Abstract
Electrically tunable permittivity of graphene provides an excellent tool in photonic device design. Many previous works on graphene-based photonic devices relied on variable absorption in graphene, which is naturally small in the optical region, and resonant structures to enhance it. Here we proposed a novel scheme to control evanescent coupling strength by inserting two graphene layers to a frustrated total internal reflection (FTIR) configuration. The resulting structure behaves in a drastically different way from the original FTIR: optical transmission though the structure can be electrically controlled from ~10−5 to ~1 with little dependency on angle of incidence. This unique feature stems from the fact that the permittivity of doped graphene can be close to zero at a certain photon energy. The electrical controllability of evanescent coupling strength can enable novel design of optical devices. As a proof-of-concept, we designed a waveguide-type optical modulator of a novel operation principle: transmission modulation depends on the electrically controlled existence of a guided-mode of the waveguide, not the variation of the ohmic loss of graphene, resulting in a low insertion loss and a small device footprint.
Highlights
Among the many graphene-based optical devices, optical modulators have attracted much interest because of their broad optical bandwidth and semi-infinite high-index (Si) waveguide compatibility[5]
When light from a high-index medium is refracted on the surface to a lower-index medium, total internal reflection will occur if the angle of incidence exceeds a certain limit known as the critical
If the low-index medium is very thin and another high-index medium is added to the end of the structure, part of the incident light can tunnel through the low-index medium and transmit to the other end; this is known as FTIR18
Summary
Among the many graphene-based optical devices, optical modulators have attracted much interest because of their broad optical bandwidth and Si waveguide compatibility[5]. In most of the previously proposed graphene-based modulators, variable ohmic loss was used to modulate optical transmission[5,6,14,17], and low field confinement in an extremely thin graphene layer required a long device because resonant nanostructures can be hardly used in waveguide-type graphene-based modulators. We propose a novel scheme to modulate optical transmission by combining the tunable permittivity of graphene, which can be very close to zero, and the frustrated total internal reflection (FTIR)[18]. We present our design of a waveguide-type optical modulator based on the proposed scheme as an example of its many possible applications. Because the GA-FTIR scheme does not rely on the ohmic loss of graphene, an optical modulator with low insertion loss and high extinction ratio (modulation depth) can be achieved with a small device footprint of several times of an operating wavelength without any resonant nanostructure
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