Source: https://www.osapublishing.org/oe/abstract.cfm?uri=oe-21-22-27326
Timestamp: 2019-04-23 06:21:50+00:00

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We propose several planar layouts of ultra-compact plasmonic modulators that utilize alternative plasmonic materials such as transparent conducting oxides and titanium nitride. The modulation is achieved by tuning the carrier concentration in a transparent conducting oxide layer into and out of the plasmon resonance with an applied electric field. The resonance significantly increases the absorption coefficient of the modulator, which enables larger modulation depth. We show that an extinction ratio of 46 dB/µm can be achieved, allowing for a 3-dB modulation depth in much less than one micron at the telecommunication wavelength. Our multilayer structures can be integrated with existing plasmonic and photonic waveguides as well as novel semiconductor-based hybrid photonic/electronic circuits.
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Fig. 1 General scheme of a compact modulator integrated with low-loss plasmonic waveguides. In this geometry, a stripe waveguide (grey) is used to bring a long ranging SPP mode to and from the modulator structure where an applied voltage modulates the SPP wave.
Fig. 2 Illustration of the low-index (a, b, c) and high-index (d, e, f) multilayer modulator designs considered in this work. They are vertically divided by their configuration. The first column, GZO only structures (a) and (d), uses the GZO as both the plasmonic layer and the dynamic layer. The second column, single interface structures (b) and (e), introduces a thick TiN layer, which supports single interface SPPs and use the GZO layer to perform modulation. Finally, the third column of thin TiN structures (c) and (f), uses a thin stripe of TiN to support the long ranging SPP mode and the GZO layer to modulate the signal.
Fig. 3 (a) GZO permittivity versus its carrier concentration, λ = 1.55 μm. The permittivity of the GZO layer was taken from  and a carrier concentration in the GZO was determined using a Drude-Lorentz model fitting: N0 = 9.426 × 1020 cm−3 (black dotted line). (b) TiN permittivity extracted from spectroscopic ellipsometry measurements.
Fig. 4 Depiction of the mode profile illustrating the definition of the mode size. Due to the complexity of the structure and high concentration of electrical energy in the GZO layer, the traditional definition of the mode size cannot be utilized. Here we define the mode size as the distance/range, which encompasses 86% of the electric field energy, a condition similar to that of the 1/e definition for a single interface waveguide.
Fig. 5 Multilayer structures along with graphs of the absorption coefficient (a, c) and mode size (b, d) versus GZO carrier concentration. Structures with high-index cladding (lower) show much higher absorption than structures with a low-index cladding (upper). The absorption maximum is accompanied by the highest mode localization, which occurs at the plasmon resonance for the structure. At lower carrier concentrations in the GZO, modes are increased due to smaller magnitude of its real permittivity.
Fig. 6 Schematic of plasmonic modulators integrated with TiN stripe waveguides providing long range SPP propagation to and from the modulator (side view). To create the electrical isolation and prevent shorting of the modulator structure, the silicon layers are doped as shown. However, even with large doping required in the n + region, the losses associated with silicon are several orders of magnitude below the plasmonic losses and are neglected in this analysis.
Fig. 7 Single interface coupling loss between the high-index waveguide and high-index “thin TiN” modulator sections versus carrier concentration in the GZO layer.
Fig. 8 Example mode profiles in the integrated modulator geometry high-index “thin TiN”. Note that the field decay outside the stripe waveguide is slow and therefore appears constant in this graph. The carrier concentration in the GZO layer used for the calculations corresponds to the maximum absorption in the modulator, i.e. plasmonic resonance condition N = Non. Under these conditions the majority of the field is localized within the GZO layer.
Table 1 Performance comparison for planar modulator designs. In the following table all of the fundamental parameters for the device characterization are listed. Among them we have Non which is the on-state carrier concentration, αmax which is the maximum absorption in the on-state, αmin which is the minimum absorption in the off-state, ER is the extinction ratio as defined in Eq. (1), woff is the off-state mode size, neff is the effective index of the mode, and FoM is the figure of merit as defined in Eq. (2).
Table 2 Summary of the performance of previous works in TCO based modulator structures. This is for means of comparison with the structures presented in this paper.
Performance comparison for planar modulator designs. In the following table all of the fundamental parameters for the device characterization are listed. Among them we have Non which is the on-state carrier concentration, αmax which is the maximum absorption in the on-state, αmin which is the minimum absorption in the off-state, ER is the extinction ratio as defined in Eq. (1), woff is the off-state mode size, neff is the effective index of the mode, and FoM is the figure of merit as defined in Eq. (2).
Summary of the performance of previous works in TCO based modulator structures. This is for means of comparison with the structures presented in this paper.

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