# Ultraviolet Light Generation through Lanthanide Upconversion

## TL;DR

This paper reviews recent progress in generating ultraviolet light using lanthanide-doped materials, highlighting their potential for applications like sterilization and solar-blind imaging.

## Contribution

The paper presents new methods for enhancing UV upconversion emission and explores its emerging applications in various fields.

## Key findings

- Er3+, Tm3+, and Pr3+ ions are effective for UV upconversion when combined with suitable host crystals.
- Techniques like dielectric coupling and plasmonic modulation can boost UV emission intensity.
- UV upconversion has promising applications in lighting, imaging, and environmental science.

## Abstract

Upconversion is a nonlinear optical process
in which long-wavelength
photons are absorbed by specific material systems and converted into
shorter-wavelength light. Yb3+–Ln3+ (Ln:
Er/Ho/Tm) pairs are the most widely studied upconversion systems,
demonstrating great success in efficient near-infrared-to-visible
light conversion. Nevertheless, further exploration of upconversion
luminescence toward shorter wavelengths, especially in the UV region,
has achieved limited progress. In comparison with visible light, UV
radiation suffers from minimized interference from natural and most
artificial light sources. By shifting the emission to the deep UV
band, for example, solar interference could be circumvented, enabling
highly valuable applications such as solar-blind imaging and labeling.
Additionally, due to the higher photon energy in this spectral range,
the system could be simultaneously employed for sterilization, phototherapy,
and plastic degradation.

To unlock the application potentials
of UV-emitting upconversion
materials, substantial research efforts have been undertaken in recent
years. Specifically, classic visible upconverting Er3+ and
Tm3+ ions have been repurposed for UV emission due to their
rich energy levels extending to the UV spectrum region. To effectively
populate the high-lying excited states, systematic investigations
into doping concentrations, host lattice compositions, and excitation
schemes have been conducted. In parallel, Pr3+typically
ineffective for near-infrared to visible upconversionhas been
established as a prominent candidate for UV upconversion under blue-light
excitation. By precisely tuning its 4f15d1 state
through host lattice engineering, both the upconversion dynamics and
emission characteristics can be strategically optimized.

In
this Account, we focus on recent advances in UV upconversion
through lanthanide-doped inorganic crystals, primarily drawing upon
our research group’s advancements over the past few years.
We begin by summarizing the methods for constructing UV upconversion
materials based on rational selection of dopant ions and host crystals,
including Er3+-, Tm3+-, and Pr3+-based
systems. Building on these foundations, we introduce emerging methods
for enhancing the UV upconversion emission intensity, encompassing
dielectric coupling, plasmonic modulation, and organic surface coating,
which all have a certain degree of universality. The subsequent section
will focus on the frontier applications of UV upconversion in lighting,
imaging, and environmental sciences. In the end, we conclude by providing
a summary and a perspective on future directions.