Simulations of nanograting-assisted light coupling in GaN planar waveguide

The numerical simulations of nanogratings integrated with gallium nitride (GaN) planar waveguides as well as the experimental in-coupling results are presented. A simulation tool based on the eigenmode expansion method and advanced boundary conditions provided a rigorous model of 400-nm-period grating couplers. A full-vectorial Maxwell solver allowed performing a number of simulations with varying grating parameters, where coupling efficiency, reflection and transmission characteristics of device were calculated. Gratings with different etch depths and arbitrary shapes were simulated using a staircase approximation, with an optimized number of steps per single slope. For the first time, an impact of dry etch processing on GaN coupler efficiency was evaluated, due to the inclusion of the sloped sidewalls, with regard to the technological constrains. Finally, the experimental results in the visible spectrum region (lambda = 633 nm), for 400-nm-deep gratings etched in GaN waveguide, were presented together with theoretical data for binary and trapezoidal profiles of a grating, for different optical mode profiles (TE(0) divided by TE(3) modes)

Nanotexturing of GaN light-emitting diode material through mask-less dry etching

We describe a new technique for random surface texturing of a gallium nitride (GaN) light-emitting diode wafer through a mask-less dry etch process. This involves depositing a sub-monolayer film of silica nanospheres (typical diameter of 200 nm) and then subjecting the coated wafer to a dry etch process with enhanced physical bombardment. The silica spheres acting as nanotargets get sputtered and silica fragments are randomly deposited on the GaN epi-layer. Subsequently, the reactive component of the dry etch plasma etches through the exposed GaN surface. Silica fragments act as nanoparticles, locally masking the underlying GaN. The etch rate is much reduced at these sites and consequently a rough topography develops. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) inspections show that random topographic features at the scale of a few tens of nanometres are formed. Optical measurements using angle-resolved photoluminescence show that GaN light-emitting diode material thus roughened has the capability to extract more light from within the epilayers

Monolithic 45-GHz mode-locked surface-etched DBR laser using quantum-well intermixing technology

The 45-GHz passively mode-locked AlGaInAs-InP 1.55-mu m lasers integrated with surface-etched distributed Bragg mirrors have been fabricated. Quantum-well intermixing was used to provide low absorption loss gratings with accurate wavelength control. The lasers produce 3.6-ps Gaussian pulses with time-bandwidth product of 0.57

10-GHz mode-locked extended cavity laser integrated with surface-etched DBR fabricated by quantum-well intermixing

The 10-GHz passively mode-locked AlGaInAs/InP 1.55- μm extended cavity lasers integrated with optimized surface-etched distributed Bragg mirrors have been fabricated. A quantum-well intermixing process was used to provide low-absorption loss gratings with accurate wavelength control. The lasers produce 2.99-ps sech<sup>2</sup>-pulses with a time-bandwidth product (TBP) of 0.51

Charge dissipation layer based on conductive polymer for electron-beam patterning of bulk zinc oxide

The ability of thin conductive polythiophene layers to dissipate electrons in electron-beam lithography (EBL) process on bulk zinc oxide (ZnO) samples is shown. High energy electron-beam exposure of relatively thick (650 nm-thick) hydrogen silsesquioxane (HSQ) negative-type resist deposited on ZnO was investigated for three different cases. In turn, no charge dissipation layer, 40 nm-thick Al and 100 nm-thick conductive polymer layers were used on the top of HSQ resist. A quick and inexpensive processing method with the use of polymer is shown for an EBL exposure of dense and high-resolution patterns in HSQ/ZnO sample

Impact of titanium adhesion layers on the response of arrays of metallic split-ring resonators (SRRs)

At higher frequencies (visible and infrared) both the dimensions and the individual metal properties play an important role in determining the resonant response of arrays of SRRs. As a result, a substantial difference between the responses of gold-and Al-based SRR arrays has been observed. Additionally, deposition of gold SRRs onto a substrate typically involves the use of an additional adhesion layer. Titanium (Ti) is the most common adhesive thin-film material used to attach gold onto dielectric/semiconductor substrates. In this paper we investigate the impact of the Ti adhesion layer on the overall response of Au-based nano-scale SRRs. The results quantify the extent to which the overall difference in the resonance frequencies between Au- and Al-based SRRs is due to the presence of the Ti. We show that even a 2-nm-thick Ti layer can red-shift the position of SRR resonance by 20 nm. Finally, we demonstrate that by intentional addition of titanium in the Au-based SRRs, their overall resonant response can be tuned widely in frequency, but at the expense of resonance magnitude