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.

Ultrafast dynamic machine vision with spatiotemporal photonic computing.

Ultrafast dynamic machine vision in the optical domain can provide unprecedented perspectives for high-performance computing. However, owing to the limited degrees of freedom, existing photonic computing approaches rely on the memory's slow read/write operations to implement dynamic processing. Here, we propose a spatiotemporal photonic computing architecture to match the highly parallel spatial computing with high-speed temporal computing and achieve a three-dimensional spatiotemporal plane. A unified training framework is devised to optimize the physical system and the network model. The photonic processing speed of the benchmark video dataset is increased by 40-fold on a space-multiplexed system with 35-fold fewer parameters. A wavelength-multiplexed system realizes all-optical nonlinear computing of dynamic light field with a frame time of 3.57 nanoseconds. The proposed architecture paves the way for ultrafast advanced machine vision free from the limits of memory wall and will find applications in unmanned systems, autonomous driving, ultrafast science, etc.

Low loss, flexible single-mode polymer photonics.

Single-mode polymer photonics is of significant interest to short-reach data communications, photonic packaging, sensing, and biophotonic light delivery. We report here experimental demonstration of mechanically flexible waveguides fabricated by using commercial off-the-shelf biocompatible polymers that claim a record low propagation loss of 0.11 dB/cm near 850 nm wavelength. We also show the excellent flexibility of the free-standing waveguides which can withstand repeated deformation cycles at millimeter bending radius without compromising their low-loss characteristics. High-performance passive optical components, such as waveguide Y-branches, multi-mode interferometers (MMIs), and waveguide crossings are also realized using the polymer photonics platform.

CdTe microwires as mid-infrared optical waveguides.

Cadmium telluride (CdTe) has been proven to be an attractive mid-infrared (MIR) material with a large refractive index (~2.68 at 4.5 µm) and broadband transparency (~1 to 25 µm). CdTe microwires (MWs) with diameters from a few to about ten micrometers were fabricated by a thermal evaporation process. MIR light was coupled into and guided through individual MWs. Excellent optical waveguiding properties of these MWs are experimentally obtained within MIR spectral range (up to 8.6 µm), with waveguiding losses from 1.3 to 13 dB/cm. Our results show that CdTe MWs can be used as wavelength or subwavelength-width waveguides for MIR microphotonics or circuits.

2D Materials for Optical Modulation: Challenges and Opportunities.

Owing to their atomic layer thickness, strong light-material interaction, high nonlinearity, broadband optical response, fast relaxation, controllable optoelectronic properties, and high compatibility with other photonic structures, 2D materials, including graphene, transition metal dichalcogenides and black phosphorus, have been attracting increasing attention for photonic applications. By tuning the carrier density via electrical or optical means that modifies their physical properties (e.g., Fermi level or nonlinear absorption), optical response of the 2D materials can be instantly changed, making them versatile nanostructures for optical modulation. Here, up-to-date 2D material-based optical modulation in three categories is reviewed: free-space, fiber-based, and on-chip configurations. By analysing cons and pros of different modulation approaches from material and mechanism aspects, the challenges faced by using these materials for device applications are presented. In addition, thermal effects (e.g., laser induced damage) in 2D materials, which are critical to practical applications, are also discussed. Finally, the outlook for future opportunities of these 2D materials for optical modulation is given.

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.

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.

Hybrid photon-plasmon nanowire lasers.

Metallic and plasmonic nanolasers have attracted growing interest recently. Plasmonic lasers demonstrated so far operate in hybrid photon-plasmon modes in transverse dimensions, rendering it impossible to separate photonic from plasmonic components. Thus only the far-field photonic component can be measured and utilized directly. But spatially separated plasmon modes are highly desired for applications including high-efficiency coupling of single-photon emitters and ultrasensitivity optical sensing. Here, we report a nanowire (NW) laser that offers subdiffraction-limited beam size and spatially separated plasmon cavity modes. By near-field coupling a high-gain CdSe NW and a 100 nm diameter Ag NW, we demonstrate a hybrid photon-plasmon laser operating at 723 nm wavelength at room temperature, with a plasmon mode area of 0.008λ(2). This device simultaneously provides spatially separated photonic far-field output and highly localized coherent plasmon modes, which may open up new avenues in the fields of integrated nanophotonic circuits, biosensing, and quantum information processing.