Parallel wavelength-division-multiplexed signal transmission and dispersion compensation enabled by soliton microcombs and microrings.

The proliferation of computation-intensive technologies has led to a significant rise in the number of datacenters, posing challenges for high-speed and power-efficient datacenter interconnects (DCIs). Although inter-DCIs based on intensity modulation and direct detection (IM-DD) along with wavelength-division multiplexing technologies exhibit power-efficient and large-capacity properties, the requirement of multiple laser sources leads to high costs and limited scalability, and the chromatic dispersion (CD) restricts the transmission length of optical signals. Here we propose a scalable on-chip parallel IM-DD data transmission system enabled by a single-soliton Kerr microcomb and a reconfigurable microring resonator-based CD compensator. We experimentally demonstrate an aggregate line rate of 1.68 Tbit/s over a 20-km-long SMF. The extrapolated energy consumption for CD compensation of 40-km-SMFs is ~0.3 pJ/bit, which is calculated as being around 6 times less than that of the commercial 400G-ZR coherent transceivers. Our approach holds significant promise for achieving data rates exceeding 10 terabits.

Continuously tunable silicon optical true-time delay lines with a large delay tuning range and a low delay fluctuation.

On-chip switchable optical true-time delay lines (OTTDLs) feature a large group delay tuning range but suffer from a discrete tuning step. OTTDLs with a large delay tuning range and a continuous tuning capability are highly desired. In this paper, we propose and experimentally demonstrate a silicon-based broadband continuously tunable OTTDL comprising a 7-bit delay line and a switch-based continuously tunable delay line. The group delay of the entire OTTDL can be continuously tuned from 0 to 1020.16 ps. A delay error within -1.27 ps to 1.75 ps, and a delay fluctuation of less than 2.69 ps in the frequency range of 2∼25 GHz are obtained. We analyze the causes of the delay fluctuation and its influence on beamforming. Moreover, we also propose a simplified non-invasive calibration method that can significantly reduce the complexity of the delay state calibration and can be easily extended to delay lines with more stages of optical switches. The high performance of our OTTDL chip and the calibration method drive practical applications of integrated OTTDLs.

On-chip passive pump-rejection long-pass filters for integrated SiC-based nonlinear and quantum photonic chips.

We present an on-chip passive pump-rejection filter on an integrated silicon carbide (SiC)-on-insulator photonic platform. Our filters exploit the optical absorption from an amorphous silicon (α-Si) thin-film layer deposited on the top surface and on the sidewalls of the SiC waveguide to reject light with a wavelength below 1.0 µm. The filter has a simple design and can be readily fabricated using a standard semiconductor wafer fabrication process and can be integrated as a pump-rejection filter component for SiC-based nonlinear and quantum photonic chips. We experimentally demonstrate a pump-rejection efficiency exceeding 230 dB/mm for 780 nm wavelengths, while we extract an insertion loss of ∼1 dB for the O-, C-, and L-bands.

A 3C-SiC-on-Insulator-Based Integrated Photonic Platform Using an Anodic Bonding Process with Glass Substrates.

Various crystalline silicon carbide (SiC) polytypes are emerging as promising photonic materials due to their wide bandgap energies and nonlinear optical properties. However, their wafer forms cannot readily provide a refractive index contrast for optical confinement in the SiC layer, which makes it difficult to realize a SiC-based integrated photonic platform. In this paper, we demonstrate a 3C-SiC-on-insulator (3C-SiCoI)-based integrated photonic platform by transferring the epitaxial 3C-SiC layer from a silicon die to a borosilicate glass substrate using anodic bonding. By fine-tuning the fabrication process, we demonstrated nearly 100% area transferring die-to-wafer bonding. We fabricated waveguide-coupled microring resonators using sulfur hexafluoride (SF6)-based dry etching and demonstrated a moderate loaded quality (Q) factor of 1.4 × 105. We experimentally excluded the existence of the photorefractive effect in this platform at sub-milliwatt on-chip input optical power levels. This 3C-SiCoI platform is promising for applications, including large-scale integration of linear, nonlinear and quantum photonics.

Integrated Si3N4 microresonator-based quantum light sources with high brightness using a subtractive wafer-scale platform.

The silicon nitride (Si3N4) platform, demonstrating a moderate third-order optical nonlinearity and a low optical loss compared with those of silicon, is suitable for integrated quantum photonic circuits. However, it is challenging to develop a crack-free, wafer-scale, thick Si3N4 platform in a single deposition run using a subtractive complementary metal-oxide-semiconductor (CMOS)-compatible fabrication process suitable for dispersion-engineered quantum light sources. In this paper, we demonstrate our unique subtractive fabrication process by introducing a stress-release pattern prior to the single Si3N4 film deposition. Our Si3N4 platform enables 950 nm-thick and 8 µm-wide microring resonators supporting whispering-gallery modes for quantum light sources at 1550 nm wavelengths. We report a high photon-pair generation rate of ∼1.03 MHz/mW2, with a high spectral brightness of ∼5×106 pairs/s/mW2/GHz. We demonstrate the first heralded single-photon measurement on the Si3N4 platform, which exhibits a high quality of conditional self-correlation gH(2)(0) of 0.008 ± 0.003.

Stress-released Si3N4 fabrication process for dispersion-engineered integrated silicon photonics.

We develop a stress-released stoichiometric silicon nitride (Si3N4) fabrication process for dispersion-engineered integrated silicon photonics. To relax the high tensile stress of a thick Si3N4 film grown by low-pressure chemical vapor deposition (LPCVD) process, we grow the film in two steps and introduce a conventional dense stress-release pattern onto a ∼400nm-thick Si3N4 film in between the two steps. Our pattern helps minimize crack formation by releasing the stress of the film along high-symmetry periodic modulation directions and helps stop cracks from propagating. We demonstrate a nearly crack-free ∼830nm-thick Si3N4 film on a 4" silicon wafer. Our Si3N4 photonic platform enables dispersion-engineered, waveguide-coupled microring and microdisk resonators, with cavity sizes of up to a millimeter. Specifically, our 115µm-radius microring exhibits an intrinsic quality (Q)-factor of ∼2.0×106 for the TM00 mode and our 575µm-radius microdisk demonstrates an intrinsic Q of ∼4.0×106 for TM modes in 1550nm wavelengths.

An optofluidic "tweeze-and-drag" cell stretcher in a microfluidic channel.

The mechanical properties of biological cells are utilized as an inherent, label-free biomarker to indicate physiological and pathological changes of cells. Although various optical and microfluidic techniques have been developed for cell mechanical characterization, there is still a strong demand for non-contact and continuous methods. Here, by combining optical and microfluidic techniques in a single desktop platform, we demonstrate an optofluidic cell stretcher based on a "tweeze-and-drag" mechanism using a periodically chopped, tightly focused laser beam as an optical tweezer to trap a cell temporarily and a flow-induced drag force to stretch the cell in a microfluidic channel transverse to the tweezer. Our method leverages the advantages of non-contact optical forces and a microfluidic flow for both cell stretching and continuous cell delivery. We demonstrate the stretcher for mechanical characterization of rabbit red blood cells (RBCs), with a throughput of ∼1 cell per s at a flow rate of 2.5 µl h-1 at a continuous-wave laser power of ∼25 mW at a wavelength of 1064 nm (chopped at 2 Hz). We estimate the spring constant of RBCs to be ∼14.9 µN m-1. Using the stretcher, we distinguish healthy RBCs and RBCs treated with glutaraldehyde at concentrations of 5 × 10-4% to 2.5 × 10-3%, with a strain-to-concentration sensitivity of ∼-1529. By increasing the optical power to ∼45 mW, we demonstrate cell-stretching under a higher flow rate of 4 µl h-1, with a higher throughput of ∼1.5 cells per s and a higher sensitivity of ∼-2457. Our technique shows promise for applications in the fields of healthcare monitoring and biomechanical studies.

Photoresponsive spiro-polymers generated in situ by C-H-activated polyspiroannulation.

The development of facile and efficient polymerizations toward functional polymers with unique structures and attractive properties is of great academic and industrial significance. Here we develop a straightforward C-H-activated polyspiroannulation route to in situ generate photoresponsive spiro-polymers with complex structures. The palladium(II)-catalyzed stepwise polyspiroannulations of free naphthols and internal diynes proceed efficiently in dimethylsulfoxide at 120 °C without the constraint of apparent stoichiometric balance in monomers. A series of functional polymers with multisubstituted spiro-segments and absolute molecular weights of up to 39,000 are produced in high yields (up to 99%). The obtained spiro-polymers can be readily fabricated into different well-resolved fluorescent photopatterns with both turn-off and turn-on modes based on their photoinduced fluorescence change. Taking advantage of their photoresponsive refractive index, we successfully apply the polymer thin films in integrated silicon photonics techniques and achieve the permanent modification of resonance wavelengths of microring resonators by UV irradiation.

Optical lattice generation using vertically embedded multimode-interference square-core polymer waveguides on a silicon chip.

We demonstrate two-dimensional optical lattice generation at 1064nm wavelength using vertically embedded multimode-interference (MMI) square-core polymer waveguides on a silicon chip. We demonstrate tuning of the effective waveguide length by longitudinally offsetting the waveguide input end-face from the input beam waist. Our measurement results of the waveguides with different cross-sectional dimensions at different effective waveguide lengths exhibit lattice patterns spanning from 4 × 4 to 10 × 10 arrays at the waveguide output end-face. Our theoretical analysis reveals that the offset causes additional mode-dependent phase changes. Our numerical modeling results using the three-dimensional beam-propagation method are consistent with our experimental results and theory.

Actively stabilized silicon microrings with integrated surface-state-absorption photodetectors using a slope-detection method.

We propose and experimentally demonstrate actively stabilized silicon microrings with integrated surface-state-absorption (SSA) photodetectors using a slope-detection method. Our proof-of-concept experiments reveal that the active stabilization using multiple discrete-step slope thresholds can effectively reduce the microring transmitted intensity variations upon various temperature modulation conditions. We demonstrate an actively stabilized microring transmission with intensity modulations within ~2.5 dB upon a 5mHz temperature modulation between 17 °C and 31 °C, which is ~7.5dB improved from without stabilization. The active alignment tolerance between the stabilized microring resonance wavelength and a carrier wavelength is ~0.16 nm over a 14°C temperature modulation. We observe open eye-diagrams at a data transmission rate of up to 30 Gb/s under temperature modulations with actively stabilized silicon microrings.

Active resonance wavelength stabilization for silicon microring resonators with an in-resonator defect-state-absorption-based photodetector.

We propose and demonstrate active resonance wavelength stabilization for silicon microring resonators with an in-resonator defect-state-absorption (DSA)-based photodetector (PD) for optical interconnects. We integrate an electro-optic (EO) tuner and a thermo-optic (TO) tuner on the microring, which are both feedback-controlled following a photocurrent threshold-detection method. Our BF(2)-ion-implanted DSA-based PIN PD exhibits a cavity-enhanced sub-bandgap responsivity at 1550 nm of 3.3 mA/W upon -2 V, which is 550-fold higher than that exhibited by an unimplanted PIN diode integrated on the same microring. Our experiment reveals active stabilization of the resonance wavelength within a tolerance of 0.07 nm upon a step increment of the stage temperature by 7 °C. Upon temperature modulations between 23 °C and 32 °C and between 18 °C and 23 °C, the actively stabilized resonance exhibits a transmission power fluctuation within 2 dB. We observe open eye diagrams at a data transmission rate of up to 30 Gb/s under the temperature modulations.

Silicon coupled-resonator optical-waveguide-based biosensors using light-scattering pattern recognition with pixelized mode-field-intensity distributions.

Chip-scale, optical microcavity-based biosensors typically employ an ultra-high-quality microcavity and require a precision wavelength-tunable laser for exciting the cavity resonance. For point-of-care applications, however, such a system based on measurements in the spectral domain is prone to equipment noise and not portable. An alternative microcavity-based biosensor that enables a high sensitivity in an equipment-noise-tolerant and potentially portable system is desirable. Here, we demonstrate the proof-of-concept of such a biosensor using a coupled-resonator optical-waveguide (CROW) on a silicon-on-insulator chip. The sensing scheme is based on measurements in the spatial domain, and only requires exciting the CROW at a fixed wavelength and imaging the out-of-plane elastic light-scattering intensity patterns of the CROW. Based on correlating the light-scattering intensity pattern at a probe wavelength with the light-scattering intensity patterns at the CROW eigenstates, we devise a pattern-recognition algorithm that enables the extraction of a refractive index change, Δn, applied upon the CROW upper-cladding from a calibrated set of correlation coefficients. Our experiments using an 8-microring CROW covered by NaCl solutions of different concentrations reveal a Δn of ~1.5 × 10(-4) refractive index unit (RIU) and a sensitivity of ~752 RIU(-1), with a noise-equivalent detection limit of ~6 × 10(-6) RIU.

Silicon nitride three-mode division multiplexing and wavelength-division multiplexing using asymmetrical directional couplers and microring resonators.

We demonstrate silicon nitride mode-division multiplexing (MDM) and wavelength-division multiplexing (WDM) using asymmetrical directional couplers and microring resonators. Our experiments reveal three-mode multiplexing and demultiplexing. We demonstrate 30Gb/s open eye diagrams with an extinction ratio of ~9 dB for each of the three modes. We observe the worst-case modal crosstalk of ~-10 dB. Our analysis of the measured transmission spectra suggests three contributions to the observed crosstalks, with the dominant cause being a compromised input-coupling at the directional couplers in the multiplexer.

Unfolding a design rule for microparticle buffering and dropping in microring-resonator-based add-drop devices.

We propose an intuitive and quantitative design rule to determine the microparticle transport processes, including buffering and dropping, on microring-resonator-based add-drop devices at cavity resonances in an integrated optofluidic chip. The design rule uses the splitting ratio, S, of the optical-field intensity at the microring feedback-arc just after the output-coupling region to that at the drop-waveguide as a figure-of-merit for particle transport to determine between particle buffering (S > 1) and dropping (S < 1). The particle transport, however, becomes probabilistic in the case that S is close to 1. The S factor thus provides a clearer physical criterion for determining the particle transport processes compared to the cavity quality (Q) factor. We experimentally investigate this design rule on four different devices with different design parameters on a silicon nitride-on-silica substrate, and show that the particle transport behaviours of 2.2 µm- and 0.8 µm-sized polystyrene particles are consistent with the S values extracted from the transmission spectra. Our numerical simulations of the four devices suggest that the S values extracted from the simulated transmission spectra are consistent with those extracted from the simulated mode-field intensity distributions. We calculate the optical force field using Maxwell stress tensor and an effective microdisk model to relate the S values to the particle transport processes. We further experimentally demonstrate the viability of the design rule by switching between deterministic particle buffering and probabilistic particle transport processes by switching the polarization modes.

Direct-modulated waveguide-coupled microspiral disk lasers with spatially selective injection for on-chip optical interconnects.

We investigate direct-modulated waveguide-coupled microspiral disk lasers for on-chip optical interconnects. Microspiral resonators, with a rotationally asymmetric shape and a waveguide directly gapless coupled to the notch, offer a compact unidirectional-emission on-chip laser source. We employ spatially selective injection by means of a ring-shaped p-contact on top of the microdisk rim region to selectively inject current to the whispering-gallery-like modes and thus enhance the laser performance. Here we report room-temperature continuous-wave electrically injected AlGaInAs/InP waveguide-coupled microspiral disk lasers with a disk radius of 30 and 40 µm. For a 30 µm microspiral disk laser gaplessly coupled with a 100 µm-long passive waveguide that is directly connected to an on-chip AlGaInAs/InP photodiode, we estimate a laser output power of at least 200 µW upon a 70 mA injection. We realize small-signal modulation with a 3dB bandwidth exceeding 10 GHz for the 30 µm microspiral disk. We demonstrate an open eye diagram at 15 Gbit/s with a bias current of 90 mA at a stage temperature of 15 °C.

Sub-bandgap linear-absorption-based photodetectors in avalanche mode in PN-diode-integrated silicon microring resonators.

We report a sub-bandgap linear-absorption-based photodetector in avalanche mode at 1550 nm in a PN-diode-integrated silicon microring resonator. The photocurrent is primarily generated by the defect-state absorption introduced by the boron and phosphorous ion implantation during the PN diode formation. The responsivity is enhanced by both the cavity effect and the avalanche multiplication. We measure a responsivity of ~72.8 mA/W upon 8 V at cavity resonances in avalanche mode, corresponding to a gain of ~72 relative to the responsivity of ~1.0 mA/W upon 3 V at cavity resonances in normal mode. Our device exhibits a 3 dB bandwidth of ~7 GHz and an open eye diagram at 15 Gbit/s upon 8 V.

Silicon-on-insulator multimode-interference waveguide-based arrayed optical tweezers (SMART) for two-dimensional microparticle trapping and manipulation.

We demonstrate two-dimensional optical trapping and manipulation of 1 µm and 2.2 µm polystyrene particles in an 18 µm-thick fluidic cell at a wavelength of 1565 nm using the recently proposed Silicon-on-insulator Multimode-interference (MMI) waveguide-based ARrayed optical Tweezers (SMART) technique. The key component is a 100 µm square-core silicon waveguide with mm length. By tuning the fiber-coupling position at the MMI waveguide input facet, we demonstrate various patterns of arrayed optical tweezers that enable optical trapping and manipulation of particles. We numerically simulate the physical mechanisms involved in the arrayed trap, including the optical force, the heat transfer and the thermal-induced microfluidic flow.

Epitaxial III-V-on-silicon waveguide butt-coupled photodetectors.

We report silicon waveguide butt-coupled p-i-n InGaAs photodetectors epitaxially grown on silicon-on-insulator substrates by metalorganic chemical vapor deposition. The InGaAs absorption layer that is lattice-matched to InP is selectively grown on patterned SOI substrates, employing metamorphic growth of GaAs and InP buffer layers. We measure a dark current of 2.5 µA and a responsivity of 0.17 A/W at 1550 nm wavelength upon -1 V bias voltage, with a 20 µm × 20 µm InGaAs photodetector area. This device exhibits a 3 dB bandwidth of 9 GHz upon -4 V bias voltage. We demonstrate an open-eye diagram at 10 Gb/s data rate upon -4 V bias voltage.

Optical trapping of microparticles using silicon nitride waveguide junctions and tapered-waveguide junctions on an optofluidic chip.

We study optical trapping of microparticles on an optofluidic chip using silicon nitride waveguide junctions and tapered-waveguide junctions. We demonstrate the trapping of single 1 µm-sized polystyrene particles using the evanescent field of waveguide junctions connecting a submicrometer-sized input-waveguide and a micrometer-sized output-waveguide. Particle trapping is localized in the vicinity of the junction. We also demonstrate trapping of one and two 1µm-sized polystyrene particles using tapered-waveguide junctions connecting a submicrometer-sized singlemode input-waveguide and a micrometer-sized multimode output-waveguide. Particle trapping occurs near the taper output end, the taper center and the taper input end, depending on the taper aspect ratio.

Planar optical tweezers using tapered-waveguide junctions.

We demonstrate planar optical tweezers using the evanescent field of a silicon nitride tapered-waveguide junction between a singlemode waveguide and a multimode waveguide. Our experiments show that the junction embedded in a fluidic channel holds up to one and two polystyrene particles of sizes of 2.2 µm and 1 µm, respectively. The trapped particles are successively substituted by the incoming particles. Our experiments and numerical modeling reveal that the junction particle trapping depends on particle size and number.