Post by williamplayer on Jan 22, 2014 12:39:47 GMT
Massachusetts Institute of Technology
High-Contrast Electrooptic Modulation of a Photonic Crystal Nanocavity by Electrical Gating of Graphene
ABSTRACT:
We demonstrate high-contrast electro-optic modulation of a photonic crystal nanocavity integrated with an electrically gated monolayer graphene. A silicon air-slot nanocavity provides strong overlap between the resonant optical field and graphene. Tuning the Fermi energy of the graphene layer to 0.85 eV enables strong control of its optical conductivity at telecom wavelengths, which allows modulation of cavity reflection in excess of 10 dB for a swing voltage ofonly 1.5 V. The cavity resonance at 1570 nm is found to undergo a shift in wavelength of nearly 2 nm, together with a 3-fold increase in quality factor. These observations enable a cavity-enhanced determination of graphene’s complex optical sheet conductivity at different doping levels. Our simple devicedemonstrates the feasibility of high-contrast, low-power, and frequency-selective electro-optic modulators in graphene-integrated silicon photonic integrated circuits.
Graphene’s unusual optical properties enable a range of promising optoelectronic applications. To enhance the light-matter interaction in this single atomic layer material, graphene has been integrated into optical waveguides and cavities, and has as well been used to support surface plasmon polariton states. In the limit of wavelength-scale confinement, we recently demonstrated a dramatic enhancement of the light-matter interaction in graphene coupled to a planar photonic crystal (PPC) nanocavity, leading to a reduction in the cavity reflectivity by more than 20 dB. Here, we demonstrate high contrast electro-optic modulation of the reflectivity of a PPC nanocavity by electrical gating of the graphene layer. Relying on the high Fermi velocity and monolayer thickness of graphene, we tune its Fermi energy to as much as 0.85 eV. We thereby obtain a cavity modulation in excess of 10 dB, which, while limited in speed for our current method of electrochemical gating, clearly shows the capability for strong modulation of light in this very compact device geometry. Furthermore, we employ efficient coupling between the cavity modes and graphene for precision spectroscopy of graphene in the near-infrared spectral range as a function ofdoping level. Our measurements yield a complex optical sheet conductivity of monolayer graphene that agrees well with a theoretical model of the material’s optical sheet conductivity.
As shown schematically in Figure 1a, the experimental device consists of an air-suspended PPC nanocavity that is coupled to a graphene field-effect transistor (FET) gated by a solid electrolyte. The optical transmission of monolayer graphene for an incident photon at frequency ω is modulated by electrostatically tuning the graphene’s Fermi energy (EF). As indicated in Figure 1b, when EF is tuned away from the Dirac point by more than half of the photon energy ℏω/2, the interband transitions are Pauli blocked, reducing the graphene absorption. Thus, the reflectivity and Q factor of the cavity can be controlled by modulating the doping level of
graphene. To improve the overlap between graphene and cavity resonance modes, we employ an air-slot PPC nanocavity with strongly confined modes in a central air-gap, as shown in Figure 1c. This design increases the coupling strength of the optical mode to the graphene sheet by almost a factor of 3 compared to the linear three-hole defect cavity used previously, where light is confined in the high index materialand experiences less overlap with the graphene layer.
Read Full Article: qplab.mit.edu/QP/files/2013.NanoLett.Gan-Englund.graphene%20modulator.pdf
High-Contrast Electrooptic Modulation of a Photonic Crystal Nanocavity by Electrical Gating of Graphene
ABSTRACT:
We demonstrate high-contrast electro-optic modulation of a photonic crystal nanocavity integrated with an electrically gated monolayer graphene. A silicon air-slot nanocavity provides strong overlap between the resonant optical field and graphene. Tuning the Fermi energy of the graphene layer to 0.85 eV enables strong control of its optical conductivity at telecom wavelengths, which allows modulation of cavity reflection in excess of 10 dB for a swing voltage ofonly 1.5 V. The cavity resonance at 1570 nm is found to undergo a shift in wavelength of nearly 2 nm, together with a 3-fold increase in quality factor. These observations enable a cavity-enhanced determination of graphene’s complex optical sheet conductivity at different doping levels. Our simple devicedemonstrates the feasibility of high-contrast, low-power, and frequency-selective electro-optic modulators in graphene-integrated silicon photonic integrated circuits.
Graphene’s unusual optical properties enable a range of promising optoelectronic applications. To enhance the light-matter interaction in this single atomic layer material, graphene has been integrated into optical waveguides and cavities, and has as well been used to support surface plasmon polariton states. In the limit of wavelength-scale confinement, we recently demonstrated a dramatic enhancement of the light-matter interaction in graphene coupled to a planar photonic crystal (PPC) nanocavity, leading to a reduction in the cavity reflectivity by more than 20 dB. Here, we demonstrate high contrast electro-optic modulation of the reflectivity of a PPC nanocavity by electrical gating of the graphene layer. Relying on the high Fermi velocity and monolayer thickness of graphene, we tune its Fermi energy to as much as 0.85 eV. We thereby obtain a cavity modulation in excess of 10 dB, which, while limited in speed for our current method of electrochemical gating, clearly shows the capability for strong modulation of light in this very compact device geometry. Furthermore, we employ efficient coupling between the cavity modes and graphene for precision spectroscopy of graphene in the near-infrared spectral range as a function ofdoping level. Our measurements yield a complex optical sheet conductivity of monolayer graphene that agrees well with a theoretical model of the material’s optical sheet conductivity.
As shown schematically in Figure 1a, the experimental device consists of an air-suspended PPC nanocavity that is coupled to a graphene field-effect transistor (FET) gated by a solid electrolyte. The optical transmission of monolayer graphene for an incident photon at frequency ω is modulated by electrostatically tuning the graphene’s Fermi energy (EF). As indicated in Figure 1b, when EF is tuned away from the Dirac point by more than half of the photon energy ℏω/2, the interband transitions are Pauli blocked, reducing the graphene absorption. Thus, the reflectivity and Q factor of the cavity can be controlled by modulating the doping level of
graphene. To improve the overlap between graphene and cavity resonance modes, we employ an air-slot PPC nanocavity with strongly confined modes in a central air-gap, as shown in Figure 1c. This design increases the coupling strength of the optical mode to the graphene sheet by almost a factor of 3 compared to the linear three-hole defect cavity used previously, where light is confined in the high index materialand experiences less overlap with the graphene layer.
Read Full Article: qplab.mit.edu/QP/files/2013.NanoLett.Gan-Englund.graphene%20modulator.pdf