Current compact emitter and receiver technologies are generally inefficient and impractical at terahertz (THz) frequencies between 0.1 and 10 THz. Hence, a gap exists between mature microwave and developed optical technologies. On-chip, integrated broadly tunable and powerful quantum sources that coherently radiate THz waves between 0.1 and 11 THz (potentially extendable to 15 THz) can be achieved based on quantum tunneling of electron pairs across the stack of intrinsic Josephson junctions (IJJs) naturally present in a single crystal of the layered high-temperature superconducting BSCCO. Such superconducting devices have been found to be especially promising solid-state THz sources capable of bridging the entire THz gap, as their wide-frequency tunability range is superior to that obtained from their semiconducting-based rivals, either single resonant-tunneling diodes (RTDs) or THz-quantum cascade lasers (QCLs). I will review our recent results on superconducting THz devices and discuss our approaches towards their integration, scale up, and performance development. I will conclude by demonstrating novel low-loss photonic integrated circuits, waveguides, tunable modulators, slow light devices and filters.