Lecture_19

Lecture_19 - 9 Semiconductor lasers Conditions Necessary...

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1 Prof. J. S. Harris 1 EE243. Semiconductor Optoelectronic Devices (Winter 2010) 9. Semiconductor lasers Conditions Necessary for Stimulated Emission Gain Phase Match Threshold Current Optical ConFnement & Coupling to EM ±ield Single ±requency Lasers Vertical Cavity Surface Emitting Lasers (VCSEL) Laser Gain Dynamics Modulation Bandwidth Integrated VCSEL/Detector--micro²uroimeter (μTAS) Prof. J. S. Harris 2 EE243. Semiconductor Optoelectronic Devices (Winter 2010) Pizza Lunch for all “Survivors” CISX-Patio Thursday Noon March 11 Final Exam Wednesday, March 17 7-10 PM Bldg 380, Rm 380X Please go to AXESS for online course evaluation
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2 Prof. J. S. Harris 3 EE243. Semiconductor Optoelectronic Devices (Winter 2010) 1 1+ exp E 2 - F 2 ( ) / kT [ ] 1- 1 1+ exp E 1 - F 1 ( ) / kT [ ] 1 E 1 - F 1 ( ) / kT [ ] 1 1 E 2 - F 2 ( ) / exp F 2 - F 1 ( ) / kT [ ] exp E 2 - E 1 ( ) / kT [ ] F 2 - F 1 E 2 - E 1 = E 21 = E g B 21 f 2 f 1 ( ) P E 21 ( ) B 12 f 1 f 2 ( ) P E 21 ( ) and since B 21 = B 12 f 2 f 1 ( ) f 1 f 2 ( ) Conditions for Stimulated Emission A photon is more likely to cause stimulated emission than absorption when then r 21 stim . ( ) r 12 Separation of Quasi- Fermi levels > E g Prof. J. S. Harris 4 EE243. Semiconductor Optoelectronic Devices (Winter 2010) E c E v F 1 F 2 E g δ c δ v Δ E h ν > F 2 F 1 h < F 2 F 1 Isotropic Spontaneous Emission Stimulated Emission τ cavity << rec We can see this schematically from the energy band diagram. When F 2 - F 1 E g , then there are no Flled levels between F 1 and E v and no empty levels between F 2 and E g . Thus a photon at energy E g will stimulate a photon (downward transition) rather than absorb it (upward transition). Spontaneous emission still occurs above threshold, but most is stimulated because Conditions for Stimulated Emission (2)
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3 Prof. J. S. Harris 5 EE243. Semiconductor Optoelectronic Devices (Winter 2010) Lasing Condition A LASER (Light AmpliFcation by Stimulated Emission Radiation) as we commonly know it, is not an ampliFer at all--there is no INPUT. It is an OSCILLATOR! (LOSER) As such, two conditions must be met: 1. The gain must exceed the losses 2. The feedback from output to input must be in phase with the starting signal for oscillation (noise or for a laser, spontaneous emission) There can thus be stimulated emission without lasing if the above criteria are not satisFed (referred to as a super-radiant LED because the emission spectra is narrower and more efFcient, but it is not coherent). Prof. J. S. Harris 6 EE243. Semiconductor Optoelectronic Devices (Winter 2010) M 1 M 2 E i t 1 E i t 1 E i e −Γ L t 1 t 2 E i e −Γ L t 1 r 2 E i e −Γ L t 1 r 2 E i e 2 Γ L t 1 2 r 2 E i e 2 Γ L t 1 r 1 r 2 E i e 2 Γ L g E ( ) gain coeff . = α E ( ) = h 3 c 2 8 π n 2 E 21 2 r stim .
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This note was uploaded on 06/05/2010 for the course EE 243 taught by Professor Harris,j during the Winter '10 term at Stanford.

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Lecture_19 - 9 Semiconductor lasers Conditions Necessary...

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