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Abstract Arrayed waveguide grating is a versatile and scalable integrated light dispersion device, which has been widely adopted in various applications, including, optical communications and optical sensing. Recently, thin-film lithium niobate emerges as a promising photonic integration platform, due to its ability of shrinking largely the size of typical lithium niobate based optical devices. This would also enable multifunctional photonic integrated chips on a single lithium niobate substrate. However, due to the intrinsic anisotropy of the material, to build an arrayed waveguide grating on X-cut thin-film lithium niobate has never been successful. Here, a universal strategy to design anisotropy-free dispersive components on a uniaxial in-plane anisotropic photonic integration platform is introduced for the first time. This leads to the first implementation of arrayed waveguide gratings on X-cut thin-film lithium niobate with various configurations and high-performances. The best insertion loss of 2.4 dB and crosstalk of −24.1 dB is obtained for the fabricated arrayed waveguide grating devices. Applications of such arrayed waveguide gratings as a wavelength router and in a wavelength-division multiplexed optical transmission system are also demonstrated.

Group velocity and impedance matches are prerequisites for high-speed Mach–Zehnder electro-optic (EO) modulators. However, not all platforms can realize matching conditions, restricting high-speed modulation in many practical conditions. Here, we propose and demonstrate a quasi-matching scheme to satisfy the group velocity and characteristic impedance matches by cascading fast-wave and slow-wave traveling wave electrodes. The effective group velocity can be flexibly adjusted by changing the ratio of fast-wave and slow-wave traveling wave electrodes. Moreover, the quasi-matching scheme is experimentally verified by demonstrating a 6 mm long EO modulator on a thin-film lithium-niobate-on-insulator platform with a silica cladding. The radio frequency signal insertion loss at the boundary of the slow-wave and fast-wave electrodes is less than 0.12 dB. The measured small signal EO response of the quasi-matched EO modulator drops less than 2 dB at 67 GHz, while the measured small-signal EO responses of conventional slow and fast traveling wave EO modulators drop 4 dB at 67 GHz. The measured 100 Gb/s on–off key signal eye-diagrams of the quasi-matched EO modulator also exhibit an overwhelming advantage over conventional schemes. Therefore, our results will open many opportunities for high-speed EO modulators in various platforms.

A matrix-vector multiplication (MVM) optical signal processor based on mode division multiplexing (MDM) was proposed and demonstrated in the current work, which is composed of a mode multiplexer, a multimode beam splitter, a mode demultiplexer, a modulator array and combiners. In addition, the characteristics of MDM obviate the need for multiple wavelengths and therefore multiple laser light sources are unneeded, which greatly reduces the complexity and cost. A 4 × 4 MDM-MVM was realized on a standard silicon-on-insulator (SOI) platform. Combined with the off-chip light source and photodetectors (PDs), 4-level modulation has been demonstrated, and each level of the output signal could represent 2 bits of information.

Recently, high-performance thin-film lithium niobate optical modulators have emerged that, together with advanced multiplexing technologies, are highly expected to satisfy the ever-growing demand for high-capacity optical interconnects utilizing multiple channels. Accordingly, in this study, a compact lithium-niobate-on-insulator (LNOI) photonic chip was adopted to establish four-channel wavelength-division-multiplexing (WDM) transmitters, comprising four optical modulators based on ultracompact 2 × 2 Fabry-Perot cavities and a four-channel WDM filter based on multimode waveguide gratings. The fabricated chip with four wavelength channels has a total footprint as compact as 0.3 × 2.8 mm2, and exhibits an excess loss of ~0.8 dB as well as low inter-channel crosstalk of < –22 dB. Using this LNOI photonic chip, high-capacity data transmissions of 320 Gbps (4 × 80 Gbps) on-off-keying signals and 400 Gbps (4 × 100 Gbps) four-level pulse amplitude signals were successfully realized with the ultra-low power consumption of 11.9 fJ/bit.

Chip-scale programmable optical signal processors are often used to flexibly manipulate the optical signals for satisfying the demands in various applications, such as lidar, radar, and artificial intelligence. Silicon photonics has unique advantages of ultra-high integration density as well as CMOS compatibility, and thus makes it possible to develop large-scale programmable optical signal processors. The challenge is the high silicon waveguides propagation losses and the high calibration complexity for all tuning elements due to the random phase errors. In this paper, we propose and demonstrate a programmable silicon photonic processor for the first time by introducing low-loss multimode photonic waveguide spirals and low-random-phase-error Mach-Zehnder switches. The present chip-scale programmable silicon photonic processor comprises a 1×4 variable power splitter based on cascaded Mach-Zehnder couplers (MZCs), four Ge/Si photodetectors, four channels of thermally-tunable optical delaylines. Each channel consists of a continuously-tuning phase shifter based on a waveguide spiral with a micro-heater and a digitally-tuning delayline realized with cascaded waveguide-spiral delaylines and MZSs for 5.68 ps time-delay step. Particularly, these waveguide spirals used here are designed to be as wide as 2 µm, enabling an ultralow propagation loss of 0.28 dB/cm. Meanwhile, these MZCs and MZSs are designed with 2-µm-wide arm waveguides, and thus the random phase errors in the MZC/MZS arms are negligible, in which case the calibration for these MZSs/MZCs becomes easy and furthermore the power consumption for compensating the phase errors can be reduced greatly. Finally, this programmable silicon photonic processor is demonstrated successfully to verify a number of distinctively different functionalities, including tunable time-delay, microwave photonic beamforming, arbitrary optical signal filtering, and arbitrary waveform generation.

A polarization-insensitive thermo-optic switch is proposed and demonstrated on the silicon-on-insulator platform with a 220 nm silicon core layer. The present device is based on the Mach–Zehnder interferometer structure, consisting of polarization-insensitive power splitters and polarization-insensitive phase shifters (PIPSs). The polarization-insensitive power splitter has been realized by employing an adiabatic directional coupler, which utilizes the fast adiabatic mode evolution by introducing cubic Bézier curves on outer contours, providing broadband 3-dB power splitting for TE and TM polarization modes with only 70 µm coupling length. For the novel PIPSs, the ridge waveguide with large aspect ratio, based on the mode hybridness property, could obtain the same power consumption (Pπ) for an optical switch working at TE and TM polarizations. Experimental results indicate that the measured insertion losses are less than 2 dB and the extinction ratios are larger than 15 dB over a 40 nm wavelength band (covering the C-band).

A compact spectrometer on silicon is proposed and demonstrated with an ultrahigh resolution. It consists of a thermally-tunable ultra-high-Q resonator aiming at ultrahigh resolution and an array of wideband resonators for achieving a broadened working window. The present on-chip spectrometer has a footprint as compact as 0.35 mm2, and is realized with standard multi-project-wafer foundry processes. The measurement results show that the on-chip spectrometer has an ultra-high resolution ∆λ of 5 pm and a wide working window of 10 nm. The dynamic range defined as the ratio of the working window and the wavelength resolution is as large as 1940, which is the largest for on-chip dispersive spectrometers to the best of our knowledge. The present high-performance on-chip spectrometer has great potential for high-resolution spectrum measurement in the applications of gas sensing, food monitoring, health analysis, etc.

Qudit-based quantum devices can outperform qubit-based ones, but a programmable qudit-based quantum computing device is still missing. Here, the authors fill this gap using a programmable silicon photonic chip employing ququart-based encoding, showing the scaling advantages compared to the qubit counterpart.

Abstract Two-dimensional materials (2DMs) have been used widely in constructing photodetectors (PDs) because of their advantages in flexible integration and ultrabroad operation wavelength range. Specifically, 2DM PDs on silicon have attracted much attention because silicon microelectronics and silicon photonics have been developed successfully for many applications. 2DM PDs meet the imperious demand of silicon photonics on low-cost, high-performance, and broadband photodetection. In this work, a review is given for the recent progresses of Si/2DM PDs working in the wavelength band from near-infrared to mid-infrared, which are attractive for many applications. The operation mechanisms and the device configurations are summarized in the first part. The waveguide-integrated PDs and the surface-illuminated PDs are then reviewed in details, respectively. The discussion and outlook for 2DM PDs on silicon are finally given.

We propose the silicon photonic optical phased array (OPA) with large scanning angle by using polarization multiplexing. A Mach-Zehnder interferometer (MZI) switch together with a polarization splitter/rotator is utilized to select the polarization mode input to the OPA. By optimizing the OPA grating emitter, the steering angle can be doubled with the input of the TE/TM polarization modes. An 1 × 16 silicon-based optical phased array with polarization multiplexing is designed as an example. The simulation results show that 77.4° lateral beam steering with no sidelobes via phase tuning and 28.2° longitudinal beam steering via wavelength tuning can be realized.

A novel Si/In2O3 hybrid plasmonic waveguide modulator was experimentally realized by using an asymmetric directional coupler (ADC), which consists of a silicon photonic waveguide and a Si/In2O3 hybrid plasmonic waveguide. All the silicon cores are covered with a silica layer, above which there is a metal–oxide–semiconductor (MOS) capacitor consisting of the In2O3/HfO2/Au layers. The Au layer sitting on the top of the MOS capacitor works as the top-electrode, while the In2O3 thin film covers the sidewall and contacts with the Au bottom-electrode. When the bias voltage is not applied, light launched from the silicon photonic waveguide is weakly coupled into the Si/In2O3 hybrid plasmonic waveguide, and thus, one has a high transmission at the through port of the ADC. On the other hand, when the bias voltage is applied, the carrier density in the In2O3 layer is changed, which introduces some modification to the refractive index of the In2O3 thin film. As a result, light is strongly cross-coupled from the silicon photonic waveguide to a Si/In2O3 hybrid plasmonic waveguide, and one has low transmission at the through port. In this Letter, an ultra-compact Si/In2O3 hybrid plasmonic waveguide modulator is realized with a 3.5-μm-long ADC. In the experiments, the fabricated waveguide modulator works well and exhibits a high modulation bandwidth of >40 GHz for the first time.

Ultra-compact silicon photonic filters with box-like responses and high extinction ratios are proposed and demonstrated by introducing tenth-order adiabatic elliptical-microrings (AEMs) with bent directional couplers. For the AEMs, the waveguides in the non-coupling regions are wide and have a minimal bending radius, while the waveguides in the coupling regions are narrowed and have a maximal bending radius. The present silicon photonic filters based on tenth-order AEMs show a free spectral range as large as 37 nm for the first time. Even fabricated with commercial complementary metal–oxide–semiconductor-compatible 248 nm deep-ultraviolet lithography processes, the present devices show high performances with excess losses as low as ∼1 dB and extinction ratios as high as >60 dB. More importantly, the present silicon photonic filters work very well all-passively without any thermal control for calibration.

Thin film lithium niobate (LN) shows great potentials for highly compact passive and active devices. As LN is an anisotropic material, waveguides made on it exhibit different mode properties from those on conventional isotropic materials. We study the effective refractive indices of fundamental modes of two polarizations in etched ridge waveguides on an X-cut LN thin film. Mode hybridization phenomenon, where the effective refractive indices of the two polarizations are close, is analyzed in detail with different structural parameters. Transmission through a 90° bend, which is a typical routing element for a photonic chip, is simulated. Significant polarization coupling related to the mode evaluation through the bend is observed, and becomes the dominant fact limiting the performance of this element. In order to ensure a low bending loss, the required bending radius is much larger than that for waveguides on an in-plane isotropic material, e.g. a Z-cut LN thin film. Mode hybridization also plays an important role in the performance of the 90° bend, which should be avoided. Generally, decreasing the thickness of the LN thin film, working at a longer wavelength, or confining the propagation angle on a chip would help to decrease the polarization coupling.

Thin-film lithium niobate (TFLN) based traveling-wave modulators maintain simultaneously excellent performances, including large modulation bandwidth, high extinction ratio, low optical loss, and high modulation efficiency. Nevertheless, there still exists a balance between the driving voltage and modulation bandwidth. Here, we demonstrate an ultra-large bandwidth electro-optic modulator without compromising the driving voltage based on the TFLN platform on a silicon substrate, using a periodic capacitively loaded traveling-wave electrode. In order to compensate the slow-wave effect, an undercut etching technique for the silicon substrate is introduced to decrease the microwave refractive index. Our demonstrated devices represent both low optical and low microwave losses, which leads to a negligible optical insertion loss of 0.2 dB and a large electro-optic bandwidth with a roll-off of 1.4 dB at 67 GHz for a 10 mm-long device. A low half-wave voltage of 2.2 V is also achieved. Data rates up to 112 Gb s−1 with PAM-4 modulation are demonstrated. The compatibility of the proposed modulator to silicon photonics facilitates its integration with matured silicon photonic components using, e.g., hybrid integration technologies.

Applying natural mineral water to skin care is a popular tendency and many cosmetics products based on thermal spring water have been developed. The special location and environmental conditions provide Dead Sea water (DSW) with unique ion composition and concentrations, which bring comprehensive positive effects on skin health. This article reviews two potential action modes of DSW, and the biological function of DSW and its related complex in dermatology and skin care. Previous studies have proved the functions of skin moisturization, anti-inflammation, skin barrier repair, and anti-pollution. Especially, the anti-aging effect of DSW and related complexes can act in three different ways: keratinocyte rejuvenation, photo-protection, and cellular energy elevation. Additionally, the issues that need further investigation are also discussed. We hope that this review will help to improve the understanding of DSW and its related complex, and further contribute to product development in the skincare industry.

We report the designs of on-chip grating couplers for the silicon hybrid plasmonic waveguides, which is the first proposal, to the best of our knowledge, for the direct coupling between a standard single-mode fiber and a hybrid plasmonic waveguide. By leveraging the apodized gratings and a two-stage-taper mode converter, we obtain a theoretical coupling efficiency of 79% (−1.03 dB) at the 1550 nm wavelength and a 3-dB bandwidth of 73 nm between the fiber and a 100 nm-wide silicon hybrid plasmonic waveguide with a bottom metal layer. We further propose grating couplers for three other sorts of silicon hybrid plasmonic waveguides with a metal cap and theoretically achieve good performances with coupling efficiencies larger than 47% and bandwidths larger than 51 nm. The proposed direct coupling scheme can avoid extra insertion losses and additional alignment processes that conventional indirect coupling schemes produce. It is believed to be a new step forward to the CMOS-compatible and large-scale integration based on the plasmonic waveguides.

The members of the Nesterenkonia genus have been isolated from various habitats, like saline soil, salt lake, sponge-associated and the human gut, some of which are even located in polar areas. To identify their stress resistance mechanisms and draw a genomic profile across this genus, we isolated four Nesterenkonia strains from the lakes in the Tibetan Plateau, referred to as the third pole, and compared them with all other 30 high-quality Nesterenkonia genomes that are deposited in NCBI. The Heaps’ law model estimated that the pan-genome of this genus is open and the number of core, shell, cloud, and singleton genes were 993 (6.61%), 2782 (18.52%), 4117 (27.40%), and 7132 (47.47%), respectively. Phylogenomic and ANI/AAI analysis indicated that all genomes can be divided into three main clades, named NES-1, NES-2, and NES-3. The strains isolated from lakes in the Tibetan Plateau were clustered with four strains from different sources in the Antarctic and formed a subclade within NES-2, described as NES-AT. Genome features of this subclade, including GC (guanine + cytosine) content, tRNA number, carbon/nitrogen atoms per residue side chain (C/N-ARSC), and amino acid composition, in NES-AT individuals were significantly different from other strains, indicating genomic adaptation to cold, nutrient-limited, osmotic, and ultraviolet conditions in polar areas. Functional analysis revealed the enrichment of specific genes involved in bacteriorhodopsin synthesis, biofilm formation, and more diverse nutrient substance metabolism genes in the NES-AT clade, suggesting potential adaptation strategies for energy metabolism in polar environments. This study provides a comprehensive profile of the genomic features of the Nesterenkonia genus and reveals the possible mechanism for the survival of Nesterenkonia isolates in polar areas.

A comprehensive multiparameter model is proposed for underwater wireless optical communication (UWOC) channels to integrate the effects of absorption, scattering, and dynamic turbulence. The simulation accuracy is further improved by the combined use of the subharmonic method and the strict sampling constraint method by comparing the phase structure function with the theoretical value. The average light intensity and scintillation index are analyzed using the channel parameters of absorption, scattering, turbulence, flow velocity, and transmission distance. Under weak or medium turbulence, the bit error rate (BER) performance can be effectively improved by increasing the transmitting light power. The power penalty of a 50 m UWOC channel is 5.8 dBm from pure seawater to ocean water and 1.0 dBm from weak turbulence to medium turbulence, with the BER threshold of 10−6.

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.

Optoelectronics: Graphene breaks limit of silicon-based UV detection A high-performance graphene/silicon ultraviolet (UV) photodetector significantly increases the upper-limit of traditional silicon-based UV detectors. A team led by Yang Xu at China’s Zhejiang University fabricated silicon-based UV detectors using graphene with unique UV absorption property, leading to ultra-long lifetime of hot carriers that contribute to photocurrent, and even to carrier multiplication. In the near- and mid-UV regime, the improved performance parameters are photocurrent responsivity (0.20 A W−1), response time (100%). The key metrics of graphene/Si detector outperform those of UV detectors based on Si, GaN, SiC, etc. These results show great promise in applications such as wearable devices, secured mobile communication, and “dissipation-less” remote sensor networks.

Mode-division multiplexing (MDM) technology has drawn tremendous attention for its ability to expand the link capacity within a single-wavelength carrier, paving the way for large-scale on-chip data communications. In the MDM system, the signals are carried by a series of higher-order modes in a multi-mode bus waveguide. Hence, it is essential to develop on-chip mode-handling devices. Silicon-on-insulator (SOI) has been considered as a promising platform to realize MDM since it provides an ultra-high-index contrast and mature fabrication processes. In this paper, we review the recent progresses on silicon integrated nanophotonic devices for MDM applications. We firstly discuss the working principles and device configurations of mode (de)multiplexers. In the second section, we summarize the multi-mode routing devices, including multi-mode bends, multi-mode crossings and multi-mode splitters. The inverse-designed multi-mode devices are then discussed in the third section. We also provide a discussion about the emerging reconfigurable MDM devices in the fourth section. Finally, we offer our outlook of the development prospects for on-chip multi-mode photonics.

A design rule for a Mach-Zehnder interferometer (MZI) sensor is presented, allowing tunable sensitivity by appropriately choosing the MZI arm lengths according to the formula given in this paper. The present MZI sensor designed by this method can achieve an ultra-high sensitivity, which is much higher than any other traditional MZI sensors. An example is given with silicon-on-insulator (SOI) nanowires and the device sensitivity is as high as 106 nm/refractive-index -unit (or even higher), by choosing the MZI arms appropriately. This makes it possible for one to realize a low-cost optical sensing system with a detection limit as high as 10−6 refractive-index-unit, even when a cheap optical spectrum analyzer with low-resolution (e.g., 1 nm) is used for the wavelength-shift measurement.

An ultra-high sensitivity double-slot hybrid plasmonic (DSHP) ring resonator, used for optical sensors and modulators, is developed. Due to high index contrast, as well as plasmonic enhancement, a considerable part of the optical energy is concentrated in the narrow slots between Si and plasmonic materials (silver is used in this paper), which leads to high sensitivity to the infiltrating materials. By partial opening of the outer plasmonic circular sheet of the DSHP ring, a conventional side-coupled silicon on insulator (SOI) bus waveguide can be used. Experimental results demonstrate ultra-high sensitivity (687.5 nm/RIU) of the developed DSHP ring resonator, which is about five-times higher than for the conventional Si ring with the same geometry. Further discussions show that a very low detection limit (5.37 × 10−6 RIU) can be achieved after loaded Q factor modifications. In addition, the plasmonic metal structures offer also the way to process optical and electronic signals along the same hybrid plasmonic circuits with small capacitance (~0.275 fF) and large electric field, which leads to possible applications in compact high-efficiency electro-optic modulators, where no extra electrodes for electronic signals are required.

As the demands of the data communications increase quickly for the applications of cloud-computing as well as data centers，optical interconnects have been very attractive because of the unique advantages on the aspects of low power consumption，high speed and large bandwidth.It is effective to enhance the link capacity by utilizing multiplexing technologies like wavelength-division-multiplexing（WDM），polarization-division-multiplexing（PDM），as well as mode-division-multiplexing（MDM）.It will become very useful when these multiplexing technologies are combined to work together as a hybrid multiplexing technology.Definitely the multiplexer is one of the most important devices for a multiplexed optical interconnect link.A review for the recent progress of ultracompact on-chip multiplexers was given，including WDM devices，PDM devices，MDM devices，and hybrid multiplexers.

Plasmonic nanostructures have attracted intensive attention for many applications in recent years because of the field enhancement at the metal/dielectric interface. First, this strong field enhancement makes it possible to break the diffraction limit and enable subwavelength optical waveguiding, which is desired for nanophotonic integrated circuits with ultra-high integration density. Second, the field enhancement in plasmonic nanostructures occurs only for the polarization mode whose electric field is perpendicular to the metal/dielectric interface, and thus the strong birefringence is beneficial for realizing ultra-small polarization-sensitive/selective devices, including polarization beam splitters, and polarizers. Third, plasmonic nanostructures provide an excellent platform of merging electronics and photonics for some applications, e.g., thermal tuning, photo-thermal detection, etc. Finally, the field enhancement at the metal/dielectric interface helps a lot to realize optical sensors with high sensitivity when introducing plasmonic nanostrutures. In this paper, we give a review for recent progresses on the utilization of field enhancement in plasmonic nanostructures for these applications, e.g., waveguiding, polarization handling, heating, as well as optical sensing.

A comparative study is given for the sensitivity of several typical Si nanophotonic waveguides, including SOI (silicon-on-insulator) nanowires, nanoslot waveguides, suspended Si nanowires, and nanofibers. The cases for gas sensing (ncl ~ 1.0) and liquid sensing (ncl ~ 1.33) are considered. When using SOI nanowires (with a SiO2 buffer layer), the sensitivity for liquid sensing (S ~ 0.55) is higher than that for gas sensing (S ~ 0.35) due to lower asymmetry in the vertical direction. By using SOI nanoslot waveguides, suspended Si nanowires, and Si nanofibers, one could achieve a higher sensitivity compared to sensing with a free-space beam (S = 1.0). The sensitivity for gas sensing is higher than that for liquid sensing due to the higher index-contrast. The waveguide sensitivity of an optimized suspended Si nanowire for gas sensing is as high as 1.5, which is much higher than that of a SOI nanoslot waveguide. Furthermore, the optimal design has very large tolerance to the core width variation due to the fabrication error (∆w ~ ±50 nm). In contrast, a Si nanofiber could also give a very high sensitivity (e.g., ~1.43) while the fabrication tolerance is very small (i.e., ∆w < ±5 nm). The comparative study shows that suspended Si nanowire is a good choice to achieve ultra-high waveguide sensitivity.