Research on antennas based on nanophotonic materials

In this study, the performance of bilayer graphene and its dual-frequency reconfigurable antenna structure on SiO2/Si substrates was explored. The research underscores that when compared to traditional radio frequency and microwave antennas, the performance of nano-optical antennas is strongly contingent on their size and shape. It is also intimately related to their intrinsic material properties, highlighting the unique physical attributes and scaling behavior of nano-photonic antennas. A salient feature of bilayer graphene is its ability to dynamically adjust its conductivity by applying an external voltage between the two layers, offering new prospects for its application in micro-nano electronics and photonics. Through a comparative analysis of radiation decay rates and quantum efficiency, it was determined that metallic materials exhibit much higher non-radiative losses than nano-optical materials. This research provides a foundational theoretical framework for future experiments and paves the way for creating secure information networks. However, the study acknowledges the potential challenges in the real-world application and production of nano-photonic antennas, suggesting further exploration in optimizing their structure to enhance efficiency.


Introduction
Ensuring the security of communication systems and preventing information from being eavesdropped or leaked has become one of today's most pressing topics.The security of quantum information and communications is guaranteed based on the fundamental physical principles of quantum mechanics, and shortly, it may serve as a vital means to realize secure information networks.Currently, research on the performance of communication systems is burgeoning, with certain nanomaterials possessing intricate structures drawing considerable attention.These refined nanostructures not only enable the directional emission of single photons but also significantly enhance the collection efficiency of single photons.These structures have a wide range of applications, and they can also serve as components in optical switches and optical routing.Nanostructures based on Surface Plasmon Polaritons (SPP) are exceptionally effective in collecting light energy, confining freely propagating light within subwavelength volumes [1,2].Such nanostructures exhibit pronounced localization and amplification effects, and therefore, they have a broad spectrum of optical applications.As is widely recognized, when metallic structures are scaled down to the nanometer dimension, they can be utilized to boost the optical properties of nearby objects of similar or smaller sizes.
In the realm of optical nano-antenna research, the team led by Masaaki Ono from Japan demonstrated a structure that combines a semiconductor nanowire with a plasmonic bowtie nanoantenna.The proposed configuration consists of a subwavelength indium phosphide (InP) nanowire precisely placed at the center of the gap between a pair of gold bowtie nano-antennas, with a nanomanipulator mounted on a focused ion beam system [3].Gleb M. Akselrod's group designed a system

Introduction to Graphene Structures
Nanophotonic antennas bear significant resemblance to radio frequency (RF) and microwave antennas, yet they exhibit profound differences in their physical properties and scaling behavior [6].Much of this distinction arises because metals are not perfect conductors in the visible light spectrum but rather function as strongly correlated plasmas of a free-electron cloud.Nanophotonic antennas are also not typically driven by transmission lines of current; instead, electronic oscillations are capacitive-driven.Furthermore, nano-optical antennas can adopt various atypical properties (e.g., tips, nanoparticles, etc.), and their characteristics might be substantially modified due to surface plasmon resonance, exhibiting material dependence.Bilayer graphene (BLG) consists of two closely bound layers of carbon atoms, each arranged in a distinctive hexagonal lattice pattern.For a more intuitive grasp of its structure, we can refer to Figure 1, which displays the crystalline configurations of single-layer graphene (SLG) and BLG.In these structures, the lattice constant between two adjacent cells is 0 2.46 A a = , while the bond length between carbon atoms is 0 3 1.42A CC aa == [7].Notably, the fundamental unit of SLG is composed of two carbon atoms, whereas, in BLG, each basic unit encompasses four carbon atoms, with two situated on the bottom layer and the other two on the top layer.Recently, significant advancements have been made in synthesizing BLG using chemical vapor deposition [8].Moreover, as described, segments of BLG separated from natural graphite have been successfully deposited on Si/SiO 2 wafers [9].Given the unique properties and immense application potential of BLG, it has garnered extensive attention in the research community.A salient feature of BLG is its ability to regulate its conductivity using an electric field effect by applying an external voltage between its two SLG layers [10].

Structure of Nanophotonic Material Antennas
A cross-sectional view of the reconfigurable plasmonic dipole antenna of bilayer graphene on a SiO 2 /Si substrate is depicted in Figure 2. The antenna has a length of 60 nm and a width of 18 nm, and it is positioned on a SiO 2 substrate.In this study, we considered a SiO 2 layer with a relative dielectric constant ε=4 and a thickness of t=2 nm, as well as a-Si layer with a relative dielectric constant ε=11.9 and a thickness of h=1 nm.The intermediary SiO 2 layer, situated between the upper and lower graphene layers, provides a convenient mechanism for modulating the conductivity of both graphene layers through the electric field effect.A prominent feature of the bilayer graphene is its ability to dynamically control the Fermi energy of the top and bottom graphene layers by applying an external voltage between them, offering an extensive tuning range.This applied voltage can adjust the electromagnetic properties of the graphene surface, thereby opening up new possibilities for further research and application of graphene in microelectronics, photonics, and other fields.

Introduction to Graphene Antennas
As demonstrated in Section 2.2, for an individual antenna rod, the top view is depicted in Figure 3.For a single antenna, the length x is 60 nm (adjustable), the width y is 18 nm (adjustable), the height z is 38 nm (adjustable), and the inter-antenna spacing d is 8 nm (adjustable).From Figure 4(a), it is evident that as the wavelength changes, the radiation attenuation rate reaches its maximum at a particular wavelength, which we term the resonance wavelength.As the antenna length increases, the value of the resonance wavelength exhibits a redshift, moving towards a larger wavelength.This shift arises as increasing the antenna length augments the resonant cavity formed by the metal nanorod, leading to an associated increase in the resonance wavelength.Moreover, the maximum value of the radiation attenuation rate increases with antenna length, attributable to the enhanced coupling strength with length.
Figure 4(b) reveals that, for the nanophotonic material antenna, the resonance wavelength is approximately 580 nm.As the antenna width, or y value, decreases, there is an increase in the maximum radiation attenuation rate, and the resonance wavelength also shows a redshift.This primarily occurs because, as the antenna width reduces, the resonant cavity narrows, concentrating energy more effectively and thereby amplifying the coupling strength and radiation attenuation rate.From Figure 4(c), it's discerned that variations in antenna height have a relatively minor impact on the radiation attenuation rate, and the trends are similar to those observed for antenna width.Specifically, as the z value decreases, the maximum radiation attenuation rate increases, along with a red shift in the resonance wavelength.The reason is analogous; a shorter resonant cavity concentrates energy more in the x direction or the length direction, intensifying the coupling and increasing radiation attenuation.From Figure 4(d), we observe that as the distance between two antennas grows, the overall radiation attenuation rate decreases.This is due to the reduced coupling strength resulting from increased interantenna spacing.

Comparison of Graphene Antennas with Antennas Made of Other Materials
As depicted in Figure 5(a), different materials possess distinct radiation attenuation rates.It is readily observable from the graph that the enhancement in radiation attenuation rate for graphene antennas due to the surrounding medium is significantly higher than that for copper.The underlying reason is that the non-radiative losses, such as ohmic losses, in metallic materials (like gold, silver, copper, etc.) are considerably greater than in nanophotonic materials.Consequently, the quantum efficiency of a metallic copper antenna is much lower than that of a graphene material antenna, as shown in Figure 5

Directionality of Graphene Antennas
Bulk graphene antennas exhibit another property: the directionality of the nanophotonic material antenna changes with the varying distance between the dipole emission source and the graphene antenna.For our analysis here, the relative dielectric constant for the graphene antenna is ε=3, the antenna edge length is 600 nm, and the distance between the dipole and the antenna increases from 50 nm to 260 nm.When a=50 nm, the peak intensity is between the nanorods and intensifies in the direction of the nano-block.In contrast, at a=260 nm, the intensity in the nano-block direction decreases, intensifying in the opposite direction.To display this outcome, we employ a normalized directional polar plot, as seen in Figure 6.Within this figure, we observe that as the distance between the dipole and the nano-block increases from 50 nm to 260 nm, the emission direction inverts.Thus, by adjusting the distance between the dipole and the nano-block, we can alter the emission direction of the nanophotonic antenna.

Summary of Research Results
The study delves into the properties of bilayer graphene and its dual-frequency reconfigurable antenna structures on a SiO 2 /Si substrate.The primary findings of the research are: 1. Nanophotonic antennas exhibit significant differences in physical properties and scaling behaviors compared to traditional RF (radio frequency) and microwave antennas.These differences arise due to factors such as surface plasmon resonance and material dependencies.
2. Bilayer graphene possesses unique properties, notably the ability to dynamically modulate its conductivity by applying an external voltage between its two layers.This characteristic presents new opportunities for graphene's applications in micro-nano electronics and photonics.
3. The study found that metallic materials have considerably higher non-radiative losses than nanooptical materials.

Discussion on the Practical Significance
The research provides a foundational theoretical framework that has significant implications for the field of secure information networks and other related applications: 1.The study's findings offer a new direction for achieving secure information networks.Given the unique properties of bilayer graphene and the distinct characteristics of nanophotonic antennas, there is potential for the development of advanced communication systems that prioritize security and efficiency.
2. The ability of bilayer graphene to dynamically adjust its conductivity opens up novel avenues for its integration into micro-nano electronics and photonics.This could lead to the development of innovative devices and systems with enhanced performance and functionalities.
In conclusion, the study on bilayer graphene and nanophotonic antennas offers promising prospects for the advancement of secure information networks and other applications.However, continued research and exploration are essential to harness the full potential of these materials and technologies.

Figure 1
illustrates the (a) single-layer graphene structure and (b) bilayer graphene structure.

Figure 3 .
Figure 3. Schematic diagram of the top view structure of two antennas.

Figure 5 .
Figure 5.Comparison of antenna performance between different materials (a) Radiation attenuation rate (b) Quantum efficiency.