The starting material for this glowing glass is far from exotic laboratory-grade chemical purity. The researchers used quartz sand from Huta Ginjang, a site on the shores of Lake Toba in North Sumatra, Indonesia. After washing, sun-drying and ball-milling, X-ray fluorescence analysis confirmed the sand contains more than 98.65 percent silica oxide, with minor amounts of potassium, calcium, chromium and iron oxides. Replacing expensive high-purity silica with this locally abundant resource cuts production costs and turns a common natural material into the backbone of a functional optical device. The processed sand was combined with ammonium dihydrogen phosphate, gadolinium oxide, calcium carbonate and barium carbonate, melted in an alumina crucible at 1,200 degrees Celsius for three hours, then cast into preheated graphite molds and annealed at 500 degrees Celsius to relieve internal stresses.

The resulting glass series, labeled POCaGE, follows the composition (15-x)QS–50P2O5–15CaO–10BaO–10Gd2O3–xEu2O3, where europium oxide content ranged from zero to 2.0 mole percent. Each component plays a deliberate structural role. Phosphorus pentoxide serves as the primary glass former, creating a transparent network that dissolves rare-earth ions readily. Calcium oxide improves chemical durability and thermal stability, addressing the hygroscopic weakness that limits pure phosphate glasses. Barium oxide, a heavy-metal oxide, raises the glass density and effective atomic number, boosting its ability to absorb ionizing radiation. Gadolinium oxide performs a dual job as both a structural modifier and a sensitizer, since gadolinium ions have a large neutron and X-ray absorption cross-section and emit strongly in the ultraviolet near 311 nanometers — precisely the wavelength that can be handed off to europium.

Optical absorption measurements across the ultraviolet, visible and near-infrared revealed six characteristic absorption bands arising from the shielded inner 4f electron transitions of the europium ions. The most prominent lies at about 394 nanometers, corresponding to the 7F0 to 5L6 transition, which is partially allowed by selection rules and serves as the most efficient pump wavelength for these ions. Additional bands appear at roughly 413, 465 and 534 nanometers, while two unusual near-infrared absorptions at 2092 and 2214 nanometers correspond to transitions ending on the 7F6 level. The band at 465 nanometers is stronger than the one at 534 because it is a hypersensitive electric-dipole transition, whereas the latter is largely magnetic-dipole in character. Overall absorbance increased steadily with europium loading, confirming that the dopant ions were genuinely incorporated into the glass network rather than segregating into clusters.

To quantify how strongly europium interacts with its glassy surroundings, the team applied Judd–Ofelt theory, the standard framework for describing the intensities of rare-earth transitions in condensed matter. Experimental oscillator strengths, derived from integrated absorption areas, agreed reasonably well with values calculated from the theory, with small root-mean-square deviations peaking at 0.406 and dipping as low as 0.191 for the 1.5 mole percent sample. Strikingly, the intensity parameter Ω2 dominated over Ω4 and Ω6 in every composition — a signature that europium ions occupy highly asymmetric, covalent coordination sites within the amorphous network. That asymmetry is not a mere structural curiosity: it directly governs emission behavior, because the hypersensitive red transition at 613 nanometers flourishes only when the europium ion sits in a site lacking inversion symmetry.

When excited with ultraviolet light, the glasses delivered exactly the kind of emission the asymmetry predicted. Under 275-nanometer excitation, which the gadolinium ions absorb, a strong ultraviolet band at 311 nanometers appeared alongside the full set of europium emission lines at 578, 592, 613, 652 and 699 nanometers. As europium concentration rose, the gadolinium ultraviolet emission faded while the europium red lines strengthened — a textbook demonstration of energy transfer from gadolinium sensitizer to europium activator. The red emission is dominated by the 5D0 to 7F2 electric-dipole transition at 613 nanometers, which overshadows the symmetry-insensitive magnetic-dipole line at 592 nanometers, again confirming non-centrosymmetric europium sites. Under direct 394-nanometer excitation, only europium emission appeared, and it grew stronger with every increase in dopant concentration, with the 2.0 mole percent sample the brightest — and notably without severe concentration quenching across the range studied.

Lifetime measurements added independent, quantitative proof of the energy-transfer mechanism. The gadolinium emission decayed as a clean single exponential, but its lifetime collapsed systematically as europium was added, falling from 4.154 milliseconds in the undoped glass to 0.845 milliseconds at 2.0 mole percent europium. The team calculated that energy-transfer efficiency climbed from 42 percent to 80 percent across the doped series, meaning that by the highest loading, four out of five excitations placed on gadolinium end up on a europium neighbor. Meanwhile, the europium lifetime at 613 nanometers remained essentially fixed at around 2.27 to 2.34 milliseconds under direct excitation, showing that the emitting state itself is barely disturbed and that europium–europium cross-relaxation is not yet a limiting factor. Photoluminescence quantum yields reached 25.5 percent at the optimal 1.0 mole percent composition under 394-nanometer pumping, indicating a healthy balance between radiative output and non-radiative losses.

Colorimetric analysis revealed that all four doped glasses emit at essentially the same point in CIE 1931 color space, with coordinates near x = 0.65 and y = 0.348, firmly in the deep red region. Color purity measured an impressive 94 percent or higher, and the correlated color temperature hovered around 2,600 kelvin — the warm, amber-red territory of candlelight and incandescent lamps. The near-identical chromaticity across compositions means that increasing europium simply makes the light brighter without shifting its hue, a stability that simplifies device engineering for red LEDs and display phosphors. Radiative analysis reinforced the picture: the 613-nanometer transition carried the highest radiative transition probability and branching ratios above 70 percent, with the strongest stimulated emission cross-section, 11.1 × 10⁻²² square centimeters, appearing at the lowest doping level of 0.5 mole percent.

The most dramatic test came when the glasses were bombarded with X-rays from a copper-target generator operating at 50 kilovolts and 30 milliamperes. Under irradiation, the glass glowed with both the gadolinium ultraviolet line at 311 nanometers and the full set of europium red emission bands, mirroring the photoluminescence spectra and confirming that the sensitizer–activator relay operates under high-energy excitation too. Compared with the bismuth germanate (BGO) crystal — the workhorse scintillator used in medical imaging and particle physics — the glasses produce lower overall intensity but far sharper, spectrally selective emission lines rather than a broad glow. That spectral sharpness is an asset, because it allows the red europium signal to be isolated cleanly against background light, exactly what detector designers want.

The work demonstrates a compelling convergence of sustainability and function: a high-performance luminescent and scintillating material built partly from lake-shore sand, with optical behavior tunable simply by adjusting dopant concentration. Because scintillators underpin X-ray imaging in medicine, security screening, radiation monitoring and high-energy physics, glass-based alternatives that can be cast into large, homogeneous, optically clear bodies hold real practical appeal over single crystals, which are costly and difficult to grow at scale. With its 80 percent energy-transfer efficiency, near-quench-free red emission, 94 percent color purity and demonstrable X-ray response, the POCaGE glass family now stands as a promising, low-cost candidate for red-emitting photonic devices and radiation detectors — proof that remarkable photonics can begin with a handful of beach sand.

Subject of Research: Europium-doped quartz sand based phosphate glasses for photoluminescence and scintillator applications

Article Title: The effect of Eu3+ ion on photoluminescence and scintillator properties in QS-P2O5-CaO-BaO-Gd2O3 glasses

Article References: Rajagukguk, J., Panggabean, J. H., Hutahaean, J., Situmorang, H., Simanullang, E., Sarumaha, C. S., Ruangtaweep, Y., & Kaewkhao, J. (2026). The effect of Eu3+ ion on photoluminescence and scintillator properties in QS-P2O5-CaO-BaO-Gd2O3 glasses. Frontiers in Photonics, 7, Article 1834287. https://doi.org/10.3389/fphot.2026.1834287

Image Credits: AI Generated

DOI: 10.3389/fphot.2026.1834287

Keywords: europium, phosphate glass, photoluminescence, scintillator, quartz sand, gadolinium, energy transfer, Judd-Ofelt analysis, X-ray luminescence, red emission, CIE chromaticity, rare-earth doping