Ultra-compact and high-performance four-channel coarse wavelength-division (de)multiplexing filters based on cascaded Mach-Zehnder interferometers with Bezier-shape directional couplers.

Using cascaded Mach-Zehnder interferometers (CMZIs) provides an attractive option for realizing coarse wavelength-division (de)multiplexing (CWDM) filters with low losses, low crosstalk, flat tops, and high scalability. However, they usually have large footprints and insufficient fabrication tolerances, due to the inferior performance of conventional directional couplers (DCs) used for MZIs. Here, a four-channel CMZI wavelength-division (de)multiplexer based on novel Bezier-shape DCs with compact footprints, broad bandwidths and decent fabrication tolerances. For the fabricated (de)multiplexer with 20-nm channel spacing, the excess loss is less than 0.5 dB and the crosstalk is lower than -19.5 dB in the 1-dB bandwidth of 12.8 nm. For the case with a core-width deviation of ±20 nm, the device still performs very well with low losses and low crosstalk. Compared to the state-of-the-art MZI-based CWDM filters, the present device has slightly high performances and a footprint of 0.012 mm2 shrunk greatly by ∼3-folds. This work can be extended for more channels and other material platforms.

Polarization-insensitive multimode interference coupler based on a subwavelength grating structure.

A multimode interference (MMI) coupler is one of the basic components for photonic integrated circuits. However, MMI couplers realized by conventional waveguides are polarization sensitive, which is undesired for many applications, such as optical switches and communications. In this Letter, we propose a polarization-insensitive MMI coupler on a 220-nm silicon-on-insulator platform by constructing different effective interference lengths for TE and TM modes assisted with subwavelength grating structures. The designed MMI coupler shows an excess loss of <0.24(0.43) dB and a power imbalance of <0.6(0.5) dB for the TE(TM) mode over the wavelength range of 1.5-1.6 µm in theory. Experimentally, the fabricated MMI exhibits low excess loss <0.64(0.53) dB and power imbalance <1(0.85) dB for the TE(TM) mode over a wavelength range of 1.55-1.61 µm.

Waveguide-integrated aluminum-MoTe2 Schottky photodetectors for the wavelength band extended to 2†µm.

Transition metal dichalcogenide (TMDC) materials with excellent optoelectronic properties have attracted much attention in the fields of reconfigurable electronic devices, next-generation FETs, and photodetectors (PDs). While normal TMDC PDs have a bandgap-limited absorption edge of ∼1.3 µm, metal-TMDC Schottky PDs based on internal photoemission provide an operation band extension strategy. In this study, we demonstrate that a TMDC PD can even operate at the wavelength band as long as 2.0 µm by judiciously choosing TMDC and metal materials to construct a low barrier height Schottky PD. Specifically, a silicon waveguide-integrated Al-MoTe2 Schottky PD was measured with responsivities of 18 mA/W and 5.5 mA/W at 1.6 µm and 2 µm, respectively. Meanwhile, the dark current is as low as 2 µA. The linear response can be maintained when the input optical power is in the mW scale. A measured 3 dB bandwidth is much larger than 1.75 MHz. These findings offer a promising avenue for expanding the detection range of the TMDC-based PDs with overall good performance in responsivity, bandwidth, sensitivity, and linearity.

Coaxial transceiving LiDAR based on a silicon photonic optical phased array.

A high performance optical phased array (OPA) combined with frequency-modulated continuous-wave (FMCW) technology is essential for coherent all-solid-state light detection and ranging (LiDAR). In this work, we propose and experimentally demonstrate a coaxial transceiver based on a single OPA for a LiDAR system, which releases the off-chip circulator and collimator. The proposed scheme is demonstrated on the commonly used silicon-on-insulator (SOI) platform. For realizing the long optical grating antenna with only one-step etching, the bound state in the continuum is harnessed to simplify the fabrication process and ease the fabrication precision. Experimental results indicate that the OPA is with 0.076° vertical beam divergence under a 1.5 mm-long grating antenna. The measured field of view (FOV) is 40° × 8° without grating lobes under a wavelength band of 60 nm. The coaxial transceiver of the single OPA is also demonstrated with the FMCW method for ranging measurement at different angles.

High performance polarizer on thin-film lithium niobate with width-tapered Euler bending.

In this Letter, we propose and demonstrate an integrated polarizer on thin film lithium niobate (TFLN). The polarizer consists of a width-tapered 180° Euler bending waveguide featuring thin thickness and bilevel mode convertors with silica cladding. Notably, the TE0 mode is efficiently confined in the waveguide, while the TM0 mode confronts significant bending losses. The measurements reveal that the excess loss remains below 1.5 dB, and the extinction ratio surpasses 19 dB within a working bandwidth spanning from 1480 to 1578 nm. The proposed polarizer holds considerable promise for enhancing polarization handling within TFLN photonic circuits.

Polarization-insensitive and high-efficiency edge coupler for thin-film lithium niobate.

In this Letter, we propose and demonstrate a fiber-to-chip edge coupler (EC) on an x-cut thin film lithium niobate (TFLN) for polarization-insensitive (PI) coupling. The EC consists of three width-tapered full-etched waveguides with silica cladding and matches well with a single-mode fiber (SMF). The measured results show that the minimum coupling losses for TE0/TM0 modes remain to be 0.9 dB/1.1 dB per facet, and the polarization dependent loss (PDL) is <0.5 dB over the wavelength range from 1260 to 1340 nm. Moreover, the EC features large misalignment tolerance of ±2 µm in the Z direction and ±1.5 µm in the X direction for both polarizations for a 1 dB penalty. To the best of our knowledge, this is the first realized O-band edge coupler on TFLN with SMF. The proposed device shows promising potential for integration into TFLN polarization diversity devices.

Cryogenic lithium-niobate-on-insulator optical filter.

Photonic integrated circuits have garnered significant attention and experienced rapid development in recent years. To provide fundamental building blocks for scalable optical classical and quantum information processing, one important direction is to develop cryogenic compatible photonic integrated devices. Here, we prepare one optical filter on a lithium-niobate-on-insulator (LNOI) platform based on a multimode waveguide grating and verify its availability at temperature from 295 to 7 K. We find that the integrated optical filter still shows good quality under cryogenic conditions, and the shift of the working wavelength at different temperatures is well explained by the index variation of the material. These results advance LNOI integrated optical devices in applications under cryogenic conditions.

Perovskite light-emitting diodes based on spontaneously formed submicrometre-scale structures.

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.

Waveguide-Integrated Light-Emitting Metal-Insulator-Graphene Tunnel Junctions.

Ultrafast interfacing of electrical and optical signals at the nanoscale is highly desired for on-chip applications including optical interconnects and data processing devices. Here, we report electrically driven nanoscale optical sources based on metal-insulator-graphene tunnel junctions (MIG-TJs), featuring waveguided output with broadband spectral characteristics. Electrically driven inelastic tunneling in a MIG-TJ, realized by integrating a silver nanowire with graphene, provides broadband excitation of plasmonic modes in the junction with propagation lengths of several micrometers (∼10 times larger than that for metal-insulator-metal junctions), which therefore propagate toward the junction edge with low loss and couple to the nanowire waveguide with an efficiency of ∼70% (∼1000 times higher than that for metal-insulator-metal junctions). Alternatively, lateral coupling of the MIG-TJ to a semiconductor nanowire provides a platform for efficient outcoupling of electrically driven plasmonic signals to low-loss photonic waveguides, showing potential for applications at various integration levels.

Grating lobe-free silicon optical phased array with periodically bending modulation of dense antennas.

A grating lobe-free silicon optical phased array with large field of view is demonstrated. Antennas with periodically bending modulation are spaced at half wavelength or less. The experimental results show that the crosstalk between adjacent waveguides is negligible at 1550 nm wavelength. Additionally, to reduce the optical reflection caused by the sudden change of refractive index at the output antenna of the phased array, tapered antennas are added to the output end face so that more light will be coupled into the free space. The fabricated optical phased array shows a field of view of 120° without any grating lobes.

Low-loss and power-efficient phase shifter based on an optimized multimode spiral silicon waveguide.

Low-loss and energy-efficient phase shifters are an effective tool to reduce the power consumption of large-scale photonic integrated circuits. In this work, a low-loss and power-efficient thermo-optic phase shifter has been demonstrated on the silicon-on-insulator platform. The multimode spiral waveguide is optimized to obtain lower power consumption and low cross talk. The waveguide width is beyond the single-mode region in consideration of low propagation loss. The optimized ultra-low loss 180° Bezier bends are used to further reduce the bending loss. The experimental results show that the excess loss of the phase shifter is only 0.36 dB at 1550-nm wavelength and the power consumption is 4.87 mW/π.

Polarization multiplexing silicon photonic optical phased array with a wide scanning range.

We propose and experimentally demonstrate a polarization multiplexed silicon optical phased array (OPA) with a wide scanning range. The two polarization states share the same power splitter tree and the phase shifter array. A polarization switch is introduced in front of the power splitter tree to manipulate the polarization state of the light in OPA. Through a polarization splitter-rotator (PSR), the light of two polarization states propagates into the superlattice grating antenna array. The wavelength tuning efficiency could be doubled by optimizing the parameters of the waveguide grating. We demonstrate the scheme on the commonly used 220 nm silicon-on-insulator (SOI) platform. Experimental results indicate that the 24.8° vertical scanning range could be realized with a high wavelength tuning efficiency of 0.31°/nm. The measured field of view (FOV) is 24.8 × 60°.

High-performance and compact integrated photonic dichroic filters and triplexer realized by an efficient inverse design.

Integrated optical filters are key components in various photonic integrated circuits for applications of communication, spectroscopy, etc. The dichroic filters can be flexibly cascaded to construct filters with various channel numbers and bandwidths. Therefore, the development of high-performance and compact dichroic filters is crucial. In this work, we develop the dichroic filters with 1.49/1.55-µm channels by an inverse design. Benefiting from a search-space-dimension control strategy and advanced optimization algorithm, our efficient design method results in two high-performance dichroic filters without and with subwavelength gratings (SWGs). The comparison suggests that SWGs in filters can be useful for loss reduction and footprint compression by dispersion engineering. The developed dichroic filter with SWGs exhibits measured bandwidths of 26/29 nm, excess losses of < 0.5 dB, and crosstalks of <-10 dB with a compact footprint of 2.5 × 22.0 µm2. It has advantages in performance or compactness compared to the previously reported counterparts. A triplexer with a footprint of 10.5 × 117 µm2 is developed based on the dichroic filters, also showing decent overall performance and compactness.

Proposal for collinear integrated acousto-optic tunable filters featuring ultrawide tuning ranges and multi-band operations.

Integrated optical tunable filters are key components for a wide spectrum of applications, including optical communications and interconnects, spectral analysis, and tunable light sources, among others. Compared with their thermo-optic counterparts, integrated acousto-optic (AO) tunable filters provide a unique approach to achieve superior performance, including ultrawide continuous tuning ranges of hundreds of nm, low power consumption of sub-mW and fast tuning speed of sub-µs. Based on suspended one-dimensional (1D) AO waveguides in the collinear configuration, we propose and theoretically investigate an innovative family of integrated AO tunable filters (AOTFs) on thin-film lithium niobate. The AO waveguides perform as tunable wavelength-selective narrow-band polarization rotators, where highly efficient conversion between co-propagating TE0 and TM0 modes is enabled by the torsional acoustic A1 mode, which can be selectively excited by a novel antisymmetric wavefront interdigital transducer. Furthermore, we systematically and quantitatively explore the possibilities of exciting modulated acoustic waves, which contain multiple frequency components, along the AO waveguide to achieve independently reconfigurable multi-band operations, with tunable time-variant spectral shapes. By incorporating a complete set of ultrawide-band polarization-handling components, we have proposed and theoretically investigated several representative monolithic AOTF configurations, featuring different arrangements of single or cascaded identical AO waveguides. One of the present AOTF designs exhibits a theoretical linewidth of ∼8 nm (∼4 nm), a sidelobe suppression ratio of ∼75 dB, and theoretically no excess loss at the center wavelength of 1550 nm (1310 nm), with an ultrawide tuning range of 1.25-1.65 µm (from O-band to L-band), a fast tuning speed of 0.14 µs, and a low power consumption of a few mW.

High-performance waveguide Ge/Si avalanche photodiode with a lateral separate-absorption-charge-multiplication structure.

A high-performance waveguide Ge/Si avalanche photodiode operating at the O-band (1310 nm) is designed with a Ge/Si ridge waveguide defined by two shallow trenches in the active region and fabricated with simplified processes. The device shows a high primary responsivity of 0.96 A/W at the unit-gain voltage of -7.5 V. It has a large 3-dB bandwidth of >27 GHz and a low dark current of 1.8 µA at a reverse bias voltage of -13 V. When the present Ge/Si APD is used for receiving 25 Gbps data, the eye-diagram is open even for an optical power as low as -18 dBm. Furthermore, 50 Gbps data receiving is also demonstrated with an input optical power of -15 dBm, showing the great potential of the present Ge/Si APD for the application in future high-speed data transmission systems.

Ultra-compact electro-optic modulator based on etchless lithium niobate photonic crystal nanobeam cavity.

Photonic crystal (PhC) cavities with high Q factor and low volume have been applied in nonlinear, electro-optic and acoustic-optic devices due to the enhancement of the light-matter interactions. However, there are few devices and research on LiNbO3 (LN) PhC cavities due to the difficulty in making hyperfine structures on LN platform. In this work, we propose a PhC nanobeam cavity on the etchless x-cut LiNbO3-On-Insulator (LNOI). The fabrication-friendly device has been designed based on photonic bound states in the continuum (BICs) exhibiting a high Q factor of over 10,000 with the device length of only about 100 µm. Utilizing the electro-optical effect γ13 of LN, we demonstrate an ultra-compact electro-optic modulator based on the PhC nanobeam cavities, which has the modulation efficiency of 1.5 pm/V and the 3 dB bandwidth of 28 GHz.

Reconfigurable add-drop filter based on an antisymmetric multimode photonic crystal nanobeam cavity in a silicon waveguide.

We have designed and demonstrated a reconfigurable channel add-drop filter (ADF) based on an antisymmetric multimode photonic crystal nanobeam cavity (AM-PCNC) in a silicon waveguide. The proposed AM-PCNC can realize channel add-drop filtering by selectively filtering and reflecting the fundamental mode (TE0) and 1st-order mode (TE1) in the multimode waveguide. A high-performance add-drop filter has been demonstrated with a high extinction ratio of 28.2 dB and an insertion loss of 0.18 dB. Meanwhile, the reconfigurable add-dropping has been realized by heating the nanobeam cavity to tune the filtering wavelength. A tuning efficiency of 0.464 nm/mW was measured. The rising and falling time are ∼6.5 µs and ∼0.6 µs, respectively, which are at microsecond time scale. The footprint of the involved nanobeam cavity is only 16.5 µm2. The device can potentially provide an integrated component for optical switch array, routers, and wavelength-division multiplexing in the optical networks.

Silicon optical phased array with calibration-free phase shifters.

Optical phased array (OPA) based on silicon photonics is considered as a promising candidate for realizing solid-state beam steering. However, the high refractive index contrast of the silicon waveguides leads to conventional silicon based OPA suffering from large random phase errors, which require complex post-processing such as time-consuming phase calibration. We propose and demonstrate a calibration-free silicon OPA with optimized optical waveguides width as well as the compact 90° waveguide bends beyond the single mode regime. By using grouped cascaded phase shifters, it is able to reduce the number of control electrodes from N to log2(N). A 16-channel OPA has been demonstrated with continuous beam steering over the field of view controlled by only four control voltages without any calibration.

High-performance lithium-niobate-on-insulator optical filter based on multimode waveguide gratings.

A high-performance optical filter is proposed and realized with multimode waveguide grating (MWG) and two-mode multiplexers on the x-cut lithium-niobate-on-insulator (LNOI) platform for the first time, to the best of our knowledge. The present optical filter is designed appropriately to avoid material anisotropy as well as mode hybridness, and has a low excess loss of 0.05 dB and a high sidelobe suppression ratio (SLSR) of 32 dB in theory with Gaussian apodization. The fabricated filters show a box-like response with 1-dB bandwidth of 6-23 nm, excess loss of ∼0.15 dB, sidelobe suppression ratio of >26 dB. The device performance is further improved with a sidelobe suppression ratio as high as 48 dB and a low excess loss of ∼0.25 dB by cascading two identical MWGs.

High-efficiency four-wave mixing in low-loss silicon photonic spiral waveguides beyond the singlemode regime.

Low-loss optical waveguides are highly desired for nonlinear photonics such as four-wave mixing (FWM), optical parametric amplification, and pulse shaping. In this work, low-loss silicon photonic spiral waveguides beyond the single-mode regime are proposed and demonstrated for realizing an enhanced FWM process. In particular, the designed 2-µm-wide silicon photonic waveguides are fabricated with standard foundry processes and have a propagation loss as low as ∼0.28 dB/cm due to the reduced light-matter interaction at the waveguide sidewalls. In the experiments, strong FWM effect is achieved with a high conversion efficiency of -8.52 dB in a 2-µm-wide and 20-cm-long silicon photonic waveguide spiral, and eight new wavelengths are generated with the pump power of ∼80 mW (corresponding to a low power density of ∼195 mW/µm2). In contrast, the FWM efficiency for the 0.45-µm-wide waveguide spiral is around -15.4 dB, which is much lower than that for the 2-µm-wide waveguide spiral. It can be seen that silicon photonics beyond the singlemode regime opens a new avenue for on-chip nonlinear photonics and will bring new opportunities for nonlinear photonic applications.