James J. Raftery

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.

Mode Control in Photonic Crystal Vertical-Cavity Surface-Emitting Lasers and Coherent Arrays

We demonstrate transverse mode control in vertical-cavity surface-emitting lasers (VCSELs) and 2-D VCSEL arrays. By etching a periodic arrangement of circular holes into the top distributed Bragg reflector mirror, we are able to control the lasing modes through index and loss confinement. Theoretical modeling of these confinement effects are shown to be consistent with experimental measurements. Photonic crystal etched patterns and ion-implanted photonic lattices have been employed to fabricate coherently-coupled 2-D arrays. Control of the array supermodes from the out-of-phase and in-phase conditions is discussed. Designs of photonic crystal coherent VCSEL arrays for high-power emission and beam steering applications are described.

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

An approach for creating two-dimensional arrays of coherently coupled vertically emitting laser cavities is demonstrated. This is achieved by creating a 2×2 array of defect cavities within the top distributed Bragg reflector of a photonic crystal vertical cavity surface-emitting laser. The optical coupling occurs laterally through coupling regions defined between the defect cavities. Modifying the index within the coupling regions, accomplished by varying the hole parameters of the photonic crystal in those regions, leads to out-of-phase coherent coupling observed in the far field. Agreement is found between the simulated and observed out-of-phase far fields.

Coherence of Photonic Crystal Vertical-Cavity Surface-Emitting Laser Arrays

We measure and compare the coherence properties of 2 times 1 arrays of photonic crystal vertical-cavity surface-emitting lasers. Antenna array theory applied to the measured far field intensity patterns is used to determine the phase of the complex degree of coherence, which is found to vary with current injection. The amplitude of the complex degree of coherence is determined by calculating the visibility from the far field patterns and making near field measurements of the relative intensities between lasing defects. We find that the amplitude and phase of the complex degree of coherence are correlated, such that coherence is maximized near in-phase and out-of-phase coupling conditions, and controllable by independent current injection to each array element

Loss and Index Guiding in Single-Mode Proton-Implanted Holey Vertical-Cavity Surface-Emitting Lasers

Wedge-shaped holes are fabricated in the top mirror of proton-implanted vertical-cavity surface-emitting lasers (VCSELs). A radially symmetric fill factor approach is used to calculate the resulting transverse index profile. To investigate both the index confinement provided by the etched pattern and its effect on optical loss, continuous-wave (CW) and pulsed experiments are performed. Under CW operation, we show proper wedge design leads to improved fundamental-mode output power, decreased threshold, and increased efficiency. We report a significant decrease in threshold under pulsed operation for the etched device compared to an unetched device, indicating a significant reduction in diffraction loss to the fundamental mode due to strong index guiding. Single-mode output is maintained over the entire operating range of the VCSEL due to increased loss for the higher order modes

Relative phase tuning of coupled defects in photonic crystal vertical-cavity surface-emitting lasers

Evanescent coupling can be achieved by patterning the top facet of a vertical-cavity surface-emitting laser (VCSEL) with a multiple defect photonic crystal pattern. We show that for 2×1 coupled defect arrays, the phase between these emission sites changes as the current is varied. The phase variation is manifest in the far field pattern and may be determined using antenna array theory. It is found that the phase varies 81 deg between transversely coupled cavities as the current to the VCSEL is varied by 24 mA. This corresponds to a far field angular change of approximately 1.5 deg.

Etch damage and deposition repair of vertical-cavity surface-emitting lasers

Dielectric layers are often employed as etch masks for mesa and trench structures during vertical-cavity surface-emitting laser (VCSEL) fabrication. The removal of these mask layers by reactive ion etching results in unavoidable exposure of the top laser facet to sputtering. This sputtering is experimentally shown to impact the device performance. After a thickness of less than a quarter wavelength (∼60nm) has been removed, the VCSELs are no longer able to achieve lasing threshold. Simulation indicates that the reason for this is a decrease in quality factor by more than an order of magnitude. Consistent with this explanation is that the damage can be partially repaired (allowing laser oscillation) by depositing SiO2 to compensate for the missing semiconductor material.