Femtosecond-laser nanofabrication onto silicon surface with near-field localization generated by plasmon polaritons in gold nanoparticles with oblique irradiation

Nanohole fabrication process with gold nanoparticles irradiated by femtosecond laser at different incident angles is investigated. Nanoparticles with diameter of 200 nm and laser irradiation with center wavelength of 800 nm are used in the present study. The analysis of the electromagnetic field distribution in the near-field zone of the particle is made by simulations based on finite-differential time domain (FDTD) method. It is shown that when gold nanoparticle is irradiated by laser pulse surface plasmon excitation can be induced, and associated with it, high-intensity near field is produced in a limited area around the particle. It is found that the change of the irradiation conditions by means of irradiation from various incident directions gives a possibility of laser nanoprocessing with tunable characteristics. Our results show that enhanced optical intensity is able to be induced on the substrate surface regardless of incident direction of the laser due to the image charge interaction with the substrate. Furthermore, the use of p-polarized laser irradiation at a certain angle gives a minimum of the spatial dimensions of the enhanced zone on the substrate which is about two times smaller than that obtained at normal incidence.     

Laser conferral of a directed character to near-field energy transfer

With sufficient intensity, an off-resonant laser beam can modify the character of electromagnetic near-field radiation produced by a nanoscale point source, narrowing its spatial distribution. The mechanism, for which the laser frequency is significantly off-resonant from the emission radiation, is detailed and analyzed through a quantum electrodynamical analysis. Results are calculated for various positions of a polarization-sensitive probe relative to the point source, sited within the throughput beam, and contour maps exhibit variations in the registered signal over a range of input intensities. A key feature is the clear exhibition of directed propagation features, usually emergent only in the wave zone, within the near-field region of the source.     

The magnesium ramsey interferometer: Applications and prospects

In this paper it will be shown that an atom interferometer, based on the coherent splitting of the atomic wavefunction by four travelling waves (Ramsey interferometer), may be explained by a purely mechanical interpretation. As our first application of this Ramsey interferometer we have measured the phase shifts respectively optical length changes in a magnesium atomic beam caused by the acceleration of the partial atomic wave in one arm of the interferometer. This acceleration was achieved by the dipole force exerted by an off-resonant crossing laser beam which interacted with the ground state part of the wavefunction only. Further applications of this interferometer and improvements due to laser cooling will be discussed.     

探测器阵列法测量激光远场光斑能量密度算法研究

针对靶场试验装备布局很难做到激光器与阵列面垂直的实际情况,分析了激光探测器阵列系统处理激光光斑能量密度算法的不足。根据激光束照射靶面的倾斜角,研究了激光束与每个探测器形成夹角的计算方法,并推导出了具体的余弦角计算公式,对激光光斑每一点的能量密度进行了相应的修正计算,通过试验应用,验证了该算法在激光远场光斑测量中能够有效提高光斑能量密度的处理精度。     

Effect of field and medium asymmetries on enhancement of signal-to-noise ratio

The effect of field and medium asymmetry on improvement of signal-to-noise ratio (SNR) in respect of homodyne detection technique is analysed theoretically. The analysis is applied to the sample of bulk GaAs irradiated by off-resonant nanosecond pulsed laser. The dependence of SNR on electric field amplitude and local oscillator (LO) phase is examined. It is found that SNR is maximum for the combination of asymmetric field and asymmetric medium.      

Near-field second-harmonic generation induced by local field enhancement

The field near a sharp metal tip can be strongly enhanced if irradiated with an optical field polarized along the tip axis. We demonstrate that the enhanced field gives rise to local second-harmonic (SH) generation at the tip surface thereby creating a highly confined photon source. A theoretical model for the excitation and emission of SH radiation at the tip is developed and it is found that this source can be represented by a single on-axis oscillating dipole. The model is experimentally verified by imaging the spatial field distribution of strongly focused laser modes.     

光伏碲镉汞探测器在波段内连续激光辐照下的非线性响应机理研究

利用波段内连续激光, 辐照禁带宽度为0.33 eV的中波光伏碲镉汞探测器. 实验结果表明, 随着辐照激光光强的逐渐增大, 探测器从线性响应过渡为非线性响应. 当探测器进入非线性状态, 探测器的开路电压随激光光强的增大而减小, 且在激光开启辐照时开路电压信号迅速下跳, 在激光停止辐照时开路电压信号迅速上跳. 通过考虑激光辐照下探测器的温度场分布以及温度对p-n结内建电场的影响, 结合考虑机械快门在开启和关闭时对激光光强变化的影响, 建立了光伏探测器在波段内连续激光辐照下的解析模型, 模型计算结果与实验结果吻合得较好. 研究表明, 激光辐照过程中的非线性响应, 主要由温度对p-n结内建电场的影响决定, 激光开启和关闭时的开路电压的幅值是由光强和温度共同决定.     

环状连续强激光下光学薄膜的瞬温和热畸变

详细研究了非冷却光学薄膜元件在环形激光光束辐照下的瞬态温度分布和随后的热畸变，并且用泰曼干涉仪实际测量了连续波氧碘激光辐照各种非冷却光学元件的热畸变量并与理论计算相比较，分析结果表明理论和实验相一致。     

基于可调谐飞秒激光的光学特性测量装置

随着激光技术的快速发展,激光光源在光谱辐射和光学特性测量中得到了广泛的应用。与传统的灯光源相比,激光光源具有高功率、宽光谱和极低的波长不确定度等优点。建立了一套基于宽波段可调谐激光光源的光电探测器响应定标装置,其中激光光源的光谱覆盖范围为190 nm～4000 nm,激光脉冲宽度约130 fs,重复频率约80 MHz。为了避免高峰值功率脉冲光对探测器定标的影响并提高测量的线性度,利用光纤的固有特性成功研制了一种可将脉冲光转换成连续光的转换器。同时基于激光光源搭建了一套激光准直和扩束光路,在波长λ=550 nm时得到了非均匀性为0.29%的单色均匀辐射场。该单色均匀辐射场在高精度的探测器定标应用中具有重要的作用,围绕其搭建的定标测量装置在计量领域具有一定的参考价值。     

三能级激光器的突变行为

本文描写了有注入外场和可饱和吸收体的三能级单模激光器于阈值附近的行为.定量的计算表明:激光电场方程的非线性,特别是由于外场的注入,使得激光定态具有标准尖点突变流形.最后讨论了尖点突变流形的物理意义,给出了产生光学双稳态的条件.     

双光子吸收材料用于高功率激光近场控制技术

 为了提高高功率激光光束质量和改善近场均匀性,在SILEX-Ⅰ超短脉冲激光装置的泵浦光系统上进行实验,研究了一种双光子吸收材料的光限幅特性及对激光近场的改善情况。利用泵浦光SAGA激光器输出的激光进行光限幅特性实验。结果表明:该材料具有明显的光功率限幅特性,且透过率较高,当入射光强小于0.60 GW/cm₂时,透过率大于90%。在SILEX-Ⅰ装置的泵浦光系统进行联机实验,研究了该材料对激光近场均匀性的改善。实验结果表明:经过材料后,激光近场均匀性和调制度有明显改善。     

Propagation offset characteristics of annular laser beams from confocal unstable resonators through the natural atmosphere

Based on the laser field from a positive confocal unstable resonator (ab initio), the propagation characteristics of the beam through atmosphere are investigated by means of fast Fourier transform (FFT), and it is assumed that the refractive index and absorption of atmosphere are only decided by the air molecules. Considering the thermal effects of the laser beam and wind velocity of air, we evaluate the beam offsets, beam spreading, and beam qualities of the laser field transmitting from the resonator through the natural atmosphere. Meanwhile, the far-field intensity distributions of the laser beam with various intensities in the atmosphere are obtained. It is shown that the far-field diffraction patterns are deformed due to absorption of air and wind velocity. β parameter and Strehl ratio, which are usually used for high-power lasers, are introduced to estimate the beam quality characteristics. With increasing the intensity of the beam, the far-field diagram patterns are outspreaded, the peak intensities reduced, the peak centers shifted, and the beam quality characteristics greatly degraded.     

Modulation instability by intense laser beam in magnetized plasma

Modulation instability of an intense right-hand elliptically polarized laser beam propagating through magnetized plasma is investigated by a new method. The nonlinear dispersion relation, in which the relativistic and ponderomotive nonlinearities are taken into account, is obtained for the laser radiation in magnetized plasma by the Lorentz transformation. The Karpman equation is firstly generalized to the case of three dimensions with three field components. When the nonlinear frequency shift of the electromagnetic field in plasma is involved, the nonlinear evolution equation for the slowly varying envelope of the laser field is obtained. Thus, modulation instability of the intense laser beam in magnetized plasma is studied and the temporal growth rate of the instability is derived. The analysis shows that the peak growth rate of self-modulation instability is increased due to the axial magnetization of plasma. It is also shown that the growth rate of modulation instability is increased significantly near the critical surface in a laser-plasma.     

Off-resonant activation of optical emission

Recent reports have identified a three-wave optically parametric mechanism for the active enhancement of fluorescence using off-resonant radiation. In this Letter it is shown by numerical simulation that the output of a laser system optically pumped just below threshold, can be strongly enhanced by this mechanism, using an ancillary beam of moderate intensity. The electrodynamics and kinetics of the nonlinear optical mechanism are analyzed, model calculations performed, and the output is illustrated graphically. The response demonstrates a novel method for achieving all-optical transistor action.     

微小孔径激光器的研制与近场测试

介绍了一种用于高密度近场存储领域的新型微小孔径激光器(VSAL)。为了解决由腔面金属膜造成的激光器PN结短路的问题,在激光器中引入了窗口隔离区,不仅降低了器件制备的难度,而且也提高了器件的性能和成品率。采用聚焦离子束刻蚀技术成功地制备了输出功率为0 3mW的激光器,利用矩阵方法通过远场测量值估算了激光器的近场分布。     

Fresnel diffraction model for mode-mismatched thermal lens with top-hat beam excitation

A theoretical model, based on the Fresnel diffraction integral, is developed to describe the thermal lens (TL) signal in a mode-mismatched collinear configuration, which is optimized for a near field detection scheme and excitation by a cw modulated laser beam with a top-hat profile. The TL amplitudes obtained with both top-hat and Gaussian beam excitations are numerically computed and compared, and the dependence of the TL amplitude on the experimental parameters is discussed. Numerical results show that the top-hat beam TL instrument is more sensitive than the Gaussian beam TL instrument, with a potential doubling of the sensitivity. The use of the top-hat beam excitation with TL detection is a significant improvement because a top-hat beam can be easily obtained with a low-cost, wide-spectral emission white-light source. The use of incoherent light sources as the excitation sources would substantially expand the applicability of the TL technique to the general area of chemical analysis.This revised version was published online in March 2005. In the previous version, the published online date was missing     

Determination of optical Kerr nonlinearity of a photonic bandgap structure by Z-scan measurement

Through analysis of the dispersion relation in a photonic bandgap structure, the effective optical Kerr nonlinearity that determines a Z-scan profile particularly near the stop-band edges, is derived. Near and inside the stop band, the nonlinear optical phase change that originates from an off-resonant response is converted into a change in nonlinear optical intensity through Bragg reflection. The Z-scan measurement of a cholesteric liquid-crystal photonic bandgap structure confirmed that off-resonant Kerr nonlinearity is responsible for the characteristic open-aperture Z-scan profiles near the stop-band edges.     

Implications of transient changes of optical and surface properties of solids during femtosecond laser pulse irradiation to the formation of laser-induced periodic surface structures

The formation of laser-induced periodic surface structures (LIPSS) upon irradiation of silicon wafer surfaces by linearly polarized Ti:sapphire femtosecond laser pulses (pulse duration 130 fs, central wavelength 800 nm) is studied experimentally and theoretically. In the experiments, so-called low-spatial frequency LIPSS (LSFL) were found with periods smaller than the laser wavelength and an orientation perpendicular to the polarization. The experimental results are analyzed by means of a new theoretical approach, which combines the widely accepted LIPSS theory of Sipe et al. with a Drude model, in order to account for transient (intra-pulse) changes of the optical properties of the irradiated materials. It is found that the LSFL formation is caused by the excitation of surface plasmon polaritons, SPPs, once the initially semiconducting material turns to a metallic state upon formation of a dense free-electron-plasma in the material and the subsequent interference between its electrical field with that of the incident laser beam resulting in a spatially modulated energy deposition at the surface. Moreover, the influence of the laser-excited carrier density and the role of the feedback upon the multi-pulse irradiation and its relation to the excitation of SPP in a grating-like surface structure is discussed.     

Laser beam patterns of an optical cavity formed by two twisted cylindrical mirrors

Propagation of an elliptical Gaussian beam in a multiple-pass optical cell formed by two twisted cylindrical mirrors has been described by means of complex curvature tensors. Using the ABCD tensor approach various light patterns were computed for the use in tunable laser absorption spectroscopy with a multiple-pass optical cell. Light patterns with high beam-spot density can be also defined for a cavity formed by two twisted cylindrical high-reflectivity mirrors. In order to achieve higher cavity output intensity, a high-reflectivity cylindrical mirror cavity with at least one mirror that has a central transparent spot for laser beam injection has been described for applications in non-resonant cavity enhanced absorption spectroscopy. The author was with TDL Sensors Ltd., when the experimental part of this work was performed.     

Surface evolution and laser damage resistance of CO2 laser irradiated area of fused silica

A 10.6 μm CO₂ laser has been reported to effectively mitigate the laser damage growth of fused silica. Two zones of the laser irradiated area are defined in this work: the distorted zone and the laser affected zone. The parameters of the two zones are studied at different CO₂ laser beam sizes, irradiation times, and powers by microscopy, profilometry, and photoelastic method. The results show that the diameter of laser affected zone is almost completely determined by the laser beam size and the distorted zone is associated with the mitigation range of CO₂ laser beam. The diameter and depth of the distorted zone increase as the laser power and irradiation time increase. The depth grows exponentially depending on the irradiation time. The maximum residual stress discrepancy is located near the boundary of the laser affected zone. The laser damage resistance test results show that the distorted zone and the laser affected zone have a better damage resistance than the original substrate.