Jin-hui Chen

Nonlinear frequency conversion of fields with orbital angular momentum using quasi-phase-matching

We propose and investigate the quasi-phase matched (QPM) nonlinear optical frequency conversion of optical vortices in periodically poled Lithium Niobate (PPLN). Laguerre-Gaussian (LG) modes are used to represent the orbital angular momentum (OAM) states, characterized with the azimuthal and radial indices. Typical three-wave nonlinear interactions among the involved OAM modes are studied with the help of coupling wave equations. Being different from normal QPM process where the energy and quasi-momentum conservations are satisfied, both of the azimuthal and radial indices of the OAM states keep constant in most of the cases. However, abnormal change of the radial index is observed when there is asynchronous nonlinear conversion in different parts of the beams. The QPM nonlinear evolution of fractional OAM states is also discussed showing some interesting properties. In comparison with the traditional birefringent phase matching (BPM), the QPM technique avoids the undesired walk-off effect to reserve high-quality LG modes. We believe the QPM is an efficient way to convert, amplify and switch OAM states in various optical vortex related applications.

Tunable Fano resonance in hybrid graphene-metal gratings

Hybrid graphene-metal gratings with tunable Fano resonance are proposed and theoretically investigated in THz band. The grating contains alternately aligned metal and graphene stripes, which could be viewed as the superposition of two kinds of gratings with the same period. Due to different material properties, the resonance coupling between the metal and graphene parts forms typical Fano-type transmitting spectra. The related physical mechanism is studied by inspecting the induced dipole moment and local surface charge distributions at different wavelengths. Both of the resonance amplitude and frequency of the structure thus are adjustable by tuning graphenes Fermi energy and the gratings geometrical parameters. Furthermore, the Fano-type spectra are also quite sensitive to environmental indices, which supply another kind of tunability. All these features should have promising applications in tunable THz filters, switches, and modulators.

Optical electrical current sensor utilizing a graphene-microfiber-integrated coil resonator

A graphene-based electrical current sensor is proposed utilizing a microfiber coil resonator. Monolayer graphene sheet with a large sheet resistance is transferred onto the surface of a glass capillary rod. A microfiber is spirally wrapped around the graphene sheet to form a coil resonator. Heat generated from electrical current shifts the resonant wavelength because of the thermal effect in the microfiber resonator. The sensor exhibits a very good performance with a high sensitivity of 67.297 μm/A2, which is two orders of magnitude higher than that reported earlier. Our results show that microfiber-graphene-integrated devices have great potential for miniature and highly sensitive fiber sensors for monitoring electrical current.

Platform for enhanced light–graphene interaction length and miniaturizing fiber stereo devices

Sufficient light–matter interactions are important for waveguide-coupled graphene optoelectronic devices. Subwavelength-diameter microfibers (MFs) with a strong evanescent field are attractive for graphene integration in fiber optics system, which can be realized by covering or wrapping a graphene sheet on a straight and thin MF. However, it is challenging to handle such a thin MF and graphene for sufficient length and strength of interaction. Using an MF-based lab-on-a-rod technique, we present a platform for ultralong light–graphene interaction and design graphene-integrated helical MF devices. Using this approach, we experimentally demonstrate polarization manipulation by wrapping an MF on a rod pretreated with a graphene sheet. The device can operate as not only a broadband polarizer but also a high-Q single-polarization resonator by tuning the geometry of MF coils. By specializing the rod surface and coil geometry, we believe the platform could contribute to advancing the research for more graphene–MF-integrated devices including modulators and photodetectors.

Microfiber-coupler-assisted control of wavelength tuning for Q-switched fiber laser with few-layer molybdenum disulfide nanoplates

Based on the liquid exfoliated method, we obtained the few-layer molybdenum disulfide (MoS2) nanoplates solution. By thermal evaporation method, we directly deposited MoS2 thin film onto the facet of a fiber patch cord. The modulation depth of the film is as high as 29%, and a Q-switched fiber laser was achieved. We also provided a new method to continuously tune the output laser with a tuning sensitivity of ∼5.5  nm/(1%  strain) by controlling the cavity loss with a strained microfiber coupler (MFC).

A Fiber Laser Using Graphene-Integrated 3-D Microfiber Coil

We demonstrate a state variable fiber pulse laser based on a graphene-integrated microfiber device. A 3-D microfiber coil integrated with graphene acts as a polarization-sensitive saturable absorber. By adjusting the polarization controllers, stable Q-switched pulses and mode-locked rectangular pulses are observed. To our knowledge, this is the first study conducted to achieve switching between rectangular pulses and Q-switched pulses while keeping the pump power constant. The rectangular pulses have a pulsewidth and repetition rate of ~10 ns and ~940 kHz, respectively, and the Q-switched pulses have a pulsewidth and repetition rate of ~40 μs and ~4.8 kHz, respectively.

Synthesis of Easily Transferred 2D Layered BiI3 Nanoplates for Flexible Visible-Light Photodetectors

Bismuth triiodide, BiI3, is one of the promising 2D layered materials from the family of metal halides. The unique electronic structure and properties make it an attractive material for the room-temperature gamma/X-ray detectors, high-efficiency photovoltaic absorbers, and Bi-based organic-inorganic hybrid perovskites. Other possibilities including optoelectronic devices and optical circuits are envisioned but rarely experimentally confirmed yet. Here, we report the synthesis of vertical 2D BiI3 nanoplates using the physical vapor deposition mechanism. The obtained products were found easy to be separated and transferred to other substrates. Photodetectors employing such 2D nanoplates on polyethylene terephthalate substrate are demonstrated to be quite sensitive to red light (635 nm) with good responsivity (2.8 A W-1), fast stable photoresponse (3/9 ms for raise/decay times), and remarkable specific detectivity (1.2 × 1012 jones), which attest to high comparability of the assembled components with many latest 2D nanostructured light sensors. In addition, such photodetectors exhibit outstanding mechanical stability and durability under different bending strains within the theoretically affordable levels, suggesting a variety of potential applications of 2D BiI3 for flexible devices.

Periodic micro-structures in optical microfibers induced by Plateau-Rayleigh instability and its applications

A periodic micro-structure on optical microfibers induced by Plateau-Rayleigh instability (PRI) was investigated and a potential application for long period gratings (LPGs) fabrication was given. The linear relation between the average periods of micro-structures and the diameters of optical microfibers was demonstrated first. By brushing a glass rod with a Teflon droplet suspended at the end tip along microfibers, a continuous film of Teflon was formed at once, then the film broke up into a series of periodic droplets due to PRI. Periodic Teflon nodes were left after the evaporation of the solvent. A LPG structure based on polymer was finally formed by this method on a microfiber with a diameter of 5.5 μm. An attenuation transmission dip of 15 dB around 1447 nm was achieved. Investigation of the strain and temperature response characteristics of the grating presented a strain sensitivity of -2.5 pm/με and a temperature sensitivity of -157 pm/°C. The technique proposed here provides a versatile technique for polymer-based LPGs fabrication. Benefiting from the high sensitivities, LPGs based on numerous polymers fabricated in this way could have potential applications in optical and biological sensing.

Miniaturized stereo fiber devices based on the wrapon-a-rod technology

3D micro-devices based on subwavelength-diameter microfibres (MFs) with a strong evanescent field have attracted increasing attention because of the recent rapid development of the so-called wrap-on-a-rod technology. Because they are fabricated from a single-mode fiber with low stiffness and micrometric bending radii, MFs can be wrapped multiple times on a thin microrod (hundreds of micrometers) and do not experience any input/output coupling problems. By tuning the pitch between turns and specializing the rod surface, a variety of functions can be realized and even multiple functions can be integrated on a single device, which is the concept of the future lab-on-a-microrod. Here we will show several kinds of lab-on-a-microrod devices including a resonator, grating, Hi-Bi device, and polarizer. The applications in sensing and lasing will also be discussed.

Single-polarization microfiber and resonator for sensing applications

With a unique 3D geometry, a broadband polarizer and single-polarization resonator are demonstrated by wrapping a microfiber on a specialized rod with a hybrid polymer-metal-dielectric nanostructure, respectively. The polarizer has an extinction ratio of more than 20 dB over the spectra range of 450 nm. The resonator has a Q-factor of ~ 80,000 with excellent suppressing of polarization noise. By functionalizing the rod surface with nanoscale silver film and tuning the coil geometry, performances can be improved by optimizing those parameters. Such kind of miniature singlepolarization microfiber and resonator are impossible to be realized by conventional fabrication process, and has the potential in gyro and current sensors.