强耦合腔-QED系统中原子的纳秒脉冲激发光谱研究

本文在多原子强耦合腔-QED系统中,利用脉冲宽度为5 ns的强脉冲光在垂直于腔轴方向直接激发原子,脉冲的峰值功率为40 mW,通过光学腔观测激发原子辐射到腔中的光子获得相应的激发光谱。我们发现当光场频率和原子跃迁失谐±80 MHz时原子激发率达到最大,而在共振时原子激发被抑制。我们建立了脉冲光与三能级原子相互作用的模型,通过缀饰态能够解释此现象。     

Investigation of a copper-vapor laser pumped by trains of damped sine pulses

A copper-vapor laser the lasing medium of which is pumped by damped sine voltage pulses is studied experimentally. It is shown that this laser can operate at megahertz lasing pulse repetition rates. Each current pulse in a train of damped sine excitation pulses generates a lasing pulse. The time between lasing pulses is no more than 224 ns, which corresponds to a lasing pulse repetition rate of more than 4.5 MHz. According to estimates, the use of the megahertz sine voltage to excite the copper-vapor laser may raise the specific power of laser radiation by at least an order of magnitude at an efficiency of ≈15%.     

近共振区超短强激光脉冲激发的等离子体尾波场

 用一维相对论粒子模拟研究了相对论超短强激光脉冲在等离子体中传播时激发的尾波场,初步获得了近共振区尾波场的峰值幅度随激光脉冲宽度变化的特点,发现在近共振区等离子体波激发出现增强。通过准静态近似下尾波激发的一维非线性方程数值求解,并与粒子模拟结果比较,得到了该非线性方程的适用范围：当激光脉冲宽度小于等离子体波波长的4倍时,该方程所得结果与粒子模拟结果一致;而当激光脉冲宽度大于该数值时,该方程不再适用。     

Generation of femtosecond vacuum-ultraviolet pulses

High power femtosecond pulses in the Vacuum Ultra Violet (VUV) have been generated through the nonlinear interaction of femtosecond KrF pulses with xenon and argon gas. Under near resonant two photon excitation of xenon by a femtosecond KrF laser, parametric four wave mixing processes lead to VUV pulses at 147 and 108 nm with pulse energies in the 10 µJ range. Tuning is demonstrated by mixing the KrF pulse with a 500 fs dye laser pulse at 497 nm, resulting in 165 nm emission. In argon, a three photon resonance leads to third harmonic generation at 83 nm and micro joule level pulses near 127 nm generated by a six wave mixing process. Since the spectra of the VUV pulses show an ionization-induced blue shift with increasing KrF laser intensity, the VUV pulses can be shown to have temporal duration less than the pulse width (450 fs) of the KrF laser. Blue shifting of the third harmonic of the KrF laser in argon is dominated by a reduction in the neutral gas density rather than by an increase in the electron density.     

孔缝对金属腔体强电磁脉冲耦合特性影响研究

核电磁脉冲和高功率微波等强电磁脉冲易造成电子设备功能失效甚至损毁,在实际工程实施中用金属腔体对电子设备进行屏蔽是常用的强电磁脉冲抑制手段。基于电磁仿真计算,对含矩形孔缝金属腔体的强电磁脉冲耦合特性进行了系统研究,阐述了孔缝宽长比、腔体尺寸等因素对多种不同类型强电磁脉冲（核电磁脉冲、宽带高功率微波、窄带高功率微波）作用下腔体内耦合场的影响;并以此为基础,重点分析了强电磁脉冲与含孔缝金属腔体之间的作用机制。研究结果表明:不同类型强电磁脉冲耦合信号差异明显,金属腔体对强电磁脉冲的响应是腔体谐振模式、孔缝谐振频率与强电磁脉冲共同作用的结果;当腔体谐振模式、孔缝谐振频率在强电磁脉冲的带内时,腔体内部的耦合场会出现增强效应;特别地,腔体与孔缝间的相互作用还可造成腔体与缝隙的谐振频率发生偏移。因此,在为电子设备设计金属屏蔽外壳时,应基于不同强电磁脉冲的频带范围,对腔体与孔缝的尺寸进行综合设计,抑制腔体、孔缝谐振及谐振频率偏移,提升其强电磁脉冲防护性能。     

三色圆偏振激光组合脉冲驱动氦原子产生椭圆偏振的阿秒脉冲

我们提出利用频率为ω,3ω和强度较弱的2ω组成的三色圆偏振激光组合脉冲驱动氦原子能够得到椭偏率较大的阿秒脉冲链的一种方法.通过强场近似方法,计算了氦原子在两色和三色圆偏振激光组合脉冲驱动下发射高次谐波谱及其合成阿秒脉冲链,比较了氦原子(初态为s态)在这两种情况下发射高次谐波谱的特点及其合成阿秒脉冲链椭偏率的大小,结果发现,在反旋的两色ω,3ω激光脉冲基础上加入了频率为2ω的第三色激光脉冲联合作用到氦原子上,所得到的阿秒脉冲链的椭偏率相对于双圆场情况下有所增加,通过调整ω,3ω激光的强度比,并且选择适当的第三色激光的强度,对初态为s态的原子,仍能够得到具有较大椭偏率的阿秒脉冲链.     

Ultralow-jitter passive timing stabilization of a mode-locked Er-doped fiber laser by injection of an optical pulse train

The pulse timing of a mode-locked Er-doped fiber laser was stabilized to a reference pulse train from a Cr:forsterite mode-locked laser by all-optical passive synchronization scheme. The reference pulses were injected into a ring cavity of the fiber laser by using a 1.3-1.5 mum wavelength-division multiplexer. The spectral shift induced by cross-phase modulation between copropagating two-color pulses realizes self-synchronization due to intracavity group-delay dispersion. The rms integration of timing jitter between the fiber laser pulse and the reference pulse was 3.7 fs in a Fourier frequency range from 1 Hz to 100 kHz.     

Pulse picking by phase-coherent additive pulse generation in an external cavity

We have implemented a simple method for generating an "amplified" phase-coherent light pulse in which a pulse train of phase-coherent, equidistant input light pulses from a mode-locked Ti:sapphire laser is coupled into a ring cavity resonator whose length is matched to the mode-locked pulse repetition frequency at 82 MHz. Pulses are thus coherently superimposed and added inside the buildup cavity and form an intense pulse that is switched out from the cavity via a fast acousto-optic modulator. The method thus provides a pulse train at a reduced and controlled repetition frequency and with higher pulse energies than the original mode-locked pulses.     

Population transfer between two quantum states by piecewise chirping of femtosecond pulses: theory and experiment

We propose and experimentally demonstrate the method of population transfer by piecewise adiabatic passage between two quantum states. Coherent excitation of a two-level system with a train of ultrashort laser pulses is shown to reproduce the effect of an adiabatic passage, conventionally achieved with a single frequency-chirped pulse. By properly adjusting the amplitudes and phases of the pulses in the excitation pulse train, we achieve complete and robust population transfer to the target state. The piecewise nature of the process suggests a possibility for the selective population transfer in complex quantum systems.     

Storing optical information as a mechanical excitation in a silica optomechanical resonator

We report the experimental demonstration of storing optical information as a mechanical excitation in a silica optomechanical resonator. We use writing and readout laser pulses tuned to one mechanical frequency below an optical cavity resonance to control the coupling between the mechanical displacement and the optical field at the cavity resonance. The writing pulse maps a signal pulse at the cavity resonance to a mechanical excitation. The readout pulse later converts the mechanical excitation back to an optical pulse. The storage lifetime is determined by the relatively long damping time of the mechanical excitation.     

Pulse energy modulation of the cw-mode-locked 1.32 μm Nd-YAG-laser by drive frequency detuning

The frequency of an acousto-optic mode-locker is detuned from the fundamental resonance frequency of the cavity resulting in undamped oscillations of the laser pulse energy. The envelope of the mode-locked pulse train consists of spikes 10 μs wide with a repetition time of 46 μs. The energy of the mode-locked pulses in a spike is increased by up to a factor of 50 compared to a cw-mode-locked pulse. The pulse width can be as short as 700 ps.     

Picosecond-pulse amplification with an external passive optical cavity

We experimentally demonstrate the amplification of picosecond pulses at high repetition rates through the coherent addition of successive pulses of a mode-locked pulse train in a high-finesse optical cavity equipped with cavity dumping. Amplification greater than 30 times is obtained at a repetition rate of 253 kHz, boosting the 5.3-nJ pulses from a commercial mode-locked picosecond Ti:sapphire laser to pulse energies of more than 150 nJ.     

基于光纤中SBS相位共轭的自调Q光纤激光器

分析了利用光纤中的受激布里渊散射(stimulated Brillouin scattering,SBS)相位共轭效应进行激光腔内调Q,产生ns量级脉冲激光的原理,并对利用该效应产生的激光脉冲波形和脉冲形成过程进行了数值模拟,得到的脉冲波形与SBS相位共轭反射率随时间变化曲线基本一致,表明利用光纤中的SBS相位共轭作用调Q具有可行性.据此,对采用单模光纤(single-mode fiber,SMF)作为SBS池的Er³⁺掺杂调Q光纤激光器进行实验研究.当单模光纤长度为1.5 m时,在45 mW的光抽运功率下得到脉宽约2.6ns,脉冲周期58.23ns,平均功率7.35mW的激光脉冲.进一步的研究表明:激光器中相位共轭镜的形成与SBS介质长度有关,SBS介质过长,斯托克斯线之间无固定的相位关系,不能形成相干叠加,SBS相位共轭腔不能形成;SBS介质过短,腔内正交偏振模光子寿命的改变使脉冲出现双峰现象.脉冲形成后其属性只与SBS动态属性有关.     

Double-line terawatt OPCPA laser system for exciting beat wave oscillations

A double-line terawatt beat laser (BEAT) is developed for exciting beat wave oscillations. BEAT consists of two oscillators and an amplification system including optical parametric chirped-pulse amplification (OPCPA) in which two individual pulses with wavelength separations of 10–35 nm are amplified, recompressed, and focused as a single beam. The recompressed pulse trace shows that a 150-fs pulse duration full width at half maximum was modulated at a beating period of 72 fs. This beating period matches a resonant excitation of plasma wave with an electron density of 2.5 × 10¹⁸ cm^?3, resulting in excitation of a beat wave in hydrogen plasma with wave amplitude of 15 GV/m. The multiple beating oscillations can amplify the plasma wave and improve its structure. This scheme would be ideal for stabilizing the plasma wave strength in the plasma cavity and for realizing a practical laser plasma accelerator.     

Ionisation of Rydberg hydrogen atom near a metal surface by short pulse laser

In the ionisation of Rydberg hydrogen atoms near a metal surface,the electron will escape from the nucleus and arrive at the detector in a time sequence.This probability flux train relies on the initial electron wave packet irradiated by the laser pulse.For simplicity,the laser pulse is usually simplified to a delta function in energy domain,resulting in a sharp initial arrival time with an exponentially decaying tail at the detector.Actually and semiclassically,the initial outgoing wave should be modeled as an ensemble of trajectories propagating away from the atomic core in all directions with a range of launch times and a range of energies.In this case,each pulse in the pulse train is averaged out rather than a sharp profile.We examine how energy and time averaging of the electron wave packet affects the resolution of escaping electron pulses and study the energy dependence of the arrival time for each pulse in the ionisation train.An optimization condition for the laser pulse shape to generate narrow ionisation electron pulse in the train is obtained.The ionisation rates with various excitation energy are calculated also,which show the excitation to higher N Rydberg states will narrow the electron pulse as well.     

Stable multigigahertz pulse-train formation in a short-cavity passively harmonic mode-locked erbium/ytterbium fiber laser

We demonstrate a short-cavity erbium-ytterbium fiber laser that is passively mode locked by a saturable Bragg reflector with a fundamental repetition rate of 235 MHz . The laser operates in the soliton regime and under passive harmonic mode locking with 11 pulses in the cavity and produces output pulse trains at 2.6 GHz with transform-limited 270-fs pulses and 1.6 mW of average power. Within the cavity the multiple pulses form a stable pattern with fixed, nearly equal pulse-to-pulse temporal spacings, causing the output pulse train to have timing offsets of less than 15 ps. A slow gain-recovery model is proposed to explain the pulse-train self-organization.     

Ultrahigh-resolution coherence spectroscopy by means of periodic excitation with picosecond pulses

The feasibility of ultrahigh-resolution coherence spectroscopy with a train of picosecond light pulses from a synchronously pumped mode-locked and cavity-dumped dye laser is demonstrated. Narrow resonances of periodic pulse excitation are observed whenever the pulse repetition rate of the laser of a higher harmonic coincides with the frequency splitting of coherently excited levels. The zero-field hyperfine splitting of the Na-ground state (1771.6 MHz) could be measured in the 2100th order of the excitation rate with a resonance halfwidth of 800 Hz.     

Wakefield generation by elliptically polarized femtosecond laserpulse in ionizing gases

Wakefield generation by a femtosecond laser pulse is described in the frame of the slowly varying amplitudes approximation. The amplitude of the wakefield A, is studied as a function of laser pulse and background gas parameters, and is compared with well-known results for preformed, completely ionized plasma A_p,i. It is found that the ionization processes can increase A_p as compared to A_p,i at comparatively high laser peak intensities. It is shown that the increase of the wakefield amplitude due to gas ionization is more pronounced for circularly polarized laser pulses than for linearly polarized laser pulses. The strongest enhancement of A_p in comparison with A_p,i takes place for longer laser pulses with a duration in excess of the plasma wave period when the resonant conditions for ponderomotive excitation of the wakefield are not matched. Thus, ionization processes can expand the region of parameters for efficient generation of the laser wakefields     

First-principles electron dynamics control simulation of diamond under femtosecond laser pulse train irradiation

A real-time and real-space time-dependent density functional is applied to simulate the nonlinear electron-photon interactions during shaped femtosecond laser pulse train ablation of diamond. Effects of the key pulse train parameters such as the pulse separation, spatial/temporal pulse energy distribution and pulse number per train on the electron excitation and energy absorption are discussed. The calculations show that photon-electron interactions and transient localized electron dynamics can be controlled including photon absorption, electron excitation, electron density, and free electron distribution by the ultrafast laser pulse train.     

Observation of dark pulse in a dispersion-managed fiber ring laser

The observation of dark pulse in a dispersion-managed fiber ring laser with net negative cavity group velocity dispersion (GVD) is reported. Both bright and dark pulses can be obtained in our fiber laser. When we appropriately adjust the cavity birefringence to achieve triple-wavelength mode-locked operation in the laser by rotating the polarization controller, the bright pulse could be switched to dark pulse. It is believed that the dark dispersion-managed (DM) pulse generation is caused by the linear and nonlinear intermodulation effects among the three wavelength pulses.