# Single-photon upconversion

## TL;DR

This paper introduces a novel single-photon upconversion mechanism that surpasses the traditional 50% quantum yield limit of multi-photon processes, potentially enhancing the efficiency of various photonic devices.

## Contribution

It proposes a new single-photon absorption-based upconversion process involving two atoms and a reservoir, enabling higher quantum yields than existing multi-photon methods.

## Key findings

- Single-photon upconversion can exceed 50% quantum yield.
- Energy transfer occurs via reservoir-induced decoherence.
- System transitions from pure to mixed states with increased entropy.

## Abstract

The phenomenon of upconversion, in which a system sequentially absorbs two or more photons and emits a photon of a higher frequency, has been used in numerous applications. These include high-resolution non-destructive bioimaging, deep-penetrating photodynamic therapy, and photovoltaic devices. Due to the multi-photon mechanism of upconversion, its quantum yield cannot exceed 50%. We propose a new mechanism of upconversion, which is based on single-photon absorption; in this process, unlike in multiple-photon upconversion, the quantum yield can be higher than 50%. We show that in a system of two atoms interacting with a reservoir, a low-frequency excitation of one atom can be upconverted into a high-frequency excitation of another atom. The energy required for such an upconversion is drawn from the reservoir, which destroys coherence. Decoherence leads to the transition of the system from the pure state with a small energy dispersion to the mixed state with greater dispersion of energy, while the system entropy increases. The phenomenon of single-photon upconversion can be used to increase the efficacy of devices utilizing upconversion.

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Source: https://tomesphere.com/paper/1706.08981