定义：
是指通常电子要从低能级跃迁到高能级去必须吸收一份相当于二个能级之差的能量。如果这份能量由光辐射来提供，只有在光子的能量为二个能级之差时才会被原子所吸收。但是在高功率的光束下，虽然一个光子的能量还达不到二个能级之差，但电子可以同时吸收二个光子达到一定的能量而完成一次跃迁,这就是双光子吸收。

双光子吸收的发生主要在脉冲激光器所产生的超强激光的焦点处，光路上其他地方的激光强度不足以产生双光子吸收，而由于所用光波长较长，能量较低，相应的单光子过程不能发生，因此，双光子过程具有良好的空间选择性，对样品伤害较小。

双光子吸收分两种类型的跃迁： 仅涉及一个（或多个）光子的跃迁，称直接跃迁；包含声子的跃迁，称为间接跃迁。直接跃迁是在两个直接能谷之间的跃迁，且由一个带到另一个带的跃迁中，仅垂直跃迁是允许的。 取价带顶为能级零点，则每一个Ei的初态对应于某个Ef的终态。在间接带隙半导体中，由于导带最低状态的k值同价带最高能量状态的k值不同，因此价带顶的电子不能直接跃迁到导带底，因为动量不守恒。为了动量守恒，必须以发射或吸收一个或多个声子的形式把动量从晶格中取出或者交给晶格，由于多声子过程比起单声子过程可能性更小，所以主要考虑单声子过程。

Acronym: TPA

Definition: a nonlinear absorption process where two photons are simultaneously absorbed

More general term: nonlinear absorption

Two-photon absorption (TPA) is a process where two photons (usually of the same energy) are absorbed simultaneously, exciting e.g. an atom or ion to a higher-lying state, with the energy increase being equal to the sum of the photon energies. This is a nonlinear absorption process, occurring with significant rates only at high optical intensities, because the absorption coefficient is proportional to the optical intensity:

with the TPA coefficient β.

The absorbed power is thus proportional to the square of the optical input power.

Two-photon absorption is the simplest variant of multiphoton absorption.

Figure 1: Two-photon absorption in a semiconductor allows for absorption of light even if the photon energy is below the band gap energy.

In a dielectric material or a semiconductor, two-photon absorption can normally occur only if the photon energy is at least half the band gap energy. Therefore, there are e.g. no losses via two-photon absorption when ultrashort pulses at 800 nm wavelength propagate in a silica fiber. On the other hand, two-photon absorption at the same wavelength can occur in semiconductors such as GaAs, having a much smaller band gap.

Applications

The phenomenon of two-photon absorption finds applications in various technical areas. Some examples:

• It is used in simple autocorrelators for pulse characterization, where TPA in a photodiode, having a bandgap energy larger than the photon energy, is exploited to obtain a nonlinear response.
• Also, two-photon absorption is often used in fluorescence microscopy (two-photon microscopy) for exciting fluorescence with an infrared laser beam, which can easily penetrate the sample.
• In other cases, TPA is exploited for optical power limiting or for microfabrication.

Under certain circumstances (illumination with ultrashort pulses), it is even possible that the human eye responds to infrared light due to two-photon absorption processes in the retina [7].

TPA can also modify the saturation characteristics of saturable absorbers such as SESAMs, leading to a roll-over of the saturation curve which can help e.g. to suppress Q-switching instabilities.

Problems with TPA

Detrimental TPA effects can occur for nonlinear frequency conversion of ultrashort pulses in nonlinear crystal materials, particularly for conversion of short wavelengths, e.g. in UV sources. The nonlinear absorption leads to additional power losses, to thermal effects and possibly also to degradation of the material (photodarkening).

Bibliography