Engineering DAST Nanoscale-Thick Films for Giant and Isotropic Second Harmonic Generation: Implications for Bioimaging and Quantum Optics

Second harmonic generation (SHG) is applied widely in frequency conversion, medical imaging, and quantum optics. 4-N,N-dimethylamino-4′-N′-methyl-stilbazolium tosylate (DAST) crystal, which is an outstanding organic nonlinear optical (NLO) material, has attracted considerable attention. To maximize its potential nonlinear properties, new degrees of freedom─the controls of the growth interface and temperature were explored to prepare DAST nanoscale-thick films with giant SHG in this work. Results reveal that a phase transition from a triclinic structure to a monoclinic structure in DAST nanoscale-thick films is initially and irreversibly induced at ∼120 °C. However, the ultrastrong SHG signal which is 4 orders of magnitude larger than those of both an unannealed DAST nanoscale-thick film and an inorganic potassium dihydrogen phosphate crystal can be measured from the DAST nanoscale-thick film deposited on the glass surface and annealed at 200 °C. This is attributed to a large second-order nonlinear susceptibility χ(2) of 7.24 nmV–1, high degree of structural ordering, formation of DAST single-crystal micro-platelets, and relaxation of the phase-matched condition in this case. Interestingly, the isotropic giant SHG response can be measured from such a DAST single-crystal nanoscale-thick film, attributed to the synergistic effects of the anisotropic DAST single-crystal micro-platelets with various preferential orientations distributed uniformly on the substrate surface and the interface interactions between DAST and the glass substrate. These findings shed light on the engineering of DAST nanostructures for significantly enhancing the SHG response, and particularly, they also pave an efficient way toward seeking the strongest nonlinear optical performances from DAST and other NLO organic materials for bioimaging and quantum optics.