Semiconductor Optics. Absorption & Gain in Semiconductors: Some Applications Semiconductor Lasers (diode lasers) Low Dimensional Materials: Quantum wells, wires & dots Quantum cascade lasers Semiconductor detectors. More Applications Light Emitters, (including lasers & LEDs)

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Semiconductor Optics

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Bandgap “Rules” The Bandgap Increases with decreasing Lattice Constant. The Bandgap Decreases with increasing Temperature.

Interband vs Intraband • Interband: • Most semiconductor devices operated based on the interband transitions, namely between the conduction and valence bands. • The devices are usually bipolar involving a p-n junction. C V • Intraband: • A new class of devices, such as the quantum cascade lasers, are based on the transitions between the sub-bands in the conduction or valence bands. The intraband devices are unipolar. Faster than the intraband devices C

Direct vs. Indirect Band Gap Direct bandgap materials: Strong luminescence Light emitters Detectors Direct bandgap materials: Weak or no luminescence Detectors

Gain and loss at T>0 K Effect of increasing temperature laser g Eg N2 >N1 N1 E At a higher temperature

A diode laser Larger bandgap (and lower index ) materials <0.2m p n <0.1 mm Substrate Cleaved facets w/wo coating Smaller bandgap (and higher index ) materials <1 mm

Wavelength of diode lasers • Broad band width (>200 nm) • Wavelength selection by grating • Temperature tuning in a small range

Typical numbers for optical gain: Gain coefficient at threshold: 20 cm-1 Carrier density: 10 18 cm-3 Electrical to optical conversion efficiency: >30% Internal quantum efficiency >90% Power of optical damage 106W/cm2 Modulation bandwidth >10 GHz

Semiconductor vs solid-state Semiconductors: • Fast: due to short excited state lifetime ( ns) • Direct electrical pumping • Broad bandwidth • Lack of energy storage • Low damage threshold Solid-state lasers, such as rare-earth ion based: • Need optical pumping • Long storage time for high peak power • High damage threshold