Optical rib waveguides based on sol-gel derived silica–titania films

We realize an AlGaAs-on-sapphire platform through $\mathrm{Al_2O_3}$ -assisted direct wafer bonding and substrate removal processes. The direct wafer bonding process is optimized concerning the intermediate layer deposition and annealing temperature to obtain a high bonding strength between the AlGaAs and sapphire wafers. High quality-factor ( Q ) microring resonators are fabricated using electron-beam lithography in which the charging effect is mitigated by applying a thin aluminum layer, and a smooth pattern sidewall definition is obtained using a multi-pass (exposure) process. We achieve an intrinsic Q of up to $\sim$ 460,000 which is the highest Q for AlGaAs microring resonators. Taking advantage of such high Q resonators, we demonstrate an ultra-efficient nonlinear four-wave mixing process in this platform and obtain a conversion efficiency of $-$ 19.8 dB with continuous-wave pumping at a power level of 380 $\mu$ W. We also investigate the thermal resonance shift of microring resonators with different substrate layouts and observe superior temperature stability for devices in the AlGaAs-on-sapphire platform. The realization of the AlGaAs-on-sapphire platform also opens new prospects for AlGaAs devices in nonlinear applications in the mid-infrared wavelength range.

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High-Quality-Factor AlGaAs-on-Sapphire Microring Resonators

A GaAs-based high-index-contrast ridge waveguide laser is fabricated using a self-aligned process of deep dry etching plus oxygen-enhanced wet thermal oxidation of low Al-content AlGaAs. Lasers operating at /spl lambda/=813 nm (CW, 300 K) with external differential quantum efficiencies as high as 78% are demonstrated, indicating effective passivation of the directly-oxidised etched active region sidewall surface.

High-efficiency native-oxide-passivated high-index-contrast ridge waveguide lasers

AlGaAs heterostructure high-index-contrast (HIC) ridge waveguide (RWG) diode lasers incorporating a folded-cavity single-facet resonator with a folding bend radius as small as r=10 mum are demonstrated. Fabricated by a self-aligned deep dry etch (through the active region) plus nonselective O2-enhanced wet thermal oxidization process, the low-index, insulating, and interface-passivating wet thermal oxide grown directly on the etch-exposed AlGaAs waveguide sidewalls yields a high lateral refractive index contrast of Deltan~1.7 and provides strong optical mode confinement. The HIC RWG device geometry also completely eliminates lateral current spreading, which results in an excellent overlap between the optical field and the gain region of the single InAlGaAs quantum-well graded-index separate confinement heterostructure. A threshold current of Ith=65mA is obtained for the r=10 mum device (a half-racetrack ring resonator), giving a threshold current density of Jth=1503 A/cm2, 3.34 times higher than that of same-length straight lasers. At a bend radius of r=150 mum, Ith=16.6 mA, and Jth is comparable to straight cavity values, indicating that at this curvature there is negligible bending and scattering loss for the lowest-order waveguide mode

Single-Facet Folded-Cavity Diode Laser With Ultrasmall Bend Radius High-Index-Contrast Oxidized AlGaAs Ridge Waveguide

Using a modified, O2-enhanced nonselective wet thermal oxidation process, deep-etched ridge waveguides in AlGaAs heterostructures containing lambda = 808 nm InAlGaAs single quantum well or aluminum-free lambda = 1.3 mum GaAsP/InGaAsN dilute nitride multi-quantum-well active regions have been directly oxidized to effectively provide simultaneous electrical isolation, interface state passivation, and sidewall roughness reduction. The resulting high- index-contrast (HIC) ridge waveguide (RWG) diode lasers show improved performance relative to conventional shallow-etched devices owing to both strong optical confinement and the complete elimination of current spreading, with 5 mum stripe width dilute- nitride devices showing up to a 2.3 times threshold reduction and strong index guiding for kink-free operation. Oxidation of an AlGaAs graded-index separate confinement heterostructure is studied for varying O2 concentrations, and the interface passivation effectiveness of the native oxide is studied through comparison with deposited SiO2 and via a study of the stripe-width dependence of internal quantum efficiency and modal loss. The HIC RWG structure is shown to enable the operation of half-racetrack-ring- resonator lasers with a bend radius as small as r = 6 mum.

Deep-Etched Native-Oxide-Confined High-Index-Contrast AlGaAs Heterostructure Lasers With 1.3 $\mu$ m Dilute-Nitride Quantum Wells

We present data showing that the addition of trace amounts of O $_2$ ( $≪1hbox%$ relative to N $_2$ ) to N $_2+$ H $_2$ O process gas during the wet thermal oxidation of Al $_ x$ Ga $_1 -x$ As enhances the oxidation rates of lower Al content $( xle 0.8)$ alloys (a tenfold increase for $ x=0.3$ ), while decreasing the oxidation rate selectivity $ R( x=0.8)/ R( x=0.3)$ by a factor of seven. An increase in the refractive index from 1.49 to 1.68, and a fourfold decrease in surface roughness, indicates the formation of a denser, higher quality oxide for $ x=0.3$ AlGaAs. Oxides are characterized by prism coupling, atomic force microscopy, and scanning electron microscopy. Thermochemical calculations show a probable mechanism in the enhancement of the dry oxidation reactions of AlGaAs for low levels of O $_2$ , while there is still an adequate quantity of H $_2$ produced to reduce As oxides in the wet oxidation process. An AlGaAs quantum well heterostructure p-n laser diode crystal is nonselectively oxidized to create a deep oxide, high-index contrast waveguide with potential applications in semiconductor photonic integrated circuits that require small bend radius, high isolation, low crosstalk optical waveguides.

D.C. Hall

Papers

Nonselective Wet Oxidation of AlGaAs Heterostructure Waveguides Through Controlled Addition of Oxygen