超快光学非线性光学.pptx
《超快光学非线性光学.pptx》由会员分享,可在线阅读,更多相关《超快光学非线性光学.pptx(32页珍藏版)》请在得力文库 - 分享文档赚钱的网站上搜索。
1、Nonlinear optics isnt something you see everyday.Sending infrared light into a crystal yielded this display of green light(second-harmonic generation,SHG):Nonlinear optics allows us to change the color of a light beam,to change its shape in space and time,and tocreate ultrashort laser pulses.Why don
2、t we see nonlinear opticaleffects in our daily life?1.Intensities of daily life are too weak.2.Normal light sources are incoherent.3.The occasional crystal we see has the wrong symmetry(for SHG).4.Phase-matching is required,and it doesnt usually happen on its own.第1页/共32页Why do nonlinear-optical eff
3、ects occur?Recall that,in normal linear optics,a light wave acts on a molecule,which vibrates and then emits its own light wave,which interfereswith the original light wave.We can also imagine thisprocess in terms of the molecular energy levels,using arrows for thephoton energies:第2页/共32页Why do nonl
4、inear-optical effects occur?(continued)Now,suppose the irradiance is high enough that many molecules are excited to the higher-energy state.This state can then act as the lower level for additional excitation.This yields vibrations at all frequencies corresponding to all energy differences between p
5、opulated states.第3页/共32页Nonlinear optics is analogous to nonlinear electronics,which we can observe easily.Sending a high-volume sine-wave(pure frequency)signal into a cheap amplifier or cheap speakers yields a truncated output signal,more of a square wave than a sine.This square wave has higher fre
6、quencies.We hear this as distortion.Sharp edges require higher frequencies in the Fourier series.第4页/共32页Nonlinear effects in atomsSo an electrons motion will also depart from a sine wave.The potential gets very flat out at infinity,so the electrons motion can easily go nonlinear!Potential due to nu
7、cleusNucleusEnergyPositionAnother way to look at nonlinear optics is that the potential of the electron is an atom is not a simple quadratic potential.第5页/共32页Nonlinear optics and anharmonic oscillatorsFor weak fields,motion is harmonic,and linear optics prevails.For strong fields(i.e.,lasers),anhar
8、monic motion occurs,and higherharmonics occur,both in the motion and the light emission.A nucleus(in a molecule)also does not have a simple quadratic potential.So its vibrational motion is also nonlinear:第6页/共32页Molecules Excited by Laser LightLaser light causes molecules to vibrate in unison.We say
9、 that laser light polarizes the medium.Accelerating charges emit light.Polarized matter emits light at the frequency at which it is oscillating.If the motion isnt sinusoidal,the medium will also emit the additional frequencies!第7页/共32页Maxwells Equations in a MediumThe polarization,P ,contains the ef
10、fect of the medium:Sine waves of all frequencies are solutions to the wave equation;its the polarization that tells which frequencies will occur.The polarization is the driving term for the solution to this equation.These equations reduce to the(scalar)wave equation:Inhomogeneous Wave Equation第8页/共3
11、2页Solving the wave equation in the presence of linear induced polarizationFor low irradiances,the polarization is proportional to the incident field:In this simple(and most common)case,the wave equation becomes:This equation has the solution:The induced polarization only changes the refractive index
12、.Dull.If only the polarization contained other frequencieswhere w=c k and c=c0/n and n=(1+c)1/2Using the fact that:Simplifying:第9页/共32页Maxwells Equations in a Nonlinear MediumNonlinear optics is what happens when the polarization is the resultof higher-order(nonlinear!)terms in the field:What are th
13、e effects of such nonlinear terms?Consider the second-order term:2w w=2nd harmonic!Harmonic generation is one of many exotic effects that can arise!第10页/共32页Sum-and difference-frequency generationSuppose there are two different-color beams present:Note also that,when wi is negative inside the exp,th
14、e E in front has a*.2nd-harmonic gen2nd-harmonic genSum-freq genDiff-freq gendc rectificationSo:第11页/共32页Complicated nonlinear-optical effects can occur.The more photons(i.e.,the higher the order)the weaker the effect,however.Very-high-order effects can be seen,but they require very high irradiance.
15、Also,if the photon energies coincide with the mediums energy levels as above,the effect will be stronger.Nonlinear-optical processesare often referred to as:N-wave-mixing processeswhere N is the number ofphotons involved(including the emitted one).This is a six-wave-mixing process.Emitted-lightfrequ
16、encyw wsig第12页/共32页Induced polarization for nonlinear optical effectsArrows pointing upward correspond to absorbed photons and contribute a factor of their field,Ei;arrows pointing downward correspond to emitted photons and contribute a factor of the complex conjugate of their field:sig第13页/共32页Solv
17、ing the wave equation in nonlinear opticsRecall the inhomogeneous wave equation:Because its second-order in both space and time,and P is a nonlinear function of E,we cant easily solve this equation.Indeed,nonlinear differential equations are really hard.Well have to make approximationsTake into acco
- 配套讲稿:
如PPT文件的首页显示word图标,表示该PPT已包含配套word讲稿。双击word图标可打开word文档。
- 特殊限制:
部分文档作品中含有的国旗、国徽等图片,仅作为作品整体效果示例展示,禁止商用。设计者仅对作品中独创性部分享有著作权。
- 关 键 词:
- 超快光学 非线性光学 光学 非线性
限制150内