Since graphene interacts with the weakly evanescent field of a Si waveguide mode, the modulation depths of the previous graphene-based Si waveguide modulators are smaller than 0.16 dB/μm. In the case of the former, graphene is placed in the region where the electric field of the IRT Si waveguide mode is mainly confined. In the case of the latter, graphene is placed on the narrow insulator where the electric field of the MISIM waveguide mode is strongly confined.
The EAM based on the IRT Si waveguide is theoretically investigated and the calculated modulation depth is 0.41 dB/μm. The working principle of the modulator is that the grating-assisted coupling generates a repulsion band in the transmission spectrum of the hybrid plasmonic waveguide. Finally, a mid-IR perfect absorber based on a metal-insulator-metal (MIM) structure is experimentally investigated to demonstrate the feasibility of the grating-assisted coupling between a GP and a MIM waveguide mode.
Propagation losses of MISIM waveguides and conventional Si waveguides with solid-electrolyte guided graphene in chip 2 and chip 3.
Introduction to Graphene
Optical Properties of Graphene
Real and imaginary part of the conductivity of graphene (a) as a function of wavelength when the chemical potential of graphene is set to 0.4 eV. b) as a function of the chemical potential of graphene when the wavelength is set to 1.55 μm. The absorption mechanism of graphene can be interpreted from the dielectric constant of graphene with a finite thickness. Since the thickness of monolayer graphene is very thin which is about 0.35 nm, graphene can be treated as a uniaxial medium which has in-plane dielectric constant and surface normal dielectric constant.
The relationship between the conductivity of graphene, σg, and the in-plane component of the dielectric constant of graphene, εg, can be written in Eq. Thus, due to this tunable property of graphene based on the chemical potential of graphene, many researches on graphene-based optical modulators have been introduced. Dielectric constant and refractive index of graphene. a) In-plane component of complex dielectric constants of graphene at 1.55 μm wavelength.
The black curve is for the real part and the red curve is for the imaginary part of the dielectric constant.
Graphene Nanophotonic Modulators for Near-Infrared Applications
Graphene-based Optical Modulators
It was also reported on a silicon waveguide with a graphene-Al2O3-graphene capacitor that the MD of the modulator was measured to be 0.16 dB/μm [44]. The modulation depths of graphene-covered Si waveguides are quite limited because the graphene is located on top of the Si waveguide, so the graphene only affects the evanescent field of the waveguide mode. Various graphene-based optical modulators. a) Graphene-based optical modulator integrated in a conventional Si waveguide [43].
Therefore, another approach to the graphene-based optical modulators needs to be investigated for large modulation depth and electrical bandwidth. In the case of the IRT Si waveguide, the graphene is placed between the ridge and the slab so that the graphene interacts with the strong electric field in the waveguide mode. In the case of the MISIM waveguide, the waveguide mode of the MISIM waveguide is strongly confined to the insulator channels.
Thus, when graphene is located at the top of the MISIM waveguide, graphene works with the highly confined electric field of the MISIM waveguide mode such that the modulation depth of the graphene-based MISIM waveguide can be expected to be larger.
![Figure 2.2.1. Diverse graphene-based optical modulators. (a) Graphene-based optical modulator integrated on the conventional Si waveguide [43]](https://thumb-ap.123doks.com/thumbv2/123dokinfo/10489305.0/28.892.181.712.402.744/diverse-graphene-modulators-graphene-modulator-integrated-conventional-waveguide.webp)
Theoretical Investigation of Inverted-Rib-Type Silicon Waveguides Integrated with
- Device Structure and Analysis Method
- Analysis Results
The purpose of the a-Si layer is the plate part of a rib waveguide and to enable strong interaction between the graphene and the IRT Si waveguide mode. Schematic of the proposed EAM based on the IRT Si waveguide. a) Structure of EAM based on IRT Si waveguide with double graphene layer. They are determined such that the propagation loss of the TE mode of the IRT Si waveguide is maximized at the wavelength of 1.55 μm for μc = 0 eV.
However, when ws is larger than 190 nm, the larger ws shows the more confined Ex in the Si stripe portion of the IRT Si waveguide. Finally, the dependence of the propagation loss of the IRT Si waveguide on ta is considered; Thus, Ta is tentatively determined to be 10 nm for the largest modulation depth of the IRT Si waveguide with double graphene layers.
Then the transmission to the TE mode of the IRT Si waveguide except graphene layers was calculated.
Experimental Investigation of the Solid-Electrolyte-Gated Graphene-Covered Metal-
- Structure and Experiment Methods of the EAM
- Experiment Results – (1) DC Characteristics
- Experiment Results – (2) Modulation Characteristics
- Theoretical Investigation of Graphene-Capacitor-Covered MISIM
Experimental investigation of the solid electrolyte-coated graphene-covered metal-insulator-silicon-insulator-metal waveguide. The structure of the MISIM waveguide is described in Figure 2.4.1(a), and the MISIM waveguide is covered with the single graphene and solid electrolyte. The mode profile of the MISIM waveguide, which is calculated using a simulation program (Mode Solutions, Lumerical), is also shown in Figure 2.4.1(b).
The electric field of the MISIM waveguide mode is strongly confined in the SiO2 layers due to the Si strip and Cu blocks. Actually, there is a surrounding SiO2 layer around the Si strip of the MISIM waveguide above the level of the Cu top surface. The relationships between the propagation loss and coupling loss of the MISIM waveguide with VG are shown in Figure 2.4.2 (a) and (b).
Propagation loss of a 450-nm-wide conventional graphene-covered Si strip waveguide dependent on a solid electrolyte shown as a function of VG in Fig. 2.4.2(c). Direct current characteristics of a MISIM waveguide dependent on a graphene-covered solid electrolyte. a) MISIM waveguide propagation loss as a function of applied voltage VG. The propagation losses of the MISIM waveguides in chips 2 and 3 are shown versus VG in Figure 2.4.3(a).
Based on the DC characteristics, the modulation characteristics of the MISIM waveguide covered with the solid electrolyte-gated graphene are observed in chip 1. A Si photonic modulator using the MISIM waveguide covered with a graphene capacitor is schematically shown in figure 2.4.7 (a). and the cross-section of the MISIM waveguide is shown in Figure 2.4.7(b). To analyze the modulator, first calculate the modulation depth of the MISIM waveguide at a wavelength of 1550 nm.
The modulation depth is shown as a function of the width of the overlap region wo in Figure 2.3.8(b). Characteristics of a MISIM waveguide covered with a graphene capacitor. a) Relation of the propagation loss to the chemical potential of graphene.
Graphene Nanophotonic Modulators for Mid-Infrared Applications -
Theoretical Investigation of Mid-Infrared Graphene Plasmonic Modulator based on
- Design of the Modulator
- Modulator Analysis
Fabrication of ZnS Sub-micrometer-scale Structure and Photonic Devices
- Dry Etching Process for ZnS
- Anti-reflection Pattern based on ZnS Nanostructures
- Post-fabrication Tuning of Mid-IR Perfect Absorber
- ZnS Metasurfaces
Modulation of Graphene Plasmon Excited by ZnS Subwavelength Grating
- Device Structure, Fabrication, and Measurement Methods
- Single Graphene on ZnS Subwavelength Grating with Solid Electrolyte Gate -
- Graphene Capacitor on ZnS Subwavelength Grating
- Single Graphene on ZnS Subwavelength Sampled Grating
In the case of ZnS subwavelength grating single graphene with solid electrolyte gate, the grating period is designed to be 250 nm, the grating duty cycle is 50%, and the grating depth is 80 nm. The polarization of the mid-IR beam from the FTIR is set vertical to the grid with the mid-IR polarizer. The reflection spectra of single graphene on ZnS subwavelength grating with solid electrolyte gate for several values of applied voltage are shown in Figure 3.4.3(a).
The dependence of the reflection spectrum on values for the mobility of graphene is shown in figure 3.4.5. Reflectance spectra of solid-electrolyte-guided graphene on ZnS subwavelength gratings. a) Reflectance spectra by changing applied voltage on the graph from 0.2 V to 2 V in 0.2 V steps. Experimental investigation of the mid-infrared free-space modulator based on graphene plasmon excitation at the ZnS subwavelength grating.
The reflection spectrum of the MIM strip array has a perfect absorption band at a certain wavelength λ due to MDR. The red shift of the reflection peak is because increasing the applied voltage or chemical potential of graphene results in increasing the imaginary part of the conductivity of graphene. However, in Figure 3.5.5 (c), the plasmon excitation of graphene is observed and the electric field confinement is weaker.
Simulation results of the mid-infrared MIM modulator. a) Calculated spectra when the chemical potential of graphene is 0 (black) and 0.6 eV (red). Analogue of the electromagnetically induced transparency of the mid-infrared metal-insulator-metal unit insulator-metal unit. The characteristic of the mid-IR MIM device can be interpreted as an analogue of the electromagnetically induced transparency (EIT).
Similar to this EIT phenomenon in the metamaterial, the characteristic of the mid-IR MIM device can be explained by EIT. The working principle of the modulator is to couple the GP excited by the 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.
Experimental Investigation of Mid-Infrared Free-Space Modulator based on Graphene
- Device Structure and Fabrication Process
- Experiment Results
- Analogue of the Electromagnetically-induced Transparency for the Mid-
Conclusion
In this thesis, graphene nanophotonic modulators are proposed for near-infrared and mid-infrared applications. Among graphene-based devices, optical modulators based on the tunable characteristics of graphene have been put in the spotlight. In the mid-infrared regime, graphene is used for plasmonic modulators since the graphene plasmon (GP) exists in the mid-infrared and its property can be tuned by controlling the chemical potential of graphene.
The EAM based on the IRT Si waveguide with dual graphene capacitor is theoretically investigated. The EAM based on the MISIM waveguide integrated with solid electrolyte integrated graphene is experimentally demonstrated. This modulator is based on coupling a hybrid plasmonic waveguide mode (HPWM) to GP through a zinc sulfide (ZnS) subwavelength grating.
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. This work is not only the proof-of-concept study for coupling GP, but also a demonstration of efficient mid-infrared free-space modulator. Tsang, “Mid-infrared suspended membrane waveguide and ring resonator on silicon-on-insulator,” IEEE Photonics J., vol.
Kwon, “Mid-infrared subwavelength modulator based on lattice-assisted coupling of a hybrid plasmonic waveguide mode to a graphene plasmon,” Nanoscale , vol. Li, “Heterogeneous integrated silicon photonics for the mid-infrared and spectroscopic sensing,” ACS Nano , vol. Deng, “Independently tunable dual-band plasmonic-induced transparency based on hybrid metal-graphene metamaterials at mid-infrared frequencies,” Opt.
