Excitation Wavelength Steers Green and Red Upconversion Light in Tetra-Doped Y2O3 Phosphors

A team of materials researchers in India has shown that simply changing the wavelength of a near-infrared laser can switch the colour of light emitted by a rare-earth-doped phosphor from red to green, and that one of their compositions doubles as a sensitive optical thermometer. Writing in Discover Condensed Matter, H. S. Deshmukh and G. G. Muley report how yttrium oxide (Y2O3) powders co-doped with four rare-earth ions, neodymium (Nd3+), ytterbium (Yb3+), erbium (Er3+) and holmium (Ho3+), respond very differently to excitation at 980 nanometres compared with 932 nanometres, a wavelength-dependent behaviour they trace to competing energy-transfer pathways inside the crystal lattice.

Upconversion phosphors work by absorbing two or more low-energy near-infrared photons and re-emitting the combined energy as a single higher-energy visible photon. The Y2O3 host is a favourite for this trick because its low phonon energy of roughly 550 per centimetre, high thermal stability and cubic lattice structure suppress non-radiative losses and accommodate lanthanide dopants efficiently. In the conventional arrangement, Yb3+ ions act as sensitizers: they absorb strongly near 980 nanometres and hand the energy on to activator ions such as Er3+ or Ho3+, which then glow green or red. The drawback is that 980-nanometre light is also absorbed by water, causing localized heating that limits biological use.

The researchers prepared four phosphor compositions by a combustion route, dissolving high-purity oxides in nitric acid, adding urea as fuel, igniting the gel at 650 degrees Celsius and calcining the resulting powders at 950 degrees Celsius for three hours. The samples were Y2O3 doped with Nd and Yb alone, Nd-Yb-Er, Nd-Yb-Ho, and all four ions together. Powder X-ray diffraction with Rietveld refinement confirmed that every sample retained the cubic Ia-3 structure of the host, with refined lattice parameters between 10.6078 and 10.6092 angstroms and no secondary phases. Goodness-of-fit values between 1.042 and 1.127 indicated reliable refinements, and scanning electron microscopy showed aggregated particles of roughly 200 nanometres whose morphology barely changed when the extra dopants were added.

Diffuse reflectance spectra recorded from 200 to 1800 nanometres revealed the expected fingerprint of each dopant. Strong Nd3+ absorption bands appeared near 585, 598, 688, 751, 814 and 899 nanometres, with the intense band near 814 nanometres closely matching a standard 808-nanometre laser. Yb3+ dominated the 940 to 1040 nanometre region with peaks near 948 and 979 nanometres, while Er3+ contributed a band near 1422 nanometres and Ho3+ one near 536 nanometres. These overlapping absorption channels, the authors note, open multiple energy-migration routes among the co-doped ions.

The central result concerns what happens when the excitation wavelength changes. Under 980-nanometre excitation at 166 milliwatts, the Nd-Yb-Er phosphor emitted dominant red light at 662 nanometres with comparatively weak green emission. Under 932-nanometre excitation the trend reversed: green emission from the Er3+ levels at 521 to 562 nanometres dominated, with only minor red contribution. The Nd-Yb-Ho sample showed strong green emission plus appreciable red emission at 980 nanometres, but under 932 nanometres only green emission remained. The tetra-doped sample behaved similarly, with red emission suppressed and enhanced green emission dominating. Because the pump powers at the two wavelengths differed, the authors caution that absolute intensities cannot be compared directly, but the relative distribution among emission bands shifted unambiguously with wavelength.

Under 932-nanometre excitation the tetra-co-doped phosphor produced an intense, nearly pure green emission centred near 550 nanometres. The authors attribute this to preferential population of the Stark sublevels responsible for the 550-nanometre line in both Er3+ and Ho3+, while the strong 562-nanometre Er3+ line seen in the triply doped sample was suppressed. They propose that the 932-nanometre photon energy of about 10,730 per centimetre matches the Stark-split Yb3+ absorption levels well enough to sustain sensitization, and that the subsequent step from the Er3+ 4I11/2 intermediate state to 4F7/2 requires about 10,130 per centimetre, favourably matched to the pump. This preferentially fills the green-emitting 2H11/2 and 4S3/2 levels. By contrast, 980-nanometre excitation, near-resonant with Yb3+ at about 10,204 per centimetre, drives strong sensitization but leaves less excess energy, allowing population to relax toward the red-emitting 4F9/2 state in Er3+ and the 5F5 state in Ho3+. Nd3+ adds further pathways, transferring energy from its 4G9/2 and 4G7/2 levels directly to Er3+ and Ho3+ excited states.

Power-dependence measurements at 932 nanometres, sweeping the laser from 276.82 to 1928.2 milliwatts, showed all emission bands intensifying with pump power while peak positions stayed fixed. Fitting the logarithmic intensity-power relation gave slopes between 2.46 and 3.26 for the dominant bands, exceeding the value of about two expected for a simple two-photon upconversion process. The authors attribute these higher effective photon numbers to multiple excitation pathways: excited-state absorption, energy back transfer from Er3+ to Yb3+ at high powers, cross-relaxation, and possible saturation of intermediate levels. The red Er3+ emission alone showed a slope of 2.72, indicating a two-photon-assisted mechanism complicated by additional energy-transfer steps.

The temperature behaviour proved equally consequential. Cooling and heating the samples from 313.15 to 473.15 kelvin under 932-nanometre excitation caused all prominent emission peaks to weaken as temperature rose, the classic thermal quenching that underpins fluorescence-intensity-ratio thermometry. For the Nd-Yb-Er phosphor, the ratio of the 562-nanometre to the 521-nanometre Er3+ emission followed the expected Boltzmann-type dependence, yielding an activation energy of 767 per centimetre and, at 313.15 kelvin, an absolute sensitivity of 0.0615 per kelvin and a relative sensitivity of 0.0113 per kelvin, the highest among the samples studied. Heating and cooling cycles between 313.15 and 413.15 kelvin produced nearly overlapping ratio curves, indicating negligible thermal hysteresis and good reversibility.

The other compositions fared less well as thermometers. The Nd-Yb-Ho sample, whose emission under 932 nanometres is confined to the green band, showed pronounced nonlinearity in the logarithmic ratio plot, signalling departure from Boltzmann equilibrium and making it unsuitable for this thermometry approach under the present conditions. The tetra-doped phosphor reached sensitivities of only 0.0342 and 0.0059 per kelvin for its best ratio, limited by the small energy separation among its closely spaced green peaks. The authors suggest the superior performance of the Nd-Yb-Er composition may stem from Nd3+ participation in energy transfer and local crystal-field modification, together with the 932-nanometre excitation itself.

The practical implications are twofold. The intense pure green emission of the tetra-doped phosphor under 932 nanometres, a wavelength that reduces water absorption and thermal loading compared with 980 nanometres, makes it a candidate for bio-imaging and anti-counterfeiting applications where selective, single-colour emission is desirable. Meanwhile the Nd-Yb-Er phosphor offers a non-contact temperature sensor with sensitivity figures that compare favourably with earlier Y2O3-based thermometers, which typically reported absolute sensitivities between 0.005 and 0.054 per kelvin under various excitation conditions. The work demonstrates that excitation wavelength is not a fixed parameter but an active design variable for tuning both emission colour and thermometric performance in multi-doped oxide phosphors.

Subject of Research: Excitation-wavelength-dependent upconversion emission and fluorescence-intensity-ratio thermometry in Nd, Yb, Er and Ho co-doped Y2O3 phosphors

Article Title: Exploring effect of excitation wavelength on upconversion emission in Nd, Yb, Er and Ho tetra co-doped Y2O3 phosphor and use in optical thermometry

Article References: Deshmukh, H. S., & Muley, G. G. (2026). Exploring effect of excitation wavelength on upconversion emission in Nd, Yb, Er and Ho tetra co-doped Y2O3 phosphor and use in optical thermometry. Discover Condensed Matter, 1(1), Article 3. https://doi.org/10.1007/s44570-026-00003-2

Image Credits: AI Generated

DOI: 10.1007/s44570-026-00003-2

Keywords: upconversion, Y2O3 phosphor, rare-earth doping, excitation wavelength, 932 nm excitation, 980 nm excitation, green emission, optical thermometry, fluorescence intensity ratio, thermal quenching, combustion synthesis, Rietveld refinement