Lecture_10

Lecture_10 - Outline of Lecture Optical refraction Near...

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1 Prof. J. S. Harris 1 EE243. Semiconductor Optoelectronic Devices (Winter 2010) Outline of Lecture Optical refraction Near Bandedge index Change of index Kramers-Kronig Plasma Contributions Semiconductor Diodes Prof. J. S. Harris 2 Gentle Reminder EE243. Semiconductor Optoelectronic Devices (Winter 2010) MIDTERM Tuesday, February 16, 2010 Material thru half of Ch 4 of class notes
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2 Prof. J. S. Harris 3 EE243. Semiconductor Optoelectronic Devices (Winter 2010) Just below the bandgap energy, we are just below a region of high absorption. While the material itself is substantially transparent, as the photon energy approaches the bandgap, the resonant denominator in Eq. (3.85) becomes stronger, leading to an increase in refractive index. Note rise as the photon energy approaches the bandgap energy. While Δ n is small, it is more than adequate to form waveguides. Note large bandgap materials tend to have lower refractive index at any given photon energy in the transparent region h ν (eV) Refractive index near the bandgap energy Prof. J. S. Harris 4 EE243. Semiconductor Optoelectronic Devices (Winter 2010) Difference in refractive indices between materials (GaAs and AlGaAs) is very useful for making waveguides in edge emitting lasers and distributed Bragg mirrors in vertical cavity surface emitting lasers (VCSELs) It is very fortunate that the lower index layers which can form the outer "cladding" of the guides are also the ones with the larger bandgap energy, hence transparent at the wavelength of interest. Usually want to use the lower bandgap energy for the active parts of the device, which form the center part (the core) of the waveguide such that the region with gain has the highest optical Feld. Refractive index near the bandgap energy (2)
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3 Prof. J. S. Harris 5 EE243. Semiconductor Optoelectronic Devices (Winter 2010) Controlled changes in refractive index can be used to make devices, such as optical modulators. Some such devices work by changing the index by carrier density in the semiconductor, others by changing the optical absorption spectrum near the bandgap energy by applying electric Felds (electroabsorption) and hence changing the index through the Kramers-Kronig relations. Other devices use so-called electro- optic effects--change in index with applied electric Feld. These mechanisms often lead to undesired changes in refractive index that cause “chirping” in lasers and modulators--i.e. and undesired frequency sweep in the optical output beam as the amplitude is modulated Temperature changes also lead to changes in refractive index and can change the operating wavelength of lasers with temperature. This can be good (for desired tuning) or bad (requiring temperature control) to maintain a Fxed wavelength. Changing the refractive index Prof. J. S. Harris 6 EE243. Semiconductor Optoelectronic Devices (Winter 2010) The change of refractive index with photon energy below the bandgap means the index also changes with temperature as the bandgap energy changes with temperature Simple estimate
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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_10 - Outline of Lecture Optical refraction Near...

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