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Received: Feb. 12, 2019
Accepted: Apr. 18, 2019
Posted: Jul. 8, 2019
Published Online: Jul. 8, 2019
The Author Email: Guangqing Du (email@example.com), Feng Chen (firstname.lastname@example.org)
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Yanhong Dong, Qing Yang, Guangqing Du, Feng Chen, Noor Uddin, Dayantha Lankanath, Xun Hou. Electronic manipulation of near-field nanofocusing in few-layer graphene-based hybrid nanotips[J]. Chinese Optics Letters, 2019, 17(7): 072501
Chinese Optics Letters, Vol. 17, Issue 7, 072501 (2019)
Electronic manipulation of near-field nanofocusing in few-layer graphene-based hybrid nanotips
Yanhong Dong1, Qing Yang2, Guangqing Du1,*, Feng Chen1,**, Noor Uddin1, Dayantha Lankanath1, and Xun Hou1
- 1State Key Laboratory for Manufacturing System Engineering and Key Laboratory of Photonics Technology for Information of Shaanxi Province, School of Electronics & Information Engineering, Xi’an Jiaotong University, Xi’an 710049, China
- 2School of Mechanical Engineering, Xi’an Jiaotong University, Xi’an 710049, China
In this Letter, we propose the electronic manipulation of localized surface plasmon resonance for active tuning in near-field nanofocusing. We theoretically studied the excited graphene tuning of the nanofocusing field in few-layer graphene (FLG)-based hybrid nanotips. It is revealed that the normalized enhanced electric field can be significantly promoted to more than 300 times. It is also observed that resonant peaks can be unprecedently modified by the electron state of excited graphene that is embedded in the substrate. It shows the possibility of flexible tuning of plasmon resonances via controlling the electron excitation state of graphene for specific advanced near-field nanofocusing applications.
Near-field nanofocusing has drawn much attention in the past few years because of its unique properties of spatially localized near-field enhancement on the nanoscale for potential applications in the fields of nanolithography, photothermal nanosurgery, and nanoimaging[1
Compared to the noble-metal-based nanotip structures, the non-metal-based nanotips, such as semiconductors and dielectrics, are mostly selected as the candidate for plasmonic nanofocusing. The intrinsic optical losses can be significantly reduced in non-metal plasmonic materials. Nevertheless, the lacking of active tuning of plasmonic near-field nanofocusing is still a major challenge due to the fact that the nanofocusing properties of plasmonic nanostructures closely depend on their shape, size, and material properties. On the other hand, it is usually in the cost of the fabrication engineering’s complexity for obtaining the ideal geometry, which presents a challenge in the nanofabrication process. In recent years, graphene, a monolayer of hexagonally arranged carbon atoms, has been widely concerned and vigorously pursued for the plasmonic applications because of its unique mechanical, electric, magnetic, and thermal properties. The electron state of excited graphene can be flexibly tuned by external excitations, such as electrical sources and optical sources, without consideration of the shape or size. The active tuning of near-field nanofocusing in graphene-based hybrid nanotips can be explained as graphene plasmonics hybridization in nanostructures, leading to the possibility of electronic manipulation of localized surface plasmon resonance (LSPR). However, the intrinsic graphene-based plasmon resonances typically exhibit limited tunability due to the fact that the graphene layer is atomically thin and thus only interacts with a very small portion of the plasmon modes. It is believed that the few-layer graphene (FLG)-based hybrid nanotips could be more beneficial for electronic manipulation of LSPR compared to atom-layered intrinsic graphene plasmonics. FLG can be treated as thin graphite, as the atom layer number is less than five. Experimentally, the FLG can be fabricated by mechanical or ion beam milling of graphite to a few atom layers. Especially, the typical metallic nanotips with respect to the excited graphene hybrid nanostructures can be interesting for manipulatable near-field nanofocusing applications. It is currently challenging to obtain more flexible and active tuning of nanofocusing based on the modified plasmon resonance process for the graphene-noble nanotip structure. To the best of our knowledge, the active tunability mechanism in excited graphene-based metallic plasmonic hybrid structures is still an open topic, especially in the FLG-based hybrid system. It is urgent to understand the LSPR tunability mechanism, which is greatly beneficial for promoting near-field nanofocusing applications.
In this Letter, by combining graphene with gold nanotips, we theoretically investigated the electronic manipulation of the LSPR for active tuning near-field nanofocusing in excited graphene-noble nanotips based on the finite element method (FEM). We started with electronic tuning of plasmons via modifying the Fermi energy of excited graphene, which may be modified by an external laser or electrostatic excitation. The potential physical mechanism of the electronic manipulation of the LSPR in the excited graphene-noble hybrid system is examined in details. The significant results of tunable near-field nanofocusing in aspects of the enhancement and resonant peak are explored. The results provide a large possibility for promoting high-performance near-field nanofocusing applications.
Figure 1. Schematic of the simulated FLG-based nanotip hybrid system. The graphene-coated Au tip is modeled as a conical taper terminated by a hemisphere of radius
In the equation set,
As for the tunability of this excited graphene-based nanotip hybrid structure, changes of the Fermi energy would lead to a simultaneous change in the interband threshold and the interband transition owing to the unique linear dispersion relation of graphene. In other words, the interband transitions are allowed at all frequencies at a Dirac point; however, as the Fermi energy is increased, some of the interband transitions are blocked, hence leading to a narrower resonance. As a result, tunable Fermi energy leads to tunable surface optical conductivity, which includes the contributions from intraband transitions and interband transitions of excited graphene. As reported, most works consider FLG to be graphene sheets with no more than five layers[21
Both the real and imaginary parts of permittivity of excited graphene can be modified via tuning the Fermi energy and the thickness of the substrate graphene. The relation does not hold for more than five layers, as interactions between adjacent layers cannot be ignored anymore. It is reasonable that the introduction of a tunable number of graphene layers would greatly enrich the diversity of both real and imaginary parts of the FLG permittivity, as well as the tuning modes of the FLG-based electronic manipulation of near-field nanofocusing.
The localized electric fields (e-fields) for near-field nanofocusing in the excited mono-graphene-based nanotip structure with respect to different excitation wavelengths are shown in Fig.
Figure 2. Cross-section of mono-graphene-based nanotip structure. Calculated images of the e-field distributions in the case of mono-graphene in the substrate (The curvature radius of the nanotip is
Figure 3. Permittivity of MLG with respect to different Fermi energies. (a) The red, blue, black, green, and orange lines are corresponding to 0.1, 0.2, 0.3, 0.4, and 0.5 eV, respectively. (b) The normalized e-field enhancement and resonant frequency of the nanotip hybrid system depending on the Fermi energy of excited graphene. (c) Real and (d) imaginary parts of graphene permittivity with respect to the layers of FLG changing from 1 to 5 (
Figure 4. Simulation results of the electronic manipulation of near-field nanofocusing in a five-layer FLG-based tip structure. (a) The normalized e-field enhancement spectra with respect to different Fermi energies (The Fermi energy decreases with 0.1 eV step size from left to right). (b) The Fermi energy of FLG (black line) and the resonant wavelength (red line) are plotted as a function of the carrier concentration of FLG. The inset illustrates the calculated image of the localized e-field.
An increase in the number of carriers inside the FLG leads to the graphene being more metal-like due to the promoted Fermi energy. As a result, the increase of the carrier concentration of FLG leads to obvious blue shifts in the resonant wavelengths, which is in agreement with the charging effect in previous theoretical studies and experimental measurements. As reported, the carrier concentration can be tuned by means such as doping and femtosecond laser ionization, and those fundamental researches provide an important foundation for the electronic manipulation of LSPR for actively tuning near-field nanofocusing. The inset in Fig.
In conclusion, we have theoretically investigated the electronic manipulation of LSPR for actively tuning the spatially highly concentrated near-field nanofocusing in FLG-based hybrid nanotips. It is revealed that the introduction of multilayered graphene leads to not only the increase of tuning ranges in resonant peaks, but also the promoted e-field enhancement for nanofocusing. It shows that the maximal normalized e-field enhancement in the nanofocusing region of the five-layer FLG-based tip structure reaches up to 304.3 times in the THz band. The resonant wavelengths of the nanofocusing hotspot can be actively modified from 17.671 to 24.876 μm. The results are important for understanding graphene-based plasmonics hybridization and promoting a wide range applications of specific advanced near-field nanofocusing, such as near-field imaging, nanolithography, and photothermal nanosurgery.
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