pumping mechanism

pumping mechanism

1. What Pumping Is

Pumping = supplying external energy to excite atoms/ions from lower to upper pump level, creating population inversion. Without pumping, system returns to thermal equilibrium $N_2<N_1$.

Pumping must be selective (populate upper laser level faster than lower) and continuous (to replenish stimulated emission loss).

2. Types of Pumping (exam list with examples)

Pumping	Energy Source	Example Lasers
Optical pumping	Flashlamp, arc lamp, another laser/diode	Ruby (flashlamp), Nd:YAG (lamp or 808 nm diode), dye
Electrical / discharge	Electric current / RF discharge excites gas	He-Ne (discharge), CO₂, Ar⁺
Electron beam / injection	Current injects e-h into semiconductor	Semiconductor diode laser (p-n junction forward bias)
Chemical	Exothermic reaction creates excited species	HF chemical laser
Thermal? Not useful	—	—

• Optical: broadband flash excites to $E_3$; fast decay to $E_2$ (Ruby, Nd:YAG).
• Electrical: electrons collide with gas atoms, excite to upper level (He-Ne: He* transfers to Ne* via collision — collisional pumping).
• Injection: in semiconductor, Forward bias injects electrons to conduction band and holes to valence ⇒ inversion at junction (homojunction/heterojunction).

3. Requirements for Efficient Pumping

• Pump rate > stimulated + spontaneous loss.
• Rapid non-radiative decay from pump level $E_3→E_2$ to keep $E_3$ empty and $E_2$ metastable populated.
• Fast depopulation of lower laser level $E_1→E_0$ for four-level.

4. Worked Example

Ruby: flashlamp pulse ~ms, pumps Cr³+ from ²A ground to ⁴F bands (E3), which relax in ~ns to ²E (E2, τ≈3 ms metastable) ⇒ inversion vs ground ; lases 694 nm.

He-Ne: discharge excites He to 2³S*, collides with Ne ground → Ne 3s,2s upper levels (E2) ⇒ inversion vs 2p,3p lower.

Diode: ~100 mA injection gives N2 electrons in conduction > N1 holes? Actually quasi-Fermi separation >Eg for inversion.

5. Exam Points

• Define pumping and why needed — 3-5 marks, frequent.
• List types with one example each — 5 marks, very frequent.
• Explain why pump must be faster than spontaneous loss and need metastable — 5 marks, frequent.
• Diagram of three/four-level with pump arrow — 3 marks.