Computer Models Show Each Atomic Defect in Boron Nitride Has Its Own Laser 'Fingerprint'
University at Buffalo researchers simulated 10 defect types in hexagonal boron nitride and found some boost light at double or triple the laser frequency, which could help pick out sources of quantum light.
Atomic-scale flaws in crystals are the raw material for some future quantum technologies, but they are notoriously hard to identify. A team at the University at Buffalo now reports a computational method that predicts how individual defects in a layered material would respond to laser light, so that each one could be recognized by a distinctive optical signature.
The material is hexagonal boron nitride, or h-BN, in which boron and nitrogen atoms form a honeycomb-like pattern. Defects in it matter because some emit quantum light, single photons released one at a time, which could carry information in future communication and sensing devices. The difficulty is matching what an experiment measures to the specific defect that caused it.
Defects change how a material responds to light, and that response can be measured as an indirect clue. The Buffalo team used computer models to predict those fingerprints for known defects, simulating how each one changes the intensity and polarization of light generated when h-BN is struck by a laser. They modeled 10 defect types, including ones with missing boron or nitrogen atoms and ones where carbon atoms replace boron or nitrogen.
Two findings stand out. First, some defects were predicted to greatly boost light emitted at double or triple the laser's frequency, depending on polarization, which should make their fingerprints easier to measure. Second, arrangement matters: defects made of the same atoms can look different depending on how those atoms sit. The team compared two arrangements of four carbon atoms. One was predicted to give a strong signal in two polarization directions, while the other gave a strong signal in only one.
"Not all atomic defects are created equal," said Vasili Perebeinos, a professor of electrical and computer engineering at Buffalo and a co-corresponding author. Paras N. Prasad, a SUNY Distinguished Professor in the Department of Chemistry and executive director of the university's Institute for Lasers, Photonics, and Biophotonics, said the distinct light responses could eventually let scientists identify which atoms are involved in a defect and how they are arranged.
Perebeinos described the long-term goal as a material in which defect placement can be precisely controlled, so that arrays of quantum light sources could be built rather than relying on single sources.
The study was led by Olga Savkina, a Ph.D. student, and Davoud Adinehloo, a postdoctoral associate, both in electrical and computer engineering. Huamin Li, an associate professor, and Ph.D. students Pranati R. Kuntla and Viktor Labuntsov are co-authors. The paper, "Nonlinear Optical Fingerprints of Defects in Hexagonal Boron Nitride," appears in Nano Letters (DOI 10.1021/acs.nanolett.6c03684).
The work is theoretical. The authors say the calculations could eventually help researchers tell defect types apart and work out which ones suit quantum technologies, but the predictions still need to be checked against laboratory measurements. Knowing which defects produce quantum light, and how to engineer them, is the next step toward practical devices.
