# Boltzmann Thermometry at Cryogenic Temperatures Exploiting Stark Sublevels in Er3+/Yb3+-Codoped Yttrium Oxide Nanoparticles

## TL;DR

This paper introduces a new optical thermometer using Er3+/Yb3+ nanoparticles that works effectively at very low temperatures, offering precise temperature measurements for quantum and superconducting technologies.

## Contribution

The paper presents a novel optical Boltzmann thermometer using Stark sublevels in Er3+/Yb3+-codoped Y2O3 nanoparticles for cryogenic temperature sensing.

## Key findings

- Thermal sensitivities of up to 1.22% K–1 were achieved at 100 K.
- Temperature resolutions as low as 0.6 K were demonstrated.
- The method confirms theoretical predictions about thermometric performance using Stark transitions.

## Abstract

The development of reliable luminescent nanothermometers
for cryogenic
applications is essential for advancing quantum technologies, superconducting
systems, and other fields that require precise, high-spatial-resolution
temperature monitoring. Lanthanide-doped systems are vastly employed
to this purpose, and typically perform optimally at or above room
temperature when manifold-to-manifold transitions are used. In this
work we exploit individual Stark sublevels to demonstrate an optical
Boltzmann thermometer based on Er3+/Yb3+ codoped
yttria (Y2O3) nanoparticles that operates effectively
across the temperature range from 25 to 175 K. This is achieved due
to the pronounced crystal field environment of the Y2O3 host matrix, leading to well-separated Stark lines in the
luminescence spectrum of the Er3+ ions. By applying the
Luminescence Intensity Ratio (LIR) method to transitions originating
from two Stark components of the 4S3/2 manifold
of the Er3+ ions, we achieve thermal sensitivities up to
1.22% K–1 at 100 K and temperature resolutions reaching
0.6 K. Our results further experimentally confirm recently published
theoretical predictions, demonstrating that thermometric performance
is not directly dependent on the peak energy separation of the resulting
spectral lines of the involved electronic energy levels when using
individual Stark transitions to evaluate the LIR. The proposed procedure
gives an energy gap calibration that matches the one determined by
sample spectroscopy for nonoverlapping lines in the luminescence spectrum.
These insights provide a robust foundation for the design of high-performance
cryogenic thermometers based on rare-earth-doped materials.