www.ietdl.org Published in IET Optoelectronics Received on 15th September 2013 Revised on 14th January 2014 Accepted on 9th February 2014 doi: 10.1049/iet-opt.2013.0108
ISSN 1751-8768
Design of flower-shaped dipole nano-antenna for energy harvesting Mohamed Hussein1,2, Nihal Fayez Fahmy Areed1,3, Mohamed Farhat Othman Hameed1,4, Salah Sabry Ahmed Obayya1 1
Centre for Photonics and Smart Materials, Zewail City of Science and Technology, Sheikh Zayed District, 6th of October City, Giza, Egypt 2 Department of Physics, Faculty of Science, Ain Shams University, Cairo 11566, Egypt 3 Department of Electronics and Communications Engineering, Faculty of Engineering, Mansoura University, Mansoura 35516, Egypt 4 Department of Mathematics and Engineering Physics, Faculty of Engineering, Mansoura University, Mansoura 35516, Egypt E-mail:
[email protected]
Abstract: In this study, a novel design of nano-antenna for energy harvesting is proposed and analysed using three-dimensional finite difference time-domain method. The new design consists of three elements nano-antenna with elliptical shape and with air gap. The numerical simulations are investigated for improving the harvesting efficiency of the nano-antennas within the wavelength range from 400 to 1400 nm. The suggested design has high efficiency of 74.6% at 500 nm where the irradiance of the sun is maximum. The proposed nano-antenna shows an improvement in the harvesting and the total harvesting efficiency over the conventional dipole antenna by 15 and 32.7%, respectively.
1
Introduction
The worldwide energy demand, during the past years, is strongly increased and as a consequence the deleterious effects because of the combustion of fossil are apparent. Recently, renewable energies give strong contributions to power generation without increasing environmental pollutions [1]. In particular, solar energy is largely used because it is a freely available source and the technology utilised to obtain electricity is relatively of low cost [2]. The solar radiation energy owes its origin to the nuclear fusion reaction in the sun. The resulting energy is emitted mainly as electromagnetic radiation in the spectral range 0.2–3 µm [3]. The spectral distribution of the solar radiation which reaches the earth can be approximated by that of a black body at a temperature of 5800 K. About ∼30% of this energy is reflected back to space from the atmosphere, 19% is absorbed by atmospheric gases and reradiated to the earth’s surface in the mid-infrared (IR) range (7–14 µm) and 51% is absorbed by the surface or organic life and reradiated at around 10 µm [4, 5]. There are several approaches that have been pursued to harvest energy from the sun. The photovoltaic (PV) technique is the most common for the conversion of solar energy to electricity [6]. The PV solar cell devices are designed to absorb solar energy in the visible region (400– 700 nm) which constitutes 46% of the solar spectrum [4, 7]. In this regard, there seems to be a strong need to harvest more solar energy by extending the absorbed range to the IET Optoelectron., 2014, Vol. 8, Iss. 4, pp. 167–173 doi: 10.1049/iet-opt.2013.0108
IR region, through developing solar cell antennas covering this region [4, 7]. One of the alternative approaches to PV technology is the ‘nano-antennas’. The concept of collecting solar energy from the sun and the earth radiation using nano-antennas essentially relies on the fact that when an electromagnetic wave is incident on a nano-antenna, a time varying current will be induced on the antenna surface, and hence a voltage will be generated at the feeding point of the antenna. The generated wave will oscillate at the frequency of the incident wave. Consequently, in order to obtain DC power, a suitable rectifier should be embedded at the feed point of antenna. These types of energy harvesting systems are called ‘rectennas’, which basically consist of antennas connected to a rectifier that converts the received signal to DC power and produce electricity [4, 8]. The nano-antennas exhibit a wider angular reception characteristic than that of PV devices. This in turn optimises the solar energy collection during day, and thus cancels the need for sun tracking systems [4]. Vandenbosch and Ma [9] introduced upper bounds for the solar energy harvesting efficiency of nano-antennas. This efficiency was investigated in terms of the dimensions of the nano-antenna and the metal (gold, silver, copper, aluminium and chromium) used as a conductor. These results set upper bounds for any possible process transforming the light into electrical energy. In this regard, the silver exhibited the highest efficiencies, both in free space and on top of a glass (SiO2) substrate, with radiation efficiencies near or slightly above 90% [9]. 167
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www.ietdl.org In this paper, a novel design of flower-shaped dipole nano-antenna for energy harvesting is introduced and analysed by using three-dimensional (3D) finite difference time-domain (FDTD) method [10]. The performance of the proposed antenna is investigated particularly in terms of the radiation efficiency and energy harvesting. The key issue in the design of nano-antenna for energy harvesting is to obtain high efficiency around 500 nm where the solar irradiance is largest. In this regard, the suggested design has maximum efficiency of 74.6% at 500 nm which is more than twice that of the traditional crystalline silicon wafer based solar cells in the market. In addition, the reported design can enhance the efficiency of harvesting the solar energy and total harvesting efficiency by 15 and 32.7%, respectively, over the dipole antenna designed by Vandenbosch and Ma [9]. The paper is organised as follows. Following this introduction, a brief description of the nano-antenna parameters will follow in Section 2. Section 3 presents the simulated design and the temporal response of the reported nano-antenna. Finally, conclusion will be drawn.
2
Nano-antenna parameters
The performance of the nano-rectennas depends on the efficiency of light capturing by the nano-antenna as well as transferring to its terminals. In addition, the capability of the rectifier to transform the captured light into low-frequency power can affect the behaviour of the nano-rectennas. Based on the reciprocity theory [11], it is deduced that the efficiency at transmission is similar to that of the antenna in converting input power, given at its terminals, into radiation, which is called the radiation efficiency η rad of the antenna [9, 11, 12]. In addition, at small scale, no quantum effects have to be taken into account, and then the antenna is able to transmit and receive electromagnetic waves rather than particles. In effect, the coupling between electromagnetic waves and nano-antenna is the same as at microwave frequencies. From the values of the extinction and scattering cross-sections, based on Mie theory [12], the optical radiation efficiency of nano particles can be expressed as Csca Csca = Cext Csca + Cabs
hrad =
(1)
where Csca, Cext and Cabs are scattering, extinction and absorption cross-sections, respectively, that can be calculated by series expansion of the internal and scattered fields into a set of partial waves described by vector harmonics [13]. For energy harvesting applications, and to characterise the antenna, namely, the total harvesting efficiency is used which is given by [9] 1
h
tot
=
0
Fig. 1 Conventional dipole nano-antenna
P(l, T ) × h (l) dl 1 P(l, T ) dl 0 rad
(2)
where λ is the wavelength and P is the Planck’s law for black body radiation defined by
2phc2 1 P(l, T) = × hc/lkT −1 e l5 168 & The Institution of Engineering and Technology 2014
(3)
a Conventional dipole structure [9] b Variations of the wavelength dependent radiation efficiency using MOM [7] and FDTD techniques at different mesh grid sizes Δ c Scattered Ez field magnitude along XY plane d Absorbed Ez field magnitude along XY plane e Current distribution along XY plane
where T is the absolute temperature of the blackbody (in K), h is the Planck’s constant (6.626 × 10−34 Js), c is the speed of light in vacuum (3 × 108 m/s), and k is the Boltzmann IET Optoelectron., 2014, Vol. 8, Iss. 4, pp. 167–173 doi: 10.1049/iet-opt.2013.0108
www.ietdl.org constant (1.38 × 10−23 J/K). In the case of solar energy harvesting, the temperature T is the temperature of the sun surface [1].
3
Simulation results
To validate the simulation results, initially the conventional dipole antenna [9] shown in Fig. 1a has been considered. The conventional dipole antenna [9] consists of two elements of equal length L and height H separated by a distance G. In this investigation, the two elements are made from gold with dipole length L = 250 nm, width W = 40 nm, height H = 40 nm and gap G = 10 nm. In this evaluation, the radiation efficiency calculated by the FDTD method is compared with that of the method of moment (MOM) technique [9]. Fig. 1b shows the variation of the wavelength
dependent radiation efficiency using MOM techniques [9] and FDTD method at different mesh grid sizes Δx = Δy = Δz = Δ. It is revealed from this figure that a good agreement between MOM and FDTD results at Δ = 10 nm. Therefore, mesh grid size Δ will be fixed to 10 nm in all simulations throughout this paper. Figs. 1c and d show the scattering and the absorbing fields along XY-plane of the conventional dipole using air substrate. It can be noted from these figures that the fields are concentrated in Ez component therefore the antenna can harvest linearly polarised light. Fig. 1e shows the induced current distribution along the metal surface of the conventional dipole. It may be noted from this figure that the majority of the field is located along the edge. Therefore the conventional rectangular dipole design is modified in this paper to elliptical shape with elliptical air gap. In addition, the dimension of the air gap is equal to
Fig. 2 Elliptical dipole nano-antenna a Proposed elliptical dipole nano-antenna b Top view of the proposed elliptical dipole nano-antenna c Scattered Ez field component along XY plane d Absorbed Ez field component along XY plane IET Optoelectron., 2014, Vol. 8, Iss. 4, pp. 167–173 doi: 10.1049/iet-opt.2013.0108
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Fig. 3 Variation of the radiation efficiency of the proposed elliptical dipole with the wavelength at different a Minor radii of the air gap b2 b Major radii of the air gap a2
the region of minimum field distribution as shown in Fig. 2a. Fig. 2b shows the top view of the proposed structure. Figs. 2c and d show the scattering and the absorbing fields along XY-plane of the proposed elliptical dipole using air substrate. The proposed design as shown in Fig. 2a consists of two elliptical shape elements with air gap separated by a distance G. Each element of height H and length L has minor and major radii a1 and b1, respectively. However, the elliptical air gap has major and minor radii a2 and b2, respectively. The structure geometrical parameters can affect the harvesting efficiency [9] of the suggested design. Therefore the effect of the antenna dimensions and gap size on the performance of the reported nano-antenna is investigated thoroughly. The impact of the major a2 and minor b2 radii of the air gap is first investigated. However, the other parameters are fixed at G = 10 nm and H = 40 nm. In addition, the major a1 and minor axes b1 of the metals are taken as 125 and 40 nm, respectively. First, the effect of the minor radius of the air gap is studied, whereas the major radius is fixed at 70 nm. Fig. 3a shows the wavelength dependent radiation efficiency at different radii of the minor radius of the air gap. It is evident from this figure that the optimum value for the minor radius b2 is equal to 20 nm which corresponds to the highest efficiency and broadest bandwidth. Next, the effect of the major radius a2 of the air gap is reported, whereas the minor radius b2 is taken as 20 nm. The numerical results reveal that the behaviour is almost the same for different major radii of the air gap as shown in Fig. 3b. In addition, maximum harvesting efficiency is obtained at a2 = 80 nm, with a slight change in the bandwidth of the proposed antenna. In microwave regime, the electrical length should be equal to an integer multiple of half the wavelength in order to obtain the resonance case which is not the case with nano-antennas. In this regard, the resonance condition is affected by the length L of each element, radii of the metal ellipse a1 and b1 and the air gap radii a2 and b2. Fig. 4 shows the wavelength dependent radiation efficiency at different values of the length of each element L. It should be noted that for the proposed elliptical nano-antenna, the length L is equal to the major diameter 2a1. In this paper, the height H and gap G are fixed to 40 and 10 nm, respectively. In addition, different cases are studied; L = 150 nm (a1 = 75 nm, b1 = 24 nm, a2 = 42 nm, b2 = 9 nm), L = 200 nm (a1 = 100 nm, b1 = 32 nm, a2 = 56 nm, b2 = 12 nm), L = 250 nm (a1 = 125 nm b1 = 40 nm, a2 = 70 nm, b2 = 15 nm), 170 & The Institution of Engineering and Technology 2014
L = 300 nm (a1 = 150 nm b1 = 48 nm, a2 = 100 nm, b2 = 18 nm), L = 350 nm (a1 = 175 nm b1 = 56 nm, a2 = 133 nm, b2 = 21 nm) and L = 400 nm (a1 = 200 nm b1 = 64 nm, a2 = 177 nm, b2 = 24 nm). It is found that the harvesting efficiency and bandwidth increase with increasing the length of each element L as shown in Fig. 4. Next, the effect of the height H on the nano-antenna performance is studied. In this evaluation, the height of the antenna element is changed from 40 to 80 nm with a step of 10 nm. However, the other parameters are fixed at a1 = 125 nm, b1 = 40 nm, G = 10 nm, a2 = 100 nm and b2 = 20 nm. Fig. 5 shows the wavelength dependent radiation efficiency at different H values. It is evident from this figure that the radiation efficiency increases with increasing the height H up to certain limit where the absorption losses will cancel the enhancement of the increase of the density wave of electrons. In addition, the harvesting efficiency shows an effective improvement by increasing the antenna height. The effect of the gap distance G between the two elliptical elements is also reported, whereas the other parameters are fixed at H = 40 nm, a1 = 125 nm, b1 = 40 nm, a2 = 100 nm and b2 = 20 nm. Fig. 6 shows the variation of radiation efficiency of the proposed elliptical dipole nano-antenna with the wavelengths at different gap distances G. It is
Fig. 4 Variation of the radiation efficiency of the proposed elliptical dipole with the wavelengths at different values of the length L of each elements IET Optoelectron., 2014, Vol. 8, Iss. 4, pp. 167–173 doi: 10.1049/iet-opt.2013.0108
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Fig. 5 Variation of the radiation efficiency of the proposed elliptical dipole with the wavelengths at different dipole height H values
evident from this figure that the radiation efficiency decreases with increasing the gap distance between the two elements. A comparison between the traditional dipole nano-antenna and the proposed elliptical dipole nano-antenna with air gap, in terms of the radiation efficiency and total harvesting efficiency, is also demonstrated. In this paper, the dimensions of the traditional dipole nano-antennas are taken as W = 40 nm, H = 40 nm and L = 250 nm. However, the dimensions of the proposed design are fixed to a1 = 125 nm, b1 = 40 nm, a2 = 100 nm b2 = 20 nm and H = 40 nm. Fig. 7 shows the variation of radiation efficiency with the wavelength for the conventional dipole and the proposed elliptical shape nano-antenna. It is evident from this figure that the harvesting efficiency of the suggested elliptical nano-antenna is greater than that of the conventional dipole antenna. In addition, the investigated nano-antenna has harvesting efficiency of 74.6% at 500 nm where maximum irradiance of the sun occurs. Moreover, the obtained harvesting efficiency is more than twice that of the traditional crystalline silicon wafer based solar cells. This is because of the metal thickness and the air gap that acts as a cavity which harvests more energy. Therefore the proposed structure improves the radiation efficiency and the total harvesting efficiency by 10 and 17.7%, respectively. In addition, the new design reduces the size of the metal used in the nano-antenna fabrication. Moreover, the suggested
Fig. 7 Variation of the radiation efficiency with the wavelength for the conventional dipole [9] and the proposed elliptical nano-antenna
antenna has a bandwidth from 450 to 1200 nm with harvesting efficiency of ∼80%. Next elliptical nano-antenna with two elliptical shapes in each element is considered. In this type, each element consists of two ellipses with the same size and with rotation angle θ between their major axes as shown in Fig. 8a. To detect the effect of the rotation angle θ on the antenna performance, the other parameters are fixed to a1 = 125 nm, b1 = 40 nm, a2 = 100 nm, b2 = 20 nm, G = 10 nm and H = 40 nm. Fig. 9 shows the wavelength dependent radiation efficiency at different rotation angles, 5, 10, 15, 20, 30 and 45°. It is found that the radiation efficiency is slightly affected by the rotation angle θ as shown in Fig. 9. To increase the capability of energy harvesting, a flower-shaped nano-antenna, shown in Fig. 8b, is suggested. The proposed flower nano-antenna dipole is composed of three ellipses in each element, with rotation angle θ = 10° to harvest energy within the wavelength range from 400 to 1400 nm. Fig. 10 shows the calculated radiation efficiency of the conventional dipole antenna [9] and proposed elliptical dipole antennas with one ellipse
Fig. 8 Schematic diagram of Fig. 6 Variation of the radiation efficiency of the proposed elliptical dipole with the wavelengths at different gap distances IET Optoelectron., 2014, Vol. 8, Iss. 4, pp. 167–173 doi: 10.1049/iet-opt.2013.0108
a Elliptical dipole nano-elements with two ellipses in each element b Flower-shaped dipole nano-antenna 171
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Fig. 9 Variation of the radiation efficiency of the proposed two elliptical elements dipole antenna with the wavelengths at different rotation angles between two ellipses
(type one), two ellipses (type two) and the three ellipses (flower shaped) in each element. In this investigation, the dimensions of the traditional dipole nano-antennas are taken as W = 40 nm, H = 40 nm and L = 250 nm. However, the dimensions of the proposed designs are θ = 10°, H = 40 nm, G = 10 nm, a1 = 125 nm, b1 = 40 nm, a2 = 100 nm and b2 = 20 nm. It is evident from this figure that the flower-shaped dipole shows an increase in the radiation efficiency and the total harvesting efficiency by 15 and 32.7%, respectively, over the conventional dipole antenna. These results considerably agree with the upper bound declared by Vadanbosch and Ma [9] for nano-antennas. In an attempt to find a metal that is more abundant and hence considerably less expensive to employ as antenna, different metals are tested such as Cr, Al, Au and Ag. Fig. 11 shows the wavelength dependent radiation efficiency of the suggested flower-shaped dipole at different metals. In this paper, the other parameters are fixed to θ = 10°, H = 40 nm, G = 10 nm, a1 = 125 nm, b1 = 40 nm, a2 = 100 nm and b2 = 20 nm. It is evident from Fig. 11 that the radiation efficiency depends essentially on the metal conductivity. In this regards, the silver possesses the highest efficiency from 450 nm to 800 nm; whereas gold has
Fig. 10 Variation of wavelength dependent radiation of the conventional dipole nano-antenna [9] and the proposed elliptical nano-antennas 172 & The Institution of Engineering and Technology 2014
Fig. 11 Variation of wavelength dependent radiation efficiency of the proposed flower-shaped dipole at different metals
obvious high efficiency from 800 nm to 1200 nm. This is in a good agreement with the results for dipole antenna studied by Vadanbosch and Ma [9]. It is also evident form Fig. 11 that metals with high electrical conductivity such as gold and silver have much better performance than metals with low electrical conductivity such as Al and Cr in the wavelength range of interest. In recent practical applications, especially in biophysics and sensorics, it is essential to take into account the influence of the surrounding medium on the optical properties of the nanoparticles [12]. As the proposed design is intended for different applications, the dependence of energy harvesting on the refractive index of the surrounding medium is essential in this scenario. Fig. 12a shows variation of the wavelength dependent radiation efficiency of the proposed flower-shaped dipole antenna and the conventional dipole antenna using different surrounding mediums, air (nm = 1), water (nm = 1.33) and fused silica (nm = 1.46). However, the other parameters are kept constant at θ = 10°, H = 40 nm, G = 10 nm, a1 = 125 nm, b1 = 40 nm, a2 = 100 nm and b2 = 20 nm. In addition, the conventional dipole parameters are taken as W = 40 nm, H = 40 nm and L = 250 nm. It is revealed from Fig. 12 that the radiation efficiency for the flower-shaped dipole antenna decreases with increasing the refractive index of the surrounding medium. Fig. 12a also shows that the index of the surrounding medium has slight effect on the radiation efficiency of the suggested nano-antenna at higher wavelengths. In addition, the behaviour of the radiation efficiency for the suggested flower-shaped nano-antenna follows the same trend as the conventional dipole antenna. At 500 nm, the radiation efficiency of the flower-shaped antenna surrounded by air (nm = 1) shows 40% increase over the conventional dipole antenna. Additionally, the suggested nano-antenna can harvest a linearly polarised light over a broad bandwidth extending from 500 nm to 1200 nm. It is also evident from Fig. 12a that 15 and 20% improvements in the radiation efficiency of the proposed nano-antenna are obtained with surrounding mediums of refractive indices 1.33 and 1.46, respectively, in the wavelength range from 800 to 1200 nm, over the conventional dipole antenna. Figs. 12b and c show the captured absorbed fields with air and fused silica mediums at 550 nm, respectively. It can be observed from these figures that fused silica substrate induces surface waves which minimise the radiation efficiency by about 20% compared with air substrate. IET Optoelectron., 2014, Vol. 8, Iss. 4, pp. 167–173 doi: 10.1049/iet-opt.2013.0108
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Fig. 12 Variation of the wavelength dependent radiation efficiency a Variation of wavelength dependent radiation efficiency of the proposed flower-shaped dipole and conventional dipole antenna at different surrounding mediums b Absorbed electric field Ez along XY plane for flower-shaped antenna using air substrate c Absorbed electric field Ez along XY plane for flower-shaped antenna using fused silica substrate
4
Conclusion
In this paper, a novel flower-shaped dipole nano-antenna for energy harvesting is presented and analysed using an accurate 3D-FDTD method. The proposed flower-shaped design exhibits higher efficiency over the conventional dipole antenna by at least 15%. In addition, the total harvesting efficiency is enhanced by 32.7%. Moreover, the suggested nano-antenna offers large bandwidth in the wavelength range from 450 to 1400 nm. Additionally, the flower-shaped dipole nano-antenna has better efficiency of 74.6% compared with the conventional solar cells at λ = 500 nm at which the sun irradiance is maximum. The obtained results pave the road to higher efficiencies in transforming light into electrical energy through such designs.
5
References
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