Layered two-dimensional materials have shown novel optoelectronic properties and are well suited to be integrated in planar photonic circuits [1][2][3] . For example, graphene has been utilized for wideband photodetection [4][5][6][7] . Because graphene lacks a band gap, however, graphene photodetectors suffer from very high dark current 6,8 . In contrast, layered black phosphorous, the latest addition to the family of 2D materials [9][10][11] , is well-suited for photodetector applications due to its narrow but finite band gap [12][13][14][15][16][17] . Here, we demonstrate a gated multilayer black phosphorus photodetector integrated on a silicon photonic waveguide operating in the near-infrared telecom band. In a significant advantage over graphene devices, black phosphorus photodetectors can operate under a bias with very low dark current and attain intrinsic responsivity up to 135 mA/W and 657 mA/W in 11.5nm and 100 nm thick devices, respectively, at room temperature. The photocurrent is dominated by the photovoltaic effect with a high response bandwidth exceeding 3 GHz. Two-dimensional (2D) materials have tremendous potential for optoelectronic applications [1][2][3] . Graphene, the most extensively investigated 2D material, has many novel optical properties such as a tunable inter-band transition and saturable absorption, and has potential for a wide range of optoelectronic applications 1,2 . However, one important optoelectronic device application where graphene is severely limited in is photodetection. Although graphene has shown broadband optical absorption, ultrafast photoresponse and reasonable responsivity, graphene photodetectors have very high dark current when they are operated in photoconductive mode

Scanning probe microscopies (SPM) and cantilever-based sensors generally use low-frequency mechanical devices of microscale dimensions or larger. Almost universally, off-chip methods are used to sense displacement in these devices, but this approach is not suitable for nanoscale devices. Nanoscale mechanical sensors offer a greatly enhanced performance that is unattainable with microscale devices. Here we describe the fabrication and operation of self-sensing nanocantilevers with fundamental mechanical resonances up to very high frequencies (VHF). These devices use integrated electronic displacement transducers based on piezoresistive thin metal films, permitting straightforward and optimal nanodevice readout. This non-optical transduction enables applications requiring previously inaccessible sensitivity and bandwidth, such as fast SPM and VHF force sensing. Detection of 127 MHz cantilever vibrations is demonstrated with a thermomechanical-noise-limited displacement sensitivity of 39 fm Hz(-1/2). Our smallest devices, with dimensions approaching the mean free path at atmospheric pressure, maintain high resonance quality factors in ambient conditions. This enables chemisorption measurements in air at room temperature, with unprecedented mass resolution less than 1 attogram (10(-18) g).

The force exerted by photons is of fundamental importance in light-matter interactions. For example, in free space, optical tweezers have been widely used to manipulate atoms and microscale dielectric particles. This optical force is expected to be greatly enhanced in integrated photonic circuits in which light is highly concentrated at the nanoscale. Harnessing the optical force on a semiconductor chip will allow solid state devices, such as electromechanical systems, to operate under new physical principles. Indeed, recent experiments have elucidated the radiation forces of light in high-finesse optical microcavities, but the large footprint of these devices ultimately prevents scaling down to nanoscale dimensions. Recent theoretical work has predicted that a transverse optical force can be generated and used directly for electromechanical actuation without the need for a high-finesse cavity. However, on-chip exploitation of this force has been a significant challenge, primarily owing to the lack of efficient nanoscale mechanical transducers in the photonics domain. Here we report the direct detection and exploitation of transverse optical forces in an integrated silicon photonic circuit through an embedded nanomechanical resonator. The nanomechanical device, a free-standing waveguide, is driven by the optical force and read out through evanescent coupling of the guided light to the dielectric substrate. This new optical force enables all-optical operation of nanomechanical systems on a CMOS (complementary metal-oxide-semiconductor)-compatible platform, with substantial bandwidth and design flexibility compared to conventional electrical-based schemes.

Ultrafast, high-efficiency single-photon detectors are among the most sought-after elements in modern quantum optics and quantum communication. However, imperfect modal matching and finite photon absorption rates have usually limited their maximum attainable detection efficiency. Here we demonstrate superconducting nanowire detectors atop nanophotonic waveguides, which enable a drastic increase of the absorption length for incoming photons. This allows us to achieve high on-chip single-photon detection efficiency up to 91% at telecom wavelengths, repeatable across several fabricated chips. We also observe remarkably low dark count rates without significant compromise of the on-chip detection efficiency. The detectors are fully embedded in scalable silicon photonic circuits and provide ultrashort timing jitter of 18 ps. Exploiting this high temporal resolution, we demonstrate ballistic photon transport in silicon ring resonators. Our direct implementation of a high-performance single-photon detector on chip overcomes a major barrier in integrated quantum photonics.

The optical binding forces between guided lightwaves in dielectric waveguides can be either repulsive or attractive. So far only attractive force has been observed. Here we experimentally demonstrate a bipolar optical force between coupled nanomechanical waveguides. Both attractive and repulsive optical forces are obtained. The sign of the force can be switched reversibly by tuning the relative phase of the interacting lightwaves. This tunable, bipolar interaction forms the foundation for the operation of a new class of light force devices and circuits.Comment: 4 figure

Black phosphorus has been the subject of growing interest due to its unique band structure that is both layer dependent and anisotropic. While many have studied the linear optical response of black phosphorus, the nonlinear response has remained relatively unexplored. Here we report on the observation of third-harmonic generation in black phosphorus using an ultrafast near-IR laser and measure χ (3) experimentally for the first time. It was found that the third-harmonic emission is highly anisotropic, dependent on the incident polarization, and varies strongly with the number of layers present due to signal depletion and phase-matching conditions. Black phosphorus (BP) recently emerged as a new two dimensional material with highly unique optical and electrical properties, including high carrier mobility, 1,2 optical and electrical anisotropy, 3-5 and importantly, a tunable, direct bandgap. 6-9 Particularly, for thicknesses greater than a few nanometers, BP's bandgap bridges the technically important mid-infrared (mid-IR) spectral range that is not currently covered by other two-dimensional semiconductors. 4 These attributes of BP make it very promising for optoelectronic devices that cover a broad spectral range from near-to mid-IR for both communication and optical sensing. 10,11 In addition to linear optical properties, the quantum confinement in 2D materials also leads to many novel nonlinear optical properties. For example, MoS2, 12-14 WSe2, 15,16 and hBN,13 all with non-centrosymmetric lattice structures, have shown strong second-order nonlinear optical effects, such as second-harmonic generation (SHG). SHG in these materials has shown strong enhancement at the exciton resonances, 15 can be electrically tuned by a local back gate, 16 and has been utilized for optically probing the crystal orientation 12 and thickness. 13,14 Additionally, third-order optical nonlinearity such as third-harmonic generation (THG) has been observed to be strong in graphene, 17,18 as well as in MoS2 thin films. 19 In terms of nonlinear optics in BP, its centrosymmetric crystalline structure only permits third-order nonlinearity but its strong anisotropy and layer dependent band structure should lead to very intriguing nonlinear optical effects. Research to date, however, has been primarily limited to the saturable absorption effect in BP, studied with z-scan 20,21 and ultrafast pump-probe 22 techniques for liquid exfoliated BP suspensions and utilized for application in mode-locked lasers. [23][24][25][26][27] The intrinsic optical nonlinearity of crystalline BP has not been experimentally investigated.In this paper, we investigate BP's optical nonlinearity by measuring both the polarization and thickness dependence of THG in multilayer BP samples. We find that the THG in BP is

For optical communication, information is converted between optical and electrical signaldomains at a high rate. The devices to achieve such a conversion are various types of electro-optical modulators and photodetectors. These two types of optoelectronic devices, equally important, require different materials and consequently it has been challenging to realize both using a single material combination, especially in a way that can be integrated on the ubiquitous silicon platform. Graphene, with its gapless band structure, stands out as a unique optoelectronic material that allows both photodetection and optical modulation.Here, we demonstrate a single graphene-based device that simultaneously provides both efficient optical modulation and photodetection. The graphene device is integrated on a silicon waveguide and is tunable with a gate made from another layer of graphene to achieve near-infrared photodetection responsivity of 57 mA/W and modulation depth of 64%. This novel multifunctional device may lead to many unprecedented optoelectronic applications.

We demonstrate spectrally tuned dispersive and reactive optical force in a cavity optomechanics system that comprises a microdisk and a vibrating nanomechanical beam waveguide. The waveguide coupled to the microdisk acts as a bosonic dissipation channel and its motion modulates the cavity's damping rate. As a result a reactive optical force arises in addition to the normal dispersive force. The cavity enhanced force is not observed at zero detuning but shows asymmetric behavior with a maximum at red-detuned offset. Such reactive cavity backaction force points to new avenues in cavity optomechanics.