定义：
由激发态原子或离子的自发衰变导致的量子效应

当一个原子（或者激光增益介质中的离子）被激发到一个较高的能级（如通过吸收光子）后，经过一段时间后会以光子的形式释放能量，并自发地跃迁到基态，或者跃迁到基态与激发态之间的态。（光子可以对应介质中原子或离子的任何传输模式）这个过程就叫做自发辐射。 

自发辐射是一种量子效应，而在半经典图像中其被描述为真空噪声导致的辐射，也就是光场在零点处的涨落。通过在微腔结构中加入原子或者离子可以抑制或者修正自发辐射，因为加入原子或者离子后会改变光场的模场结构，这一事实可以支持以上观点。

自发辐射的速率（以及激发能级的寿命）是由原子的性质和周围介质的模场结构决定的。对于在自由空间（或者均匀介质）中的原子，可以利用跃迁截面来计算自发辐射的速率（参阅辐射寿命）。

通常来说，当上能级原子可以跃迁到较低能级时，其寿命在几个纳秒左右；对于禁戒跃迁的情况而言，其寿命会长很多。 
自发辐射产生的光是荧光。由激光增益介质辐射的荧光的能量通常约为激光输出功率的十分之一到三分之一。荧光光谱由跃迁截面决定。

Definition: a quantum effect, causing the spontaneous decay of excited states of atoms or ions

Opposite term: stimulated emission

When an atom (or a laser ion in a gain medium) is excited into a higher-lying energy level, e.g. by absorption of a photon, it may after some time spontaneously return to its ground state, or to some intermediate energy level, by releasing the energy in the form of a photon, which carries the energy in some random direction. (More precisely, the photon can correspond to any propagation mode of the medium surrounding the atom or ion.) This process is called spontaneous emission.

Spontaneous emission is a quantum effect.

Spontaneous emission is a quantum effect, which in a semiclassical picture can be described as an emission which is stimulated by vacuum noise, i.e. by the zero point fluctuations of the optical field. This view is supported by the fact that spontaneous emission can be suppressed or modified by placing an atom or ion in a microcavity structure, which modifies the mode structure of the optical field. Therefore, one should not see spontaneous emission as something “freely” done by atoms or ions, but as an effect which results from an interaction with the quantum-mechanical electromagnetic field, which is influenced by their environment.

The rate of spontaneous emission (and thus the lifetime of the excited level) is determined both by the properties of the atom and by the mode structure of the surrounding medium. For an atom in free space (or in a homogeneous medium), transition cross sections can be used for calculating the rate of spontaneous emission events (→ radiative lifetime). Typical upper-state lifetimes of atoms are a few nanoseconds if there are allowed transitions to lower levels; much longer values can occur for forbidden transitions.

Light produced by spontaneous emission is called luminescence. The luminescence emitted by a laser gain medium typically carries a total power which equals around one-tenths to one-third of the laser's output power. The optical spectrum of the luminescent light is essentially governed by transition cross sections.

Effect of Spontaneous Emission in Lasers

Spontaneous emission is important during the start-up phase of a laser, e.g. when generating pulses with Q switching. It provides the first “seed” for the build-up of laser radiation in the laser resonator.

After the start-up phase of a continuous-wave laser, spontaneous emission still has the important effect of causing a substantial power loss: a significant part of the applied pump power is needed just to replace the power lost by spontaneous emission. (An exception would be a Q-switched laser with pulsed pumping very shortly before the pulse generation.)

The direct influence of spontaneous emission on the generated laser radiation (after start-up) is quite weak in terms of optical power, but one can partly interpret the occurrence of laser noise as an effect of spontaneous emission.

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