Peter M. Livingston

Semiconductor electrorefractive modulators

Experimental and analytical results for semiconductor electro-refractive modulators will be presented. Modulation structures investigated include quantum wells, coupled quantum wells and quantum dots.

Quantum dots: a new approach to low V π optical modulators

Preliminary analysis has shown that quantum dots enable tens of millivolt-range operation of phase-shifters in a semiconductor Mach-Zehnder interferometer modulator. Our methodology based upon the quantum dot experimental work of Hse et al, makes use of his measured exciton line shapes to estimate refractive index changes in a PIN structure in which the intrinsic laser is loaded with self-organizing quantum dots and their associated wetting layers. We consider both forward and reversed bias cases; in the former, the interferometer phase shift sections become DFB lasers, and in the latter, the phase shift is caused by the quantum-confined Stark effect (QCSE). With the latter, we found a trade-off between low operating voltage and modulating bandwidth. For a phase shifter insertion loss of 5 dB, a 250-micron long phase section will yield a pi/2 control voltage of 50 mV at a bandwidth of around 18 GHz. Ifi 90 mV control voltage swing can be tolerated, the modulator bandwidth increases to 30 GHz. If a resonant tunneling diode (RTD) is made part of the assembly, the local E-field is enhanced by a factor of 5 to 10, thereby reducing the drive requirements even further. Similar, though narrower bandwidth results were noted for the DFB laser phase modulator concept.

Compact phonon-pumped terahertz laser

This paper describes a concept to generate coherent THz radiation in a semiconductor diode device using phonon generation via high-mobility electrons in semiconductor quantum well heterostructures. The theoretical basis for pumping both acoustic and optical phonons by high-mobility, two-dimensionally confined electrons has been established over the past decade. The electrons drift parallel to the quantum well heterojunction, and because their drift velocity exceeds either the local velocity of sound or the phase velocity of optical phonons in the crystal, energy is transferred from the electrons to the phonons (Cherenkov radiation). Strong confinement of both electrons and optical phonons in the quantum well leads to highly efficient energy transfer from high mobility electrons to coherent phonon waves. Plasmon oscillations created by coherent phonons in a polar material (such as GaAs or InP) create propagating THz electromagnetic fields. This process is analogous to the physical process that is the basis of a laser: multi-level pumping, stimulated emission, and a selection of one mode at the expense of the other modes. This paper describes a design approach to design structures that will produce required electron velocities and bias fields, for phonon generation through electrical pumping. This paper will also discuss the applicability of incorporating acoustic mirrors for a high finesse phonon cavity, and approaches for outcoupling the THz radiation.