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Semiconductor Lasers (Pdf) Semiconductor Lasers Direct-Gap vs. Indirect-Gap E(energy) k (momentum) A Brief Introduction to Semiconductors Energy Bands E Conduction band (empty) Eg (bandgap energy) 1 0 Valance band 1 (full) fFD EE f 1 exp kT Equilibrium Fermi-Dirac Statistics http://britneyspears.ac/lasers.htm !!!??? Conduction band Eg Valance band Nonequilibrium Electron-Hole Injection 104 cm-1 Example Gain Gain GaAs Eg=1.4 eV Absorption or or Absorption Absorption (g=850 nm) 0 hw E g Section 12.1 p.2 p-n junctions Doping with Impurities n-type p-type Ef Eg Eg Ef Examples: GaAs doped with Br Examples: GaAs doped with Zn Si doped with P Si doped with Al Semiconductor junction lasers Forward-Biased p-n Junction (LED) n+-GaAs p+-GaAs + V d Inversion (Gain) Layer Neh, ed Current Density Threshold J th r Recombination time Section 12.2 p.2 Edge-Emitting Homojunction Laser Diodes +v Polished facets p-GaAs n-GaAs Lg100 m •Waveguide Modes Homojunction Lasers have very high current threshold mainly because. •Electrons and holes are free to diffuse and therefore dilute the gain (no carrier confinement) •Optical mode has poor overlap with gain (no opticalNeh, ed confinement or guiding) J th r 12.3 Heterojunction Lasers Diodes A fortunate coincidence: n when Eg electrons n+ p+ AlGaAs GaAs AlGaAs Energy Carrier confinement holesholes 100 nm Index of Refraction Mode confinement Section 12.3 p.2 Edge-Emitting Heterojunction Laser Diodes Edge-Emitting Buried Heterojunction Laser Diodes Cleaved reflecting surface W L Stripe electrode Oxide insulator p-GaAs (Contacting lay er) p-Al Ga As (Confining lay er) x 1-x p-GaAs (Active lay er) n-Al Ga As (Confining lay er) 2 x 1-x 1 3 Substrate n-GaAs (Substrate) Current pa thsSubst rate Electrode Elliptic al Cleaved reflecting surface laser Active region where J > J . be am th (Emission region) Schematic illustration of the the structure of a double heterojunction stripe contact laser diode © 1999 S.O. Kasap,Optoelectronics (Prentice Hall) 12.4 Quantum Well Lasers Multiple Quantum Well (MQW) Lasers +v n+ p+ p+-AlGaAs AlGaAs 10 nm AlGaAs n+-AlGaAs n-GaAs Substrate mode confinement Epitaxial Growth Section 12.4 p.2 High Power Diode Bars •P>100 W (cw) •Diode-pumping solid-state lasers (DPSS) •Material Processing •… Section 12.4 p.3 Vertical Cavity Surface Emitting Lasers (VCSEL) Mirror MQW (gain) Mirror •Good mode quality couples to fiber efficiently for telecom applications •Single mode operation •2-D structures cam be made •Low power Section 12.4 p.4 Laser Diodes Cover the Spectrum Compound Spectral Region Notes AlxGa1-xN uv GaN uv (350 nm) InxGa1-xN blue (480-400 nm) data storage, display GaxI1-xP (x=0.5) 670 nm display GaxAl1-xAs (x=0-0.45) 620-895 nm GaAs 904 nm diode pumping solid-state and fiber lasers. InxGa1-xAs (x=0.2) 980 nm InxGa1-xAsyP1-y 1100-1650 Telecom (x=0.73, y=0.58) 1310 nm (x=0.58, y=0.9) 1550 nm PbSSe 4200-8000 nm cryogenic PbSnTe 6300-29,000 nm cryogenic 12.5 Recent Advances: Quantum Cascade Lasers Section 12.5 p.2 THE END ! Final Exam Thursday, Dec. 16 Comprehensive, Closed Book Review Session Thursday, Dec. 9 .
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