Reversible Thermally Driven Phase Change of Layered In2Se3 for Integrated Photonics.

Two-dimensional In2Se3, an unconventional phase-change material, has drawn considerable attention for polymorphic phase transitions and electronic device applications. However, its reversible thermally driven phase transitions and potential use in photonic devices have yet to be explored. In this study, we observe the thermally driven reversible phase transitions between α and ß' phases with the assistance of local strain from surface wrinkles and ripples, as well as reversible phase changes within the ß phase family. These transitions lead to changes in the refractive index and other optoelectronic properties with minimal optical loss at telecommunication bands, which are crucial in integrated photonic applications such as postfabrication phase trimming. Additionally, multilayer ß'-In2Se3 working as a transparent microheater proves to be a viable option for efficient thermo-optic modulation. This prototype design for layered In2Se3 offers immense potential for integrated photonics and paves the way for multilevel, nonvolatile optical memory applications.

Integrated Flexible Microscale Mechanical Sensors Based on Cascaded Free Spectral Range-Free Cavities.

Photonic mechanical sensors offer several advantages over their electronic counterparts, including immunity to electromagnetic interference, increased sensitivity, and measurement accuracy. Exploring flexible mechanical sensors on deformable substrates provides new opportunities for strain-optical coupling operations. Nevertheless, existing flexible photonics strategies often require cumbersome signal collection and analysis with bulky setups, limiting their portability and affordability. To address these challenges, we propose a waveguide-integrated flexible mechanical sensor based on cascaded photonic crystal microcavities with inherent deformation and biaxial tensile state analysis. Leveraging the advanced multiplexing capability of the sensor, for the first time, we successfully demonstrate 2D shape reconstruction and quasi-distributed strain sensing with 110 µm spatial resolution. Our microscale mechanical sensor also exhibits exceptional sensitivity with a detected force level as low as 13.6 µN in real-time measurements. This sensing platform has potential applications in various fields, including biomedical sensing, surgical catheters, aircraft and spacecraft engineering, and robotic photonic skin development.

Integrated Bragg grating filters based on silicon-Sb2Se3 with non-volatile bandgap engineering capability.

Integrated optical filters show outstanding capability in integrated reconfigurable photonic applications, including wavelength division multiplexing (WDM), programmable photonic processors, and on-chip quantum photonic networks. Present schemes for reconfigurable filters either have a large footprint or suffer from high static power consumption, hindering the development of reconfigurable photonic integrated systems. Here, a reconfigurable hybrid Bragg grating filter is elaborately designed through a precise, modified coupling mode theory. It is also experimentally presented by integrating non-volatile phase change material (PCM) Sb2Se3 on silicon to realize compact, low-loss, and broadband engineering operations. The fabricated filter holds a compact footprint of 0.5 µm × 43.5 µm and maintains a low insertion loss of < 0.5 dB after multiple levels of engineering to achieve crystallization. The filter is able to switch from a low-loss transmission state to the Bragg reflection state, making it a favorable solution for large-scale reconfigurable photonic circuits. With a switching extinction ratio over 30 dB at 1504.85 nm, this hybrid filter breaks the tradeoff between insertion loss and tuning range. These results reveal its potential as a new candidate for a basic element in large-scale non-volatile reconfigurable systems.

Flexible waveguide integrated thermo-optic switch based on TiO2 platform.

Mechanically flexible photonic devices are critical components of novel bio-integrated optoelectronic and high-end wearable systems, in which thermo-optic switches (TOSs) as optical signal control devices are crucial. In this paper, flexible titanium oxide (TiO2) TOSs based on a Mach-Zehnder interferometer (MZI) structure were demonstrated around 1310 nm for, it is believed, the first time. The insertion loss of flexible passive TiO2 2 × 2 multi-mode interferometers (MMIs) is -3.1 dB per MMI. The demonstrated flexible TOS achieves power consumption (Pπ) of 0.83 mW, compared with its rigid counterpart, for which Pπ is decreased by a factor of 18. The proposed device could withstand 100 consecutive bending operations without noticeable degradation in TOS performance, indicating excellent mechanical stability. These results provide a new perspective for designing and fabricating flexible TOSs for flexible optoelectronic systems in future emerging applications.

Waveguide-Integrated PdSe2 Photodetector over a Broad Infrared Wavelength Range.

Hybrid integration of van der Waals materials on a photonic platform enables diverse exploration of novel active functions and significant improvement in device performance for next-generation integrated photonic circuits, but developing waveguide-integrated photodetectors based on conventionally investigated transition metal dichalcogenide materials at the full optical telecommunication bands and mid-infrared range is still a challenge. Here, we integrate PdSe2 with silicon waveguide for on-chip photodetection with a high responsivity from 1260 to 1565 nm, a low noise-equivalent power of 4.0 pW·Hz-0.5, a 3-dB bandwidth of 1.5 GHz, and a measured data rate of 2.5 Gbit·s-1. The achieved PdSe2 photodetectors provide new insights to explore the integration of novel van der Waals materials with integrated photonic platforms and exhibit great potential for diverse applications over a broad infrared range of wavelengths, such as on-chip sensing and spectroscopy.

Fast thermo-optical modulators with doped-silicon heaters operating at 2 µm: erratum.

We correct the errors in the performance of the MRR modulator in our paper [Opt. Express29, 23508, (2021)10.1364/OE.430756]. The FWHM of the MRR device should be 0.22nm instead of 0.11nm. And thus, the Q factor, power consumption, and FOM need to be corrected. After the correction, the performance of our devices was still the best among 2µm-waveband TO modulators. All the conclusions are not changed.

Flexible passive integrated photonic devices with superior optical and mechanical performance.

Flexible integrated photonics is a rapidly emerging technology with a wide range of possible applications in the fields of flexible optical interconnects, conformal multiplexing sensing, health monitoring, and biotechnology. One major challenge in developing mechanically flexible integrated photonics is the functional component within an integrated photonic circuit with superior performance. In this work, several essential flexible passive devices for such a circuit were designed and fabricated based on a multi-neutral-axis mechanical design and a monolithic integration technique. The propagation loss of the waveguide is calculated to be 4.2 dB/cm. In addition, we demonstrate a microring resonator, waveguide crossing, multimode interferometer (MMI), and Mach-Zehnder interferometer (MZI) for use at 1.55 µm, each exhibiting superior optical and mechanical performance. These results represent a significant step towards further exploring a complete flexible photonic integrated circuit.

High-performance silicon PIN diode switches in the 2-µm wave band.

The 2-µm wave band has attracted significant research interest due to its potential applications for next-generation high-capacity optical communication and sensing. As the key component, fast optical switches are essential for an advanced and reconfigurable optical network. Motivated by this prospect, we propose and demonstrate two typical silicon PIN diode switches at 2 µm. One is based on a coupled microring resonator (CMRR), and the other is based on a Mach-Zehnder interferometer (MZI) with a push-pull-like configuration. The measured insertion loss of the CMRR switch is <2.5 dB, and the cross talk is <-10.8 dB. The insertion loss of the MZI switch is <2 dB, and the cross talk is <-15.6 dB. The switch times of these two structures are both lower than 12.5 ns.

Fast thermo-optical modulators with doped-silicon heaters operating at 2 µm.

The 2-µm-waveband has been recognized as a potential telecommunication window for next-generation low-loss, low-latency optical communication. Thermo-optic (TO) modulators and switches, which are essential building blocks in a large-scale integrated photonic circuit, and their performances directly affect the energy consumption and reconfiguration time of an on-chip photonic system. Previous TO modulation based on metallic heaters at 2-µm-waveband suffer from slow response time and high power consumption. In this paper, high-performance thermo-optical Mach-Zehnder interferometer and ring resonator modulators operating at 2-µm-waveband were demonstrated. By embedding a doped silicon (p++-p-p++) junction into the waveguide, our devices reached a record modulation efficiency of 0.17 nm/mW for Mach-Zehnder interferometer based modulator and its rise/fall time was 3.49 µs/3.46 µs which has been the fastest response time reported in a 2-µm-waveband TO devices so far. And a lowest Pπ power of 3.33 mW among reported 2-µm TO devices was achieved for a ring resonator-based modulator.

High-speed Fourier ptychographic microscopy for quantitative phase imaging.

Fourier ptychographic microscopy (FPM), as an emerging computational imaging method, has been applied to quantitative phase imaging with resolution bypassing the physical limit of the detection objective. Due to the weak illumination intensity and long image acquisition time, the achieved imaging speed in current FPM methods is still low, making them unsuitable for real-time imaging applications. We propose and demonstrate a high-speed FPM method based on using laser illumination and digital micro-mirror devices for illumination angle scanning. In this new, to the best of our knowledge, FPM method, we realized quantitative phase imaging and intensity imaging at over 42 frames per second (fps) with around 1 µm lateral resolution. The quantitative phase images have revealed membrane height fluctuations of red blood cells with nanometer-scale sensitivity, while the intensity images have resolved subcellular features in stained cancer tissue slices.

Monolithic back-end-of-line integration of phase change materials into foundry-manufactured silicon photonics.

Monolithic integration of novel materials without modifying the existing photonic component library is crucial to advancing heterogeneous silicon photonic integrated circuits. Here we show the introduction of a silicon nitride etch stop layer at select areas, coupled with low-loss oxide trench, enabling incorporation of functional materials without compromising foundry-verified device reliability. As an illustration, two distinct chalcogenide phase change materials (PCMs) with remarkable nonvolatile modulation capabilities, namely Sb2Se3 and Ge2Sb2Se4Te1, were monolithic back-end-of-line integrated, offering compact phase and intensity tuning units with zero-static power consumption. By employing these building blocks, the phase error of a push-pull Mach-Zehnder interferometer optical switch could be reduced with a 48% peak power consumption reduction. Mirco-ring filters with >5-bit wavelength selective intensity modulation and waveguide-based >7-bit intensity-modulation broadband attenuators could also be achieved. This foundry-compatible platform could open up the possibility of integrating other excellent optoelectronic materials into future silicon photonic process design kits.

On-chip silicon electro-optical modulator with ultra-high extinction ratio for fiber-optic distributed acoustic sensing.

Ultra-high extinction ratio (ER) optical modulation is crucial for achieving high-performance fiber-optic distributed acoustic sensing (DAS) for various applications. Bulky acousto-optical modulators (AOM) as one of the key devices in DAS have been used for many years, but their relatively large volume and high power consumption are becoming the bottlenecks to hinder the development of ultra-compact and energy-efficient DAS systems that are highly demanded in practice. Here, an on-chip silicon electro-optical modulator (EOM) based on multiple coupled microrings is demonstrated with ultra-high ER of up to 68 dB while the device size and power consumption are only 260 × 185 µm2 and 3.6 mW, respectively, which are at least two orders of magnitude lower than those of a typical AOM. Such an on-chip EOM is successfully applied to DAS with an ultra-high sensitivity of -71.2 dB rad2/Hz (4 pε/√Hz) and a low spatial crosstalk noise of -68.1 dB rad2/Hz, which are very similar to those using an AOM. This work may pave the way for realization of next-generation ultra-compact DAS systems by integration of on-chip opto-electronic devices and modules with the capability of mass-production.

Graphene/silicon heterojunction for reconfigurable phase-relevant activation function in coherent optical neural networks.

Optical neural networks (ONNs) herald a new era in information and communication technologies and have implemented various intelligent applications. In an ONN, the activation function (AF) is a crucial component determining the network performances and on-chip AF devices are still in development. Here, we first demonstrate on-chip reconfigurable AF devices with phase activation fulfilled by dual-functional graphene/silicon (Gra/Si) heterojunctions. With optical modulation and detection in one device, time delays are shorter, energy consumption is lower, reconfigurability is higher and the device footprint is smaller than other on-chip AF strategies. The experimental modulation voltage (power) of our Gra/Si heterojunction achieves as low as 1 V (0.5 mW), superior to many pure silicon counterparts. In the photodetection aspect, a high responsivity of over 200 mA/W is realized. Special nonlinear functions generated are fed into a complex-valued ONN to challenge handwritten letters and image recognition tasks, showing improved accuracy and potential of high-efficient, all-component-integration on-chip ONN. Our results offer new insights for on-chip ONN devices and pave the way to high-performance integrated optoelectronic computing circuits.

Silicon Thermo-Optic Switches with Graphene Heaters Operating at Mid-Infrared Waveband.

The mid-infrared (MIR, 2-20 µm) waveband is of great interest for integrated photonics in many applications such as on-chip spectroscopic chemical sensing, and optical communication. Thermo-optic switches are essential to large-scale integrated photonic circuits at MIR wavebands. However, current technologies require a thick cladding layer, high driving voltages or may introduce high losses in MIR wavelengths, limiting the performance. This paper has demonstrated thermo-optic (TO) switches operating at 2 µm by integrating graphene onto silicon-on-insulator (SOI) structures. The remarkable thermal and optical properties of graphene make it an excellent heater material platform. The lower loss of graphene at MIR wavelength can reduce the required cladding thickness for the thermo-optics phase shifter from micrometers to tens of nanometers, resulting in a lower driving voltage and power consumption. The modulation efficiency of the microring resonator (MRR) switch was 0.11 nm/mW. The power consumption for 8-dB extinction ratio was 5.18 mW (0.8 V modulation voltage), and the rise/fall time was 3.72/3.96 µs. Furthermore, we demonstrated a 2 × 2 Mach-Zehnder interferometer (MZI) TO switch with a high extinction ratio of more than 27 dB and a switching rise/fall time of 4.92/4.97 µs. A comprehensive analysis of the device performance affected by the device structure and the graphene Fermi level was also performed. The theoretical figure of merit (2.644 mW-1µs-1) of graphene heaters is three orders of magnitude higher than that of metal heaters. Such results indicate graphene is an exceptional nanomaterial for future MIR optical interconnects.

Topological Nanophotonic Wavelength Router Based on Topology Optimization.

The topological nanophotonic wavelength router, which can steer light with different wavelength signals into different topological channels, plays a key role in optical information processing. However, no effective method has been found to realize such a topological nanophotonic device. Here, an on-chip topological nanophotonic wavelength router working in an optical telecom band is designed based on a topology optimization algorithm and experimentally demonstrated. Valley photonic crystal is used to provide a topological state in the optical telecom band. The measured topological wavelength router has narrow signal peaks and is easy for integration. This work offers an efficient scheme for the realization of topological devices and lays a foundation for the future application of topological photonics.

High-Performance Waveguide-Integrated Bi2O2Se Photodetector for Si Photonic Integrated Circuits.

Due to the excellent electrical and optical properties and their integration capability without lattice matching requirements, low-dimensional materials have received increasing attention in silicon photonic circuits. Bi2O2Se with high carrier mobility, narrow bandgap, and good air stability is very promising for high-performance near-infrared photodetectors. Here, the chemical vapor deposition method is applied to grow Bi2O2Se onto mica, and our developed polycarbonate/polydimethylsiloxane-assisted transfer method enables the clean and intact transfer of Bi2O2Se on top of a silicon waveguide. We demonstrated the Bi2O2Se/Si waveguide integrated photodetector with a small dark current of 72.9 nA, high responsivity of 3.5 A·W-1, fast rise/decay times of 22/78 ns, and low noise-equivalent power of 15.1 pW·Hz-0.5 at an applied voltage of 2 V in the O-band for transverse electric modes. Additionally, a microring resonator is designed for enhancing light-matter interaction, resulting in a wavelength-sensitive photodetector with reduced dark current (15.3 nA at 2 V) and more than a 3-fold enhancement in responsivity at the resonance wavelength, which is suitable for spectrally resolved applications. These results promote the integration of Bi2O2Se with a silicon photonic platform and are expected to accelerate the future use of integrated photodetectors in spectroscopy, sensing, and communication applications.