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We demonstrate the first high-speed LiNbO3 thin-film optical modulators operating at 2 μm wavelength. The measured L is 3.67 V·cm, and the electro-optic bandwidth is > 22 GHz (beyond the measurement limit of photodetector).

Scaling laws of mode confinement and lasing threshold in plasmonic nanolasers far beyond the diffraction limit are proposed, by which we revisit the reported realizations and discuss the future prospects in further plasmonic nanolaser miniaturization.

A novel silicon photonic MEMS switch based on total internal reflection (TIR) is proposed for the first time. The key element is a multimode interference (MMI) waveguide crossing with a switchable gap. This photonic switch works very well with a low excess loss of ~0.2 dB and a high extinction ratio of >23 dB in an ultra-broad wavelength range of 1250–1650 nm in the OFF state. The excess loss is still as low as 0.18 dB and the extinction ratio is as high as 58 dB in the ON state. © 2020 The Author(s)

We present a silicon racetrack-resonator based on a modified Euler curve for high-performance microwave photonic filters. It has a high Q-factor of 7×105, free spectral range of 0.73 nm, and tuning range of 92 GHz. © 2020 The Author(s)

A Mech-Zehnder switch (MZS) with very low random phase errors is proposed and demonstrated for the first time by introducing broadened arm waveguides. The fabricated thermo-optic MZS shows high an extinction ratio of >35 dB at the central wavelength. © 2020 The Author(s)

Fast electro-optic modulators with an ultracompact footprint and low power consumption are always highly desired for optical interconnects. Here we propose and demonstrate a high-performance lithium niobate electro-optic modulator based on a new 2 × 2 Fabry–Perot cavity. In this structure, the input and reflected beams are separated by introducing asymmetric multimode-waveguide gratings, enabling TE 0 − TE 1 mode conversion. The measured results indicate that the fabricated modulator features a low excess loss of ∼ 0.9 dB , a high extinction ratio of ∼ 21 dB , a compact footprint of ∼ 2120 μm 2 , and high modulation speeds of 40 Gbps OOK and 80 Gbps PAM4 signals. The demonstrated modulator is promising for high-speed data transmission and signal processing.

All-optical silicon-photonics-based LiDAR systems allow for desirable features in scanning resolution and speed, as well as leverage other advantages such as size, weight, and cost. Implementing optical circulators in silicon photonics enables bidirectional use of the light path for both transmitters and receivers, which simplifies the system configuration and thereby promises low system cost. In this work, to the best of our knowledge, we present the first experimental verification of all-passive silicon photonics conditional circulators for monostatic LiDAR systems using a nonlinear switch. The proposed silicon nonlinear interferometer is realized by controlling signal power distribution with power-splitting circuits, allowing the LiDAR transmitter and receiver to share the same optical path. Unlike the traditional concept requiring a permanent magnet, the present device is implemented by using common silicon photonic waveguides and a standard foundry-compatible fabrication process. With several additional phase shifters, the demonstrated device exhibits considerable flexibility using a single chip, which can be more attractive for integration with photodetector arrays in LiDAR systems.

Multimode silicon photonics is attracting more and more attention because the introduction of higher-order modes makes it possible to increase the channel number for data transmission in mode-division-multiplexed (MDM) systems as well as improve the flexibility of device designs. On the other hand, the design of multimode silicon photonic devices becomes very different compared with the traditional case with the fundamental mode only. Since not only the fundamental mode but also the higher-order modes are involved, one of the most important things for multimode silicon photonics is the realization of effective mode manipulation, which is not difficult, fortunately because the mode dispersion in multimode silicon optical waveguide is very strong. Great progresses have been achieved on multimode silicon photonics in the past years. In this paper, a review of the recent progresses of the representative multimode silicon photonic devices and circuits is given. The first part reviews multimode silicon photonics for MDM systems, including on-chip multichannel mode (de)multiplexers, multimode waveguide bends, multimode waveguide crossings, reconfigurable multimode silicon photonic integrated circuits, multimode chip-fiber couplers, etc. In the second part, we give a discussion about the higher-order mode-assisted silicon photonic devices, including on-chip polarization-handling devices with higher-order modes, add-drop optical filters based on multimode Bragg gratings, and some emerging applications.

Light-emitting diodes (LEDs), which convert electricity to light, are widely used in modern society—for example, in lighting, flat-panel displays, medical devices and many other situations. Generally, the efficiency of LEDs is limited by nonradiative recombination (whereby charge carriers recombine without releasing photons) and light trapping1–3. In planar LEDs, such as organic LEDs, around 70 to 80 per cent of the light generated from the emitters is trapped in the device4,5, leaving considerable opportunity for improvements in efficiency. Many methods, including the use of diffraction gratings, low-index grids and buckling patterns, have been used to extract the light trapped in LEDs6–9. However, these methods usually involve complicated fabrication processes and can distort the light-output spectrum and directionality6,7. Here we demonstrate efficient and high-brightness electroluminescence from solution-processed perovskites that spontaneously form submicrometre-scale structures, which can efficiently extract light from the device and retain wavelength- and viewing-angle-independent electroluminescence. These perovskites are formed simply by introducing amino-acid additives into the perovskite precursor solutions. Moreover, the additives can effectively passivate perovskite surface defects and reduce nonradiative recombination. Perovskite LEDs with a peak external quantum efficiency of 20.7 per cent (at a current density of 18 milliamperes per square centimetre) and an energy-conversion efficiency of 12 per cent (at a high current density of 100 milliamperes per square centimetre) can be achieved—values that approach those of the best-performing organic LEDs. The formation of submicrometre-scale structure in perovskite light-emitting diodes can raise their external quantum efficiency beyond 20%, suggesting the possibility of both high efficiency and high brightness.

A monolithically integrated 32-channel hybrid (de)multiplexer on silicon is proposed and experimentally demonstrated to enable wavelength-division multiplexing (WDM) and mode-division multiplexing (MDM) simultaneously for a large capacity of optical interconnect links. The proposed (de)multiplexer consists of a four-channel mode (de)multiplexer based on cascaded asymmetric directional couplers and eight-channel wavelength-division-(de)multiplexers based on two microring resonator (MRR) arrays with a channel spacing of 2 nm. These MRRs are realized with a bending radius of as small as $4~\mu \text{m}$ by introducing a multimode microring waveguide so that a large free spectral range (~24 nm) is enabled for transverse magnetic polarization. Furthermore, each MRR works bi-directionally and provides two drop ports from which the same wavelength-channel from two of the mode-channels is dropped separately. For the fabricated 32-channel hybrid MDM-WDM (de)multiplexer, the excess loss is

A compact polarization beam splitter (PBS) is presented for the case with a 340-nm-thick silicon core layer by using an asymmetric evanescent coupling system. It consists of a narrow input waveguide, a narrow output waveguide, and a wide middle waveguide between them. In this design, the widths of these three waveguides are chosen optimally, so that the phase matching condition is satisfied for the fundamental (TM0) mode in the narrow input/output waveguide and the first higher order (TM1) mode in the wide middle waveguide. Meanwhile, there is moderate phase mismatch for TE polarization. Then, the length of the coupling region is also optimized, so that the launched TE 0 mode gets out from the through port while the launched TM0 mode is coupled completely to the cross port with the assistance of the first higher order (TM1) mode in the wide middle waveguide. The designed PBS has a compact footprint of 38 × 3.5 μm2. In theory, the designed PBS works very well with a low excess loss ( 15 dB) in a broad band of 1510–1570 nm. For the fabricated PBS, the extinction ratios are >10 dB and >20 dB in the wavelength range of 1535–1610 nm, for TM and TE polarizations, respectively.

A novel hybrid multiplexer for wavelength-division- multiplexing (WDM) and polarization-division-multiplexing (PDM) is proposed and realized by integrating a polarization-splitter-rotator (PSR) and an optical-filter array based on novel microring resonators (MRRs). With the PSR, the launched TM-polarized light is rotated to be TE-polarized and outputs from the cross port while the launched TE-polarized light outputs from the through port directly. In the proposed novel configuration, the cross- and through-ports of the PSR are connected through the bus waveguide for all the MRR-based optical filters. In this way, each MRR-based optical filter works bidirectionally and has two drop ports, from which the same wavelength channel of TE- and TM-polarizations are dropped separately. As an example, a 16-channel hybrid WDM-PDM multiplexer is designed and realized with eight wavelength channels and dual polarizations by integrating a PSR and eight MRR-based optical filters with box-like responses. The fabricated PSR has an excess loss of ∼0.6 dB and an extinction ratio (ER) of ∼20 dB in the wavelength range from 1550 to 1580 nm. The optical filters are designed by using cascaded MRRs with bent directional couplers, which works for TE polarization only with very high ER (>35 dB) and, thus, reduces the polarization crosstalk greatly. For the MRR-based optical filters of the present hybrid (de)multiplexer, the channel spacing is 400 GHz (3.2 nm), the crosstalk between the adjacent channels is μ m × 300 μ m. The present hybrid (de)multiplexer can be extended for more channels by reducing the channel spacing as well as increasing the free-spectral range of the MRRs.

A compact and high-performance coarse wavelength-division multiplexing (CWDM) device is introduced with a footprint of 2.1 mm × 0.02 mm using an angled multimode interferometer structure based on a thin-film lithium niobate (TFLN) platform. The demonstrated device built on a 400 nm thick x -cut TFLN shows ultra-low insertion losses of < 0.72 dB . Measured 3 dB bandwidths are 12.1 nm for all channels, and cross talks from adjacent channels are better than 18 dB. Its peak wavelength positions comply with the CWDM standard with a channel spacing of 20 nm. The filter bandwidth of the proposed CWDM device can be tuned by adjusting the structural parameters. This demonstrated CWDM device will promote future realization of multi-channel and multi-wavelength transmitter chips on TFLN.

The O-band coarse wavelength-division (de)multiplexing (CWDM) has been extensively used in data-center optical communications, whereas it’s still challenging to reduce crosstalk and enhance fabrication tolerances for a CWDM filter. In this paper, we propose and experimentally demonstrate a low-crosstalk and fabrication-tolerant four-channel CWDM filter by utilizing dispersion-engineered Mach-Zehnder interferometers. The multi-sectional phase shifters are exploited to eliminate the phase errors induced by width deviations, leading to ultra-precise phase shifts and ultra-large width-error tolerances. The random-phase errors are also inhibited by using multi-mode waveguides as phase-shifting sections. The two-stage-coupler scheme is utilized to flatten the strong coupling-ratio dispersions for directional couplers, so that low crosstalk can be achieved over the whole O-band. The experimental results show both low insertion losses (< 1.2 dB) and low crosstalk (< −22.2 dB) over the whole working wavelength range. The measured width-error tolerance is also as large as ≈ 70 nm.

An ultra-sensitive temperature sensor without sacrificing detection range is demonstrated on the silicon-on-insulator (SOI) platform using cascaded Mach–Zehnder interferometers (MZIs). The sensitivity enhancement is achieved by tailoring the geometric parameters of the two MZIs to have similar free spectral ranges (FSRs) but quite different sensitivities. The proposed sensor only needs single lithography for the sensing unit, without introducing negative thermo-optic coefficient (TOC) materials. The measured sensitivity is 1753.7 pm/°C from 27°C to 67°C, which is higher than any reported results on a silicon platform and about 21.9 times larger than conventional all-silicon temperature sensors.

Silicon photonics has been developed successfully with a top-down fabrication technique to enable large-scale photonic integrated circuits with high reproducibility, but is limited intrinsically by the material capability for active or nonlinear applications. On the other hand, free-standing nanowires synthesized via a bottom-up growth present great material diversity and structural uniformity, but precisely assembling free-standing nanowires for on-demand photonic functionality remains a great challenge. Here we report hybrid integration of free-standing nanowires into silicon photonics with high flexibility by coupling free-standing nanowires onto target silicon waveguides that are simultaneously used for precise positioning. Coupling efficiency between a free-standing nanowire and a silicon waveguide is up to ~97% in the telecommunication band. A hybrid nonlinear-free-standing nanowires–silicon waveguides Mach–Zehnder interferometer and a racetrack resonator for significantly enhanced optical modulation are experimentally demonstrated, as well as hybrid active-free-standing nanowires–silicon waveguides circuits for light generation. These results suggest an alternative approach to flexible multifunctional on-chip nanophotonic devices., Precisely assembling free-standing nanowires for on-demand photonic functionality remains a challenge. Here, Chen et al. integrate free-standing nanowires into silicon waveguides and show all-optical modulation and light generation on silicon photonic chips.

A polarization splitter-rotator (PSR) based on the supermode evolution in a dual-core adiabatic taper with multiple segments is designed. For this dual-core adiabatic taper, one core becomes narrow and the other one becomes wide. The gap between them is kept constant except the input/output parts where these dual cores are separated gradually. When choosing some specific widths for the dual cores, mode hybridization between the TM0 supermode and the TE 1 supermode happens. As a result, when light propagates along the dual-core adiabatic taper, the TM0 supermode launched from the input port can be efficiently converted to the TE1 supermode, which is mainly confined in the core connected to the cross port, and finally one has the TE0 mode output from the cross port. On the other hand, the TE0 mode launched from the input port is confined tightly in the core connected to the through port and finally outputs from the through port with a very low loss. For the structural design of the adiabatic taper, three operation wavelengths of 1450, 1550, and 1650 nm are considered simultaneously so that the PSR works well in a very broad band from 1450 to 1650 nm. Here, the widths of the multiple segments in the taper section are chosen optimally according to the mode hybridization regions of the dual-core waveguide when operating at the wavelengths of 1450, 1550, and 1650 nm. With this design method, the designed PSR based on silicon-on-insulator nanowires is about 240 μm long. The bandwidth is even as large as 300 nm (1450–1750 nm) for an extinction ratio of >16 dB and an excess loss of

A fabrication-tolerant high-performance polarizing beam splitter (PBS) is proposed and demonstrated with silicon-on-insulator nanowire waveguides. This PBS is realized with cascaded bent direction couplers, which is designed to make the phase-matching condition satisfied for the transverse-magnetic (TM) polarization mode. There is significant phase-mismatch for the transverse-electric (TE) polarization mode. As a result, TM-polarization mode is cross coupled efficiently to the cross port, while TE-polarization mode outputs from the through port with a low loss. The present PBS has a footprint of as small as $\sim 4\times 15\,\,\mu \text{m}^{2}$ . The extinction ratio at the center wavelength is 32.6 and 36.8 dB for TE and TM polarizations, respectively. The bandwidth for achieving a >25-dB extinction ratio is ~32nm, and the excess loss is

A silicon-based microdonut resonator consisting of a multimode microdonut waveguide is realized. The random shift of the resonance wavelength due to the fabrication deviation is greatly reduced to be ~0.91 nm compared with that (~4.93 nm) of those regular microring resonators (on the same chip). For the coupling between the access waveguide and the microdonut waveguide, bent directional couplers are used to suppress the undesired higher-order modes according to the phase-matching condition. A local laser-trimming process with a ~1.1-W blue laser is further applied and can introduce a maximal wavelength shift of ~0.46 nm for the resonance alignment. This letter combines the present microdonut resonator design technique and the local laser-trimming process, which potentially provides a promising approach to realize a micro-resonator array with excellent resonance-wavelength alignment.

A review is given on our recent progresses in silicon nanophotonic integrated devices for multiplexing and switching, which are key elements in a multichannel multiplexed photonic networks-on-chip. On-chip (de)multiplexers include wavelength-division-multiplexing filters based on arrayed-waveguide gratings and microring resonators, polarization-division-multiplexing devices like polarizers, polarization-beam splitters and polarization rotators, mode-division-multiplexing devices, and some novel hybrid (de)multiplexers enabling more than one multiplexing technologies simultaneously. Thermal-switchable silicon photonic devices are also discussed regarding the increasing demands for reconfigurable photonic networks-on-chip, including high-performance optical switches and reconfigurable add-drop multiplexers.

An optical 90° hybrid consisting of a polarization splitter–rotator (PSR) and a multimode interferometer (MMI) is proposed to realize insensitive optical frequency mixing for TE/TM states of polarization. The PSR is based on the mode hybridization and mode coupling between waveguides; input light at a fundamental transverse electric (TE) mode and a transverse magnetic (TM) mode are output at separate ports, but both output signals are in the fundamental TE mode. These two output fields are then guided to the MMI, where the optical frequency mixing with the light of a local oscillator takes place. This mixing is then independent of the state of polarization of the hybrid input light. We designed such an optical hybrid based on silicon-nanowire waveguides. At a wavelength of 1540 nm, simulation results for the hybrid show a transmission efficiency of 98.6% or 95.8% for TE- or TM-polarized input light, respectively, while the transmission efficiencies for both polarizations are above 90% in a 30 nm wavelength range, namely [1525 nm, 1555 nm]. Further, in the wavelength range [1510 nm, 1565 nm], the transmission imbalance between the output fields is below 0.85 dB for TE- or TM-polarized input light, respectively, while the phase errors in both cases are less than 4°.

We demonstrate microring lasers based on Si3N4 optical waveguides cladded with MAPbBr3 quantum dots composite film. Si3N4 microrings are designed and fabricated with electron beam (E-beam) lithography and inductively-coupled plasma reactive-ion-etching followed by spin coating a MAPbBr3 quantum dots composite film on them. We clearly observe the lasing modes with narrow linewidths for both TE- and TM-polarization modes when the microrings are optically pumped with a nanosecond laser. The experimental results show that the laser has a typical linewidth of 0.23 nm and a minimum pump power of 8.46 μJ/cm2.

A graphene photodetector based on ultra-thin silicon waveguide at 1.55μm is proposed. By reducing the silicon core thickness, the fundamental TE waveguide mode is less confined and light-graphene interaction is enhanced. Benefiting from the ultrathin silicon waveguide and reflector structure, the graphene absorption coefficient reaches 0.36 dB/μm. A 10nm-thick CVD-grown hexagonal boron nitride is covered on the graphene to improve the device performance. With the help of metal-graphene-metal structure, the contact resistance is reduced dramatically. The devices have shown a responsivity of 1.4 mA/W at 0 V bias and 23.1 mA/W at 0.3 V bias with 0.24 mW input optical power. The measured 3-dB bandwidth is 17GHz under 0V bias voltage at 1550 nm.

Silicon photonics has become very popular because of the compatibility with mature CMOS technologies. However, it is still very difficult for pure silicon to realize various photonic devices for large-scale photonic integrated circuits due to the material intrinsic properties. Currently silicon-plus photonics, which introduces some specific optoelectronic materials on silicon to overcome the shortcomings of silicon, is becoming more and more attractive a very important role. This paper gives a review and discussion on silicon-plus photonics, including the structures, devices and applications for on-chip light-manipulation and photodetection.

It is still challenging to realize an on-chip optical sensor that can detect humidity and temperature at the same time. In this paper, we demonstrate a silicon-based dual-polarization micro-ring resonator (MRR) with a polyvinyl-alcohol (PVA) upper-cladding, which enables the simultaneous measurement of humidity and temperature. Due to the significant polarization-dependence of the silicon-on-insulator (SOI) nanophotonic waveguide, the transverse electric (TE) and transverse magnetic (TM) polarization modes have quite different sensitivities towards the changes of ambient relative humidity (RH) and temperature. Sensitivity, resolution, stability and cross-sensitivity are analyzed for the present dual-parameter sensor. The RH and temperature response sensitivity are measured to be 97.9 pm/%RH, 325.1 pm/%RH, 69.0 pm/°C and 30.6 pm/°C for TE and TM polarizations, respectively. To the best of our knowledge, it is the first on-chip optical sensor enabling the simultaneous measurement of RH and temperature.

A silicon-based polarization-insensitive optical filter is designed and demonstrated by using dual multimode-waveguide-gratings. For the ~300μm-long device, the 3dB-bandwidth is ~11nm and the excess loss is ~1.5dB for both polarizations.

Silicon photonics has attracted much attention because of the CMOS compatibility, high integrated density, etc. However, silicon is not a good option for active photonic integrated devices due to its indirect-bandgap structure. Recently, two-dimensional (2D) materials, such as graphene and black phosphorus (BP), exhibit excellent optical and electronic properties, and thus provide a potential option for realizing active photonic integrated devices on silicon. In this paper, we present our recent works on silicon/2D-materials photonic integrated devices for photodetection and thermal-tuning.

Optical communication wavelength is being extended from the near-infrared band of 1.31/1.55 µm to the mid-infrared band of 2 µm or beyond for satisfying the increasing demands for high-capacity long-distance data transmissions. An efficient electro-optic (EO) modulator working at 2 µm is highly desired as one of the indispensable elements for optical systems. Lithium niobate (LiNbO3) with a large second-order nonlinear coefficient is widely used in various EO modulators. Here, we experimentally demonstrate the first Mach-Zehnder EO modulator working at 2 µm based on the emerging thin-film LiNbO3 platform. The demonstrated device exhibits a voltage-length product of 3.67 V·cm and a 3-dB-bandwidth of >22 GHz which is limited by the 18 GHz response bandwidth of the photodetector available in the lab. Open eye-diagrams of the 25 Gb/s on-off keying (OOK) signals modulated by the fabricated Mach-Zehnder EO modulator is also measured experimentally with a SNR of about 14 dB.

An on-chip quadplexer is proposed and demonstrated with four wavelength-channels of 1270, 1310, 1490, and 1577 nm. The present quadplexer consists of four cascaded filters based on multimode waveguide grating (MWG), which are composed of a two-mode (de)multiplexer and an MWG. For the fabricated quadplexer on silicon, all four wavelength channels have flat-top responses with low excess losses of < 0.5 dB as well as the desired bandwidths, which are about 16, 38, 19, and 6 nm, respectively. The cross-talk for both upstream channels and downstream channels is less than − 24 dB . Moreover, the data transmission of 10 Gb / s of the present silicon quadplexer is also successfully demonstrated.

On-chip silicon polarizers have been widely used in polarization controllers. However, there is limited research on all-silicon polarizer covering the whole optical communication band due to the strong waveguide dispersion for silicon waveguides. In this Letter, we demonstrated an all-silicon TE polarizer with high polarization extinction ratio and low insertion loss, working for the whole optical communication band. The device is based on a shallow-etched waveguide realized on a silicon-on-insulator (SOI) platform. The optical field of TE polarization is designed to be tightly confined in the shallow-etched silicon waveguide, while that of TM polarization is weakly confined. As a result, TE polarization propagates through the waveguide with low loss, while TM polarization leaks into the substrate and decays finally. The measurements show that a maximum insertion loss < 0.25 d B and polarization extinction ratio ( P E R ) > 20 d B over an ultrabroad operation band from 1260–1675 nm have been achieved for the proposed polarizer.

A Mach–Zehnder silicon photonic switch with low random phase errors is proposed and demonstrated for the first time, to the best of our knowledge, by incorporating judiciously widened and shortened phase shifter waveguides. With a 180 nm complementary metal–oxide–semiconductor (CMOS) foundry process, more than one hundred 2 × 2 thermo-optic Mach–Zehnder switches (MZSs) with varied phase shifter widths have been designed, fabricated, and characterized on 14 silicon chips. The mean and standard deviation of the random phase errors of the MZSs with phase shifters widened to 2 µm are less than a third of those of the conventional design with 0.45-µm-wide single-mode phase shifters. This validates the improved fabrication tolerance and results in considerable reduction of the power consumption for the phase error compensation. Such elegant methodology paves the way to further scaling up N × N silicon thermo-optic switches and can be generalized for other phase-sensitive integrated photonic devices as well.

A compact thermally reconfigurable optical add/drop multiplexer (ROADM) is realized with 500 $\text {nm}\times 220$ nm silicon-on-insulator (SOI) strip waveguides. The demonstrated ROADM has eight channels with a channel spacing of 400 GHz and consists of two identical arrayed-waveguide gratings integrated with eight thermo-optic switches on the same chip. These thermo-optic switches have a broad bandwidth with low loss ( 20 dB) for covering all wavelength channels by utilizing point-symmetric Mach–Zehnder interferometers with bent directional couplers. The ROADM’s footprint is about $770~\mu \text {m}\times 880~\mu \text{m}$ only due to the ultra-strong confinement ability of SOI strip waveguides. The total on-chip loss of the fabricated ROADM is $\sim 15$ and $\sim 7$ dB for the transmitted signals and the added/dropped signals, respectively.

Silicon photonics has become very popular because of their compatibility with mature CMOS technologies. However, pure silicon is still very difficult to be utilized to obtain various photonic functional devices for large-scale photonic integration due to intrinsic properties. Silicon-plus photonics, which pluses other materials to break the limitation of silicon, is playing a very important role currently and in the future. In this paper, we give a review and discussion on the progresses of siliconplus photonics, including the structures, devices and applications

Directional couplers (DCs) have been playing an important role as a basic element for realizing power exchange. Previously most work was focused on symmetric DCs and little work was reported for asymmetric directional couplers (ADCs). In recently years, silicon nanophotonic waveguides with ultra-high index contrast and ultra-small cross section have been developed very well and it has been shown that ADCs based on silicon-oninsulator (SOI) nanophotonic waveguides have some unique ability for polarization-selective coupling as well as mode-selective coupling, which are respectively very important for polarization-related systems and modedivision-mulitplexing systems. In this paper, a review is given for the recent progresses on silicon-based ADCs and the applications for power splitting, polarization beam splitting, as well as mode conversion/(de)multiplexing.

A configuration with cascaded ring-resonators is designed and demonstrated for realizing multi-channel optical sensing with reduced temperature sensitivity. The present configuration integrates a microring-based $1\times N$ wavelength-selective power splitter and $N$ microring transducers. The $1\times N$ wavelength-selective power splitter serves as a $1\times N$ power splitter and $N$ bandpass optical filters, which helps realize a compact chip with an optical sensor array. As an example, a sensor array with three microring transducers is demonstrated by using silicon-on-insulator nanowires. The present multi-channel optical sensor tolerates a temperature variation within $\sim 10^{\circ }\text{C}$ .

A low power-consuming variable optical attenuator (VOA) on silicon is demonstrated by using a reflective Mach–Zehnder interferometer (MZI), which consists of broadband highly-efficient Bragg-grating reflectors at the ends of the MZI arms. The present reflective-MZI VOA enables an ultra-high extinction ratio intrinsically even when the 2×2 3 dB coupler is not balanced perfectly, which make it very robust to realize a VOA array. The power consumption is reduced by half in comparison with the regular MZI-type VOA. For the fabricated reflective MZI-type VOA with 125 μm-long heaters, the power consumption is only about 10.8 mW for a high attenuation of 35.5 dB.

Photonic integrated circuits (PICs) technologies in China have made rapid progress in the past decade. In this paper, recent advances and future prospective of PICs in China are introduced and discussed, respectively. More than nine major projects in the field of PICs have been issued. Various material platforms and technologies have been developed to realize photonic integrated devices for many applications focusing on broadband communication, such as optical and wireless network, optical interconnects, and optical coherent communication. Many research achievements and breakthrough have been made under the support of these projects, such as silicon photonics transceiver, InP-based transceiver, and optical and microwave signal processing chips. It can be expected that the PICs technology will be continuously supported in the next five years, since it has been considered as one of the key enabling technologies for supporting several national major assignments and strategies, such as broadband communication, supercomputing and datacenter.

A multichannel mode-selective silicon photonic add/drop multiplexer is proposed by introducing asymmetric directional couplers (ADCs) assisted with phase change material (PCM) strips. Each ADC consists of a multimode bus waveguide and a single-mode access waveguide with a thin PCM strip on the top. For the present mode-selective add/drop multiplexer, the PCM of G e 2 S b 2 S e 4 T e 1 (GSST) is used, and the PCM-loaded access waveguide is designed optimally by engineering the effective index and the modal field profile. When the switching state of the add/drop multiplexer is OFF with crystal GSST, all the mode channels go through with very low excess losses. Otherwise, when it is ON, by switching the GSST-PCM strip to be amorphous, the desired TE mode channel can be selectively added to or dropped from the multimode bus waveguide, while the other mode channels still stay and propagate in the multimode bus waveguide. The simulated results show that the designed mode-selective silicon photonic add/drop multiplexer utilizing a single ADC has low excess losses of < 0.5 d B and low intermodal crosstalk of < − 15 d B over the C-band for all the mode channels. Furthermore, using the design with cascaded ADCs for each mode-selective add/drop switch, the crosstalk can be suppressed to < − 20 d B , and the excess losses are less than 1 dB over an 80 nm broad wavelength band from 1505 to 1585 nm.

A broadband dual-mode 2 × 2 3 d B power splitter on silicon is proposed by utilizing multimode interference (MMI) couplers with a shallowly etched multimode region. The present MMI coupler is designed optimally to be with reduced phase errors for all the excited higher-order modes in the MMI region by engineering the refractive-index profile with the assistance of a shallowly etched multimode region. Simulation results show the device works well for the T E 0 and T E 1 input modes over the C-band. For the T E 0 input mode, the excess losses (ELs) and the imbalances (IBs) are, respectively, less than 0.4 and 0.12 dB in the wavelength range of 1.5–1.6 µm, while the ELs and the IBs for the T E 1 input mode are, respectively, less than 0.5 and 0.2 dB in the wavelength range of 1.53–1.58 µm. Besides, the intermode cross-talk is less than − 22 d B for T E 0 and T E 1 modes in the wavelength range of 1.5–1.6 µm. The proposed dual-mode MMI coupler can be extended for more than two input modes.

A novel wavelength-selective 2 × 2 optical switch based on a Ge 2 Sb 2 Te 5 (GST)-assisted microring-resonator (MRR) is proposed. The present GST-assisted MRR consists of two access optical waveguides and an MRR coupled with a bent GST-loaded silicon photonic waveguide. The 2 × 2 optical switch is switched ON or OFF by modifying the GST state to be crystalline or amorphous. In particular, the microring waveguide and the bent GST-loaded waveguide are designed to satisfy the phase-matching condition when the GST is crystalline. As a result, the MRR becomes highly lossy and the resonance peak is depressed significantly. On the other hand, when it is off, there is little coupling due to the significant phase mismatching. Consequently, one has a low-loss transmission at the drop port for the resonance wavelength. In this paper, the simulation using the three-dimensional finite-difference method shows that the extinction ratio of the designed photonic switch is ∼ 20 dB at the resonance wavelength, while the excess losses at the through port and drop port are 0.9 dB and 2 dB. In particular, the resonance wavelength changes little between the ON and OFF states, which makes it suitable for multichannel wavelength-division-multiplexing systems.

Radio frequency (RF) switches are essential for implementing routing of RF signals. However, the increasing demand for RF signal frequency and bandwidth is posing a challenge of switching speed to the conventional solutions, i.e., the capability of operating at a sub-nanosecond speed or faster. In addition, signal frequency reconfigurability is also a desirable feature to facilitate new innovations of flexible system functions. Utilizing microwave photonics as an alternative path, we present here a photonic implementation of an RF switch providing not only the capability of switching at a sub-nanosecond speed but also options of frequency doubling of the input RF signals, allowing for flexible output waveforms. The core device is a traveling-wave silicon modulator with a device size of 0.2 mm × 1.8 mm and a modulation bandwidth of 10 GHz. Using microwave frequencies, i.e., 15 GHz and 20 GHz, as two simultaneous RF input signals, we experimentally demonstrated their amplitude and frequency switching as well as that of the doubled frequencies, i.e., 30 GHz and 40 GHz, at a switching frequency of 5 GHz. The results of this work point to a solution for creating high-speed RF switches with high compactness and flexibility.

The multimode power splitter is a basic component in mode-division multiplexing systems. In this paper, we propose an ultra-broadband silicon multimode power splitter enabling arbitrary power splitting ratios. The proposed multimode splitter is designed based on a waveguide crossing with an obliquely embedded subwavelength grating (SWG) transflector. The incident multiple guided-modes can be split into two beams with low excess losses and low crosstalk by the SWG transflector where the thin-film interference effect happens. As an example, a silicon multimode power splitter is designed to work with the three lowest-order modes of TE polarization. Any desired splitting ratio ranging from 0% to 100% can be achieved by engineering the structural parameters of the SWG. Moreover, the desired splitting ratio can be very uniform over an extremely broad bandwidth of ≥ 415 nm, covering O-, E-, S-, C-, L- and U-bands. The intermodal crosstalk is < −20 dB for all the input modes in theory. To the best of our knowledge, the proposed structure is the first multimode power splitter enabling any desired power splitting ratios in all the optical communication bands.

An ultrahigh-Q silicon racetrack resonator is proposed and demonstrated with uniform multimode silicon photonic waveguides. It consists of two multimode straight waveguides connected by two multimode waveguide bends (MWBs). In particular, the MWBs are based on modified Euler curves, and a bent directional coupler is used to achieve the selective mode coupling for the fundamental mode and not exciting the higher-order mode in the racetrack. In this way, the fundamental mode is excited and propagates in the multimode racetrack resonator with ultralow loss and low intermode coupling. Meanwhile, it helps achieve a compact 180° bend to make a compact resonator with a maximized free spectral range (FSR). In this paper, for the chosen 1.6 μm wide silicon photonic waveguide, the effective radius R eff of the designed 180° bend is as small as 29 μm. The corresponding FSR is about 0.9 nm when choosing 260 μm long straight waveguides in the racetrack. The present high-Q resonator is realized with a simple standard single-etching process provided by a multiproject wafer foundry. The fabricated device, which has a measured intrinsic Q-factor as high as 2.3 × 10 6 , is the smallest silicon resonator with a > 10 6 Q-factor.

Graphene has emerged as a promising solution for on-chip ultrafast photodetection for its advantages of easy integration, high mobility, adjustable chemical potential, and wide operation wavelength range. In order to realize high-performance photodetectors, it is very important to achieve efficient light absorption in the active region. In this work, a compact and high-speed hybrid silicon/graphene photodetector is proposed and demonstrated by utilizing an ultra-thin silicon photonic waveguide integrated with a loop mirror. With this design, the graphene absorption rate for the fundamental mode of TE polarization is improved by ∼5 times compared to that in the conventional hybrid silicon/graphene waveguide with hco=220 nm. One can achieve 80% light absorption ratio within the active-region length of only 20 µm for the present silicon/graphene waveguide photodetector at 1550 nm. For the fabricated device, the responsivity is about 25 mA/W under 0.3V bias voltage and the 3-dB bandwidth is about 17 GHz. It is expected to achieve very high bandwidth by introducing high-quality Al2O3 insulator layers and reducing the graphene channel length in the future.

A silicon-based on-chip ultra-sharp multimode waveguide bend (MWB) is proposed to work for dual polarizations by introducing modified Euler-curves and shallowly-etched non-uniform subwavelength-gratings (SWGs). For the designed 90° MWB with a core width of 1.01 μm and an effective bending radius as small as 10 μm, the excess losses are less than 0.23 dB and the inter-mode crosstalk is lower than

Silicon photonics have been developed very successfully in the past decades. In order to break the limitation of the silicon material for realizing photonic devices, more and more special optical materials are introduced to silicon photonics, and silicon-plus photonics is being developed. In this paper, we give a review on recent progresses of siliconplus photonics for light manipulation and photodetection by introducing metals, germanium, 2D materials, semiconductor nanowires, etc.

Reconfigurable photonic integrated devices are playing in smart photonic networks, so that it is possible to utilize the resources of the bandwidth/channels optimally and flexibly. Since silicon photonics has become one of the most popular technologies for realizing photonic chips currently, in this paper we focus on our recent work for reconfigurable photonic integrated devices on silicon. It includes the following three parts. The first part is for thermally-tunable optical filters based on micro-ring resonators (MRRs) and waveguide gratings. The second part is for thermo-optic switches, which are designed to be ultra-broad band and polarization-insensitive. The third part is for all-optically reconfigurable photonic integrated devices on silicon.