LED导论2015 秋冬Introduction to Light Emitting Diode Chapter 4 Laser action in semiconductors
主讲教员：陈家豪
北京大学软件与微电子学院无锡产学研合作教育基地
?雷射: LASER
Light Amplification by Stimulated Emission of Radiation
?1960年5月16日，美国加利福尼亚州休斯实验室的科学家梅曼宣布获得了波长为0.6943微米的雷射，这是人类有史以来获得的第一束雷射，梅曼因而也成为世界上第一个将雷射引入实用领域的科学家。
?1960年7月7日，梅曼宣布世界上第一台雷射器
诞生，利用一个高强闪光灯管，来刺激红宝石。
?在一块表面镀上反光镜的红宝石的表面钻一个孔，
使红光可以从这个孔溢位，从而产生一条相当集中
的纤细红色光柱，当它射向某一点时，可使其达到
比太阳表面还高的温度。
美军使用雷射器的实验
Comparison between an LD and LED
?Laser Diode
?Stimulated radiation
?narrow linewidth
?coherent
?higher output power
? a threshold device
?strong temperature dependence
?higher coupling efficiency to a fiber ?LED
?Spontaneous radiation
?broad spectral
?incoherent
?lower output power
?no threshold current
?weak temperature dependence
?lower coupling efficiency
雷射光特性:
?高同调性(Coherence)
?高单色性(Monochromatity)
?高亮度(Brightness)
?高方向性(Directionality)
Incoherence
Coherence
Laser is an optical-frequency oscillator
?A laser is an optical-frequency oscillator constructed from an optical-frequency amplifier with positive feedback.
?Light waves which become amplified on traversing the amplifier are returned through the amplifier by the reflectors and grow in intensity, but this intensity growth does not continue indefinitely because the amplifier saturates.
oscillator
?An oscillator is an amplifier with positive feedback at a particular frequency.
?For oscillators circuit, an electronic amplifier provides the signal gain, a filter determines the frequency, and feedback results from connecting the amplifier output back to its input.
To make a laser
1. Stimulated emission
2. Population inversion –a criterion to provide gain
3. Optical feedback
4. Laser threshold
To make a laser
受激发射(Stimulated emission)与自发輻射(Spontaneous Emission) 自发輻射(Spontaneous Emission)
能量较高的物质放出电磁波，而降低其能量的过程称为自发放射。物质吸收光子
能量后，电子跃迁到激发态，当入射光消失时，电子经过一小段时间后，会自
然回到稳定的基态。这时能量将以光子方式释放，且朝各方向放射的机率相同，
一般的光源放射就是这种方式。
受激发射(Stimulated emission)与自发輻射(Spontaneous Emission) 受激辐射(Stimulated Emission)
当原子处于准稳态，而能量等于准稳态与基态之差的光子，经过此原子时，原子立即从亚稳态跃迁至基态，且将发射一个新光子。
居量反转(Population inversion)
电子能暂存于其上，使原子本身居于特定能态的数量高于其居于另一较低能态的原子数量，造成所谓的居量反转(population inversion)，便能产生雷射增益而进行光放大作用，此种物质便可做为雷射增益物质。
居量反转(Population inversion)
?2阶系统只含有两个能态，被激发的和回到稳态的原子数量相同，在总原子数量守恒的情形下，无法在持续状况(steady-state)下达到居量反转，因此无法提供雷射增益。
?在3阶系统中，原子被激发至上层能态后，快速跃迁至次一能态，存于此能阶的原子具有较长的存活期(life time)，不易跃迁至基态，因此可在此累积，持续达到居量反转，产生雷射增益。红宝石雷射即属此一类型。
?在4阶系统中，原子被激发至上层能阶，同样也会快速跃迁至次一存活期较长的能阶，而存在于此能态的原子会再度跃迁至另一短存活期之能阶，然后快速回到基态能阶，于是存于长存活期能阶的原子越来越多，短存活期能阶的原子越来越少，其间的居量反转便更容易达成，也因此4阶系统的雷射增益物质通常具有较高的增益，或具有较低的激发阈值，更容易产生雷射。Nd:YAG雷射即属此一类型。
居量反转(Population inversion) 「居量反转」是产生雷射光的重要条件，就是上能阶的电子密度必须
高于下能阶的电子密度。热平衡下，
上能阶的电子密度一般都低于下能
阶的电子密度，也就是說，如果将
能阶看成是樓层，电子当做是人，
这裡的人喜欢居住在低的樓层，而
不喜欢住在高樓层。但居量反转发
生时，高樓层住的人多，低樓层则
住的少，这与热平衡下的分布趋势
相反，因为这缘故，当雷射发生时，
才能够放大放射光。
居量反转(Population inversion)
?In thermal equilibrium the stimulated emission is essentially negligible, since the density of electrons in the excited state is very small, and optical emission is mainly because of the spontaneous emission. Stimulated emission will exceed absorption only if the population of the excited states is greater than that of the ground state.
This condition is known as Population Inversion. Population inversion is achieved by various pumping techniques.
?In a semiconductor laser, population inversion is accomplished by injecting electrons into the material to fill the lower energy states of the conduction band.
居量反转(Population inversion)
?In thermal equilibrium the stimulated emission is essentially negligible, since the density of electrons in the excited state is very small, and optical emission is mainly because of the spontaneous emission. Stimulated emission will exceed absorption
only if the population of the excited states is greater than that of the ground state.
This condition is known as Population Inversion. Population inversion is achieved by various pumping techniques.
?In a semiconductor laser, population inversion is accomplished by injecting electrons into the material to fill the lower energy states of the conduction band.
?The population inversion is usually attained by electric-current injection in some form of a p+-n+ junction diode (also possible by optical pumping for basic research)?a forward bias voltage causes carrier pairs to be injected into the junction region, where they recombine by means of stimulated emission.
居量反转(Population inversion)
Population inversion by carrier injection
In a semiconductor, population inversion can be obtained by means of high carrier injection which results in simultaneously heavily populated electrons and holes in the same spatial region.
居量反转(Population inversion)
Population inversion by carrier injection
Incident photons with energy Eg< h?< (E Fc-E Fv) cannot be absorbed. Because the necessary conduction band states are occupied! (and the necessary valance band states are empty
居量反转(Population inversion)
Population inversion by carrier injection
Instead, these photons can induce downward transitions of an electron from a filled conduction band state into an empty valence band state. => emitting coherent photons!
The condition for stimulated emission under population inversion:
居量反转(Population inversion)
Population inversion in a forward-biased heavily doped p+-n+ junction
Upon high injection carrier density in a heavily-doped p+-n+ junction there exists an active region near the depletion layer, which contains simultaneously heavily populated electrons and holes –population inverted!