Enhanced ultrafast optical nonlinearity of porous anodized aluminum oxide nanostructures

Revealing the truncated conical geometry of nanochannels in anodic aluminium oxide membranes

Anodic aluminium oxide (AAO) membranes with self-ordered nanochannels have become promising candidates for applications in the aspects such as structural coloration, photonic crystals, upconversion luminescence and nanofluidic transport. Also, self-ordered AAO membranes have been extensively used for the fabrication of functional nanostructures such as nanowires, nanotubes, nanoparticles, nanorods and nanopillars. Geometries of nanochannels are crucial for the applications of AAO membranes as well as controlling growth (e.g., nucleation, direction and morphology) and in applications (e.g., optics, magnetics, thermoelectrics, biology, medicine, sensing, and energy conversion and storage) of the functional nanostructures fabricated via AAO template-based methods. However, observation of whole nanochannels with nanometer-resolution in thick AAO membranes remains a fundamental challenge, and the nanochannel geometry has not yet been sufficiently elucidated. Here, for the first time, we use depth-profiling transmission electron microscopy to reveal the truncated conical geometry of whole nanochannels of 70 μm in length. Such shape nonuniformity of the nanochannels leads to different reflectance properties of the different depths of the nanochannels along their long axis for one AAO membrane, which suggests that the nonuniformity result in some effects on applications of the nanostructures. Furthermore, we introduce a shape factor to evaluate the shape nonuniformity and demonstrate that the nonuniformity can be remarkably removed by an effective etching method based on a temperature gradient regime.

Plasmon-enhanced versatile optical nonlinearities in a Au-Ag-Au multi-segmental hybrid structure

A Au-Ag-Au multi-segmental hybrid structure has been synthesized by using an electrodeposition method based on an anodic aluminum oxide (AAO) membrane. The third-order optical nonlinearities, second harmonic generation (SHG) and photoluminescence (PL) properties containing ultrafast supercontinuum generation and plasmon mediated thermal emission have been investigated. Significant optical enhancements have been obtained near surface plasmon resonance wavelength in all the abovementioned nonlinear processes. Comparative studies between the Au-Ag-Au multi-segmental hybrid structure and the corresponding single-component Au and Ag hybrid structures demonstrate that the Au-Ag-Au multi-segmental hybrid structure has much larger optical nonlinearities than its counterparts. These results demonstrate that the Au-Ag-Au hybrid structure is a promising candidate for applications in plasmonic devices and enhancement substrates.

Near-UV-enhanced broad-band large third-order optical nonlinearity in aluminum nanorod array film with sub-10 nm gaps

Plasmonic nanostructures with sub-10 nm gaps possess intense electric field enhancements, leading to their high reputation for exploring various functional applications at nanoscale. Till now, although large amounts of efforts have been devoted into investigation of such structures, few works were emphased on the nonlinear optical properties in near-ultraviolet (UV) region. Here, by combining sputtering technique and an optimized anodic aluminum oxide (AAO) template growing method, we obtain aluminum (Al) nanorod array film (NRAF) with average rod diameter and gap size of 50 and 7 nm, respectively. The Al-NRAF exhibits large third-order optical nonlinear susceptibility (χ⁽³⁾) and high figure of merit (χ⁽³⁾/α) over a broad wavelength range from 360 to 900 nm, and reaches their maximums at the shortest measured wavelength. In addition, comparisons with Au-NRAF and Ag-NRAF samples further confirm that Al-NRAF has better nonlinear optical properties in the blue and near-UV wavelength regions. These results indicate that Al nanostructures are promising candidates for nonlinear plasmonic applications at blue and near-UV wavelengths.

Pump power dependence of femtosecond two-color optical Kerr shutter measurements

We investigated the pump power dependence of femtosecond two-color optical Kerr shutter (OKS) signals, which showed a damped sinusoidal variation with increasing pump power. The sinusoidal dependence was attributed to the polarization rotation caused by light-induced birefringence effect. The numerical analysis indicated that, the damping of OKS signal intensity could be attributed to the temporal profile change of probe pulse passing through the OKS setup, due to the non-uniform transient refractive index change induced by pump pulse. Because of the large phase shift of probe pulse, the time-resolved OKS signals showed modulated temporal intensity when pump power was increased.