Deep inside a diamond, a misplaced atom or an empty spot in the crystal lattice could hold the key to better quantum technologies. However, changing one such defect without disturbing others while keeping the material useful has been a big challenge.
Now, in a new study, researchers have used nanosecond ultraviolet laser pulses to modify diamond’s optical properties while leaving its existing nitrogen-vacancy (NV) centers largely unchanged. This finding offers a potential way to engineer diamond’s defects more selectively.
“Our study demonstrates that nanosecond laser pulses can achieve selective engineering of point defects in diamond and is promising for deterministic control of diamond color centers for quantum applications,” the researchers stated.
Using ultraviolet light to manipulate diamond defects
Diamonds consist of carbon atoms arranged in a crystal lattice. When atoms are displaced, missing or replaced by other elements, they create defects that can give the material new optical and electronic properties.
One example is the nitrogen-vacancy center, which forms when a nitrogen atom replaces a carbon atom next to a vacant spot in the lattice. NV centers can function as quantum bits, or qubits, and serve as highly sensitive sensors of magnetic and electric fields.
However, modifying one type of defect can also affect others nearby, making it difficult to tailor diamonds for specific applications.
To investigate whether light could offer greater control, researchers irradiated a single-crystal diamond grown using chemical vapor deposition (CVD). They used 266-nanometer ultraviolet laser pulses, each lasting six nanoseconds, to expose localized regions of the crystal.
The laser fluence—the energy delivered per unit area—ranged from 2.2 to 8.6 joules per square centimeter. Before irradiation, the team characterized the diamond using confocal photoluminescence spectroscopy, ultraviolet-visible absorption spectroscopy and Fourier-transform infrared spectroscopy.
These techniques helped establish the crystal’s initial optical defects, transparency and impurity content. The starting material contained very little substitutional nitrogen, in which nitrogen atoms replace carbon atoms in the lattice.
Establishing this baseline was important: without it, researchers could not confidently distinguish defects associated with laser treatment from those already present in the diamond.
New optical signals, but existing NV centers survive
After irradiation, the researchers detected a previously absent photoluminescence signal near 563 nanometers in the exposed regions. A second emission appeared near 579 nanometers.
Previous studies have associated these emissions with defects related to carbon self-interstitials, in which carbon atoms occupy positions outside their normal sites in the lattice.
The more significant observation, however, was what did not change. The diamond already contained NV centers, and their optical signals remained largely unchanged after laser treatment compared with untreated regions.
“We show that these nanosecond laser pulses can selectively engineer the 563-nm center, a self-interstitial-related defect, without affecting the background nitrogen-vacancy (NV) center,” the study authors said.
This suggests that the laser can modify one part of the defect population without substantially disturbing another under the tested conditions.
The distinction matters because the 563-nanometer and 579-nanometer centers have not been established as newly demonstrated qubits. Instead, the experiment points toward a possible way to engineer particular defects while preserving existing centers relevant to diamond quantum sensors.
The researchers also found that the 563-nanometer emission did not increase indefinitely with continued exposure. Its signal could decline after reaching a maximum, suggesting that irradiation can both generate and transform defect configurations.
The emission’s intensity showed a near-linear relationship with laser fluence, with a measured power-law exponent of 1.10 ± 0.16. This is consistent with a single-photon excitation process, rather than one requiring multiple photons to act together.
What happens inside the diamond remains a mystery
The precise atomic-scale mechanism is unresolved. The researchers propose that ultraviolet photons may excite electronic states associated with defects, with subsequent energy transfer potentially triggering rearrangements of nearby carbon atoms.
Processes involving trapped charge carriers or excitons may also play a role, but the experiments did not establish the complete pathway or identify the exact atomic structures responsible for both emissions.
The control over diamond’s optical properties as claimed in the study could eventually benefit quantum sensing and other devices that depend on carefully chosen defects.
However, the study does not yet demonstrate a working quantum device or prove that a specific defect can be created on demand with complete reproducibility.
The next step is to determine exactly how the defects transform and how consistently those changes can be controlled by adjusting laser conditions.
If researchers can connect irradiation parameters to predictable atomic changes, they may eventually be able to design diamonds with tailored defect populations rather than treating their imperfections as an inseparable mixture.
The study is published in the journal Diamond and Related Materials.