سنجش شاخص انکساری و شفافیت ناشی از پلاسمون (PIT) در موجبر پلاسمونیک مبتنی بر گرافن

ABSTRACT Plasmon induced transparency and refractive index sensing in a new type of graphene-based plasmonic waveguide

In recent years, the effect of quantum interference has become one of the research hot spots in the field of physics because many physical phenomena are generated in quantum optics and atomic physics, such as electromagnetically induced transparency (EIT) and electromagnetically induced absorption (EIA). The EIT is generated by quantum interference between pumping and probing field, which occurs in three level atomic system [1,2]. Compared with the EIT in atomic systems, the plasmonic analogue of EIT or plasmon induced transparency (PIT) has attracted much attention due to its significant advantages and wide practical applications, such as sensor [3,4], active plasmonic switch [5,6], polarization conversion [7], and so on. However, most of these structures mentioned above are composed of metallic materials and in real application, it is often needed to change their geometrical parameters to realize the dynamic control of the PIT-like window, which significantly limit the scope of their applications. Meanwhile, not only the metallic structures will have larger propagation losses but also it will have difficulties in controlling the permittivity, and it will also result in a lower ability of active modulation. To overcome these shortcomings, many approaches have been focused on the structures using graphene material [8–11] in recent years to achieve dynamic tunability of PIT phenomenon.

Graphene, known as a type of two-dimensional material of which the carbon atoms are packed in honeycomb like crystal lattice [12,13], has attracted particular attention in recent years due to its remarkable electronic and optical properties [14–16]. The latest research results show that enhanced performance of light-controlled conductive switching can be achieved in hybrid cuprous oxide/reduced graphene oxide (Cu2O/rGO) nanocomposites [17]. In [18], tunable photoluminescence of water-soluble AgInZnS-graphene oxide (GO) nanocomposites and their application in-vivo bioimaging are reported. It is well-known that in the mid-infrared and terahertz frequency ranges, graphene can be used as an alternative to the traditional noble-metal plasmonics. To excite the plasmons in graphene-based waveguide, the periodically patterned graphene structures [19], sub-wavelength dielectric gratings [20], have been used in the experiments. Compared with traditional noble metal materials working in visible and near infrared frequencies, graphene has some major advantages [21] such as low loss, extreme mode confinement capacity and dynamic tunability [10,11]. The stronger confinement of plasmons in graphene can create strong light–matter interactions and can be potentially used to build different types of optical devices. Moreover, in contrast to the noble metals, the most notable property of graphene is that its surface conductivity can be flexibly altered by either chemical doping or gate voltage [22–25] without refabricating the structure. Based on these unique characteristics, the graphene has certainly turned to be a very promising material for optical devices analogue to those using noble metals, including absorber [26], sensor [27] and PIT applications [10,28–30].

In [31], plasmon-induced absorption in a single graphene sheet with two air cavities side-coupled to a graphene nanoribbon is investigated. In [32], the authors demonstrated that PIT can be achieved by placing a flat monolayer graphene under a sinusoidally curved graphene layer. In [33], multi-mode PIT in dual graphene ring resonators is studied, and in [34], multicavity-coupled graphene-based waveguide system as well as its PIT transmission is demonstrated. In [35], the authors have theoretically investigated the PIT characteristics using coupled mode theory (CMT) in integrated graphene waveguides with direct and indirect couplings. Recently, Wang et al. investigated the Fano resonance and its sensing application in an improved grating-coupled graphene structure [36].

However, as far as we know, few results have been found for the research of PIT in a graphene-based waveguide coupled to a vertically placed graphene strip resonator except for Ref. [30], where the authors use two graphene sheets with finite length placed symmetrically on both sides of the graphene bus waveguide as two detuned side-coupled resonators to achieve the PIT-like phenomenon. However, the two resonators are both standing-wave (SW) cavity, and it is well-known that the couplings between a bus waveguide and a traveling-wave (TW) cavity or SW cavity are different [35]. Thus inspired by the special properties of graphene and the reported research results, this paper is to design a new type of graphene-based PIT waveguide system, where a vertical placed graphene strip is used as a resonator, and at the same time, it is side-coupled with a graphene ring on the top and with a graphene bus waveguide at the bottom. Meanwhile, the ring resonator works as a TW cavity while the strip resonator works as a SW cavity. Then in this paper, the effects of the structural parameters and chemical potential of the graphene on the transmission characteristics are studied in detail. In addition, the refractive index sensing performance based on the PIT-like effect is also to be calculated. This proposed compact plasmonic structure may pave a new way for the flexible and tunable PIT-like properties in the application of compact integrated photonic devices.