Large Kerr effect in bulk Se-based chalcogenide glasses

High-speed optical communication requires ultrafast all-optical processing and switching capabilities. The Kerr nonlinearity, an ultrafast optical nonlinearity, is often used as the basic switching mechanism. A practical, small device that can be switched with ~1-pJ energies requires a large Kerr effect with minimal losses (both linear and nonlinear). We have investigated theoretically and experimentally a number of Se-based chalcogenide glasses. We have found a number of compounds with a Kerr nonlinearity hundreds of times larger than silica, making them excellent candidates for ultrafast all-optical devices.     

Ultrafast optical switching in Kerr nonlinear photonic crystals

Nonlinear photonic crystals made from polystyrene materials that have Kerr nonlinearity can exhibit ultrafast optical switching when the samples are pumped by ultrashort optical pulses with high intensity due to the change of the refractive index of polystyrene and subsequent shift of the band gap edge or defect state resonant frequency. Polystyrene has a large Kerr nonlinear susceptibility and almost instantaneous response to pump light, making it suitable for the realization of ultrafast optical switching with a response time as short as a few femtoseconds. In this paper, we review our experimental progress on the continual improvement of all-optical switching speed in two-dimensional and three-dimensional polystyrene nonlinear photonic crystals in the past years. Several relevant issues are discussed and analyzed, including different mechanisms for all-optical switching, preparation of nonlinear photonic crystal samples by means of microfabrication and self-assembly techniques, characterization of optical switching performance by means of femtosecond pump-probe technique, and different ways to lower the pump power of optical switching to facilitate practical applications in optical information processing. Finally, a brief summary and a perspective of future work are provided.     

Ultrafast optical switching in Kerr nonlinear photonic crystals

Nonlinear photonic crystals made from polystyrene materials that have Kerr nonlinearity can exhibit ultrafast optical switching when the samples are pumped by ultrashort optical pulses with high intensity due to the change of the refractive index of polystyrene and subsequent shift of the band gap edge or defect state resonant frequency. Polystyrene has a large Kerr nonlinear susceptibility and almost instantaneous response to pump light, making it suitable for the realization of ultrafast optical switching with a response time as short as a few femtoseconds. In this paper, we review our experimental progress on the continual improvement of all-optical switching speed in two-dimensional and three-dimensional polystyrene nonlinear photonic crystals in the past years. Several relevant issues are discussed and analyzed, including different mechanisms for all-optical switching, preparation of nonlinear photonic crystal samples by means of microfabrication and self-assembly techniques, characterization of optical switching performance by means of femtosecond pump-probe technique, and different ways to lower the pump power of optical switching to facilitate practical applications in optical information processing. Finally, a brief summary and a perspective of future work are provided.     

Nonlinear All-Optical Switching Device by Using the Spatial Soliton Collision

We propose a new nonlinear all-optical switching device by using the spatial solitons collision. This is 1 × N switching device controlled by two control beams. The numerical results show that this device could really function as a 1 × N all-optical switching device. This device is a potential key component in the application of optical signal processing and optical computing systems.     

Optical bistability in a single-bus InGaAs micro-ring resonator array

In this paper, we investigate the optical bistability induced by optical nonlinearity in a compact parallel array of micro-ring resonators with radius of 5 μm. Due to the nature of perfect light confinement in this structure, resonance and accumulation process in a ring resonator and optical nonlinear effects, are observable even at small optical power (a few milliwatts). Different optical applications such as all-optical switching, optical memory devices, logic gates and modulators are possible, due to optical bistability in a ring resonator. Using alternative semiconductor compounds, instead of silicon that have weak nonlinear optical properties, we improve the performance of ring-resonator based devices. Here we use a polymer cladding layer with negative thermo-optic coefficient. Using this structure we can eliminate the temperature created nonlinearity which is a very slow process. Therefore, the switching speed increases from a few MHz to several tens of GHz. By plotting the transfer function of the resonator, a hysteresis loop is observed in a few milliwatts. Although, using a ring resonator array the bandwidth reduces, however, the width of the hysteresis loop and resolution between both steady state increases.     

Nonlinear All-Optical Switching Device by Using the Spatial Soliton Collision

We propose a new nonlinear all-optical switching device by using the spatial solitons collision. This is 1 × N switching device controlled by two control beams. The numerical results show that this device could really function as a 1 × N all-optical switching device. This device is a potential key component in the application of optical signal processing and optical computing systems.     

Phase- and polarization-insensitive all-optical switching by self-guiding in quadratic media

We demonstrate numerically that a frequency-degenerate process through typeII downconversion in a quadratic nonlinear waveguide leads to the formation of spatial simultons and all-optical switching in the presence of a control signal. This novel effect is phase independent, generally insensitive to polarization, and weakly affected by the magnitude of the control input. Multiple solitary waves can also be obtained, depending on the details of the interaction.     

Nonlinear dual-core photonic crystal fiber couplers

We study nonlinear modes of dual-core photonic crystal fiber couplers made of a material with the focusing Kerr nonlinearity. We find numerically the profiles of symmetric, antisymmetric, and asymmetric nonlinear modes and analyze all-optical switching generated by the instability of the symmetric mode. We also describe elliptic spatial solitons controlled by the waveguide boundaries.     

Highly nonlinear chalcogenide fibres for all-optical signal processing

Chalcogenide glasses offer many attractive properties for all-optical signal processing including large Kerr nonlinearity (up to 500 × silica glass), an intrinsic ultrafast response time and low to moderate two-photon absorption (TPA). These properties together with the convenience of a fibre format allow us to achieve all-optical signal processing at low peak power and in a very compact form. In this paper, several nonlinear processing functions will be demonstrated including: femto-second pedestal free, pulse compression; all-optic wavelength conversion; and all-optical regenerator. In addition, we show enhanced nonlinearity for more efficient signal processing by tapering the As₂Se₃ fibre. These applications show chalcogenide glass fibres are very promising candidate materials for nonlinear all-optic signal processing.     

Tunable interaction-free all-optical switching in a five-level atom-cavity system

A scheme is proposed for tunable all-optical switching based on the double-dark states in a five-level atom-cavity system. In the scheme, the output signal light of the reflection and the transmission channels can be switched on or off by manipulating the control field. When the control light is coupled to the atom-cavity system, the input signal light is reflected by the cavity. Thus, there is no direct coupling between the control light and the signal light. Furthermore, the position of the double-dark states can be changed by adjusting the coherent field, and,thus, the switching in our scheme is tunable. By presenting the numerical calculations of the switching efficiency,we show that this type of the interaction-free all-optical switching can be realized with high switching efficiency.     

An all-optical switch of Mach－Zehnder interferometer type using an active fibre ring resonator

We propose an all-optical switch of the Mach－Zehnder interferometer type using an active nonlinear ring resonator and analyse the significance of the parameter A, a product of gain and total loss, for performing an ideal 1 by 2 switch. We found that in the range of 1-κ≤A≤\sqrt{1-κ}, the increment of A can compensate the losses inside the ring, therefore increase the finesse of the ring and enhance the nonlinearity contribution to reduce the switching power threshold effectively. We also emphasize the importance of the initial switching point and discuss the feasibility of utilizing a high-nonlinear fibre in the ring.     

Nonlinear optical switching and all-figures of merit in Bi_2S_(3-x)Se_x/PMMA nanocomposite films investigated by Z scan under visible CW laser

Bi2S3-xSex/poly(methyl methacrylate)(PMMA)nanocomposite films were prepared using microwave assisted synthesis with different compositions of x.Crystal structure,surface morphology,and optical properties were investigated to characterize the prepared nanocomposite films.The crystallinity and optical band gap of the prepared Bi2S3-xSex/PMMA were affected by x.The prepared samples showed a blue shift in the absorption edge.The laser power dependent nonlinear refraction and absorption of Bi2S3-xSex/PMMA nanocomposite films were investigated by using the Z-scan technique.The optical nonlinearity of the nanocomposite films exhibited switchover from negative nonlinear refraction to positive nonlinear refraction to negative nonlinear refraction effects,and from saturable absorption to reverse saturable absorption to saturable absorption with an increase and decrease in the composition.An interesting all-optical figure of merit was reported to assess the nanocomposite films for a practical device.It was calculated that the device all-figures of merit were based on the nonlinear response,which is important for the all-optical switching device.The results demonstrate that the optimized all-optical figures of merit can be achieved by adjusting the composition and input laser power,which can be used for the design of different all-photonic devices,and the results of nonlinear switching behavior can open new possibilities for using the nanocomposite films in laser Q-switching and mode-locking.     

C60分子瞬态全光开关

本文以YAG脉冲激光为控制光,He-Ne连续激光为信号光,在C_(60)甲苯溶液中首次实现瞬态全光开关.由光开关时间推断出C_(60)分子三重态的寿命.     

Switching behavior of a nonlinear Mach-Zehnder interferometer: Saturating nonlinearity

This paper uses the Beam Propagation Method to investigate numerically the switching behavior of a Nonlinear Mach-Zehnder Interferometer (NMZI). A saturating-type nonlinearity has been considered for the present investigations. It is shown that the input versus output characteristics change drastically when a Kerr type nonlinear medium is replaced by a saturating type nonlinear medium. In contrast to an NMZI with Kerr nonlinearity, where only quantitative behavior changes with NMZI length, quantitative as well as qualitative behaviors change in the case of a saturating nonlinearity. We propose an all-optical stabilizer and MZI with stable “ON” and “OFF” states on the basis of our investigation.     

Demonstration of an all-optical switching operation using an optical fiber grating coupler

An optical fiber grating coupler (FGC) is a fused optical fiber coupler with a tapered region in which refractive index-modulated gratings are written. In the FGC, the light with specific wavelength satisfying the Bragg condition of the grating can be dropped to one output port and other lights are transmitted to another output port when lights with various wavelengths are launched into the input port. The FGC can operate as an all-optical switch by controlling the Bragg wavelength of the grating using a third order nonlinear optical effect caused by a control light that are launched with a signal light. In this paper, an all-optical switching operation due to a third order nonlinear optical effect in an FGC is first demonstrated for a signal light with 1.55 μm-wavelength to be changed from one port of the FGC to another one by the 720 W peak of a control light from a Nd:YAG laser with 1.06 μm-wavelength. The switching efficiency obtained was 7%. It was clarified that a longer pulse length of the control light compared to the grating length is required to obtain a large Bragg wavelength shift for the switching. It was also clarified that the Bragg wavelength shift is caused by a third order nonlinear effect and a photothermal effect. A contribution of the photothermal effect was estimated. We also estimated the switching efficiency for pump power in the FGC switch.     

Nonlinear optics on the nanoscale

We review the concept of achieving a large optical nonlinearity via a light-induced phase transformation in a confined solid. Nanoscale films and nanoparticles of gallium are examined as prime examples of materials displaying this type of optically broadband, fast and bright nonlinear response, which show promise for future all-optical switching applications.     

All-optical steering of the interactions between multiple spatial solitons in isotropic polymers

All-optical steering of the nonlinear interactions between multiple spatial solitons can be performed in an isotropic photoisomerization polymer, by propagating an external control beam in perpendicular direction. Fusing, giving birth to another new soliton, and transferring energy can take place in the interactions of signal beams, which can be achieved by changing the incident position of the control beam, the initial relative phase and the power ratio between the signal beams and the control beam. These phenomena are physically explained, and they have significantly potential applications in optical signal readdressing, logic gating, and all-optical switching, etc.     

Dynamical switching and memory via incoherent pump assisted optical bistability

We analyze the dynamical behavior of all-optical switching and memory based on tunable optical bistability in a three-level Λ-type atomic system driven by a probe field circulating in an optical ring cavity, a coherent coupling field and an incoherent pump field outside cavity. Owing to the incoherent pump process, the absorption and Kerr nonlinearity near the atomic resonance change dramatically as a result of varied population distribution and atomic coherence, and then the switch-up and switch-down thresholds, as well as the width (or area) of bistable curve, can be manipulated effectively. By tuning the intensity of either coherent coupling field or incoherent pump field, we can make the system output flip between the lower and upper branches of different bistable hysteresis loops while the cavity input keeps to a constant value. Accordingly, all-optical switching and memory can be implemented via dynamical control of the bistable behavior under the assistance of incoherent pump. The proposed scheme can find potential applications in all-optical communication and computation.     

All optical switching in henna thin film

The optical nonlinearity in henna (Lawson (2- hydroxyl-1,4 naphthoquinone) film was utilized to demonstrate all optical switching. The nonlinear absorption of the henna film was calculated by measuring the transmission of the laser beam (λ = 488 nm) as a function of incident light intensities. The observed nonlinear absorption is attributed to a two-photon absorption process. The pump and probe technique was used to demonstrate all optical switching. The switching characteristics can be utilized to generate all-optical logic gates such as simple inverter switches (NOT) NOR, AND NAND logic functions.     

Linear and nonlinear optical properties of Ag-As-Se chalcogenide glasses for all-optical switching

We prepared Ag(x)(As0.4Se0.6)(100-x) chalcogenide glasses by a melt-quenching method and measured their linear and nonlinear optical properties to evaluate their potential applications to all-optical ultrafast switching devices. Their nonlinear refraction and absorption were measured by the Z-scan method at 1.05 microm. The addition of Ag to As2Se3 glass led to an increase in the nonlinear refractive index without introducing an increase in the nonlinear absorption coefficient. The glass with a Ag content of x = 20 at. % revealed high nonlinearity ranging from 2000 to 27,000 times that of fused silica, depending on the incident optical intensity.