Aaron J. Danner

Two-dimensional photonic crystal confined vertical-cavity surface-emitting lasers

The two-dimensional photonic crystal (2-D PhC) structure has been investigated as a method for lateral mode control of vertical-cavity surface-emitting lasers (VCSELs). The 2-D PhC structures were designed using an equivalent index model developed for photonic crystal fibers combined with a plane wave expansion method. The etching depth dependence of the PhC structure was incorporated for the first time to design practical devices. 2-D PhC-confined VCSELs are demonstrated to operate in single PhC-confined mode using either a single- or seven-point defect.

Etching depth dependence of the effective refractive index in two-dimensional photonic-crystal-patterned vertical-cavity surface-emitting laser structures

A vertical-cavity surface-emitting laser (VCSEL) having a two-dimensional (2-D) photonic crystal structure on its surface has been investigated for single-lateral-mode operation. We evaluated the effective index change of a VCSEL cavity introduced by a 2-D pattern. Our experimental results showed good agreement with a theoretical model in which the influence of a finite etching depth was taken into consideration. The etching-depth dependence parameter γ, which can be explained by the optical power distribution inside a VCSEL structure, will be helpful for controlling the lateral mode of VCSEL devices.

Visible-Frequency Metasurface for Structuring and Spatially Multiplexing Optical Vortices

A multifocus optical vortex metalens, with enhanced signal-to-noise ratio, is presented, which focuses three longitudinal vortices with distinct topological charges at different focal planes. The design largely extends the flexibility of tuning the number of vortices and their focal positions for circularly polarized light in a compact device, which provides the convenience for the nanomanipulation of optical vortices.

TiO2 Thin Films Prepared via Adsorptive Self-Assembly for Self-Cleaning Applications

Low-cost controllable solution-based processes for preparation of titanium oxide (TiO(2)) thin films are highly desirable, because of many important applications of this oxide in catalytic decomposition of volatile organic compounds, advanced oxidation processes for wastewater and bactericidal treatments, self-cleaning window glass for green intelligent buildings, dye-sensitized solar cells, solid-state semiconductor metal-oxide solar cells, self-cleaning glass for photovoltaic devices, and general heterogeneous photocatalysis for fine chemicals etc. In this work, we develop a solution-based adsorptive self-assembly approach to fabricate anatase TiO(2) thin films on different glass substrates such as simple plane glass and patterned glass at variable compositions (normal soda lime glass or solar-grade borofloat glass). By tuning the number of process cycles (i.e., adsorption-then-heating) of TiO(2) colloidal suspension, we could facilely prepare large-area TiO(2) films at a desired thickness and with uniform crystallite morphology. Moreover, our as-prepared nanostructured TiO(2) thin films on glass substrates do not cause deterioration in optical transmission of glass; instead, they improve optical performance of commercial solar cells over a wide range of incident angles of light. Our as-prepared anatase TiO(2) thin films also display superhydrophilicity and excellent photocatalytic activity for self-cleaning application. For example, our investigation of photocatalytic degradation of methyl orange indicates that these thin films are indeed highly effective, in comparison to other commercial TiO(2) thin films under identical testing conditions.

Annular aperture array based color filter

In this letter, we propose and experimentally demonstrate a color filter based on an annular aperture geometry working in the visible range. The device is built by configuring an array of annular apertures in a gold film suitable for transmission measurement. We show effective fine tuning of resonance peaks through precise geometric control of the aperture dimensions. Selective transmission through annular apertures of various sizes leads to continuous color tuning of transmitted electromagnetic waves. This may find potential for application in high-definition displays, optical filters, ultrafast switching, and bio-sensing.

Focused Ion Beam Nanopatterning for Optoelectronic Device Fabrication

Recent photonic device structures, including distributed Bragg reflectors (DBRs), one-dimensional (1-D) or two-dimensional (2-D) photonic crystals, and surface plasmon devices, often require nanoscale lithography techniques for their device fabrication. Focused ion beam (FIB) etching has been used as a nanolithographic tool for the creation of these nanostructures. We report the use of FIB etching as a lithographic tool that enables sub-100-nm resolution. The FIB patterning of nanoscale holes on an epitaxially grown GaAs layer is characterized. To eliminate redeposition of sputtered materials during FIB patterning, we have developed a process using a dielectric mask and subsequent dry etching. This approach creates patterns with vertical and smooth sidewalls. A thin titanium layer can be deposited on the dielectric layer to avoid surface charging effects during the FIB process. This FIB nanopatterning technique can be applied to fabricate optoelectronic devices, and we show examples of 1-D gratings in optical fibers for sensing applications, photonic crystal vertical cavity lasers, and photonic crystal defect lasers.

Transverse modes of photonic crystal vertical-cavity lasers

The control of lateral mode operation using a photonic crystal in a vertical-cavity surface-emitting laser (VCSEL) is analyzed and confirmed experimentally. By controlling design parameters of the photonic crystal pattern, we have produced photonic crystal VCSELs that operate in higher order defect modes in addition to the fundamental defect mode. The transverse modal behavior is consistent with the predictions of a theoretical model in which the etching depth dependence of the air holes of the photonic crystals is considered. We also have determined the lower limit of optical confinement required from the photonic crystal pattern to influence the output beam of the laser.

Single mode photonic crystal vertical cavity lasers

We report the accuracy of the photonic crystal model in describing the characteristics of vertical cavity surface-emitting lasers with lateral optical confinement consisting of a periodic array of etched circular holes. Experiments were carried out to compare predictions of the photonic crystal model to observed modal device characteristics, and the oxide aperture size was optimized to give maximum output power and lower threshold. The role of loss in improving modal properties was also investigated. Optimized lasers exhibit submilliamp threshold current and operate in the fundamental lateral mode for all currents.

In-phase evanescent coupling of two-dimensional arrays of defect cavities in photonic crystal vertical cavity surface emitting lasers

In-phase evanescent coupling in 2×1 and 2×2 arrays of defect cavities in photonic crystal (PhC) vertical cavity surface emitting lasers (VCSELs) is reported. Two-dimensional PhC patterns of air holes containing multiple defects are etched into the top distributed Bragg reflector of VCSELs. The resulting modification of the effective index and optical loss results in evanescent coupling between the multiple defect cavities of the PhC VCSEL. Far field measurements and simulations show good agreement and demonstrate the in-phase results.

Vertical-cavity surface-emitting laser operating with photonic crystal seven-point defect structure

A vertical-cavity surface-emitting laser with a two-dimensional photonic crystal (PC) structure has been investigated for single lateral mode operation. The PC confined mode can be controlled by the lattice constant, the hole diameter, the etching depth, and the defect structure. A seven-point defect structure is proposed to enhance the confinement effect, which is diluted by finite hole depth of the PC structure. We obtained a pure PC confined mode under room-temperature cw conditions with a PC lattice constant of 2 μm, hole diameters of 1.0 and 1.4 μm, and depths of 1.85 and 1.92 μm, respectively.