Graphene is an optical material of unusual characteristics because of its linearly dispersive conduction and valence bands and the strong interband transitions. It allows broadband light-matter interactions with ultrafast responses and can be readily pasted to surfaces of functional structures for photonic and optoelectronic applications.

Recently, graphene-based optical modulators have been demonstrated with electrical tuning of the Fermi level of graphene. Their operation bandwidth, however, was limited to about 1 GHz by the response of the driving electrical circuit. Clearly, this can be improved by an all-optical approach. Here, we show that a graphene-clad microfiber all-optical modulator can achieve a modulation depth of 38% and a response time of ∼2.2 ps, limited only by the intrinsic carrier relaxation time of graphene. This modulator is compatible with current high-speed fiber-optic communication networks and may open the door to meet future demand of ultrafast optical signal processing. Graphene is known to exhibit a variety of exceptional electronic and photonic properties. Because of its unique electronic structure, a graphene monolayer can have a constant absorption coefficient of 2.3% over a wide spectral range from the visible to the infrared with the low-frequency part tunable by external fields (e.g., electrical-bias tuning of the Fermi level or optical excitation of carriers leading to Pauli blocking of part of the interband transitions).

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The relaxation time of the photoexcited carriers is only a few picoseconds, dominated by electron–phonon interactions and cooling of hot phonons. Compared to many other materials for ultrafast optics, graphene has the unique merit of possessing exceptionally high nonlinearity over a broad spectral range with ultrafast response. Being atomically thin, it is also highly flexible to be incorporated into other photonic structures. Recently, by electrically tuning the Fermi level of a graphene film, pasted onto a planar waveguide to modify the interband transitions of graphene, Liu et al.

Have successfully demonstrated a high-speed graphene-based optical modulator. The modulation bandwidth was however limited to ∼1 GHz by the response time of the bias circuit. For future optical data processing, a modulation rate larger than 100 GHz is needed. Obviously, the “electrical bottleneck” on the modulation rate can be circumvented by an all-optical scheme but to date graphene-based ultrafast all-optical modulation (e.g., bandwidth >100 GHz) has not yet been reported. Here, taking the advantage of the mature platform of fiber optics, we report a graphene-clad microfiber (GCM) all-optical modulator at ∼1.5 μm (the C-band of optical communication) with a response time of ∼2.2 ps (corresponding to a calculated bandwidth of ∼200 GHz for Gaussian pulses with a time-bandwidth product of 0.44) limited only by the intrinsic graphene response time. The modulation comes from the enhanced light-graphene interaction due to optical field confined to the wave guiding microfiber and can reach a modulation depth of 38%. Our GCM all-optical modulator is illustrated in Figure a.