Second harmonic generation in the centrosymmetric antiferromagnet NiO

In spite of the fact that inversion is a symmetry operation of both the crystalline and the magnetic lattice of NiO, second harmonic generation (SHG) has been observed below the Néel temperature. A spectroscopic study shows that the signal is due to combined magnetic-dipole and electric-dipole transitions between the (3d)(8) levels of the Ni(2+) ion in the crystal field. The SHG is resonant in both the incoming and the outgoing light waves and thus greatly enhanced. A quadratic coupling of the nonlinear polarization to the order parameter was found. This allows the investigation of individual domains.     

Tunable second harmonic generation by phase-modulated ultrashort laser pulses

We report on the generation of tunable light around 400?nm by frequency-doubling ultrashort laser pulses whose spectral phase is modulated by a sum of sinusoidal functions. The linewidth of the ultraviolet band produced is narrower than 1?nm, in contrast to the 12?nm linewidth of the non-modulated incident spectrum. The influence of pixellation of the liquid crystal spatial light modulator on the efficiency of the phase-modulated second harmonic generation is discussed.     

Tunable resonant third harmonic generation of picosecond pulses in sodium vapour

High efficient (η 8%) phase-matched third harmonic generation of 12 ps pulses in sodium vapour has experimentally been investigated. The characteristics of the ultraviolet signal at 200 nm depending on tuning near two-photon resonance and the incident laser power were measured. A saturation effect of the picosecond pump at substantially higher intensity (10² GW/cm²) in comparison with the corresponding intensity of nanosecond pulses was observed. A theoretical treatment of these results is presented.     

The bistable mode of second harmonic generation by femtosecond pulses

The possibility of controlling the efficiency of second harmonic generation by femtosecond pulses is demonstrated. By the phase shift in a certain cross section of the medium, switching from the low-efficient to the high-efficient generation state (and vice versa) is realized. The switching contrast in terms of the conversion efficiency is up to 4 (or more) and in terms of the peak intensity at the center of the pulse it is as high as 9. A new class of optical switches based on this effect is suggested.     

Optical second harmonic generation in ammonium tartrate

Second harmonic generation was observed in ammonium tartrate single crystals by a phase matching technique at the fundamental wavelength 1.153 μm. The nonlinear coefficients were found to be comparable in magnitude to those of KDP. The optical transmission and the refractive indices were also measured.     

Optical second harmonic generation in ammonium malate

Second harmonic generation in ammonium malate has been investigated in a wavelength region between 492 nm (short wavelength limit of the harmonic beam at 246 nm) and 640 nm. Phase match angle, refractive indices and all coefficients of the nonlinear susceptibility tensor have been measured; several possibilities for phase matching are discussed.     

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。     

Ab initio treatment of optical second harmonic generation in NiO

In this Letter we develop a new systematic approach to study optical second harmonic generation in NiO, on both the (001) surface and the bulk. NiO is modeled as a doubly embedded cluster on which two highly correlated quantum chemistry methods are applied in order to obtain the wave functions of all the intragap d states and the low lying charge transfer states. The optical gap is calculated and the electric dipole, magnetic dipole, and electric quadrupole contributions to the second order susceptibility tensor are computed for the first time from first principles. Going beyond the electric dipole approximation gives new insight into the experimentally observed spectrum. A method is proposed for monitoring the spin dynamics of the NiO(001) surface.     

Optical second harmonic generation from Pt nanowires

We have measured optical second harmonic intensity from arrays of Pt nanowires of 20 nm and 9 nm average widths, as a function of the incident and output light polarizations, the azimuthal angle, and the excitation photon energy. The nanowires were fabricated through shadow deposition on self-organized NaCl(1 1 0) faceted templates. The anisotropy of the SH intensity from the Pt nanowires was found to be stronger than that from the Au nanowires reported previously. The effective nonlinear susceptibility element , with the suffix 2 indicating the direction , was observed for Pt nanowires, although it was not observed for Au nanowires. This difference is suggested to be due to the weaker suppression of the incident fundamental fields by the depolarization field in the Pt nanowires and the larger anisotropy in the nonlinearity of Pt nanowires due to the thinner widths.     

Optical system aberration effect on the second harmonic generation

The results of observation of the aberration effect representation by the focusing optical system in the angular structure of the frequency-transformed radiation are presented. The experiments have been performed on lithium iodate crystals.     

Optical second harmonic generation in asymmetric double triangular quantum wells

The second harmonic generation (SHG) in the asymmetric double triangular quantum wells (DTQWs) is investigated theoretically. The dependence of the SHG coefficient on the right-well width of the DTQWs is studied, and the influence of the applied electric field on SHG coefficient is also taken into account. The analytical expression of the SHG coefficient is analyzed by using the compact density-matrix approach and the iterative method. Finally, the numerical calculations are presented for the typical GaAs/Al_xGa_1−xAs asymmetric DTQWs. The results show that the calculated SHG coefficient in this coupled system can reach the magnitude of 10⁻⁵ m/V, 1–2 orders of magnitude higher than that in step quantum well, and that in double square quantum wells. Moreover, the SHG coefficient is not a monotonic function of the right-well width, but has complex relationship with it. The calculated results also reveal that an applied electric field has a great influence on the SHG coefficient. Applying an appropriate electric field to a DTQW with a wider right well can induce a sharper peak of the SHG coefficient due to the double-resonant enhancement.     

Optical second harmonic generation with surface plasmons in piezoelectric crystals

We report the first observation of reflected optical second harmonic generation of light due to excitation of fundamental and harmonic surface plasmon modes at the interface between a metallic film and a piezoelectric crystal. Excitation of the fundamental surface plasmon mode produces a large enhancement of the piezoelectric generated reflected harmonic light whereas excitation of the nonlinear mode produces only a small minimum in the background surface generated harmonic light.     

Deep-ultraviolet picosecond flat-top pulses by chirp-matched sum frequency generation

Picosecond, flat-top, deep-UV pulses are needed to generate high-brightness electron beams to efficiently drive x-ray free electron lasers. Current metal photocathodes have low efficiency and therefore require high-energy pulses, and the generation of high-energy, flat-top pulses in the deep UV is still challenging. The low efficiencies of both the harmonic generation and deep-UV pulse shapers restrict the accessible pulse energy. Moreover, the acceptance bandwidth of the harmonic generation limits the minimum rise time of the flat-top profile. We present the generation of few-hundred microjoule, picosecond, deep-UV pulses using chirp-matched sum frequency generation. This scheme combined with IR spectral manipulation is a novel approach for deep-UV pulse shaping. It permits flat-top pulses with high energy and fast rise time, highly suited for high-brightness photoelectron beam production.     

Direct ninth harmonic conversion of picosecond laser pulses

Direct ninth harmonic conversion of picosecond laser pulses is obtained for the first time. Using sodium vapor, phase- matched ten-photon process by Nd³⁺: glass laser pumping is realized. Effective ninth-order nonlinear susceptibility of the order of x⁽⁹⁾_eff ～ 10^-64 ESU is estimated.     

空气中激光等离子体通道的三次谐波辐射研究

对超强飞秒激光脉冲在空气中形成的长等离子体通道的三次谐波辐射进行了研究．实验研究发现三次谐波的转换效率在通道范围内基本保持不变.还对等离子体通道锥角辐射中的三次 谐波的辐射角进行了研究，结果表明其辐射角接近6mrad，与理论计算结果符合得很好． 关键词： 三次谐波 等离子体通道     

Properties of periodic multicrystal configurations in walk-off-compensating second harmonic generation of ultrashort pulses

This work designs a four-platelet periodic multicrystal configuration in the second harmonic generation of ultra-short pulses as a new walk-off-compensating device. It theoretically investigates a proposed active and a typical passive compensating scheme with the undepleted-pump approximation. The result shows that the angular and spectral band-widths are proportional to the number of crystal pairs as expected,but the temperature tunability is basically unaltered owing to inter-plate pulse interference. At the same time,an analysis reveals that a misuse of the phase mismatch factor is responsible for a historic controversy about pulse interference. A real design of an ultraviolet second harmonic generation (262.5 nm) is considered in a passive periodic β-Barium Borate-calcite configuration,where the inter-plate pulse interference is found to form an azimuthal tuning restriction and to lower plate length tolerance. A subsequent numerical simulation with pump depletion is in good accordance with theoretical prediction.     

Magnetic-field induced second harmonic generation in CuB2O4

Three types of optical magnetic-field induced second harmonic (MFISH) generation are observed in CuB2O4. Unusually sharp and intense electronic transitions in MFISH and linear absorption spectra provide selective access to the two nonequivalent Cu2+ sublattices. The magnetic phase diagram for both sublattices is determined by MFISH. Magnetic structure is dominated by antiferromagnetic order at the 4b site. Sublattice interactions transfer it to the 8d site where it coexists with a discoupled paramagnetic component.     

Efficient generation of narrow-bandwidth picosecond pulses by frequency doubling of femtosecond chirped pulses

We demonstrate efficient generation of picosecond narrow-bandwidth pulses by frequency mixing of broadband opposite chirped pulses in a type I doubling crystal. This procedure allows us to produce picosecond pulses that are perfectly synchronized with femtosecond pulses. The experiment shows a decrease of the initial bandwidth by a factor of more than 30, while a high conversion efficiency is maintained.     

利用康普顿散射实现太赫兹皮秒脉冲的分析

研究了利用微波与电子束团的康普顿散射实现太赫兹的方法、光子产额和辐射功率。推导出了单个电子产生的太赫兹的光子产额和辐射功率表达式,也推导出了电子束团产生的太赫兹的光子总产额和辐射总功率表达式。结果发现：利用微波与电子束团发生康普顿垂直散射,可以产生太赫兹皮秒脉冲;单个电子产生的太赫兹光子产额与微波功率、微波波长成正比,与微波束截面积成反比;单个电子产生的太赫兹辐射功率与微波功率、电子Lorentz因子的平方成正比,与微波束截面积成反比;电子束团产生的太赫兹光子总产额与微波功率的平方、微波波长的平方成正比,与微波束截面积的平方成反比;电子束团产生的太赫兹辐射总功率与微波功率的平方、微波波长以及电子Lorentz因子的平方成正比,与微波束截面积的平方成反比。     

利用康普顿散射实现太赫兹皮秒脉冲的分析

 研究了利用微波与电子束团的康普顿散射实现太赫兹的方法、光子产额和辐射功率。推导出了单个电子产生的太赫兹的光子产额和辐射功率表达式,也推导出了电子束团产生的太赫兹的光子总产额和辐射总功率表达式。结果发现：利用微波与电子束团发生康普顿垂直散射,可以产生太赫兹皮秒脉冲;单个电子产生的太赫兹光子产额与微波功率、微波波长成正比,与微波束截面积成反比;单个电子产生的太赫兹辐射功率与微波功率、电子Lorentz因子的平方成正比,与微波束截面积成反比;电子束团产生的太赫兹光子总产额与微波功率的平方、微波波长的平方成正比,与微波束截面积的平方成反比;电子束团产生的太赫兹辐射总功率与微波功率的平方、微波波长以及电子Lorentz因子的平方成正比,与微波束截面积的平方成反比。