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Graphene based new physics phenomena are leading to a variety of stimulating graphene-based photonic devices. In this study, the enhancement of surface evanescent field by graphene cylindrical cladding is observed, for the first time, by using a graphene-coated microfiber multi-mode interferometer (GMMI). It is found theoretically and experimentally that the light transmitting in the fiber core is efficiently dragged by the graphene, hence significantly enhancing the evanescent fields, and subsequently improving the sensitivity of the hybrid waveguide. The experimental results for gas sensing verified the theoretical prediction, and ultra-high sensitivities of ~0.1 ppm for NH3 gas detection and ~0.2 ppm for H2O vapor detection are achieved, which could be used for trace analysis. The enhancement of surface evanescent field induced by graphene may pave a new way for developing novel graphene-based all-fiber devices with compactness, low cost, and temperature immunity.
© 2014 Optical Society of America
1. Introduction
Graphene, an array of carbon atoms densely formed in a honeycomb crystal lattice with one atom thickness, is attracting worldwide attentions for its unique electronic and photonic properties [1–3]. Many useful optical effects of graphene have been discovered, such as good transparency, strong nonlinearity, surface scattering, photovoltaic effects, etc [4–7], which have led to the demonstration of a number of graphene-based optical devices, e.g. polarizer, saturation absorber, wavelength converter, modulator, switch, photo-detector, LED, antenna, etc [8–16]. Recent theoretical studies also show that, graphene waveguide supports both TE and TM surface plasmonic modes [17–21]. Such a unique property makes it possible for ultra-sensitive refractive index detections [22–24]. Moreover, as graphene is also a smart material with large specific surface area for molecular adsorption, high aspect ratio and high carrier mobility, it also shows the potential for bio-chemical sensing [25, 26].
In this study, by coating the graphene around the surface of a chemically etched-microfiber using the CVD method proposed in Ref [27], we demonstrate that the surface evanescent field of the microfiber can be significantly enhanced by the graphene, via adopting a graphene coated microfiber multi-mode interferometer (GMMI). Though graphene coated microfiber has been theoretically discussed and experimentally applied as saturable absorbers [16, 28, 29], here graphene is utilized as the cladding material for the enhancement of the surface evanescent field. Experimental results show that comparing with common microfiber multi-mode interferometer (MMI), the extinction ratio (ER) of the GMMI is ~6dB higher. Such an enhancement would be of great potential in various photonic applications, especially in bio/chemical sensing. For instance, by applying the GMMI to detect the gas concentrations of NH3 and H2O, sub ppm resolutions are achieved in our gas sensing experiment. This research provides not only a new way to investigate the graphene based photonics in optical fiber, but also propose a high performance graphene enhanced all-fiber sensor with compact structure, fast response and temperature insensitivity.
2. Fabrication of the GMMI
The GMMI is shown in Fig. 1(a) schematically. A monolayer graphene is wrapped around a microfiber tightly attached onto a piece of MgF2 substrate. The microfiber with diameter of d = ~10μm is chemically etched from a single mode fiber (SMF) with core diameter of 8μm, and the length of the tapered section is ~3cm. At 1550nm, the refractive indexes of the core of SMF, the cladding of SMF, the MgF2 substrate are 1.45, 1.44 and 1.37, respectively. The graphene coated length LG is ~3 mm. Figure 1(b) shows the sectional view of the GMMI, in which the red dashed curve presents the graphene cladding.
Fig. 1 (a) Schematic diagram of the GMMI: Monolayer graphene film (honeycombs) coated on the microfiber (red) set on the MgF2 substrate (blue). (b) Sectional view.
Figure 2(a) indicates the fabrication process of the GMMI. In this work, the graphene film was first grown on the surface of Cu foil (Alfa Aesar, No. 13382) by chemical vapor deposition (CVD) [27]. Then the PMMA was spin-coated on the surface of graphene/Cu foil, forming the PMMA/graphene/Cu sandwich like structure. Subsequently the underlying Cu foil was etched with 1M FeCl3 solution. The PMMA/graphene was ultrasonically washed in DI water several times and then covered on the microfiber, which was tightly attached onto the MgF2 substrate. Finally, the PMMA was removed by acetone. The microfiber adopted was etched in hydrofluoric acid from a standard SMF (SMF-28, Corning Inc.), under control of a computer [30]. Figure 2(b) shows the optical microscope image (500X) of the GMMI, the graphene is coated on the left side of the white dashed line. In Fig. 2(c), by launching a 633nm light in the GMMI, the graphene cladded area was clearly visible in the dark, because of the graphene based scattering light enhancement. The Raman spectrum of the GMMI is shown in Fig. 2(d), in which the Raman characterizations of the GMMI and the MgF2 substrate are located at the light-blue circle and the orange circle shown in Fig. 2(b). For the graphene on the GMMI, its G-to-2D intensity ratio is ~0.23, and the full width at half maximum of the 2D-peak is ~36.3 cm−1.
Fig. 2 (a) Fabrication process of the GMMI. (b) Optical microscope image of the GMMI, here the orange circle shows the location of the graphene on GMMI, while the light-blue circle shows the location of the MgF2. (d) Scattering of the GMMI in dark. (d) Raman spectrum of the GMMI (red curve) and the MgF2 substrate.
3. Surface evanescent field enhanced by graphene
According to Fig. 1, for the MMI, firstly light is launched from SMF and only the HE11 mode is supported in the fiber core. Then, light propagates along the microfiber zone where the cladding is very thin to excite the cladding modes. In conventional MMI, the interference between the cladding modes and the core mode have been demonstrated [31–33]. However, in this GMMI, as graphene works as a nanolayer with high index, transmitting light energy would be drawn by the graphene from the fiber core to be enhanced surface evanescent fields [34]. The spatial distribution of mode field of the GMMI is described in Eq. (1) and Eq. (2), which is determined by the effective refractive index of the GMMI. Here fx and fy are the x- and y- component of field intensity, neffk0 is the propagation constant, ω is the photonic frequency, μ0 = 4π × 10−7H/m, σg is the conductivity of the graphene, fd(ε) = {exp[(ε-μ)/kBT] + 1}−1 is the Fermi-Dirac distribution, kB is the Boltzmann’s constant, j is the imaginary unit and e is the unit charge [35]. Utilizing the finite element method (CMOSOL) [36] to solve the equation, at 1550nm, the electric field distributions of conventional microfiber and the GMMI are shown in Fig. 3, here the diameters of the microfiber is fixed at 10μm, the refractive index of the graphene cladding is fixed as 3-i14 [37]. Figure 3(a) and 3(b) presents the x and y polarized electric field of the fundamental mode of the MMI, respectively, where the light field is confined in the core. Figure 3(c) and 3(d) presents the x and y polarized electric field of the fundamental mode of the GMMI, respectively. It can be seen that the mode field of the microfiber is altered by graphene, and the light is drawn by graphene to form the enhanced surface fields around the surface of the GMMI. Figure 3(e) presents the distributions of the x- and y- polarized components in the MMI, where the red solid curve corresponds to Fig. 3(a), while the blue dashed curve corresponds to Fig. 3(b). Figure 3(f) presents the distributions of the x- and y- polarized components in the GMMI, where the red solid curve corresponds to Fig. 3(c), while the blue dashed curve corresponds to Fig. 3(d). The simulated results show, comparing with the MMI, the evanescent fields distributing on the surface of the GMMI is much stronger. For the MMI, the intensity ratio inside the core η = Icore/(Icore + Iclad) is ~95%, while for the GMMI η is ~80% [38].
Fig. 3 (a) (b) x and y polarized field of the fundamental mode of the microfiber, (c) (d) x and y polarized field of the fundamental mode of the GMMI. (e) (f) Field distribution in the MMI and the GMMI.
To experimentally verify the surface field enhancement, we compared the resonant spectra of the MMI and the GMMI. The light energy transformation from the fiber core to the surface brings the enhancement of the interference, as shown by Eq. (3) and Eq. (4). Here Icore, Iclad are the intensities in the core and cladding. neff, clad = 1.44 is the fiber cladding index, neff, core = 1.45 is the fiber core index, neff, GCM is the index of the graphene coated zone. LG is the length of the graphene cladding, z is the length of the whole microfiber, N is a natural number. For the MMI, Icore/Iclad >> 1, if Iclad was enhanced by graphene, the extinction ratio (ER) of the resonant spectrum would be increased. Moreover, as neff, GCM is smaller than neff, clad, the free spectral range (FSR) of the GMMI should be wider. Figure 4(a) and 4(b) respectively illustrates the calculated results of “η-ER” and “neff, GCM-FSR” correlations. For the MMI, the calculated ER and FSR are 4.1dB and 5.4nm, marked by red circles, while for the GMMI, the calculated ER and FSR are 9.8dB and 12.4nm, marked by blue circles.
Fig. 4 (a) Calculated correlation of η and ER. (b) Calculated correlation of neff, GCM and FSR. (c) Experimentally measured transmission spectra of the MMI (red dashed) and the GMMI (blue solid).
With a tunable laser (81960A, Agilent, USA), we directly observed the transmission spectra of the MMI and the GMMI on an optical spectrum analyzer (OSA) (N744A, Agilent, USA), as shown in Fig. 4(c). According to Fig. 4(c), the ER and FSR of the MMI and the GMMI are ~6.5dB, ~5nm and ~11.3dB, ~13nm, respectively. The graphene cladding induced loss of the core mode is very small. The experiment verifies that the intensity of the cladding modes is significantly enhanced by the graphene cladding.
4. GMMI highly sensitive gas sensing
The graphene enhanced surface evanescent field is ultra-sensitive to local refractive index so that it greatly improves the GMMI’s ability to perceive local environment. In the GMMI, the phase difference between the interfering modes is shown in Eq. (5). Here c is the light velocity in vacuum. Thus the location of a random resonant dip is shown in Eq. (6). When gas molecules, especially polar gas molecules (e.g. NH3, NO2 and H2O) are adsorbed on the graphene, they cause charge transfer from adsorbed molecules to graphene or a local electrostatic gating effect, which results in the permittivity change of graphene, hence, the effective index neff, GMMI would be altered, consequently, the resonant dips of the GMMI would shift [25, 39].
To investigate how the graphene based surface evanescent field enhancement along the GMMI improves the sensitivity for gas detection, we setup an experimental system as shown in Fig. 5. Both the GMMI and MMI with diameter of 10μm attached onto the same MgF2 substrate were simultaneously placed in a gas chamber. In air, their transmission spectra are shown in Fig. 4(c). Light launched from the laser was sent into the two interferometers via a direct coupler (DC). The light signals were collected by the OSA via two separate channels (channel 1 for the GMMI, channel 2 for the MMI), respectively.
Figure 6(a) and 6(b) illustrates the spectral shifts of the GMMI in the gas sensing experiment. In Fig. 6(a), the GMMI was exposed in NH3 gas. When the concentrations of NH3 were 0 ppm, 100 ppm, 200 ppm and 400 ppm, the locations of the measured dip were 1552.9 nm, 1553.7 nm, 1554.1 nm and 1554.4 nm, while the local ER was 11.3 dB, 11 dB, 10.8 dB and 10.4 dB, respectively. In Fig. 6(b), the GMMI was exposed in H2O vapor. For the H2O vapor with concentrations of 0ppm, 100ppm, 200ppm, and 400ppm, the locations of a dip were 1552.9 nm, 1553.3 nm, 1553.5 nm and 1553.7 nm, corresponding to ER of 11.3 dB, 11.2 dB, 11.1 dB and 11 dB. In summary, for both NH3 and H2O vapor gas, when the concentration increases, the resonant dips of the GMMI red shift and the ER decreases.
Fig. 6 Transmission spectra of (a) the GMMI exposed in NH3, (b) the GMMI exposed in H2O, (c) the MMI exposed in NH3, (d) the MMI exposed in H2O.
For comparison, the spectral shifts of the MMI exposed in NH3 gas and H2O vapor are shown in Fig. 6(c) and 6(d), respectively. For either NH3 or H2O, it is clear that the when the gas concentration changed from 0ppm to 400ppm, the spectrum of the MMI was almost unchanged. In Fig. 6, blue curve, red curve, green curve and purple curve indicate the spectra for gas concentration of 0ppm, 100ppm, 200ppm and 400ppm, respectively.
By comparing the experimental results of the GMMI and MMI, we can see that the sensitivity of the GMMI is much higher, as shown in Fig. 7. Here the red filled cubs and the red dashed boxes show the GMMI for sensing NH3 and H2O, respectively, and the blue filled dots and the blue dashed circles show the MMI for sensing NH3 and H2O, respectively. In Fig. 7(a), for NH3 gas detecting, the sensitivity of the MMI is <0.1 pm/ppm, mainly determined by the microfiber diameter [38], while the sensitivity of the GMMI is as high as 8 pm/ppm, for the concentration of <100 ppm. For H2O vapor detecting, the sensitivity of the MMI is < 0.05 pm/ppm, while the GMMI, the sensitivity could be ~4 pm/ppm, for the concentration of <100 ppm. They are 80 times higher than those of the MMI. Considering the resolution of the OSA < 1 pm, the limited resolution of the GMMI sensor for NH3 gas and H2O vapor detection is ~0.1 ppm and ~0.2 ppm, respectively. Figure 7(b) shows the correlation between the gas concentration and the ERs. For the GMMI, the ER decreases ~0.003 dB/ppm for NH3 gas, and ~0.001 dB/ppm for H2O vapor. Such a weak attenuation for the resonant spectrum in the sensing process indicates that this GMMI sensor could work over a wide range. Figure 7(c) presents the recoverability of the GMMI sensor, by cyclically exposing the GMMI in NH3 and H2O with concentration of 100 ppm, it demonstrates that this sensor has good repeatability. Figure 7 (d) is an amplified figure of Fig. 7(c), which reveals the response of the GMMI is fast, depending on the diffusion of the gas molecules. Moreover, Fig. 7(e) verifies the temperature insensitivity of the GMMI sensor. No matter in the air, 100ppm NH3 or 100ppm H2O, when temperature rises from 20°C to 40°C, the locations of the resonant dips are stable. The thermal immunity makes the GMMI sensor practical.
Fig. 7 (a) Correlation of the concentration and dip shifts, for the GMMI (solid curves) and the MMI (dashed curves). (b) Correlation of the concentration and the ER, for GMMI (solid curves) and MMI (dashed curves). Here red cubs show the results for NH3 sensing while blue dots show the results for H2O sensing. (c) GMMI's recoverability in the sensing processes. (d) Zoomed-in results of (c). (e) Spectra remain stable when temperature varies from 20°C to 40°C.
5. Conclusions
In conclusion, by coating a layer of graphene around a microfiber multimode interferometer, the surface evanescent field enhancement due to the cylindrical graphene cladding is demonstrated. Such a phenomenon not only supports previous theoretical analysis, but also is used as a highly sensitive fiber-optic gas sensor with experimentally achieved sensitivities of ~0.1 ppm for NH3 gas and ~0.2ppm for H2O vapor detections, which are ~2 orders higher than those of the microfiber-based multi-mode interferometer without graphene. Moreover, by doping other atoms, growing acceptors on the graphene cladding, such a structure could be optimized to be selective bio/chemical sensors for specific applications in the future. This work extends the study of optical fields for graphene based planar waveguides to cylindrical waveguides and may open a new window for development of novel graphene-based all-fiber devices with compactness, thermal immunity, low cost and insertion loss.
Acknowledgments
This work was supported by National Natural Science Foundation of China under Grant 61290312, 61107072, 61107073, and 61475032. It was also supported by Program for Changjiang Scholars and Innovative Research Team in Universities of China (PCSIRT) and the “111 Project” of China Education Ministry.
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