A. Fiore

Electric field induced interband second harmonic generation in GaAs/AlGaAs quantum wells

We investigate the second order optical susceptibility χ(2) of symmetric GaAs/AlGaAs quantum wells under applied bias, in the frequency region near half the band gap. The second harmonic is generated in a transmission geometry, and is separated from bulk contribution by a lock‐in detection technique. As opposed to previous reports, we find that χxzx(2) is much higher than χzxx(2). For 50 A quantum wells, χxzx(2)≊25 pm/V and χzxx(2)≊0 at λ=1.66 μm under an applied electric field E=50 kV/cm.

Postgrowth tuning of semiconductor vertical cavities for multiple-wavelength laser arrays

Combined lateral-vertical oxidation of AlGaAs is investigated as a means of tuning the resonant wavelength of a semiconductor microcavity after the epitaxial growth. It is shown that this technique can provide arrays with a wavelength spread equal to the cavitys free spectral range with a single postgrowth processing step. Design issues for multiple-wavelength vertical-cavity laser arrays using this postgrowth tuning technique are discussed, comparing the performance of devices with all-semiconductor and partially or totally oxidized Bragg mirrors. Experimental results are presented on arrays with a 48-nm lasing span around 970 nm, using partially and totally oxidized mirrors.

Surface emitted second-harmonic generation from a quasi-phase matched waveguide in an Al x Ga 1-x As/Al 2 O 3 microcavity

A nonlinear Al(x)Ga(1-x)As waveguide consisting of a quasi-phase matched heterostructure embedded in a microcavity has been designed and fabricated. The microcavity resonator is formed by Al(2)O(3)/Al(0.32)Ga(0.68)As multilayer mirrors located above and below the waveguide core. The cavity resonantly enhances the surface emitting second-harmonic generation. The SH conversion efficiency has been measured for wavelengths between l = 1525 and 1575 nm. A simple waveguide loss measurement technique based on the SH autocorrelation of short optical pulses in a III-V waveguide is also demonstrated.

Quasiphase matched surface emitting second harmonic generation in periodically reversed asymmetric GaAs/AlGaAs quantum well waveguide

We experimentally demonstrate surface-emitting second harmonic generation in a waveguide containing asymmetric coupled GaAs/AlGaAs quantum wells. The nonlinear conversion efficiency is enhanced by reversing the asymmetric well orientation every coherence length, in order to quasiphase match the vertical second harmonic generation process. The measured spectrum of the asymmetric quantum well susceptibility is dominated by an excitonic peak at a pump frequency corresponding to half of the first electron-heavy hole transition energy.

Patterning the second-order optical nonlinearity of asymmetric quantum wells by ion implantation enhanced intermixing

The change in the second-order nonlinear susceptibility of an asymmetric quantum well (AQW) superlattice induced by ion beam-enhanced intermixing has been measured. The surface-emitted second-harmonic intensities radiated from implanted and masked areas of an AQW waveguide were measured and compared for incident wavelengths between λ=1480 and 1600 nm. Intermixing resulted in a 60 meV blueshift of the AQW band edge and a uniform suppression of the AQW second-order susceptibility, while the masked AQWs were unchanged.

Multiple-wavelength vertical-cavity laser arrays based on postgrowth lateral-vertical oxidation of AlGaAs

We demonstrate that combined lateral-vertical oxidation of AlGaAs can be used to change the resonant wavelength of an optical cavity after the single epitaxial growth. A multiple-wavelength array of vertical-cavity surface-emitting lasers with a 48 nm wavelength span has been realized using this technique.

Parametric processes in GaAs/Alox structures

We discuss here the feasibility of an optical parametric oscillator integrated on a GaAs chip, after reviewing the recent frequency conversion experiments using from birefringence in GaAs/oxidized-AlAs (Alox) waveguides. Recently, phase-matching has been demonstrated for the first time in a GaAs-based waveguide, using form birefringence in multilayer heterostructures GaAs/Alox. Birefringence n(TE)- n(TM) from 0.15 to 0.2 have been measured for different GaAs/Alox waveguides, which is sufficient to phase match mid-IR generation between 3 micrometers and 10 micrometers by difference frequency generation form two near-IR beams. A second step was the observation of parametric fluorescence. Results on parametric fluorescence at 2.1 micrometers will be described, in an oxidized AlGaAs form-birefringent waveguide, consisting of a high-index, strongly birefringent GaAs-Alox core embedded in an AlGaAs cladding. One of the most existing perspectives opened with this new type of nonlinear material is the realization of an optical parametric oscillator on a GaAs chip. To this aim, minimization of losses is the most crucial point. A typical calculated value of this threshold is less than 70 mW for 1 cm-1 losses, and with 90 percent reflection coefficients. The level of losses has been reduced from 2 cm-1 in ridges obtained by a standard reactive ion etching technique, to less than 0.5 cm-1 in ridges realized with a more refined reactive ion etching process, using a three layer mask. There is still a need for an improvement of the waveguide fabrication process, before reaching the oscillation threshold.

Postgrowth tuning of cavity resonance for multiple-wavelength laser and detector arrays

Multiple-wavelength arrays of vertical-cavity lasers (VCLs) and detectors are important for wavelength-division-multiplexing systems. In-plane variation of the cavity resonant wavelength usually requires growth on patterned substrates or two successive growths and a complicated processing. It was recently observed that the lateral wet oxidation of an AlGaAs layer can result in partial vertical oxidation of an adjacent layer. We propose to use this combined lateral-vertical oxidation to change the optical thickness of a cavity after the epitaxial growth of both Bragg mirrors.

Birefringence phase-matched frequency doubling at /spl lambda/=1.6 /spl mu/m in GaAs-based waveguides

Frequency conversion in GaAs waveguides is a promising approach to the realization of tunable sources in the mid-IR and wavelength shifters for optical-fiber communications. The high second-order nonlinear susceptibility of GaAs and the possibility of integration with sources imply high conversion efficiencies and may lead to all-semiconductor integrated optical parametric oscillators. It was recently demonstrated that very high form birefringence can be obtained in (Al)GaAs/Al/sub 2/O/sub 3/ multilayers, realized by selective wet oxidation of (Al)GaAs/AlAs. This has led to demonstration of mid-IR sources by difference-frequency generation. Here we demonstrate that the same concept can be used to phase match frequency conversion in the 1.55-/spl mu/m telecommunication window. In this case, a much higher birefringence and a new design for the cladding layers are needed to compensate for the high dispersion of AlGaAs near the bandgap.

Mid-IR difference-frequency generation in GaAs/oxidized AlAs waveguides

As a semiconductor with a centrosymmetric crystal structure, bulk GaAs is nonbirefringent and is therefore rarely used for nonlinear frequency conversion despite its attractive nonlinear coefficient. By building a multilayer system GaAs/oxidized AlAs (Alox), we have shown that high form birefringences can be obtained in the composite material. To obtain a birefringence sufficient for phase matching in frequency conversion, a high index contrast is required between the two materials in the multilayer stack, which can be obtained by the oxidation of AlAs. We show phase-matched frequency conversion processes using form birefringence in composite GaAs/Alox waveguides. We believe these experiments are the first experimental demonstration of Van der Ziels proposal (1975) of using form birefringence for nonlinear phase matching.