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Chapter 13-14

Chapter 13-14. Lasers. Atoms, Molecules, and Solids. 选择定 则 : 角 动量守恒和宇称守 恒. 晶体里能带的形成. 人工的晶体结构. Quantum wells/wires/dots. 利用不同材料 中不同的导带和价带，人工构建电子的势阱结构. Intra-Band Transition. Inter-Band Transition. 人工的晶体结构. Quantum wells/wires/dots. Interactions of Photons with Atoms.

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Chapter 13-14

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  1. Chapter 13-14 Lasers

  2. Atoms, Molecules, and Solids

  3. 选择定则: 角动量守恒和宇称守恒

  4. 晶体里能带的形成

  5. 人工的晶体结构 Quantum wells/wires/dots

  6. 利用不同材料中不同的导带和价带，人工构建电子的势阱结构利用不同材料中不同的导带和价带，人工构建电子的势阱结构

  7. Intra-Band Transition Inter-Band Transition

  8. 人工的晶体结构 Quantum wells/wires/dots

  9. Interactions ofPhotons with Atoms

  10. Spontaneous Emission because the transition is independent of the number of photons that may already be in the mode

  11. Stimulated Emission

  12. Absorption Depends on the density of states n

  13. Quantum Background? • Upper level: n1 • Down Level: n0 • Photon: n • Photon is harmonic oscillation 考虑到产生与消灭算符： 前两项：Stimulated Emission and Absorption 最后一项： Spontaneous Emission 谐振子（比如声子）跃迁时：

  14. Spontaneous Emission 与三维腔的态密度有关。 可以通过调整腔的构型（二维腔、亚波长），控制该关系式

  15. Stimulated Emission and Absorption 与态密度相关 黑体辐射中有同样的表示！

  16. Line Broadening • Lifetime Broadening • Collision Broadening • Inhomogeneous Broadening

  17. Thermal Light

  18. Thermal Equilibrium Between Photons and Atoms

  19. Blackbody Radiation Spectrum

  20. Thermography

  21. Luminescence Light

  22. (α) Rayleigh, (b) Raman (Stokes), (c) Raman (anti-Stokes), and (d) Brillouin

  23. 荧光与Raman散射的特点 荧光：出射波长固定 Raman散射：上能级为虚能级，出射波长与入射波长相关

  24. Comments • Amplitude • Phase • Polarization • Mode Structure

  25. Phase = kz+ 0 = zv/c + 0

  26. The Laser Amplifier

  27. LASER Amplification The probability density for stimulated emission is the same as that for absorption. A population inversion is required

  28. Gain Coefficient

  29. Lorentzian Shape

  30. WhyLorentzian？ • Density Matrix Approach • 双能级体系中电子状态随外加电磁场E(t)驱动下的响应 • ρij表征双能级态上电子波函数的关联，或电子密度

  31. Remember: Lorentz oscillator model 与前面章节中Lorentz Osillation结论类似，包含实部和虚部 Phase shift Gain

  32. Phase Shift 问题：是否与激光要求的“保持位相”矛盾？ • 写入Exp(…)形式 • 实部：增益 • 虚部：相移

  33. AMPLlFIER PUMPING

  34. Rate Equations

  35. Rate Equations in the Absence of Amplifier Radiation

  36. Rate Equations in the Presence of Amplifier Radiation

  37. 越小越好 • 当入射光的光强增大到一定的程度后，粒子数反转/增益系数随光强的增大而减少 • 粒子总数有限 • 入射光的驱动，等效于缩短了21（增强了跃迁效率）

  38. Four-Level Pumping

  39. Three-Level Pumping

  40. 更优！

  41. Comments • 粒子数反转的条件（上能级寿命>下能级寿命）是否为必须？ • 否定的答案 • 只针对稳态情况，dN/dt=0 • 对于瞬态，有可能在不满足粒子数反转条件下，实现短脉冲激光的发射 • 强的短脉冲泵浦 • 量子光学中，也存在Lasing without Inversion (LWI)效应 (Coherently driven three- and multilevel systems) • potential to obtainlaser light in spectral domains, e.g. the x-ray range, whereconventional methods based on population inversion are notavailable or are difficult to implement.

  42. COMMON LASER AMPLlFIERS

  43. 1961，William Bennett and Ali Javan invented the helium-neon laser–the first gas laser and the first laser to operate continuously, rather than in pulses. HeNe gas laser

  44. Amplifier Nonlinearity and Gain Saturation

  45. Saturable Absorbers • the absorption of light decreases with increasing light intensity • Applications in shorter laser pulses

  46. Theory of Laser Oscillation

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