Insulating moiré homobilayers lack a threefold symmetric second-harmonic generation

Journal Article · 14 November 2024 · Physical Review Materials

DOI: https://doi.org/10.1103/physrevmaterials.8.116203 · OSTI ID:2477405

Rosas-Hernandez, Luis Enrique  ^[1]; Cabellos, Jose Luis  ^[2]; Huamán, Angiolo  ^[3]; Mendoza, Bernardo  ^[4]; Barraza-Lopez, Salvador  ^[3]

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1. University of Arkansas, Fayetteville, AR (United States); University of Arkansas
2. Tapachula, Codigo Postal (Mexico)
3. University of Arkansas, Fayetteville, AR (United States)
4. Centro de Investigaciones en Óptica, Codigo Postal (Mexico); Max Planck Institute for the Structure and Dynamics of Matter Hamburg (Germany)

Atoms within moiré bilayers relax in plane to minimize elastic energy; such relaxation brings their space group symmetries down to P1. Here, the ab initio second harmonic generation (SHG) of twisted and atomistically optimized hBN bilayers was determined at four twist angles (θ = 38.21°, 60.00°, 73.17°, and 98.21°) and for three displacements τ measured away from the ground state AA' configuration. All moiré bilayers have a P1 space symmetry after structural optimization. This situation is quite different to monolayers with hexagonal lattices, which retain a threefold symmetry. We point out that the actual symmetries of the SHG reported for hBN bilayers on two experimental works do not coincide with the sixfold symmetric theoretical profiles they provide [either sin²⁡(3⁢Φ) or cos²⁡(3⁢Φ)], and show that the intrinsic low structural symmetry of (atomically optimized) hBN bilayer moirés can in fact be read out from experimental SHG intensity profiles—which are tunable by θ and by the frequency of light ω: The SHG is most definitely not sixfold symmetric because moirés do not retain a threefold symmetry. Furthermore, an extrinsic twofold symmetry of the SHG emission is realized by tilting the pump by an angle α away from the 2D material's normal, regardless of θ and ω. Furthermore, the design of in-plane and ultrathin sources of SHG with low symmetry could be useful for the eventual creation of entanglement sources from 2D materials.

• Research Organization: University of Arkansas, Fayetteville, AR (United States)
• Sponsoring Organization: National Science Foundation (NSF); USDOE Office of Science (SC), Basic Energy Sciences (BES)
• Grant/Contract Number: SC0022120
• OSTI ID: 2477405
• Journal Information: Physical Review Materials, Journal Name: Physical Review Materials Journal Issue: 11 Vol. 8; ISSN 2475-9953
• Publisher: American Physical Society (APS)Copyright Statement
• Country of Publication: United States
• Language: English