Two-Photon Excited Nonlinear Optical Properties Of Semiconductor Nanostructures

Semiconductor micro/nanostructures have attracted great attention due to their unique optical and electrical properties, as well as their potential applications for developing the miniaturized optoelectronic components. Specially, the typical Ⅱ-Ⅵ semiconductor materials, cadmium sulfide (CdS) and zinc oxide (ZnO) have been extensively investigated due to their optoelectronic applications, such as nanolasers, optoelectronic detector, and photomodulator in the visible spectrum range and ultraviolet light range, respectively. Up to now, the photonluminescence (PL) and stimulated emission properties in CdS nanowires and nanoribbons have been widely investigated. According to previous reports, the optical properties of CdS nanostuctures are mostly investigated under one-photon excitation (OPE). Compared with OPE, the two-photon excitation (TPE) not only provides a larger penetration depth, but also offers three-dimensional resolution. In addition, the near-infrared excitation source is needed for TPE, which makes CdS having more extensive applications in optical communications and biomedicine. Therefore, it is significant to investigate the two-photon excited optical properties of CdS nanostuctures. In this thesis, the two-photon excited fluorescence, lasing emission and second hamonic generation (SHG) of an individual CdS nanostructure were investigated using confocal optical microscopy system under the excitation of a femtosecond pulsed laser with a near-infrared wavelength. The excitation light polarization-dependent PL intensity and the influences of sample morphology on PL efficiency were discussed. The nonlinear absorption properties of ZnO nanohole array were also studied. The main contents are listed as follows.CdS nanowires and nanosheets have been successfully synthesized on Si and quartz substrates via chemical vapor deposition method. The surface morphology of a culster and sparse-distributed CdS nanostructures was characterized by field-emission scanning electron microscope. The crystal structure of the nanostructure was characterized by X-ray diffraction. And the room temperature photoluminescence (PL) properties were investigated using the laser confocal Raman spectrometer with the wavelength of325nm.The home-built confocal optical microscope measurement configuration, which is flexible, stable, and is of high resolution, has been designed and set up by ourselves. The configuration can be used to measure and image a single micro/nano sample precisely. The nonlinear optical properties of single CdS nanostructure were investigated using the configuration under the excitation of seed femtosecond laser and amplified femtosecond laser with the wavelength of800nm～820nm.The two-photon absorption induced fluorescence and lasing properties of an individual CdS nanowire have been investigated. When the seed femtosecond laser was used, the two-photon fluorescence and SHG were observed. Meanwhile, the excitation light polarization-dependent fluorescence intensity and the polarization state of fluorescence were discussed. When the amplified femtosecond laser was used, the transition from spontaneous emission to laser oscillation was observed as the excitation energy increased. Furthermore, the dependence of lasing emission on the polarization direction of the excitation light in the single CdS nanowire was also demonstrated. The polarization-related lasing emission is further confirmed by performing the finite-difference time-domain (FDTD) simulations under the same excitation wavelength.The two-photon fluorescence and lasing characteristics from individual CdS nanosheets of different shapes were also investigated using the same measurement system and excitation light source as described above. The two-photon fluorescence and SHG from irregular nanosheets were observed under the excitation of seed femtosecond laser. It is found that the PL efficiency of nanosheet is higher than that of nanowire. The lasing emission with a lower laser threshold and higher quality factor from a singel CdS nanosqure was realized under the excitation of amplified femtosecond laser. Different lasing spctra can be obtained by changing the angle betwween nanosheet and the collected fiber, which indicates the different emission direction of lasing action.The third-order nonlinear optical properties of ZnO nanohole were measured. Highly ordered ZnO nanohole array was synthesized using pulsed laser deposition method on the prepared template substrate fabricated by nanosphere lithography and reactive ion etching. The nonlinear absorption and nonlinear refractive properties of ZnO nanohole were measured using Z-scan technique under the excitation of amplified femtosecond laser with a wavelength of800nm. It is found that the nanohole array shows strong reverse saturable absorption, which can be attributed to the two-photon absorption (TPA), and weak nonlinear refractive effect. The TPA coefficient of the nanohole is about two orders of magnitude larger than that of the ZnO film fabricated under the same conditions. Furthermore, the cause of enhanced TPA in the nanohole array was analyzed by a linear absorption measurement and FDTD simulations.