High-temperature and stress response behavior of femtosecond laser pulses inscribed eccentric fiber Bragg gratings.

Eccentric fiber Bragg grating (EFBG) is inscribed in standard communication single-mode fiber using femtosecond laser pulses, and the temperature and strain sensing characteristics are experimentally demonstrated and analyzed. The EFBG exhibits strong thermal stability and good robustness in high-temperature measurement up to 1000 °C, and undergoes different thermal sensitivities during Bragg peak and the strong resonance coupled cladding spectral comb. The temperature sensitivity linearly increases with respect to the effective index of the resonant modes. Such a situation also occurs in axial strain measurement. These characteristics are of high interest for multiparametric sensing at high temperatures.

A High Precision and Multifunctional Electro-Optical Conversion Efficiency Measurement System for Metamaterial-Based Thermal Emitters.

In this study, a multifunctional high-vacuum system was established to measure the electro-optical conversion efficiency of metamaterial-based thermal emitters with built-in heaters. The system is composed of an environmental control module, an electro-optical conversion measurement module, and a system control module. The system can provide air, argon, high vacuum, and other conventional testing environments, combined with humidity control. The test chamber and sample holder are carefully designed to minimize heat transfer through thermal conduction and convection. The optical power measurements are realized using the combination of a water-cooled KBr flange, an integrating sphere, and thermopile detectors. This structure is very stable and can detect light emission at the µW level. The system can synchronously detect the heating voltage, heating current, optical power, sample temperatures (both top and bottom), ambient pressure, humidity, and other environmental parameters. The comprehensive parameter detection capability enables the system to monitor subtle sample changes and perform failure mechanism analysis with the aid of offline material analysis using scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-ray diffraction. Furthermore, the system can be used for fatigue and high-low temperature impact tests.

Terahertz Sensor via Ultralow-Loss Dispersion-Flattened Polymer Optical Fiber: Design and Analysis.

A novel cyclic olefin copolymer (COC)-based polymer optical fiber (POF) with a rectangular porous core is designed for terahertz (THz) sensing by the finite element method. The numerical simulations showed an ultrahigh relative sensitivity of 89.73% of the x-polarization mode at a frequency of 1.2 THz and under optimum design conditions. In addition to this, they showed an ultralow confinement loss of 2.18 × 10-12 cm-1, a high birefringence of 1.91 × 10-3, a numerical aperture of 0.33, and an effective mode area of 1.65 × 105 µm2 was obtained for optimum design conditions. Moreover, the range dispersion variation was within 0.7 ± 0.41 ps/THz/cm, with the frequency range of 1.0-1.4 THz. Compared with the traditional sensor, the late-model sensor will have application value in THz sensing and communication.

Tailoring topological edge states with photonic crystal nanobeam cavities.

The realization of topological edge states (TESs) in photonic systems has provided unprecedented opportunities for manipulating light in novel manners. The Su-Schrieffer-Heeger (SSH) model has recently gained significant attention and has been exploited in a wide range of photonic platforms to create TESs. We develop a photonic topological insulator strategy based on SSH photonic crystal nanobeam cavities. In contrast to the conventional photonic SSH schemes which are based on alternately tuned coupling strength in one-dimensional lattice, our proposal provides higher flexibility and allows tailoring TESs by manipulating mode coupling in a two-dimensional manner. We reveal that the proposed hole-array based nanobeams in a dielectric membrane can selectively tailor single or double TESs in the telecommunication region by controlling the coupling strength of the adjacent SSH nanobeams in both transverse and axial directions. Our finding provides an additional degree of freedom in exploiting the SSH model for integrated topological photonic devices and functionalities based on the well-established photonic crystal nanobeam cavity platforms.

Design and optical characterization of an efficient polarized organic light emitting diode based on refractive index modulation in the emitting layer.

Luminescent liquid Crystal (LC) material is regarded as the most promising material for polarized organic light emission due to their intrinsic characteristics including orderly alignment and luminescence. Nevertheless, the optical extraction efficiency of LC based organic light emitting diodes (OLEDs) devices still requires significant effort and innovation towards real-world applications. In this paper, we propose the design of a highly linearly polarized light-emission from OLEDs with integrated refractive index nanograting in the emissive layer (EML) based on photo aligned luminescent liquid crystal material. The simulation results indicate that the geometrically optimized polarized device yields an external quantum efficiency (EQE) up to 47% with a polarized ratio up to 28 dB at a 550 nm emission wavelength. This conceptual design offers a new opportunity to achieve efficient polarized organic luminescence, and it is (to the best of our knowledge) the first approach that enhances the light extraction of OLEDs based on luminescent liquid crystal via index grating in the EML.

Highly efficient and broadband mid-infrared metamaterial thermal emitter for optical gas sensing.

Development of a novel, cost-effective, and highly efficient mid-infrared light source has been identified as a major scientific and technological goal within the area of optical gas sensing. We have proposed and investigated a mid-infrared metamaterial thermal emitter based on micro-structured chromium thin film. The results demonstrate that the proposed thermal light source supports broadband and wide angular absorption of both TE- and TM-polarized light, giving rise to broadband thermal radiation with averaged emissivity of ∼0.94 in a mid-infrared atmospheric window of 8-14 µm. The proposed microphotonic concept provides a promising alternative mid-infrared source and paves the way towards novel optical gas sensing platforms for many applications.

Strong plasmonic confinement and optical force in phosphorene pairs.

The plasmonic responses in the spatially separated phosphorene (single-layer black phosphorus) pairs are investigated, mainly containing the field enhancement, light confinement, and optical force. It is found that the strong anisotropic dispersion of black phosphorus gives rise to the direction-dependent symmetric and anti-symmetric plasmonic modes. Our results demonstrate that the symmetrical modes possess stronger field enhancement, higher light confinement, and larger optical force than the anti-symmetric modes in the nanoscale structures. Especially, the light confinement ratio and optical force for the symmetric mode along the armchair direction of black phosphorus can reach as high as >90% and >3000 pN/mW, respectively. These results may open a new door for the light manipulation at nanoscale and the design of black phosphorus based photonic devices.

Optical sensors using chaotic correlation fiber loop ring down.

We have proposed a novel optical sensor scheme based on chaotic correlation fiber loop ring down (CCFLRD). In contrast to the well-known FLRD spectroscopy, where pulsed laser is injected to fiber loop and ring down time is measured, the proposed CCFLRD uses a chaotic laser to drive a fiber loop and measures autocorrelation coefficient ring down time of chaotic laser. The fundamental difference enables us to avoid using long fiber loop as required in pulsed FLRD, and thus generates higher sensitivity. A strain sensor has been developed to validate the CCFLRD concept. Theoretical and experiment results demonstrate that the proposed method is able to enhance sensitivity by more than two orders of magnitude comparing to the existing FLRD method. We believe the proposed method could find great potential applications for chemical, medical, and physical sensing.

Spoof four-wave mixing for all-optical wavelength conversion.

We present for the first time an all-optical wavelength conversion (AOWC) scheme supporting modulation format independency without requiring phase matching. The new scheme is named "spoof" four wave mixing (SFWM) and in contrast to the well-known FWM theory, where the induced dynamic refractive index grating modulates photons to create a wave at a new frequency, the SFWM is different in that the dynamic refractive index grating is generated in a nonlinear Bragg Grating (BG) to excite additional reflective peaks at either side of the original BG bandgap in reflection spectrum. This fundamental difference enable the SFWM to avoid the intrinsic shortcoming of stringent phase matching required in the conventional FWM, and allows AOWC with modulation format transparency and ultrabroad conversion range, which may have great potential applications for next generation of all-optical networks.

Numerical investigation of an all-optical switch in a graded nonlinear plasmonic grating.

We have proposed and numerically investigated an all-optical switch based on a metal-insulator-metal waveguide with graded nonlinear plasmonic gratings. The influences of grating depth and refractive index of a Kerr nonlinear medium on the transmission of the switch are exactly analyzed by utilizing transmission line theory. The finite-difference time-domain simulation results show that the highly compact structure possesses excellent switch function by tuning the incident electric field intensity. In addition, the simulation results show that this all-optical switch has an ultrawide operating frequency regime and femtosecond-scale response time (~130 fs). Such a switch can find potential applications for all-optical signal processing and optical communication.

Perfect absorber supported by optical Tamm states in plasmonic waveguide.

Based on a two-dimensional plasmonic metal-dielectric-metal (MDM) waveguide with a thin metallic layer and a dielectric photonic crystal in the core, a novel absorber at visual and near-infrared frequencies is presented. The absorber spectra and filed distributions are investigated by the transfer-matrix-method and the finite-difference time-domain method. Numerical results show that attributing to excitation of the optical Tamm states in the MDM waveguide core, the optical wave is trapped in the proposed structure without reflection and transmission, leading to perfect absorption as high as 0.991. The proposed absorber can find useful application in all-optical integrated photonic circuits.

Optical bistability in metal-insulator-metal plasmonic waveguide with nanodisk resonator containing Kerr nonlinear medium.

We numerically investigate the optical bistability effect in the metal-insulator-metal waveguide with a nanodisk resonator containing a Kerr nonlinear medium. It is found that the increase of the refractive index, which is induced by enhancing the incident intensity, can cause a redshift for the resonance wavelength. The local resonant field excited in the nanodisk cavity can significantly increase the Kerr nonlinear effect. In addition, an obvious bistability loop is observed in the proposed structure. This nonlinear structure can find important applications for all-optical switching in highly integrated optical circuits.

Induced transparency in nanoscale plasmonic resonator systems.

An optical effect analogous to electromagnetically induced transparency (EIT) is observed in nanoscale plasmonic resonator systems. The system consists of a slot cavity as well as plasmonic bus and resonant waveguides, where the phase-matching condition of the resonant waveguide is tunable for the generation of an obvious EIT-like coupled resonator-induced transparency effect. A dynamic theory is utilized to exactly analyze the influence of physical parameters on transmission characteristics. The transparency effect induced by coupled resonance may have potential applications for nanoscale optical switching, nanolaser, and slow-light devices in highly integrated optical circuits.

Enhancement of transmission efficiency of nanoplasmonic wavelength demultiplexer based on channel drop filters and reflection nanocavities.

We propose and numerically investigate a novel kind of nanoscale plasmonic wavelength demultiplexing (WDM) structure based on channel drop filters in metal-insulator-metal waveguide with reflection nanocavities. By using finite-difference time-domain simulations, it is found that the transmission efficiency of the channel drop filter can be significantly enhanced by selecting the proper distance between the drop and reflection cavities. The result can be exactly analyzed by the temporal coupled-mode theory. According to this principle, a nanoscale triple-wavelength demultiplexer with high drop efficiencies is designed. The proposed structure can find more applications for the ultra-compact WDM systems in highly integrated optical circuits.

Multiple responses of TPP-assisted near-perfect absorption in metal/Fibonacci quasiperiodic photonic crystal.

Absorption properties in one-dimensional quasiperiodic photonic crystal composed of a thin metallic layer and dielectric Fibonacci multilayers are investigated. It is found that a large number of photonic stopbands can occur at the dielectric Fibonacci multilayers. Tamm plasmon polaritons (TPPs) with the frequencies locating at each photonic stopband are excited at the interface between the metallic layer and the dielectric layer, leading to almost perfect absorption for the energy of incident wave. By adjusting the length of dielectric layer with higher refractive-index or the Fibonacci order, the number of absorption peaks can be tuned effectively and enlarged significantly.

Highly flexible all-optical metamaterial absorption switching assisted by Kerr-nonlinear effect.

A three-dimensional metamaterial nanostructure for realizing all-optical absorption switching is proposed and investigated. The structure consists of dual metallic layers for allowing near-perfect absorption due to electric and magnetic resonances, and a nonlinear Kerr-dielectric layer for actively manipulating the nanostructure absorption. The finite-difference time-domain simulation results demonstrate that, by adjusting the incident optical intensity, the metamaterial absorption can be flexibly tuned from near unity to zero. The all-optical absorption switching structure can find potential applications in actively integrated photonic circuits for thermal sensing, photo detecting, and optical imaging.

Optical bistability in metal-insulator-metal plasmonic Bragg waveguides with Kerr nonlinear defects.

We numerically investigate the characteristics of the defect mode and the nonlinear effect of optical bistability in metal-insulator-metal (MIM) plasmonic Bragg grating waveguides with Kerr nonlinear defects. By means of finite-difference time-domain simulations, we find that the defect mode peak exhibits a blueshift and height-rise by enlarging the width of the defect layer, and it has a redshift and height-fall with the increase of the dielectric constant of defect layer. Obvious optical bistability is obtained in our waveguides with a length of less than 2 µm. The results show that our structure could be applied to the design of all-optical switching in highly integrated optical circuits.

Observations of four types of pulses in a fiber laser with large net-normal dispersion.

Four different types of pulses are experimentally obtained in one erbium-doped all-fiber laser with large net-normal dispersion. The proposed laser can deliver the rectangular-spectrum (RS), Gaussian-spectrum (GS), broadband-spectrum (BS), and noise-like pulses by appropriately adjusting the polarization states. These kinds of pulses have distinctly different characteristics. The RS pulses can easily be compressed to femtosecond level whereas the pulse energy is restricted by the trend of multi-pulse shaping with excessive pump. The GS and BS pulses always maintain the single-pulse operation with much higher pulse-energy and accumulate much more chirp. After launching the pulses into the photonic-crystal fiber, the supercontinuum can be generated with the bandwidth of >700 nm by the BS pulses and of ~400 nm by the GS pulses, whereas it can hardly be generated by the RS pulses. The physical mechanisms behind the continuum generation are qualitatively investigated relating to different operating regimes. This work could help to a deeper insight of the normal-dispersion pulses.

Ultrafast all-optical switching in nanoplasmonic waveguide with Kerr nonlinear resonator.

A novel ultrafast all-optical switching based on metal-insulator-metal nanoplasmonic waveguide with a Kerr nonlinear resonator is proposed and investigated numerically. With the finite-difference time-domain simulations, it is demonstrated that an obvious optical bistability of the signal light appears by varying the control-light intensity, and an excellent switching effect is achieved. This bistability originates from the intensity-dependent change induced in the dielectric constant of Kerr nonlinear material filled in the nanodisk resonator. It is found that the proposed all-optical switching exhibits femtosecond-scale feedback time.

Tunable band-pass plasmonic waveguide filters with nanodisk resonators.

A novel and simple plasmonic filter based on metal-insulator-metal plasmonic waveguides with a nanodisk resonator is proposed and investigated numerically. By the resonant theory of disk-shaped nanocavity, we find that the resonance wavelengths can be easily manipulated by adjusting the radius and refractive index of the nanocavity, which is in good agreement with the results obtained by finite-difference time-domain (FDTD) simulations. In addition, the bandwidths of resonance spectra are tunable by changing the coupling distance between the nanocavity and waveguides. This result achieved by FDTD simulations can be accurately analyzed by temporal coupled mode theory. Our filters have important potential applications in high-density plasmonic integration circuits.