Christophe Levallois

Impact of the capping layers on lateral confinement in InAs∕InP quantum dots for 1.55μm laser applications studied by magnetophotoluminescence

We have used magnetophotoluminescence to study the impact of different capping layer material combinations (InP, GaInAsP quaternary alloy, or both InP and quaternary alloy) on lateral confinement in InAs∕InP quantum dots (QDs) grown on (311)B orientated substrates. Exciton effective masses, Bohr radii, and binding energies are measured for these samples. Conclusions regarding the strength of the lateral confinement in the different samples are supported by photoluminescence at high excitation power. Contrary to theoretical predictions, InAs QDs in quaternary alloy are found to have better confinement properties than InAs∕InP QDs. This is attributed to a lack of lateral intermixing with the quaternary alloy, which is present when InP is used to (partially) cap the dots. The implications of the results for reducing the temperature sensitivity of QD lasers are discussed.

Time-resolved pump probe of 1.55μm InAs∕InP quantum dots under high resonant excitation

We have performed time-resolved resonant pump-probe experiment to study the dynamic response of InAs∕InP quantum dot transitions. A 72-stacked InAs∕InP quantum dot layer sample is grown on (311)B substrate. Photoluminescence at high excitation power reveals ground and excited transitions. Carrier radiative lifetimes and differential transmission are determined under strong excitation powers. The variation of measured carrier radiative lifetimes with increasing excitation powers is attributed to the exciton and biexciton lifetimes difference. The implications of such a difference on differential transmission are discussed, and finally exciton and biexciton lifetimes are measured to be about 1720 and 530ps, respectively.

A direct comparison of single-walled carbon nanotubes and quantum-wells based subpicosecond saturable absorbers for all optical signal regeneration at 1.55 μm

Subpicosecond optical transmission experiments are used to compare saturable absorber (SA) based on bundled single-walled carbon nanotubes (SWNT) and iron-doped InGaAs/InP epitaxial multiple quantum wells (MQW) at 1.55 μm telecom wavelength. The SA key parameters (contrast ratio, saturation fluence, and recovery time) relevant for high speed all optical signal regeneration (AOSR) are extracted from the normalized differential transmission (NDT). Although both SA exhibit good contrast ratios, SWNT show a full signal recovery as well as a much faster response time than MQW. This original work on SA shows that SWNT are excellent candidates for future low cost AOSR.

Tunable semiconductor vertical-cavity surface-emitting laser with an intracavity liquid crystal layer

A tunable vertical-cavity surface-emitting laser is fabricated where tunability is achieved with an intracavity layer of nematic liquid crystal and gain is provided by a semiconductor quantum well structure. The anisotropic liquid crystal layer enables a continuously tunable single-mode emission along the extraordinary axis of the layer. Polarization control is achieved when the layer thickness is such that the ordinary modes are out of the spectral gain region. Laser emission in the 1.5μm telecom wavelength range is demonstrated under optical pumping with a tuning range of more than 30 nm for an applied voltage of less than 3 V.

Polarization control of 1.6 μm vertical-cavity surface-emitting lasers using InAs quantum dashes on InP(001)

The authors report the demonstration of a polarization-controlled vertical-cavity surface-emitting laser (VCSEL), emitting at the telecommunication wavelength. VCSELs are based on an active medium constituted of well elongated InAs quantum dashes (QDHs) nanostructures grown on conventional (001) oriented InP substrate. QDHs present important optical polarization anisotropies according to the [11¯0] crystallographic orientation. Inserted into a VCSEL microcavity, QDH VCSELs show a continuous wave laser operation at 1.6 μm, at room temperature, with a reduced 13 kW/cm2 optical excitation density threshold. The QDH VCSEL output laser polarization is fixed along the same [11¯0] direction. Power and temperature dependant measurements do not show any polarization instabilities and switching on all QDH VCSELs. A polarization extinction ratio as high as 30 dB is deduced from experiments.

Long-wavelength vertical-cavity surface-emitting laser using an electro-optic index modulator with 10nm tuning range

We demonstrate an original approach to achieving a tunable 1.55μm vertical-cavity surface-emitting laser. The tunability is based on an electro-optic index modulator using nanosized droplets of liquid crystal as a phase layer. Such an approach can produce a robust and a low-cost device. A 10nm tuning range with less than 170V applied voltage has been demonstrated. The device is formed by a conventional InP-based active region with an epitaxial and a dielectric Bragg mirror. This optically pumped device exhibits an excellent side-mode suppression ratio of higher than 20dB over the whole spectral range.

Monolithic Integration of Diluted-Nitride III–V-N Compounds on Silicon Substrates: Toward the III–V/Si Concentrated Photovoltaics

Abstract GaAsPN semiconductors are promising material for the development of high-efficiency tandem solar cells on silicon substrates. GaAsPN diluted-nitride alloy is studied as the top-junction material due to its perfect lattice matching with the Si substrate and its ideal bandgap energy allowing a perfect current matching with the Si bottom cell. The GaP/Si interface is also studied in order to obtain defect-free GaP/Si pseudo-substrates suitable for the subsequent GaAsPN top junctions growth. Result shows that a double-step growth procedure suppresses most of the microtwins and a bi-stepped Si buffer can be grown, suitable to reduce the anti-phase domains density. We also review our recent progress in materials development of the GaAsPN alloy and our recent studies of all the different building blocks toward the development of a PIN solar cell. GaAsPN alloy with energy bandgap around 1.8 eV, lattice matched with the Si substrate, has been achieved. This alloy displays efficient photoluminescence at room temperature and good light absorption. An early-stage GaAsPN PIN solar cell prototype has been grown on a GaP(001) substrate. The external quantum efficiency and the I–V curve show that carriers have been extracted from the GaAsPN alloy absorber, with an open-circuit voltage above 1 eV, however a low short-circuit current density obtained suggests that GaAsPN structural properties need further optimization. Considering all the pathways for improvement, the 2.25% efficiency and IQE around 35% obtained under AM1.5G is however promising, therefore validating our approach for obtaining a lattice-matched dual-junction solar cell on silicon substrate.

Enhancement of the polarization stability of a 1.55 µm emitting vertical-cavity surface-emitting laser under modulation using quantum dashes

Polarization controlled quantum dashes (QDHs) Vertical Cavity Surface Emitting Lasers (VCSELs) emitting at 1.6 µm grown on InP(001) are investigated and compared with a quantum well (QW) similar VCSEL. Polarization stability of optically-pumped VCSELs under a low frequency modulation is investigated. While major fluctuations of the polarization-resolved intensity are observed on QW-based structures, enhanced polarization stability is reached on QDH-based ones. Statistical measurements over a large number of pulses show an extremely low variation in QDH VCSEL polarized output intensity, related to the intrinsic polarization control. This makes QDH VCSEL ideal candidate to improve telecommunication networks laser performances.

Polymer tunable microlens arrays suitable for VCSEL beam control

We report on a simple method for the collective fabrication of polymer tunable microlens arrays suitable for VCSEL active beam shaping. Its principle is based on a SU-8 suspended membrane, surmounted by a polymer microlens, and thermally actuated to achieve a vertical displacement of lens plane. SU-8 resist presents many advantages for MOEMS fabrication, as this resist allows for high aspect ratio patterns and high transparency. In addition, it exhibits a thermal expansion coefficient suitable for thermal actuation. Moreover, this kind of polymer MOEMS can be fabricated on VCSEL arrays with footprints as low as 500x500μm2 enabling a rapid, low cost and wafer-scale integration technology. We have successfully fabricated this MOEMS on a glass substrate by means of a SU-8 double exposure method and we report on a vertical displacement of 8μm under an applied power of 43mW (3V). A good agreement with the theoretical thermo-mechanical behavior is found. Moreover, optical measurements of microlens focus displacement under actuation are presented. We evaluate analytically the focus properties of the system under coherent laser illumination, using the classical ABCD matrix formalism of Gaussian transformation optics. The same approach enables one to assess its tolerance to opto-geometrical parameters, such as refractive index or dioptre curvature. As a wide range of initial gaps between the membrane and the substrate can be chosen, this MOEMS technology opens new insights for dynamic control of VCSEL beam or for tunable VCSELs fabrication.

Multijunction photovoltavics: integrating III–V semiconductor heterostructures on silicon

Gallium arsenide phosphide nitride shows promise for developing highefficiency tandem solar cells on low-cost silicon substrates