Laser Polarization Effect on Molecular Harmonic and Elliptically Polarized Attosecond Pulse Generation

Molecular harmonic spectra of H+2driven by the linearly polarized laser pulses with different polarized angles have been theoretically investigated through solving the two-dimensional time-dependent Schr¨odinger equation.(i) Below-threshold harmonic spectra show a visible enhanced peak around the 7th harmonic(H7), which produces a red-shift phenomenon as the internuclear distance increased. Theoretical analyses show the red-shift enhanced peak is caused by the laser-induced electron transfer between the ground state and the 1st excited state of H+2.(ii) Due to the two-centre interference phenomenon, the above-threshold harmonic spectra exhibit many maxima and minima.(iii)With the introduction of the polarized angle, the anomalous elliptically polarized harmonics can be found. But, with the introduction of the spatial inhomogeneous effect, not only the ellipticities of the harmonics are equal to a stable value of ε～ 0.1–0.3, but also the harmonic cutoffs are extended. As a result, four super-bandwidths of 407 e V, 310 e V,389 e V, and 581 e V can be obtained. Time profiles of the harmonic generations have been shown to explain the harmonic characteristics. Finally, a series of elliptically polarized(ε～ 0.1–0.3) attosecond X-ray pulses with durations from 18 as to 25 as can be directly produced through Fourier transformation of the spectral continuum.     

Polarization switching in a quasi-isotropic microchip Nd:YAG laser induced by optical feedback

This paper demonstrates the influence of external optical feedback on the polarization state of longitudinal modes in quasi-isotropic microchip Nd:YAG lasers. Under optical feedback, the polarization state of longitudinal modes in quasi-isotropic lasers relies strongly on the intracavity anisotropy loss and mode competition. When the intracavity anisotropy loss is small, external optical feedback can cause polarization switching and strong mode competition between two orthogonal linearly polarized eigenstates of one laser longitudinal mode, which leads to the distortion of laser intensity modulation waveform. The polarization switching is independent of the initial external cavity length. By increasing the intracavity anisotropy loss, one polarization eigenstate can be suppressed and the laser works in single-polarization state. A theoretical analysis based on the compound cavity model is presented, which is in good agreement with the experimental results. The results offer guidance to the development of laser feedback interferometers.     

Theoretical study and optimization of a high power mid-infrared erbium-doped ZBLAN fibre laser

Based on the rate equations describing the erbium-doped fluoride glass(ZBLAN) fibre lasers with different pumping configurations being taken into account,this paper presents theoretical calculations related to the dynamic population density and the operation performance of a high power mid-infrared all-fibre erbium-doped ZBLAN fibre laser.It shows that the ground-state absorption,excited-state absorption and energy-transfer-upconversion processes co-exist and produce a population balance,causing the laser to operate stably at a continuous wave state.A good agreement between the theoretical results and recent experimental measurement is obtained.Furthermore,the laser structure parameters including fibre length,reflectance of output fibre Bragg grating and pumping configurations are quantitatively optimised to achieve the best performance.The results show,as expected,that the slope efficiency of the fibre laser can be improved significantly through optimisation,which then provides an important guide for the design of high-performance mid-infrared erbium-doped ZBLAN fibre lasers.     

Anomalously Large Deformation in Some Medium and Heavy Nuclei

We investigate the properties of the Ce isotopes with neutron number N =60 - 90 and the properties of the heavy nuclei near 242Am within the framework of deformed relativistic mean-field （RMF） theory. A systematic comparison between theoretical results and experimental data is made. The calculated binding energies, two-neutron separation energies, and two-proton separation energies are in good agreement with experimental ones. The variation trend of experimental quadrupole deformation parameters on the Ce isotopes can be approximately reproduced by the RMF model. It is found that there exists an abnormally large deformation in the ground state of proton-rich Ce isotopes. This phenomenon can be the general behavior of proton-rich nuclei on the neighboring isotopic chains such as Nd and Sin isotopes. For the heavy nuclei near ^242 Am the properties of the ground state and superdeformed isomeric state can be approximately reproduced by the RMF model. The mechanism of the appearance of anomalously large deformation or superdeformation is analyzed and its influence on nuclear properties is discussed. Parther experiments to study the anomalously large deformation in some proton-rich nuclei are suggested.     

Polarization switching and synchronization of mutually coupled vertical-cavity surface-emitting semiconductor lasers

Polarization switching (PS) dynamics and synchronization performances of two mutually coupled vertical-cavity surface-emitting lasers (VCSELs) are studied theoretically in this paper. A group of dimensionless rate equations is derived to describe our model. While analysing the PS characteristics, we focus on the effects of coupling rate and frequency detuning regarding different mutual injection types. The results indicate that the x-mode injection defers the occurrence of PS, while the y-mode injection leads the PS to occur at a lower current. Strong enough polarization-selective injection can suppress the PS. Moreover, if frequency detuning is considered, the effects of polarization-selective mutual injection will be weakened. To evaluate the synchronization performance, the correlation coefficients and output dynamics of VCSELs with both pure mode and mixed mode polarizations are given. It is found that performance of complete synchronization is sensitive to the frequency mismatch but it is little affected by mixed mode polarizations, which is opposite to the case of injection-locking synchronization.     

Anomalously Large Deformation in Some Medium and Heavy Nuclei

We investigate the properties of the Ce isotopes with neutron number N = 60 ～ 90 and the properties of the heavy nuclei near 242 Am within the framework of deformed relativistic mean-field (RMF) theory. A systematic comparison between theoretical results and experimental data is made. The calculated binding energies, two-neutron separation energies, and two-proton separation energies are in good agreement with experimental ones. The variation trend of experimental quadrupole deformation parameters on the Ce isotopes can be approximately reproduced by the RMF model. It is found that there exists an abnormally large deformation in the ground state of proton-rich Ce isotopes.This phenomenon can be the general behavior of proton-rich nuclei on the neighboring isotopic chains such as Nd and Sm isotopes. For the heavy nuclei near 242 Am the properties of the ground state and superdeformed isomeric state can be approximately reproduced by the RMF model. The mechanism of the appearance of anomalously large deformation or superdeformation is analyzed and its influence on nuclear properties is discussed. Further experiments to study the anomalously large deformation in some proton-rich nuclei are suggested.     

Resolving multi-orbital effects on high harmonic generation from aligned N_2 molecules in linearly and elliptically polarized intense laser fields

We perform an experimental study of the multi-orbital effect on the high-order harmonic generation(HHG) from aligned N_2 molecules in both linearly and elliptically polarized intense laser fields.Measured by a home-built extreme ultraviolet(XUV) flat grating spectrometer with the pump–probe method, the angular distributions of different orders of HHG are obtained, which show distinctive behaviors for harmonics in the plateau and the cut-off regions.The ellipticity dependence of HHG is investigated by aligning the molecular axis parallel or perpendicular to the laser polarization.Our results indicate that both the highest occupied molecular orbital(HOMO) as well as the lower one(HOMO-1) contribute to the HHG of N_2 molecules, in either linearly or elliptically polarized intense laser field.The study paves the way for understanding the ultrafast electron dynamics of molecules exposed to an intense laser field.     

Phase-related noise characteristics of 780 nm band single-frequency lasers used in the cold atomic clock

We propose a method to directly measure phase-related noise characteristics of single-frequency lasers in the 728–980 nm band based on a 120°phase difference interferometer.Differential phase information of the laser under test is demodulated via the interferometer.Other parameters related to the phase noise characteristics such as linewidth at different observation time, phase/frequency noise, power spectrum density of phase/frequency fluctuation, and Allan deviation are further obtained.Frequency noise as low as 1 Hz~2/Hz can be measured using our system.Then the phase-related noise characteristics of two commercial lasers frequently used in cold atomic clocks are studied systematically by the method.Furthermore, several influencing factors and their relative evolution laws are also revealed, such as the pump current and frequency-locking control parameters.This would help to optimize the laser performance, select laser sources, and evaluate the system performance for cold atomic physics applications.     

Spectral filtering of dual lasers with a high-finesse length-tunable cavity for rubidium atom Rydberg excitation

We propose and demonstrate an alternative method for spectral filtering and frequency stabilization of both 780-nm and 960-nm lasers using a high-finesse length-tunable cavity(HFLTC). Firstly, the length of HFLTC is stabilized to a commercial frequency reference. Then, the two lasers are locked to this HFLTC using the Pound–Drever–Hall(PDH) method which can narrow the linewidths and stabilize the frequencies of both lasers simultaneously. Finally, the transmitted lasers of HFLTC with each power up to about 100 μW, which act as seed lasers, are amplified using the injection locking method for single-atom Rydberg excitation. The linewidths of obtained lasers are narrowed to be less than 1 k Hz, meanwhile the obtained lasers' phase noise around 750 k Hz are suppressed about 30 d B. With the spectrally filtered lasers, we demonstrate a Rabi oscillation between the ground state and Rydberg state of single-atoms in an optical trap tweezer with a decay time of(67 ± 37) μs, which is almost not affected by laser phase noise. We found that the maximum short-term laser frequency fluctuation of a single excitation lasers is at ～ 3.3 k Hz and the maximum long-term laser frequency drift of a single laser is ～ 46 k Hz during one month. Our work develops a stable and repeatable method to provide multiple laser sources of ultra-low phase noise, narrow linewidth, and excellent frequency stability, which is essential for high precision atomic experiments, such as neutral atom quantum computing, quantum simulation, quantum metrology, and so on.     

Optical Feedback Characteristics in He-Ne Dual Frequency Lasers

Optical feedback characteristics in He-Ne dual frequency lasers are studied systematically in different feedback power ratios with a variable attenuator. Feedback power ratios vary from 0.010 up to 0.998. Five distinct regimes of self-interference effects are found and defined as regimes Ⅰ, Ⅱ,Ⅲ,Ⅳand Ⅴ. Accordingly, five optical feedback levels have been put forward in He-Ne dual frequency lasers. Strong mode competitions are observed in regimes Ⅲ and Ⅳ. In regime Ⅴ, multiple feedback effects are investigated. The basic theoretical analysis is also presented. Our results can advance the research of self-mixing interferometer and displacement sensor of He-Ne orthogonally polarized dual frequency lasers.     

基于两正交互耦1550nm垂直腔面发射激光器获取多路随机数

提出将正交互耦1550 nm垂直腔面发射激光器(1550 nm-VCSEL)在优化条件下输出的多路平均功率可比拟、延时特征(TDS)得到有效抑制的混沌信号作为混沌熵源,经8位模数转换器(ADC)量化和最低有效位(m-LSB)后续处理获取多路物理随机数的方案,并研究了系统参量对最终获取的比特序列随机性的影响.首先,基于VCSEL的自旋反转模型分析耦合强度和频率失谐对两个正交互耦合1550 nm-VCSEL输出动力学的影响,初步确定利用该系统产生四路平均功率可比拟、TDS得到抑制的混沌信号所需的耦合强度和频率失谐优化范围;在此基础上,选择一个耦合强度值,利用处于优化范围内的不同频率失谐下获取的四路混沌信号作为熵源,经8位ADC量化和m-LSB后续处理得到最终的比特序列;最后,采用NIST Special Publication800-22统计测试套件对获取的最终比特序列的随机性能进行测试,确定了同时获取四路高质量随机数所需的参数范围.     

Instability of linearly polarized lasing modes of a single-mode VCSEL

Conditions of stability of linearly polarized lasing modes are analyzed within the framework of the spin-flip theory of VCSELs over a wide range of parameters in relation to the linear dichroism and linear birefringence introduced into the system by an optically anisotropic semiconductor crystal.     

Mechanisms of polarization switching in single-transverse-mode vertical-cavity surface-emitting lasers: thermal shift and nonlinear semiconductor dynamics

We analyze polarization switching in vertical-cavity surface-emitting lasers, taking into account a proper semiconductor frequency-dependent complex susceptibility and spin-flip processes. Thermal effects are included as a varying detuning, and gain differences arise from birefringence splitting. We find that, for large birefringence, gain differences between the two linearly polarized modes are preponderant, and switching occurs owing to thermal shift. For small birefringence polarization switching from the high- to the low-gain mode occurs owing to the combined effect of birefringence and semiconductor phase-amplitude-coupled dynamics for a finite value of the carrier spin-flip rate.     

Polarization manipulated solid-state lasers with crystalline-orientations

The polarization states of 〈111〉-cut Yb:YAG crystal microchip lasers were investigated by pumped with the elliptically polarized pump beam from fiber-coupled laser-diode. The manipulated polarized lasers were achieved in laser-diode pumped Yb:YAG microchip laser by controlling the crystalline-orientations in 〈111〉-growth Yb:YAG crystal. Generally elliptically polarized lasers were obtained in laser-diode pumped Yb:YAG microchip lasers. However, crystalline-orientation manipulated linearly polarized laser was obtained when six different sites with different crystalline orientations were set to parallel to the major axis direction of the elliptically polarized pump beam. Six different sites in Yb:YAG crystal were separated with 30° and 90°, which were responsible for the linearly polarized laser oscillations. Circularly polarized lasers were observed when a Yb:YAG crystal was aligned to a special position between two sites responsible for linearly polarized laser oscillation. Effects of the polarization states of pump source on the laser polarization states of Yb:YAG microchip lasers and polarization direction of different polarized lasers with respect to Yb:YAG crystal rotation was addressed.     

1550 nm垂直腔面发射激光器的特征参量随温度的变化

在采用自旋反转模型分析垂直腔面发射激光器（VCSELs）动力学行为的过程中,为了正确预测VCSELs的动力学行为,需要准确给出自旋反转模型中光场衰减速率k、总反转载流子衰减速率γ_N、线性二向色性系数γ_a、线性双折射系数γ_p、自旋反转速率γ_s和线宽增强因子α这6个特征参量.本文对1550 nm VCSELs在自由运行和平行光注入下的输出特性进行实验分析,获取了这6个特征参量的值,并着重研究了当激光器温度在10.00–30.00 ℃范围内变化时,这6个特征参量呈现的变化趋势.研究结果表明,随着温度的逐渐升高,γ_p整体呈现逐渐增加的趋势,γ_a,γ_s,γ_N和k呈现复杂的变化趋势,而α则呈现逐渐减小的趋势.     

Propagation of polarized light in opals: Amplitude and phase anisotropy

The interaction of linearly polarized light with photonic crystals based on bulk and thin-film synthetic opals is studied. Experimental transmission spectra and spectra showing the polarization state of light transmitted through opals are discussed. A change in polarization is found for waves experiencing Bragg diffraction from systems of crystallographic planes of the opal lattice. It is shown that the polarization plane of the incident linearly polarized wave at the exit from photonic crystals can be considerably rotated. In addition, incident linearly polarized light can be transformed to elliptically polarized light with the turned major axis of the polarization ellipse. Analysis of polarization states of transmitted light by using the transfer-matrix theory and homogenization theory revealed good agreement between calculated and experimental spectra.     

用Stokes参量法实现数字同轴偏振全息的研究

提出了一种基于Stokes参量的数字同轴偏振全息方法.在实验中用一束线偏振光和一束椭圆偏振光作为参考光, 分别与物光进行干涉,通过拍摄在两个相互垂直方向上的全息图,计算出物光在这两个方向的振幅和相位信息, 从而得到物光的Stokes参量和物体的全偏振信息,实现对各向异性物体偏振态空间分布的图像重建. 实验结果表明,该方法可用于物体的全偏振特性的测量.这种方法在求出物光Stokes参量的同时, 也可消除零级像和共轭像的干扰,因此也可用于同轴或离轴全息.     

光电负反馈下单向耦合注入垂直腔表面发射激光器的混沌同步特性研究

基于自旋反转模型（SFM）,对光电负反馈下单向耦合注入垂直腔表面发射激光器（VCSEL）的混沌同步特性进行了数值仿真和理论分析.研究结果表明：随着反馈强度的增加,激光器输出功率呈现两个不同的混沌区域.在单一偏振模区域,能实现很好的同步性,但接近两模共存时,同步质量急剧下降;在两模共存区域,偏振模的连续性差,同步质量不理想.两模共存时的同步质量比单一偏振模差的这一特性,与已报道的基于光反馈的情景相反.产生这一特殊现象的原因是光电负反馈诱发了偏振模竞争效应.此外,还发现内部参数的变化对两个混沌区域的选取有较大 关键词： 线性偏振模 光电负反馈 垂直腔表面发射激光器（VCSEL） 混沌同步     

Reflection and absorption of HF radiation by a turbulent plasma

Reflection and absorption of a linearly polarized wave by a plasma with developed ion-acoustic turbulence is studied. The dependences of the absorption coefficient and Stokes parameters of the reflected elliptically polarized wave on the anisotropic effective frequency of electron collisions are established.     

Nonlinear dynamics and localized synchronization in mutually coupled VCSELs

Nonlinear dynamics of mutually coupled vertical-cavity surface-emitting lasers (VCSELs) are studied theoretically. We consider both identical and nonidentical cases, respectively. For identical cases, where the device parameters and the operating conditions are the same, both lasers show the same behaviors with an identical output, and some typical nonlinear states, including period-one, period-two, multi-period and chaos, are observed. However, for nonidentical cases, the symmetric behaviors are broken. Due to the mismatch of the bias current of these two lasers, the system nonlinearity is increased and the synchronization performance is degraded greatly. Nevertheless, it is found that the coupled VCSELs can still exhibit a form of localized synchronization when they operate under the periodic state introduced by either the weak coupling or the frequency detuning.