A. Passaseo

Capture and thermal re-emission of carriers in long-wavelength InGaAs/GaAs quantum dots

We investigate the ultrafast carrier dynamics in metalorganic chemical vapor deposition-grown InGaAs/GaAs quantum dots emitting at 1.3 μm. Time-resolved photoluminescence upconversion measurements show that the carriers photoexcited in the barriers relax to the quantum-dot ground state within a few picoseconds. At low temperatures and high carrier densities, the relaxation dynamics is dominated by carrier–carrier scattering. In contrast, at room temperature, the dominant relaxation process for electrons is scattering between quantum-dot levels via multiple longitudinal optical (LO)-phonon emission. The reverse process, i.e., multiple LO-phonon absorption, governs the thermal re-emission of electrons from the quantum-dot ground state.

Wavelength control from 1.25 to 1.4 μm in InxGa1−xAs quantum dot structures grown by metal organic chemical vapor deposition

This letter reports on the realization of long-wavelength InGaAs quantum dots (QDs) fabricated by metal organic chemical vapor deposition. By controlling the In incorporation in the QD layers and/or in the barrier embedding the QDs, we are able to tune the wavelength emission continuously from 1.25 to 1.4 μm at room temperature. Efficient stacking of dots emitting at 1.3 μm is also demonstrated.

Fabrication of 3D metallic photonic crystals by X-ray lithography

Photonic crystals (3D) represent one of the most important building blocks towards the achievement of a full optics communication technology. So far the largest interest has been attracted by two-dimensional photonic crystals because they are potentially more amenable to fabrication and much closer to application. Straightforward application of the photonic band gap concept is generally thought to require three-dimensional (3D) photonic crystals that, however, represent a challenge from a fabrication point of view. Recent works have shown that 3D metallic PC can be fabricated and that they can be advantageous in the low frequency region where the metals become almost completely reflectors. In this work we show the possibility to fabricate 3D PC structures by X-ray lithography. Gold and nickel 3D photonic crystals with threefold (Yablonovite) and fourfold rotation symmetry have been fabricated with a lattice parameter ranging from 1 µm down to 300 nm. The total thickness of the 3D PC is of the order of 10 µm, a value which should allow to achieve a complete bulk behavior. This is supported by variable angle reflectance measurements, which have shown clear indications for true 3D dimensionality of our samples.

3-D FEM Modeling and Fabrication of Circular Photonic Crystal Microcavity

In this paper, we study an unconventional kind of quasi-three-dimensional (3-D) photonic crystal (PhC) with circular lattice pattern: it consists of air holes in a GaAs material (n=3.408) along circular concentric lines. This particular PhC geometry has peculiar behavior if compared with the traditional square and triangular lattices, but it is difficult to model by using conventional numerical approaches such as wave expansion method. The resonance and the radiation aspects are analyzed by the 3-D finite-element method (FEM). The model, based on a scattering matrix approach, considers the cavity resonance frequency and evaluates the input-output relationship by enclosing the photonic crystal slab (PhCS) in a black box in order to define the responses at different input-output ports. The scattering matrix method gives important information about the frequency responses of the passive 3-D crystal in the 3-D spatial domain. A high sensitivity of the scattering parameters to the variation of the geometrical imperfection is also observed. The model is completed by the quality factor (Q-factor) estimation. We fabricated the designed circular photonic crystal over a slab membrane waveguide embedding InAs/GaAs quantum dots emitting around 1.28 mum. Good agreement between numerical and experimental results was found, thus validating the 3-D FEM full-wave investigation.

Structural study of InGaAs/GaAs quantum dots grown by metalorganic chemical vapor deposition for optoelectronic applications at 1.3 μm

We have studied the influence of difference growth conditions on the two-dimensional to three-dimensional growth mode transition for a specific class of InGaAs/GaAs quantum dots (QDs) optimized for applications to optical devices operating around 1.3 μm (In content x≈0.5). The dots are grown by low-pressure metalorganic chemical vapor deposition on GaAs substrates. We demonstrate that the critical layer thickness corresponding to optimized single-QD layer structures (i.e., with reduced wetting layer thickness and high uniformity) can be controlled by kinetic effects. The optimized growth conditions allow us to grow six-layers stacked QD structures as active material for the fabrication of a light emitting devices operating around 1.3 μm at room temperature.

Long wavelength emission in InxGa1−xAs quantum dot structures grown in a GaAs barrier by metalorganic chemical vapor deposition

We demonstrate a method to obtain room temperature long wavelength emission from InGaAs quantum dots (QDs) growth directly into a binary GaAs matrix. The wavelength is tuned from 1.26 up to 1.33 μm by varying the V/III ratio during growth of the GaAs cap layer, without using a seeding layer or InGaAs wells. Strong improvement in terms of line-shape narrowing and efficiency is obtained. In addition to the shift in wavelength we observe an impressive reduction of temperature dependent quenching of the emission efficiency, which decreases only by a factor of 3 between cryogenic temperatures and room temperature, very good for QD structures emitting at 1.3 μm. Photoluminescence spectroscopy and theoretical modeling were combined for interpretation of the results.

High-modal gain 1300-nm In(Ga)As-GaAs quantum-dot lasers

A semiconductor laser containing seven InAs-InGaAs stacked quantum-dot (QD) layers was grown by molecular beam epitaxy. Shallow mesa ridge-waveguide lasers with stripe width of 120 mum were fabricated and tested. A high modal gain of 41 cm-1 was obtained at room temperature corresponding to a modal gain of ~6 cm-1 per QD layer, which is very promising to enable the realization of 1.3-mum ultrashort cavity devices such as vertical-cavity surface-emitting lasers. Ground state laser action was achieved for a 360-mum-cavity length with as-cleaved facets. The transparency current density per QD layer and internal quantum efficiency were 13 A/cm2 and 67%, respectively

Structural and optical studies of InxGa1−xAs/GaAs multiple quantum wells

Strained multiple quantum wells of InxGa1−xAs/GaAs were grown by low pressure metalorganic chemical vapor deposition (LP‐MOCVD) and characterized by secondary ion mass spectrometry, x‐ray diffraction, and optical spectroscopy. The structural analysis demonstrates the excellent control of the interface morphology and composition achieved by MOCVD growth. Temperature dependent optical absorption, photoluminescence, and magnetotransmission were used to evaluate the well‐width dependence of the major excitonic properties. The samples show sharp excitonic resonances with distinct excited states evolving into Landau‐type excited states in high magnetic field. The well‐width dependence of the excitonic eigenstates and of the exciton binding energy as well reproduced by envelope function and variational calculations, also in the presence of external electric field. Finally, nonlinear electro‐optic modulation induced by the quantum confined Stark effect is demonstrated in a Schottky diode with extremely low switch...

Dependence of the emission wavelength on the internal electric field in quantum-dot laser structures grown by metal–organic chemical-vapor deposition

We show that the combination of different electric fields in In0.5Ga0.5As/GaAs quantum-dot electroluminescent devices dramatically blueshifts the emission wavelength even though the photoluminescence occurs at the expected value of 1.3 μm at room temperature. Systematic photoluminescence (PL), electroluminescence (EL), and photocurrent measurements demonstrate that the electric field associated with the built-in dipole in the dots, directed from the base of the dots to their apex, and the device junction field lead to the depletion of the ground state. As a consequence, structures grown on n-type GaAs substrates exhibit electroluminescence only from the excited states (whereas the photoluminescence comes from the ground level). Instead, by growing the same device structure on p-type GaAs substrates, i.e., by reversing the direction of the built-in electric field of the device, the effect of the permanent dipole is strongly reduced, thus allowing us to obtain EL emission at the designed wavelength of 1.3 μ...

Structural and optical properties of vertically stacked triple InAs dot-in-well structure

The authors report a detailed investigation of the structural and optical properties of vertically stacked InAs quantum dots embedded in an (In,Ga)As quantum well by means of transmission electron microscopy and time resolved photoluminescence based on the upconversion technique. By comparing the optical features of quantum dot samples of different barrier thicknesses (nominal values between 5 and 65nm), they have found evidence for electronic coupling among the quantum dots, featured by an increase of radiative lifetime and a relatively blueshifted emission peak for the thinnest spacer layer sample.