【作者】 徐晗；

【导师】 曾和平；

【作者基本信息】 华东师范大学 ， 光学， 2007， 博士

【摘要】 超短激光脉冲在正常色散的二阶非线性介质中传输过程中，在级联二阶非线性效应以及衍射、色散等线性效应的相互作用下，输入脉冲的波包能自发转化为X光子弹。所谓“光子弹”即输出脉冲在时间和空间维度被极大的压缩且在传输过程中能保持脉冲包络的形状不变。而“X”是指其时空频谱的特征曲线为一系列双曲线。X光子弹可以广泛的应用于光激发粒子加速、光信息存储、光信息传输以及紫外阿秒脉冲产生等领域。此外，X光子弹的优良的可调谐性使得它有可能成为在量子相干控制技术中理想的光源。本论文主要围绕二阶非线性介质中X光子弹的产生条件以及光强、相位失匹、群速度失匹、衍射不对称等参数对X光子弹的调制作用展开了详细讨论。论文的主要内容包括：1．以正常色散的BBO倍频晶体为原型，建立了超短脉冲激光传输的三维理论模型。综合考虑了衍射、色散、群速度失匹等线性效应和二阶非线性效应。编写了能够数值求解三维激光脉冲传输模型的软件。数值模拟显示入射的高斯型光束能够自发的转化为X光子弹。并得出了X波产生的必要条件：1．入射激光能量足够大。2．入射脉宽足够短。3．合适大小的自聚焦类型的相位失匹。2．研究了X光子弹在不对称衍射情况下的稳定性。发现了在输入椭圆光束导致的衍射不对称情况下，选择合适的相位失匹以及光强等参数，X光子弹仍能稳定存在而不发生分裂。输入的椭圆高斯光束演化为椭圆的X光子弹。并提出稳定传输的机制：脉冲传播过程中产生类似于波导功能的虚拟光纤通道能够对光脉冲起到束缚的作用。3．研究了谐波间群速度失匹对X光子弹的调制作用。发现了在合适的相位失匹和入射光强等参数条件下，基波和一部分二次谐波能够克服群速度失匹而形成移动的X光子弹。群速度失匹的调制作用使基波和二次谐波的频谱都发生了显著的频移，频移方向决定于相位失匹类型以及群速度失匹的符号。数值模拟结果显示，移动X光子弹形成过程中能够自发产生倾斜的波阵面，可以抵消两个谐波波包之间由于群速度不同而导致的相互吸引作用，使得X光子弹能够静止的在以基波群速度移动的坐标系中运动。4．实验中观察到了X波的产生。在合适的参数下，输出脉冲最短可被压缩到15fs。空间上形成了与数值模拟一致的峰结构能量分布。自制的时空频谱仪显示X波的时空频谱具有明显的双曲线型特征。实验结果验证了数值模拟有关椭圆X光子弹和移动X光子弹的结论。5．数值模拟了光稠密三能级系统中选择性共振激发。由于光稠密介质中的偶极—偶极相互作用，入射光场与介质相互作用时可以产生跃迁频率的移动。频移量的大小是可以通过调节输入脉冲的包络、相位以及粒子数密度等参数加以控制的，进而使得我们能够控制三能级系统的跃迁途径，实现选择性激发。更多还原

【Abstract】 During the propagation of ultra-short laser pulse in normal dispersive quadratic nonlinear media, the input wave-packet can be spontaneously converted into X-shaped light bullets, where "light bullet" is a kind of propagation invariant wave-packet and localized both in space and time, and "X" refers to the hyperbolic structured spatiotemporal spectrum. X-shaped light bullet can be used, for example, light induced particle acceleration, optical information storage and transmission and ultra-violent attosecond pulse generation, etc. In addition, the tunability of X-shaped light bullet makes it becomes a possible pump source for quantum control.This article focused on the discussion of the conditions under which X-shaped light bullets forms, together with the modulations on the output wave-packet induced by input parameters, such as pump intensity, phase mismatch, group velocity mismatch, and anisotropic diffraction etc. The main contents are listed below:1. We establish 3-D model of ultra-short pulse propagating in normal dispersive quadratic nonlinear media whose prototype is type-I BBO crystal, and this 3-D model can be numerically resolved by our home-made software. The numerical simulation reveals that the input Gaussian wave-packet can be converted into X-shaped light bullets under the conditions of (1). Large enough input intensity, (2). Short enough input pulse width, and (3). Self-focusing type phase mismatch with proper value.2. Numerically investigate the stability of X-shaped light bullet with anisotropic diffraction, where we find the input elliptic Gaussian beam can be converted into stable elliptic X-shaped light bullet and no beam splitting occurs. This originates from the virtual waveguide which forms accompany with elliptic X-shaped light bullet and prevents the wave-packets from splitting during propagation.3. Numerically investigate the modulation effects of group velocity mismatch. Wefind that with unvanishing group velocity mismatch, wave-packets of fundamental wave and a part of second harmonic wave can still combine together in the form of walking X-shaped light bullets. And the output spectrum is strongly modulated, where the direction of spectrum shifting is governed by the sign of phase mismatch and group velocity mismatch. The numerical simulation indicate that the spontaneously generated phase front tilting can balance the attracting force between harmonics induced by group velosity mismatch and lead to zero-velocity (view in retarded frame) walking X-shaped light bullets.4. Experimental research of X-shaped light bullet. We observe the compression of output pulse both in time and space with proper selected parameters. And the hyperbolic structured spatiotemporal spectrum of output wave-packet is also observed via home-made spatiotemporal spectrometer. The experimental results agree well with numerical simulations on elliptic and walking X-shaped light bullets.5. Selective resonant excitations can be realized with ultrashort pulse in an optical dense collection of three-level individual atoms, where the near dipole-dipole interactions induce intrinsic frequency shifts of the atomic resonance. The intrinsic frequency shifting can be controlled by modulating the wave-packet profile and phase of excitation pulses or varying the density of optical media, which will lead to constructive or destructive quantum interference between different excitation pathways towards a desired selective resonant excitation.更多还原