| Semiconductor quantum structures (quantum well/wire/dot) have attracted intense interest for making electrooptic (EO) modulators because of their high EO coefficient due to the quantum confined stark effect (QCSE). This capability can lead to devices with low-driving voltages, high speed operations, and potential low cost.;In this dissertation, InGaAsP/InP quantum well (QW) and In(Ga)As/GaAs quantum dot (QD) material systems have been studied for high frequency applications. For the QW modulator, both EO and electroabsorption (EA) effects are observed for the same modulator. For high bias, the modulator performs as an EA modulator. At relatively low bias, the device acts like an EO modulator. When the modulator is bias at -18.6V, the modulation efficiency is measured to be 4.2 V · mm at 1550nm. While the reverse bias is -23V, the modulation efficiency is 3.3V · mm. The modulation signal can be measured up to 10GHz for 1mm-long device. The problem for these QW modulators is the high reverse bias, and high insertion loss (>7dB/mm).;Our second round devices are based on QD materials. The InAs/GaAs QD modulators were designed and fabricated. These modulators show a ∼ 9 V · mm modulation efficiency, and with a reverse bias at -6V for operation at 1.55µm. The insertion loss is 6.5dB/mm, which is dominated by waveguide sidewall scattering. The 3dB-bandwidth for an 800µm-long device is ∼10GHz. The EO coefficient is measured to be 35pm/V, which is 27 times as large as bulk GaAs. To further reduce the optical loss, and improve the high frequency performance, aligned QD chain structures have been designed and fabricated. These QD chain modulators show anisotropic properties along [011] and [01-1] directions. At 1.55µm, the modulation efficiencies are ∼ 7.3V · mm and 8V · mm for [011] device and [01-1] device. The QD chains give ∼10% higher EO effect along QD chain [01-1] direction. The 800µm-long modulator has a 3dB-bandwidth of ∼12.2GHz, and optical insertion loss reduces to ∼4.5dB/mm. This means that for a 2mm-long Mach Zehnder modulator using push-pull drive, the half wave voltage Vπ can be reduced to less than 2V.;Because of the small size and easy integration with other photonic devices, these semiconductor modulators have great potential in next generation photonic integrated circuits. |
