PHOTODETECTOR

PROBLEM TO BE SOLVED: To provide a photo-detector provided with a quantum cascade structure capable of taking advantage of the optical absorption of transition between quantum sub-bands and having a sensitive wavelength band expanded.SOLUTION: A photo-detector 1A is provided with a layered structure 3 having a first layer 4 comprising a first metal or first semiconductor, a semiconductor structure layer 5 stacked on top of the first layer 4 and having electrons excited by Plasmon resonance, and a second layer 5 stacked on top of the semiconductor structure layer 5 and comprising a second metal or second semiconductor. The semiconductor structure layer 5 has a quantum cascade structure, the quantum cascade structure including an active region 5a having a first quantum higher level and a second quantum higher level lower than the first quantum higher level, and an injector region 5b for transporting electrons excited in the active region 5a.