The Environmentalist

Integrated sensing and communication (ISAC), a key technology for next-generation wireless networks (e.g., 5G-A and 6G), aims to provide both large-capacity wireless communication and high-resolution microwave sensing/ranging simultaneously. Microwave photonics (MWP)-ISAC, with its unique features such as high frequency, large bandwidth, low frequency-dependent loss, flat frequency response, fast analog signal processing, and strong immunity to electromagnetic interference, offers superior performance in terms of data rate and range/imaging resolution compared to traditional electronic technologies. This paper presents a comprehensive overview of the latest advancements in MWP-ISAC techniques, covering multi-domain resource multiplexing (MDRM) and integrated waveform (IW) strategies. We review four MDRM methods: time division multiplexing, frequency division multiplexing, space division multiplexing, and hybrid resource division multiplexing. In addition, we discuss sensing-centric IWs (including phase modulated continuous-wave and linear frequency modulation-based parameter modulation) and communication-centric IWs (such as orthogonal frequency division multiplexing and orthogonal chirp division multiplexing).

In the past decade, remarkable advances in integrated photonic technologies have enabled table-top experiments and instrumentation to be scaled down to compact chips with significant reduction in size, weight, power consumption, and cost. Here, we demonstrate an integrated continuously tunable laser in a heterogeneous gallium arsenide-on-silicon nitride (GaAs-on-SiN) platform that emits in the far-red radiation spectrum near 780 nm, with 20 nm tuning range, <6 kHz intrinsic linewidth, and a >40 dB side-mode suppression ratio. The GaAs optical gain regions are heterogeneously integrated with low-loss SiN waveguides. The narrow linewidth lasing is achieved with an extended cavity consisting of a resonator-based Vernier mirror and a phase shifter. Utilizing synchronous tuning of the integrated heaters, we show mode-hop-free wavelength tuning over a range larger than 100 GHz (200 pm). To demonstrate the potential of the device, we investigate two illustrative applications: (i) the linear characterization of a silicon nitride microresonator designed for entangled-photon pair generation and (ii) the absorption spectroscopy and locking to the D1 and D2 transition lines of 87Rb. The performance of the proposed integrated laser holds promise for a broader spectrum of both classical and quantum applications in the visible range, encompassing communication, control, sensing, and computing.

Increasing the energy efficiency and reducing the footprint of on-chip photodetectors enable dense optical interconnects for emerging computational and sensing applications. While heterojunction phototransistors (HPTs) exhibit high energy efficiency and negligible excess noise factor, their gain-bandwidth product (GBP) has been inferior to that of avalanche photodiodes at low optical powers. Here, we demonstrate that utilizing type-II energy band alignment in an Sb-based HPT results in six times smaller junction capacitance per unit area and a significantly higher GBP at low optical powers. These type-II HPTs were scaled down to 2 μm in diameter and fully integrated with photonic waveguides on silicon. Thanks to their extremely low dark current and high internal gain, these devices exhibit a GBP similar to the best avalanche devices (∼270 GHz) but with one order of magnitude better energy efficiency. Their energy consumption is about 5 fJ/bit at 3.2 Gbps, with an error rate below 10−9 at −25 dBm optical power at 1550 nm. These features suggest new opportunities for creating highly efficient and compact optical receivers based on phototransistors with type-II band alignment.

Light eigenmodes with cylindrical symmetry, such as the Laguerre–Gaussian (LG) modes, are characterized by the radial and angular quantum numbers indicating radial nodes and orbital angular momentum (OAM), which are independent and invariant upon beam propagation. Here, we connect these two quantum numbers and produce a configurable optical vortex ladder. An LG vortex ladder that consists of multiple LG modes with different radial quantum numbers is proposed, whose OAM state of the mainlobe can change step by step upon propagation. By controlling radial quantum numbers within the LG vortex ladder, every step change of the OAM state can be configured arbitrarily, such as topological charge of OAM state increasing by two in every step. Manipulating the evolution of photonic OAM states is of great significance for quantum information processing and longitudinal manipulation of OAM have potential applications in communications, all-optical switch, and optical tweezers.

Quantum entanglement is a vital resource in quantum information processing. High-dimensional quantum entanglement offers advantages that classical systems cannot surpass, particularly in enhancing channel capacity, improving system noise resilience, and increasing sensitivity to external environments. The construction of multimode entanglement in the spectral domain is well-suited for fiber-optic systems. Here, we present a straightforward scheme for generating multimode frequency-bin entanglement using a semiconductor chip through a simple mode conversion. A general model for Hong–Ou–Mandel (HOM) interference with a multimode frequency-bin entangled state is presented and applied to the experiments. The multimode entangled photons we produced exhibit HOM interference with a high-visibility beating pattern, demonstrating a strong relationship with the mode number, mode spacing, and the profile of the single mode. Building on the Fisher information analysis, we explore the relationship between the features in multimode entangled state interference traces and the precision of interferometric measurements even in the presence of experimental nonidealities. This work may deepen the understanding of multimode frequency-bin entanglement and advance the application of multimode HOM interference in quantum sensing.

Healthcare has rapidly evolved in the last decades, driven by the demand for personalized therapies and advancements in enabling technologies. Among many solutions, fiber Bragg grating (FBG) sensors have gained significant acceptance in the medical field, due to their good static and dynamic performance, small dimensions, biocompatibility and immunity to electromagnetic interferences. The integration of artificial intelligence (AI) with FBGs is emerging as a breakthrough approach, enabling the design of smart systems for medical applications, like minimally invasive surgery, physiological monitoring, biomechanics, and medical biosensing. These systems harness the potential of FBGs and the advanced data processing capabilities of AI to improve diagnostics and therapeutic procedures. This perspective provides an overview of the sensing systems that combine FBG and AI technologies in medicine, focusing on their working principle, potentials, and challenges. It also explores the open research directions for encouraging further investigations in this field.

Continuous wave (cw) terahertz (THz) radiation has a wide array of applications, ranging from sensing to next-generation wireless communication links. Industrial applications frequently require THz systems that are broadband, highly efficient, and compact. Photomixer-based solutions hold promise in meeting these demands, offering extremely broadband operation and the potential for miniaturization through photonic integration. However, current photoconductive antenna (PCA) receivers used in these systems are top-illuminated, which strongly limits their efficiency and renders them incompatible with photonic integration. To overcome these limitations, we developed optical waveguide-integrated photoconductive antennas (win-PCAs) for cw-THz detection. These antennas not only facilitate integration into photonic integrated chips (PICs) but also allow us to explore new device geometries to optimize the PCA’s responsivity. By optimizing the absorber geometry of the win-PCAs, we achieve a 22-fold increase in photoresponse, a 500-fold improvement in THz responsivity, and a 4.7-fold reduction in noise-equivalent power compared to state-of-the-art top-illuminated PCAs. In a coherent cw-THz spectrometer, these improvements enable measurements with a peak dynamic range of 123 dB for 300 ms averaging, which is 11 dB higher than what is achievable with comparable top-illuminated receivers. The presented win-PCAs represent a significant step toward fully integrated, high-performing photonic cw-THz systems for both spectroscopy and high-capacity wireless links.

Optical simulation plays an important role in photonic hardware design flow. The finite-difference time-domain (FDTD) method is widely adopted to solve time-domain Maxwell equations. However, FDTD is known for its prohibitive runtime cost as it iteratively solves Maxwell equations and takes minutes to hours to simulate a single device. Recently, AI has been applied to realize orders-of-magnitude speedup in partial differential equation solving. However, AI-based FDTD solvers for photonic devices have not been clearly formulated. Directly applying off-the-shelf models to predict the optical field dynamics shows unsatisfying fidelity and efficiency since the model primitives are agnostic to the unique physical properties of Maxwell equations and lack algorithmic customization. In this work, we thoroughly investigate the synergy between neural operator designs and the physical property of Maxwell equations and introduce a physics-inspired AI-based FDTD prediction framework PIC2O-Sim. PIC2O-Sim features a causality-aware dynamic convolutional neural operator as its backbone model that honors the space–time causality constraints via careful receptive field configuration and explicitly captures the permittivity-dependent light propagation behavior via an efficient dynamic convolution operator. Meanwhile, we explore the trade-offs among prediction scalability, fidelity, and efficiency via a multi-stage partitioned time-bundling technique in autoregressive prediction. Multiple key techniques have been introduced to mitigate iterative error accumulation while maintaining efficiency advantages during autoregressive field prediction. Extensive evaluations on three challenging photonic device simulation tasks have shown the superiority of our PIC2O-Sim method, showing 51.2% lower roll-out prediction error, 23.5 times fewer parameters than state-of-the-art neural operators, providing 133–310× or 31–89× higher simulation speed than an open-source single-process or eight-process parallel FDTD numerical solver.

Light sources monolithically integrated with optical filters, modulators, and detectors are necessary components for photonic systems on a chip. For broadband applications such as chemical or biological sensing using absorption spectroscopy, white light sources are preferred over lasers or amplified spontaneous emission sources. In particular, thermal sources offer a straightforward means for broadband optical emission. However, to date, there have been few reports of waveguide-coupled thermal sources. In this work, we demonstrate a suspended nanophotonic waveguide-coupled broadband thermal source. It is heated by an adjacent resistive heater that permits temperatures in excess of 1000 °C at electrical powers of tens of milliwatts. We measure the waveguide-coupled emission, confirming broadband operation from 875 to 1600 nm (instrumentation limited). Thermal simulations show good agreement with measurements, and optical modeling accurately describes the heater–waveguide coupling and polarization.

Due to the overlapping emission and detection spectra of quantum well (QW) diodes, they inherently possess the dual functions of light emission and detection. In this paper, we integrate a 4 × 4 array of QW diodes and combine it with a programmable circuit and a convolutional neural network algorithm, ultimately proposing a simultaneous display-communication system. This system not only displays visual content but also receives external signals via wireless light communication and classifies and recognizes the signal content with an accuracy exceeding 95%. The QW diode array operates within a temperature range of −40–85 °C and is easily scalable, making it suitable for both on-chip and off-chip integration. Moreover, the channels are mutually independent, meaning the channel capacity is theoretically proportional to the number of QW diodes. This system has significant potential for secure transmission and intelligent display applications: while the screen displays a certain image, it may also be secretly transmitting other information in the background.

Silicon photonics, with its CMOS compatibility and high integration density, has enabled a wide range of novel applications. Harnessing stimulated Brillouin scattering (SBS), an optomechanic interaction between optical and gigahertz acoustic waves, in silicon-on-insulator (SOI) platforms attracts great interest for its potential in narrow-linewidth lasers and microwave photonics. However, the poor optoacoustic overlap in silicon nanowires on conventional SOI platforms has previously restricted the observation of SBS signals to suspended silicon waveguide structures. In this work, we report, for the first time, the SBS response in a non-suspended ultra-low-loss thick-SOI waveguide platform. The backward SBS process in this 3 μm thick SOI platform is enabled by a leaky acoustic mode that coexists with the optical mode in the waveguide core, resulting in an enhanced optoacoustic overlap. We measured Brillouin gain coefficients of 2.5 and 1.9 m−1 W−1 at 37.6 GHz for the rib and strip waveguides, respectively. This work paves the way for Brillouin-based applications in non-suspended ultra-low-loss silicon photonics systems.

Thin-film lithium niobate (TFLN) has emerged as a promising platform for the realization of high-performance chip-scale optical systems, spanning a range of applications from optical communications to microwave photonics. Such applications rely on the integration of multiple components onto a single platform. However, while many of these components have already been demonstrated on the TFLN platform, to date, a major bottleneck of the platform is the existence of a tunable, high-power, and narrow-linewidth on-chip laser. Here, we address this problem using photonic wire bonding to integrate optical amplifiers with a TFLN feedback circuit. We demonstrate an extended cavity diode laser with an excellent side mode suppression ratio exceeding 60 dB and a wide wavelength tunability over 43 nm. At higher currents, the laser produces a high maximum on-chip power of 76.2 mW while maintaining 51 dB side mode suppression. The laser frequency stability over short timescales shows an ultra-narrow intrinsic linewidth of 550 Hz. Long-term recordings indicate a high passive stability of the photonic wire bonded laser with 58 hours of mode-hop-free operation, with a trend in the frequency drift of only 4.4 MHz/h. This work verifies photonic wire bonding as a viable integration solution for high performance on-chip lasers, opening the path to system level upscaling and Watt-level output powers.

We present a novel approach to cavity ringdown spectroscopy utilizing an optical frequency comb as the direct probe of the Fabry–Pérot cavity, coupled with a time-resolved Fourier transform spectrometer for parallel retrieval of ringdown events. Our method achieves a high spectral resolution over a broad range, enabling precision measurements of cavity losses and absorption line shapes with enhanced sensitivity. A critical advancement involves a stabilization technique ensuring complete extinction of comb light without compromising cavity stabilization. We demonstrate the capabilities of our system through precision spectroscopy of carbon monoxide rovibrational transitions perturbed by argon.

The transmission line is one of the most fundamental components for the implementation of electromagnetic systems, such as electric cables and optical fibers for microwave and optic applications, respectively. The terahertz band, sandwiched between those two well-developed spectra, is not an exception. To meet such essential demand, low-loss, flexible, wideband terahertz fibers and corresponding functional devices have witnessed a blooming interest in the past two decades, being considered as a promising candidate for building compact, robust terahertz systems thus advancing the practicality and commercialization of terahertz science and technology. In this tutorial, we will provide a concise introduction to the fundamental characteristic parameters and prevalent hosting materials of terahertz fibers. Subsequently, we will look backward over the developments of terahertz hollow-core and solid-core fibers, as well as fiber-based terahertz functional devices for communication, sensing, spectroscopy, and imaging applications. Moreover, we will discuss several remaining challenges hampering the practical utilizations of terahertz fiber devices and propose some potential solutions to current major bottlenecks.