Beyond the Free Spectral Range: On-Chip Spectrometer with Multi-color Cascaded Colloidal Quantum-Dot Photodiodes

Abstract

Colloidal quantum dots (QDs), as a low-cost, flexibly tunable and fabrication scalable semiconductor material, have demonstrated exceptional capabilities in many complex applications including high-quality displays, high-resolution infrared imaging, advanced spectrometry and integration into photonic circuits. This study explores combining the tunable spectral response features of QD-based photodiodes (QDPDs) with dispersive photonic integrated circuits (PICs), demonstrating a spectrometer with operational spectral range extended beyond its original free spectral range (FSR). Experimentally, two types of PbS QDPDs, with different absorption features, were integrated in cascade on the output channels of a planar concave grating (PCG) with a 90 nm FSR. The differential responses of these QDPDs to two adjacent diffraction orders of the PCG enabled the creation of a spectrometer with a spectral range of approximately 180 nm, effectively decoupling two FSRs of the PCG. The proposed cascaded QDPDs, with diverse spectral photodetection capabilities, present great potential when integrated into complex optical systems.