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Valley photonic crystals (VPCs) provide an intriguing approach to suppress backscattering losses and enable robust transport of light against sharp bends, which could be utilized to realize low-loss and small-footprint devices for on-chip optical communications. However, there are few studies on how to achieve power-efficient tunable devices based on VPCs, which are essential for implementing basic functions such as optical switching and routing. Here, we propose and experimentally demonstrate a thermally tunable add-drop filter (ADF) based on VPCs operating at telecommunication wavelengths. By leveraging the topological protection of the edge state and the distinct property of negligible scattering at sharp bends, a small footprint of 17.4 × 28.2 μm2 and a low insertion loss of 2.7 dB can be achieved for the proposed device. A diamond-shaped microloop resonator is designed to confine the light and enhance its interaction with the thermal field generated by the microheater, leading to a relatively low power of 23.97 mW needed for switching the output signal from one port to the other. Based on the thermally tunable ADF under the protection of band topology, robust data transmission is implemented with an ultrahigh data rate of 132 Gb/s. Our work shows great potential for developing high-performance topological photonic devices with the thermally tunable silicon-based VPCs, which offers unprecedented opportunities for realizing topologically protected and reconfigurable high-speed datalinks on a chip.

Silicon photonics has gained great success mainly due to the promise of realizing compact devices in high volume through the low-cost foundry model. It is burgeoning from laboratory research into commercial production endeavors such as datacom and telecom. However, it is unsuitable for some emerging applications which require coverage across the visible or mid infrared (mid-IR) wavelength bands. It is desirable to introduce other wideband materials through heterogeneous integration, while keeping the integration compatible with wafer-scale fabrication processes on silicon substrates. We discuss the properties of silicon-family materials including silicon, silicon nitride, and silica, and other non-group IV materials such as metal oxide, tantalum pentoxide, lithium niobate, aluminum nitride, gallium nitride, barium titanate, piezoelectric lead zirconate titanate, and 2D materials. Typical examples of devices using these materials on silicon platform are provided. We then introduce a general fabrication method and low-loss process treatment for photonic devices on the silicon platform. From an applications viewpoint, we focus on three new areas requiring integration: sensing, optical comb generation, and quantum information processing. Finally, we conclude with perspectives on how new materials and integration methods can address previously unattainable wavelength bands while maintaining the advantages of silicon, thus showing great potential for future widespread applications.

Metalens has emerged as an important optical block in free-space optical systems, which shows excellent performance. Even the metalens based on gradient index (GRIN) profiles can be implemented for on-chip beam focusing behavior. However, for most previous schemes, the GRIN metalenses can only achieve on-chip beam focusing behavior in one dimension, which limits their applications in low-loss waveguide interconnecting or fiber-to-chip coupling. In this paper, an on-chip half Maxwell’s fisheye lens based on GRIN profiles with subwavelength features, integrated with silicon waveguides, is experimentally demonstrated. Benefitting from the index distribution and beam focusing characteristics of the half Maxwell’s fisheye lens, an on-chip beam transforming can be achieved for transverse electric (TE) fundamental mode in two waveguides with different heights and widths. The simulated 1 dB bandwidth can reach 1100 nm, which exhibits great prospects in integrated photonic circuits. The measured insertion loss of an on-chip 5.4 μm-length lens is less than 1 dB to connect a 220 nm-height, 8 μm-wide waveguide, and a 60 nm-height, 0.5 μm-wide waveguide in the wavelength range of 1280–1620 nm.

Graphene is a 2D material which has attracted tremendous interest from academia and industry, due to its attractive electrical and optical properties. By integrating graphene onto silicon photonics platform, various high-performance electro-optic and thermo-optic modulators have been demonstrated. In this paper, such graphene-based optical modulators are reviewed. The concept and principle of optical modulations are firstly analyzed and then an overview of the developing trends of optical modulators is presented. Meanwhile, the performances of graphene-based optical modulators, including power consumptions and speeds of electro-optic modulators as well as tuning efficiencies of thermo-optic modulators, are then evaluated and discussed in detail. All these optical modulators would play important roles in a wide range of applications including telecom, interconnects, computing, quantum information processing, and beam steering.

We review the integration techniques for incorporating various materials into silicon-based devices. We discuss on-chip light sources with gain materials, linear electro-optic modulators using electro-optic materials, low-power piezoelectric tuning devices with piezoelectric materials, highly absorbing materials for on-chip photodetectors, and ultra-low-loss optical waveguides. Methodologies for integrating these materials with silicon are reviewed, alongside the technical challenges and evolving trends in silicon hybrid and heterogeneously integrated devices. In addition, potential research directions are proposed. With the advancement of integration processes for thin-film materials, significant breakthroughs are anticipated, leading to the realization of optoelectronic monolithic integration featuring on-chip lasers. [ABSTRACT FROM AUTHOR]

Integrated tunable optical filters are essential components in photonic signal processors, telecom systems, sensors, and quantum optical devices. Two of the most important features of a tunable filter are its dimensions and power consumption. Herein, the design and experimental validation of an on‐chip optical filter composed of a 1D topological photonic crystal cavity based on a hybrid integrated lithium tantalite‐silicon platform is presented. The strong optical confinement of the boundary state allows the fabrication of a tunable filter with an ultra‐compact size of 1.14 × 75 µm2 is demonstrated. Moreover, lithium tantalite has excellent electro‐optic properties and enables ultra‐low‐power wavelength tuning of the topological boundary state. The measured power consumption and tuning efficiency of the device are 0.0218 nW pm−1 and 6.64 pm V−1, respectively. The anisotropy of thin‐film lithium tantalite is verified by evaluating its tuning efficiency at different optical angles. The device can compensate for thermally induced refractive index changes ≈20 °C, exhibiting operational robustness. High‐speed transmission experiments confirm the stability of the developed tunable filter. This optical filter implements a topological structure with a compact size and can potentially be applied in on‐chip quantum optics, nonlinear optics, and optical sensing. [ABSTRACT FROM AUTHOR]

Integrated photonics is demanded in applications operating at ultraviolet to visible wavelengths, such as atomic/quantum systems, on‐chip broadband receivers, and far‐field structured illumination autofluorescence microscopy. A fundamental challenge in these applications is efficient edge coupling from a single‐mode fiber (SMF) to on‐chip photonic components, which is critical for on‐chip integration. In this paper, a high‐efficiency edge coupler based on an alumina‐on‐insulator platform is introduced and experimentally validated. The coupler employs a symmetric double‐tip taper and a multimode interference (MMI)‐based optical combiner. The double‐tip taper effectively expands the mode field diameter at the chip facet to match that of the SMF at the initial stage. Then the MMI‐based combiner efficiently combines the two channels in the taper into a highly confined strip waveguide. A coupling loss of 2.85 dB/facet has been achieved for the transverse magnetic mode at the wavelength of 407 nm, which is the lowest insertion loss for fiber‐chip coupling on this platform to the best of the knowledge. The design can significantly reduce the insertion loss associated with fiber‐chip coupling, offering a key component for diverse areas ranging from atomic/quantum photonic integrated circuits and ultra‐high capacity communications to optical microscopes. [ABSTRACT FROM AUTHOR]

Ultracompact and low-power-consumption optical switches are desired for high-performance telecommunication networks and data centers. Here, we demonstrate an on-chip power-efficient 2 × 2 thermo-optic switch unit by using a suspended photonic crystal nanobeam structure. A submilliwatt switching power of 0.15 mW is obtained with a tuning efficiency of 7.71 nm/mW in a compact footprint of 60 μm × 16 μm. The bandwidth of the switch is properly designed for a four-level pulse amplitude modulation signal with a 124 Gb/s raw data rate. To the best of our knowledge, the proposed switch is the most power-efficient resonator-based thermo-optic switch unit with the highest tuning efficiency and data ever reported.

Subwavelength structures such as subwavelength gratings (SWGs) and subwavelength metamaterials are capable of tailoring the optical properties of materials and controlling the flow of light at the nanoscale. The effective indices of the subwavelength structured strip and slab waveguides can be changed in a wide range by choosing an appropriate duty cycle or a filling factor of silicon, which provides an effective method to manipulate the optical field and achieve effective index matching for functional devices. Recent advances in nanofabrication techniques have made it possible to implement subwavelength structures in silicon strip and slab waveguides. Here we review various approaches used to design subwavelength structures and achieve exotic optical responses and discuss how these structures can be used to realize high-performance silicon photonic devices. Both one-dimensional SWG devices and two-dimensional subwavelength metamaterial devices are covered in this review, including subwavelength structure–based polarization handling devices, mode manipulation devices, and building blocks for integrated optical interconnects. Perspectives on subwavelength structured silicon photonic devices are also discussed.

A novel non-volatile optical filter with a large bandwidth (BW) and extinction ratio (ER) tunability is first experimentally demonstrated by introducing an all-optical phase change of Ge2Sb2Te5 (GST). The Si-GST hybrid device promises flexible multi-level regulation of essential filter parameters in an ultra-compact footprint of 30 μ m × 13 μ m. Ultra-low power consumption is realised on account of the saving of external static power that is required in other electric-optic or optic-optic-driven filters. The GST is loaded onto two triple-waveguide directional couplers located at the coupling regions of an add-drop microring resonator. By initiating the GST phase transition with pump optical pulses, the transmitted optical power to the cross port of the triple-waveguide coupler is adjustable, hence influencing the coupling efficiency states of the microring filter. Consequently, a tunable on-off ER from 0.7 dB to 18.2 dB and a tunable BW from 0.6 nm to 3.3 nm are experimentally obtained with the aid of optically manipulating the crystallization degree of GST. Our device potentially enables the realisation of high-density photonic integrated circuits, especially in dense wavelength division multiplexing networks. [ABSTRACT FROM AUTHOR]

AR enables the integration of virtual digital information on the real world. However, most AR displays suffer from accommodation vergence conflict, which results in 3D visual fatigue. In this work, our works on 3D visual fatigue‐free display techniques would be presented. [ABSTRACT FROM AUTHOR]

In this paper, we aimed to address the accommodation‐vergence conflict by developing a liquid crystal (LC) lens that provides faster and more responsive visual effects. Specifically, we used the azo‐dye SD1 based on photoalignment techniques and fabricated a fast‐response Pancharatnam‐Berry (PB) phase LC lens using a single‐exposure photoalignment method. The response time of the LC lens was approximately 1ms. [ABSTRACT FROM AUTHOR]

In recent years, the emergence of virtual reality (VR) and augmented reality (AR) has revolutionized the way we interact with the world, leading to significant advancements in 3D display technology. However, some of the currently employed 3D display techniques rely on stereoscopic 3D display method, which may lead to visual discomfort due to the vergence-accommodation conflict. To address this issue, several true 3D technologies have been proposed as alternatives, including multi-plane displays, holographic displays, super multi-view displays, and integrated imaging displays. In this review, we focus on planar liquid crystal (LC) devices for different types of true 3D display applications. Given the excellent optical performance of the LC devices, we believe that LC devices hold great potential for true 3D displays. [ABSTRACT FROM AUTHOR]

Optical interconnects have emerged as promising solutions for assisting electrical interconnects in short‐reach scenarios, where high bandwidth density and energy efficiency are of particular importance. Mode‐division multiplexing (MDM), capable of enhancing the bandwidth density and energy efficiency of optical systems, has drawn tremendous interest. However, the chip‐to‐chip MDM optical interconnects are impeded by the multimode chip‐fiber interfaces, where the transverse modes on the MDM chip mismatch with the linear polarization modes in the circular core few‐mode fiber (CCF). Moreover, the high differential group delays (DGDs) of a conventional CCF make the digital signal processing (DSP) computation complex thus consuming high power. To overcome these bottlenecks, a new multimode coupling solution based on a rectangular core few‐mode fiber with ultra‐low DGDs and an integrated multimode coupler is proposed. Based on this coupling scheme, a chip‐to‐chip MDM transmission experiment is performed on the TE00, TM00, TE10, and TM10 modes, and the highest bandwidth density of 19.4 Tb s mm −1 is realized at the chip facet. The DSP power consumption is ≈6% of a conventional CCF‐based system. It is believed that this work will pave the way for ultra‐compact, high density, low cost, and low computation‐complexity optical interconnects, such as data centers, cloud computing, and telecommunications. [ABSTRACT FROM AUTHOR]

Polarization manipulation is essential in photonic integrated circuits and has numerous applications in various fields, such as optical communication, nonlinear optics, and quantum optics. Advances in nanofabrication have enabled the integration of subwavelength‐structured metamaterials on optical waveguides, providing unprecedented optical manipulation capabilities beyond classical waveguide‐based architectures. In this paper, the polarization space is demonstrated to be fully manipulated by a dielectric metamaterial composed of nanoholes and nanoslots. This approach offers competitive performances for key polarization components, including the polarizer, polarization beam splitter, and polarization‐splitter‐rotator, while maintaining ultra‐compact coupling regions of 18×1 µm2, 16×1.1 µm2, and 13×1 µm2 respectively. The devices are designed by manipulating the phase and amplitude of all possible eigenmodes supported in the waveguide, which is inherently scalable and versatile for on‐chip mode and wavefront manipulation. The unique properties of metamaterials provide powerful tools for on‐chip polarization manipulation and offer new possibilities for the development of compact and high‐performance photonic integrated circuits. [ABSTRACT FROM AUTHOR]

Grating‐assisted contra‐directional couplers are important optical components in photonic integrated circuits (PICs), which can serve as fundamental building blocks for optical (de)multiplexers, switches, filters, and splitters, among others. Recently, lithium niobate on insulator (LNOI) has emerged as an attractive PIC platform with many important active photonic circuit components demonstrated, including electro‐optic modulators and second‐harmonic wavelength converters. Nevertheless, the LNOI component toolbox still lacks some passive photonic circuit components to fulfill the requirements of future high‐performance PICs. In this contribution, a subwavelength grating‐assisted contra‐directional coupler (GACDC) is proposed, designed, and experimentally demonstrated in a silicon nitride‐loaded LNOI waveguide platform. As an example of its applications, a three‐channel wavelength‐division (de)multiplexer is fabricated and experimentally demonstrated by cascading several GACDCs, with a narrow wavelength spacing of 3.2 nm (400 GHz). The measured device insertion loss of each channel is about 2.5 dB with low interchannel crosstalk below −20 dB. A fabrication tolerance analysis is also carried out, indicating that the proposed GACDCs are tolerant to waveguide width variations. [ABSTRACT FROM AUTHOR]

Silicon photonics is becoming a competitive technology to settle the issues of power and speed in data centers and computing systems. However, the absence of an on‐chip light source restricts the wide and deep application of silicon photonics. Essentially, efficient edge coupling from a Si‐based III‐V laser to silicon photonic components remains the major obstacle for on‐chip integration. Here, two compact edge couplers with ultra‐low coupling loss and eased fabrication for the light transmission from a Si‐based III‐V laser to Si3N4 waveguide are realized. An ultra‐low laser‐to‐chip coupling loss of only 1.175 dB for the butt coupling of a Si‐based InAs QD laser is experimentally demonstrated with high alignment tolerance. The strategies presented here provide a competitive solution for realizing ultra‐efficient integration of Si‐based light sources and silicon photonic components. [ABSTRACT FROM AUTHOR]

A multi‐focal waveguide near‐eye display (WNED) system for augmented reality (AR) based on a tunable Pancharatnam‐Berry (PB) phase lens. The tunable lens is made of a stack of switchable PB lens elements which can be switched on and off by voltage. Combined with a waveguide display module, a prototype of the WNED system with images generated at different depths is demonstrated. [ABSTRACT FROM AUTHOR]

The development of metaverse has the potential to revolutionize people's life. As the entry to metaverse, virtual reality (VR) and augmented reality (AR) technologies are gaining ever‐increasing attention. However, there is vergence‐accommodation conflict problem for most VR or AR 3D displays. In this paper, we propose to realize vari‐focal near‐eye displays based on tunable geometry phase liquid crystal lenses to address this problem. Experimental results validate that images can be rendered at different depth as the tunable lenses change their focal lengths. Thus, correct accommodation cue can be induced, and 3D visual fatigue could be significantly suppressed. [ABSTRACT FROM AUTHOR]

Power consumption of photonic integrated circuits becomes a critical consideration. A new platform is proposed for ultralow‐power tuning in silicon photonics via piezo‐optomechanical coupling using hafnium‐oxide actuators. As an example of the potential of the platform, a tunable silicon‐hafnium‐oxide hybrid microring, where hafnium‐oxide film acts as an active optical and piezoelectric layer, is demonstrated. The hybrid microring is capable of linear bidirectional tuning with a wavelength tuning efficiency of 8.4 pm V−1 and a power efficiency of 0.12 nW pm−1. The estimated power consumption for tuning a free spectral range (FSR) in hybrid microring is 3.07 µW per FSR. The hybrid silicon‐hafnium‐oxide technology with complementary metal‐oxide‐semiconductor (CMOS) compatibility advances the field of ultralow‐power integrated photonic devices and can find applications in optical communications, computing, and spaces under cryogenic temperatures. [ABSTRACT FROM AUTHOR]

Coupled‐waveguide devices are of central importance in on‐chip photonic circuits. However, their performance usually suffers from high wavelength and structure sensitivity, which makes it challenging to achieve broadband and fabrication tolerant optical functions. Topological pumping of edge states has emerged as a promising route for robust light transport in coupled waveguides. Here, the Thouless pumping process is explored in finite Rice‐Mele modeled silicon waveguide arrays and the robust power coupling and mode‐order conversion are experimentally demonstrated in this system. Thanks to the topological protection of light transport, the directional coupler and mode‐order converter based on the waveguide arrays show an ultrabroad bandwidth of 120 nm and are quite tolerant to significant structural deviations (−50–150 nm). As compared to the robust optical coupling in other topological waveguide arrays reported in recent works, the approach is more advantageous in terms of robustness against the variations in both waveguide width and gap distance, which is greatly favored in large‐scale photonic integration. This work could serve as a design principle for a new class of broadband and fabrication tolerant coupled‐waveguide devices, which can find applications in many fields including optical simulations of condensed matter physics, on‐chip optical communications, and quantum information processing. [ABSTRACT FROM AUTHOR]

We propose an ultracompact parallel label-free biosensor based on concentric silicon-on-insulator microring resonators. Our numerical studies show that the proposed biosensor offers higher sensitivity and a larger detection area than conventional single-ring-based sensors. We analyze the detection sensitivity of the DNA hybridization by immobilizing the probe on top of the double concentric ring resonators and in the ring-ring air gap. For a concentric double-ring system with an area of 27.646 [micro][m.sup.2], the bulk detection sensitivity is 683 nm [RIU.sup.-1]. The sensitive uniformity of the system related to the optical field distribution is also studied. OCIS codes: 130.6010, 230.4555.