Micro/nanofiber-based Graphene Ultrafast All-optical Modulators

Owing to its linearly dispersive conduction and valence bands and the strong interband transitions, the atomic-thickness two-dimensional graphene film allows broadband light-matter interactions with ultrafast responses, and can be readily pasted to surfaces of functional structures for a variety of photonic and optoelectronic applications including mode-locked lasers, ultrafast optical modulators, broadband polarizers and ultrafast photodetectors. Among the above-mentioned possibilities, graphene ultrafast optical modulation is one of the most promising and challenging techniques for device applications. Recently, by electrically tuning the Fermi level of a graphene film to modify the interband transitions of graphene, Liu et al. successfully demonstrated a high-speed graphene-based optical modulator. The modulation bandwidth was however limited to～1GHz by the response time of the bias circuit. Obviously, the "electrical bottleneck" on the modulation rate can be circumvented by an all-optical scheme.Biconical silica optical micro-/nanofibers (MNFs) tapered down from standard telecom single-mode fibers, have been used for launching light into and collecting signal out from micro/-nano scale components or devices. With proper taper geometries, light from the standard single-mode fiber can be guided through the taper region and propagation along the subwavelength-diameter microfiber in single-mode (fundamental HE11mode) with low transmission loss, which does not only enhance the light-matter interaction on the MNF surface, but also realize highly efficient optical connection between micro/nanostructures and standard optical fibers.In this work, we propose, for the first time to our knowledge, an ultrafast all-optical modulator based on a graphene-cladded MNF. Relying on significantly enhanced evanescent light-graphene interaction in a tightly confined MNF waveguiding structure, we experimentally demonstrated a graphene optical modulation with modulation time down to2.2-ps (corresponding to a calculated bandwidth of450GHz; for Gaussian pulses with a time-bandwidth product of0.44, the calculated bandwidth is～200GHz.) in a single-mode optical fiber around1.5-μm wavelength (the C-band of optical communication). The maximum modulation depth is about38%, which is two orders of magnitude larger than that in free-space measurement. This compact modulator is compatible with fiber-optic communication networks, and may find applications in optical communications, optical computing and optical logic devices. The thesis consists of five chapters, as introduced below:The first part is an introductory chapter, which briefly introduces the motivation and aims of this study, followed by the research background of optical MNFs and graphene.The second chapter investigates optical properties and fabrication of MNFs. Firstly, we introduce optical properties of MNFs, including mode distribution, surface power density, single-mode condition and waveguide dispersion. Secondly, we study the flame-heated taper drawing fabrication and optical characterization of biconical MNFs with low optical loss. Then, based on the multimode-interference effect in single microfiber and fusion spliced MNFs via CO2laser heating, we demonstrate the approaches to structural functionalization of MNFs for strain sensing and closed-loop ring lasing.The third chapter focuses on the fabrication and optical characterization of graphene-clad-microfiber (GCM). Here, we propose, for the first time, an ultrafast all-optical graphene modulator based on the hybrid GCM structure. We first prepare a few layer graphene flake by micromechanical exfoliation of highly oriented pyrolytic graphite, and then transfer and wrap a graphene flake around the microfibers via micromanipulation. Besides, we model the surface power density of GCM, and experimentally measure the linear and nonlinear transmission of the GCM, which shows excellent saturable absorption properties of GCMs.In the fourth chapter, we experimentally demonstrate graphene ultrafast all-optical modulation. Firstly, by co-propagating a train of1064-nm5-ns nanosecond laser pulses and a1550-nm CW light in a GCM, we show graphene all-optical modulation around1550-nm wavelength. Secondly, by employing an in-fiber optical pump-probe technique, we measure the ultrafast dynamic response of the GCM modulator. We obtain a measured response time down to2.2ps, corresponding to a calculated modulation rate of～200GHz for Gaussian pulses. The maximum modulation depth is～38%, which is about2orders of magnitude higher than previous free-space normal incident optical differential transmission measurements.Finally, in the last chapter, we give a brief summary and outlook of our work.Overall, the graphene-clad-microfiber ultrafast all-optical modulator demonstrated in this work, represents a successful example of merging graphene photonics and fiber optics, which does not only extend the reach of fiber optics and graphene photonics for ultrafast optical technology, but also opens a new opportunity for future ultrafast optical signal processing.