Experiment studies on the polarized gamma-rays generation at KEK-ATF

Polarized positrons can be created through electron-positron pair creation from circularly polarized gamma-rays. Laser-Compton scattering is an efficient method to generate circularly polarized gamma-rays. A high finesse 2-mirror optical stacking cavity had been installed on the straight section of the electron storage ring at KEK-ATF. A 1064 nm circularly polarized pulsed laser beam was stacked in the cavity. Polarized gamma-rays with a maximum energy of 28.3 MeV were produced via inverse Compton scattering of the enhanced laser pulse off an electron beam of 1.28 GeV. The number of generated gamma photons per collision was estimated by a photon detector. It was found that the experimental result was in agreement with the simulated value.     

Experiment studies on the polarized gamma-rays generation at KEK-ATF

Polarized positrons can be created through electron-positron pair creation from circularly polarized gamma-rays. Laser-Compton scattering is an effcient method to generate circularly polarized gamma-rays. A high finesse 2-mirror optical stacking cavity had been installed on the straight section of the electron storage ring at KEK-ATF. A 1064 nm circularly polarized pulsed laser beam was stacked in the cavity. Polarized gamma-rays with a maximum energy of 28.3 MeV were produced via inverse Compton scattering of the enhanced laser pulse off an electron beam of 1.28 GeV. The number of generated gamma photons per collision was estimated by a photon detector. It was found that the experimental result was in agreement with the simulated value.     

Atomic high-order harmonic generation from a circularly polarized laser pulse

We demonstrate theoretically that the high-order harmonic of an atom can be generated by a circularly polarized laser pulse.The harmonic spectrum shows a clear cutoff with an energy Ip + 2Up.In particular,the high-order harmonic generation comes from the multiple recombination of the ionized electron with non-zero initial velocity.These results are verified by the classical model theory and the time-frequency analysis of a harmonic spectrum.     

Optical rotation quasi-phase-matching for circularly polarized high harmonic generation

A scheme for quasi-phase-matching high harmonic generation of circularly polarized radiation is proposed: optical rotation quasi-phase-matching (ORQPM). In ORQPM, propagation of the driving radiation in a system exhibiting circular birefringence causes its plane of polarization to rotate; by appropriately matching the period of rotation to the coherence length, it is possible to avoid destructive interference of the generated radiation. It is shown that ORQPM is approximately five times more efficient than conventional QPM, and half as efficient as true phase-matching.     

相对论圆偏振激光与固体靶作用产生高次谐波

超短超强激光与固体靶表面等离子体相互作用可以通过高次谐波的方式产生从极紫外到软X射线波段的相干辐射,获得飞秒甚至阿秒量级的超短脉冲,可用于观测原子或分子中的电子运动等超快动力学过程.本文实验研究了相对论圆偏振飞秒激光与固体靶相互作用的高次谐波产生过程,实验结果表明,在较大入射角下,圆偏振激光也可以有效地产生高次谐波辐射.通过预脉冲控制靶表面的预等离子体密度标长,发现高次谐波的产生效率随密度标长的增加而单调下降.进一步通过二维粒子模拟程序,分析了激光的偏振以及预等离子体密度标长对高次谐波产生的影响,很好地解释了实验观测结果.     

Semi-integer harmonic generation from an argon atom by bichromatic counter-rotating circularly polarized laser field

We theoretically investigated the properties of the high-order harmonic generation from an argon atom by bichromatic counter-rotating circularly polarized(BCCP) laser field. The harmonic emission processes have been illustrated by numerically solving the two-dimensional time-dependent Schr ¨odinger equation of an atom in intense laser fields. It is found that with the decrease of the right-circularly polarized laser wavelength, the harmonic spectra are gradually splitting and the harmonic orders move towards the higher frequency. Meanwhile, the integer and semi-integer harmonic emission will be generated when the frequency ratios of right-and left-circularly polarized lasers are semi-integer. The emission mechanism of the semi-integer-order harmonics has been investigated by using the rules of photon absorption and emission.     

Isolated attosecond pulse generation with few-cycle two-color counter-rotating circularly polarized laser pulses

Most of the schemes for generating isolated attosecond pulses(IAP) are sensitive to the carrier-envelope phase(CEP)of the driving lasers. We propose a scheme for generating IAP using two-color counter-rotating circularly polarized(TCCRCP) laser pulses. The results demonstrate that the dependence of the IAP generation on CEP stability is largely reduced in this scheme. IAP can be generated at most of CEPs. Therefore, the experiment requirements become lower.     

High-order harmonic generation of the N2 molecule in two-color circularly polarized laser fields

The generation of high-order harmonics and the attosecond pulse of the N₂ molecule in two-color circularly polarized laser fields are investigated by the strong-field Lewenstein model. We show that the plateau of spectra is dramatically extended and a continuous harmonic spectrum with the bandwidth of 113 eV is obtained. When a static field is added to the x direction, the quantum path control is realized and a supercontinuum spectrum can be obtained, which is beneficial to obtain a shorter attosecond pulse. The underlying physical mechanism is well explained by the time-frequency analysis and the semi-classical three-step model with a finite initial transverse velocity. By superposing several orders of harmonics in the combination of two-color circularly polarized laser fields and a static field, an isolated attosecond pulse with a duration of 30 as can be generated.     

Anisotropic gratings recorded from two circularly polarized coherent waves

A constant intensity, rectilinear light vibration, with spatially varying orientation, is obtained from superposition of two orthogonal circularly polarized waves propagating, in slightly different directions. Such an exposure on a photoanisotropic medium, prints a grating characterized only by spatial variations in the orientation of the induced optic axis. The diffraction properties of such a grating are investigated. It behaves as a uniformly transparent isotropic medium when illuminated by the same waves as during the recording process. Illumination by a circular wave, generates a transmitted and a single diffracted beam, while an incident elliptically polarized one would produce, apart from the central order, two diffracted beams with orthogonal circular polarizations.     

High-order harmonic generation of N2molecule in two-color circularly polarized laser fields

The generation of high-order harmonics and the attosecond pulse of the N_2 molecule in two-color circularly polarized laser fields are investigated by the strong-field Lewenstein model. We show that the plateau of spectra is dramatically extended and a continuous harmonic spectrum with the bandwidth of 113 e V is obtained. When a static field is added to the x direction, the quantum path control is realized and a supercontinuum spectrum can be obtained, which is beneficial to obtain a shorter attosecond pulse. The underlying physical mechanism is well explained by the time–frequency analysis and the semi-classical three-step model with a finite initial transverse velocity. By superposing several orders of harmonics in the combination of two-color circularly polarized laser fields and a static field, an isolated attosecond pulse with a duration of 30 as can be generated.     

Helicity of harmonic generation and attosecond polarization with bichromatic circularly polarized laser fields

We theoretically investigate the high-order harmonic generation(HHG) of helium atom driven by bichromatic counterrotating circularly polarized laser fields. By changing the intensity ratio of the two driving laser fields, the spectral chirality of the HHG can be controlled. As the intensity ratio increases, the spectral chirality will change from positive-to negativevalue around a large intensity ratio of the two driving fields when the total laser intensity keeps unchanged. However, the sign of the spectral chirality can be changed from positive to negative around a small intensity ratio of the two driving fields when the total laser intensity changes. At this time, we can effectively control the helicity of the harmonic spectrum and the polarization of the resulting attosecond pulses by adjusting the intensity ratio of the two driving laser fields. As the intensity ratio and the total intensity of the driving laser fields increase, the relative intensity of either the left-circularly or right-circularly polarized harmonic can be enhanced. The attosecond pulses can evolve from being elliptical to near linear correspondingly.     

Emission of circularly polarized recombination radiation from p-doped GaAsP bombarded with longitudinally polarized electrons

We report measurements of circularly polarized recombination radiation from a GaAs_0.6P_0.4 crystal bombarded with longitudinally spin polarized electrons of 10 eV energy. Qualitatively, this process can be considered as the inverse of the photoemission utilized in GaAs and GaAsP polarized-electron sources. The measured degree of circular polarization is 0.29%±0.06% if the crystal is cooled with liquid nitrogen and the incident electrons are 15%±3% polarized.     

双色反向旋转圆偏振激光场下氩原子高次谐波的产生

通过二阶分裂算符法数值求解了二维含时薛定谔方程,研究了双色反向旋转圆偏振激光场作用下氩原子的高次谐波的产生.研究表明,双色反向旋转圆偏振激光场的幅值比对高次谐波发射的效率有较大影响,当幅值比为1.4时高次谐波的发射效率最高,并应用时频分析方法、经典三步模型以及利萨茹图形等解释了高次谐波发射的物理机制.研究了不同幅值比情况下谐波场的旋转偏振特性,发现对不同频段的谐波谱的电场进行叠加,谐波场的偏振特性随幅值比的不同而发生变化,但谐波场的旋向性与入射激光脉冲的旋向性相同,不受幅值比变化的影响.     

Monoenergetic ion beam generation by driving ion solitary waves with circularly polarized laser light

Experimental data from the Trident Laser facility is presented showing quasimonoenergetic carbon ions from nm-scaled foil targets with an energy spread of as low as ±15% at 35?MeV. These results and high-resolution kinetic simulations show laser acceleration of quasimonoenergetic ion beams by the generation of ion solitons with circularly polarized laser pulses (500?fs, λ=1054 nm). The conversion efficiency into monoenergetic ions is increased by an order of magnitude compared with previous experimental results, representing an important step towards applications such as ion fast ignition.     

Magnetic field generation in a plasma produced through atomic ionization by circularly polarized radiation

We have established the dependences of the maximum Weibel instability growth rate and the corresponding wavenumber on the degree of anisotropy in the photoelectron distribution formed through tunnel atomic ionization in the field of a circularly polarized short laser pulse. We show how the relaxation of the initial distribution of photoelectrons due to their collisions with ions affects the pattern of generation of a quasi-static magnetic field.     

Harmonic generation by atoms in circularly polarized laser fields: far-off and near resonance regimes

The generation of harmonics by atoms interacting with two laser fields having coplanar circular polarizations and an integral frequency ratio is addressed through ab initio numerical simulations. A detailed characterization of a few specific harmonics is given. In particular, the two different cases where the total energy absorbed through photons is far off or close to the energy gap between different atomic states are investigated. It is found that the conversion efficiency in the harmonic generation is strongly dependent on the inner atomic structure and in certain specific cases it can be significantly enhanced within a small frequency range. PACS 42.50.Hz; 42.65.Ky     

Spin polarization of photoelectrons from solid cesium by circularly polarized light

The spin polarization of photoelectrons excited in cesium metal by circularly polarized light is discussed using group theoretical methods. Calculations of the spin polarization have been performed for direct transitions at the-point and between, and states. The results suggest that photoelectrons excited in monocrystalline Cs could have a high degree of spin polarization which depends both on the wave vector of the electrons and on the energy of the absorbed photon. Measurement of this polarization should allow the spin-orbit splitting of different energy bands to be determined.     

High-order harmonic generation of the cyclo[18]carbon molecule irradiated by circularly polarized laser pulse

High-order harmonic generation of the cyclo[18]carbon (C₁₈) molecule under few-cycle circularly polarized laser pulse is studied by time-dependent density functional theory. Compared with the harmonic emission of the ring molecule C₆ H₆ having similar ionization potential, the C₁₈ molecule has higher efficiency and cutoff energy than C₆ H₆ with the same laser field parameters. Further researches indicate that the harmonic efficiency and cutoff energy of the C₁₈ molecule increase gradually with the increase of the laser intensity of the driving laser or decrease of the wavelength, both are larger than those of the C₆ H₆ molecule. Through the analysis of the time-dependent evolution of the electronic wave packets, it is also found that the higher efficiency of harmonic generation can be attributed to the larger spatial scale of the C₁₈ molecule, which leads to a greater chance for the ionized electrons from one atom to recombine with others of the parent molecule. Selecting the suitable driving laser pulse, it is demonstrated that high-order harmonic generation in the C₁₈ molecule has a wide range of applications in producing circularly polarized isolated attosecond pulse.     

Ultra-thin circularly polarized lens antenna based on single-layered transparent metasurface

Circularly polarized(CP) lens antenna has been applied to numerous wireless communication systems based on its unique advantages such as high antenna gain, low manufacturing cost, especially stable data transmission between the transmitter and the receiver. Unfortunately, current available CP lens antennas mostly suffer from high profile, low aperture efficiency as well as complex design. In this paper, we propose an ultra-thin CP lens antenna based on the designed singlelayered Pancharatnam–Berry(PB) transparent metasurface with focusing property. The PB metasurface exhibits a high transmissivity, which ensures a high efficiency of the focusing property. Launched the metasurface with a CP patch antenna at its focal point, a low-profile lens antenna is simulated and measured. The experimental results show that our lens antenna exhibits a series of advantages including high radiation gain of 20.7 dB, aperture efficiency better than 41.3%, and also narrow half power beam width(HPBW) of 13°at about 14 GHz. Our finding opens a door to realize ultra-thin transparent metasurface with other functionalities or at other working frequencies.