Photonic Integrated Circuits Research Articles

Ultrathin BIC Metasurfaces Based on Ultra-Low-Loss Sb2Se3 Phase-Change Material.

Ultrathin and low-loss phase-change materials (PCMs) are highly valued for their fast and effective phase transitions and applications in reconfigurable photonic chips, metasurfaces, optical modulators, sensors, photonic memories, and neuromorphic computing. However, conventional PCMs mostly suffer from high intrinsic losses in the near-infrared (NIR) region, limiting their potential for high quality factor (Q-factor) resonant metasurfaces. Here we present the design and fabrication of tunable bound states in the continuum (BIC) metasurfaces using the ultra-low-loss PCM Sb2Se3. Our BIC metasurfaces, only 25 nm thick, achieve high modulation depth and broad resonance tuning in the NIR with high Q-factors up to 130. Experimentally, we employ these BIC metasurfaces to modulate photoluminescence in rare earth-doped upconversion nanoparticles, reducing the excitation power for multiphoton photoluminescence and enabling emission polarization manipulation. This work offers a promising platform for developing active resonant metasurfaces, with broad applications including optical modulation, ultrafast switches, color filtering, and optical sensing.

Low cross talk, low P π silicon optical switch based on highly balanced couplers and folded phase shifters

The cross talk and power consumption of the 2 × 2 optical switch is a key metric in the design of large-scale photonic integrated circuits (PICs). We build a theoretical model of a 2 × 2 Mach–Zehnder interferometer (MZI) optical switch, taking into account both imbalances in the arm loss and the coupler splitting ratio. The splitting ratio imbalance requirement for a given switch cross talk is summarized, which provides a guideline for the switch design. A 2 × 2 multimode interference (MMI) coupler design with low excess loss (<0.14 dB) and low imbalance (<0.13 dB) over the C-band is given. Its robustness to width variations up to ±40 nm is demonstrated. The 2 × 2 MZI switches with such an MMI are fabricated on a silicon-on-insulator (SOI) substrate using both electron beam (EB) and deep ultraviolet (DUV) lithography. The EB and DUV MMI switches exhibit cross talk of <−35 dB and <−30 dB over 40 nm, respectively. Folded configurations are used for both the silicon waveguides and the metal heaters to increase the power efficiency, resulting in a Pπ of ∼8 mW.

Monolithically Integrated Ultra‐Low Threshold GeSn‐on‐Insulator Laser Using Rapid Melting Growth

AbstractA low‐threshold, monolithically integrated laser on Si is considered a crucial missing ingredient in realizing efficient fully functional photonic‐integrated circuits (PICs). Owing to its compatibility with complementary metal‐semiconductor‐oxide (CMOS) processes, direct bandgap GeSn alloy has recently been studied intensively in hopes of making GeSn lasers the mainstream technology for PICs. However, the inevitable formation of harmful defects in GeSn directly grown on Si has thus far required the use of non‐monolithic approaches such as wafer bonding to obtain high‐quality GeSn layers, preventing the realization of practical, low‐threshold GeSn lasers. Here, ultra‐low threshold lasing in a monolithically‐grown, nearly‐defect‐free GeSn single‐crystal layer is demonstrated. The rapid melting growth method used in this study allows the fabrication of a compact, integrated laser that simultaneously achieves an ideal GeSn gain medium with built‐in tensile strain and an excellent GeSn optical cavity on an insulating layer. The measured threshold is ≈0.52 kW cm−2 under the optical pumping scheme at 10 K, which is the lowest among all the reported GeSn lasers. This work provides a new solution for building a truly CMOS‐compatible, monolithic laser that can complete the device library of the mature Si photonics foundries.

On-chip high-dimensional entangled photon sources

Abstract High-dimensional quantum entanglement is an important resource for emerging quantum technologies such as quantum communication and quantum computation. The scalability of metres-long experimental setups limits high-dimensional entanglement in bulk optics. Advancements in quantum technology hinge on reproducible, and reconfigurable quantum devices — including photon sources, which are challenging to achieve in a scalable manner using bulk optics. Advances in nanotechnology and CMOS-compatible integration techniques have enabled the generation of entangled photons on millimeter-scale chips, significantly enhancing scalability, stability, replicability, and miniaturization for real-world quantum applications. In recent years we have seen several chip-scale demonstrations with different degrees of freedom including path, frequency-bin, time-bin, and transverse modes, on many material platforms. A complete quantum photonic integrated circuit requires the generation, manipulation, and detection of qudits, involving various active and passive quantum photonic components which further increase the degree of complexity. Here, we review and introduce the nonlinear optical processes that facilitate on-chip high-dimensional entangled photon sources and the currently used material platforms. We discuss a range of current implementations of on-chip high-dimensional entangled photon sources and demonstrated applications. We comment on the current challenges due to the limitations of individual material platforms and present future opportunities in hybrid and heterogeneous integration strategies for the next generation of integrated quantum photonic chips.

Drastic enhancements of direct and indirect light emissions from Ge microbridge web-structures via efficient light confinements

Abstract Drastic enhancements of both direct and indirect light emissions are demonstrated from Ge microbridge web-structures fabricated based on the strained Ge-on-Si (110). This enhancement is concluded to be caused more predominantly by the light confinement effect in several-µm-scale microbridges than the induced strain. It is also remarkable that this effect works very well with as small strain as 0.4%, making the fabrication procedure very easy and bridges very stable and rigid. These findings will, therefore, widen the applicability and raise the potentiality of here-proposed Ge web-structure microbridges toward Si optical integrated circuits.

Micro-Transfer printing of InGaAs/InP Avalanche Photodiode on Si Substrate

Abstract We report on the fabrication and micro-transfer printing of InGaAs/InP avalanche photodiodes (APDs) onto silicon substrates. A process flow was developed to suspend the devices using semiconductor tethers. The developed process reduces the number of fabrication steps required compared to methods based on the use of photoresist tethers. Furthermore, our process is compatible with devices that may be susceptible to damage induced by the photoresist removal process. APDs were characterised in linear mode operation both before suspension and after printing. Despite the additional fabrication steps required to suspend the APD membranes and the physical nature of the micro-transfer printing process, the electrical characteristics of the devices were preserved. No degradation in the optical performance of the devices was measured. Our work represents the first demonstration of micro-transfer printing of InGaAs/InP avalanche photodiodes onto silicon substrates. The results highlight the viability of micro-transfer printing for effective heterogeneous integration of InGaAs/InP avalanche photodiodes with silicon photonic integrated circuits for optical and quantum communication and other light detection applications.

Heterogeneous integration of GaInAsSb-GaSb photodiodes on SOI photonic integrated circuits for SWIR applications

Heterogeneous integration of GaInAsSb-GaSb photodiodes on SOI photonic integrated circuits for SWIR applications

Ultra-Low Loss and Ultra-Compact Polarization-Insensitive SOI Multimode Waveguide Crossing Based on an Inverse Design Method

Polarization-insensitive waveguide crossings are indispensable components of photonic integrated circuits (PICs), enabling the concurrent computing of optical signals from diverse waveguides inside the limits of a restricted spatial footprint. Leveraging mirror symmetry direct binary search, we successfully demonstrate an ultra-compact and ultra-low loss polarization-insensitive waveguide crossing that achieves insertion losses below −0.11 dB and crosstalk levels beneath −22.6 dB for transverse electric (TE) mode, as well as insertion losses below 0.05 dB and crosstalk levels beneath −24.5 dB for transverse magnetic (TM) mode across the C-band with a footprint of 3 × 4 μm2. The results confirm that this mirror symmetry optimization method yields high-efficiency devices while reducing computational time. We believe this high-efficiency polarization-insensitive waveguide crossing can have potential applications in dense PIC systems.

When do we need Pbps photonic chips and what are the challenges?

We estimate the capacity requirement for optical input/output chips boosted by artificial intelligence in computing centers. It is expected that petabits per second (Pbps) capacity on a single chip may be achieved in ten years or so in laboratories. We then study the key challenges in the implementation of such photonic chips, focusing on technical bottlenecks such as laser integration, modulation speed, MUX/DEMUX scaling, photodetector efficiency, and packaging density. We also discuss potential solutions, including novel materials and integration techniques, to enhance performance and reduce power consumption. Our analysis suggests that significant innovations in these areas could lead to the development of compact and efficient Pbps photonic chips by 2035, paving the way for next-generation optical interconnect systems.

Effect on the spectral purity of photon-pairs due to on-chip temporal manipulation of the pump

Photonic Integrated Circuits (PICs) are a promising contender for quantum information technologies. The spectral purity of photons is one of the key attributes of photon-pair sources based on nonlinear optics. We can obtain >99% purity in PIC ring-resonator photon-pair sources by manipulating the pump pulse in the time domain. Here, we have developed a PIC to shape a pulse into dual, triple, and quadruple pulses and investigated the effect of these pump pulse configurations on the purity. Our results show that more complex configurations, compared to a two-pulse configuration, do not result in comparatively higher purity but allow more accurate control over choosing arbitrary values of the purity.

Suppression of parasitic lasing in erbium doped thin film lithium niobate waveguide amplifier by integrated wavelength division multiplexer

Photonic integrated circuits based erbium doped amplifiers have attracted great interest due to their compact footprint, high gain in the telecom C-band, and high scalability for functional integration. In this work, a wavelength division multiplexer integrated erbium doped waveguide amplifier fabricated on the thin film lithium niobate on insulator platform is demonstrated. An on-chip saturated power of 10 dBm with the net gain around 10 dB is achieved from the monolithically integrated amplifier chip with the footprint of only 3 × 5 mm2. In particular, the suppression of parasitic lasing in the waveguide amplifier is realized thanks to the spectral response of the integrated wavelength division multiplexer. Theoretical analysis of parasitic lasing on amplifier performance is also conducted. The demonstrated integrated erbium doped waveguide amplifier will find great use in various applications based on the thin film lithium niobate on an insulator platform.

Frequency agile photonic integrated external cavity laser

Recent advances in the development of ultra-low loss silicon nitride integrated photonic circuits have heralded a new generation of integrated lasers capable of reaching fiber laser coherence. However, these devices are presently based on self-injection locking of distributed feedback laser diodes, increasing both the cost and requiring tuning of laser setpoints for their operation. In contrast, turn-key legacy laser systems use reflective semiconductor optical amplifiers (RSOAs). While this scheme has been utilized for integrated photonics-based lasers, so far, no cost-effective RSOA-based integrated lasers exist that are low noise and simultaneously feature fast, mode-hop-free, and linear frequency tuning as required for frequency modulated continuous wave (FMCW) LiDAR or for laser locking in frequency metrology. Here we overcome this challenge and demonstrate a RSOA-based, frequency agile integrated laser, that can be tuned with high speed, with high linearity at low power. This is achieved using monolithic integration of piezoelectrical actuators on ultra-low loss silicon nitride photonic integrated circuits in a Vernier filter-based laser scheme. The laser operates at 1550 nm, features a 6 mW output power and a 400 Hz intrinsic laser linewidth, and allows ultrafast wavelength switching within 7 ns rise time and 75 nW power consumption. In addition, we demonstrate the suitability for FMCW LiDAR by showing laser frequency tuning over 1.5 GHz at 100 kHz triangular chirp rate with a nonlinearity of 0.25% after linearization and use the source for measuring a target scene 10 m away with a 8.5 cm distance resolution.

High-performance full-etched fiber-to-chip grating couplers at 3.7-micron wavelength on silicon

Mid-infrared (MIR) silicon photonics have attracted great interest for potential applications in on-chip spectroscopy, free-space communication, and remote imaging. Currently, the high-performance and fabrication-friendly fiber-to-chip grating couplers operating at 3–4 μm wavelength band are still desired urgently. Here, we propose an efficient method for designing a 2D subwavelength grating coupler. The few analytical fitting parameters defining the subwavelength grating periods and fill factors in two dimensions enable a high design degree of freedom and a low search space dimension, resulting in high figure-of-merit and fast convergence simultaneously. The developed 3.7 μm grating coupler showcases record-high coupling efficiency both theoretically (−2.2 dB) and experimentally (−4.4 dB) among the 3–4 μm MIR counterparts. This study provides an effective method for designing fiber-to-chip grating couplers, by which the MIR grating couplers with high performance and fabrication ease are developed, thus paving the way for developing high-performance silicon photonic integrated circuits operating in the MIR wavelength band.

Discussing the future of high-capacity photonic chips

Looking at growth rates of optical input/output chips along with demand from AI training provides an estimate of future performance.

Radio Front-End for Frequency Agile Microwave Photonic Radars

Recent advancements in photonic integrated circuits (PICs) have paved the way for a new era of frequency-agile coherent radar systems. Unlike traditional all-electronic RF radar techniques, fully photonic systems offer superior performance, overcoming bandwidth limitations and noise degradation when operating across S (2–4 GHz), X (8–12 GHz), and K-band (12–40 GHz) frequencies. They also exhibit excellent phase noise performance, even at frequencies exceeding 20 GHz. However, current state-of-the-art PICs still suffer from high processing losses in the optical domain, necessitating careful design of the electrical RF domain. This study delves into the critical challenges of designing RF front-ends for microwave photonic radars, including stability, noise minimization, and intermodulation distortion reduction. To demonstrate the feasibility of the proposed design, a functional prototype is constructed, achieving a total power gain of 107 dB (radar system at 10 GHz) while minimizing signal noise degradation. Furthermore, a comprehensive demonstration of the RF front-end, encompassing both optical RF signal generation and experimental measurements of a rotor blade’s Doppler fingerprint with 0.5 Hz resolution, validates the proposed system’s performance.

Efficient mode coupling/(de)multiplexing between a few-mode fiber and a silicon photonic chip

Mode-division multiplexing (MDM) has attracted much attention due to its ability to further increase the transmission capacity of optical interconnects. While further developments of MDM optical interconnects are hindered by the coupling of few-mode fibers (FMFs) and silicon photonic chips, a high-efficiency, broadband, and scalable multimode FMF-chip interface is still eagerly desired. To address this challenge, a novel scheme for efficient multimode coupling is proposed by introducing a silica planar lightwave circuit as an intermediate. The core idea is to couple and demultiplex higher-order modes by leveraging the superiorities of silica optical waveguides for manipulating LP modes, facilitated through tailoring the mode conversion related to different mode symmetric properties. The demultiplexed modes are consequently butt-coupled to the silicon photonic chip in single-mode manner, thus being available for fulfilling further data transmitting/receiving/routing directly. As a proof of concept, a six-channel FMF-chip coupler working with the LP01-x/y, LP11a-x/y, and LP11b-x/y modes is designed with low coupling losses of 0.77–1.39 dB and low intermode crosstalk of <−27.2 dB in a broad bandwidth (>150 nm). Minimum coupling losses of 1.36–2.48 dB are experimentally demonstrated. It is the first demonstration for the integrated multimode FMF-chip coupler enabling the simultaneous coupling of six mode-channels, to the best of our knowledge. We believe that this work has the great potential for developing energy-efficient and low-cost chip-to-chip MDM interconnections in the future.

Gauge-flux-controlled orbital angular momentum mode conversion in silicon waveguides.

We propose a method to convert fundamental modes into orbital angular momentum (OAM) modes through chiral dynamics induced by gauge fluxes in silicon waveguides. By integrating a trench into a few-mode waveguide, we induce the rotation of TE10 and TE01 modes, naturally generating the gauge flux for the synthesized OAM modes. By precisely controlling the gauge flux, we achieve chiral dynamics that optimize the conversion efficiency of OAM modes at specific propagation distances, addressing challenges posed by mode degeneracy. Additionally, we demonstrate an on-chip OAM mode (de)multiplexer based on flux-controlled mode conversion. Our findings offer new, to the best of our knowledge, strategies for creating artificial gauge fluxes in straight waveguides and open up possibilities for manipulating OAM modes on photonic chips.

Reconfiguration of Quantum Photonic Integrated Circuits Using Auxiliary Fields

Reconfiguration of Quantum Photonic Integrated Circuits Using Auxiliary Fields

(Invited) Graded Index Silicon Germanium Photonics Circuits in the Mid-Infrared

Mid-infrared (mid-IR) spectroscopy can be used to identify chemical and biological substances. Indeed, most molecules absorb light at specific wavelengths in the mid-infrared (mid-IR). Mid-IR photonic circuits on silicon chips have recently gained a lot of attention. Indeed, they can provide high-performance with a low power consumption, while being cheap, compact, and light. Germanium (Ge) and silicon-germanium (SiGe) alloys with a high Ge concentration are particularly interesting because of the wide transparency window of Ge extending up to 15 µm. It has been demonstrated for a few years now that a Ge-rich graded SiGe platform relying on a graded SiGe layer epitaxially grown on a Si substrate can be used as a photonics platform for mid-IR operation in a wide spectral range.In this invited talk, recent developments concerning graded SiGe photonic integrated circuits will be presented. First, passive devices will be reviewed. It will be shown that graded-SiGe waveguides can be used in an unprecedented spectral range, e.g. up to 11 µm. Mach Zehnder interferometers, resonators and integrated Fourier transform spectrometers will be discussed. Then, a large bandwidth light source based on non-linear optical effects in SiGe waveguides [1] will be presented, together with results on integrated mid-IR optoelectronic modulators [2, 3]. Tunable electro-optical frequency-comb generation around 8 µm wavelength based on such integrated modulators will also be discussed [4]. Finally, the recent scale-up of this platform will be shown, with the first demonstration of low-loss mid-infrared waveguides fabricated on a Ge-rich SiGe strain-relaxed buffer grown on an industrial-scale 200 mm wafer [5].Acknowledgment:This work was co-funded by ANR Light-up Project (ANR-19-CE24-0002-01) and by European Union (ERC, Electrophot,101097569). This work was partly supported by the French RENATECH network.

Advancing Graphene Integration in Si CMOS Technology: Challenges, Solutions, and Modulator Exploration

The potential of graphene-based devices in enhancing Si CMOS technology functionalities has been widely recognized [1,2]. Nevertheless, the successful integration of graphene into novel microelectronic devices encounters significant challenges. These include: i) The attainment of wafer-scale, high-quality graphene on CMOS-compatible materials (e.g., dielectrics or semiconductors) and subsequent transfer. ii) Developing reliable methods for applying a passivation layer on the graphene surface. iii) Ensuring the entire wafer processing with graphene can adhere to the standard Si CMOS pilot line protocols. This study is dedicated to addressing these challenges comprehensively within a 200mm pilot line at IHP. We have devised several conceptual frameworks aimed at overcoming these obstacles, exemplified by the fabrication of test proof-of-concept devices such as TLMs, Hall bars, or Kelvin bridge structures. The electrical properties of the transferred graphene layers (were determined to be Rs ~1500 ±100 Ω/sq and μ ~800 ±20 cm2/V∙s. The extracted contact resistances at graphene/metal interface were measured at 60 Ω.Moreover, recent years have seen a surge in interest in graphene-based modulators due to their potential for high-speed and low-power operation, rendering them promising for optical communication and computing systems. These modulators leverage graphene as the active material to modulate light intensity or phase. With their outstanding characteristics encompassing high speed, low power consumption, and compatibility with silicon-based technology, graphene modulators possess the capability to transform optical communication and computing landscapes. Therefore, simulations and fabrication of the graphene ring modulators have been conducted at both the component and device levels, incorporating realistic graphene properties. These results indicate a modulation depth of 1.6 dB/µm and a 3dB bandwidth of 7 GHz, showcasing the potential of graphene-based photonic devices for high-speed communication applications. In addition, this study describes the development of silicon nitride photonic integrated circuits (Grating couplers and waveguides) on a 200mm wafer scale as a basis for the graphene-based optical modulators. It's crucial to highlight that the experiments conducted herein occurred within a standard BiCMOS pilot line, rendering this research particularly distinctive. The insights gleaned from this study hold the potential to significantly expedite the integration of graphene and the prototyping of graphene-based devices within mainstream Si technologies.[1] L. Kim et.al., ,Science 479 (2011) 338.[2] W. Mehr et.al., Electr. Dev. Lett., 33, (2012) 691. AcknowledgmentThis research was funded by the European Union’s Horizon 2020 research and innovation programme under Graphene Flagship grant agreement No 952792 and by the European Commission through the HORIZON.2.4 – Digital, Industry and Space programme under grant agreement n° 101070482