Recently, wireless optical communication system is developing toward the chip level. Optical nanoantenna array in optical communication system is the key component for radiating and receiving light. In this chapter, we propose a sub-wavelength plasmonic nanoantenna with high gain operating at the standard optical communication wavelength of 1550 nm. The designed plasmonic antenna has a good matching with the silicon waveguide in a wide band, and light is fed from the bottom of the nanoantenna via the silicon waveguide. Furthermore, we design two kinds of antenna arrays with the proposed plasmonic nanoantenna, including one- and two-dimensional arrays (1 × 8 and 8 × 8). The radiation characteristics of the antenna arrays are investigated and both arrays have high gains and wide beam steering range without grating lobes.
- plasmonic nanoantenna
- localized surface plasmon
- integrated optical antenna arrays
- integrated photonic devices
- radiation characteristics
In recent years, silicon-based integrated photonic devices have been developing rapidly. In particular, integrated optical antenna arrays have broad application prospect in many fields, such as optical communication, light detection and ranging (LiDAR), vehicle autonomous driving, security monitoring, and display advertising [1, 2, 3, 4, 5]. Nanoantenna is a key part of the optical antenna array for converting guided light and free space light with specific directivity. Based on the light interference principle, beam steering is realized by controlling the phase of the light radiated by each nanoantenna in the optical antenna array. In order to realize optical phase control, the concept of optical phased array (OPA) is proposed [6, 7, 8]. In the field of silicon-based photonics, OPA is a highly integrated on-chip system, which consists of light division network, phase shifters, and optical antenna array [9, 10, 11]. In optical communication, OPA is required to have high gain, narrow beam, and wide steering range. However, at present, monolithic integrated OPAs suffer from low gain, small beam steering range, and wide beam width, which are mainly due to the low radiation efficiency of the nanoantenna, the large element spacing (the spacing between adjacent antenna in the antenna array), and the limited scale of the optical antenna array [12, 13].
The most commonly used silicon-based nanoantenna in the integrated optical antenna array is dielectric grating antenna. Generally, dielectric grating antenna refers to the periodic micro/nanostructure etched on dielectric substrate. The existing dielectric grating antenna suffers from large footprint and bidirectional radiation, which result in large element spacing and waste of radiation energy of the optical nanoantenna array [14, 15, 16, 17, 18]. In a uniform antenna array, the element spacing larger than the operating wavelength will lead to the appearance of grating lobes in the radiation pattern of the antenna array, which will limit the steering range of the optical nanoantenna array.
In order to obtain a miniaturized optical antenna with high radiation efficiency, plasmonic nanoantenna is proposed [19, 20, 21, 22]. Plasmonic nanoantenna is composed of metal and dielectric. When electromagnetic wave impinges on the interface of metal and dielectric, it will couple and oscillate with the surface electrons of the metal, and surface plasmon polarization (SPP) is generated. When SPP is unable to transmit along the interface and is confined, the SPP is called localized surface plasmon (LSP). LSP can confine the electromagnetic wave into a space far less than a wavelength. Based on the LSP resonance effect, electromagnetic wave will be enhanced and radiated into free space by plasmonic antenna. Taking advantage of this character, plasmonic nanoantennas can realize a tiny size . However, the traditional plasmonic nanoantennas [20, 22] are fed by plasmonic waveguides, in which the impedance matching band is narrow and high loss is introduced. In addition, the traditional plasmonic nanoantenna does not radiate light along the direction perpendicular to the plane where the antenna is located, which also limits the light steering range of the optical nanoantenna array [1, 2, 20, 22].
In this chapter, we propose a plasmonic nanoantenna with sub-wavelength footprint and high gain operating at the standard optical communication wavelength of 1550 nm, i.e., 193.5 THz . The proposed plasmonic nanoantenna consists of a silver block and a silicon block, and its footprint is much smaller than that of dielectric grating antenna. Unlike recent studies on the plasmonic nanoantennas, an impedance matching between the proposed plasmonic nanoantenna and a silicon waveguide is achieved in a wide band. Light is fed from the bottom of the plasmonic nanoantennas by the silicon waveguide and is radiated vertically upward without bidirectional radiation. This kind of bottom fed plasmonic nanoantenna is suitable for the expansion of the nanoantenna array. Based on the proposed plasmonic nanoantenna, two plasmonic nanoantenna arrays including 1 × 8 and 8 × 8 arrays are designed. The radiation characteristics of the plasmonic nanoantenna arrays are simulated and discussed in detail.
2. Plasmonic nanoantenna
2.1 Radiation characteristics of the designed plasmonic nanoantenna
Figure 1 illustrates the schematic diagram of the proposed plasmonic nanoantenna. The plasmonic nanoantenna is composed of silicon (Si) block, silver (Ag) block, and a silicon waveguide with a silicon dioxide (SiO2) coating. As shown in Figure 1, a silicon waveguide through the silver block is connected with the silicon block for feeding light into the plasmonic nanoantenna. The lengths of the silicon waveguide along the
Some electromagnetic simulations for investigating the radiation characteristics of the plasmonic nanoantenna are performed with the commercial software of CST-Microwave Studio. For simulation, a beam of light with TE polarization (
Figure 2 shows the simulated optical field distributions in the cross-sections of the designed plasmonic nanoantenna at the frequency of 193.5 THz. Figure 2(a) and (b) illustrates the optical fields in the
The optical vector distribution maps in the
Radiation characteristics of the plasmonic nanoantenna in Figure 1 including far-field radiation pattern, gain, and return loss are analyzed by using electromagnetic simulation. Figure 3 displays the far-field radiation pattern of the designed plasmonic nanoantenna at 193.5 THz. The parameters
Gain is an important radiation characteristic of the optical antennas, which represents the ability of an antenna to radiate optical power in a given direction. Figure 5 shows the calculated gain of the designed plasmonic nanoantenna. It is clearly seen that at the center frequency of 193.5 THz, the gain of the antenna reaches its maximum value of 8.45 dB.
2.2 Parameter analysis of the plasmonic nanoantenna
In order to understand the impacts of the geometric parameters of the silicon and silver blocks on the radiation characteristics of the plasmonic nanoantenna, a series of electromagnetic simulations are performed and the simulation results are analyzed in detail. We choose the plasmonic nanoantenna mentioned above (
Firstly, the influences of the width (
Secondly, the influences of the width (
Finally, the influences of the heights of the silicon block (
The simulation results show that the geometric parameters of the silver and silicon blocks will affect the radiation characteristics of the antenna. For TE polarized incident wave, the widths of the silicon and silver blocks have significant effect on the gain, and the lengths of the silicon and silver blocks have little effect on the gain. It is necessary to optimize these parameters to obtain a plasmonic nanoantenna with high gain.
3. Plasmonic nanoantenna arrays
In practical applications, the antenna array is used to realize beam steering on the basis of the optical field superposition principle. The beam deflection is realized by changing the phase of the light radiated by each antenna in the array. Usually, radiation characteristics of the antenna array, including steering angle, beam width, gain, return loss, and mutual coupling between each antenna  should be considered. Utilizing the proposed plasmonic nanoantenna, one-dimensional (1-D) and two-dimensional (2-D) arrays are designed, and their radiation characteristics are investigated in detail.
3.1 1 × 8 plasmonic nanoantenna array
In the research of 1-D array, a 1 × 8 array is designed using the proposed plasmonic nanoantenna, as shown in Figure 9. In the 1 × 8 array, the nanoantennas are arranged in a row along the
The return loss of each waveguide port in the designed 1 × 8 array is calculated and shown in Figure 10. The curves S11 to S88 represent the return losses of Port1 to Port8, respectively. The results in Figure 10 show that the return losses of all ports are less than −22.5 dB in the frequency region 191.5–196.2 THz. It means that the reflectivity of each port is less than 0.6%. Such low return losses also prove that there is a good impedance match between the designed plasmonic nanoantenna and the silicon waveguide in a wide bandwidth. To research the coupling effect among the ports, the mutual couplings between other ports and Port1 are shown in Figure 11. The mutual coupling decreases as the distance between Port1 and other ports increases at 193.5 THz.
In the simulation, the radiation pattern of the 1 × 8 array is studied by feeding optical signals with the equal amplitude and the same phase into each port simultaneously. Figure 12(a) and (b) display the far-field radiation patterns of the 1 × 8 array in the planes of
According to the filed superposition principle, beam steering can be realized by changing the phase of light radiated by each antenna in the antenna array. In theory, for a 1-D array along the
where the parameter
We further study the beam steering characteristics of the 1 × 8 array in Figure 9 by using the methods of electromagnetic simulation and theoretical calculation. At 193.5 THz, the far-field radiation pattern of the 1 × 8 array in the plane of
In a uniformly arranged array, element spacing is the critical factor for determining the radiation characteristics of the array. Numerical simulations of the 1 × 8 array with different element spacing
The calculated S-Parameters of the 1 × 8 array with different element spacing are displayed in Figures 15 and 16. Figure 15(a) and (b) show the return loss of Port1 (S11) and Port4 (S44) when
The calculated far-field radiation patterns of the 1 × 8 array with the phase difference in the
The far-field radiation patterns of the 1 × 8 array with
3.2 8 × 8 plasmonic nanoantenna array
In this section, we take the 1 × 8 array mentioned above as a sub-array and extend it along the
where and represent the far field radiation patterns of the 2-D array and the 1-D sub-array, respectively. The function indicates the array factor of the 2-D array. The array factor is determined by the 1-D sub-array spacing and the number of the 1-D sub-arrays in the 2-D array along the extend direction, and the amplitude and phase of the light fed in each antenna. According to Eq. (2), the far-field radiation pattern of the 8 × 8 array with constant amplitude and same phase of light in each antenna at 193.5 THz is obtained. The calculated far-field radiation patterns in the planes of
Similar to the 1-D beam steering, 2-D beam steering of the 8 × 8 array is realized by changing the phases of light in the nanoantennas along the two orthogonal directions. The parameters and are used to represent the phase differences between two lights fed in the adjacent nanoantennas along the
Figure 22 shows the far-field beam steering of the 8 × 8 array in the
Therefore, the designed 8 × 8 array can be used to realize the beam steering with a steering range of ±44.0° × ±45.0° by controlling the differences of and . Calculated by Eq. (1), an 8 × 8 array with the element spacing of 0.7λ0 along the
In this chapter, we review the silicon-based optical nanoantennas and their applications in OPA for beam steering. In order to obtain an OPA with high gain and wide beam steering range, we propose a sub-wavelength plasmonic nanoantenna with an operating wavelength of 1550 nm. The proposed plasmonic nanoantenna consists of a silver block and a silicon block with a standard silicon waveguide for feeding light into the nanoantenna. On the basis of LSPR, the plasmonic nanoantenna radiates light vertically upward with a high gain of 8.45 dB at 1550 nm. There is a good impedance match between the plasmonic nanoantenna and the silicon waveguide in a frequency range from 176.7 to 248.5 THz. Furthermore, two nanoantenna arrays (1 × 8 and 8 × 8) with the element spacing of 0.7λ0 composed of the proposed plasmonic nanoantennas are designed, and their beam steering radiation patterns are studied in detail. The simulation results show that the 1 × 8 array can be used to realize 1-D beam steering from −44.0° to +44.0° with a gain of 14.5 dB at 1550 nm, and the 8 × 8 array can achieve a 2-D beam steering from −44.0° to +44.0° in one dimension and from −45.0° to +45.0° in the other dimension with a gain of 24.2 dB at 1550 nm.
The plasmonic nanoantenna we proposed is a good candidate for the extension of the nanoantenna array used in a large-scale OPA. Utilizing the proposed plasmonic nanoantenna, a 3-D array extend mode can be adopted to form an OPA with thousands of optical nanoantennas. We first make a 1-D OPA as a sub-layer, in which the optical power division network, phase shifters, and a 1-D plasmonic nanoantenna array are integrated in a plane. After that, such 1-D OPA layers are extended longitudinally. Therefore, a highly integrated OPA containing thousands of optical nanoantennas with sub-wavelength element spacing can be obtained theoretically to steer beam in a wide angle without grating lobes. However, the processing of the OPA with multilayer structure is limited by our micro/nanofabrication technology. We believe that with the development of micro/nanoprocessing technology, the large-scale OPA will be applied in various fields of optical communication, LiDAR, security monitoring, and display advertising, which will bring great benefits to human life.
The authors would like to thank Dr. Zhihui Liu for her support in reviewing. This work is supported by Innovation Funds of China Aerospace Science and Technology (No. Y-Y-Y-GJGXKZ-18, No. Z-Y-Y-KJJGTX-17) and the 2017 Open Research Fund of Key Laboratory of Cognitive Radio and Information Processing, Ministry of Education, Guilin University of Electronic Technology (No. CRKL170202).