Laser beam alignment by fast feedback control of both linear and angular drifts

A new way to enhance the directional stability of laser beams with alternating intensity by fast feedback control of both linear and angular drifts has been proposed for alignment of laser beams at higher accuracy. Both linear and angular drifts of laser beams, processed through light intensity modulation and primary alignment using single-mode optical fiber (SMOF), are separated using light path arrangement and detected using phase-lock technique, and are controlled using fast feedback control mechanisms according to their detected magnitudes, so that both linear and angular drifts are suppressed to enhance the directional stability of the emitting laser beams. Theoretical analyses and preliminary experimental results indicate that the approach proposed can be used to achieve an alignment accuracy of more than 10⁻⁸ rad.     

Evidence for shape coexistence in186Pt

High-spin states in¹⁸⁶Pt have been populated by the¹⁸⁸Os (α, 6n) reaction and were investigated with the OSIRIS spectrometer. A shape coexistence at high spins was established in the nucleus¹⁸⁶Pt, which lies on the border between light prolate and heavy oblate Pt nuclei. Two bands corresponding to predominantly prolate shapes and one band of predominantly oblate shape have been observed. For prolate shapes a (π h _9/2)² alignment and for oblate shapesa (vi _13/2)² alignment has been found.     

All‐Dielectric Meta‐Reflectarray for Efficient Control of Visible Light

Wide‐bandgap material based all‐dielectric metasurfaces have been ideal platforms for the realization of arbitrary phase control in visible spectrum. While TiO₂ metasurfaces are very promising in broadband and high‐efficiency anomalous transmission, meta‐hologram, and meta‐lenses et al., the current realizations are strongly dependent on the sophisticated fabrication technique to fabricate TiO₂ nano‐pillars with aspect ratio > 10. Herein we experimentally demonstrate a much simpler approach to realize efficient phase control of visible light. By exploiting TiO₂ nano‐blocks as meta‐atoms on a ground metal plane, we find that TiO₂ metasurface with aspect ratio around 1‐1.5 is good enough to produce phase changes covering ‐π to π and high reflection efficiency simultaneously. Based on the phase control of the meta‐reflectarray, anomalous reflection with a ratio between anomalous reflection and normal reflection ～ 74/26 have been experimentally realized using a combination of typical electron‐beam lithography, electron‐beam evaporation, and a simple lift‐off process. Similarly, high performance TiO₂ metasurface in form of hologram has also been demonstrated for red (633 nm), green (520 nm), and blue (445 nm) wavelengths. We believe this research shall route a new way to cost‐effective all‐dielectric metasurfaces and advance their applications in encryption, anti‐counterfeiting, and wearable displays.     

Giant Three‐Photon Absorption in Monolayer MoS2 and Its Application in Near‐Infrared Photodetection

For decades, there has been extensive research on exploring fundamental physical mechanisms for strong and fast optical nonlinearities. One of the important nonlinear‐optical mechanisms is multiphoton absorption which has a wide range of photonic applications. Herein, a theoretical model is proposed for three‐photon absorption (3PA) in monolayer MoS₂. The model shows that the 3PA coefficients are on the order of 0.1 cm³/GW². As compared to bulk semiconductors, these coefficients are enhanced by several orders of magnitude due to excitonic effects. Such exciton‐enhanced 3PA is validated by light‐intensity‐dependent photocurrent measurements on a monolayer MoS₂ photodetector with femtosecond laser pulses. These results lay both theoretical and experimental foundation for developing sensitive near‐infrared MoS₂‐based three‐photon detectors.     

Active long pulse shaping technique of pulsed CO2 laser using multi-pulse discharge control

We describe the pulse forming of pulsed CO₂ laser using multi-pulse superposition technique. Various pulse shapes, high duty cycle pulse forming network (PFN) are constructed by time sequence. This study shows a technology that makes it possible to make various long pulse shapes by activating SCRs of three PFN modules consecutively at a desirable delay time with the aid of a PIC one-chip microprocessor. The power supply for this experiment consists of three PFN modules. Each PFN module uses a capacitor, a pulse forming inductor, a SCR, a high voltage pulse transformer, and a bridge rectifier on each transformer secondary. The PFN modules operate at low voltage by driving the primary of HV pulse transformer. The secondary of the transformer has a full-wave rectifier, which passes the pulse energy to the load in a continuous sequence.We investigated various long pulse shapes as different trigger time intervals of SCRs among three PFN modules. As a result, we could obtain laser beam with various pulse shapes and durations from about 250 to 1000 μs.     

Angular distributions of promptγ-rays in the light charged particle accompanied and binary fission of252Cf

Double-differential emission distributions W(B,E_x) in angle 8 and energy E_x of prompt x-rays released in binary and light charged particle (LCP) accompanied fission of 252-Cf have been measured. The x-ray anisotropies are rather similar for both fission modes but the shapes of the angular distributions differ considerably. The maxima of the angular distributions are shifted from zero degree in the binary mode to 8≈20.30 deg. in the LCPac. fission. This behaviour may be understood as an influence of the α-particle (predominantly emitted in equatorial direction) on the alignment of angular momenta produced in bending modes.     

Frequency doubling and Fourier domain shaping the output of a femtosecond optical parametric amplifier: easy access to tuneable femtosecond pulse shapes in the deep ultraviolet

Tuneable, shaped, ultraviolet (UV) femtosecond laser pulses are produced by shaping and frequency doubling the output of a commercial optical parametric amplifier (OPA). A reflective mode, folded, pulse shaping assembly employing a spatial light modulator (SLM) shapes femtosecond pulses in the visible region of the spectrum. The shaped visible light pulses are frequency doubled to generate phase- and amplitude-shaped, ultrashort light pulses in the deep ultraviolet. This approach benefits from a simple experimental setup and the potential for tuning the central frequency of the shaped ultraviolet waveform. A number of pulse shapes have been synthesised and characterised using cross-correlation frequency resolved optical gating (XFROG). This pulse shaping method can be employed for coherent control experiments in the ultraviolet region of the spectrum where many organic molecules have strong absorption bands. D.S.N. Parker and A.D.G. Nunn contributed equally to this work.     

Interrelationship between the polarization moments of different parity upon optical pumping of atoms under magnetic resonance conditions: II. Theory

The mutual coupling between the polarization moments with ranks of different parity is theoretically considered. The manifestation of this mutual coupling has been revealed previously in experiments on magnetic resonance of optically oriented cesium atoms. The two well-known types of the coupling between the polarization moments are considered: the field coupling of these moments that occur due to the breaking of the hyperfine coupling between the electronic and nuclear moments of the alkali atom by the magnetic field and the light coupling of the moments due to the absorption of the pumping light by polarized atoms. The experimentally observed similarity in the shape of resonance signals of alignment and orientation upon circularly polarized pumping can be explained by the fact that, for alkali atoms, the generation of alignment by light at the wavelength of the D ₁ line is of low efficiency. Therefore, alignment arises mainly from orientation by means of either the field or the light coupling of polarization moments. For metastable 2³ S ₁ ⁴He atoms, no influence of the orientation on the alignment was observed because, in these atoms, the field coupling between the polarization moments is absent and the light coupling is not displayed because the generation of alignment by the circularly polarized pumping light is more efficient than the creation of alignment from orientation by means of light coupling of polarization moments.     

Angular distributions of CH3I fragment ions under the irradiation of single pulse and trains of ultrashort laser pulses

The angular distribution of CH₃I is investigated experimentally using a single Fourier transform-limited laser pulse and a pulse train, where a 90-fs 800-nm linearly polarized laser field with a moderate intensity of 2.8×10¹³ W/cm² is used. The dynamic alignment is demonstrated in a single pulse experiment. Moreover, a pulse train is used to optimize the molecular alignment, and the alignment degree is almost identical to that with the single pulse. The results are analysed by using chirped femtosecond laser pulses, and it demonstrates that the structure of pulse train rather than its effective duration is crucial to the molecular alignment.     

Probing the acoustic vibrations of single metal nanoparticles by ultrashort laser pulses

The strong interaction of metal nanoparticles with light makes it possible to detect individual particles by far‐field optical methods. In this article, the interaction of a metal nanoparticle with a short laser pulse is discussed, with the emphasis on the coherent excitation of mechanical (acoustic) modes and the optical detection of these vibrations. The literature on acoustic vibrations of single metal nanoparticles of different shapes (spheres, dumbbells, rods, cubes, wires, prisms) is reviewed, and the modes that have been excited and detected in these particles are discussed. Finally, the insights and potential applications enabled by these studies are summarized.     

修整放大池对双布里渊放大池控制脉冲波形的影响

在传统独立双池结构的受激布里渊相位共轭镜系统中,引入修整放大池,采用主放大池与修整放大池相结合的双布里渊放大池放大控制脉冲波形.主放大池控制脉冲的整体波形,而修整放大池对脉冲波形,特别是脉冲前沿进行微调.修整放大池池长对双布里渊放大池放大控制脉冲波形的影响进行了理论研究和实验验证,得到了不同情况下脉冲波形随双池间距的变化规律.研究表明修整放大池池长决定放大光前沿的放大效率,是控制脉冲波形的重要参量.     

A flexible blue light sensitive organic photodiode with high properties for the applications in low‐voltage‐control circuit and flexion sensors

Low‐voltage‐control circuit is one of the most important parts of the modern electrical control system due to the avoidance of operation risk and easy automation. Here, based on a C₆₀: m‐MTDATA bulk heterojunction, a blue‐light‐sensitive organic photodiode (OPD) is explored for the development of flexible low‐voltage‐control circuit. The control of circuit under 2000 V high voltage is achieved. The influences of the organic‐layer thickness, the donor/acceptor volume ratio and the matching of energy levels on the photocurrent are investigated. The maximum light/dark current ratio and current transfer ratio of 1.3 × 10⁴ and 1.3% are achieved, respectively. The highest photoresponse is up to 130 mA/W, markedly higher than some commercial inorganic photodiodes. This device could also be used as flexion and mechanical force sensors with the current density changing under different bending conditions. Therefore, this sort of OPD has a promising application in low‐voltage‐controlled, high‐voltage‐endurable hands for intelligent robots.     

On the Theory of Hydrogen Atom Ionization by Ultra‐Short Electromagnetic Pulses

An investigation of the probability of hydrogen atom ionization by ultra‐short electromagnetic pulses is carried out in the frame of perturbation theory We consider the case when the electric field strength amplitude E₀ in a pulse by two orders lower than characteristic atomic field strength E_a (E_a ? 5.1 · 10⁹ V/cm). A detailed investigation of the dependence of the probabilities on the pulse duration was performed for Gaussian pulse shapes. In the case where the carrier frequency of the Gaussian pulse is larger than the atomic ionization potential, the probability goes to the standard limit of perturbation per unit time. At the same pulse durations, the probabilities for carrier frequencies less than the ionization potential go to zero. The frequency dependence of the ionization probability becomes equal to the standard threshold dependence with increasing pulse duration time. A comparison between the ionization effects caused by wavelet pulses without carrier frequency and Gaussian pulses with carrier frequency shows that the same ionization probability values are achieved when the pulse carrier frequency is detuned by about 20% from the ionization threshold. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Laser pulses designed in time by adaptive hydrodynamic modeling for optimizing ultra-fast laser–metal interactions

Determining optimal temporal pulse shapes is an essential aspect for controlling the nature and the energetic characteristics of the ablation products following laser irradiation of materials on ultra-fast scales. In this respect, adaptive feedback loops based on temporal pulse manipulation have been inserted into a hydrodynamic code. The procedure enables us to reach the theoretical maximal temperature at a certain energy input. Several regimes have been considered with fluences ranging from the ablation threshold (F _th=0.34 J/cm²) up to 10 J/cm², proposing an optimal coupling for laser–solid and laser–plasma interactions in these fluence regimes. We determine shapes of optimal pulses on ultra-short and short scales (up to 42 ps) and forecast optimized interaction scenarios with fundamental control factors difficult to access experimentally. Simulations performed on aluminum reveal that ultra-short pulses are the natural better solution for localizing energy in space and time for F～F _th. For higher fluences, pulses spread over tens of picoseconds and ended by a final peak enable a better impulsive coupling with the nascent plasma, optimizing its maximal temperature.     

Studying condensed phase formed upon interaction between a high-power laser pulse and crystal aluminum oxide

Forms of the condensed phase created under the action of a high-power CO₂ laser pulse (power, 4 J/pulse; λ = 10.6 μm; pulse duration, 1.5 μs) on optical sapphire (Al₂O₃) solid targets are investigated. The ablation products emitted from the laser crater and the particles formed in the space above the surface of the target are collected using witness samples oriented in a predetermined manner with respect to the laser crater. Forms of the condensed phase are studied via electron microscopy and atomic force microscopy. Conclusions are drawn as to the mechanisms of formation of particles with different shapes and structures, including Al₂O₃ vacuum hollow microspheres (hollow bubbles).     

On the application of a single‐crystal κ‐diffractometer and a CCD area detector for studies of thin films

A multipurpose six‐axis κ‐diffractometer, together with the brilliance of the ESRF light source and a CCD area detector, has been explored for studying epitaxial relations and crystallinity in thin film systems. The geometrical flexibility of the six‐axis goniometer allows measurement of a large volume in reciprocal space, providing an in‐depth understanding of sample crystal relationships. By a set of examples of LaAlO₃ thin films deposited by the atomic layer deposition technique, the possibilities of the set‐up are presented. A fast panoramic scan provides determination of the crystal orientation matrices, prior to more thorough inspection of single Bragg nodes. Such information, in addition to a broadening analysis of families of single reflections, is shown to correlate well with the crystallinity, crystallite size, strain and epitaxial relationships in the thin films. The proposed set‐up offers fast and easy sample mounting and alignment, along with crucial information on key features of the thin film structures.     

Interrelationship between the polarization moments of different parity upon optical pumping of atoms under magnetic resonance conditions: III. Comparison of theory with experiment

The optical pumping of Cs atoms with light of the D ₁ line under magnetic resonance conditions is numerically calculated. This calculation is done to check the suggestion that an unexpectedly strong influence of the polarization of the pumping light on the resonance signals of transverse alignment, which we observed experimentally, is caused by the coupling between orientation and alignment by means of the magnetic field and/or the pumping light. This suggestion has been confirmed: upon circularly polarized pumping, the shape of the signal line of transverse alignment proved to be similar, as well as in the experiment, to the shape of the signal line of transverse orientation and was sharply different from the shape of the line of alignment observed upon linearly polarized pumping. For metastable 2³ S ₁ ⁴He atoms, in accordance with the experimental data, the calculated shape of the signal line of transverse alignment is found to be independent of the polarization of the pumping light. The calculations also confirm the possibility of a reverse effect: the influence of alignment on orientation, which manifests itself in the occurrence of orientation upon pumping with unpolarized light under magnetic resonance conditions. For Cs atoms exposed to a field of ～0.6 Oe, the largest contribution to this effect is yielded by the field coupling of orientation and alignment, whereas, for metastable 2³ S ₁ ⁴He atoms, the largest contribution is made by the light coupling of these polarization moments.     

飞秒光丝中等离子体密度时间演化特征

采用能够较为清晰、完整描述强飞秒激光等离子体通道内带电粒子产生过程及其演化的物理模型,进一步研究了飞秒光丝中等离子体密度的时间演化特征。计算结果表明:对于不同时间线型的脉冲,在等离子体通道形成过程中,氧气分子的电离贡献率及氮气分子的贡献率明显不同,不同线型的脉冲对高效维持高密度等离子体的寿命具有较大的影响。有效控制成丝脉冲线型能够达到对等离子体通道的高效利用。长脉冲、短波长虽能够获得较高密度等离子体通道,但其存活寿命却完全受限于通道的后期演化。     

Self‐Referenced Measurement of Light Waves

The wave nature of light has been known for more than 100 years. However, it is still very difficult to directly sample an optical field transient on a time scale below the oscillation period. It has been naturally believed that the field oscillation can be detected only by using a reference pulse that has a shorter duration than the period of the oscillation. In this Letter, an experimental demonstration of a self‐referenced light wave measurement is shown. Frequency‐resolved optical gating capable of carrier‐envelope phase determination was used to measure the complete electric field evolution of a 21‐fs few‐cycle pulse without any reference pulses. A second harmonic generation frequency‐resolved optical gating spectrogram and an interferogram between second harmonic and self‐diffraction signals were simultaneously recorded. The phase‐controlled light waves of few‐cycle pulses were clearly observed by using this method.     

Pulse evolution and mode selection characteristics in a TEA-CO2 laser perturbed by injection of external radiation

A grating-tunable TEA-CO₂ laser with an unstable resonator cavity, modified to allow injection of cw CO₂ laser radiation at the resonant transition line by means of an intracavity NaCl window, has been used to study the coupling requirements for generation of single frequency pulses. The width and shape of the mode selection region, and the dependence of the gain-switched spike buildup time and the pulse shapes on the intensity and detuning frequency of the injected radiation are reported. Comparisons of the experimental results with previously reported mode selection behavior are discussed.