Broadband polarization splitter-rotator on a thin-film lithium niobate with conversion-enhanced adiabatic tapers.

In this work, we propose and experimentally demonstrate a broadband polarization splitter-rotator (PSR) on the lithium niobate on insulator (LNOI). With multiple sequentially connected adiabatic tapers for waveguide mode conversion and directional coupling, the PSR shows a 160-nm bandwidth covering the C and L bands, an insertion loss of less than 2 dB, and an extinction ratio of more than 11 dB. Benefiting from the conversion-enhanced adiabatic tapers, the broadband device has a short length of 405 µm. Further optimization is performed to reduce the device length to 271 µm and comparable performances are achieved, demonstrating the feasibility of higher device compactness. The proposed design and principle can contribute to high-performance polarization management for integrated lithium niobate photonics.

Single-Crystalline Li2Sn(IO3)6 Microwires: Combining Optical-Waveguiding and Frequency-Doubling Functions.

High-quality single-crystalline Li2Sn(IO3)6 microwires (MWs) have been successfully prepared by using a facile hydrothermal method. The as-synthesized Li2Sn(IO3)6 MWs exhibit regular hexagonal prism morphology, excellent surface smoothness, and remarkable diameter uniformity. The optical propagation loss has been determined to be as low as 0.026 dB µm-1 at 785 nm wavelength, implying the low-loss optical waveguiding capability of the Li2Sn(IO3)6 MWs. The effective frequency-doubling conversions of the fundamental frequency light source in the wavelength range from 916 to 1560 nm have been observed, and the second-harmonic generation (SHG) conversion efficiency has been measured to be 2.1% with a 1560 nm fundamental pump source (pulse duration of 10 ns, and average power of 9.06 nW) transmitted through a 1.32-µm-diameter and 300-µm-length Li2Sn(IO3)6 MW. These intriguing optical waveguiding and strong SHG conversion capabilities of the Li2Sn(IO3)6 MWs suggest its potential applications for photonic devices in micrometer scale.

Direct observation of multimode interference in rare-earth doped micro/nanofibers.

Modal inspection of optical fibers is important for multimode application but it is challenging to collect in-situ information of propagating modes for evaluation and manipulation. Here we demonstrate direct observation of multimode interference in Er3+/Yb3+ co-doped micro/nanofibers. Luminescent interference patterns are visualized by visible up-conversion of Er3+ ions and are used for establishing the existence of higher order modes co-propagating with fundamental modes. We use fast Fourier transform to analyze the patterns in detail and obtain excellent agreement between experiment and calculation on beat lengths of the interference. Effective index differences among higher order modes and a fundamental mode of a microfiber are also experimentally investigated with the assistance of interference patterns, revealing the characteristic of modal dispersions.

Hybrid Photon-Plasmon Coupling and Ultrafast Control of Nanoantennas on a Silicon Photonic Chip.

Hybrid integration of nanoplasmonic devices with silicon photonic circuits holds promise for a range of applications in on-chip sensing, field-enhanced and nonlinear spectroscopy, and integrated nanophotonic switches. Here, we demonstrate a new regime of photon-plasmon coupling by combining a silicon photonic resonator with plasmonic nanoantennas. Using principles from coherent perfect absorption, we make use of standing-wave light fields to maximize the photon-plasmon interaction strength. Precise placement of the broadband antennas with respect to the narrowband photonic racetrack modes results in controlled hybridization of only a subset of these modes. By combining antennas into groups of radiating dipoles with opposite phase, far-field scattering is effectively suppressed. We achieve ultrafast tuning of photon-plasmon hybridization including reconfigurable routing of the standing-wave input between two output ports. Hybrid photonic-plasmonic resonators provide conceptually new approaches for on-chip integrated nanophotonic devices.

Real-time monitoring and gradient feedback enable accurate trimming of ion-implanted silicon photonic devices.

Fabrication errors pose significant challenges on silicon photonics, promoting post-fabrication trimming technologies to ensure device performance. Conventional approaches involve multiple trimming and characterization steps, impacting overall fabrication complexity. Here we demonstrate a highly accurate trimming method combining laser annealing of germanium implanted silicon waveguide and real-time monitoring of device performance. Direct feedback of the trimming process is facilitated by a differential spectroscopic technique based on photomodulation. The resonant wavelength trimming accuracy is better than 0.15 nm for ring resonators with 20-µm radius. We also realize operating point trimming of Mach-Zehnder interferometers with germanium implanted arms. A phase shift of 1.2π is achieved by annealing a 7-µm implanted segment.

Single CdTe Nanowire Optical Correlator for Femtojoule Pulses.

On the basis of the transverse second harmonic generation (TSHG) in a highly nonlinear subwavelength-diameter CdTe nanowire, we demonstrate a single-nanowire optical correlator for femto-second pulse measurement with pulse energy down to femtojoule (fJ) level. Pulses to be measured were equally split and coupled into two ends of a suspending nanowire via tapered optical fibers. The couterpropagating pulses meet each other around the central area of the nanowire, and emit TSHG signal perpendicular to the axis of the nanowire. By transferring the spatial intensity profile of the transverse second harmonic (TSH) image into the time-domain temporal profile of the input pulses, we operate the nanowire as a miniaturized optical correlator. Benefitted from the high nonlinearity and the very small effective mode area of the waveguiding CdTe nanowire, the input energy of the single-nanowire correlator can go down to fJ-level (e.g., 2 fJ/pulse for 1064 nm 200 fs pulses). The miniature fJ-pulse correlator may find applications from low power on-chip optical communication, biophotonics to ultracompact laser spectroscopy.

Graphene decorated microfiber for ultrafast optical modulation.

We demonstrate ultrafast optical modulation using a single 1-µm-diameter graphene-decorated microfiber, which is fabricated with a convenient and controllable evanescent-field-induced deposition method. Benefitting from the significantly enhanced light-graphene interaction of the subwavelength transvers dimension of the microfiber and accumulation of the saturable absorption of the piled graphene flakes, the microfiber shows nonlinear saturable absorption with a peak power threshold down to 1.75 W (60 MW/cm(2)), with a measured response time of about 3.5 ps.

Ultrafast all-optical graphene modulator.

Graphene is an optical material of unusual characteristics because of its linearly dispersive conduction and valence bands and the strong interband transitions. It allows broadband light-matter interactions with ultrafast responses and can be readily pasted to surfaces of functional structures for photonic and optoelectronic applications. Recently, graphene-based optical modulators have been demonstrated with electrical tuning of the Fermi level of graphene. Their operation bandwidth, however, was limited to about 1 GHz by the response of the driving electrical circuit. Clearly, this can be improved by an all-optical approach. Here, we show that a graphene-clad microfiber all-optical modulator can achieve a modulation depth of 38% and a response time of ∼ 2.2 ps, limited only by the intrinsic carrier relaxation time of graphene. This modulator is compatible with current high-speed fiber-optic communication networks and may open the door to meet future demand of ultrafast optical signal processing.

Graphene coated ZnO nanowire optical waveguides.

We report the fabrication and characterization of freestanding graphene coated ZnO nanowires (GZNs) for optical waveguiding. The GZNs are fabricated using a tape-assist transfer under micromanipulation. Owing to the deep-subwavelength diameter and high index contrast of the ZnO nanowire waveguide, light-graphene interaction is significantly enhanced by the strong surface optical fields, resulting in a linear absorption as high as 0.11 dB/µm in a 606-nm-diameter GZN at 1550-nm wavelength. Launched by 1550-nm-wavelength femto-second pulses, a 475-nm-diameter GZN with a graphene coating length of merely 24 µm exhibits evident nonlinear saturable absorption with a peak power threshold down to 1.3 W. In addition, we also demonstrate a transmission modulation for 1550-nm-wavelength signal with a 590-nm-diameter GZN, showing the possibility of using GZN waveguides as nanoscale bulding blocks for nanophotonic devices.

On-chip silicon electro-optical modulator with ultra-high extinction ratio for fiber-optic distributed acoustic sensing.

Ultra-high extinction ratio (ER) optical modulation is crucial for achieving high-performance fiber-optic distributed acoustic sensing (DAS) for various applications. Bulky acousto-optical modulators (AOM) as one of the key devices in DAS have been used for many years, but their relatively large volume and high power consumption are becoming the bottlenecks to hinder the development of ultra-compact and energy-efficient DAS systems that are highly demanded in practice. Here, an on-chip silicon electro-optical modulator (EOM) based on multiple coupled microrings is demonstrated with ultra-high ER of up to 68 dB while the device size and power consumption are only 260 × 185 µm2 and 3.6 mW, respectively, which are at least two orders of magnitude lower than those of a typical AOM. Such an on-chip EOM is successfully applied to DAS with an ultra-high sensitivity of -71.2 dB rad2/Hz (4 pε/√Hz) and a low spatial crosstalk noise of -68.1 dB rad2/Hz, which are very similar to those using an AOM. This work may pave the way for realization of next-generation ultra-compact DAS systems by integration of on-chip opto-electronic devices and modules with the capability of mass-production.

Ge Ion Implanted Photonic Devices and Annealing for Emerging Applications.

Germanium (Ge) ion implantation into silicon waveguides will induce lattice defects in the silicon, which can eventually change the crystal silicon into amorphous silicon and increase the refractive index from 3.48 to 3.96. A subsequent annealing process, either by using an external laser or integrated thermal heaters can partially or completely remove those lattice defects and gradually change the amorphous silicon back into the crystalline form and, therefore, reduce the material's refractive index. Utilising this change in optical properties, we successfully demonstrated various erasable photonic devices. Those devices can be used to implement a flexible and commercially viable wafer-scale testing method for a silicon photonics fabrication line, which is a key technology to reduce the cost and increase the yield in production. In addition, Ge ion implantation and annealing are also demonstrated to enable post-fabrication trimming of ring resonators and Mach-Zehnder interferometers and to implement nonvolatile programmable photonic circuits.

Ultrafast perturbation maps as a quantitative tool for testing of multi-port photonic devices.

Advanced photonic probing techniques are of great importance for the development of non-contact wafer-scale testing of photonic chips. Ultrafast photomodulation has been identified as a powerful new tool capable of remotely mapping photonic devices through a scanning perturbation. Here, we develop photomodulation maps into a quantitative technique through a general and rigorous method based on Lorentz reciprocity that allows the prediction of transmittance perturbation maps for arbitrary linear photonic systems with great accuracy and minimal computational cost. Excellent agreement is obtained between predicted and experimental maps of various optical multimode-interference devices, thereby allowing direct comparison of a device under test with a physical model of an ideal design structure. In addition to constituting a promising route for optical testing in photonics manufacturing, ultrafast perturbation mapping may be used for design optimization of photonic structures with reconfigurable functionalities.

Flexible integration of free-standing nanowires into silicon photonics.

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.

Electrical tuning of surface plasmon polariton propagation in graphene-nanowire hybrid structure.

We demonstrate a dynamic surface plasmonic modulation based on graphene-nanowire (grapheme-NW) hybrid structures in the visible light range. A static modulation depth of as high as 0.07 dB/µm has been achieved experimentally. Through careful simulation and systematical experimental investigation, we found that the dual-confinement effect of charge density and electromagnetic energy around the vicinity of the NW will dramatically enhance the light-matter interaction and increase the Fermi level shifting, which are the key roles for bringing the optical response of the device to the visible range. The carrier concentration near the vicinity of a Ag NW is estimated to reach 0.921×10(14) cm(-2) after applying more than 25 V voltages, which is enough to shift the Fermi level for visible light. Furthermore, the modulation behaviors near the Dirac point of monolayer graphene and the singularity of gap-induced bilayer graphene are investigated. Calculated optical conductivity as a function of Fermi level predicts a minimum value near the Dirac point, which is consistent with the experimental results.