III. Graphene Nanophotonic Modulators for Mid-Infrared Applications -
3.4. Modulation of Graphene Plasmon Excited by ZnS Subwavelength Grating
3.4.4. Single Graphene on ZnS Subwavelength Sampled Grating
The sampled grating, consisting of grating part and sampling part, makes a comb-like reflection spectrum and it is widely used for distributed feedback (DFB) laser nowadays [145]. The wavelength spacing by the sampled grating Δλ is given by Δλ = Δλ02/(2neffΛ). neff is effective index of the waveguide mode and Λ is sampling period. Figure 3.4.7(a) shows the measured reflectance spectra of the fabricated sampled grating for three values of the applied voltage on graphene. When the applied voltage is 2 V, the reflectance peaks are at the wavelengths of 7.04 μm and 8.49 μm respectively. When applied voltage is reduced to 1 V, the reflectance peaks are red-shifted to the wavelengths of 7.8 μm and 9.94 μm. In order to verify these results, the simulation was conducted. Figure 3.4.7(b) shows the measured and simulated reflectance spectra for the applied voltage of 2 V. Both results have the first two peaks and their locations are similar. However, there is no third peak in the measurement which exists in the simulation at the wavelength of 10.9 μm. Even the peak levels and bandwidths in the measurement data are worse than those in the simulation, which may be due to the fabrication error in grating fabrication..
Figure 3.4.7. Solid-electrolyte-gated graphene on the ZnS sampled grating. (a) Reflectance spectra of the ZnS sampled grating with graphene for the voltages of 0, 3, 6, 9, 12 and 15 V. (b) For comparison, reflectance spectrum of the simulation for the voltage of 15 V is shown with the measured reflectance with a red curve.
3.5. Experimental Investigation of the Mid-Infrared Free-Space Modulator based on Graphene Plasmon Excitation by the ZnS Subwavelength Grating
In previous chapters, the mid-infrared modulator based on coupling the graphene plasmon from the hybrid plasmonic waveguide mode was theoretically investigated, dry etching techniques for ZnS nanopatterns are provided with several ZnS photonic devices, and graphene plasmon excitation with ZnS subwavelength grating are experimentally demonstrated. In this chapter, by combining those researches, the mid-infrared modulator based on graphene plasmon excitation by ZnS Subwavelength grating is experimentally demonstrated. The concept of the modulator is described in Figure 3.5.1.
Initially, without applied voltage on graphene, there is a magnetic dipole resonance (MDR) by a metal- insulator-metal (MIM) strip array, or a MIM waveguide mode (MWM). The reflectance spectrum of the MIM strip array has a perfect absorption band at a certain wavelength λ due to the MDR. When the voltage is applied on graphene in order to excite the graphene plasmon by subwavelength grating at λ, then the perfect absorption by MDR is broken down due to excited graphene plasmon and GP waveguide mode (GPWM) as well, then the reflectance at λ becomes large.
Based on this phenomenon, the MIM strip array integrated with ZnS subwavelength grating has been demonstrated. The MIM strip array is designed to have a perfect absorption band at the wavelength near 8 μm and the ZnS subwavelength grating is designed to excite the graphene plasmon at the same wavelength as possible. In order to apply voltage on graphene, the capacitor, which is based on Al-HfO2-graphene, is placed. The modulation efficiency of the modulator is about 65% at the wavelength of 7.84 μm. It is expected that this device can be used for free-space mid-infrared modulators.
Figure 3.5.1. Operation principle of the mid-infrared MIM modulator. Without applied voltage, there is a magnetic dipole resonance (MDR) inside the metal strip such that there is a perfect absorption. However, when the voltage is enough to excite the graphene plasmon, the MDR is broken. Then reflectance becomes higher.
3.5.1. Device Structure and Fabrication Process
The structure of the device is described in Figure 3.5.2. 150-nm-thick Al layer is placed on the Si wafer.
Up on the Al layer, ZnS layer is formed with a thickness of 150 nm. ZnS subwavelength grating is fabricated by surface-relief grating of ZnS layer. The etching depth and period of ZnS grating are set to 80 nm and 150 nm and the duty cycle of the grating is 40%, in other words, the length of intact ZnS region is 60 nm and the length of etched ZnS region is 90 nm. The number of ZnS subwavelength grating is designed to 10. Graphene is transferred on ZnS subwavelength grating, and 20-nm-thick HfO2 layer is deposited on graphene. Finally, there is Al strip array on HfO2 layer which width is 1.67 μm and the period is 4.8 μm.
The fabrication step of the modulator is explained in following sentences. Al layer was deposited by DC sputter on Si wafer. Then, ZnS layer was deposited by sputter as well. The first e- beam lithography was applied for pattering ZnS subwavelength grating. For the e-beam lithography,
Figure 3.5.2. Structure of the mid-infrared MIM modulator. On a silicon substrate, there is a vertical stack of Al-ZnS-HfO2. The thicknesses of Al and ZnS are 150 nm. And, the thickness of HfO2 is 20 nm. The subwavelength grating with the period of P is formed inside the ZnS layer. The duty cycle and the number of the grating is designed to 0.4 and 10 respectively. On HfO2 layer, there is an Al strip which makes MDR in this structure. The graphene layer is sandwiched between HfO2 and ZnS, and its width is identical to the width of Al strip.
with RIE and the residual e-beam resist was removed by the e-beam resist remover. On the graphene pattern, 1.5-nm-thick Hf was deposited by e-beam evaporator as a seed layer of HfO2 and oxidized naturally. Then, HfO2 was deposited by ALD to have a total thickness of 20 nm. The final e-beam lithography with same e-beam resist was applied to fabricate the Al strip array. 100-nm-thick Al was deposited by the e-beam evaporation. To form a strip array, Al-deposited sample was dipped into the e- beam resist remover to lift off the metal outside of strip. Before forming contact electrodes, the HfO2 at graphene contact region was removed by buffered oxide etch 6:1 for 25 seconds. Then the contact electrodes for graphene and Al strip array were patterned by photolithography with an AZ5214E photoresist. The 5-nm-thick Cr and 100-nm-thick Au were deposited by the e-beam evaporation and lifted-off by dipping the sample in acetone. The optical microscope image of the fabricated device is shown in Figure 3.5.3
.
Figure 3.5.3. Optical microscope image of the mid-infrared MIM modulator.
3.5.2. Experiment Results
The device is measured with same methods in chapter 3.4. Figure 3.5.4 shows the reflectance spectra of the device for the applied voltage on graphene. When the applied voltage is 0, the reflectance spectrum has a rejection band at the wavelength of 7.85 μm, and the reflectance at that wavelength is 0.02. This perfect absorption is resulted by the magnetic dipole resonance (MDR) of Al strip array. This rejection band is slightly distorted when the voltage is on. For examples, when the applied voltage is 9 V, there is small peak inside the rejection band at the wavelength of 9.5 μm, and when the applied voltage is increased to 12 V, the peak inside the rejection band is located at 8.4 μm. Then when the applied voltage is 15 V, there is a strong reflectance peak at the wavelength of 7.84 μm. At that wavelength, the reflectance peak level is 65.2%, which means the wavelength for exciting graphene plasmon is almost identical to the perfect absorption wavelength, then MDR is broken by coupled graphene plasmon. The red-shift of the reflectance peak is because the increase of the applied voltage or the chemical potential of graphene results in the increase of the imaginary part of conductivity of graphene. So the excitation wavelength of graphene plasmon is blue-shifted when the chemical potential of graphene is increased by enhancing the applied voltage.
Figure 3.5.5(a) shows the simulated reflectance spectra when the chemical potential of graphene is 0.6 eV which is value when the applied voltage on graphene is 15 V for this device. When the applied voltage is zero, there is a perfect absorption at the wavelength of 8 μm. However, when the chemical potential of graphene is set to 0.6 eV, the reflectance peak exists at the wavelength of 8.02 μm. The peak level is 71.6%. Figure 3.5.5(b) and 3.5.5(c) show the electric field distribution when the applied voltage is zero and 15 V respectively. In Figure 3.5.5(b), the strong electric field confinement due to MDR is observed. However, in the Figure 3.5.5(c), the excitation of graphene plasmon is observed and the confinement of electric field is weaker. The major reasons for wider band of the experimental result may be mainly due to ZnS film absorption which is not considered in the simulation and the randomness or fabrication errors of ZnS subwavelength grating and alignment.
Figure 3.5.5. Simulation results of the mid-infrared MIM modulator. (a) Calculated spectra when chemical potential of graphene is 0 (black) and 0.6 eV (red). (b) Intensity of the magnetic field of the structure when chemical potential is 0. Strong magnetic field confinement due to magnetic dipole resonance. (c) Intensity of the magnetic field when the chemical potential of graphene is 0.6 eV. Since graphene plasmon is exicted, the magnetic dipole resonance is broken.
0 eV 0.6 eV
3.5.3. Analogue of the electromagnetically-induced transparency for the mid-infrared metal- insulator-metal device
The characteristic of the mid-IR MIM device can be interpreted as an analogue of the electromagnetically-induced transparency (EIT). EIT is a phenomenon that resulted from a destructive interference of two energy levels by an electromagnetic wave such that it eliminates the effect of the optical medium and makes transparency [146],[147]. Several metamaterials consisting of two resonators are investigated for the EIT in the infrared and terahertz range [148]-[151]. Those resonators in the metamaterials have two distinct energy levels. One is called as the bright mode and the other is called as the dark mode. When two resonators are close enough to interact each other, there is the destructive interference between energy levels such that the structure becomes transparent. Similar to this EIT phenomenon in the metamaterial, the characteristic of the mid-IR MIM device can be explained by the EIT. Figure 3.5.6(a) shows the reflectance spectra of the mid-IR MIM modulator for the applied voltage of 0 V and 15 V. Figure 3.5.6(b) is the schematic diagram of the analog of EIT for the mid-IR MIM device. The ground state which is denoted as 0 is excited to the energy state 1 by the magnetic dipole resonance (MDR) in the MIM structure. This energy level transition is denoted as Ω01. Then, there is graphene plasmon (GP) mode denoted as 2 excited by ZnS subwavelength grating from the MDR in the MIM structure, which is a metastable state. This transition is denoted as Ω12 in the Figure 3.5.6(b). Finally, the destructive interference between Ω01 and Ω12 makes the transparency which is shown in reflectance spectrum in Figure 3.5.6(a) for the applied voltage of 15 V. So, the mid- IR MIM device is the dynamically-controlled EIT analogue using graphene plasmon.
IV. Conclusion
In this thesis, the graphene nanophotonic modulators for near-infrared and mid-infrared applications are proposed. The graphene has unique properties such as zero bandgap, tunable absorption characteristic, and high carrier mobility. Based on these properties, graphene-based photonic devices have been researched. Among graphene-based devices, the optical modulators based on the tunable characteristics of graphene have been spotlighted. In the near-infrared regime, graphene is used for the electroabsorption modulators (EAMs) since the chemical potential of graphene controls the absorption.
In the mid-infrared regime, graphene is used for the plasmonic modulators since graphene plasmon (GP) exists in the mid-infrared and its property can be tuned by controlling the chemical potential of graphene.
In the near-infrared, popular platform for the graphene optical modulators is the conventional silicon (Si) waveguide. Conventional Si waveguides integrated with graphene layers have been investigated as the EAMs. The modulation depth of one of those EAMs is just 0.16 dB/μm. In order to make the EAM more compact and faster, other approaches should be investigated. In this thesis, two approaches are suggested. One is using an inverted-rib-type (IRT) Si waveguide and the other is using a metal-insulator-silicon-insulator-metal (MISIM) waveguide. The EAM based on the IRT Si waveguide with double graphene capacitor is theoretically investigated. The modulation depth of the IRT Si waveguide is 0.41 dB/μm and its electrical bandwidth and optical bandwidth are calculated to 46.4 GHz and over 100 nm. The EAM based on the MISIM waveguide integrated with solid-electrolyte- gated graphene is experimentally demonstrated. The modulation depth of the MISIM waveguide is 0.276 dB/μm. For the faster operation, the MISIM waveguide with graphene capacitor would be a solution. The ideal electrical bandwidth of the MISIM waveguide with graphene capacitor would be expected to 185 GHz.
In the mid-infrared, GP exists, which is a collective electron oscillation at the graphene. The electromagnetic wave associated with GP is called as GP polariton (GPP). Many researches about GP- based or GPP-based devices are introduced theoretically. However, those devices do not consider the integration with other mid-infrared devices. Also, another approaches for exploiting GP are essential.
In this thesis, the mid-infrared graphene plasmonic modulator has been designed. This modulator is based on a coupling a hybrid plasmonic waveguide mode (HPWM) to GP by zinc sulfide (ZnS) subwavelength grating. The modulation depth of the modulator is simulated to 25.25 dB. And the electrical and optical bandwidth would be expected to be 29.3 GHz and 154 GHz, respectively.
explained and the optical devices such as ZnS perfect absorber, ZnS anti-reflection, and ZnS metasurfaces are fabricated and characterized. In addition, based on this ZnS nanopattern fabrication, excitation of GP by ZnS subwavelength grating is demonstrated in various grating and graphene structures. There are ZnS subwavelength grating and ZnS sampled grating integrated with solid- electrolyte-gated single graphene, and ZnS subwavelength grating with double graphene capacitor. By measuring the reflectance spectra of these grating structures with applying electric voltage on the graphene, the peak shift and the reflectance modulation have been demonstrated well.
Finally, the mid-infrared metal-insulator-metal (MIM) modulator is demonstrated as a proof- of-concept study for the coupling GP from the waveguide mode. The operation principle of the modulator is coupling GP excited by ZnS subwavelength grating from the MIM waveguide mode, then the magnetic dipole resonance (MDR) which results in the perfect absorption is hindered by the GP.
Therefore, the reflectance at the wavelength of MDR becomes higher. The modulation of reflectance is about 65% when applied voltage on graphene is 15 V. This modulator behavior can be thought like a plasmon-induced transparency. This work is not only the proof-of-concept study for coupling GP, but also demonstration of efficient mid-infrared free-space modulator. Also, this mid-IR MIM device can be interpreted as the analogue of electromagnetically-induced transparency. There is the destructive interference between the MDR and the GP.
I sincerely expect that studies in this thesis would be a foundation stone for novel graphene nanophotonic modulators with compact size, large electrical and optical bandwidth, high operation speed, and low energy consumption for next-generation optical communications in the future.
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