Milliwatt terahertz harmonic generation from topological insulator metamaterials

Achieving efficient, high-power harmonic generation in the terahertz spectral domain has technological applications, for example in sixth generation (6G) communication networks. Massless Dirac fermions possess extremely large terahertz nonlinear susceptibilities and harmonic conversion efficiencies. However, the observed maximum generated harmonic power is limited, because of saturation effects at increasing incident powers, as shown recently for graphene. Here, we demonstrate room-temperature terahertz harmonic generation in a Bi$_2$Se$_3$ topological insulator and topological-insulator-grating metamaterial structures with surface-selective terahertz field enhancement. We obtain a third-harmonic power approaching the milliwatt range for an incident power of 75 mW - an improvement by two orders of magnitude compared to a benchmarked graphene sample. We establish a framework in which this exceptional performance is the result of thermodynamic harmonic generation by the massless topological surface states, benefiting from ultrafast dissipation of electronic heat via surface-bulk Coulomb interactions. These results are an important step towards on-chip terahertz (opto)electronic applications

Composition of surface layer bottom sediments from the Chukchi Sea

Abundance of noble metals and bulk chemical composition have been studied in bottom sediments of the Chukchi Sea. Distribution of noble metals and their correlation with major and trace elements in the sediments have been analyzed using multicomponent statistics. It was established that average contents of noble metals in the bottom sediments of the Chukchi Sea significantly exceed those both in shelf terrigenous sediments and stratisphere. Osmium and iridium enrich mixed and pelitic sediments relative to shallow-water sediments and their influx is presumably determined by erosion of coastal and bottom unconsolidated deposits. High Ag, Ru, Au, and Pt contents were identified in clayey sediments enriched in biogenic elements in the some areas of the Southern Chukchi plain (Chukchi Sea) confined to intersection zones of submeridional and sublatitudinal structures of the graben-rift system, which formed in Mesozoic and activated in Late Cenozoic

Reactivity of <i>C</i>‑Amino-1,2,4-triazoles toward Electrophiles: A Combined Computational and Experimental Study of Alkylation by Halogen Alkanes

A combination of computational and experimental methods was used to examine the structure–reactivity relationships in the reactions of <i>C</i>-amino-1<i>H</i>-1,2,4-triazoles with electrophiles. The global nucleophilicity of 3-amino- and 3,5-diamino-1<i>H</i>-1,2,4-triazoles was predicted to be higher than that of 5-amino-1<i>H</i>-1,2,4-triazoles. Fukui functions and molecular electrostatic potential indicate that reactions involving an amino group should occur more easily for the 3-amino- than for the 5-amino-1<i>H</i>-1,2,4-triazoles. Increasing electrophile hardness should increase the probability of attack at the N-4 atom of the triazole ring, whereas increasing softness should enhance the probability of attack at the N-2 atom and 3-NH₂ group. Calculated transition state energies of model S_N2 reactions and experimental studies showed that quaternization of 1-substituted 3-amino- and 3,5-diamino-1<i>H</i>-1,2,4-triazoles by many alkyl halides proceeds with low selectivity and can involve the N-2 and N-4 atoms as well as the 3-NH₂ group as reaction centers. A new method for the selective synthesis of 1,4-disubstituted 3-amino- and 3,5-diamino-1,2,4-triazoles based on quaternization of readily available 1-substituted 3-acetylamino-1,2,4-triazoles with subsequent removal of the acetyl protecting group by acid hydrolysis was developed

Facile Hydrolysis of Nickel(II) Complexes with N‑Heterocyclic Carbene Ligands

Metal complexes with N-heterocyclic carbene ligands (NHC) are ubiquitously used in catalysis, where the stability of the metal–ligand framework is a key issue. Our study shows that Ni-NHC complexes may undergo facile decomposition due to the presence of water in organic solvents (hydrolysis). The ability to hydrolyze Ni­(NHC)₂X₂ complexes decreases in the order of NHC = 1,2,4-triazolium > benzimidazolium ≈ imidazolium. Depending on the ligand and substituents, the half reaction time of the complex decomposition may change from several minutes to hours. The nature of the halogen is also an important factor, and the ability for decomposition of the studied complexes decreases in the order of Cl > Br > I. NMR and MS monitoring revealed that Ni-NHC complexes in the presence of water undergo hydrolysis with Ni–C_carbene bond cleavage, affording the corresponding <i>N</i>,<i>N</i>′-dialkylated azolium salts and nickel­(II) hydroxide. These findings are of great importance for designing efficient and recyclable catalytic systems, because trace water is a common contaminant in routine synthetic applications