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在不考虑背景辐照和扩散非线性,而仅考虑一维和傍轴近似的情况下,光伏光折变晶体中的光束传播归一化非线性Schrödinger方程可以描述为[44]:
$$ i\frac{{\partial q}}{{\partial \xi }} + \frac{1}{2}\frac{{{\partial ^2}q}}{{\partial {\eta ^2}}} - \beta \frac{q}{{1 + {{\left| q \right|}^2}}} + \alpha \frac{{{{\left| q \right|}^2}q}}{{1 + {{\left| q \right|}^2}}} = 0 $$ (1) 式中:q为光场复振幅包络;经光束宽度x0和Rayleigh长度
$ kx_0^2 $ 归一化处理过的横纵坐标分别用η及ξ表示,k = k0ne为介质中的波数,其中ne为介质折射率,k0 = 2π/λ是真空中的波数,λ是入射光的波长;公式(1)左侧第三、四项分别描述的是漂移和光伏非线性效应,$\; \beta = {({k_0}{x_0})^2}(n_e^4{r_{eff}}/2){E_0} $ ,$ \alpha = {({k_0}{x_0})^2} (n_e^4{r_{eff}}/2){E_p} $ ,其中reff为有效电光系数,E0为外加电场,Ep为光伏场常数。若不考虑非线性作用,公式(1)存在艾里函数解$q(\eta ,\xi ) = {{Ai}}(\eta - {\xi ^2}/4)\exp (i\eta \xi /2 - i{\xi ^3}/12)$ ,式中艾里函数用Ai(∙)表达。虽然理想艾里光束在演化过程中能在横向方向进行自加速,但由于存在无穷能量振荡末尾,实际中不能实现。为此,参考文献[1]中提出了有限能量艾里光束解为:$q(\eta ,\xi = 0) = {{Ai}}(\eta )\exp (a\eta )$ ,其中a是衰减系数。若将两加速方向相反的有限能量艾里光束作为入射光场,对其交互作用进行研究时,光场表达式为:$$ \begin{split} q\left( {\eta ,\xi = 0} \right) =& {A_1}{{Ai}}\left( {\eta - B} \right)\exp \left[ {\left( {a + iv} \right)\left( {\eta - B} \right)} \right] +\\ &{A_2}\exp \left( {iQ} \right){{Ai}}\left( { - \eta - B} \right)\exp \left[ { - \left( {a + iv} \right)\left( {\eta + B} \right)} \right] \end{split}$$ (2) 式中:A1与A2是可以调整两初始入射有限能量艾里光束功率的振幅参量;v是正比于光束入射角度的控制参量,v > 0表示光束加速被增强,v < 0表示光束加速被减弱;B为光束初始相对间隔;Q为两束有限能量艾里光束的相位差,则同相位用Q = 0表示,而反相位则用Q = π进行表示。
同时,引入加速坐标
$ \eta = s - {\xi ^2}/4 $ ,s为坐标变换后的横坐标,将该表达式代入公式(1)可得非线性加速解满足方程:$$ i\frac{{\partial q}}{{\partial \xi }} - i\frac{\xi }{2}\frac{{\partial q}}{{\partial s}} + \frac{1}{2}\frac{{{\partial ^2}q}}{{\partial {s^2}}} - \left( {\beta - \alpha {{\left| q \right|}^2}} \right)\frac{1}{{1 + {{\left| q \right|}^2}}}q = 0 $$ (3) 设
$ q(s,\xi ) = u(s)\exp [i(s\xi /2 + {\xi ^3}/24)] $ 为所求加速解的表达形式,加速光的包络用u(s)来表示,将其代入公式(3)得:$$ \frac{{{\partial ^2}u}}{{\partial {s^2}}} - su - \left( {\beta - \alpha {{\left| u \right|}^2}} \right)\frac{1}{{1 + {{\left| u \right|}^2}}}u = 0 $$ (4) 由于当
$ s $ 趋于$\infty $ 时光场的振幅趋于0,取公式(4)边界条件为$u(s) = \gamma {{Ai}}(s)$ 和$u'(s) = \gamma {{Ai'}}(s)$ ,其中s > 0,γ表示非线性强度系数。非线性加速光束的解通过求解公式(4)即可得到。为分析加速光束的交互作用,构造如下形式初始光束:$$ u\left( {s,\xi = 0} \right) = {u_1}\left( {s - B} \right) + \exp \left( {iQ} \right){u_2}\left[ { - \left( {s + B} \right)} \right] $$ (5) 式中:横向对称的非线性加速光束包络由u1与u2来表示,光束间隔由B调控,光束的相位差由Q调控。文中以LiNbO3晶体为例,参数取值:ne = 2.208,reff = 33 pm/V,Ep = –5 kV/cm,其他参数: λ = 0.488 μm,x0 = 0.04 mm。求得α = –5.2,通过调节偏压可以对参数β进行操控。
Theoretical study on interaction effect of self-accelerating beams in a biased photovoltaic photorefractive crystal
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摘要: 采用分步傅里叶法理论探究了有限能量艾里光束和非线性加速光束在有偏压光伏光折变介质中的交互效应。结果表明:调节光束初始间隔和入射角度可使同相位或反相位有限能量艾里光束相互吸引或排斥。同相位时不仅产生呼吸孤子和孤子对,在适当参数条件下还可形成分叉孤子;反相位时仅有孤子对产生;同相位非线性切趾加速光束交互可以产生奇数个呼吸孤子,反相位非线性切趾加速光束交互可以产生偶数个呼吸孤子对。此外,呼吸孤子的峰值强度、呼吸周期和相互作用力的大小均可以通过外部偏压和入射角度进行有效调控。研究结果可为艾里光束交互调控提供理论基础,同时在全光信息处理和光学网络器件制备等领域具有潜在的应用前景。Abstract: Investigation on the interactions of Airy and nonlinear accelerating beams in a biased photovoltaic-photorefractive crystal was presented theoretically by means of split-step Fourier method. The results shown that, by adjusting the initial beams interval and incident angle, two finite-energy Airy beams in the in-phase or out-of-phase case can attract or repel each other. In the in-phase case, not only single breathing solitons and soliton pairs can generate, but also splitting solitons with oscillation are obtained. While only soliton pairs can be formed in the out-of-phase case. Interaction of two in-phase nonlinear truncated accelerating beams can generate an odd number of breathing solitons, and an even number of soliton pairs can be produced in the out-of-phase case. Moreover, the peak intensity, breathing period and magnitude of the interaction force of the breathing solitons can be effectively regulated by adjusting the external bias and the incident angle. The results can provide a theoretical basis for the interaction regulation of Airy beams, and also have potential application prospects in the fields of all-optical information processing and optical network device fabrication.
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Key words:
- nonlinear optics /
- Airy beam /
- interaction /
- light field regulation
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