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首页> 外文会议>2017 Conference on Lasers and Electro-Optics Europe amp; European Quantum Electronics Conference >Second-order cascading-assisted controllable supercontinuum generation in birefringent media
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Second-order cascading-assisted controllable supercontinuum generation in birefringent media

机译：双折射介质中二阶级联辅助可控超连续谱的产生

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摘要

Summary form only given. The propagation of intense ultrashort light pulses in transparent dielectric media gives rise to one of the most spectacular nonlinear optical phenomenon - supercontinuum generation (SC). The nature of this process is fairly well understood and involves intricate coupling between temporal and spatial effects leading to filamentation and an explosive growth of the pulse spectrum [1]. In recent years the rapidly growing field of applications has seen a rising demand for achieving broadband radiation with desired temporal and spectral properties. However, the spectral extent of the SC is essentially determined by the input wavelength and the linear or nonlinear properties of the medium such as material dispersion, bandgap and the nonlinear refractive index [2], all of which remain fixed in an isotropic medium for a given input wavelength. On the other hand, the use of birefringent nonlinear media, which possess both, quadratic and cubic nonlinearities, may provide the desired degree of control. The second-order cascading, which results from the phase-mismatched second harmonic (SH) generation [3] produces a cascaded-quadratic Kerr-like nonlinearity, ncasc2 ∝ -d2eff/Δk, which contributes to the nonlinear index of refraction. By setting an appropriate phase mismatch, Δk = k2ω - 2kω, the interplay between the cascaded and cubic nonlinearities may be exploited to either enhance or suppress the nonlinear effects [4, 5] and to access qualitatively different regimes of nonlinear propagation.In this work, we demonstrate fully controllable SC generation with 120 fs, 800 nm laser pulses in beta barium borate (BBO) crystal in the regime of second-order cascading-assisted filamentation in both, positive and negative phase mismatch regions, with various input pulse energies. In particular, we show that in the range of negative phase mismatch (see Fig. 1(a)), the blue-shifted spectral broadening and the short wavelength cut-off of the SC spectrum is fully controlled in the 410 - 700 nm range by simply varying the phase mismatch parameter, i.e. by changing the propagation angle with respect to the optical axis. The achieved spectral control is attributed to efficient generation of the self-phase-matched SH, which introduces a considerable energy loss and distortion of the trailing sub-pulse shape, counteracting the joint effect of cascaded-quadratic and cubic selfsteepenings. In contrast, in the range of positive phase mismatch (Fig. 1(b)), where self-phase matching is not possible, the control of the SC spectrum is explained by the competition between the cubic and cascadedquadratic self-steepenings. Moreover, we uncover a number of interesting findings regarding the SH generation. We observe filamentation of the SH pulses at perfect phase matching, which leads to extraordinarily polarized SC spanning from 300 nm to 610 nm and inducing spectral broadening around the fundamental wavelength. We expect that our results may by applied to any nonlinear crystal possessing both quadratic and cubic nonlinearities, opening the route to perform ultrafast nonlinear interactions over a wide spectral range in a simple and fully controlled fashion.
机译：仅提供摘要表格。强烈的超短光脉冲在透明介电介质中的传播引起了最壮观的非线性光学现象之一-超连续谱产生（SC）。此过程的性质已被很好地理解，并且涉及时间和空间效应之间的复杂耦合，从而导致细丝化和脉冲频谱的爆炸性增长[1]。近年来，快速增长的应用领域对实现具有所需时间和光谱特性的宽带辐射的需求不断增长。但是，SC的光谱范围主要由输入波长和介质的线性或非线性特性（例如材料色散，带隙和非线性折射率[2]）决定，它们对于各向同性介质均保持固定不变。给定输入波长。另一方面，使用同时具有二次和三次非线性的双折射非线性介质可以提供所需的控制程度。由相位不匹配的二次谐波（SH）产生[2]导致的二阶级联产生级联二次克尔状非线性，n casc 2 ∝ -d 2 eff /Δk，它有助于非线性折射率。通过设置适当的相位失配，Δk= k 2ω-2k ω，可以利用级联和三次非线性之间的相互作用来增强或抑制非线性效应[4]。 ，5]并访问定性上不同的非线性传播机制。在这项工作中，我们证明了在二阶级联辅助丝化过程中，β硼酸钡（BBO）晶体中以120 fs，800 nm激光脉冲完全可控的SC生成。在正负相位失配区域中，具有各种输入脉冲能量。特别是，我们表明，在负相位失配的范围内（参见图1（a）），蓝移光谱的加宽和SC光谱的短波长截止被完全控制在410-700 nm范围内通过简单地改变相位失配参数，即通过改变相对于光轴的传播角度。所实现的频谱控制归因于自相位匹配SH的高效生成，该自相位匹配的SH引入了相当大的能量损耗和尾随子脉冲形状的失真，从而抵消了级联二次方和三次自增强的联合效应。相反，在正相位不匹配的范围内（图1（b）），不可能进行自相位匹配，SC谱的控制由立方和级联二次自增强之间的竞争来解释。此外，我们发现了许多有关SH世代的有趣发现。我们观察到SH脉冲在完美的相位匹配处出现丝状化，这会导致SC的偏振范围从300 nm扩展到610 nm，并引起基本波长附近的光谱加宽。我们希望我们的结果可以应用于具有二次和三次非线性的任何非线性晶体，从而以简单且完全受控的方式开辟了在宽光谱范围内执行超快速非线性相互作用的途径。

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