Finite-length Nanowire Research Articles

Nanowire-array-based photonic crystal cavity by finite-difference time-domain calculations

Stimulated by recent reports on successful preparation of periodic nanowire arrays, we propose a class of photonic crystal (PC) cavities based on such nanowire arrays. This concept combines the advantages of PC's for tight confinement of light with the demonstrated suitability of nanowires for fabricating lasers. Unlike a previously used two-dimensional analysis, the three-dimensional finite-difference time-domain technique employed here allows us to study real spatial structures with nanowires of finite length. We discuss the realistic aspects of the cavity design, including the leakage in the vertical direction, aspect ratio of the wires, and thickness of the insulating layer between wires and substrate. The results show it is feasible to achieve microcavites with mode volumes from $\ensuremath{\sim}10{(\ensuremath{\lambda}∕n)}^{3}$ to $\ensuremath{\sim}2{(\ensuremath{\lambda}∕n)}^{3}$ and $Q$ values as high as ${10}^{4}$ using only about 80 nanowires with a proper design. The electromagnetic field of the mode concentrates in the nanowires, allowing their use as possible active materials for lasing and nonlinear operation. In addition, we analyze the influence of optical properties of the materials on the performance of the cavities, including absorption and dispersion. A simple formula is derived to calculate the $Q$ value with the presence of absorption. We predict a frequency splitting in dispersive materials based on numerical simulation. The results reported here establish the viability of the concept of using nanowires as building blocks to design microcavities and provide guidelines for designing compact and tightly confined optical modes in these cavities.

Structural transformation of ZnO nanostructures

Many nanostructures experience structural transformation, either from one type of crystalline structure to another or from one orientation to another orientation within the same crystalline structure. Using a combination of ab initio calculations and analytical formulation, the authors show that such structural transformation is a direct result of surface reconstruction. In particular, the ab initio results show that infinite large ZnO nanoplates and infinite long ZnO nanowires transform from wurtzite to graphitic structure below a critical thickness or diameter. The analytical formulation further shows that nanowires of finite length can go through the same structural transformation at larger critical diameters.

Distribution of optical emission between guided modes and free space in a semiconductor nanowire

We study the distribution of the emitted power between the free-space modes and guided modes in a semiconductor nanowire. We analyze all possible dipole orientations and nanowire radii in the range from very small to comparable to the wavelength. Our theoretical approach is based on the Fourier transform technique and equivalent to the construction of Green’s function for a dipole at an arbitrary location inside the nanowire. We show that the total emitted power can exhibit rather pronounced oscillations as a function of the frequency and radius. The far-field pattern is also very sensitive to the frequency and radius, especially in the regime when leaky (or whispering gallery) modes with finite axial wave numbers are excited. We discuss the enhancement of emission into guided modes due to formation of Fabry-Pérot cavity in a finite length nanowire. Our results yield directly the extraction efficiencies and angular distribution of radiation of light-emitting diodes made of nanowires.