Optical interconnection between different spatial modes using double phase conjugate mirror with polarization alignment system

We propose a fundamental interconnection method using a polarization alignment system for waveguides having different spatial modes. In addition, as an example for the verification of the fundamental operation, we demonstrate an interconnection between a photonic crystal fiber and a laser that have obviously different spatial modes. The polarization alignment system operates synergistically with a self-written waveguide formed with a double phaseconjugate mirror. This technique enables us to interconnect a photonic crystal fiber with a laser source without complicated and time-consuming optical alignment. In this method, although it is not necessary to perform an external control for interconnection, the waveguide most suitable for connection is formed autonomously in a Sn₂P₂S₆:Sb crystal developed for this purpose. There was a marked reduction in the polarization dependence of coupling efficiency, compared with that observed using a stand-alone double phase-conjugate mirror.     

Numerical Simulations of femtosecond-laser-induced dynamic alignment of molecules in the high-frequency off-resonance regime

The motion equation for ? between the molecular axis and laser polarization direction in a high-frequency off-resonance femtosecond laser field is deduced while simultaneously examining the effects of a permanent dipole moment and field-induced polarizability and hyperpolarizability to molecular rotation. Femtosecond-laser-induced dynamic alignment of CO, N₂, and Br₂ molecules are investigated by numerically solving the obtained rotation equation for the angle ?. The effects of the molecular permanent dipole moment and the field-induced polarizability and hyperpolarizability on the degree of alignment are presented at different intensities. Our computational results show that the dynamic alignment of molecules is primarily determined by field-induced polarizability and the second hyperpolarizability for the laser intensity range from 5 × 10¹⁴ to 5 × 10¹⁶ W/cm². The contributions of higher order correction terms to molecular alignment can usually be neglected. The polarizability-field interaction makes the angular distributions of a molecule have a maximum along the polarization axis and a minimum perpendicular to it. The role of the second hyperpolarizability keeps the molecular counts maximum along the laser polarization direction but minimum at an angle of 45° between the molecular axis and the polarization direction. There is also a second maximum of molecular counts perpendicular to the polarization axis. For CO, N₂, and Br₂ molecules, the dependences of laser-induced dynamic alignment on laser intensity exhibit completely different characteristics.     

Field-free three dimensional molecular axis alignment

We investigate strategies for field-free three dimensional molecular axis alignment using strong nonresonant laser fields under experimentally realistic conditions. Using the polarizabilites and rotational constants of an asymmetric top rotor molecule (ethene, C2H4), we consider three different methods for axis alignment of a Boltzmann distribution of rotors at 4 K. Specifically, we compare the use of impulsive kick laser pulses having both linear and elliptical polarization to the use of elliptically polarized switched laser pulses. We show that an enhanced degree of field-free three dimensional alignment of ground vibronic state molecules obtains from the use of two orthogonally polarized, time-separated laser pulses.     

Laser-induced Alignment and Coulomb Explosion of CO2

实验研究了CO₂分子在飞秒强激光脉冲作用下的动力学过程,包括分子取向,隧穿电离和库仑爆炸,激光强度从1×10¹³W/cm²变化到6×10¹⁴W/cm². 当激光强度小于分子的电离阈值时,CO₂分子的非绝热转动激发形成一个相干转动波包,波包演化导致分子沿激光电场方向取向. 激光脉冲结束后,分子取向可以周期性地再现,利用另一束激光可以对取向结构进一步进行修饰. 当激光强度大于分子     

Amplitude and rotation of the ellipticity of harmonicsfrom a linearly polarized laser field

We simulate the dynamic response of H₂⁺ in a linearly polarized laser field by numerically solving the time-dependent Schrödinger equation. The elliptically polarized high-order harmonics generated by H₂⁺ irradiated by the linearly polarized laser field are systematically investigated. The result shows that the amplitude and rotation of the ellipticity of harmonics are affected by the alignment angle and internuclear distance of the molecule. Analyzing the change in forces acted on the ionized electrons and the trajectories of the electrons, the phenomena are found to be due to the change in the direction of the total Coulomb forces from the two nuclei felt by the recollided ionized electrons in the direction perpendicular to the laser polarization direction. Based on the influence law, we can select the harmonics with a specific frequency band under different alignment angles and then synthesize the isolated attosecond pulses with different rotations, which can be continuously converted from right-handed circular polarization, linear polarization, and left-handed circular polarization by changing the alignment angle. This study provides a new possible approach to the real-time detection of molecular states by using attosecond pulses and obtaining more optimized harmonics with molecular properties.     

Closed aperture z-scan technique using the Fresnel–Kirchhoff diffraction theory for materials with high nonlinear refractions

We present for the first time to the best of our knowledge a computerized characterization and alignment scheme of an erbium all-fiber laser passively mode-locked by nonlinear polarization rotation. This scheme can replace the common trial and error alignment procedure with manual polarization controllers and therefore greatly reduce the time to start up an ultrashort pulsed fiber laser. Based on the mode-locking maps generated by automated characterization we also demonstrate reproducible selection of pulses with different pulse durations and central wavelengths. These settings were stable over several days allowing us to choose that pulsed operation mode that is more advantageous for a specific application.     

Signal of collisional beatings of polarization of light as a function of the angular momentum of an excited atomic state

A comparative study of the beats of circular polarization generated by anisotropic atomic collisions is carried out on the basis of theoretical calculations for the states J=1, 3/2, and 2 of neon atoms and ions excited by monochromatic laser radiation in xenon atmosphere. Contributions made by ordinary alignment and higher polarization moments (octupole orientation and dodecapole alignment) in the collisional generation of orientation and the signal of beatings of circular polarization are compared.     

Surface and flexoelectric polarization in a nematic liquid crystal 5CB

The temperature dependence of the surface polarization has been measured for both the planar and homeotropic orientation of a nematic liquid crystal at a solid substrate. A conventional liquid crystal 5CB, pure and doped with an azo-dye, was used in cells with controlled asymmetry for light absorption. The measurements have been made by a pyroelectric technique using short pulses of a YAG laser to create a temperature increment. The latter, in turn, was measured independently by a novel time-resolved “optical thermometer” technique monitoring temperature-dependent birefringence by a He- Ne laser beam. In accordance with the symmetry of the order parameter, the surface polarization has different sign for the two orientations, its magnitude ranges from -4 to +2pC/m. The same technique has been used for the measurement of the flexoelectric polarization in hybrid cells. The sum of the flexoelectric coefficients is e ₁ + e ₃ = - 13pC/m at 25°C. Received 28 February 2000 and Received in final form 5 September 2000     

Orientation dependence in high harmonics of ZnO with polarization corrections to counteract the birefringent effect

We investigate the influence of the birefringence on the high-order harmonics in an a-cut Zn O crystal with midinfrared laser pulses. The high harmonics exhibit strong dependence on the alignment of the crystal with respect to the laser polarization. We introduce the Jones calculus to counteract the birefringent effect and obtain the harmonics with polarization corrections in Zn O. We show that the birefringent effect plays an important role in the orientation dependence of HHG.     

Design and implementation of polarization filter for quantum states discriminator in optical quantum communication

In optical quantum communication, quantum state measurement is necessary. This paper proposes a new technique for realization of polarization filter based on planar lightwave circuit (PLC). This filter is used for quantum state discriminator in quantum communication and also as a Bell-state analyzer in quantum repeater. Electro-optics interferometer has been used in design and implementation of polarization filter. We use lithium niobate as a wafer material and Ti:LiNbO₃ for waveguide. Two directional couplers have been used in this device. The length and spacing of these directional couplers have been designed so that each polarization is routed in specific output. The proposed device has one input and two outputs. If polarization of the input photon is vertical, then this photon will appear in output 1, otherwise if the input photon has horizontal polarization, it appears in output 2. For vertical polarization input, the power overlaps integral (POI) shows that isolation between two outputs is 14.96 dB. As to horizontal polarization input, the isolation between two outputs is 13.8 dB. The designed polarization filter has length of 33 mm and width of 60 μm. This device is very suitable for use in integrated optics.     

Tuning band alignment and optical properties of 2D van der Waals heterostructure via ferroelectric polarization switching

Favourable band alignment and excellent visible light response are vital for photochemical water splitting. In this work, we have theoretically investigated how ferroelectric polarization and its reversibility in direction can be utilized to modulate the band alignment and optical absorption properties. For this objective, 2D van der Waals heterostructures (HTSs) are constructed by interfacing monolayer MoS₂ with ferroelectric In2Se3. We find the switch of polarization direction has dramatically changed the band alignment, thus facilitating different type of reactions. In In₂Se₃/MoS₂/In₂Se₃ heterostructures, one polarization direction supports hydrogen evolution reaction and another polarization direction can favour oxygen evolution reaction. These can be used to create tuneable photocatalyst materials where water reduction reactions can be selectively controlled by polarization switching. The modulation of band alignment is attributed to the shift of reaction potential caused by spontaneous polarization. Additionally, the formed type-II van der Waals HTSs also significantly improve charge separation and enhance the optical absorption in the visible and infrared regions. Our results pave a way in the design of van der Waals HTSs for water splitting using ferroelectric materials.     

Strong-field molecular ionization: determination of ionization probabilities calibrated with field-free alignment

We report an original optical method providing the probability of molecular ionization induced by femtosecond laser pulses. The approach consists of exploiting molecular alignment to extract reliable information about ionization. The cross defocusing technique implemented for this purpose reveals a sensitivity with respect to postpulse alignment, as well as to the free electron density induced by the ultrashort laser pulse. The analysis of the resulting signal thus gives access to absolute single-ionization probabilities calibrated through the degree of alignment, provided that free electrons are produced mainly by single ionization. The relevance of the method is assessed in N2.     

Controlling high harmonic generation with molecular wave packets

We show that, by controlling the alignment of molecules, we can influence the high harmonic generation process. We observed strong intensity modulation and spectral shaping of high harmonics produced with a rotational wave packet in a low-density gas of N2 or O2. In N2, where the highest occupied molecular orbital (HOMO) has sigma(g) symmetry, the maximum signal occurs when the molecules are aligned along the laser polarization while the minimum occurs when it is perpendicular. In O2, where the HOMO has pi(g) symmetry, the harmonics are enhanced when the molecules are aligned around 45 degrees to the laser polarization. The symmetry of the molecular orbital can be read by harmonics. Molecular wave packets offer a means of shaping attosecond pulses.     

Symmetry-breaking instability of multimode vector solitons

We show experimentally that the two-component multimode spatial optical vector soliton, i.e., a two-hump self-guided laser beam, exhibits in Kerr media a sharp space-inversion symmetry-breaking instability. The experiment is performed in a CS2 planar waveguide using the orthogonal circular polarization states of light as the two components of the vector soliton.     

β-Ray angular distribution from purely nuclear spin aligned 20F

The alignment correlation term in the β-ray angular distribution from purely nuclear spin aligned ²⁰F has been measured to test the G-parity conservation law which is one of the fundamental symmetries in the weak nucleon current. We utilized the hyperfine interaction of ²⁰F in an MgF₂ single crystal and successfully created the pure alignment from the polarization by means of the spin manipulation technique based on the β-NMR method.     

离轴八程放大器自动准直技术研究

展示了基于离轴八程激光放大器的闭环自动准直技术研究,该项技术旨在用自动准直系统取代手动光路准直的方式,明显提高了该构型复杂的多程激光放大器的运行效率、准直精度与其输出光束质量。该技术利用主激光照明和像传递系统实现离轴八程激光放大器中滤波器小孔空间位置的精确标定,通过边缘检测处理远场光斑得到其指向中心。基于光斑中心与基准间的差值,对特定反射镜架进行二维控制进行光束指向补正,从而实现离轴八程放大器系统的闭环自动准直。研究结果表明,实验结果契合离轴八程放大器系统对光束准直准确率与效率的要求,验证了该准直技术在离轴八程激光放大光路中应用的可行性。     

激光核聚变装置中基于像传递的激光自动准直技术研究

基于像传递的激光自动准直技术是提高准直准确度、增加稳定性的好方法。本文在理论上用两快速傅里叶变换（ＦＦＴ）分析了准直中基于像传递的近远场信号对图像处理影响。在光束近无场像面上引入十字叉丝并以“神光”装置的某一级空间滤波器为例,计算出光束直径一定时合适的十字叉丝尺寸;最后通过实验确定了基于像传递的自动直技术的具体实现系统。     

The multielectron dissociative ionization dynamics of N2 molecule in intense femtosecond laser fields with arbitrary polarization

The field-ionization Coulomb explosion model is extended to investigate the multielectron dissociative ionization process of N₂ molecule irradiated by an intense femtosecond laser field with an arbitrary polarization. The ionization process of N₂ molecule is found to be optimal at the critical internuclear distance R_c=7a.u., which is independent of the laser polarization state, the molecular explosion channel and the angle between the molecular axis and the direction of laser electric field. The kinetic energies of the ion fragments are identical in the cases of linear and circular polarizations at the same incident laser intensity. However,the probability of electron ionization is very sensitive to the above three parameters. At the critical distance R_c=7a.u. the angular dependence of the threshold intensity for the over-the-barrier ionization leads to the geometric alignment of molecules in the case of linear polarization. The threshold intensity in the case of circular polarization is apparently higher than that in the case of linear polarization, which can well explain the significant decrease of ionization in the case of circular polarization. The numerical calculations are compared with the experimental measurements.     

Origin for ellipticity of high-order harmonics generated in atomic gases and the sublaser-cycle evolution of harmonic polarization

We investigate numerically and analytically the polarization properties of high-order harmonics generated by an atom in intense elliptically polarized laser field. The offset angle of the harmonic polarization ellipse can be well described with the semiclassic "simple-man" high-harmonic generation model. The harmonic ellipticity itself, however, can be hardly understood within this model. We show that this ellipticity originates from quantum-mechanical uncertainty of the electron motion. We develop a theoretical approach describing this ellipticity and, more generally, the time evolution of the high-harmonic polarization state within the laser cycle. The analytical results are verified with the exact numerical solution; to make the comparison accurately, we develop a specific technique for separating the contributions of quantum paths in the numerical calculation.