Introduction to Photonic Bandstructure Devices
Photonic bandstructure engineering in semiconductor photonics has the potential to change the way that optical communication hardware is designed and built. Photonic crystals offer the possibility of engineering the electromagnetic fields of these devices at the subwavelength scale in order to design the optical mode’s size, shape, frequency, and polarization. This group is working to design and nanofabricate ultra-compact optical component hardware for next-generation high data rate communication systems.

The nanophotonic devices being developed here are much smaller than existing device technology.  This device technology is immature, but it has a great deal of potential.  The goal of the ongoing work is to begin to use this potential to develop optical components that can be densely integrated into functional systems that help satisfy our ever increasing communication bandwidth needs.

In the previous decade, electronic bandstructure engineering has led to dramatic improvements in the performance of electronic and optical devices.  This was done by engineering the electron and hole wave functions by controlling the semiconductor alloy composition at the atomic scale.  In the area of photonics, for example, the ability to engineer the occupied electronic states in a semiconductor, through alloying and strain, has produced a reduction in threshold current of semiconductor lasers by more than an order of magnitude, and a simultaneous increase in their modulation bandwidth.

By allowing greatly increased control of the electromagnetic fields of photonic devices, photonic crystals and photonic bandstructure engineering offer a similar possibility for device and communication system improvement.  In our work, we engineer the electromagnetic eigenstates by engineering the spatial dielectric constant in these semiconductor devices.  This can be done by patterning the semiconductor at the 100 nm length scale.  One of the main components of photonic bandstructure engineering are photonic crystals.  Photonic crystals are materials in which the dielectric constant is periodic.  The periodicity opens up gaps in the electromagnetic spectrum, through Bragg reflection, in which there are no propagating modes.  The length scale on which these materials are periodic is the wavelength of the light.  In practice, the semiconductor dielectric constant is made periodic by defining a pattern by electron beam lithography on the surface and then transferring this pattern into the semiconductor by a sequence of reactive ion etching and electron cyclotron etching steps.  The resulting semiconductor/air lattice then modifies the electromagnetic modes allowed to propagate through the material.

The Photonic Crystal Defect Laser

One of the projects that we are currently working on is to develop microcavity photonic crystal lasers.  Previously, in a collaboration between Caltech and USC, we have demonstrated an optically-pumped semiconductor laser with a resonant cavity formed by a single defect in a two-dimensional photonic crystal .  The mode volume of this resonant cavity is about 2.5 (l/2n)³.  In other words, the resonant cavity is a disk about 500 nm in diameter and about 200 nm thick.  This laser has the smallest mode-volume yet demonstrated.  At USC, in collaboration with Professor Dan Dapkus' group, we are now focusing on obtaining room temperature CW laser operation in a VCSEL pumped configuration.