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1. Sub-volt high-speed silicon MOSCAP microring modulator driven by high-mobility conductive oxide

   https://doi.org/10.1038/s41467-024-45130-4  

   Hsu, Wei-Che ; Nujhat, Nabila ; Kupp, Benjamin ; Conley, Jr, John F. ; Rong, Haisheng ; Kumar, Ranjeet ; Wang, Alan X. ( January 2024 , Nature Communications)

   Silicon microring modulator plays a critical role in energy-efficient optical interconnect and optical computing owing to its ultra-compact footprint and capability for on-chip wavelength-division multiplexing. However, existing silicon microring modulators usually require more than 2 V of driving voltage (V_pp), which is limited by both material properties and device structures. Here, we present a metal-oxide-semiconductor capacitor microring modulator through heterogeneous integration between silicon photonics and titanium-doped indium oxide, which is a high-mobility transparent conductive oxide (TCO) with a strong plasma dispersion effect. The device is co-fabricated by Intel’s photonics fab and our in-house TCO patterning processes, which exhibits a high modulation efficiency of 117 pm/V and consequently can be driven by a very low V_ppof 0.8 V. At a 11 GHz modulation bandwidth where the modulator is limited by the RC bandwidth, we obtained 25 Gb/s clear eye diagrams with energy efficiency of 53 fJ/bit.

    

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2. Low-loss composite photonic platform based on 2D semiconductor monolayers

   Ipshita Datta, Sang Hoon ( January 2020 , Nature photographer)

   Two dimensional (2D) materials such as graphene and transition metal dichalcogenides (TMDs) are promising for optical modulation, detection, and light emission since their material properties can be tuned on-demand via electrostatic doping1–21. The optical properties of TMDs have been shown to change drastically with doping in the wavelength range near the excitonic resonances22–26. However, little is known about the effect of doping on the optical properties of TMDs away from these resonances, where the material is transparent and therefore could be leveraged in photonic circuits. Here, we probe the electro-optic response of monolayer TMDs at near infrared (NIR) wavelengths (i.e. deep in the transparency regime), by integrating them on silicon nitride (SiN) photonic structures to induce strong light -matter interaction with the monolayer. We dope the monolayer to carrier densities of (7.2 ± 0.8) × 1013 cm-2, by electrically gating the TMD using an ionic liquid [P14+] [FAP-]. We show strong electro-refractive response in monolayer tungsten disulphide (WS2) at NIR wavelengths by measuring a large change in the real part of refractive index ∆n = 0.53, with only a minimal change in the imaginary part ∆k = 0.004. We demonstrate photonic devices based on electrostatically gated SiN-WS2 phase modulator with high efficiency ( ) of 0.8 V · cm. We show that the induced phase change relative to the change in absorption (i.e. ∆n/∆k) is approximately 125, that is significantly higher than the ones achieved in 2D materials at different spectral ranges and in bulk materials, commonly employed for silicon photonic modulators such as Si and III-V on Si, while accompanied by negligible insertion loss. Efficient phase modulators are critical for enabling large-scale photonic systems for applications such as Light Detection and Ranging (LIDAR), phased arrays, optical switching, coherent optical communication and quantum and optical neural networks27–30. 

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3. Hybrid electro-optic modulator combining silicon photonic slot waveguides with high-k radio-frequency slotlines

   https://doi.org/10.1364/OPTICA.411161  

   Ummethala, Sandeep ; Kemal, Juned N. ; Alam, Ahmed S. ; Lauermann, Matthias ; Kuzmin, Artem ; Kutuvantavida, Yasar ; Nandam, Sree H. ; Hahn, Lothar ; Elder, Delwin L. ; Dalton, Larry R. ; et al ( April 2021 , Optica)

   Electro-optic (EO) modulators rely on the interaction of optical and electrical signals with second-order nonlinear media. For the optical signal, this interaction can be strongly enhanced using dielectric slot–waveguide structures that exploit a field discontinuity at the interface between a high-index waveguide core and the low-index EO cladding. In contrast to this, the electrical signal is usually applied through conductive regions in the direct vicinity of the optical waveguide. To avoid excessive optical loss, the conductivity of these regions is maintained at a moderate level, thus leading to inherentRClimitations of the modulation bandwidth. In this paper, we show that these limitations can be overcome by extending the slot–waveguide concept to the modulating radio-frequency (RF) signal. Our device combines an RF slotline that relies on$\mathrm{B}\mathrm{a}\mathrm{T}\mathrm{i}{\mathrm{O}}_3$as a high-k dielectric material with a conventional silicon photonic slot waveguide and a highly efficient organic EO cladding material. In a proof-of-concept experiment, we demonstrate a 1 mm long Mach–Zehnder modulator that offers a 3 dB bandwidth of 76 GHz and a 6 dB bandwidth of 110 GHz along with a small$\mathrm{\pi <\#comment/>}$voltage of 1.3 V ($U_{\mathrm{\pi <\#comment/>}}L=1.3\mathrm{V}\mathrm{m}\mathrm{m}$). We further demonstrate the viability of the device in a data-transmission experiment using four-state pulse-amplitude modulation (PAM4) at line rates up to 200 Gbit/s. Our first-generation devices leave vast room for further improvement and may open an attractive route towards highly efficient silicon photonic modulators that combine sub-1 mm device lengths with sub-1 V drive voltages and modulation bandwidths of more than 100 GHz.

    

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4. Miniature, highly sensitive MOSCAP ring modulators in co-optimized electronic-photonic CMOS

   https://doi.org/10.1364/PRJ.438047  

   Gevorgyan, Hayk ; Khilo, Anatol ; Wade, Mark T. ; Stojanović, Vladimir M. ; Popović, Miloš A. ( January 2022 , Photonics Research)

   Convergence of high-performance silicon photonics and electronics, monolithically integrated in state-of-the-art CMOS platforms, is the holy grail for enabling the ultimate efficiencies, performance, and scaling of electronic-photonic systems-on-chip. It requires the emergence of platforms that combine state-of-the-art RF transistors with optimized silicon photonics, and a generation of photonic device technology with ultralow energies, increased operating spectrum, and the elimination of power-hungry thermal tuning. In this paper, in a co-optimized monolithic electronics-photonics platform (GlobalFoundries 45CLO), we turn the metal-oxide-semiconductor (MOS) field-effect transistor’s basic structure into a novel, highly efficient MOS capacitor ring modulator. It has the smallest ring cavity (1.5 μm radius), largest corresponding spur-free free spectral range ( FSR = 8.5    THz ), and record 30 GHz/V shift efficiency in the O-band among silicon modulators demonstrated to date. With 1 V pp RF drive, we show an open optical eye while electro-optically tuning the modulator to track over 400 pm (69 GHz) change in the laser wavelength (using 2.5 V DC range). A 90 GHz maximum electro-optic resonance shift is demonstrated with under 40 nW of power, providing a strong nonthermal tuning mechanism in a CMOS photonics platform. The modulator has a separately optimized body layer but shares the gate device layer and the gate oxide with 45 nm transistors, while meeting all CMOS manufacturability design rules. This type of convergent evolution of electronics and photonics may be the future of platforms for high-performance systems-on-chip. 

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5. Silicon nitride stress-optic microresonator modulator for optical control applications

   https://doi.org/10.1364/OE.467721  

   Wang, Jiawei ; Liu, Kaikai ; Harrington, Mark W. ; Rudy, Ryan Q. ; Blumenthal, Daniel J. ( August 2022 , Optics Express)

   Modulation-based control and locking of lasers, filters and other photonic components is a ubiquitous function across many applications that span the visible to infrared (IR), including atomic, molecular and optical (AMO), quantum sciences, fiber communications, metrology, and microwave photonics. Today, modulators used to realize these control functions consist of high-power bulk-optic components for tuning, sideband modulation, and phase and frequency shifting, while providing low optical insertion loss and operation from DC to 10s of MHz. In order to reduce the size, weight and cost of these applications and improve their scalability and reliability, modulation control functions need to be implemented in a low loss, wafer-scale CMOS-compatible photonic integration platform. The silicon nitride integration platform has been successful at realizing extremely low waveguide losses across the visible to infrared and components including high performance lasers, filters, resonators, stabilization cavities, and optical frequency combs. Yet, progress towards implementing low loss, low power modulators in the silicon nitride platform, while maintaining wafer-scale process compatibility has been limited. Here we report a significant advance in integration of a piezo-electric (PZT, lead zirconate titanate) actuated micro-ring modulation in a fully-planar, wafer-scale silicon nitride platform, that maintains low optical loss (0.03 dB/cm in a 625 µm resonator) at 1550 nm, with an order of magnitude increase in bandwidth (DC - 15 MHz 3-dB and DC - 25 MHz 6-dB) and order of magnitude lower power consumption of 20 nW improvement over prior PZT modulators. The modulator provides a >14 dB extinction ratio (ER) and 7.1 million quality-factor (Q) over the entire 4 GHz tuning range, a tuning efficiency of 162 MHz/V, and delivers the linearity required for control applications with 65.1 dB·Hz^2/3and 73.8 dB·Hz^2/3third-order intermodulation distortion (IMD3) spurious free dynamic range (SFDR) at 1 MHz and 10 MHz respectively. We demonstrate two control applications, laser stabilization in a Pound-Drever Hall (PDH) lock loop, reducing laser frequency noise by 40 dB, and as a laser carrier tracking filter. This PZT modulator design can be extended to the visible in the ultra-low loss silicon nitride platform with minor waveguide design changes. This integration of PZT modulation in the ultra-low loss silicon nitride waveguide platform enables modulator control functions in a wide range of visible to IR applications such as atomic and molecular transition locking for cooling, trapping and probing, controllable optical frequency combs, low-power external cavity tunable lasers, quantum computers, sensors and communications, atomic clocks, and tunable ultra-low linewidth lasers and ultra-low phase noise microwave synthesizers.

    

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