Tunable hot-carrier photodetection beyond the bandgap spectral limit

The spectral response of common optoelectronic photodetectors is restricted by a cutoff wavelength limit lambda(c) that is related to the activation energy (or bandgap) of the semiconductor structure (or material) (Delta) through the relationship lambda(c) = hc/Delta. This spectral rule dominates device design and intrinsically limits the long-wavelength response of a semiconductor photodetector. Here, we report a new, long-wavelength photodetection principle based on a hot-cold hole energy transfer mechanism that overcomes this spectral limit. Hot carriers injected into a semiconductor structure interact with cold carriers and excite them to higher energy states. This enables a very long-wavelength infrared response. In our experiments, we observe a response up to 55 mu m, which is tunable by varying the degree of hot-hole injection, for a GaAs/AIGaAs sample with Delta = 0.32 eV (equivalent to 3.9 mu m in wavelength).