بهبود حساسیت فله و FoM در آرایه آنتن نانودیپل پلاسمونیک
محورهای موضوعی : فناوری اطلاعات و ارتباطات
سمیرا امیری
1
(Electrical Engineering, Shiraz University of Technology, Shiraz, Iran)
نجمه نزهت
2
(Electrical Engineering, Shiraz University of Technology, Shiraz, Iran)
کلید واژه: شکل شایستگی (FoM) , نانو آنتن , پاسخ نوری , حساسیت , حسگر,
چکیده مقاله :
در این مقاله ، حساسیت آرایه آنتن نانودیپل پلاسمونیک برای مواد مختلف فلز نانودیفل و بستر با تغییر ضریب شکست محیط اطراف محاسبه می شود. عملکرد آرایه پیشنهادی ما در دو طول موج 1310 و 1550 نانومتر ، طول موج پنجره های مخابراتی دوم و سوم مورد مطالعه قرار گرفته است. نشان داده شده است که با استفاده از نانوذرات نقره (Ag) به جای طلا (Au) ، حساسیت فله ساختار نانو بهبود می یابد. با جایگزینی ماده بستر از Si به SiO2 ، حساسیت به ترتیب در طول موجهای 1310 و 1550 نانومتر تا 1220 و 1150 نانومتر / RIU افزایش می یابد ، که برای کاربردهای سنجش بسیار مناسب است. علاوه بر این ، رقم شایستگی (FoM) حسگر پلاسمونیک برای هر دو لایه زیرین و مواد نانودیفلی محاسبه می شود. حداکثر مقدار FoM برای آرایه نانو آنتن با بستر SiO2 و نانودیفل Ag بدست می آید و برابر با 35/14 است. علاوه بر این ، نشان داده شده است که با افزایش ضخامت نانو قطب ، حساسیت ساختار نانو و FoM افزایش می یابد
In this paper, the sensitivity of a plasmonic nanodipole antenna array for different materials of the metal nanodipole and substrate is calculated by changing the refractive index of the surrounding medium. The performance of our proposed array is studied at two wavelengths of 1310 and 1550 nm, the wavelengths of the second and third telecommunications windows. It is shown that by using the silver (Ag) nanodipole instead of the gold (Au) one, the bulk sensitivity of the nanostructure is improved. By replacing the substrate material from Si to SiO2, the sensitivity increases up to 1220 and 1150 nm/RIU at the wavelengths of 1310 and 1550 nm, respectively, that is very suitable for sensing applications. Moreover, the figure of merit (FoM) of the plasmonic sensor is calculated for both substrates and nanodipole materials. The maximum value of the FoM is obtained for the nanoantenna array with SiO2 substrate and Ag nanodipole and it is equal to 14.35. Furthermore, it is shown that by increasing the thickness of the nanodipole, the nanostructure sensitivity and FoM are enhanced
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Iranian Journal of Information Technology & Communication | No.33-34, Vol.9, Fall & Winter 2018 |
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Improvement of the bulk sensitivity and FoM of the plasmonic nanodipole antenna array
* Samira Amiri * Najmeh Nozhat
* Faculty of Electrical Engineering, Shiraz University of Technology, Shiraz, Iran
Abstract
In this paper, the sensitivity of a plasmonic nanodipole antenna array for different materials of the metal nanodipole and substrate is calculated by changing the refractive index of the surrounding medium. The performance of our proposed array is studied at two wavelengths of 1310 and 1550 nm, the wavelengths of the second and third telecommunications windows. It is shown that by using the silver (Ag) nanodipole instead of the gold (Au) one, the bulk sensitivity of the nanostructure is improved. By replacing the substrate material from Si to SiO2, the sensitivity increases up to 1220 and 1150 nm/RIU at the wavelengths of 1310 and 1550 nm, respectively, that is very suitable for sensing applications. Moreover, the figure of merit (FoM) of the plasmonic sensor is calculated for both substrates and nanodipole materials. The maximum value of the FoM is obtained for the nanoantenna array with SiO2 substrate and Ag nanodipole and it is equal to 14.35. Furthermore, it is shown that by increasing the thickness of the nanodipole, the nanostructure sensitivity and FoM are enhanced.
Keywords: Figure of Merit (FoM), Nanoantenna, Optical Response, Sensitivity, Sensor.
1. Introduction
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These resonances are equivalent to the sharp peaks in the absorption spectrum of the nanostructure. With any changes in the surrounding medium of the nanostructure, the resonance wavelength is shifted, and so design of nanosensor is based on these changes. The bulk sensitivity of the sensor that is the ratio of wavelength shift to the variation of the refractive index of the medium is given as follows [7]:
(1) (1)
This means that by any changes in the vicinity of the nanostructure, the resonance wavelength is altered. These changes can predict the possible events around the nanostructure. The structure material is one of the main factors that affect the performance of the nanosensor and is investigated in this article. We have shown that by choosing an appropriate material, the sensitivity of our proposed sensor based on the plasmonic nanodipole antenna array is improved.
2. Geometry
A schematic diagram of our proposed plasmonic nanodipole antennas array and its unit cell are shown in Fig. 1. This structure consists of three main parts: the substrate, the nanodipole, and the surrounding medium. The parameters of the structure are length (l), width (w), thickness (t) and gap of the nanodipole (g). The surrounding medium is chosen to be water that can be considered as an advantage in medical applications such as diagnosis of diseases from the saliva.
(a)
(b)
Fig. 1: a) The schematic view of the proposed plasmonic nanodipole antennas array, and (b) the wide view of a unit cell of the array
In this paper, two different metals of silver (Ag) and gold (Au) are selected for nanodipole. Also, Silicon (Si) and Silica (SiO2) materials are used for the substrate. An infinite array can be created by repeating the unit cell along the x and y directions with the pitch dimensions of p and q, respectively. The pitch size is 300 nm 
300 nm. All simulations are based on the finite-difference time-domain (FDTD) numerical method, with a 1 nm × 1 nm × 1 nm mesh in the region around the nanodipole. The array is illuminated by an x‑polarized plane wave source that is embedded in the substrate and is 2.46 µm bellow the substrate-nanodipole interface. The refractive indices of gold and silver are chosen from Johnson and Christy’s model that some of these data at different wavelengths are depicted in Table. 1 [8]. Also, the refractive index of water at different wavelengths is extracted from Segelstein’s data [9].
Wavelength (nm) | Silver | Gold |
1215 | 0.09+j8.80 | 0.35+j8.20 |
1393 | 0.13+j10.1 | 0.43+j9.50 |
1610 | 0.15+j11.8 | 0.56+j11.2 |
1937 | 0.24+j14.1 | 0.92+j13.8 |
Table. 1. The refractive indices of gold and silver at some wavelengths based on Johnson and Christy’s data [8]
3. Theory and Method
Surface plasmon resonances (SPPs) are the collective oscillations of electrons propagating through the metal-dielectric interfaces. These collective oscillations can be defined by the absorption power of the nanostructure that normally occurs at a specific wavelength. Therefore, the absorption spectrum is useful for computing the nanostructure sensitivity and can be expressed as [10]:
(2) (2)
where T and R are the transmission and reflection powers. In all simulations, the monitors for measuring the transmission and reflection spectra are located 2.5 µm above and below the substrate-nanodipole interface, respectively.
For calculating the nanostructure sensitivity, the refractive index of the surrounding medium at the resonance wavelength 
, the wavelength that the value of the absorption spectrum is maximum, is extracted. When the refractive index of the surrounding medium is changed by the value of 
, a resonance shift in the absorption spectrum occurrs. By measuring the value of the resonance shift, the bulk sensitivity can be achieved by approximating the differential via central finite difference formula [11]:
(3) (3)
where 
is the refractive index of water at the resonance wavelength. In this approximation, the smaller , leads to more accuracy in the sensitivity. In this article the value of is assumed to be 0.005.
4. Simulation Results and Discussions
4.1. Optical Response
As mentioned, the absorption spectrum is used for calculating the bulk sensitivity. The optical response of a structure includes the transmission, the reflection, and the absorption spectra. The optical responses of a periodic array of gold and silver nanodipoles with the dimensions of 
, 
, 
, 
and 
are shown in Fig. 2. It can be seen that the absorption spectrum of the silver nanodipole is much narrower than the gold one. This is because of the differences between the real and imaginary parts of the refractive indices of gold and silver at different wavelengths as depicted in Table. 1. Since the variation of the imaginary part of the refractive index of silver as a function of frequency is very slow compared to gold, the narrower and sharper absorption spectrum can be achieved that is appropriate for sensing applications.
(a)
(b)
Fig. 2. Transmission (line with square), reflection (line with triangle) and absorption (line with circle) of a periodic array of nanodipole antenna with a) gold and b) silver nanodipoles and the dimensions of 
, 
, 
, 
and 
4.2. Bulk Sensitivity
The resonance wavelength of the nanostructure depends on its geometry. By increasing the nanodipole length (l), the resonance wavelength shifts to the higher values [10]. In order to calculate the sensitivity at the resonance wavelengths of 1310 and 1550 nm, the wavelengths of the second and third telecommunication windows, the nanodipole dimensions are optimized according to the method of Ref. [12]. By changing the refractive index of the surrounding medium of about 0.005, the value of the wavelength shift is attained from the absorption spectrum and the sensitivity is calculated according to Eq. (3). The sensitivities of the periodic array of Fig. 1 versus the nanodipole thickness for gold and silver nanodipoles at 1310 and 1550 nm wavelengths are demonstrated in Fig. 3.