Ring-Down Spectroscopy of Semiconductor Mirrors

Susan Lehman


Composite image of different laser cavity modes.

Distributed Bragg reflectors (DBRs) are mirrors made of semiconductor materials like GaAs and AlAs. They consist of alternating thin layers of materials with different refractive indexes; the reflections from successive interfaces sum together for very high total reflectivities. For example, a stack of 30 pairs of alternating layers of AlAs and GaAs exhibits a reflectivity of 0.99993. These mirrors are mainly of interest for their use in vertical-cavity, surface-emitting semiconductor lasers. A modified version called a saturable Bragg reflector also is used as a key element in the pulsed lasers being developed for ultra-fast spectroscopy.

Modeling the reflectivity of a DBR is straightforward, but achieving the modeled result is unusual. Impurities (dopants) in the layered materials affect the index of refraction and thus the reflectivity of the mirror. Optimization of these mirror structures has been difficult because the DBR reflectivity is difficult to measure accurately.

Cavity ring-down spectroscopy is a technique originally designed to measure ultra-high reflectivities, but it has apparently not been previously applied to the DBR problem. The essence of the technique is simple. An optical cavity (similar to a Fabry-Perot cavity) may be filled with light if the frequency of light and the length of the cavity are in resonance. If we then shutter the input light off, the light already in the cavity will gradually decay as light leaks through the mirrors. If the mirrors are very good, the light will decay slowly; poorer mirrors will cause a more rapid decay. Thus, by measuring the decay time of light in the cavity, we can accurately measure the mirror reflectivity.

The ring-down cavity is currently being constructed. We will optimize this cavity and measure the reflectivity of a distributed Bragg reflector. The work is primarily experimental but may require some computer modeling of the laser beam in order to efficiently fill the cavity with light. We will compare our results to the predicted reflectivity of the DBR and study how the reflectivity varies over the DBR wafer.