Since the first experimental observation in 2011, graphene plasmons have shown great potential in nanowaveguides, electrically modulated lasers, as well as molecular sensors. Graphene plasmons (GPs) with tunable resonance frequency can shrink the wavelength of light to the nanoscale (Nature 487, 81, 2012; Nature 487, 85, 2012). The consequent optical field enhancement is of great significance in nanophotonics (Science 349, 168, 2015; Science 351, 248, 2016; Science 357, 191, 2017). However, a necessity for realizing a plasmonic device based on graphene is the long plasmonic propagation length (at least one communication wavelength at room temperature, i.e., 1.5 μm), which is notoriously difficult to achieve due to the undesired interactions between graphene and holding substrates. Besides, nanoscale energy transfer and management by plasmons at the interface of two media are vital for the design of nanophotonic devices such as plasmonic switches, filters, and circuitry.
The leading nanophotonics groups around the world have competitively worked on the dielectric environment design of GPs since 2012. Various substrates are proposed, such as SiO2, SiC and h-BN encapsulated graphene (Science 344, 1373, 2014; Nature Mater. 14, 421, 2015), but the propagation distance of GPs is limited to 400 nm. The low temperature (60 K) can significantly extend the propagation performance to 10 μm by reducing the intrinsic electron-phonon scattering (Nature 557, 530, 2018), which is however not applicable for practical use.
Recently, DAI Qing’s group of Chinese Academy of Sciences (CAS) Key Laboratory of Nanophotonic Materials and Devices collaborates with several international research institutes, such as Prof. Javier García de Abajo of The Institute of Photonic Sciences(ICFO) in Spain, Prof. LIU Mengkun of Stony Brook University in New York, Prof. SUN Zhipei from Aalto University in Finland, Prof. CHEN Jianing from the Institute of Physics of Chinese Academy of Sciences, and Prof. Pablo Alonso-González from Oviedo University in Spain, to successfully achieve micrometer plasmon propagation in suspended graphene at room temperature. The suspended graphene plasmon exhibits highly tunable performance by introducing the graphene suspension height as a novel tuning knob, which is subsequently used as a plasmonic switch for the first time. The significance of this study is also summarized below:
*Novel structures. The authors have achieved suspended graphene structure for plasmon by fine control of gas composition/pressure to eliminate flexural phonons and increase graphene stiffness (Figure 1). The suspended graphene structure can not only avoid the losses of the dielectric environment but also provide a unique undisturbed plasmonic environment. This grants a better understanding and tuning of the physical interaction between plasmons and dielectric environments, such as manipulating the plasmon-phonon hybridization between GPs and extrinsic phonons.
Figure 1. (a) An atomic force microscopy imaging of suspended graphene with a diameter of 15 μm. The upper left illustration is the corresponding microscope photograph. (b) Near-field imaging of suspended graphene over a circular hole. Concentric fringes reveal the electric field amplitude of the propagating plasmon. (Image by DAI Qing et al)
*Outstanding performance. The current state-of-the-art GP propagation length is typically limited to a few hundred nanometers, far from the practical application. This work reports a record-long propagation length of GP up to＞3 μm (Figure 2) at room temperature. Further, the authors have demonstrated a Quality factor of ~ 33, which approaches the theoretical limit of GPs (~38) at ambient conditions. This is quite surprising given the fact the graphene is gas-doped and not encapsulated.
Figure 2. A comparison of plasmon among various graphene systems. Suspended structures have a 6-fold longer propagation length (a), a 7-fold broader wavelength-tunable range (b), and a ~3-fold increase in the operating frequency bandwidth (c). (Image by DAI Qing et al)
*High tunability. Although many studies have achieved GP's tunability, the tunable range is very limited because of the inefficient electrostatic gating (~0.4 eV). The suspended structure can shift the graphene Fermi energy up to ~0.9 eV, making accessible a wide range of operating frequency (up to 1400 cm-1) band and plasmon wavelengths (~850 nm to 1560 nm) that cover the technologically important telecom windows. More importantly, this work shows the first demonstration that the height of suspended graphene can be employed as a novel plasmonic tuning knob that enables in situ modification of the dielectric environment and substantially tunes the plasmon wavelength, propagation length, and group velocity.
*New functions. The control of GP energy transmission has always been of broad interest, which however has not been fully demonstrated before. Here, for the first time, the authors present an effective route to fully control plasmonic energy flow by applying electromechanical concepts to plasmonic devices (Figure 3), in which the suspended graphene can be deformed by a gate voltage or varying gas pressure. Furthermore, the authors discuss how the deformation can be tailored to create a plasmonic switch (by gas) and transistor (by a gate). The control of micrometer-propagation plasmon facilitates near-unity-order manipulation of plasmonic energy flow that is served as a new plasmonic switch with an on-off ratio >14, 4-time better than that of traditional plasmon transistors (e.g., <50% modulation depth by near-surface permittivity change).
Figure 3. Plasmonic switch based on tunable control of GP transmission at air-dielectric substrate interfaces. (a) Schematic of the plasmonic switch. (b) AFM topography images of suspended graphene with different heights of the graphene bubbles. (c) Near-field IR images of suspended graphene. (d) Tunable plasmon reflectance (red), transmittance (blue), and scattering (green) in the switch device. (Image by DAI Qing et al)
In summary, the current state-of-the-art of GPs is still far from the real-world applications due to the short propagation length, limited tunability, and lack of control of energy flow. This work has fully addressed the aforementioned challenges in one system with a record-high low-loss, a novel plasmonic tuning knob, and efficient energy flow switching.
This work was published in Nature Communications (2022) 13:14.65 on March 18, 2022. HU Hai, an associate research fellow of the National Center for Nanoscience and Technology (NCNST), is the first author and one of the corresponding authors of this article. DAI Qing from NCNST and Javier García de Abajo from ICFO in Spain are the co-corresponding authors. This work was supported by the National Key Research and Development Program of China, the National Natural Science Foundation of China, the Beijing Municipal Natural Science Foundation, and the Strategic Priority Research Program of the Chinese Academy of Sciences.
Contact: DAI Qing National Center for Nanoscience and Technology E-mail:daiq@nanoctr.cn
