Quantum theory of whispering gallery modes in a cylindrical optical microcavity

The quantum interpretation of whispering gallery modes in a cylindrical optical microcavity is presented in analogy with quantum mechanical theory. The formulas for the resonance shifts caused by a small change in correlative refractive index are derived. Then they are applied to two applications of the cylindrical microcavity to probe the change of surrounding medium and the nonlinear effect of cavity's surface.     

Formation of ultrashort electron pulses on scattering of an electron beam by a standing laser wave of ultrashort duration

The scattering of a well collimated electron beam by a strong standing laser wave of ultrashort duration, giving rise to a great number of scattered photons, is considered. This type of scattering is found to cause an electron beam to divide effectively into two parts. Ultrashort laser pulses are shown to be capable of forming ultrashort electron bunches whose length is governed by the laser beam diameter.     

A polariton laser based on a bulk GaN microcavity

Simulations of polariton relaxation in a bulk GaN microcavity are performed by solving semi-classical Boltzmann equations. The structure which is considered is that of a recently grown microcavity showing strong coupling at 300 K. A kinetic phase diagram is calculated. We show that such a cavity can operate as a low-threshold polariton laser at room temperature.     

Simulation of the expansion of a crystal heated by an ultrashort laser pulse

Molecular dynamics (MD) simulation of expansion of quickly heated Lennard-Jones (LJ) crystalline thin film is performed. The heating time is assumed to be much shorter than the characteristic expansion time. Such situation occurs when a femtosecond laser pulse is absorbed by a crystal. We established that plastic rarefaction wave is formed and propagates from the vacuum boundary into solid.     

基于回音壁微腔拉曼激光的纳米粒子探测

回音壁模式光学微腔由于其品质因子高、模式体积小等优点, 近年来在非标记性的纳米粒子探测方面得到了广泛的重视, 开展了大量的研究, 取得了重要的进展. 利用回音壁微腔的拉曼激光, 通过测量纳米粒子造成的模式劈裂的拍频, 可以实现不同环境下纳米粒子的实时探测. 与传统的稀土离子掺杂法不同, 这种方法采用腔的内禀增益, 不仅提高了应用回音壁模式微腔进行纳米粒子探测的极限, 而且避免了传统方法中稀土离子能级对泵浦光的限制, 拓展了应用范围. 这种方法还可以应用于其他材料的回音壁微腔, 如硅基微环腔等, 以及光子晶体结构、超材料等受损耗限制的系统中. 本文简单介绍了回音壁模式光学微腔进行纳米粒子探测的基本原理以及最新研究进展.     

To measure chirped ultrashort laser pulse by the Talbot effect of a grating

We study the Fresnel diffraction of a phase grating under the illumination of a chirped femtosecond laser pulse. Using spatial spectrum theory, first we extract the central wavelength signal from the diffraction wave of the chirped pulse through a diaphragm. Then the central wavelength waves are directed to interfere in remote regions by two reflectors. The interference stripes at different distance will be exactly the same in interference region. Since the intensity distribution of the stripes is related to the chirp parameter of the femtosecond pulse, through measuring the stripe intensity distribution, we can indirectly detect the chirp parameter of the femtosecond pulse.     

Distortion of ultrashort laser pulses in a spatial filter

By means of Collins diffraction integral, the propagation expression of an ultrashort laser pulse passing through a spatial filter is derived. The effects of the magnification of spatial filter on pulse broadening and distortion of pulse-front for the ultrashort optical pulse are analytically deduced and numerically simulated. It is found that pulse broadening and propagation time difference of a laser pulse getting through spatial filter is proportional to the magnification. As a conclusion, in an ultrashort pulse laser system with a large aperture, the effect of pulse broadening and distortion of pulse-front should be considered.     

Whispering‐gallery microcavities with unidirectional laser emission

Optical whispering‐gallery mode (WGM) microcavities featuring ultrahigh Q factors and small mode volumes enhance significantly the interaction between light and matter, becoming an excellent platform for achieving ultralow‐threshold microlasers. However, the emission of traditional WGMs is isotropic due to the rotational symmetry of cavity geometries, which hinders the potential photonics applications. In this review, the progress in WGM microcavities towards unidirectional laser emission is summarized. When a subwavelength scatterer is placed on the boundary of the microcavity, the unidirectional emission occurs due to the collimation effect of the microcavity‐enhanced scattering field. Furthermore, microcavities deformed from the circular shapes can not only produce the chaos‐assisted unidirectional emission, but also maintain high Q factors by special design and fabrication processes. Finally, gratings along the circumference of the WGM microdisk or microring can scatter the WGMs in the vertical direction. The review also lists several important applications of these types of microcavities, such as wide‐band laser illumination source, free‐space coupling, evanescent‐field enhancement, optical energy storage, and sensing.

     

啁啾超短脉冲光波照射下光栅Talbot效应的研究

应用傅里叶频谱分析法，给出了光栅在啁啾超短脉冲光波照射下菲涅耳衍射方程并分析了其Talbot效应. 数值计算表明，在Talbot距离处的衍射光强不仅与照射的超短脉冲光波的宽度有关，而且与脉冲光波的啁啾有关. 根据光栅在Talbot距离处衍射光强的变化，给出了一种检验超短脉冲光波是否含有啁啾的简单方法. 关键词： 光栅 菲涅耳衍射 Talbot效应 啁啾超短脉冲激光     

Characteristics of Er and Er-Yb-Cr doped phosphate microsphere fibre lasers

In this paper both Er and Er-Yb-Cr doped phosphate microspheres have been successfully created through precise melting of the ends of fibre tapers, drawn, respectively, from Er and Er-Yb-Cr doped phosphate glasses. When coupled with a fibre taper, a microsphere fibre laser cavity can thus be configured creating a system pumped by a 980 nm laser diode and using an optical spectrum analyzer to monitor the spectral characteristics of the laser output. The performance and characteristics of the Er and Er-Yb-Cr microsphere lasers thus created are discussed in detail and cross-compared in this paper. Both lasers have shown low-threshold in terms of the pump power and the laser output wavelengths and a close investigation of the system has shown that the output power and laser stability are closely related to the size of the microsphere, the pump power and the microsphere material composition.     

Bimodal temporal distribution of photons in ultrashort laser pulse passed through a turbid medium

The criteria were determined for simultaneous observation of scattered and unscattered (ballistic) peaks in temporal distribution of a narrow pulsed laser beam passed through a strongly scattering medium. Conditions were found for such an observation on the basis of non-stationary two-flux model for radiation transport. Calculation results permitted the definition of a compromise between the parameters of the initial laser pulse and the properties of a scattering medium. This provided the separate observation of various types of photons. The theoretical calculations were confirmed by experiments on recording pulse profiles of passed radiation of fs-laser in a water–milk solution.     

Sensitivity to external optical feedback of circular-side hexagonal resonator microcavity laser

The sensitivity to fault reflection is very important for larger dynamic range in fiber fault detection technique. Using time delay signature (TDS) of chaotic laser formed by optical feedback can solve the sensitivity limitation of photodetector in fiber fault detection. The TDS corresponds to the feedback position and the fault reflection can be detected by the laser diode. The sensitivity to feedback level of circular-side hexagonal resonator (CSHR) microcavity laser is numerically simulated and the feedback level boundaries of each output dynamic state are demonstrated. The peak level of TDS is utilized to analyze the sensitivity. The demonstration is presented in two aspects:the minimum feedback level when the TDS emerges and the variation degree of TDS level on feedback level changing. The results show that the CSHR microcavity laser can respond to the feedback level of 0.07%, corresponding to -63-dB feedback strength. Compared to conventional distributed feedback laser, the sensitivity improves almost 20 dB due to the shorter internal cavity length of CSHR microcavity laser. Moreover, 1% feedback level changing will induce 1.001 variation on TDS level, and this variation degree can be influenced by other critical internal parameters (active region side length, damping rate, and linewidth enhancement factor).     

近共振区超短强激光脉冲激发的等离子体尾波场

 用一维相对论粒子模拟研究了相对论超短强激光脉冲在等离子体中传播时激发的尾波场,初步获得了近共振区尾波场的峰值幅度随激光脉冲宽度变化的特点,发现在近共振区等离子体波激发出现增强。通过准静态近似下尾波激发的一维非线性方程数值求解,并与粒子模拟结果比较,得到了该非线性方程的适用范围：当激光脉冲宽度小于等离子体波波长的4倍时,该方程所得结果与粒子模拟结果一致;而当激光脉冲宽度大于该数值时,该方程不再适用。     

相位匹配下锥形光纤激发出的回廊模谐振

分别计算不同直径下锥形光纤基模和玻璃微球谐振腔内最低阶径向回廊模的传播常量,利用相位匹配条件,作出了锥形光纤与石英玻璃微球腔的直径对应关系曲线.在此基础上,选择锥腰直径2.8 μm左右的低损耗锥形光纤与直径143.1 μm球形度很好的玻璃微球腔进行近场耦合以激发球内的最低阶径向回廊模谐振,在锥形光纤的两端进行通光测试,在输出端获得了等间距分布的窄线宽滤波谱线,其吸收峰位置与利用Mie理论计算的球内最低阶径向回廊模谐振峰位置相一致.     

Surface ripple changes during Cr film ablation with a double ultrashort laser pulse

Surface modification is investigated experimentally by varying the time separation of double femtosecond laser radiation and surface ripples by varying the time separation and polarization direction of double pulses train. Nanometer-sized particles are formed during resolidification of the molten region when the second pulse arrives within 10 ps and the molten material is ejected much after 10 ps. The ripple in the outer region remains oblique to the sum of the vector direction of the two pulses when the time delay is zero. With time delay ranging from 0.5 to 10 ps and different polarization directions of the laser radiation, the ripple generally aligned perpendicular to the polarization direction of the electric field with multiple pulses in the vicinity of ablation threshold is effectively eliminated without fragments at the edge. Furthermore, remnant ripples on irradiated area at higher energies with the same polarization direction are removed by irradiation at a lower energy with each different polarization direction of double pulse. Based on morphological observations for different time delays, possible mechanisms of ripple formations and eliminations are suggested.     

超短激光蚀除金属机制的分子动力学研究

 采用耦合电子热传导方程的分子动力学方法,研究了飞秒激光辐照下金属Ni的熔化及蚀除动力学。分析了靶材内部温度分布特征及蚀除产物的构成,主要包含单个原子及大团簇。确定了断裂位置和蚀除开始的标志,即该处温度分布出现小的峰值,且粒子数密度急剧下降。模拟结果表明：强烈的蒸发及靶材内部所产生的拉应力分别是单个原子及大团簇喷射的机制。同时,深入探讨了激光诱导压力波的传播规律,预测了压力波的波速,约为4.97 km/s。将不同脉冲能量密度下的蚀除速率同实验数据加以对比,结果相差16%~20%。预测了熔深随时间的变化规律,基本随时间的延续而呈上升的趋势。发现过热现象的存在。     

基于回音壁模式微球腔的PDH稳频技术

光学微球腔因其回音壁模式可获得极高的品质因数而受到广泛关注.本文分析了Fabry-Perot腔和微球腔的基本原理,通过CO2激光熔融光纤实验制得了直径为1.2 mm的微球腔,并测试了微球腔和锥形光纤耦合结构的耦合特性.采用典型的PDH稳频系统设计了基于微球腔的稳频系统,分析了用于鉴频的误差曲线的吸收特性和色散特性,对比了不同调制频率、微球腔直径、耦合损耗、传输损耗下与误差曲线斜率的关系.结果表明:耦合状态下最大Q值可达到1.1×108,调节微球腔内横磁模和横电模的转换可优化耦合效率,匹配微球腔和锥形光纤的尺寸得到了径向二阶模式的透射谱,误差曲线效率达到15.4A mW/MHz.球腔在提高PDH稳频技术灵敏度上具有巨大潜力.     

Magnetic/non-magnetic nanoparticles films with peculiar properties produced by ultrashort pulsed laser deposition

Films of magnetic nanoparticles uniformly mixed with non-magnetic nanoparticles have been produced by ultrashort pulsed laser deposition. These films present innovative characteristics with respect to their counterparts produced by standard techniques, as for example nanosecond laser ablation or sputtering, due to the peculiar shape and preferential distribution of their constituent nanoparticles. In the present investigation, the difficult coalescence among the deposited nanoparticles, specific characteristic of the ultrashort pulsed laser deposition, is particularly stressed for what concerns its effect on the collective magnetic behaviour. In particular, we observed that, even for a significant fraction of magnetic particles, the films exhibit an unusual high remanent magnetization, together with relatively low values of saturation and coercive fields, showing a strong squareness of the hysteresis loops. In perspective, these nanogranular films appear very promising for potential application as permanent magnets and in magnetic recording.     

超短超强激光与稀薄等离子体相互作用背向受激Raman谱特性模拟

 利用LPIC++程序模拟了超短超强脉冲与稀薄等离子体相互作用产生的背向受激Raman谱。结果证明：在极端相对论条件下,背向受激Raman谱不再是通常所指的弱耦合模式,而进入强耦合模式。频谱加宽,并融合在一起;各谱峰之间的频移不再以等离子体波的频率为间隔,而是小于电子等离子体波的频率。模拟了各种条件下的背向散射Raman谱特性,结果显示：随着密度的提高,背向受激Raman谱的强度也将提高,这与理论结果符合得较好。     

Self-localization of whispering-gallery modes in a spherical microcavity filled with a cold electron plasma

We have studied the mechanisms and conditions for self-localization of laser radiation in a cold collisionless electron plasma for a spherical microcavity in the quasistatic approximation, taking into account the appearance of striction nonlinearity and also relativistic effects connected with forced oscillatory motion of the electrons. We have shown that when whispering-gallery modes interact nonlinearly with the electron plasma, a spherical plasmon is excited which is localized within the finite volume of the microsphere. We have established that realization of self-localized spherical structures, in the form of an electromagnetic field and spherical plasmons within a microcavity filled with a cold electron plasma, is primarily due to appearance of relativistic nonlinearity of the medium. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 1, pp. 59–64, January–February, 2006.