Laser coupling to nuclei via collective electronic oscillations—a simple heuristic model study

The induction field coupling of laser-driven collective electronic oscillations has been proposed as an interlevel transfer mechanism for a gamma-ray laser. Using a heuristic classical model for the electron oscillations, we verify in the model six general features which we conjecture are valid for laser-induced collective oscillations: (1) enhancement of the electric field seen by the nucleus, (2) harmonic generation, (3) generation of higher multipoles, (4) odd (even) order multipoles couple to odd (even) harmonics, (5) there exists an optimum laser intensity for each multipole order and harmonic, and (6) for a given multipole order, the optimum intensity increases with harmonic number. We briefly discuss the relevance of these results to a proposed interlevel transfer experiment in ²³⁵U.     

Generation of the third harmonic of near IR femtosecond laser radiation tightly focused into the bulk of a transparent dielectric in the regime of plasma formation

The generation of the third harmonic of femtosecond laser radiation with a wavelength of 1.24 μm tightly focused into the bulk of fused silica was simulated numerically for the regime with light intensity of 3 × 10¹³ W/cm², which is extreme for solid bodies. The efficiency of third harmonic generation (THG) was found to be restricted to 0.1% in the regime of plasma formation. This is determined by two competitive processes: a decrease of THG efficiency due to an increase of wave detuning and an increase of THG efficiency due to a growth of focusing asymmetry. In an isotropic medium, determination of the threshold of plasma formation by use of the third harmonic signal is shown to be a more sensitive method as compared with the standard scheme of nonlinear transmittance detection.     

原子在红外激光场中产生高次谐波及阿秒脉冲随波长的变化规律

利用强场近似理论,研究了在长波红外激光场(波长800—2000 nm)驱动下氢原子产生的高次谐波,分析了在截止位置附近高次谐波的转换效率随激光波长的变化规律.发现在截止位置附近原子发射高次谐波的转换效率比平台区域的转换效率低,但获得的阿秒脉冲的宽度会随波长的增大而缩短. 关键词： 强场近似 高次谐波 阿秒脉冲     

V型高双折射光子晶体光纤超连续谱产生的实验研究

高非线性高双折射光子晶体光纤是超连续谱产生的最有效介质之一, 因此本文选取V型光子晶体光纤作为研究对象. 通过多极理论数值模拟的结果, 确定V型光纤具有高双折射、高非线性等特性. 通过实验手段, 发现入射光中心波长和光纤的双折射效应对产生的超连续谱有很大的影响: 当入射光波长处于光子晶体光纤大的反常色散区时, 脉冲相对展得比较宽, 长轴要比短轴方向的超连续谱更宽, 频谱成分更加丰富. 在同一波长下光偏振方向越接近45°时, 超连续谱谱宽范围越大. 随着入射脉冲功率的增加, 超连续谱展得越宽, 但是当功率比较大时会达到功率饱和. 关键词： 光子晶体光纤 高双折射 多极理论 超连续谱     

氩气中高次谐波辐射的研究

报道了在４５ｆｓ－２ＴＷ激光装置上以Ａｒ惰性气体为介质产生高次谐波的实验结果,实验中通过研究气体密度和激光能量对谐波辐射的影响,找到了适当的气体密度和激光能量范围,并在Ａｒ气中观察到８１次（９．７ｎｍ）的皆波辐射,这时迄今为止,人们在Ａｒ气中所能观测到的最短波长的谐波辐射,经分析表明,高于５７次的谐波是由Ａｒ离子产生的。     

An extremely narrow UV pulse generation using a micro- and nano-ring system for pico-lithographic resolution

We propose a new technique of an extremely narrow ultraviolet (UV) pulse width generation for pico-lithography technology using a nonlinear ring resonator system. A system consists of three micro- and a nano-optical ring resonators, which can be used to generate the 50 pm (10⁻¹² m) optical spectral width at the ultraviolet wavelength. By using a soliton pulse with a pulse width of 50 ns, 1 W peak power, center wavelength at 1550 nm, after the soliton pulse is launched into a first ring device, the chaotic pulses are generated within the first ring. The chaotic filtering behaviors are performed by using the second and third ring devices, whereas the extremely short pulse, i.e. narrow spectral width, can be generated by using the extended nano-ring device. The broad spectrum of the harmonic waves is generated and filtered, especially the generation of third harmonic wave, which is known as the ultraviolet wavelength, is achieved, which is capable of forming pico-lithographic resolution. Results obtained have shown that the generation of the spectral width of 50 pm at a wavelength of 511.125 nm, with peak power at 35 mW is achieved.     

Cascaded Second Harmonic Generation for Deep-UV Radiations with a 2D Nonlinear Photonic Quartz Crystal

Harmonic generation can broaden the laser wavelength in a great scope and is rich in physics, but the efficient cascaded second harmonic generation especially those directly deep into deep-ultraviolet (DUV) with one single crystal seems to be impossible, due to the extremely strict phase-matching conditions and relative narrow bandgap of present nonlinear optical crystals. Here, this work designs a nonlinear photonic quartz crystal where the phase difference of interacted light waves is manipulated in two dimensions and realizes an unprecedented cascaded phase-matched 2nd–5th harmonic generation simultaneously with the photon energy up to 6 eV, representing the shortest wavelength by cascaded second harmonic generation in the solid-state nonlinear medium. The generated light is located in the visible-DUV region with a total conversion efficiency of 2.7%. The present nonlinear photonic quartz scheme not only provides a multi-wavelength solid-state laser source applied in modern devices but also develops a new route for studying the light-matter nonlinear interaction in the DUV region.     

Optical harmonic generation in amorphous silicon nitride microcavities

Second harmonic (SH) and third harmonic (TH) generation in amorphous silicon nitride microcavity are experimentally investigated. The transmitted SH and TH signals are measured in the 0.9-1.4 μm spectral range, showing enhanced nonlinear conversion efficiency corresponding to resonant wavelength and optical band edges. The efficiencies of the SH and TH generation processes are found to be enhanced by about two and one orders of magnitude, respectively, in comparison with the case of reference amorphous silicon nitride sample. The SH spectra can be reasonably interpreted as due to surface/interface harmonic generation, while the TH signal is related to bulk isotropic third-order polarization. The results obtained for the TH signal are discussed in terms of the linear optical properties of amorphous silicon nitride thin films.     

基于角色散的非共线匹配宽带三倍频技术

宽带光打靶可以有效降低激光等离子体相互作用过程中非线性效应。提出一种基于角色散的非共线匹配宽带三倍频方案,利用宽带基频与窄带二倍频的非共线和频产生宽带三倍频,和频过程中通过特殊设计的渐变光栅实现不同频率的基频光束以特定角度入射,补偿了波长差异引入的位相失配使得全波段满足位相匹配条件。理论模拟表明,采用KDP晶体Ⅱ类位相匹配,将中心波长为1058 nm、带宽10 nm的宽带基频光与526.5 nm的二倍频光进行非共线匹配和频,可以实现高效宽带三倍频转换。     

Wideband wavelength conversion using double-pass cascaded χ:χ interaction in lossy waveguides

We report the wavelength conversion based on double-pass cascaded nonlinear interaction (χ⁽²⁾:χ⁽²⁾) of sum and difference frequency generation in quasi-phase matched lithium niobate waveguides and compare it with double-pass cascaded second harmonic generation and difference frequency generation with and without waveguide loss. It is shown that the efficiency decreases considerably even for the low-loss waveguide compared to the lossless one especially for long waveguides and to achieve the higher efficiency for the same length, the amount of the extra power to compensate the loss increases. Also, an increased detuning of pump wavelength is proposed to flatten the response with a small efficiency penalty. The detuning- and loss-compensating pump powers can be found using the design diagrams in which the criteria for the design of waveguide length and the assignment of pumps power to obtain the desired efficiency, ripple and bandwidth are presented assuming a 75-nm pump wavelength difference.     

基于空间啁啾的宽带激光倍频技术

提出一种新型的宽带倍频方案,利用时空耦合效应将宽带的时间啁啾光转换成空间啁啾光,采用多块晶体并联、各晶体独立调谐的技术途径对空间啁啾光进行谐波转换,因此倍频效率与窄带激光倍频相当。理论研究表明,采用KDP晶体I类位相匹配,对中心波长为1 053 nm的宽带基频光实现了带宽约30 nm、转换效率大于60%的高效率宽带二倍频。而且倍频光仍为线性啁啾宽带光,具备可压缩性。     

Frequency dependence of quantum path interference in non-collinear high-order harmonic generation

High-order harmonic generation(HHG) driven by two non-collinear beams including a fundamental and its weak second harmonic is numerically studied. The interference of harmonics from adjacent electron quantum paths is found to be dependent on the relative delay of the driving pulse, and the dependences are different for different harmonic orders.This frequency dependence of the interference is attributed to the spatial frequency chirp in the HHG beam resulting from the harmonic dipole phase, which in turn provides a potential way to gain an insight into the generation of high-order harmonics. As an example, the intensity dependent dipole phase coefficient α is retrieved from the interference fringe.     

Corrugated-enhanced second harmonic generation in metal–insulator–metal plasmonic waveguides

Nonlinear effects such as second-harmonic generation (SHG) are important for applications such as switching and wavelength conversion. In this study, the generation of second harmonic in metal–insulator–metal (MIM) plasmonic waveguides was investigated for both symmetric and asymmetric structures. Symmetric means that the metals at the top and bottom of the dielectric layer are the same and asymmetric means that the metals at the top and bottom of the dielectric layer are different. Two different structures are considered here as plasmonic waveguide for generation of second harmonic and analyzed using finite-difference time domain method. Besides the structure has grating on both sides for more coupling between photons and plasmons. The wavelength duration of grating per length unit (number of grooves) will be optimized to reach the highest second harmonic generation. To perform this optimization, the wavelength of operation λ = 458 nm is considered. It was shown that field enhancement in symmetric MIM waveguides can result in enhancement of SHG magnitude compared to the literature values and asymmetric device results in more than two orders of magnitude enhancement in SHG compared to symmetric structure. It is also shown that the electric field of second harmonic depends on the thickness of crystal (insulator). So, its thickness is optimized to achieve the highest electric field.     

Isolated-attosecond-pulse generation by suppressing multiple rescatterings with plasmon-enhanced laser fields

An efficient scheme is proposed to generate broadband supercontinuum spectra and isolated attosecond laser pulses by suppressing multiple rescatterings with plasmon-enhanced laser fields. High-order harmonic generation from helium atoms in long wavelength is theoretically investigated. It is found that a quantum path interference and a multiple rescatterings interference are two fundamental limitations against the smooth degree of harmonic spectra. Numerical results show that the coherent superposition state in combination with plasmon-enhanced laser fields is robust for broadband supercontinuum generation. Then an isolated 78-as pulse can be directly generated.     

双波长双脉冲激光三次谐波的产生方法

提出双波长双脉冲激光产生三次谐波的方法。波长不同的两脉冲激光束由反射使它们合拢后通过两块非线性的BBO晶体 ,激光脉冲第一次通过BBO晶体产生两波长的二次谐波 (SHG) ,它们由各自的反射镜反射再次通过BBO又产生了二次谐波 ,这两次产生的二次谐波和基波通过用于产生三次谐波的BBO晶体可产生双波长的三次谐波 ,它们由 45°斜置的耦合镜输出。文中分析了满足二次谐波和三次谐波的四个相位匹配的条件 ,该方法也可用于腔内双波长双脉冲的三次谐波激光的产生 ,给出了相应的实验结果。     

原子在双色组合场中产生高次谐波的转换效率与激光波长的关系

利用分裂算符法求解速度规范下的含时薛定谔方程,研究了一维氦原子处于单色红外场、红外场与紫外场形成的双色组合场中产生的高次谐波谱,分析了在截止位置附近高次谐波的转换效率与激光波长(800—2000 nm)的关系,发现在双色组合场驱动下截止位置附近高次谐波的转换效率随波长的变化为η(λ)∝λ^－x,其中〈x〉的数值取决于激光场的强度,但是只要选取合适场强的组合场就能提高截止位置附近高次谐波的转换效率. 关键词： 双色组合场 分裂算符 高次谐波 转换效率     

晶体面形畸变对三倍频转换效率的影响

在惯性约束聚变（ICF）领域,为了满足物理实验需求,需要采用短波长的紫外激光打靶。目前,国内外高功率固体激光装置普遍采用谐波转换的方式来获得三倍频紫外激光。高效的频率转换必须满足相位匹配条件,而晶体的反射面形畸变过大对准直精度和倍频效率均匀性都会产生不利影响。通过实验验证了晶体准直光斑的质量退化主要来源于晶体因夹持和重力导致的反射面形畸变,证明了改进晶体的夹持方式可以有效改善晶体面形,提高准直精度和准直光斑质量,并显著提升三倍频转换效率。     

Magnetization-induced second harmonic generation in magnetophotonic microcavities based on ferrite garnets

Magnetization-induced second harmonic generation was observed in magnetophotonic microcavities consisting of a ferromagnetic yttrium-iron garnet layer surrounded by nonmagnetic photonic crystals (Bragg reflectors). At resonance between the fundamental radiation and the microcavity mode in the geometry of polar magnetooptical Kerr effect, the polarization rotation for the second harmonic was found to be (18.5±0.5)°/μm for the fundamental radiation with a wavelength of 825 nm.     

Generation of Broadband Attosecond Pulse via Controlling Quantum Path by Two Color Field in Long Wavelength Driving Regime

In this paper, we have investigated theoretically the high harmonic generation form helium atom in long wavelength driving regime at2000 nm through solving time-dependent Schrödinger equation. By adding asecond harmonic pulse (1000~nm) and a UV attosecond pulse (200 nm) to the driving field, an efficient method for picking out and enhancing ionizationpath to generate high-yield supercontinuum harmonics is realized, and thenan isolated sub-100 as pulse with a bandwidth of 190 eV is significantly obtained.     

Frequency doubling of phase-modulated femtosecond laser pulses in periodically poled and chirped nonlinear crystals

The second harmonic generation (SHG) at a wavelength of 0.8 μm by 50-and 10-fs pulses with and without phase modulation (PM) was systematically studied in LiNbO₃ crystals with regular domain structure and linearly varied domain thickness. The main results were obtained by numerical method, taking into account the difference between group velocities of interacting pulses and the group-velocity dispersion. In the approximation of the given field of the fundamental radiation, an analytical expression was derived for the spectral density of the second harmonic in the periodically poled nonlinear crystal (PPNC) under nonstationary excitation conditions. It was numerically found that the conversion efficiency of about 90% can be achieved by doubling the frequency of 50-fs laser pulses without PM in the LiNbO₃ PPNC. The maximum conversion efficiency for the SHG by PM pulses is achieved at a certain optimum chirp step in the crystal domain length, which depends on both the value and sign of frequency modulation.