Electroabsorption spectroscopy of effective-mass AlxGa1-xAs/GaAs Fibonacci superlattices

Effective-mass Fibonacci superlattices are composed of quasiperiodic sequences of weakly coupled quantum wells in which the transfer-matrix element is significantly smaller than the confinement energy, which is significantly smaller than the confining potential. We identify several distinct classes of quasiperiodic superlattices that are distinguished by diagonal vs off-diagonal quasiperiodicity and by compositional vs structural quasiperiodicity. The spatial quasiperiodicity in Fibonacci superlattices leads to a fractal set of critical points in the one-electron spectra. In the hierarchy of quasiband critical points, the critical points associated with earlier generations are dominant and terminate the widest quasigaps. The interplay of the one-electron critical points with many-electron excitonic enhancements creates complex spectra. Excitonic interactions concentrate the oscillator strength to the lower-energy transitions, and exciton linewidths of late-generation critical points are broadened by Fano resonance with the continua of lower-energy critical points. Using electromodulation spectroscopy, we identify the first several generations of critical points in experimental absorption and electroabsorption spectra of several Fibonacci superlattices that differ in their structure and coupling strength.