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The second harmonic generation (SHG) in the asymmetric coupled quantum wells (ACQWs) is studied theoretically for different widths of the right-well and the barrier. The analytical expression of the SHG susceptibility is deduced by using the compact density matrix approach and the iterative method. Numerical calculations are presented for typical GaAs/AlxGa1−xAs ACQWs. The results show that the calculated SHG susceptibility in this coupled system can reach a magnitude of 10−5 m V−1, 1–2 orders higher than that in single quantum systems. Moreover, the SHG susceptibilities are not monotonic functions of the widths of the right-well and the barrier, but have complex relationships with them. The calculated results also reveal that by adjusting the widths of the right-well and the barrier respectively, a set of optimal structural parameters can be found for obtaining a strong SHG susceptibility.

Carbonicnanomaterials such as nanodiamonds, fullerenes, carbon nanotubes, graphene sheets, and carbon quantum dots (CQDs) have attracted extensive research efforts because of their unique properties and great potential for a wide variety of applications. [1][2][3][4][5][6][7] The accidental discovery of CQDs during the separation and purification of single-walled carbon nanotubes triggered subsequent studies to exploit the fluorescence properties of CQDs which comprise discrete nanoparticles with diameters below 10 nm and contain considerable amounts of carboxyl moieties at their surfaces. [8] Fluorescent CQDs are considered as the most promising candidates for replacing semiconductor quantum dots containing heavy metals in bioimaging because of their low toxicity, biocompatibility, low cost, and chemical inertness. [9][10][11][12][13][14] In addition, the large amounts of carboxyl moieties on the surfaces of CQDs impart excellent water solubility and suitable chemically reactive groups for surface passivation and functionalization. Moreover, the interesting photoinduced electron transfer properties of CQDs would offer exciting opportunities for light energy conversion, photovoltaic devices, and related applications. [1,[15][16][17] Basically, CQDs can be synthesized by either the top-down route or the bottom-up one. [18][19][20][21] So far, the production of CQDs via laser ablation of a carbon target has become a very important method and the synthesis of fluorescent CQDs by laser irradiation of a suspension of carbon materials in organic solvent has been demonstrated. [22][23][24] By properly selecting organic solvents, the surface states of CQDs could be modified to achieve tunable light emission and the origin of the luminescence was attributed to the surface states related to the ligands on the surfaces of CQDs. A review of the fabrication methods for CQDs, however, reveals some common limitations such as the long production time and the low controllability of the formed CQDs because only colloid CQDs are obtained. From the viewpoint of many practical applications, including but not limited to optical data storage, optical displays, and biosensing, the rapid and controllable formation of CQDs and especially CQD arrays is highly desirable. Rapid and controllable formation of fluorescent carbon quantum dots (CQDs) is highly desirable in the fields of nanophotonics and biophotonics.Here, a novel strategy for creating CQDs, which emit white light efficiently under the excitation of either laser light or a mercury lamp, is proposed and demonstrated. The luminescent CQDs are generated by irradiating a poly(vinyl alcohol) (PVA) film doped with dense gold nanoparticles (AuNPs) with femtosecond laser pulses. The creation of CQDs from PVA is a twostep dehydration process mediated by AuNPs which act not only as heat sources but also as catalytic agents. The formation of CC, CC, and CO bonds is confirmed by infrared Fourier transformation spectroscopy and X-ray photoelectron spectroscopy. It is revealed both numerica...