June 2015
Spotlight Summary by Ergun Simsek
Spatially periodic modulation of optical conductivity in doped graphene by two-dimensional diffraction grating
Normally, diffraction gratings are large optical components (with respect to the wavelength of the incident light) composed of periodic structures. They have been used for many decades in lasers, monochromators, spectrometers, and several other optical applications to split light into several beams travelling in different directions. Contrarily, graphene is an extremely thin and practically new material with extraordinary optical, electrical, and mechanical properties. One of the interesting properties of this one-atom-thick sheet of carbon atoms arranged in a hexagonal lattice is that we can change its conductivity by changing its Fermi energy, in other words by electric gating or chemical doping. However, there is no energy difference between the insulating and conducting states of the graphene. This lack of a band-gap prevents graphene’s straightforward use in ultrafast opto-electronic devices to replace the “ohmic-loss-limited” semiconductor technologies.
In this work, the author brings a diffraction grating and doped graphene together, such that the overall system exhibits photonic band-gaps. Assume that we have a doped graphene layer on the xy-plane and underneath it we place a dielectric grating, which is an array of dielectric nanowires that are aligned along the z-axis. As the gap between the graphene layer and the grating gets smaller and smaller, we start to see a change in the charge distribution along the graphene layer. Since the diffraction grating is a periodic structure, this charge re-distribution occurs in a periodic manner and leads to a spatially modulated optical conductivity inside the graphene. What does this mean?
First of all, this means we can change the optoelectronic properties of graphene locally without performing a physical operation on it. Considering the fact that the optoelectronic properties of nano-graphene disks created with ablation, etching, or lithography, are not same as ones belonging to large graphene sheets, this approach removes the difficulties created by the edge effects.
Secondly and more importantly, this spatially modulated optical conductivity of graphene leads to new graphene-based systems with photonic band-gaps, which can be utilized in both linear and nonlinear optoelectronic devices operating at terahertz frequencies.
The author first theoretically studies how charge density along the graphene sheet is affected by the presence of a dielectric periodic structure (diffraction grating) in its vicinity. Then, he creates a theoretical framework to estimate the frequency dependency of the light scattering from these doped graphene-dielectric grating systems.
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In this work, the author brings a diffraction grating and doped graphene together, such that the overall system exhibits photonic band-gaps. Assume that we have a doped graphene layer on the xy-plane and underneath it we place a dielectric grating, which is an array of dielectric nanowires that are aligned along the z-axis. As the gap between the graphene layer and the grating gets smaller and smaller, we start to see a change in the charge distribution along the graphene layer. Since the diffraction grating is a periodic structure, this charge re-distribution occurs in a periodic manner and leads to a spatially modulated optical conductivity inside the graphene. What does this mean?
First of all, this means we can change the optoelectronic properties of graphene locally without performing a physical operation on it. Considering the fact that the optoelectronic properties of nano-graphene disks created with ablation, etching, or lithography, are not same as ones belonging to large graphene sheets, this approach removes the difficulties created by the edge effects.
Secondly and more importantly, this spatially modulated optical conductivity of graphene leads to new graphene-based systems with photonic band-gaps, which can be utilized in both linear and nonlinear optoelectronic devices operating at terahertz frequencies.
The author first theoretically studies how charge density along the graphene sheet is affected by the presence of a dielectric periodic structure (diffraction grating) in its vicinity. Then, he creates a theoretical framework to estimate the frequency dependency of the light scattering from these doped graphene-dielectric grating systems.
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Article Information
Spatially periodic modulation of optical conductivity in doped graphene by two-dimensional diffraction grating
Tetsuyuki Ochiai
J. Opt. Soc. Am. B 32(4) 701-707 (2015) View: Abstract | HTML | PDF