Enhancement of the second harmonic generation from monolayer WS2 coupled with a silica microsphere

Monolayer transition metal dichalcogenides (TMDs) are widely used for integrated optical and photoelectric devices. Owing to their broken inversion symmetry, monolayer TMDs have a large second-order optical nonlinearity. However, the optical second-order nonlinear conversion efficiency of monolayer TMDs is still limited by the interaction length. In this work, we theoretically study the second harmonic generation (SHG) from monolayer tungsten sulfide (WS₂) enhanced by a silica microsphere cavity. By tuning the position, size, and crystal orientation of the material, second-order nonlinear coupling can occur between the fundamental pump mode and different second harmonic cavity modes, and we obtain an optimal SHG conversion efficiency with orders of magnitude enhancement. Our work demonstrates that the microsphere cavity can significantly enhance SHG from monolayer 2D materials under flexible conditions.     

非线性腔中横模的时空不稳定

通过数值模拟研究了非线性腔的横向效应，非线性表现在光场通过放大介质时所引起的相位变化与强度有关，模拟结果显示，随着非线性系数的变化，腔中的光场分布表现丰富的时空不稳定行为，其中包括横模跳变，时空周期行为以及光学涡旋的出现。     

Normal Mode Splitting in a Cavity Optomechanical System with a Cubic Anharmonic Oscillator

The strong coupling between a macroscopic mechanical oscillator and a cavity field is essential for many quantum phenomena in a cavity optomechanical system. In this work, we discuss the normal mode splitting in a cavity optomechanical system with a cubic nonlinear movable mirror. We study how the mechanical nonlinearity affects the normal-mode splitting behavior of the movable mirror and the output field. We find that the mechanical nonlinearity can increase the peak separation in the spectra of the movable mirror and the output field. We also find that the heights and linewidths of the two peaks are very sensitive to the mechanical nonlinearity.

     

Intracavity weak nonlinear phase shifts with single photon driving

We investigate a doubly resonant optical cavity containing a Kerr nonlinear medium that couples two modes by a cross phase modulation. One of these modes is driven by a single photon pulsed field, and the other mode is driven by a coherent state. We find an intrinsic phase noise mechanism for the cross phase shift on the coherent beam which can be attributed to the random emission times of single photons from the cavity. An application to a weak nonlinearity phase gate is discussed.     

Enhanced Kerr nonlinearity via atomic coherence in a three-level atomic system

We measure the Kerr-nonlinear index of refraction of a three-level Lambda-type atomic system inside an optical ring cavity. The Kerr nonlinearity is modified and greatly enhanced near atomic resonant conditions for both probe and coupling beams. The Kerr nonlinear coefficient n(2) changes sign when the coupling beam frequency detuning switches sign, which can lead to interesting applications in optical devices such as all-optical switches.     

Analysis of Nonlinear TE Waves in a Periodic Refractive Index Structure with a Nonlinear Cladding

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Necessary conditions for nonlinear excitation of a high-order mode in a single-mode optical fiber

The necessary conditions for nonlinear excitation of higher-order modes in a single-mode step-index optical fiber are analyzed. The cutoff conditions for such an optical waveguide are investigated taking into account Kerr nonlinearity. The minimal power of optical pulses required for fulfilling the necessary conditions for nonlinear excitation of higher-order modes in a single-mode step-index fiber is calculated as a function of the normalized frequency. The allowed ranges of variation of the normalized frequency and optical-radiation power are estimated. It is demonstrated that the conditions necessary for nonlinear excitation of a higher-order mode in a step-index single-mode optical fiber can be created for optical pulses shorter than 500 fs.     

Branching behavior of field dynamics of resonators of left-handed materials

In this paper, we study the spatiotemporal dynamics of left-handed materials that contain nonlinear cavities. These materials, having a negative refractive index, exhibit a nonlinear electromagnetic behavior. The insertion of the left-handed material into an optical cavity results into a variety of branching behavior of optical oscillators that are degenerate. The optical Kerr cavity considered in this work contains a cubic nonlinearity. The objective of this work was to investigate how branching behavior can yield useful information about the relative amount of left-handed and right-handed material controlling the diffraction in cavity. We will apply the reductive perturbation method-based multi-equation bifurcation theory to a scalar nonlinear Schrodinger equation and show how period-doubling occurs during optical wave propagation.     

Low-threshold self-induced modulational instability ring laser in highly nonlinear fiber yielding a continuous-wave 262-GHz soliton train

Modulational instability (MI) is employed in a self-induced ring cavity configuration based on highly nonlinear dispersion-shifted fiber (HNL DSF) and an erbium-doped fiber amplifier to generate a continuous-wave 262-GHz train of 365-fs optical solitons. The laser operates around 1540 nm, with an average output power of 15 mW. MI is achieved at a low threshold as a result of low average cavity dispersion and high fiber nonlinearity. It is shown that, because of the normal dispersion of the HNL DSF, the solitons exist in the average soliton regime.     

基于高非线性微结构光纤的全光再生研究

提出了利用高非线性微结构光纤自相位调制效应进行全光再生的研究方案。分析了一组微结构光纤的色散和非线性特性。结果显示光纤的非线性系数与光纤结构有密切关系。通过减小有效模面积,可以提高光纤的非线性系数。采用一种高空气填充比的高非线性微结构光纤作为非线性介质,进行了基于自相位调制效应的全光再生研究。结果表明,由于微结构光纤的高非线性,采用较短的光纤长度就可以实现较好的再生效果。同时,输入微结构光纤的峰值功率、滤波器的参量选择对光再生的效果有重要的影响,它们必需满足一定要求,才能实现光再生。此外,对再生器的传输特性进行了研究。通过调整输入峰值功率和滤波器的参量,可以对不同宽度的光脉冲信号进行全光再生。     

Propagation characteristics of nonlinear optical fibers with complex refractive index. A transmission line model approach

The transmission line model has been modified in order to evaluate nonlinear waves guided by optical fibers with complex refractive index. It is proven that the proposed model is able to tackle fibers with losses or gain, under the presence of arbitrary nonlinearity. The method is applied for the calculation of weakly guided lossy nonlinear optical fibers considering various configurations and Kerr nonlinearity. Power depended propagation characteristics have been calculated and plotted for the fundamental mode and the potential use of the method in optical fiber amplifiers with index nonlinearity is also discussed.     

Cavity nonlinear optics at low photon numbers from collective atomic motion

We report on Kerr nonlinearity and dispersive optical bistability of a Fabry-Perot optical resonator due to the displacement of ultracold atoms trapped within. In the driven resonator, such collective motion is induced by optical forces acting upon up to 10(5) 87Rb atoms prepared in the lowest band of a one-dimensional intracavity optical lattice. The longevity of atomic motional coherence allows for strongly nonlinear optics at extremely low cavity photon numbers, as demonstrated by the observation of both branches of optical bistability at photon numbers below unity.     

Harmonic mode locking in a sliding-frequency fiber laser

We demonstrate a sliding-frequency mode-locked (SFM) erbium fiber laser generating 20 ps pulses with center wavelengths rapidly sweeping across a spectral range of 50 nm. Excess optical nonlinearity in the laser cavity leads to multipulsing, with a tendency to tight pulse bunching (<3 ns) at the fundamental cavity frequency of 25 MHz. The addition of a parallel optical delay line, with a path difference equal to a rational fraction of the cavity length, distributes the pulses uniformly across the entire cavity and achieves a harmonic SFM up to 1 GHz. The result establishes cavity nonlinearity as a critical design parameter for picosecond wavelength-swept lasers.     

含负折射率介质非线性Bragg腔的双稳态特性

研究了含负折射率介质一维光子晶体非线性Bragg腔的透射特性、光学增强和双稳态特性.对负折射率介质无色散和有色散非线性Bragg腔及由两种正折射率介质构成的非线性Bragg腔的透射谱、光学增强、缺陷模的线宽、入射光的红移量、双稳态开关阈值进行了比较.含负折射率介质的非线性Bragg腔可显著增大腔内光学增强效应,降低缺陷模的线宽、入射光的红移量和双稳态开关阈值.     

Optomechanical self-channeling of light in a suspended planar dual-nanoweb waveguide

It is shown that optomechanical forces can cause nonlinear self-channeling of light in a planar dual-slab waveguide. A system of two parallel silica nanowebs, spaced ~100 nm and supported inside a fiber capillary, is studied theoretically and an iterative scheme developed to analyze its nonlinear optomechanical properties. Steady-state field distributions and mechanical deformation profiles are obtained, demonstrating that self-channeling is possible in realistic structures at launched powers as low as a few mW. The differential optical nonlinearity of the self-channeled mode can be as much as 10×10(6) times higher than the corresponding electronic Kerr nonlinearity. It is also intrinsically broadband, does not utilize resonant effects, can be viewed as a consequence of the extreme nonlocality of the mechanical response, and in fact is a notable example of a so-called accessible soliton.     

Optical bistability in Coumarin-450 doped polymeric samples

Experimental observations of optical bistability in Coumarin-450 dye molecule entrapped in polymeric (PMMA) host inside Fabry–Perot cavity have been reported. Efforts have been made to demonstrate the nonlinear behavior of optical materials in solid form using 532 nm of Nd: YAG laser. The nonlinearity in terms of optical bistability and excited state absorption (ESA) has been discussed. The nonlinearity in Coumarin-450 dye molecule originated from unique intensity dependence of the complex index of refraction. It has been observed that nonlinear refraction dominates over nonlinear absorption in giving rise to the optical bistability. Nonlinear refractive index in both polymeric and liquid samples has been calculated.     

基于双芯光子晶体光纤的低非线性宽带色散补偿光纤的设计

采用矢量光束传输法数值模拟了基于模式耦合的双芯光子晶体光纤的色散和非线性与其结构的关系。结果表明:通过在包层中移除一层空气孔以形成外纤芯并调整内外纤芯之间的距离及包层空气孔的占空比,内外纤芯间的模式耦合可以在宽带范围内发生,导致产生大负色散。同时,由于光场分布在两个纤芯内,增大了模场面积,产生低非线性,可以实现低非线性宽带色散补偿。     

Pattern formation by spatially incoherent light in a nonlinear ring cavity

Modulation instability and pattern formation by spatially incoherent light is investigated experimentally in a nonlinear ring cavity using a photorefractive strontium barium niobate crystal as the nonlinear medium. A step-like threshold for the onset of pattern formation is observed experimentally for the case of high optical feedback. When compared to the case without feedback, this threshold is shifted towards smaller nonlinearities and a significant increase of the modulation degree of the obtained patterns is obtained. Our measurements also show that, above threshold, the dominating spatial frequency of the patterns decreases monotonically with both increasing nonlinearity and increasing feedback. PACS 42.65.Tg; 42.65.Sf; 89.75.Kd     

Stabilization of a multiwavelength erbium-doped fiber laser using a nonlinear silicon waveguide

We propose and demonstrate a multiwavelength erbium-doped fiber laser stabilized by four-wave mixing (FWM) in a nonlinear silicon-on-insulator (SOI) waveguide. The optical gain was provided by an erbium-doped fiber amplifier, and the wavelength selectivity was achieved by a Fabry–Pérot comb filter in the ring cavity. The FWM in the SOI waveguide was enhanced by applying a reverse-biased p-i-n diode structure to reduce free-carrier absorption. Making use of the nonlinearity of the SOI waveguide, a multiwavelength laser with six output wavelengths at 0.8 nm spacing was achieved. The power difference among modes was equalized within a range of 1.8 dB. The power fluctuation of each mode was stabilized to <0.65 dB during 20 min observation at room temperature.     

一种新的低非线性宽带色散补偿微结构光纤的设计

采用矢量光束传输法对空气孔包层呈正六边形分布的微结构光纤的色散和非线性特性进行了数值模拟。通过分别调节内三层空气孔的直径和包层空气孔节距,设计了一种低非线性宽带色散补偿微结构光纤。该光纤在波长1.55 μm处具有－3 235.8 ps/nm/km的大负色散,可在以1.55 μm为中心的100 nm宽带波长范围对相当于自身长度190倍的普通单模传输光纤进行宽带色散补偿(色散补偿率偏移在0.5%以内),同时该光纤可在此宽带波长范围内保持非线性系数低于5 W^-1·km^-1。