Nonlinear properties of a graded-index photonic heterostructure

The optical properties of a one-dimensional (1D) photonic heterostructure with graded-index nonlinear materials are demonstrated theoretically. The influence of the gradation profile of the graded-index nonlinear layers on the linear and nonlinear responses of the structure are analysed. It is shown that the Q-factor of the defect mode and the threshold input intensity to achieve the optical bistability in the used photonic heterostructure depend on the gradation profile of the graded-index nonlinear layers.     

Analysis and optimization of optical bistability in one-dimensional nonlinear photonic crystal with (HL)(D)(LH) and (LH)(D)(HL) structures

Although one-dimensional nonlinear photonic crystal (1D-NPC) has been widely studied, there is no comprehensive analysis on decreasing the bistability threshold power.In this paper, conditions required to create bistability have been specified for two types of structures of alternative high and low refractive index layers and defect layers with Kerr nonlinearity effect, in the order of (HL)^p(D)^q(LH)^p and (LH)^p(D)^q(HL)^p, where L and H denote low and high refractive index layers, respectively, D stands for the defect layers, p is the number of LH or HL layers and q is the number of defect layers.One of the essentials for the bistability is appropriate shifts of frequency of the defect mode, so the effect of the order of layer's arrangement in (1D-NPC) structures have been studied. Different structures have been introduced and the best structure for the lowest bistability threshold power has been proposed. Nonlinear finite-difference time-domain (NFDTD) in C++ has been employed for simulation.     

Dispersive medium’s normal coupled modes and low threshold optical bistability in a symmetric one-dimensional photonic crystal

We present a detailed study of the dispersive medium’s normal coupled modes in a symmetric one-dimensional photonic crystals (1D PCs). The splitting, homogeneous broadening effect, and the detuning effect of the normal coupled modes can be well controlled by adjusting the parameters of the dispersive medium. The bistable behavior of the PCs with Kerr medium is also investigated and it is found that the threshold intensity of nonlinear optical bistability (OB) of 1D PCs with dispersive medium is much lower than that of structure without dispersive medium. The threshold intensity is strikingly sensitive to the coupling strength of the dispersive medium.     

具有非线性缺陷的光子晶体的局域模

采用传输矩阵的方法，严格导出一维具有克尔非线性缺陷的光子晶体的局域模频率方程。局域模频率依赖于局域光强。取负克尔系数时，随着局域光强的增加，局域模频率从下带边出现，在带隙间上升，最后消失在上带边。在给定人射光频率下，随着人射光强的变化，系统呈现出双稳态。这种光学稳态性质则由局域模频率的移动引起的。     

Properties of defect modes in one-dimensional photonic crystals containing two nonlinear defects

Transmission properties of one-dimensional photonic crystal with two nonlinear defects were investigated based on the nonlinear transfer matrix method. From the inspection of the optical bistabilities around different defect modes, it was found that the threshold of bistability for the bonding mode is always lower than that for the anti-bonding mode. When intensity of the incident wave increases bistability could happen at any wavelength larger than the resonance wavelength of the linear anti-bonding mode in the gap. The threshold of bistability increases initially with the wavelength, then decreases sharply in reaching the resonance wavelength of the linear bonding mode. The coupling behavior of the two defects was also discussed, and it was found that the threshold of bistability increases with the distance between the two defects. A flat spectrum similar to that formed in the photonic crystal molecules was obtained under certain incident intensity.     

Optical bistability via coherent and incoherent fields in an Er-doped yttrium-aluminum-garnet crystal

We investigated the optical bistability in an Er³⁺-doped yttrium-aluminum-garnet (YAG) crystal inside a unidirectional ring cavity. We find that the intensity and the frequency detuning of the coherent field as well as the rate of incoherent field can affect the optical bistability dramatically, which can be used to manipulate efficiently the threshold intensity and the hysteresis loop. The effect of the cooperation parameter on the OB is also studied.     

Spectra of secondary emission during generation of optical harmonics in globular photonic crystals

Secondary emission spectra of globular silica photonic crystals when their surfaces were exposed to laser pulses 250 fs long at a power density to 1 TW/cm² have been studied. Optical harmonics and plasma emission were detected in this case. For the opal matrix containing pores filled with air, in the reflection mode, the third optical harmonic with a conversion efficiency of ～10% arises. The highest conversion efficiency for exciting radiation with wavelengths of 1026 or 513 nm is implemented when the frequencies of the exciting radiation or the second harmonic are near the stop band edge. In globular photonic crystals filled with sodium nitrite or barium titanate ferroelectrics, the second optical harmonic is observed. The exciting radiation conversion efficiency to the second optical harmonic was a few percent and depended on the frequency of exciting radiation and photonic crystal globule diameters. It is found that the plasma emission intensity increases with the exciting radiation power density. The dependences of the intensity of the second and third optical harmonics on the pump intensity are constructed for various photonic crystal globule diameters.     

含色散与非线性媒质的光子晶体的简正耦合模

利用传输矩阵方法及对色散媒质采用洛伦兹振子模型，研究了一维含色散媒质的光子晶体的简正耦合模，并计算了该结构的复有效折射率。在光学双稳态中，与仅含非线性媒质的光子晶体相比，得到了低得多的阈值入射光强。     

非线性一维光子晶体波导光双稳

利用非线性折射率系数较大且非线性时间响应较快的CdS_xSe_1-x玻璃为材料,设计并制备了非线性一维光子晶体波导光双稳器件,该器件的折射率空间分布呈正弦形式。实验测得双稳开关的阈值功率密度为1.60×10⁵W/cm²,开关时间为63ps。采用时域有限差分方法讨论了光子晶体带隙随入射光强变化而移动的情况,随着入射光功率密度的增加,光子晶体的带隙中心向短波方向移动。同时计算了该器件的双稳特性,理论计算得到双稳开关的阈值功率密度为1.40×10⁵W/cm²,开关时间约为50ps。获得了理论与实验基本一致的结果。     

Fast bistable all-optical switch and memory on a silicon photonic crystal on-chip

We demonstrate extremely low-power all-optical bistability by utilizing silicon photonic crystal nanocavities, based on the plasma effect of carriers generated by two-photon absorption. Owing to the high quality factor and the small volume of the nanocavities, the photon density inside the cavity becomes extremely high, which leads to a large reduction in operation power. Optical bistable operation in a single nanocavity permits optical read-write memory operation, which opens the possibility of an integrated optical logic circuit on a single chip, based on photonic crystals. The demonstrated bistable threshold power is 0.4 mW with a set pulse energy of 74 fJ, at a switching speed of <100 ps.     

A comparative study of dispersion relation of EM waves in ternary one-dimensional plasma photonic crystals having two different structures

In this paper, a rigorous theoretical analysis has been made to study the dispersion relation of EM waves in periodic ternary one-dimensional photonic crystal having two different structures. In one case we have chosen glass-plasma and ZnS in one unit cell and in other case we have considered glass-plasma and M_gF₂ in one unit cell. Using Kronig-Penney model the dispersion relation for proposed structures has been obtained and numerical results are presented in the form of dispersion curves. The dependence of photonic band gap (PBG) characteristics on plasma frequency, plasma width and the width of dielectric media are discussed in the light of frequency gap and cutoffs of binary one-dimensional plasma photonic crystal. An attempt has been made to show how the PBG characteristic of a particular structure changes when the dielectric materials of its unit cell is changed by the other dielectric material. It is found that the structure having glass-plasma-ZnS in unit cell is more useful for broad band filtering and other plasma functioning devices compared to the structure having glass-plasma-MgF₂ in one unit cell.     

All-optical switch based on photonic crystal microcavity with multi-resonant modes

We present a design of all-optical switches based on one-dimensional photonic crystals (1D PhC) doped with nonlinear optical materials. The 1D PhC switch structure is composed of a PhC cavity sandwiched by two accessional PhC microcavities. The center PhC cavity has two resonant frequencies with nearly the same quality factors (Q), while the accessional PhC cavities have the same resonant frequency, which is equal to one of the resonant frequencies of the center cavity. The two accessional PhC cavities cause reduction of Q value in this resonant frequency and result in different Q values of two modes. We realize all-optical switch effect by selecting pump light wavelength at the low Q mode and probe light wavelength at the other mode. The theoretical simulations by using the finite difference time domain method show that the pump light intensity required to realize optical switch effect in the designed switch is 50 times smaller than that in one-dimensional photonic crystals cavity with only one resonant mode.     

Optical multistability in 1D photonic crystals with nonlinear Thue–Morse structure

A 1D photonic crystal structure contains 16 layers, 8 common dielectric layers and 8 layers with nonlinear medium, with an arrangement similar to the 5th generation, Thue–Morse multilayer is presented. The properties of photonics band gap, field distribution and optical multistability are investigated. On the band gap, 5 resonant modes is observed. On the behavior of output intensity versus input intensity around resonant frequencies, the multistability response observed. When the frequency of incident field is close to the resonant frequency, the threshold intensity of multistability is decreased.     

掺Yb钒酸盐晶体激光振荡中的光学双稳态现象研究

研究了三种掺Yb钒酸盐晶体Yb:LuVO4,Yb:YVO4和Yb:GdVO4的激光振荡中所呈现的光学双稳态效应.以晶体所吸收的抽运功率表征的双稳区宽度ΔPabs可超过1W,在双稳区高功率一侧的边界,即激光振荡的上阈值点,出现激光输出功率的不连续变化或跃变,同时发生亚毫秒时间尺度上的大幅度强度涨落.谐振腔的输出耦合透过率、晶体长度、晶体中的热效应等对双稳态效应均具有重要影响.     

介质层厚对含负折射率介质Bragg微腔的影响

研究了介质层厚对含负折射率介质一维光子晶体Bragg微腔的缺陷模和双稳态的影响.在中心频率附近将传输矩阵各矩阵元采用泰勒级数展开并取一级近似,得到了缺陷模频率与介质层厚的关系式及品质因子公式.研究结果表明：一级近似法能很好地解释中心频率附近介质层厚对缺陷模频率的影响.理想Bragg微腔结构的缺陷模品质因子最大;递增正折射率介质层厚和增大缺陷层介质层厚、递减负折射率介质层厚及同时等量递减正和负折射率介质层厚,均可使缺陷模红移,双稳态阈值降低.     

Properties of omnidirectional photonic band gap in one-dimensional staggered plasma photonic crystals

In this paper, the properties of the omnidirectional photonic band gap (OBG) realized by one-dimensional (1D) photonic crystals (PCs) with a staggered structure which is composed of plasma and isotropic dielectric layer have been theoretically studied by the transfer matrix method (TMM). From the numerical results, it has been shown that such OBG is insensitive to the incident angle and the polarization of electromagnetic wave (EM wave), and the frequency range and central frequency of OBG can be effectively controlled by adjusting the plasma frequency, the average thickness of plasma layer, the average thickness of dielectric layer and staggered parameters, respectively. The frequency range of OBG can be notably enlarged with increasing the plasma frequency, average thickness of plasma layer, respectively. Moreover, the bandwidth of OBG can be narrowed with increasing the average thickness of dielectric layer. Changing staggered parameters of dielectric and plasma layer means that the OBG can be tuned. It is shown that 1D plasma dielectric photonic crystals (PPCs) with such staggered structure have a superior feature in the enhancement of frequency range of OBG compared with the conventional 1D binary PPCs. This kind of OBG has potential applications in filters, microcavities, and fibers, etc.     

Dispersion properties of two-dimensional plasma photonic crystals with periodically external magnetic field

Dispersion properties of two types of two-dimensional periodically magnetized plasma array structures with square lattices have been investigated by using plane wave expansion method. It is found that two different regions of flatbands and photonic band gaps occur in the TE polarization due to the external magnetic field. The two types of system can be seen as a kind of unusual plasma photonic crystals. The results show that not only the location of flatbands but also the position and bandwidth of photonic band gaps can be tuned by external magnetic field. The cutoff frequency decreases as external magnetic field increases. The edge of two different of flatbands regions and cutoff frequency shift downward to lower frequencies obviously with increasing plasma collision frequency, while they shift upward to higher frequencies notably with increasing plasma frequency. The filling factor has little effect on the location of flatbands regions. The width of flatbands regions and photonic band gaps are almost unchanged by increasing filling factor, but the number of ominidirectional photonic band gap for type-1 structure plasma photonic crystals can be effectively controlled by adjusting filling factor. It is worth to be noted that the first band gaps above the cutoff frequencies in Г–X and Г–M direction for two types of PPCs can be modulated by the parameters as mentioned above, and the relative bandwidth of band gap in Г–X direction is wider than the one in Г–M direction. The results may provide theoretical instructions to design new tunable photonic crystals devices.     

Tunable photonic crystals with semiconducting constituents

We propose that the photonic band structure (PBS) of semiconductor-based photonic crystals (PCs) can be made tunable if the free-carrier density is sufficiently high. In this case, the dielectric constant of the semiconductor, modeled as varepsilon(omega) = varepsilon(0)(1-omega(2)(p)/omega(2)), depends on the temperature T and on the impurity concentration N through the plasma frequency omega(p). Then the PBS is strongly T and N dependent; it is even possible to obliterate a photonic band gap. This is shown by calculating the 2D PBS for PCs that incorporate either intrinsic InSb or extrinsic Ge.     

Sensitivity enhanced all-optical switching using prism-grating coupled surface plasmon modes

A sensitivity enhanced all-optical-switch design using prism-sinusoidal grating coupled surface plasmon modes is demonstrated numerically. Owing to the simultaneous excitation of surface plasmons for the pump and signal at the interface between a nonlinear waveguide layer and a metal layer, a sensitivity enhanced bistability effect in the structure can be obtained. The nonlinear refractive index of the waveguide layer experiences an abrupt change of the local pump field owing to this bistability effect. Such a characteristic greatly improves the performance of optical switch by reducing the threshold pump intensity. An all-optical-switch design with a pump threshold as low as 0.9 GW/cm² is presented.     

掺Tm激光晶体的可见与红外发光转换及双稳多色开关

理论研究了648nm激光抽运掺Tm晶体的可见与红外发光转换和发光双稳特性.根据系统的耦合速率方程理论,推导了发光转换阈值的解析表达式,数值研究了Tm³⁺能级布居数,辐射光谱以及发光强度等随抽运光强的变化.数值结果表明,648nm激光抽运下可以实现452/469nm蓝光与1716 nm红外光谱的发光转换.在光子雪崩阈值附近,可以观察到掺Tm晶体的辐射光谱双稳,荧光色度双稳和发光强度双稳等现象.利用掺Tm晶体的可见与红外发光转换与双稳特性,可以实现抽运功率控制的全固态发光转换器和双稳波长 关键词： 光学双稳 多色开关 掺Tm晶体 光子雪崩