定义：
一种量子效应，受其它光子的激发而产生的光子辐射。

当激光活性原子或离子处于激发态时（量子力学能级），随后在一段时间内可能会自发的衰变到一个低能级，以光子的形式释放出能量，并且辐射的方向是随机的。这一过程被称为自发辐射。然而，光子辐射也可以受入射光子的激发，如果入射光子的能量合适（或者光频率），这被称为受激辐射。
在这种情况下，辐射的光子与入射光子模式相同。于是入射光子的功率被放大。这是激光放大器和激光振荡可以放大光的物理基础。 

图1：受激辐射示意图。一个入射光子激发原子或者离子使其从激发态跃迁到基态。 
当介质中太多激光活性原子处于激光跃迁中的低能级时，受激辐射的放大效应会被降低甚至完全被抑制，因为这些原子吸收光子因此使光强变小。在一个简单的二能级系统，激光放大过程需要粒子数反转。 
激发态原子受激辐射过程速率等于辐射截面和光通量的乘积（单位面积单位时间的光子数）。 
在阈值以上工作的激光器中，受激辐射占主导地位比自发辐射强，因此泵浦效率很高。如果满足这种条件，入射光强需要比饱和强度要大。

Definition: a quantum effect, where photon emission is triggered by other photons

Opposite term: spontaneous emission

If a laser-active atom or ion is in an excited state (quantum-mechanical energy level) (e.g. by optical pumping), it may after some time spontaneously decay into a lower energy level, releasing energy in the form of a photon, emitted in a random spatial direction. That process is called spontaneous emission. It is also possible that the photon emission is stimulated (provoked) by incoming photons [1], if these have a suitable photon energy (or optical frequency); this is called stimulated emission (see Figure 1). In that case, a photon is emitted into the mode of the incoming photon. In effect, the power of the incoming radiation is amplified. This is the physical basis of light amplification in laser amplifiers and laser oscillators.

Figure 1: Illustration of stimulated emission. An incoming photon stimulates an excited atom or ion to undergo a transition from the excited state to the ground state.
 

The physics of stimulated emission can be described in the context of quantum optics. There are also semiclassical descriptions (treating the interaction of an oscillating dipole or a higher-order multipole with an electromagnetic field), and the original idea of stimulated emission was published by Einstein [1] before quantum mechanics and quantum optics were fully developed.

Note that the amplification effect of stimulated emission can be reduced or entirely suppressed in a medium where too many laser-active atoms are in the lower state of the laser transition, because these atoms absorb photons and thus attenuate light. In a simple two-level system, laser amplification requires a so-called population inversion.

In rate equation modeling, the rate of stimulated emission processes for an excited atom can be calculated as the product of the so-called emission cross section and the photon flux density (number of photons per unit area and time). Such terms are regularly used in rate equation modeling. The photon flux density can be calculated as the optical intensity divided by the photon energy.

In a laser operated well above threshold, stimulated emission dominates over spontaneous emission, and the power efficiency can be high. For that condition to be fulfilled, the incident optical intensity must be higher than the saturation intensity.

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