High-speed optical modulators based on intersubband transitions
Abstract: Optical modulators are essential components in high speedoptical communication. They are employed to circumvent theinherent speed limitations of direct modulated semiconductorlasers, and may also extend the transmission distance onoptical fibers by providing optical pulses with lowerchirp.This thesis is mainly devoted to design and theoreticalinvestigations of high-speedRC-limited electro-absorption modulators based onintersubband (IS) transitions (ISTs) in quantum wells. Themodulators cover both the important fiber optics communicationwavelength λ=1.55 μm, and mid-IR wavelengths,potentially useful for interconnects. The high-speed propertiesof electrically controlled IS modulators are presented for thefirst time, and are predicted to be very competitive comparedto present state-of-the-art modulators. In contrast to presentmodulators the high-speed capability is shown to be readilycombined with a small negative chirp parameter and a very highabsorption saturation power Pst50 mW. ISTs were first demonstrated in quantumwells in 1985 and during the last decade have been successfullyimplemented in quantum cascade lasers and quantum well infraredphototransistor (QWIP) arrays.IS absorption modulation based on carrier density, Starkeffect and quantum interference is considered. It is shown thatthe IS absorption linewidth Γ is decisive for theachievable modulation speed through C~Γ3. A tightly confining optical waveguide is animportant part of a high-speed modulator. It is proposed thatthe plasma effect is employed to achieve a tight opticalconfinement at mid-IR wavelengths, with a comparison to surfaceplasmons. It is further predicted that efficient plasma effectwaveguides in III-nitrides should be viable even at λ=1.55μm.For λ=1.55 μm GaN/AlGaN/AlN andInGaAs/InAlAs/AlAsSb structures are assesed and RC-limitedspeeds up to ƒ3dB~90 GHz at Vpp=2.0 V are predicted. This is on apar with state-of-the-art travelling wave interband (QCSE)electro-absorption modulators. However given that ISTs atλ=1.55 μm have only very recently been demonstrated,and the strong dependence on the linewidth Γ, there is aconsiderable uncertainty in both directions, in the predictedƒ3dB. It is shown that in the mid-IR using matureIII-V materials on InP and GaAs even higher speeds arepossible, e.g. ƒ3dB~200 GHz at Vpp=0.9 V and λ=6.6 μm.In this thesis ISTs are also evaluated as a basis forelectromagnetically induced transparency (EIT). In contrast tothe bulk of this work EIT is based on coherent ISpolarizations, and can entail low power dissipation andimplementation of a number of optical device functions. Howeverrapid dephasing in quantum wells most likely introducesprohibitive obstacles.A comprehensive simulation program to simulateself-consistently the active region and adjoining layers, andthe modal optical properties has been developed. Some initialexperimental work has also been performed including epitaxy,processing of electro-absorption structures, and structural andoptical characterization.
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