2D material platform for overcoming the amplitude-phase tradeoff in ring modulators

Compact, high-speed electro-optic phase modulators play a vital role in various large-scale applications including phased arrays, quantum and neural networks, and optical communication links. Conventional phase modulators suffer from a fundamental tradeoff between device length and optical loss that limits their scaling capabilities. High-finesse ring resonators have been traditionally used as compact intensity modulators, but their use for phase modulation have been limited due to the high insertion loss associated with the phase change. Here, we show that high-finesse resonators can achieve a strong phase change with low insertion loss by simultaneous modulation of the real and imaginary parts of the refractive index, to the same extent i.e. $\frac{\Delta n}{\Delta k} \sim 1$. To implement this strategy, we utilize a hybrid platform that combines a low-loss SiN ring resonator with electro-absorptive graphene (Gr) and electro-refractive WSe$_2$. We achieve a phase modulation efficiency ($V_{\frac{\pi}{2}} \cdot L_{\frac{\pi}{2}}$) of 0.045 V $\cdot$ cm with an insertion loss (IL$_{\frac{\pi}{2}}$) of 4.7 dB for a phase change of $\frac{\pi}{2}$ radians, in a 25 $\mu$m long Gr-Al$_2$O$_3$-WSe$_2$ capacitor embedded on a SiN ring of 50 $\mu$m radius. We find that our Gr-Al$_2$O$_3$-WSe$_2$ capacitor can support an electro-optic bandwidth of 14.9 $\pm$ 0.1 GHz. We further show that the $V_{\frac{\pi}{2}} \cdot L_{\frac{\pi}{2}}$ of our SiN-2D platform is at least an order of magnitude lower than that of electro-optic phase modulators based on silicon, III-V on silicon, graphene on silicon and lithium niobate. This SiN-2D hybrid platform provides the impetus to design compact and high-speed reconfigurable circuits with graphene and transition metal dichalcogenide (TMD) monolayers that can enable large-scale photonic systems

Integrated Photonics for NASA Applications

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Platform based on two-dimensional (2D) materials for next-generation integrated photonics

Electro-optic phase modulators play a vital role in various large-scale photonic systems including Light Detection and Ranging (LIDAR), quantum circuits, optical neural networks and optical communication links. The key requirement of these modulators include strong phase change with low modulation induced optical loss, low electrical power consumption, small device footprint and low fabrication complexity. Conventional silicon phase modulators have either high power consumption (thermo-optic effect) or high optical loss (plasma-dispersion effect). On the other hand, low-loss phase modulation can be achieved using electro-optic ² effect such as LiNbO₃ which has a large device footprint (in mm's) and requires complex fabrication. The need of the hour is a material or a device that is strongly tunable with low optical loss and capable of picosecond switching speed. Transition metal dichalcogenides (TMDs) have been widely studied for optoelectronic applications due to their strong and tunable excitonic response. In fact, TMDs have been shown to experience massive changes of upto 20 % in their refractive index with doping, but this modulation is accompanied with large absorption change (60 %), which greatly limits their utility in photonic applications. In contrast, very litte is known about the effect of doping on the electro-optic response of TMDs at energies far below the exciton resonances, where the material is transparent and therefore could be used for photonic circuits. In this work, we first probe the electro-optic properties of TMDs in the near-infrared using a dielectric SiN microring resonator platform. We measure a strong doping induced change in the refractive index (Δn) of 0.52 in WS2 with minimal induced absorption (Δk) of 0.004. The |Δn/Δk| of 125, is an order of magnitude higher than the measured |Δn/Δk| for 2D materials including graphene and TMD monolayer at excitonic resonances, and for bulk electro-refractive materials commonly employed in silicon photonics. We next utilize this strong electro-refractive response to demonstrate low power, lossless optical phase modulation based on a composite SiN-TMD platform. The WS₂ based photonic modulator achieves a modulation efficiency (V_π⋅L) of 0.8 V ⋅ cm with a RC limited bandwidth of 0.3 GHz and DC electrical power consumption of 0.64 nW. The measured index change in monolayer TMDs (∼ 15%) in TMDs is unprecedented, considering the change in index of bulk (LiNbO₃) - the 'gold standard' for photonics - is typically 0.04 %. Despite the observed strong electro-refractive effect in TMDs and the enhanced light-matter interaction, the change in effective index of the propagating mode is 6.5 × 10⁻⁴ RIU, thereby requiring WS₂ phase modulators that are 1.3 mm long. This is due to the low optical mode overlap of 0.03 % with the monolayer that necessitates long phase shifter length. There is an urgent need for a compact, low-loss and high-speed optical phase shifter. Conventional phase modulators with low optical loss require long lengths to achieve strong phase change. On the contrary, traditional intensity modulators leverage compact high-finesse ring resonators to modulate output intensity. However, such cavities with conventional electro-refractive materials such as silicon where Δn/Δk = -20 cannot be used for phase modulation, owing to the high insertion loss associated with the phase change. Here, we show that we can leverage high-finesse ring resonators to achieve strong phase change with low optical loss. We achieve this by simultaneously modulating both the real and imaginary part of the effective index in the cavity to the same extent i.e. Δn/Δk ≈1. We design a hybrid SiN-2D platform that modulates the complex effective index of the propagating mode, by tuning the loss and index in monolayer graphene (Gr) and WSe₂ embedded on a SiN waveguide, respectively. We engineer the Gr-WSe₂ capacitor design to achieve a linear phase change of (0.50 ± 0.05)π radians with a low transmission modulation of 1.73 ± 0.20 dB and insertion loss of 2.96 ± 0.34 dB. We measure a 3 dB electro-optic bandwidth of 14.9 ± 0.1 GHz in the SiN-2D hybrid platform. We measure a phase modulation efficiency (V_(π/2)⋅L) of 0.045 V ⋅ cm with an insertion loss of 4.7 dB for a phase change of π/2 radians in the 25 μm SiN-2D platform. We show that the V_(π/2)⋅L for our SiN-2D hybrid platform is significantly lower than V_(π/2)⋅L of electro-refractive phase modulators based on silicon PN, PIN and MOS capacitors with comparable insertion loss. The TMD or TMD-graphene capacitor is incorporated as a post-fabrication process, transforming any passive substrate into an active photonic platform. The demonstrated enhanced light-matter interaction in monolayer TMDs could open up routes to a range of novel applications with these 2D materials and enable highly reconfigurable photonic circuits with low optical loss and power dissipation. We estimate that the efficiency of our TMD platform can be improved by optimizing the optical mode overlap with the monolayer through photonic mode optimization or reducing the dielectric thickness. For large-scale photonic systems, wafer-scale integration of TMD materials with silicon photonics can be done either as a direct TMD growth process on silicon wafers or a post-processing step where large wafer-scale TMD films are transferred onto a silicon photonics platform fabricated in a standard foundry

Supplementary document for 2D material platform for overcoming the amplitude-phase tradeoff in ring resonators - 6659810.pdf

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Supplementary document for 2D material platform for overcoming the amplitude–phase tradeoff in ring resonators - 6731816.pdf

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