Ethan L. Blansett

Ultrafast polarization dynamics and noise in pulsed vertical-cavity surface-emitting lasers

We have measured on ps time scales the temporal behavior of the intensity noise of, and correlations between, orthogonally polarized modes in an optically pumped VCSEL. Measurements were made in both the circular and the linear bases. Sub-ps optical pumping with circular polarization leads to positively correlated intensity noise for emission in orthogonal linear polarizations. Optical pumping with linear polarization leads to anti-correlated intensity noise for emission in orthogonal linear polarizations, due to random orientation of linearly polarized emission. Intensity noise for circularly polarized emission is uncorrelated or anti-correlated depending on spin-flip rates which determine the strength of gain competition. We have generalized the theoretical treatment of San Miguel, Feng, and Moloney to successfully model these phenomena.

Final report on LDRD project 105967 : exploring the increase in GaAs photodiode responsivity with increased neutron fluence.

A previous LDRD studying radiation hardened optoelectronic components for space-based applications led to the result that increased neutron irradiation from a fast-burst reactor caused increased responsivity in GaAs photodiodes up to a total fluence of 4.4 x 10{sup 13} neutrons/cm{sup 2} (1 MeV Eq., Si). The silicon photodiodes experienced significant degradation. Scientific literature shows that neutrons can both cause defects as well as potentially remove defects in an annealing-like process in GaAs. Though there has been some modeling that suggests how fabrication and radiation-induced defects can migrate to surfaces and interfaces in GaAs and lead to an ordering effect, it is important to consider how these processes affect the performance of devices, such as the basic GaAs p-i-n photodiode. In this LDRD, we manufactured GaAs photodiodes at the MESA facility, irradiated them with electrons and neutrons at the White Sands Missile Range Linac and Fast Burst Reactor, and performed measurements to show the effect of irradiation on dark current, responsivity and high-speed bandwidth.

Ultrafast field-polarization dynamics in an optically pumped semiconductor microcavity laser

Summary form only given. The polarization properties of vertical cavity surface emitting lasers (VCSEL) originate in the change of electron total angular momentum J/sub z/ projected along the vertical axis for transitions from the conduction to the valence band. For quantum wells, J/sub z/=/spl plusmn/1/2 for the conduction band, and J/sub z/=/spl plusmn/3/2 for the heavy hole valence band, the highest energy valence band in unstrained quantum wells. The dipole allowed transitions are between J/sub z/=-1/2 and J/sub z/=-3/2 producing right circularly polarized light and between J/sub z/=+1/2 and J/sub z/=+3/2 producing left circularly polarized light. In CW, electrically pumped VCSELs, polarization states and fluctuations are well described by a model based on these transitions (San Miguel et al, 1995). We are investigating the polarization properties of VCSELs optically pumped with ultrashort pulses (<1 ps) which allows us to control the relative population of the angular momentum states and monitor the development of the intensity, noise, and correlations of the polarized-modes as a function of time.