Active control of all-fibre graphene devices with electrical gating

Abstract

Active manipulation of light in optical fibres has been extensively studied with great interest because of its compatibility with diverse fibre-optic systems. While graphene exhibits a strong electro-optic effect originating from its gapless Dirac-fermionic band structure, electric control of all-fibre graphene devices remains still highly challenging. Here we report electrically manipulable in-line graphene devices by integrating graphene-based field effect transistors on a side-polished fibre. Ion liquid used in the present work critically acts both as an efficient gating medium with wide electrochemical windows and transparent over-cladding facilitating light-matter interaction. Combined study of unique features in gate-variable electrical transport and optical transition at monolayer and randomly stacked multilayer graphene reveals that the device exhibits significant optical transmission change (>90%) with high efficiency-loss figure of merit. This subsequently modifies nonlinear saturable absorption characteristics of the device, enabling electrically tunable fibre laser at various operational regimes. The proposed device will open promising way for actively controlled optoelectronic and nonlinear photonic devices in all-fibre platform with greatly enhanced graphene-light interaction.

Figure 1. Schematic view and images of gate-controlled all-fibre graphene devices.

(a) Schematic diagram of gate-variable all-fibre graphene device. The SPF was fabricated using a standard single-mode optical fibre (SMF-28e) where two metal electrodes were deposited with 50-nm thickness at both sides of the side-polished region. After transferring the graphene layer, ion liquid was applied to the graphene. Two electrodes and a Pt wire were used as source, drain and gate, respectively. The electrical transport property as a function of applied gate voltages was monitored through the measurement of I_D. Optical power in the fibre was measured using a laser diode at 1,550 nm, an in-line polarization controller and an optical power metre. (b) Schematic of side view of the all-fibre graphene devices. The field tail of the guided mode interacts with the graphene. Ion liquid acts as the gate medium as well as a transparent cladding that enhances the field strength at the graphene surface. The applied gate voltage makes the ion form an EDL between the ion liquid and graphene with a thickness of about 1 nm, which efficiently modifies the Fermi level in graphene. (c) Optical image of the fabricated device without ion liquid. The graphene sheet (7 × 5 mm²) covers the region of interaction with the fibre (∼5 mm). Inset: microscope image of side-polished surface where red light was launched from fibre end to visualize the core at polished surface.

Figure 2. Gate-variable properties of the all-fibre graphene device using a monolayer graphene.

(a) Optical transition properties of the device. Initial fibre-to-fibre insertion loss of the SPF was 0.5 dB (grey solid line). The insertion loss slightly increased by 0.2 and 0.27 dB for TM and TE mode, respectively, after transferring the monolayer graphene (blue and red dash lines). Blue and orange solid lines show gate-variable optical transmission for TM and TE mode, respectively, after applying the ion liquid. (b) Electrical transport properties of the device. The on–off I_D current ratio was 16.7 for the applied V_G range of ±1.8 V. The estimated V_CNP was 1.16 V (blue vertical dash line) where the Fermi level of graphene is close to zero. Inset: normalized change of optical transmission (ΔT/ΔI_D) as a function of drain current. The experimental result (black solid square) is displayed with fitted line (red solid line). Maximum of transmission change occurs at I_D of 6.55 μA (corresponding gate voltage is −0.49 V (red vertical dash line)) where the Fermi energy level shift is half the incident photon energy.

Figure 3. Device performance and statistical analysis of Raman spectrum in multilayer graphene.

Gate-controlled optical properties of all-fibre devices with (a) bi- and (b) quad-layer graphenes. Stronger graphene–light interaction leads to enhanced non-resonant optical transmission change of 72.9% (bilayer) and 90.1% (quad-layer). Corresponding electrical transport properties of devices with (c) bi- and (d) quad-layer graphenes. (e) Statistical analysis of Raman spectroscopic measurements. Histograms show the relative peak intensity of the Raman signal between 2D-peaks and G-peaks (I_2D/G). The Raman spectra at several sampling points are shown in the figures on the right side. The monolayer graphene has an almost uniform ratio of 2.9–3.6. The results of multilayer graphene exhibit broad distribution, which indicates that there is significant interlayer coupling in our stacked multilayer graphene.

Figure 4. Fibre laser operation using the all-fibre graphene device.

Gate-variable properties of fibre laser operation. (a) Fibre laser configuration including fabricated all-fibre device with bilayer graphene. (b–d) Characteristics of passively mode-locked fibre laser at an applied V_G of −1.05 V; (b) Measured pulse duration of 423 fs at a repetition rate of 30.9 MHz (inset). (c) Laser output spectrum with a spectral bandwidth of 8 nm at 3 dB. (d) Measured radio frequency spectrum of the laser output (e) and (f) Q-switched characteristics of fibre laser at an applied V_G of −0.18 V; (e) Measured output pulse duration of 3.5 μs at a repetition rate of 25.4 kHz (inset) and (f) its optical spectrum.