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Abstract
Plasmon induced transparency (PIT) has been numerically investigated and experimentally realized by two parallel gold strips on graphene for the mid-infrared (MIR) range. The PIT response is realized by the weak hybridization of two bright modes of the gold strips. The response of the device is adjusted with the lengths of two strips and tuned electrically in real time by changing the Fermi level (Ef) of the graphene. Ef is changed to tune the resonance frequency of the transparency window. A top gating is used to achieve high tunability and a 263 nm shift is obtained by changing the gate voltage from −0.6 V to 2.4 V. The spectral contrast ratio of our devices is up to 82%.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Electromagnetically induced transparency (EIT) is a phenomenon that reduces the absorption of an optically thick medium over a narrow spectrum by the quantum interference effect [1–4]. This can be used for slow light applications [5], optical switching [6], quantum information processing [6], optical storage [7], and nonlinear optical enhancement [8]. A plasmonic analogue of EIT has been observed in metallic nanoparticles [9, 10], integrated photonic structure [11], metamaterial structures [12–16] and is known as plasmon induced transparency (PIT). In most of these studies, PIT has been investigated by the destructive interference of dark-bright modes and detuning of two bright modes. In destructive interference, there is a coherent coupling between narrow and broad band resonances of two dipole antennas [14]. Another way to obtain PIT is the weak hybridization of two bright modes having different resonances due to different dimensions [17]. Dynamically tunable PIT can offer promising applications, such as electrically switchable camouflage systems and tunable enhanced sensing.
Recently, tunable PIT has been numerically investigated by several groups. Their designs are either based on graphene structures, such as Fabry-Perot resonators [18,19], ring resonators [20,21], and cascaded pi-shaped structures [22], or on hybrid metal-graphene metamaterials [13]. However, the successful realization of these numerical investigations has not been achieved due to the fabrication difficulties of the suggested structures. Graphene based structures require high quality graphene layer on substrates without any cracks, which is limited by transfer methods. Therefore, hybrid metamaterials are an efficient strategy to realize tunable PIT. We have experimentally demonstrated dynamically tunable PIT with simple gold (Au) strips on graphene. By modifying the lengths of Au strips we are able to adjust the PIT response of the device. Moreover, our design allows us to electrically tune the response of device in real time by changing the Fermi level (Ef) of graphene.
Graphene is a single atomic layer thick material (2-D Material) in which carbon atoms form a honeycomb lattice [23], discovered by Novoselov et al. in 2004 [24]. Graphene has remarkable properties, which has made it an active research area for the last decade [25–27]. At mid-infrared (MIR) graphene has tight field confinement [28] and a controllable Ef with gate voltage [29–32], which makes graphene a suitable candidate for tuning resonances electrically. The interaction of light with graphene can be explained by the concept of Pauli Blocking [33], as graphene forms a conical band diagram. There are two types of transition(s) in graphene depending upon the energy of the photons: the two transitions are actually always present regardless of the photon energy. However, when the energy is less than 2Ef the intra-band transition is dominant and if the energy is higher than 2Ef the inter-band becomes dominant. Devices with high tunability mostly operate near 2Ef [34]. Graphene Ef is a direct function of charge density as shown in Equation 1. Where ~ is reduced Planck’s constant, n is charge density and νF is Fermi velocity that is −1.1×108 cm s−1 [35]
The surface conductivity of graphene can be changed by a variety of gating methods, and results in the change of Ef that can be used to tune the optical response of plasmonic structures on graphene. Falkovsly presented a mathematical model for the surface conductivity of graphene as:
where complex conductivity (σ) is a function of frequency (ω) and Fermi function f0(ϵ)={exp[(ϵ−µc)/(kBT)]+1}−1, (kB is Boltzmann’s constant). In these equations, intra-band electron photon scattering, and inter-band electron transitions are represented by first and second terms, respectively. Integrating the first term leads to equation 3 which shows the intra-band conductivity of graphene. In the second term, where δ→0 is the infinitesimal quantity determining the bypass around the integrand pole, owes its origin to the direct inter-band electron transitions.In equation 3, µc is chemical potential, τ is scattering time, and T is temperature, as the graphene chemical potential can be controlled by gating and therefore graphene is highly tunable by gating in intra-band region.
2. Method
We have investigated and realized two parallel Au strips on graphene with BaF2 substrate as shown in Fig. 1. Each of these strips has their own plasmonic resonance that serves as two different bright modes. The detuning of these bright modes creates a PIT response in the MIR region, which is tuned by changing Ef of graphene. First, we numerically investigated the tunable PIT using the Finite Difference Time-Domain (FDTD) method in commercially available software (Lumerical FDTD Solutions). The unit cell of the design as shown in Fig. 1(a) has periodic boundaries along the x and y-axes and Perfect Matched Layers (PML) in the z-axis (propagation direction). Two devices are investigated with different lengths of strips, L1 & L2, which are selected as 3.2 & 2.4 µm for the first device and 3.4 & 2.6 µm for the second device, respectively. However, the thickness (T=70 nm), width (W= 0.5 µm), and distance (D= 0.2 µm) between the two strips are kept constant in both of the devices. Graphene is introduced as a 2D dispersive material between the BaF2 and the Au strips. In our calculations, the optical constant of the gold is considered based on Palik model and the applied optical parameters of the BaF2 is extracted from Reference [36]. The unit cell is illuminated by a plane wave source along the z direction having electric component (E) parallel to x-axis. Scattering rate of graphene was set as 0.01 eV (2.4 × 1012 sec−1), T was set as 300 K and µc was changed from 0.5 eV to 0.8 eV, which matches the experimental results from −0.6 V to 2.4 V and are consistent with previous studies [37, 38].
Fig. 1 (a) Unit cell of PIT structure. Two Au strips (golden) with lengths of L1 and L2 on graphene (grey) are presented. Two devices are designed with two different lengths of strips. Px and Py show periodicity of unit cell. (b) Schematic of ionic gating of PIT device. Two BaF2 substrates are used. The first one with Au strips on graphene (purple), which is connected to the source by conductive tape (black). The second one with a window for FTIR measurements and coated with Ti/Au (golden), which is connected to second terminal of the source. Ionic liquid (light blue) is injected between these two substrates. (c) SEM image of PIT structure.
The device was fabricated based on simulation design, using a commercially available chemical vapor deposition (CVD) grown graphene on Cu. The graphene was transferred onto BaF2 substrates by the wet transfer method and spin coated with positive photo resist (PMMA A4) and aqua save. Coated sample was exposed to a dose of 2000 pC/cm2 under E-beam to form strips. The exposed samples were developed in a developer (MIBK: IPA) for 1 minute. 5/65 nm of Ti/Au were deposited by E-beam evaporator at constant rate (1 Å/sec) for both metals, structures appeared after the lift-off process of 24 hours in acetone. An SEM image of a fabricated device is shown in Fig. 1(c). In order to apply voltage to the graphene, the top gating method was used [37, 38]. For this, we used other BaF2 substrates with metal contacts at the corners and a transparent window at the center, formed by photolithography. Contact with graphene is made using conductive tape. An Ionic liquid (Diethylmethyl(2-methoxyethyl) ammonium bis(trifluoromethylsulfonyl) imide, [deme][Tf2N]) was inserted between two substrates which were insulated by tape. A schematic is shown by Fig. 1(b). Ionic liquid is used for low operational voltage, in which ions accumulate at the surface of graphene and change the Ef of graphene.
Normalized transmission measurements for two different devices were obtained by using a Fourier transform infrared (FTIR) instrument integrated with microscope. Measurements were done from 6 to 14 µm at different gate voltage using the ionic gating. By changing the gate voltage surface carrier concentration, Ef of graphene was observed in PIT phenomenon. For the measurement, samples were illuminated by x polarized field. The background measurements were taken from graphene on BaF2 substrates with no PIT structures.
3. Results and discussion
The transmission simulation for each of the Au strips shows strong responses as presented in the insets of Fig. 2(a) and 2(b). As described above, when we bring two strips close to each other, they result in a weak hybridization and give the PIT response presented in Fig. 2(a) and 2(b). This response is not observed by the excitation of single strip. By carefully designing the lengths and distance between the strips, we investigated the PIT phenomenon for two different strips. Furthermore, this PIT response was tuned by changing Ef of graphene in simulations presented in Fig. 2(a) and 2(b). This response is shifted toward shorter wavelengths as Ef of graphene is increased from 0.5 eV to 0.8 eV. For reflection and absorption results of shorter dimension at 0.5 eV see Appendix A. Fig. 2(c) shows the measured transmission of the device with shorter lengths of strips (3.2 and 2.4 µm), SEM image shown in the inset. Similarly measured transmission of the device with a longer dimension of strips (3.4 and 2.6 µm) is presented in Fig. 2(d), with an SEM image in the inset. As the gate voltage is increased from −0.6 V to 2.4 V, a blue shift is observed in PIT for both devices. We have observed good agreement with simulations for changing Ef from 0.5 eV to 0.8 eV. The minimum gate voltage was selected based on charge neutrality point (CNP), measured previously to be −0.6 V for a similar graphene structure [37, 38].
Fig. 2 Simulated and FTIR measurement of PIT structures. Simulated results of (a) shorter dimension and (b) longer dimension for different Ef. Normalized transmission for (c) shorter dimension and (d) longer dimension at −0.6 V, 0.9 V and 2.4 V, SEM image with dimension in the inset.
Total shift of 235 nm was observed for shorter dimension and 263 nm for longer dimension by changing gate voltage of 3.0 V. Shift in PIT transmission peak at the applied gate voltage with respect to CNP is obtained for both the devices and shown in the insets of Fig. 2(c) and 2(d). Investigation of E-field for two strips that resulted in resonances at different wavelengths is investigated for a device with shorter dimensions (Fig. 3). Electric field magnitudes of two resonance wavelengths in the transmission spectrum (7.75 µm and 9.06 µm) is shown in Fig. 3(a) and 3(c). In addition, the E-field magnitude at a PIT wavelength is shown in Fig. 3(b). These results show that each of the strips are excited separately at the resonance wavelengths and act as bright mode resonances. However, at 8.22 µm, both strips are excited simultaneously resulting in the PIT phenomenon. Major localization is at the corner of the strips at their own resonance wavelengths and between two strips for transmission peak of PIT. Spectral contrast ratio (Scon) is used to evaluate the performance of PIT response in optical applications [13] and described as:
where Tpeak is intensity of transmission peak and Tdip is intensity of resonance dip. Spectral contrast ratios of our devices are 82% and 78% for shorter and longer dimensions, respectively. Both devices with a high spectral contrast ratio and high tunability are suitable for filtering and switching applications.4. Conclusion
In this work, tunable PIT devices are numerically investigated and experimentally realized. Resonance frequencies of two strips were tailored by adjusting the length of the strips and tuned by changing the Ef of graphene layer. A large tuning range was demonstrated for the FTIR measurements of PIT structures by applying gate voltage. We were able to obtain 263 nm of shift in transmission window by applying gate voltage from −0.6 V to 2.4 V. This concept of real time tuning PIT is exciting for novel devices in the field of optical switching, modulation, slow light applications, tunable sensors, filters, photoluminescence, and enhancing nonlinear interactions.
Appendix A: Simulation results of reflection and absorption
As it is evident from the presented simulation results in Fig. 4, in the designed structure the resonances of PIT response are compensated by both reflection and absorption peaks.
Funding
Academy of Finland, Competitive Funding to Strengthen University Research Profiles (301820).
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Acknowledgments
The authors acknowledge O. Ozdemir for sharing useful information. E.O. acknowledges the partial support from the Turkish Academy of Sciences.
References and links
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