Ceramic Material based Optical Antenna for Multiband Photonics Applications

In this article, design and development of a multiband ceramic materialbased Optical antenna is discussed. Square shaped aperture is utilized to excited silicon based dielectric resonator. This type of excitation system provides the capability to create triple hybrid mode (HEM11δ, HEM12δ and HEM11δ+2) inside the cylindrical shaped ceramic material. Due to this feature, the proposed aerial is operating over diverse frequency bands i.e. 117.5-140 THz, 158-165.5 THz and 175.2-190.5 THz respectively. Stable radiation characteristics as well as good value of gain (about 4.0 dBi) makes the proposed Nano-radiator applicable for hyper spectral imaging system (125 THz) and VLC for wireless LAN (160/180THz). Keywords-Optical Antenna, Multi Resonance, Hybrid Mode, Broadsided Radiation Pattern


I. INTRODUCTION
In the recent years, the main development in field of nanotechnology is the design and analysis of antennas in optical domain. There are wide used of optical antennas such as, imaging, biosensors, optical transmitter as well as receiver [1][2][3]. It is also used as feeding technique for waveguide and transmission line at optical spectrum [4][5][6]. Optical antenna, in general, is downscaled version of radiators in RF domain. But, the design procedure is not as easy as said. There are large number of challenges occurs as the frequency of operations moves from GHz to THz, such as high metallic losses due to skin effect, high surface wave losses and soon [7]. Use of dielectric resonator based radiators is the easiest remedy to all these problems. It is due to the natural potential of ceramic based radiators such as, absence of metallic losses, able to create multiple mode patterns, absence of surface wave losses and easy to achieve wider impedance bandwidth [8].
Literature of antenna design at the optical spectrum is not very mature. Very less number of research articles is present in open literature. Diverse types of aerials have been structured by researchers at optical spectrum such as dipole radiator [9], Yagi-Uda radiator [10], graphene based radiators [11], aperture antennas [12] and so-on. However, ceramic based antennas are widely used at THz frequencies because of its inherent potential (already discussed). In 2013, Zou et.al. proposed dielectric resonator-based array at 633 nm. It supports single frequency band and operate at the fundamental mode i.e. HEM11δ. It also supports the broadsided radiation pattern [13]. In the same year, Silveira et.al. presented stripline fed ceramic for Nano-photonics uses. It is also a single band antenna and operated at 193.5 THz with broadsided far-field pattern [14]. Sethi et.al. proposed microstrip line fed equilateral triangle based ceramic antenna at 195 THz. It is designed for C-band of optical spectrum [15]. Same research group has again designed as hexagonal shaped DR for 1550 nm optical communication. It operates on HEM20δ mode and produces end fire radiation pattern [16].
Varshney et.al. proposed a ceramic based CP radiator. In the aforementioned antenna structure, cylindrical shaped ceramic material is excited by microstrip line and operated at 195 THz [17].
In this article, ceramic material based Nano-antenna is proposed. It is the first time, when authors are proposed for multiband application in optical spectrum. In this antenna design, square shaped slot is utilized to excite the Nano cylindrical shaped ceramic material. Because of such type of feed design, three hybrid mode patterns are excited inside the cylindrical ceramic i.e. HEM 11δ , HEM 12δ and HEM 11δ+2 . This aerial structure supports three diverse frequency bands i.e. 117.5-140 THz, 158-165.5 THz and 175. 2-190.5 THz respectively. In order to proper understanding the working of proposed radiator, the article is divided into subsections: (i) antenna layout; (ii) its detail analysis; (iii) final outcomes; and (iv) conclusion.  [11]. The cross-sectional area of the substrate is 4 × 4 2 . Nano metallic strip line at the bottom of the substrate is made up of a silver material. Similarly, square shaped aperture has also been etched from the silver based metallic strip over the upper part of the substrate. However, gold has better conductivity and less oxidization rate as compare to silver. But, silver is cost effective and for indoor application, it does not have instant oxidation. So, silver is the most suitable material for proposed antenna. Dispersive stuffs of silver material used in the proposed antenna design have been decided by Drude's model. It is simply a mathematical model and given as follow [17]:

II. STRUCTURAL LAYOUT OF PROPOSED NANO ANTENNA
In eqn. (1), ε ∞ (real part of dielectric constant) = 5; fP (plasma frequency) =2175 THz; and γ π ⁄ =4.35 THz [14]. 'ε Ag ' and 'γ' is the dielectric constant of silver and collision frequency respectively. 'f' and 'ε 0 ' is the operating frequency and permittivity of free space. In Fig. 1, 'h1'=0.145 µm, 'h2'=0.020 µm and 'h3'=0.010 µm are the depth of the substrate on which the silver strip placed, height of the silver strip and gap between silicon based ceramic and silver strip respectively. Width of Nano silver based strip line 'w' is 0.340µm with effective reflective index of about 1.66. All these dimensions are taken from ref. [14]. Width (WA) and length (LA) of Upper silver strip is 2.0µm and 2.5µm respectively. Edges of square aperture

III. DETAILED ANALYSIS OF PROPOSED NANO RADIATOR
In this subsection, theoretical in addition to mathematical investigation has been carried out with the help of Ansys HFSS EM simulator. In the proposed antenna design, two different resonating assemblies have been utilized: (i) aperture; and (ii) silicon based dielectric resonator. To confirm the responsibility of resonant peaks produced in the optical spectrum,  [18,19].
The resonant peak due to HEM 11δ mode can also be confirmed mathematically by using the succeeding formulation [20]: On the other hand, the resonant peak of HEM 11δ+2 mode can be projected mathematically as follow [21]: f r,HEM 11δ +2 = 1.5 × f r,HEM 11δ In eqn. (3), the scaling factor is 1.5 because HEM 11δ+2 mode is second higher order mode of HEM 11δ mode. Cylindrical shape always follow Bessel functions, so scaling factor is 1.25, 1.5 for first and second higher order mode respectively [21]. From eqn. (2) and (3), the resonant frequency of HEM 11δ and HEM 11δ+2 mode is found as 124 THz and 186 THz respectively. It is well known fact that if the feeding structure is act as magnetic dipole, then only HEM 11δ and its higher order modes are created within the Si based cylinder [20]. Fig. 5 shows electric and magnetic field distribution over the rectangular slot extended towards Y-axis at 125 THz.
After seeing the field distribution i.e. electric field is maximum at the middle of aperture, it can be said that aperture supports TE10 mode, which behaves as a magnetic dipole [22]. This is the theoretical reason behind the formation of HEM 11δ mode. Wide slot as well as large height of cylindrical ceramic provides the appropriate boundary conditions for HEM 11δ+2 mode [19]. Fig. 6 displays the reflection coefficient (|S11|) variation for silicon ceramic excited with rectangular slot extended towards X-axis with changes WSA. From Fig. 6, it is confirmed that silicon ceramic excited with rectangular aperture (slot) extended towards Xaxis supports only single frequency band centred at 160 THz. In order to find out the mode at 160 THz, Fig. 7 displays the vectored E-field orientation over the Si based ceramic at 160 THz. After seeing the Fig. 7, it can be said that HEM 12δ mode is accountable for resonant peak at 160 THz [18]. THz, which is nearer to the software generated outcome. Theoretically, excitation method must behave as electric dipole for creating HEM 12δ mode inside the Si based ceramic [20]. principle, this is confirmed that rectangular aperture extended towards X-axis behaves as an electric dipole [22]. Therefore; it generates HEM 12δ mode in Si based cylindrical ceramic.
(a) (b) Fig. 8 Field Distribution over rectangular aperture extended towards X-axis at 160 THz (a) Electric Field (b) Magnetic Field From aforementioned discussion, it is confirmed that Y-and X-oriented rectangular aperture is accountable for HEM 11δ /HEM 11δ+2 and HEM 12δ mode respectively. In order to create all three modes in the same Si based ceramic, it is important that feed structure must behave as both electric and magnetic dipole. In order to fulfil this requirement, rectangular aperture is converted into square aperture and its |S11| variation is displayed in Fig. 9. Two vital comments on Fig. 9 are: (i) square shaped aperture is able to create all three mode pattern in Si based ceramic; and (ii) Optimum result is obtained at 0.5 um.  Fig. 10 is that the gain rises as the frequency of operation increases. It is because of the existence of higher order mode. Antenna gain is proportional to the square of frequency i.e. ∝ 2 [22].

VI. FUTURE POSSIBILITIES
Fabrication and measurement of such type of Nano antennas is one of the future possibilities.
Some fabrication methods available in literature for antennas at Nano scale i.e. ion beam milling and electron-beam lithography [12]. In built of circular polarization as well as MIMO feature with multiband characteristics is the another important future aspects.