Photonic Engineering Enables All‐Passive Upconversion Imaging with Low‐Intensity Near‐Infrared Light

Abstract Frequency upconversion, which converts low‐energy photons into higher‐energy ones, typically requires intense coherent illumination to drive nonlinear processes or the use of externally driven optoelectronic devices. Here, an upconversion system is demonstrated that converts low‐intensity (down to ≈10 −6 W cm −2 across an input aperture with 23 mm diameter) incoherent near‐infrared (NIR) light into the visible, reaching intensities perceptible by the human eye, without the use of any external power input. The upconverting element is enabled by the following ingredients: i) photon upconversion via triplet‐triplet annihilation in a bulk heterojunction of the organic semiconductors Y6 and rubrene; ii) plasmonic enhancement of absorption and field intensity in the heterojunction layer; iii) collection enhancement using a dichroic thin‐film assembly. To enable high‐resolution imaging, the upconverting element is inserted at an intermediate image plane of a dual‐wavelength telescope system, which preserves the relative directionality of rays between the incident NIR light and output visible light. This all‐passive upconversion imaging system will enable NIR imaging and sensing in low‐light environments under energy constraints.