光脉冲在一维负折射材料光子晶体中的传播

讨论了一维负折射光子晶体对光脉冲传播的影响.通过对光脉冲的透射场强的数值计算发现,频谱处于一维负折射率材料光子晶体禁带中的短脉冲通过光子晶体后基本保持形状不变,而对同样的短脉冲通过传统的一维光子晶体的透射脉冲则出现严重畸变.当考虑负折射率材料的色散时,讨论了位相时间随光脉冲载波频率的变化情况.     

Ultra-short pulse propagation in 3D GaAs photonic crystals

We have investigated ultra-short pulse propagation through 3D GaAs photonic crystals with a complete photonic band gap in the optical wavelength region. The pulse propagation was calculated by using the finite-differential time-domain (FDTD) method. This is the first time pulse shape measurements have been made using femtosecond pulses. From the experimental results, the shapes of the ultra-short pulses were found to change when the frequency was above the photonic band-gap after the propagation through the photonic crystal, corresponding to the simulation results.     

Nonlinear propagation delay and pulse distortion resulting from dual frequency band transmit pulse complexes

A method of acoustic imaging is discussed that potentially can improve the diagnostic capabilities of medical ultrasound. The method, given the name second order ultrasound field imaging, is achieved by the processing of the received signals from transmitted dual frequency band pulse complexes with at least partly overlapping high frequency (HF) and low frequency (LF) pulses. The transmitted HF pulses are used for image reconstruction whereas the transmitted LF pulses are used to manipulate the elastic properties of the medium observed by the HF imaging pulses. In the present paper, nonlinear propagation effects observed by a HF imaging pulse due to the presence of a LF manipulation pulse is discussed. When using dual frequency band transmit pulse complexes with a large separation in center frequency (e.g., 1:10), these nonlinear propagation effects are manifested as a nonlinear HF propagation delay and a HF pulse distortion different from conventional harmonic distortion. In addition, with different transmit foci for the HF and LF pulses, nonlinear aberration will occur.     

Broadening compensation for ultrashort pulses in photonic crystals

Extremely large group velocity dispersion of both signs can be achieved at the band edges of guided modes in photonic crystal waveguides. The selection of a proper value and sign of this parameter allows the design of short-length waveguides to compensate for pulse broadening. This pulse broadening can be caused, for instance, when a photonic crystal delay line is introduced. We present a theoretical study of the possibilities of using photonic crystal waveguides as intra-circuital dispersion compensation elements for ultrashort pulses, so the width of a transmitted pulse can be reduced. However, we also demonstrate that recovering the original pulse shape is not possible for these large-bandwidth pulses due to higher-order dispersion terms.     

Distortion-free delivery of nanojoule femtosecond pulses from a Ti:sapphire laser through a hollow-core photonic crystal fiber

We demonstrate propagation of femtosecond pulses in the 800-nm range through a hollow-core photonic crystal fiber with preserved temporal and spectral profiles for pulse energies up to 4.6 nJ. Without the use of a prechirping unit, 170-fs pulses were transmitted essentially undistorted at 812 nm, near the zero-dispersion wavelength. Because of the air guidance of pulses, intensity-dependent nonlinear effects were minimal, with only 15% pulse broadening occurring at 350-mW average output power. This fiber thus is excellently suited for applications that require single-mode delivery of high-energy ultrashort pulses to the fiber output face such as, for example, miniaturized multiphoton microscopes.     

Ultrashort pulse propagation and nonlinear frequency conversion in superconducting and magnetic photonic crystal

We study the propagation and second harmonic generation of ultrashort pulse in nonlinear photonic crystal using a combination of Fourier transform and transfer matrix method. The focus is on the reflected and transmitted output of fundamental pump pulse and second harmonic pulse in frequency and time domains, the temperature dependence of the reflection and transmission spectra where the superconducting transition frequency is close to the magnetic resonance. Interesting features include output pump and second harmonic pulses that can be strongly modulated with the transmitted pulses being delayed by slow light effect.     

Anisotropy of light propagation in thin opal films

Thin opal films are prepared by crystallization in a moving meniscus, and their optical transmission spectra are recorded in polarized light and studied. It is shown that the anisotropy of light propagation in the films is unambiguously related to the photonic band structure of opal and depends on the angle of incidence, the orientation of the incidence plane with respect to the opal lattice, and the wavelength and polarization of the incident light. Azimuthal diagrams of transmitted polarized light are constructed in the range of photonic band gaps of three orders for oblique incidence of a light beam. The anisotropy is found to vary with the light wave-length independently in perpendicular polarizations. A model of the band structure of opal wherein opal is represented as an fcc lattice of close-packed spheres adequately describes the optical transmission of opal films only in the range of the first-order photonic band gap.     

Coherently induced double photonic band gaps in a driven N-type atomic system

In an N-type atomic system with double electromagnetically induced transparency, when the traveling-wave coupling field is replaced by a standing-wave, double fully developed photonic band gaps open up due to space-modulated refractive index within the two electromagnetically induced transparency widows. The dynamically induced band gaps can be tuned coherently, and have applications in manipulating the propagation dynamics of light pulses.     

等离子体光子晶体的FDTD分析

等离子体光子晶体是等离子体和介质（真空）构成的人工周期性结构.用分段线性电流密度 递归卷积时域有限差分（PLCDRC-FDTD）算法分析了等离子体光子晶体和缺陷等离子体光子 晶体.从时域的角度分析了高斯脉冲在等离子体光子晶体中的传播过程，给出了时域反射和 透射波形.然后，从频域的角度分析了等离子体光子晶体和带缺陷的等离子体光子晶体的电 磁反射系数和透射系数.计算表明，等离子体光子晶体对频率小于等离子体频率的低频电磁 波几乎完全反射，而透射的电磁波则为频率高于等离子体频率的电磁波.在高频，等离子体 光子晶体则出现类似一般光子晶体的光子带隙特性. 关键词： 等离子体 光子晶体 时域有限差分法 等离子体光子晶体     

磁化等离子体光子晶体的FDTD分析

磁化等离子体光子晶体是磁化等离子体和介质(真空)构成的人工周期性结构.本文用磁化等离子体的分段线形电流密度卷积(PLCDRC)时域有限差分(FDTD)算法分析了磁化等离子体光子晶体特性.分析了磁化等离子体参数对电磁带隙的影响.从时域的角度分析了高斯脉冲在磁化等离子体光子晶体中的传播过程，给出了时域反射和透射波形.从频域的角度给出了磁化等离子体光子晶体的电磁反射系数和透射系数，并对结果进行了分析. 关键词： 磁化等离子体 光子晶体 时域有限差分法     

Tuning the frequency of ultrashort laser pulses by a cross-phase-modulation-induced shift in a photonic crystal fiber

Femtosecond pulses of fundamental Cr:forsterite laser radiation are used as a pump field to tune the frequency of copropagating second-harmonic pulses of the same laser through cross-phase modulation in a photonic crystal fiber. Sub-100-kW femtosecond pump pulses coupled into a photonic crystal fiber with an appropriate dispersion profile can shift the central frequency of the probe field by more than 100 nm, suggesting a convenient way to control propagation and spectral transformations of ultrashort laser pulses.     

Optical properties of chiral photonic crystals with linear profiles of modulation parameters

The oblique propagation of light through a layer of chiral photonic crystal (PC) with linear profiles of modulation parameters is considered. The problem is solved by the method of Ambartsumyan layer addition. The wavelength dependences of the amplitude characteristics have been studied at different angles of incidence in two cases, i.e., in a chiral PC with a linear profile of modulation period and a chiral PC with a linear profile of modulation depth. It is shown that the photonic band gap is broadened in both cases and that omnidirectional reflection occurs under certain conditions. The nonreciprocity features are investigated for these two cases and it is shown that a chiral PC with a linear profile of modulation period can operate as an ideal optical diode in a certain frequency range. The features of the effect of change in the modulation parameters on the photon density of states, group velocity, and group velocity dispersion are analyzed. It is shown that, when the modulation parameters change in space, the photon density of states at the photonic band gap edges significantly decreases. It is proposed to use PCs with a spatially changing modulation period can be used to compress (expand) light pulses.     

TM模式下二维非磁化等离子体光子晶体的禁带调制特性分析

采用时域有限差分法(FDTD)中的分段线性电流密度卷积(PLCDRC)算法研究了TM波入射时二维非磁化等离子体光子晶体的禁带特性.从频域角度分析得到微分高斯脉冲的透射系数,并讨论该光子晶体的介质圆柱的介电常数、晶格常数、介质圆柱半径,周期常数和等离子体参数对其禁带特性的影响.结果表明,增加周期常数和等离子体碰撞频率不会改变禁带宽度,增加介质圆柱的相对介电常数和等离子体频率可以展宽禁带的宽度. 当填充率一定时,减小介质圆柱的半径和晶格常数可以实现禁带的拓展. 关键词： 等离子体 光子晶体 禁带 PLCDRC算法     

晶格数目对面心立方结构光子晶体带隙的影响

对晶格数目对面心立方结构光子晶体带隙的影响进行了详细的实验研究.用不同厚度的材料制作面心立方结构光子晶体，并测量了其禁带的光谱特性.得到了对实际应用有指导作用的规律.一个可实际应用的光子晶体至少应有50个晶格. 关键词： 面心立方结构 光子带隙 晶格数目     

Nonlinear propagation acoustics of dual-frequency wide-band excitation pulses in a focused ultrasound system

In this article, acoustic propagation effects of dual-frequency wide-band excitation pulses in a focused ultrasound system are demonstrated in vitro. A designed and manufactured dual-frequency band annular array capable of transmitting 0.9/7.5 MHz center frequency wide-band pulses was used for this purpose. The dual-frequency band annular array, has been designed using a bi-layer piezo-electric stack. Water tank measurements demonstrate the function of the array by activating the low- and high-frequency layers individually and simultaneously. The results show that the array works as intended. Activating the low- and high-frequency layers individually, results in less than -50 dB signal level from the high- and low-frequency layers respectively. Activating both layers simultaneously, produce a well defined dual-frequency pulse. The presence of the low-frequency pulse leads to compression, expansion, and a time delay of the high-frequency pulse. There is a phase shift between the low- and high-frequency pulse as it propagates from the array to the focus. This makes the latter described effects also dependent on the array configuration. By varying the low-frequency pressure, a shift of up to 0.5 MHz in center frequency of a 8.0 MHz transmitted high-frequency pulse is observed at the array focus. The results demonstrate the high propagation complexity of dual-frequency pulses.     

Measurement of glycerol concentration in B–H–G solution using 3D photonic crystal structure

A novel technique is used to measure the concentration of glycerol in blood-hemoglobin-glycerol (B–H–G) solution using 3D photonic crystal structure is presented in this paper. Glycerol concentration is estimated accurately by measuring the intensities of transmitted light. Here, reflection as well as absorption losses is considered to measure the transmitted intensity of light having wavelength 540 nm. The principle of measurement is based on the linear variation of photonic band gap with respect to glycerol concentration. Simulations for photonic band gap are made using plane wave expansion (PWE) method. An experimental set up is theoretically designed to measure the concentration of glycerol at different hemoglobin concentration.     

Hypersonic modulation of light in three-dimensional photonic and phononic band-gap materials

The elastic coupling between the a-SiO2 spheres composing opal films brings forth three-dimensional periodic structures which besides a photonic stop band are predicted to also exhibit complete phononic band gaps. The influence of elastic crystal vibrations on the photonic band structure has been studied by injection of coherent hypersonic wave packets generated in a metal transducer by subpicosecond laser pulses. These studies show that light with energies close to the photonic band gap can be efficiently modulated by hypersonic waves.     

外磁场作用下一维光子晶体的光传输特性

采用光学传输矩阵方法,研究了外磁场作用下一维光子晶体的光传输特性.在外磁场作用下,介质介电函数在回旋频率ω.附近受到强烈的调制,使组分材料的色散关系发生明显改变,导致光子晶体的能带发生变化,透射谱出现复杂结构.在光子带隙中出现窄通带,窄带中的光是局域的.这表明,在不改变光子晶体组分材料的条件下,可以通过改变外磁场的大小,调制光子晶体的能带及其光传输性质.     

均匀等离子体光栅的色散特性研究

最近研究表明,两束交叉激光脉冲在等离子体中传播时,可以诱导生成周期等离子体密度调制(或称等离子体布拉格光栅). 分别利用传输矩阵法和耦合模理论,推导了激光斜入射时等离子体光栅的色散关系. 两种方法均表明,均匀等离子体光栅存在着光子带隙结构,并且在带隙附近有强烈的色散. 当激光斜入射时,带隙结构会呈现不同的偏振特性：S偏振光的带隙宽度随着入射角的增大而逐渐变宽,而P偏振光的带隙宽度随着入射角的增大先迅速变窄,在布儒斯特角入射时带隙消失,然后又随着入射角的增大而迅速变宽. 超宽的光子带隙和超高的激光损伤阈值,使得等离子体光栅有望成为一种新型的操纵强激光脉冲的光子器件. 关键词： 等离子体光栅 传输矩阵法 耦合模理论 光子带隙     

正负折射率交替一维光子晶体窄带梳状滤波器

利用传输矩阵法计算了正负折射率交替一维光子晶体的带隙特性。结果表明这种光子晶体具有宽而平坦的禁带,窄而尖锐的通带。这种带隙特性是受到布拉格散射、法布里-珀罗谐振和n-=0禁带共同影响的结果,通带位置决定于法布里-珀罗谐振。在理论分析基础上设计出密集波分复用用窄带梳状滤波器,给出信道间隔为0.8nm的梳状滤波器的仿真结果。仿真结果表明这种梳状滤波器具有信道间隔窄、禁带平坦、通带极窄的特点。信道间隔可以通过改变一维光子晶体单元周期光学厚度调节。