Applications of non-collinear second harmonic generation in one-dimensional periodically poled crystals

Whereas the commonly used collinear quasi-phase-matched interaction is usually suitable for phase matching only a single nonlinear process, non-collinear interactions offer rich variety of phase matching possibilities, employing different elements of the nonlinear tensor, and various orders of quasi phase matching. By studying the dependence of the second harmonic process on the angle and the phase matching order in a periodically poled LiNbO₃ crystal, we can non-destructively derive the duty cycle of the structure. The obtained duty cycle agrees well with surface scan measurements using an atomic force microscope. Furthermore, by proper selection of the non-collinear angle, we demonstrate simultaneous second harmonic generation of two cross polarized waves.     

Broadband second harmonic generation of an optical frequency comb produced by four-wave mixing in highly nonlinear fibers

We demonstrate broadband second harmonic generation of low-energy pulses produced by injecting two single-frequency lasers into a highly nonlinear fiber. Full nonlinear conversion of the corresponding spectra, consisting of broadband (∼200 nm) optical frequency combs at ∼1580 nm, were obtained by using conventional birefringence phase-matching in two BIBO crystals (2-mm and 100-μm long) with a normal incidence configuration. The crystals were not tilted and the pulses were not compressed. This broadband conversion results from the large phase-matching bandwidth of the nonlinear BIBO crystals at ∼1550 nm, but also seems to be a consequence of a fundamental comb with small spectral phase variation.     

超短脉冲倍频中三阶非线性效应的影响及补偿

 为了提高超短脉冲二倍频的转换效率和改善出射倍频光的脉冲形状,对超短脉冲倍频中三阶非线性效应的影响进行了理论分析和数值模拟,并采用初始相位失配的方法来补偿三阶非线性效应的影响。结果表明：用KDP晶体二倍频中心波长为800 nm的超短脉冲,当入射功率密度大于100 GW/cm²时,三阶非线性效应是倍频转换效率的主要影响因素。对脉宽为50 fs,入射功率密度为250 GW/cm²的超短脉冲在KDP晶体（2 mm）中的二次谐波变换,当初始相位失谐0.9 mrad时,转换效率提高了10%,同时由三阶非线性效应引起的强度调制得到明显抑制,出射基频光和倍频光的脉冲形状得到明显改善。     

Quadripartite entanglement from coupled type I second harmonic generation

We present a feasible protocol of continuous variable quadripartite entanglement from the coupled type I second harmonic generation (SHG) below threshold. According to the sufficient inseparability criteria for multipartite continuous variable (CV) entanglement, the four output fields are proved to be multicolored entangled beams in separable locations with four‐mode amplitude quadratures correlation and relative phase quadratures correlation. It shows that the coupled system can produce a compact tunable multimode entangled source that can be applied into the quantum communication.

     

A comparative study of laser second harmonic generation in some crystals

Comparison of the conversion efficiency for efficient second-harmonic generation of Nd: YAG laser radiation is reported for KDP, LAP, KTP, BBO and LBO crystals. Conversion efficiencies as high as 50% and 46% were obtained for our laboratory-grown KDP and LAP crystals respectively, for power densities well below their damage thresholds.     

Generation of green light in a thermally poled silica fiber by quasi-phase-matched second harmonic generation

Thermal poling is an attractive method to induce a nonlinear coefficient in silica materials. We have investigated potential silica materials with respect to their suitability for thermal poling and analyzed the achievable depletion thickness in the poling process. Based on this investigation, a fiber was designed and prepared for second harmonic generation. The generation of light in the visible range at a wavelength of 532 nm was successfully demonstrated with a poled fiber and with quasi-phase matching using a pump wavelength of 1064 nm.     

High-order harmonic generation by self-guided femtosecond laser pulses in argon

High-order harmonic generation (HHG) with 40 fs laser pulse in a quasistatic gas cell filled with argon gas was studied. Experiment results in several aspects were given and analyzed. As a result of balancing diffraction and plasma defocusing, and a large harmonic-generation cross-section, the spatial quality of the 23rd harmonic is successfully optimized with a divergence angle of 0.38 mrad. The modulation of spectral spatial divergence angle by the gas pressure was given, which was explained by the formation of plasma channel. The effect of initial laser pulse chirp on the harmonic generation and the harmonic spectral blueshift under different gas pressure were given and analyzed.     

Efficient second, third, fourth, and fifth harmonic generation of a Yb-doped fiber amplifier

We report generation of the second, third, fourth, and fifth harmonics of the output of a Yb-doped fiber amplifier seeded by a passively Q-switched Nd:YAG microchip laser. We obtained high conversion efficiencies using a simple optical arrangement and critically phase-matched nonlinear crystals. Starting with 320 mW of average power at 1064 nm (86 μJ per pulse at a 3.7 kHz repetition rate), we generated 160 mW at 532 nm, 38 mW at 355 nm, 69 mW at 266 nm, and 18 mW at 213 nm. The experimental results are in excellent agreement with calculations.     

Role of topological phase-defects in the parametric generation process

We show that topological phase-defects are spontaneously generated from noise fluctuations in the degenerate configuration of the parametric interaction. These localized coherent structures are shown to affect the coherence properties of the parametrically generated field. It is shown that the emergence of coherence in the fundamental field relies on a previously unrecognized process of mutual annihilation of pairs of neighboring phase-defects. More precisely, the density of phase-defects N, and the time correlation τ_c of the generated field, are shown to exhibit a power-law behavior with the propagation length, i.e., .     

Sum rules for arbitrary-order harmonic generation susceptibilities

Dispersion theory for harmonic generation susceptibilities is considered with the aid of complex analysis. New sum rules are obtained for the powers of arbitrary-order harmonic generation susceptibilities. The theory is based on the holomorphic properties and asymptotic behaviour of nonlinear susceptibilities. The present sum rules are reportedly important in nonlinear optical spectra analysis. Received 19 September 2002 Published online 31 December 2002     

Influence of phase transitions on the second harmonic generation in metal nanoparticles

In this work we use nonlinear optical properties of Ga nanoparticles monolayers of different average sizes embedded in dielectric matrices to investigate the liquid-solid phase transitions in these materials. Ga nanoparticles, formed by exploiting the partial wetting of liquid Ga over a SiO_x surface are irradiated with fs laser pulses from a Ti:sapphire source. The resulting Second Harmonic (SH) generated in the reflection and transmission directions is measured along the phase transitions by cooling the sample from 320 K down to liquid nitrogen temperature. Hysteresis cycles are observed in the nonlinear transmission, which exhibit a strong amplification from the solid to liquid values as compared to the linear optical results. A simple model for SH generation, based on Mie scattering calculations which includes the effect of surface plasmon resonance provides a fair key for the interpretation of the observed effects.     

Thermal effects observable during quasi-phasematched second harmonic generation in periodically poled crystal fibres

The impact of transient and steady-state thermal effects on the conversion efficiency of quasi-phasematched second harmonic generation in periodically poled LiNbO₃ fibres is discussed. The response time and wavelength variation of the quasi-phasematching condition depend on the thermal conductivity of the fibre, its enclosure and the thermal coefficient of the crystal’s refractive index.     

利用宽带倍频特性精确测量准相位匹配晶体的极化周期

 提出了利用准相位匹配晶体进行宽带脉冲倍频,再根据准相位匹配晶体的极化周期与倍频脉冲输出中心波长的相位匹配关系来精确测量准相位匹配晶体极化周期的新方法。理论分析了准相位匹配晶体的极化周期与倍频脉冲的光谱关系、倍频容许带宽与晶体长度的关系。研究表明：宽带脉冲倍频后输出的脉冲峰值波长对应于角度的调谐曲线具有对称性;对于长度为10 mm的极化晶体,其倍频容许带宽不到0.2 nm。实验中,采用了国产PPKTP晶体试验片,晶体尺寸为10 mm×7 mm×1 mm,理想极化周期9 300 nm。结果表明：对于中心波长为537.25 nm的倍频光,其对应的实际晶体极化周期为9 303.9 nm,大于晶体加工时的理想值;当倍频脉冲光谱测量精度为0.01 nm时,准相位匹配晶体的极化周期测量精度达到0.1 nm,远高于一般光学显微镜的观测精度。     

Enhancement of the second harmonic generation from monolayer WS2 coupled with a silica microsphere

Monolayer transition metal dichalcogenides (TMDs) are widely used for integrated optical and photoelectric devices. Owing to their broken inversion symmetry, monolayer TMDs have a large second-order optical nonlinearity. However, the optical second-order nonlinear conversion efficiency of monolayer TMDs is still limited by the interaction length. In this work, we theoretically study the second harmonic generation (SHG) from monolayer tungsten sulfide (WS₂) enhanced by a silica microsphere cavity. By tuning the position, size, and crystal orientation of the material, second-order nonlinear coupling can occur between the fundamental pump mode and different second harmonic cavity modes, and we obtain an optimal SHG conversion efficiency with orders of magnitude enhancement. Our work demonstrates that the microsphere cavity can significantly enhance SHG from monolayer 2D materials under flexible conditions.     

Effect of a periodic magnetic field on phase matching condition in second harmonic generation at interactions of laser-plasma

In this paper, the process of second harmonic generation (SHG) is studied in underdense plasma in the presence of a periodic magnetic field. It is shown that the difference of momentums of photon of second harmonic and two photons of main wave can be provided by momentum of everyone of Fourier components of periodic magnetic field so that momentum of nth Fourier component can be chosen by . It is also proven that the highest efficiency of second harmonic generation will be provided by the first Fourier component of periodic magnetic field . It is revealed that periodic magnetic field can produce longitudinal waves at and as well.     

Asymptotic behaviour and general properties of harmonic generation susceptibilities

We use the Kubo response function formalism to derive the asymptotic behaviour of the harmonic generation susceptibilities to all orders n. The results show a stringent correspondence with the ones previously obtained from the classical anharmonic oscillator model. They are characterized by a dependence and a coefficient proportional to the trace of the (n+1)th derivative of the potential energy on the equilibrium density matrix. Using the above results we derive new Kramers-Kr?nig relations and sum rules for all orders of harmonics susceptibilities. Received 17 April 2000     

二倍频晶体温升分布对输出光场分布的影响

利用分步快速傅里叶变换和四阶龙格-库塔法,对具有一定温升分布的倍频晶体的二次谐波转换过程进行了研究。综合考虑了谐波转换过程中的离散、衍射、二阶、三阶非线性等效应,着重讨论了倍频晶体吸收光能后,晶体内温升分布对晶体内o光和e光的折射率分布的影响,定量分析了温度分布引起的相位失配量、输出光场分布、二次谐波转换效率随倍频晶体温度分布变化的规律。结果表明:在高功率倍频系统中,倍频晶体温升分布引起基频光、倍频光的相位失配,相位失配导致输出光场光强分布的变化以及谐波转换效率的降低。     

Role of polytypism and degree of hexagonality on the photoinduced optical second harmonic generation in SiC nanocrystalline films

Photoinduced optiсal second harmonic generation was studied in nanocrystalline SiC films prepared by the method of direct ion deposition. For the studies were chosen three types of polytypes (with different degree of hexagonality) – 24R with degree hexagonality G=25, 27R-G=44, 33R with – G=36. The bicolor photoinduced treatment was performed by the wavelengths 1064nm/532 nm by 15 ns YAG:Nd laser. The efficiency of the output SHG was evaluated by ratio of the corresponding signal intensities with respect to the references and by the time delay between the SHG and the fundamental maxima. Explanation of the observed effect is given within a framework of the occurrence of the nano-trapping levels in the film crystalline interfaces.     

Tunable wavelength conversion of picosecond pulses based on cascaded sum- and difference-frequency generation in quasi-phase-matched LiNbO3 waveguides

Tunable wavelength conversion for picosecond pulses is proposed and demonstrated exploiting cascaded sum- and difference-frequency generation in quasi-phase-matched LiNbO₃ waveguides. The influences of initial pulse widths and injected pulse powers on the conversion efficiency and converted pulse width are theoretically analyzed. Arbitrarily tunable wavelength conversion is performed for the signal pulse with the temporal width of 1.57 ps and repetition rate of 40 GHz. Approximately −18.9 dB conversion efficiency and 25 nm variable region of the input signal are achieved under the lower launched signal power. The results imply that simultaneous wavelength conversion and pulse compression can be potentially obtained by using the pulsed control wave or designing longer waveguides.