Planar-lightwave-circuit (PLC)-type graphene polarizers are fabricated by using a low loss optical polymer waveguide. The optical characteristics are investigated at a wavelength of 1.31 µm. By interface engineering with a UV-curable perfluorinated acrylate polymer resin, the graphene’s electrical properties are tuned to support a transverse-magnetic (TM) or transverse-electric (TE) surface wave. Thus, the fabricated PLC-type graphene polarizer serves alternatively as a TM-pass or TE-pass polarizer depending on the absence or presence of the upper-cladding layer. The proposed planar-type graphene polarizer can be exploited further for on-chip photonic integrated circuit and devices.
© 2012 OSA
Graphene, a flat monolayer of carbon atoms packed into a two-dimensional honeycomb lattice, has attracted a great attention in photonics and electronics [1, 2]. Graphene’s extraordinary electron mobility has been allowed to develop high-speed complementary metal oxide semiconductor (CMOS) transistors [3–5]. Numerous graphen-based photonic devices have been also investigated to date. Graphene has been used as a transparent conductor for photovoltaic devices, organic light emitting diodes, and touch screens [6–8]. For development of novel optical communication components and systems, graphene-based high-speed optical photo-detector (PD) and modulator have been demonstrated [9, 10]. Based on the graphene’s ability to guide electromagnetic waves, graphene polarizer has been investigated using conventional optical fiber . Very recently, a graphene-based plasmonic waveguide has been demonstrated its ability to transmit optical data of 2.5 Gbps . With an aid of graphene’s linear dispersion between energy and momentum, these devices work in much broader wavelength range.
For development of on-chip photonic integrated circuits (PICs) and CMOS-compatible next-generation PICs, all photonic components should provide high compatibility with other functional planar optical waveguide devices on a single chip. Fortunately, graphene has a high compatibility to conventional CMOS devices and fabrication processes [2, 9–11]. Compared to metal, the functionality (e.g. conductivity) of graphene can be modified by means of chemical doping, electric filed, magnetic field, and/or gate bias voltage [13–15]. Therefore, graphene hold huge potential for the development of novel next-generation convergence optoelectronic components if we take numerous extraordinary advantages of graphene’s optical and electrical characteristics.
As an extended application of graphene in photonics, we developed polymer-based planar-lightwave-circuit (PLC)-type graphene polarizer and the optical characteristics were investigated at a wavelength of 1.31 µm. An interface engineering with a UV-curable perfluorinated acrylate polymer resin tunes graphene’s electrical properties such as conductivity and carrier density. This modification leads graphene to support a transverse-magnetic (TM) or transverse-electric (TE) surface wave alternatively. Thus, the fabricated PLC-type graphene polarizer serves alternatively as a TM-pass or TE-pass polarizer depending on the absence or presence of the upper-cladding layer.
2. Experiment and discussions
Schematic views of the proposed PLC-type graphene polarizers are shown in Fig. 1 . The proposed waveguide polarizer consists of an under-cladding, a core with a rectangular cross-section, and a graphene strip that is placed on the waveguide core, as shown in Fig. 1(a). The refractive indices of the cladding and core are 1.37 and 1.39, respectively. The waveguide thickness is 5 μm and its width ranges from 5 to 9 μm. Since the upper side of the waveguide is opened to air, the waveguide structure is an air-cladding waveguide. In order to tune the electrical characteristics of the graphene strip on the waveguide core, a UV-curable polymer rein having the same refractive index of the under-cladding is formed additionally, as shown in Fig. 1(b).
To fabricate the proposed graphene-based polymeric waveguide polarizer, we used a commercial UV-curable polymer, Exguide LFR from ChemOptics (www.chemoptics.co.kr). The propagation loss and the birefringence (nTE – nTM) of the optical polymer material at a wavelength of 1.31 μm are 0.06 dB/cm and 0.0003, respectively. First, 20 µm-thick under-cladding layer is spin-coated on a silicon wafer, and then cured with UV light. Subsequently, the core material was dispensed on the under-cladding to form 5 μm-thick cores and cured with UV light. Graphene film grown by a thermal chemical vapour deposition (CVD) method using 300 nm-thick Ni sputtered on SiO2/Si substrates is transferred mechanically on the core layer. Then, the waveguide cores are fabricated by the O2 plasma reactive ion etch process by using photoresist (PR) etching mask that is defined by a standard photo-lithographic technique. Finally, convex straight waveguide cores with a graphene strip are obtained. After finishing the measurement of the optical characteristics of the air-cladding polymer-based graphene polarizer, the under-clad materials were spin-coated additionally to form the upper-cladding layer. Then, the optical properties of the modified graphene polarizer are measured again.
Figure 2 shows the fabricated polymer-based PLC-type graphene polarizer and the Raman shift of the graphene on the SiO2. As shown in Fig. 2(a), a graphene strip is placed on the waveguide core. The length of the fabricated graphene polarizer is 10 mm. The graphene strip is located at the center of the waveguide core and its length is about 7 mm. Figure 2(b) exhibits representative Raman spectra with a 532 nm excitation laser. The detection of the G peak (1,580 cm−1) and the 2D peak (2,700 cm−1) corresponds the presence of graphene film . Based on the 2D/G intensity ratios obtained from several different locations in a transferred graphene film, we concluded that the CVD-grown graphene film consists of various domains having 1 to ~10 layers of graphene . Although there are several discontinuities with gaps, ripples, wrinkles, and contaminants in the transferred graphene, a graphene film with a good crystalline quality was successfully synthesized and transferred to the polymer dielectric.
To investigate the characteristics of the fabricated polymer-based PLC-type graphene polarizers, the TE- or TM-polarization light is launched at the input facet of the fabricated waveguides by using a single-mode polarization maintaining fiber (PMF). The infrared images of the guided mode were measured by a charge-coupled device (CCD). After measuring the infrared images, the output light were collected by a PMF, and the transmitted powers were measured with an optical power meter to evaluate insertion loss.
Figure 3 exhibits the measured infrared images at the output port of the fabricated waveguide polarizer depending on the polarization. For the air-cladding graphene polarizer, the intensity of the TE-polarization light is brighter than that of the TM-polarization light (Fig. 3(a)). This means that the air-cladding waveguide polarizer works as a TE-pass polarizer. On the contrary, a brighter light spot is obtained with the TM-polarization light for the modified graphene polarizer whose up-side is covered with a UV-curable polymer resin (the upper-cladding). The light intensity of the TE-polarization light is dimly visible, as shown in Fig. 3(b). A slab mode is observed because of the subtle refractive index difference between the under- and upper-cladding layers. This means that the waveguide serves as a TM-pass polarizer.
To investigate the characteristics of the detected guided mode further, we measured the insertion loss of the fabricated waveguide polarizers. Figure 4(a) and 4(b) exhibits the insertion loss of the two different graphene polarizer depending on the polarization and the waveguide width. The insertion loss includes the propagation loss of 0.6 dB/cm and coupling loss of about 0.5 dB/facet for both TM- and TE-polarization. For a polymer waveguide without graphene strip on the waveguide core, the averaged insertion loss and the extinction ratio are 1.5 dB and 0.3 dB, respectively. The investigations on the both polymer waveguides without graphene result in the observation of no polarizing effect.
If the graphene strips are placed on the waveguide core, the insertion losses depending on the polarization change significantly. For the air-cladding graphene polarizer (Fig. 4(a)), the averaged insertion losses for the TM- and TE-polarization light are 20.7 dB and 10.9 dB, respectively. This reveals clearly that the waveguide polarizer works as a TE-pass polarizer with an extinction ratio of about 10 dB. Similar to an in-line fiber-to-graphene coupler , the TE-polarization light in the waveguide core couple into a two dimensional graphene strip that supports a TE-polarization mode. If the graphene is considered as ultra thin two-dimensional metal, a TM-polarization light may couple to graphene strip on the waveguide core. However, the attenuation of a TM-polarization mode is larger than that of TE polarization. This is similar to the behavior of the surface plasmon polaritons (SPP) in metal films that exhibits high radiative insertion loss when a thin metal strip is embedded in a dielectric having an asymmetry between the index of refraction of the top clad and the bottom clad .
In contrast, the modified graphene polarizer with a UV-curable polymer resin upper-cladding shows different results. The TE insertion loss increases significantly up to 50 dB in average, as shown in Fig. 4(b). This can be attributed to the fact that most of the optical power is radiated out of the channel waveguide as slab mode, as shown in Fig. 3(b). Compared to the TM-polarization, the TE insertion loss is 19.8 dB higher. Thus, the modified graphene polarizer serves as a TM-pass polarizer. The graphene strip embedded in a homogeneous dielectric supports a TM-polarization guided mode with the extinction ratio of 19 dB . Thus, the TM-polarization light in the waveguide core couples into graphene strip.
The fully opposite experimental results for the fabricated two different polymer-based PLC-type graphene polarizers may be attributed to the change of the electrical characteristics of the graphene strip depending on the absence or presence of the upper-cladding on the graphene strip. Electrons in graphene behave as massless Dirac fermions with gapless energy band structure. Complex conductivity of graphene is modeled base on the Kubo formalism, consisted of intraband and interband contributions: σ(ω, µc, Γ, T) = σintra + σinter, where ω is radiant frequency, µc is chemical potential, Γ is scattering rate, and Τ is temperature [19, 20]. For kBT << | µc | and 0 < |µc| < ћω/2 (kB is Boltzmann’s constant, ћ = h/2π is the reduced Planck’s constant), the negative imaginary part of the interband contribution becomes dominant and the TE mode is supported by graphene . By using this weakly damped TE-mode surface wave along the graphene sheet, a TE-pass graphene polarizer has been developed based on the side-polished optical fiber . The experimental results of the air-cladding graphene polarizer shown in Fig. 3(a) are coincident with those of the previous work showing the optical characteristics of a TE-pass polarizer . The graphene strip on the waveguide core supports a TE-mode surface wave and the TE-polarization light propagates along the graphene strip on the waveguide core with low attenuation. Therefore, the graphene-based waveguide polarizer without an upper-cladding serves as a TE-pass polarizer.
However, when graphene is doped to a relatively high level (|µc| > ћω/2), the interband conductivity becomes imaginary and the imaginary part of the intraband conductivity is positive. Consequently, graphene behaves as an ultra thin metal film and supports a TM-mode surface wave [22, 23]. The TM-mode surface wave on graphene film is similar to that of the surface plasmon polariton (SPP) surface wave excited at the metal-dielectric interface . Interface engineering using UV-curable polymer resin increases the carrier density of graphene, which is coincident with the increase of the chemical potential. As a result, graphene behaves ultra thin metal film that supports a TM-polarization guided mode. The TM-polarization light in the wave guide core couples into graphene strip and propagates along the two-dimensional metal-like graphene strip. Since the TE-mode is not supported by a thin metal strip, the TE-polarization light is not coupled to metal-like graphene strip.
Interface engineering can control carrier type and density of graphene with polymethyl methacrylate or self-assembled monolayer (SAM) [14, 15]. For the graphene film used in this work, the sheet resistance (Rs) is 324 Ω/sq on the SiO2 substrate as measured by a four-point probe instrument. If the upper side of the graphene is covered with the UV-curable perfluorinated acrylate polymer cladding (Exguide LFR), the sheet resistance reduced to 270 Ω/sq. The conductivity of the graphene film increases by simply embedding the graphene film in a polymer resin. When the UV-curable polymer resin exposed to UV light, photo-initiators in the resin generate free radicals with unpaired electrons. Radicals attack double bonds and accept free electrons. Free electrons may be supplied by graphene when graphene is embedded in the UV-curable resin. Since the graphene on the SiO2 substrate is p-type, the donation of free electrons in graphene leads the increase of the hole carrier density in graphene. Consequently, the conductivity of graphene film on the SiO2 increases. The increment of the conductivity is proportional to the increase of the graphene’s chemical potential, and consequently, the graphene strip embedded in the waveguide core and the upper-cladding dielectrics in Fig. 1(b) supports a TM-polarization surface wave. Guidance of the TM-polarization light along a graphene strip embedded in a homogenous polymer dielectric has been demonstrated experimentally in our previous work . Based on the aforementioned analysis, we concluded that the graphene polarizer having a graphene strip embedded in a UV-curable polymer resin serves as a TM-pass polarizer.
The number of graphene layer and the uniformity of the transferred graphene film on the waveguide polarizer have a great effect on the extinction ratio. Uniform large-area monolayer graphene can be grown by a thermal CVD-method using Cu as well as Ni catalysis . If the non-uniform graphene film grown using Ni is substituted with an uniform monolayer graphene that is synthesized using Cu catalysis, the extinction ratio of the fabricated graphene-based polymer waveguide polarizer becomes higher . Minimization of discontinuity with gaps, ripples, wrinkles, and contaminants in the transferred graphene may contribute to the improvement of the performance of TE-pass graphene polarizer. The chemical potential depends on the carrier density and can be controlled by means of chemical doping, electric filed, magnetic field, and/or gate bias voltage. By modulating the chemical potential, the characteristics of the graphene-based polymer waveguide polarizer can be modified. Further rigorous theoretical and experimental analysis is highly necessary to fully understand the behavior of the graphene film with various thicknesses and morphologies on the UV-curable polymer-based waveguide polarizer, and we are investigating on it as another research.
We have been realized planar-lighwave-circuit (PLC)-type graphene polarizers by using a low loss optical polymer waveguide and investigated the optical characteristics at a wavelength of 1.31 µm. By placing a graphene strip on the waveguide core, the planar-type waveguide polarizer serves as a TE-pass polarizer because the graphene strip supports TE-mode surface wave. However, if the electrical properties of the graphene strip are tuned by interface engineering using a UV-curable polymer rein, graphene embedded in the polymer supports TM-mode surface waves. Thus, the modified graphene polarizer works as a TM-pass polarizer. Based on the experimental results, we concluded that the proposed graphene-based planar waveguide device can be exploited further for development of on-chip photonic integrated circuits (PICs) by taking extraordinary advantages of graphene’s optical and electrical characteristics.
This work was supported by the Creative Research Program of the ETRI (11YF1110), Korea and a grant (Code No. 2011-0031660) from the Center for Advanced Soft Electronics under the Global Frontier Research Program of the Ministry of Education, Science and Technology, Korea.
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