. Express 15(12), 7439–7447 (2007). [CrossRef]
11. P. Thitirat, L. Chutiparn, I. Ryousuke, S. Kazunari, K. Keizo, E. Sanong, O. Kontad, and B. Akira, “Enhanced organic solar cell performance: Multiple surface plasmon resonance and incorporation of silver nanodisks into a grating-structure electrode,” Opto Electronic Advances 2, 190010 (2019).
12. F. A. Banville, J. Moreau, M. Sarkar, M. Besbes, M. Canva, and P. G. Charette, “Spatial resolution versus contrast trade-off enhancement in high-resolution surface plasmon resonance imaging (SPRI) by metal surface nanostructure design,” Opt. Express 26(8), 10616–10630 (2018). [CrossRef]
13. T. Jinchao, S. Fei, M. Junhuizhi, Y. M. T. Landobasa, Q. Li, and Z. Dao Hua, “Surface plasmon enhanced infrared photodetection,” Opto-Electron. Adv. 2, 180026 (2019).
14. B. D. Gates, Q. Xu, M. Stewart, D. Ryan, C. G. Willson, and G. M. Whitesides, “New approaches to nanofabrication: molding, printing, and other techniques,” Chem. Rev. 105(4), 1171–1196 (2005). [CrossRef]
15. A. Dhawan, M. Gerhold, and T. Vo-Dinh, “Theoretical simulation and focused ion beam fabrication of gold nanostructures for surface-enhanced raman scattering (SERS),” Nanobiotechnol 3(3-4), 164–171 (2007). [CrossRef]
16. H. Im, K. C. Bantz, N. C. Lindquist, C. L. Haynes, and S.-H. Oh, “Vertically oriented sub-10-nm plasmonic nanogap arrays,” Nano Lett. 10(6), 2231–2236 (2010). [CrossRef]
17. N. Gisbert Quilis, M. Lequeux, P. Venugopalan, I. Khan, W. Knoll, S. Boujday, M. Lamy de la Chapelle, and J. Dostalek, “Tunable laser interference lithography preparation of plasmonic nanoparticle arrays tailored for SERS,” Nanoscale 10(21), 10268–10276 (2018). [CrossRef]
18. B. Alfakes, J. Villegas, H. Apostoleris, R. Devarapalli, S. Tamalampudi, J.-Y. Lu, J. Viegas, I. Almansouri, and M. Chiesa, “Optoelectronic tunability of hf-doped ZnO for photovoltaic applications,” The J. Phys. Chem. C 123(24), 15258–15266 (2019). [CrossRef]
19. P. B. Johnson and R. W. Christy, “Optical Constants of the Noble Metals,” Phys. Rev. B 6(12), 4370–4379 (1972). [CrossRef]
20. M. Lequeux, D. Mele, P. Venugopalan, R. Gillibert, S. Boujday, W. Knoll, J. Dostalek, and M. Lamy de la Chapelle, “Plasmonic Properties of Gold Nanostructures on Gold Film,” Plasmonics 15(6), 1653–1660 (2020). [CrossRef]
21. J. Liu, B. Xu, J. Zhang, and G. Song, “Double plasmon-induced transparency in hybrid waveguide-plasmon system and its application for localized plasmon resonance sensing with high figure of merit,” Plasmonics 8(2), 995–1001 (2013). [CrossRef]
22. B. D. Viezbicke, S. Patel, B. E. Davis, and D. P. Birnie III, “Evaluation of the Tauc method for optical absorption edge determination: ZnO thin films as a model system,” Phys. Status Solidi 252(8), 1700–1710 (2015). [CrossRef]
23. E. Burstein, “Anomalous optical absorption limit in InSb,” Phys. Rev. 93(3), 632–633 (1954). [CrossRef]
24. T. S. Moss, “The Interpretation of the Properties of Indium Antimonide,” Proc. Phys. Soc. B 67(10), 775–782 (1954). [CrossRef]
25. J. Zhang, L. Cai, W. Bai, and G. Song, “Hybrid waveguide-plasmon resonances in gold pillar arrays on top of a dielectric waveguide,” Opt. Lett. 35(20), 3408–3410 (2010). [CrossRef]
26. A. Brolo, J. Ferreira, M. Santos, C. Escobedo, D. Sinton, F. Eftekhari, and R. Gordon, “Development of plasmonic substrates for biosensing,” Proc. SPIE 7035, 703503 (2008). [CrossRef]
27. S. K. Patel and C. J. E. A. M. Argyropoulos, “Plasmonic nanoantennas: enhancing light-matter interactions at the nanoscale,” EPJ Appl. Metamater. 2, 4 (2015). [CrossRef]
28. M. Mesch, T. Weiss, M. Schäferling, M. Hentschel, R. S. Hegde, and H. Giessen, “Highly sensitive refractive index sensors with plasmonic nanoantennas−utilization of optimal spectral detuning of Fano resonances,” ACS Sens. 3(5), 960–966 (2018). [CrossRef]
29. W. Su, Y. Ding, Y. Luo, and Y. Liu, “A high figure of merit refractive index sensor based on Fano resonance in all-dielectric metasurface,” Results Phys. 16, 102833 (2020). [CrossRef]
30. N. Strohfeldt, J. Zhao, A. Tittl, and H. Giessen, “Sensitivity engineering in direct contact palladium-gold nano-sandwich hydrogen sensors [Invited],” Opt. Mater. Express 5(11), 2525–2535 (2015). [CrossRef]
31. M. Mesch, C. Zhang, P. V. Braun, and H. Giessen, “Functionalized hydrogel on plasmonic nanoantennas for noninvasive glucose sensing,” ACS Photonics 2(4), 475–480 (2015). [CrossRef]
32. C. Rosman, J. Prasad, A. Neiser, A. Henkel, J. Edgar, and C. Sönnichsen, “Multiplexed plasmon sensor for rapid label-free analyte detection,” Nano Lett. 13(7), 3243–3247 (2013). [CrossRef]
