The key issue for light emission strength of GaN-based LEDs is the high defect density and strain in MQWs causing the electric polarization fields. In this work, we construct 3D confocal microspectroscopy to clarify strain distribution and the relationship between photoluminescence (PL) intensity and pattern sapphire substrate (PSS). From 3D construction of E<sub>2</sub><sup>high</sup> Raman and PL mapping, the dislocation in MQW can be traced to the cone tip of PSS and the difference in E<sub>2</sub><sup>high</sup> Raman mapping between substrate and surface is also measured. The ability to measure strain change in 3D structure nondestructively can be applied to explore many structural problems of GaN-based optoelectronic devices.
Magnetization-induced second-harmonic generation (MSHG) technique in transverse Kerr configuration is used to explore the nonlinear magneto-optical properties of cobalt and cobalt oxide nanoparticles at room temperature. The nanoparticles are deposited on Si (100) substrate by a RF magnetron sputtering system at room temperature. The STM image of the studied sample indicates that the cobalt particle size is in the range of 10 nm to 30 nm. MSHG technique investigates their magnetic anisotropy property and determines their crystal and magnetic symmetry. The pure cobalt nanoparticle thin films have a large nonlinear Kerr rotation response with a low external magnetic field. The increase of coercivity field, H<sub>c</sub>, in the cobalt oxide nanoparticle thin films, can be attributed to the strong interaction between the ferromagnetic cobalt and anti-ferromagnetic cobalt oxide interface. It is found that the magnetic anisotropy is due to the shape anisotropy of cobalt nanoparticle thin film and their interaction. The magnetization reversal process of cobalt nanoparticles involves the rotation of the magnetic moment in the surface plane but not the surface normal plane. This study aims to reveal the important information regarding the magnetization orientation in the ferromagnetic materials and their heterostructures.
Coherent phonons of semiconductor-metal interfaces are impulsively generated and detected with time-resolved second-harmonic generation. Coherent longitudinal optical phonons are launched in the near surface depletion regions of GaP/Au and GaAsP/Au Schottky photodiodes. Photoexcited electrons ballistically transport from the metal layer into the semiconductor region rapidly screen the near surface depletion field and launches these coherent LO phonons. The dephasing of these coherent LO phonons is mainly due to the anharmonic decay at zero or reverse bias. Their dephasing times decrease significantly as a forward bias is applied to the heterointerface. This effect is attributed to the strong carrier-phonon scatterings induced by the electrical driven electrons flowing across the heterointerface. Meanwhile, coherent longitudinal acoustic wave packet is observed in the GaP/Au heterointerface via transient thermal absorption in the gold thin layer. This acoustic wave packet propagates into the GaP bulk with the sound velocity ~5.8×10<sup>5</sup> cm/sec of GaP LA phonon.
The coupling between photoexcited plasma and coherent LO phonons in n-type GaAs (100) was investigated via time-resolved second harmonic generation. In addition to standard pump-probe setup, a time-delayed second pump was applied to inject excess plasma in the sample. The initial pump impulsively launched coherent LO phonons and the following second pump photoexcited excess plasma in the near surface depletion region, where the excess plasma could interact with the coherent LO phonons. It was found that the coherent LO phonon mode dephased rapidly as excess plasma was injected. Meanwhile, the coherent LO-electron and LO-hole coupling modes could be clearly observed in the Fourier spectra. These coupling modes showed plasma density dependent frequencies. Their coupling frequencies and dephasing process were studied in detail. The experimental results agree with the simulation of carrier and phonon dynamics in the near surface depletion region of GaAs on sub-picosecond time scale.