All-passive upconversion of incoherent near-infrared light at intensities down to 10$^{-7}$ W/cm$^2$

TL;DR

This paper presents an all-passive system that converts extremely low-intensity incoherent near-infrared light into visible light, enabling human-eye perception without external power, using organic semiconductors, plasmonic enhancement, and optical collection techniques.

Contribution

It introduces a novel all-passive upconversion method for incoherent NIR light at very low intensities, combining organic semiconductors, plasmonics, and optical collection for low-light imaging.

Findings

Successfully upconverted NIR to visible at 10$^{-7}$ W/cm$^2$

Enabled human-eye visible perception of low-intensity NIR light

Integrated into a dual-wavelength telescope for imaging

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, we demonstrate an upconversion system that converts low-intensity (down to ~10-7 W/cm$^2$) incoherent near-infrared (NIR) light into the visible, reaching intensities perceptible by the human eye, without the use of any external power input. Our upconverting element is enabled by the following ingredients: (1) photon upconversion via triplet-triplet annihilation in a bulk heterojunction of the organic semiconductors Y6 and rubrene; (2) plasmonic enhancement of absorption and field intensity in the heterojunction layer; (3) 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. Our all-passive upconversion imaging system will enable NIR imaging and sensing in low-light environments under energy constraints.