Boosting the Efficiency of Passive Upconversion Imaging using Engineered Nanostructures

Most existing technologies to enable vision in the near infrared (NIR) region rely on cameras, displays, and other powered optoelectronics. These devices capture NIR images and convert them to the visible. In contrast, we are working on a passive frequency-upconversion imaging system that directly converts NIR light to visible light, thereby eliminating the need for external power sources or electronic components. We use a photon upconversion process that involves a sequential transfer of energy from two low-energy incident photons, resulting in the emission of a higher-energy photon via triplet-triplet annihilation [1], enabling solid-state upconverting thin films [2]. However, the previously demonstrated efficiency of this upconversion process is not sufficient for passive night-vision imaging systems.<br/>We designed and demonstrated several types of nanostructures that enhance the absorption, emission, and collection efficiencies of upconverting films, with the ultimate goal of enabling a wearable, all-passive, NIR-to-visible imaging system.<br/>In our design, the upconverting material is a 100-nm thick bulk heterojunction comprising organic molecules Y6 (sensitizer) and rubrene (emitter; here, doped with DBP [2]), similar to the materials in the bilayer structure in ref. [3]. Our bulk heterojunction passively converts incident NIR light to orange light with peak emission at ~600 nm; however, the emission is incoherent and isotropic. To enable imaging, the directionality of rays needs to be preserved through the upconversion process. We address this issue by positioning the upconverter at the focal plane of a Keplerian telescope system, a strategy that we previously demonstrated for a down-conversion imaging system [4].<br/>To enhance light extraction while operating in transmission mode, we developed a dichroic substrate that reflects visible light while transmitting NIR light, which allows us to recover visible light otherwise emitted backwards towards the source. To further improve the efficiency of the upconverter, we designed a superstrate that is transparent in the visible range but reflects in the NIR, enabling the recycling of unabsorbed NIR light back into the upconverting film, thereby enhancing the absorption and the subsequent emission.<br/>We designed the multilayer substrates and superstrates such that they provide a consistent spectral response across a broad range of viewing angles (here, up to 30 degrees) for the desired wavelengths (500 – 650 nm in the visible, and 750 – 900 nm in the NIR). This functionality was implemented using niobium pentoxide (Nb₂O₅) and silicon dioxide (SiO₂) layers. To maintain our imaging resolution, we limited the total thickness of the combined structure to 2 microns, thus ensuring that we remain within the depth of focus of our imaging system.<br/>Our approach has yielded an approximately 2-fold enhancement in collection efficiency and a 2.5-fold increase in absorption. As of this writing, we have verified the collection-efficiency enhancement in experiments using both a laser source (at 852 nm) and a broadband NIR source. We anticipate presenting the absorption-enhancement data at the conference.<br/>In addition to power measurements, we have demonstrated imaging through the upconverter with the engineered substrate, observing an approximate doubling in brightness without compromising spatial resolution. We anticipate that our approach will be useful for a variety of incoherent upconverting schemes beyond the all-organic chemistry presented here (e.g., those based on inorganic nanocrystals [2, 4]) and can enable all-passive vision even under low-power NIR illumination and longer wavelengths.<br/>We acknowledge funding from DARPA under Grant No. HR00112220010<br/>[1] T. N. Singh-Rachford et al, Coord. Chem. Rev. 254, 2560–2573 (2010).<br/>[2] M. Wu et al., Nat. Photonics 10, 31–34 (2016).<br/>[3] S. Izawa et al, Nat. Photon. 15, 895-900 (2021).<br/>[4] J. Salman et al., J. Opt. 23, 054001 (2021).