Nonlinear Optical Properties of Hetero N-I Device Structures.

The integration of rm GaAs/Al_ {x}Ga_{1-x}As and rm In_{x}Ga_{1-x}As/GaAs multiple quantum well optical interconnects on Silicon wafers (using flip-chip bonding) requires that optical modulators operate with very low voltages (< 3V) compatible with electronic circuits. To this end, I studied the feasibility of using hetero n-i-p-i (alternating n- and p-doped regions between which are intrinsic regions that contain quantum wells) modulators by resolving several pertinent design issues. Firstly, using picosecond spectroscopy, I temporally - and spectrally-resolved the formation (optical turn-on time) of the changes in absorption coefficient in hetero n-i-p-i's. The optical turn-on time is determined by the time for the carriers to escape the wells ({ ~}3 ps) and drift to screen the built -in electric field (<10 ps), thus reducing the quantum confined Stark-effect. Secondly, to quantify the effectiveness of the carriers to screen the built-in electric field, I defined and measured the per-carrier nonlinearity, sigma_{eh}, which when maximized guarantees the smallest optical switching fluence. Moreover, I demonstrated that sigma_{rm eh} can be improved by optimizing the in-well bias field and by increasing the number of wells per intrinsic region. Thirdly, using picosecond transient grating techniques, I measured the ambipolar transport coefficient in these devices. I found that the in-plane transport is enhanced in certain hetero n-i-p-i's. Furthermore, in-plane transport limits how much the area of modulators, which are typically mesas, can be reduced without increasing the switching fluence (carriers transport to the sidewalls of the mesas and recombine before screening the built-in field). Finally, I modeled and measured the non-exponential optical decay (optical turn-off time) of the spatially separated carriers in hetero n-i-p-i's. The carrier lifetime is elongated in hetero n-i-p-i's and limits the frequency response of devices based on these structures. Therefore, it is imperative to apply contacts to hetero n-i-p-i devices to allow fast recovery (high frequency operation). I then used all of my experimental results to design an interdigitated hetero n-i-p-i optical modulator with a contrast ratio of 3.5:1, an operating voltage of 2 V, a maximum operating frequency of 1GHz and an optical throughput of ~27% in the off state.