P. J. Poole

Photonic integrated circuits fabricated using ion implantation

Intermixing the wells and barriers of quantum-well (QW) laser heterostructures generally results in an increase in the bandgap energy and is accompanied by changes in the refractive index. A technique, based on ion implantation-induced QW intermixing, has been developed to enhance the quantum-well intermixing (QWI) rate in selected areas of a wafer. Such processes offer the prospect of a powerful and simple fabrication route for the integration of discrete optoelectronic devices and for forming photonic integrated circuits.

Optical spectroscopy of single, site-selected, InAs/InP self-assembled quantum dots

We present optical spectroscopy measurements on a single InAs/InP quantum dot emitting around λ=1.55 μm. The dot is produced using a nanotemplate deposition technique that allows precise a priori control of quantum dot position and electronic configuration. Clear evidence of excitonic shell structure and many-body renormalization effects are observed.

QUANTUM-WELL INTERMIXING FOR OPTOELECTRONIC INTEGRATION USING HIGH ENERGY ION IMPLANTATION

The technique of ion‐induced quantum‐well (QW) intermixing using broad area, high energy (2–8 MeV As4+) ion implantation has been studied in a graded‐index separate confinement heterostructure InGaAs/GaAs QW laser. This approach offers the prospect of a powerful and relatively simple fabrication technique for integrating optoelectronic devices. Parameters controlling the ion‐induced QW intermixing, such as ion doses, fluxes, and energies, post‐implantation annealing time, and temperature are investigated and optimized using optical characterization techniques such as photoluminescence, photoluminescence excitation, and absorption spectroscopy.

Spatially controlled, nanoparticle-free growth of InP nanowires

A technique for the growth of InP nanowires, which does not rely on the vapor–liquid–solid growth mechanism, is demonstrated using selective-area chemical beam epitaxy. The nanowires are precisely positioned on an InP wafer and are always aligned along the available substrate 〈111〉A directions. They have diameters as small as 40 nm, and typical lengths of 600 nm. They are found to be optically active, with thin embedded InAs layers showing quantum-dot-like behavior with well-defined excited states.

Observation of strongly entangled photon pairs from a nanowire quantum dot

A bright photon source that combines high-fidelity entanglement, on-demand generation, high extraction efficiency, directional and coherent emission, as well as position control at the nanoscale is required for implementing ambitious schemes in quantum information processing, such as that of a quantum repeater. Still, all of these properties have not yet been achieved in a single device. Semiconductor quantum dots embedded in nanowire waveguides potentially satisfy all of these requirements; however, although theoretically predicted, entanglement has not yet been demonstrated for a nanowire quantum dot. Here, we demonstrate a bright and coherent source of strongly entangled photon pairs from a position-controlled nanowire quantum dot with a fidelity as high as 0.859±0.006 and concurrence of 0.80±0.02. The two-photon quantum state is modified via the nanowire shape. Our new nanoscale entangled photon source can be integrated at desired positions in a quantum photonic circuit, single-electron devices and light-emitting diodes.

COMPOSITION OF ALGAAS

Although the AlxGa1−xAs alloy system has been extensively investigated, there are still considerable uncertainties in measuring the value of x. Here a new AlxGa1−xAs calibration structure, grown by molecular beam epitaxy, has been used to establish unambiguous alloy compositions. Such “standard’’ AlxGa1−xAs layers were measured by high-resolution x-ray diffraction, photoluminescence, and Raman spectroscopy to determine the compositional variations of the measured physical parameters. The phenomenological equations derived from these measurements can now be used to establish the Al content of unknown alloys with confidence. In addition, the results show that Vegard’s law does not hold for the variation of the AlxGa1−xAs lattice constant with x. The small quadratic term has very important implications for a correct analysis of x-ray results.

312-fs pulse generation from a passive C-band InAs/InP quantum dot mode-locked laser.

For the first time, we report femtosecond pulses from a passive single-section InAs/InP quantum-dot (QD) mode-locked laser (MLL) with the active length of 456 microm and ridge width of 2.5 microm at the C-band wavelength range. Without any external pulse compression, the transform-limited Gaussian-pulses are generated at the 92 GHz repetition rate with the 312 fs pulse duration, which is the shortest pulse from any directly electric-pumping semiconductor MLLs to our best knowledge. The lasing threshold injection current and external differential quantum efficiency are 17.2 mA and 38%, respectively. We have also investigated the working principles of the proposed QD MLLs.

Passivation of InGaAs surfaces and InGaAs/InP heterojunction bipolar transistors by sulfur treatment

The surface properties of InGaAs(100) after ex situ treatment with (NH4)2S solution were investigated by photoluminescence (PL) and high-energy resolution x-ray photoelectron spectroscopy. The As 3d, Ga 2p3/2, and In 3d5/2 core level studies show that the surface is free of native oxides and is terminated by S after treatment. A dramatic increase (∼40 times) in the PL efficiency was observed on undoped InGaAs(100) surfaces after sulfur passivation. This S treatment has also been applied to the passivation of the extrinsic base of InGaAs/InP heterojunction bipolar transistors (HBTs). The effectiveness of the sulfur passivation treatment was confirmed by the resulting devices which exhibited dc current gain values of up to 200 at very low collector currents (nA). Further, the sulfur passivated HBTs do not show any dependence on the perimeter-to-area (P/A) ratio of the emitter junction which is of interest for high frequency characteristics while maintaining high current gain.

InAs self-assembled quantum-dot lasers grown on (100) InP

Five stacked layers of InAs quantum dots (QDs) embedded in quaternary InGaAsP are grown on (100) InP substrate to form a laser diode. The QD ensemble has a density of 1.5×1010 cm−2 and emits light at ∼1.6 μm at 77 K. Lasing wavelength and threshold current density can be shifted by changing the cavity length of the laser diode and the latter reaches a value as low as 49 A/cm2 at 77 K for a gate size of 2000 μm×150 μm. Temperature dependence of the threshold current is observed implying the presence of thermionic emission increasing with temperature.

Resonant scattering and second-harmonic spectroscopy of planar photonic crystal microcavities

The resonant modes of two-dimensional planar photonic crystal microcavities patterned in a free-standing InP slab are probed in a novel fashion using a long working distance microscope objective to obtain cross-polarized resonant scattering and second-harmonic spectra. We show that these techniques can be used to do rapid effective assays of large arrays of microcavities that do not necessarily contain resonant light-emitting layers. The techniques are demonstrated using microcavities comprised of single missing-hole defects in hexagonal photonic crystal hosts formed with elliptically shaped holes. These cavities typically support two orthogonally polarized resonant modes, and the resonant scattering and harmonic spectra are well fitted using a coherent sum of Lorentzian functions. The well-defined coherence between the two resonant features is explained in terms of a microscopic harmonic oscillator model. The relative merits of these techniques are quantitatively compared with the more commonly used cavi...