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1. 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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2. 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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3. Electrically addressable tungsten doped phase change device in a through pixel configuration

   https://doi.org/10.1364/OME.486073  

   Burrow, Joshua A. ; Lawandi, Roseanna G. ; Sarangan, Andrew ; Agha, Imad ( March 2023 , Optical Materials Express)

   In this paper, we propose and demonstrate electrical switching of a 4% tungsten-doped Ge₂Sb₂Te₅(W-GST) pixel in a lateral configuration without the need for an auxiliary resistive heater. The phase transition between an amorphous and poly-crystalline state is achieved by Joule heating directly through the 4μm × 4μm × 350 nm active volume of the chalcogenide phase change pixel. While undoped GST would be challenging to switch in a lateral configuration due to very large resistance in the amorphous state, W-GST allows for switching at reasonable voltage levels. The pixel temperature profile is simulated using finite element analysis methods to identify the pulse parameters required for a successful electrical actuation. Experimentally, a 1550 nm light source is used for in-situ optical reflection measurements in order to verify the crystallization and re-amorphization of the pixel. As a result of the W doping, we identify volatile and non-volatile regimes with respect to bias voltage and pulse width during crystallization. During amorphization, we observe irreversible material failure after one complete cycle using in-situ optical monitoring, which can be attributed to a migration or segregation process. These results provide a promising path toward electrically addressed devices that are suitable for optical applications requiring amplitude modulation in a reflective geometry, such as spatial light modulators.

    

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4. Unique prospects of phase change material Sb 2 Se 3 for ultra-compact reconfigurable nanophotonic devices

   https://doi.org/10.1364/OME.435979  

   Jia, Wei ; Menon, Rajesh ; Sensale-Rodriguez, Berardi ( August 2021 , Optical Materials Express)

   In this work, we explore inverse designed reconfigurable digital metamaterial structures based on phase change material Sb₂Se₃for efficient and compact integrated nanophotonics. An exemplary design of a 1 × 2 optical switch consisting of a 3 µm x 3 µm pixelated domain is demonstrated. We show that: (i) direct optimization of a domain containing only Si and Sb₂Se₃pixels does not lead to a high extinction ratio between output ports in the amorphous state, which is owed to the small index contrast between Si and Sb₂Se₃in such a state. As a result, (ii) topology optimization, e.g., the addition of air pixels, is required to provide an initial asymmetry that aids the amorphous state's response. Furthermore, (iii) the combination of low loss and high refractive index change in Sb₂Se₃, which is unique among all phase change materials in the telecommunications 1550 nm band, translates into an excellent projected performance; the optimized device structure exhibits a low insertion loss (∼1.5 dB) and high extinction ratio (>18 dB) for both phase states.

    

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5. Artifact propagation in electrocorticography stimulation

   Lim, J. ; Wang, P.T. ; Shaw, S.J. ; Armacost, M. ; Gong, H. ; Liu, C.Y. ; Heydari, P. ; Do, A.H. ; Nenadic, Z. ( May 2018 , Seventh International BCI Meeting, Abstract Book)

   Introduction:Current brain-computer interfaces (BCIs) primarily rely on visual feedback. However, visual feedback may not be sufficient for applications such as movement restoration, where somatosensory feedback plays a crucial role. For electrocorticography (ECoG)-based BCIs, somatosensory feedback can be elicited by cortical surface electro-stimulation [1]. However, simultaneous cortical stimulation and recording is challenging due to stimulation artifacts. Depending on the orientation of stimulating electrodes, their distance to the recording site, and the stimulation intensity, these artifacts may overwhelm the neural signals of interest and saturate the recording bioamplifiers, making it impossible to recover the underlying information [2]. To understand how these factors affect artifact propagation, we performed a preliminary characterization of ECoG signals during cortical stimulation.Materials/Methods/ResultsECoG electrodes were implanted in a 39-year old epilepsy patient as shown in Fig. 1. Pairs of adjacent electrodes were stimulated as a part of language cortical mapping. For each stimulating pair, a charge-balanced biphasic square pulse train of current at 50 Hz was delivered for five seconds at 2, 4, 6, 8 and 10 mA. ECoG signals were recorded at 512 Hz. The signals were then high-pass filtered (≥1.5 Hz, zero phase), and the 5-second stimulation epochs were segmented. Within each epoch, artifact-induced peaks were detected for each electrode, except the stimulating pair, where signals were clipped due to amplifier saturation. These peaks were phase-locked across electrodes and were 20 ms apart, thus matching the pulse train frequency. The response was characterized by calculating the median peak within the 5-second epochs. Fig. 1 shows a representative response of the right temporal grid (RTG), with the stimulation channel at RTG electrodes 14 and 15. It also shows a hypothetical amplifier saturation contour of an implantable, bi-directional, ECoG-based BCI prototype [2], assuming the supply voltage of 2.2 V and a gain of 66 dB. Finally, we quantify the worstcase scenario by calculating the largest distance between the saturation contour and the midpoint of each stimulating channel.Discussion:Our results indicate that artifact propagation follows a dipole potential distribution with the extent of the saturation region (the interior of the white contour) proportional to the stimulation amplitude. In general, the artifacts propagated farthest when a 10 mA current was applied with the saturation regions extending from 17 to 32 mm away from the midpoint of the dipole. Consistent with the electric dipole model, this maximum spread happened along the direction of the dipole moment. An exception occurred at stimulation channel RTG11-16, for which an additional saturation contour emerged away from the dipole contour (not shown), extending the saturation region to 41 mm. Also, the worst-case scenario was observed at 6 mA stimulation amplitude. This departure could be a sign of a nonlinear, switch-like behavior, wherein additional conduction pathways could become engaged in response to sufficiently high stimulation.Significance:While ECoG stimulation is routinely performed in the clinical setting, quantitative studies of the resulting signals are lacking. Our preliminary study demonstrates that stimulation artifacts largely obey dipole distributions, suggesting that the dipole model could be used to predict artifact propagation. Further studies are necessary to ascertain whether these results hold across other subjects and combinations of stimulation/recording grids. Once completed, these studies will reveal practical design constraints for future implantable bi-directional ECoG-based BCIs. These include parameters such as the distances between and relative orientations of the stimulating and recording electrodes, the choice of the stimulating electrodes, the optimal placement of the reference electrode, and the maximum stimulation amplitude. These findings would also have important implications for the design of custom, low-power bioamplifiers for implantable bi-directional ECoG-based BCIs.References:[1] Hiremath, S. V., et al. "Human perception of electrical stimulation on the surface of somatosensory cortex." PloS one 12.5 (2017): e0176020.[2] Rouse, A. G., et al. "A chronic generalized bi-directional brain-machine interface." Journal of Neural Engineering 8.3 (2011): 036018 

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