Spatially resolved femtosecond pump-probe spectroscopy in broad-area semiconductor lasers

We investigate the ultrafast gain dynamics in broad-area semiconductor lasers with particular emphasis on spatial and spatiotemporal effects. We present a spatially resolved femtosecond pump-probe experiment which allows us to measure the compression and recovery of the gain with 250-fs temporal and 15 μm spatial resolution. We find a significant spatial variation of the gain recovery time across the lateral laser coordinate indicating an influence of the extended laser structure on the ultrafast carrier relaxation. Moreover, we are able to follow the spatiotemporal relaxation of the ultrafast spatiospectral gain saturation within the extended semiconductor active area. We find diffusion-like broadening of the locally suppressed gain on two distinct ultrafast timescales, within several picoseconds and several tens of picoseconds, resulting from an interplay between intraband relaxation, spatial holeburning, and light propagation. Supported by microscopic modeling, our results provide insight into the different mechanisms and timescales associated with the spatiotemporal carrier dynamics. These findings are essential for the design of laterally extended semiconductor active devices for ultrafast optical signal processing applications.