Seiya Suzuki

Broadband graphene electro-optic modulators with sub-wavelength thickness

Graphenes featureless optical absorption, ultrahigh carrier mobility, and variable optical absorption by an applied gate voltage enable a new breed of optical modulators with broad optical and electrical bandwidths. Here we report on an electro-optic modulator that integrates single-layer graphene in a sub-wavelength thick, reflective modulator structure. These modulators provide a large degree of design freedom, which allows tailoring of their optical properties to specific needs. Current devices feature an active aperture ~100 µm, and provide uniform modulation with flat frequency response from 1 Hz to >100 MHz. These novel, low insertion-loss graphene-based modulators offer solutions to a variety of high-speed amplitude modulation tasks that require optical amplitude modulation without phase distortions, a flat frequency response, or ultra-thin geometries, such as for controlling monolithic, high-repetition rate mode-locked lasers or active interferometers.

Frequency comb stabilization with bandwidth beyond the limit of gain lifetime by an intracavity graphene electro-optic modulator

Intracavity loss modulation enables offset-frequency control with bandwidths beyond what is possible by pump power modulation. To demonstrate this new method, we use a subwavelength thick graphene electro-optic modulator to stabilize the offset frequency in a Tm:fiber frequency comb at 1.95 μm wavelength. Record-low residual phase noise of 144 mrads was achieved with this new locking scheme.

Macroscopic, freestanding, and tubular graphene architectures fabricated via thermal annealing.

Manipulation of individual graphene sheets/films into specific architectures at macroscopic scales is crucially important for practical uses of graphene. We present herein a versatile and robust method based on annealing of solid carbon precursors on nickel templates and thermo-assisted removal of poly(methyl methacrylate) under low vacuum of ∼0.6 Pa for fabrication of macroscopic, freestanding, and tubular graphene (TG) architectures. Specifically, the TG architectures can be obtained as individual and woven tubes with a diameter of ∼50 μm, a wall thickness in the range of 2.1-2.9 nm, a density of ∼1.53 mg·cm(-3), a thermal stability up to 600 °C in air, an electrical conductivity of ∼1.48 × 10(6) S·m(-1), and field emission current densities on the order of 10(4) A·cm(-2) at low applied electrical fields of 0.6-0.7 V·μm(-1). These properties show great promise for applications in flexible and lightweight electronics, electron guns, or X-ray tube sources.

Threefold atmospheric-pressure annealing for suppressing graphene nucleation on copper in chemical vapor deposition

Chemical vapor deposition (CVD) is a promising method of producing a large single-crystal graphene on a catalyst, especially on copper (Cu), and a further increase in domain size is desirable for electro/optic applications. Here, we report on threefold atmospheric-pressure (ATM) annealing for suppressing graphene nucleation in atmospheric CVD. Threefold ATM annealing formed a step and terrace surface of the underlying Cu, in contrast to ATM annealing. Atomic force microscopy and Auger electron mapping revealed that Si-containing particles existed on threefold-ATM- and ATM-annealed surfaces; particles on Cu had a lower density after threefold ATM annealing than after ATM annealing. The formation of a step and terrace surface and the lower density of particles following the threefold ATM annealing would play a role in reducing graphene nucleation. By combining threefold ATM annealing and electropolishing of Cu, the nucleation of graphene was effectively suppressed, and a submillimeter-sized hexagonal single-crystal graphene was successfully obtained.

Nondegradative Dielectric Coating on Graphene by Thermal Evaporation of SiO

The deposition of dielectric materials onto graphene without introducing atomic defects is challenging owing to the unavoidable exposure of carbon–carbon bonds to plasma, energetic ions, or high-energy photons that are present during deposition. Here, we report a simple and nondegradative method of depositing a silicon oxide layer on graphene based on the thermal evaporation of silicon monoxide (SiO). Raman spectroscopy and mapping showed that this method does not form defects in graphene, whereas depositing silicon oxide by e-beam evaporation severely damages graphene. The SiO-coated graphene also showed excellent resistance to ozone and hot air. Since SiO is transparent to visible light and infrared light, the coating may serve as a protective layer for graphene optical devices such as photodetectors and electro-optic modulators. Also noted that the present method is much simpler than atomic layer deposition, which requires additional functionalization of graphene.

Effect of Hydrogen on Carbon Nanowall Growth by Microwave Plasma-Enhanced Chemical Vapor Deposition

The effect of hydrogen on the growth of carbon nanowalls (CNWs) with different hydrogen-to-carbon (H2/CH4) flow ratios in chemical vapor deposition was investigated using scanning electron microscopy (SEM) and Raman spectroscopy. The CNWs were synthesized on catalyst-free substrates (Si or SiO2) by microwave plasma-enhanced chemical vapor deposition (MPECVD). At low H2/CH4 flow rate ratios (0?2), the wall size increased with increasing H2/CH4 flow rate ratio while the IG/ID remained constant; however, at high H2/CH4 flow rate ratios (2?4), the wall size decreased and IG/ID increased with increasing H2/CH4 flow rate ratios. This indicates that hydrogen plays different roles in the two ranges: reduction of nucleation density resulting in the larger CNWs for the former range and removal of amorphous carbon resulting in a higher IG/ID for the latter range. Note that the present growth method provides an extremely high growth rate of CNWs reaching 23 ?m/h.

Stabilization of carrier-envelope phase with MHz bandwidth by an intra-cavity graphene electro-optic modulator

An intra-cavity, sub-micrometer thick graphene electro-optic modulator enables orthogonal offset-frequency control far beyond the stimulated lifetime of the gain medium. We demonstrate this new locking scheme in a Tm:fiber comb with record-low residual phase noise.