Arturo Chavez-Pirson

SPIN RELAXATION OF EXCITONS IN ZERO-DIMENSIONAL INGAAS QUANTUM DISKS

We report the observation of spin relaxation of excitons in zero-dimensional semiconductor nanostructures. The spin relaxation is measured in InGaAs quantum disks by using a polarization dependent time-resolved photoluminescence method. The spin relaxation time in a zero-dimensional quantum disk is as long as 0.9 ns at 4 K, which is almost twice as long as the radiative recombination lifetime and is considerably longer than that in quantum wells. The temperature dependence of the spin relaxation time suggests the importance of exciton–acoustic phonon interaction.

NEAR-FIELD OPTICAL SPECTROSCOPY AND IMAGING OF SINGLE INGAAS/ALGAAS QUANTUM DOTS

We use near-field optical probing at low temperatures (T=5 K) to image and examine the linear and nonlinear luminescence properties of single InGaAs/AlGaAs quantum dots grown on (311)B oriented GaAs substrates. The high spatial resolution of near-field “nanoprobing,” which is typically 200 nm or less, makes the observation of single dots at different locations on the sample possible, even though the spatial density of quantum dots is on the order of 100/μm2. We observe narrow excitonic emission lines at low excitation powers and, with increasing excitation, we observe biexcitonic emission strongly shifted (3 meV) to the low-energy side of the exciton emission.

Polarization properties of a vertical cavity surface emitting laser using a fractional layer superlattice gain medium

We investigate the polarization properties of a vertical cavity surface emitting laser that uses an (Al0.5Ga0.5As)1/2(GaAs)1/2 fractional‐layer superlattice (FLS) as an anisotropic gain medium. The anisotropy in the gain enables us to both control and switch the polarization state of the optically pumped lasing output. We obtain room‐temperature lasing for wavelengths from 690 to 720 nm. The output is linearly polarized and the polarization direction is fixed, either parallel or perpendicular to the FLS layers. By tuning the cavity resonance wavelength, we demonstrate high contrast switching between two orthogonal linear polarization states in the FLS surface emitting laser.

DIRECT OBSERVATION OF OPTICAL ANISOTROPY IN A GAAS/ALAS QUANTUM WELL WIRE ARRAY

Using reflectance difference spectroscopy and transmittance difference spectroscopy, optical anisotropy in a quantum well wire array is directly observed at room temperature. The quantum wire array is a fractional‐layer superlattice grown by metalorganic chemical vapor deposition on a vicinal (001)GaAs substrate. The size for quantum confinement of the wires is nominally 4×4 nm. Polarization dependence of optical absorption and refractive index is clearly observed between the directions parallel and perpendicular to the wires.

Quantum wire microcavity laser made from GaAs fractional layer superlattices

We report the first demonstration of lasing action in a quantum wire microcavity semiconductor laser made from an array of (AlAs)1/4(GaAs)3/4 fractional‐layer superlattice (FLS) quantum wires. The FLS growth method produces uniform, densely packed, damage‐free arrays of nanometer‐size quantum wires which are integrated into an optical microcavity that is the size of the wavelength of the light. We obtain room temperature optically pumped lasing for wavelengths from 670 to 690 nm. The lasing output is linearly polarized parallel to the quantum wires, reflecting the higher optical gain for polarization direction parallel to the wires. The combination of a semiconductor quantum wire active material with an optical microcavity offers the possibility of ultimately compact, highly efficient laser sources.

Effects of Dimensionality on Radiative Recombination Lifetime of Excitons in Thin Quantum Boxes of Intermediate Regime between Zero and Two Dimensions

We report on effects of dimensionality on radiative recombination lifetime in thin quantum boxes of intermediate regime between 0D and 2D. The temperature dependence of the recombination lifetime is calculated using a theoretical analysis that rigorously treats the electron-hole Coulomb interaction. We show how the dependence evolves from 2D to 0D with a decrease in the lateral width of the box. We also examine the effects of exciton localization, which arises from structural defects in the boxes, on the radiative recombination lifetime. These theoretical results are compared with experimental data obtained from InGaAs quantum disks on a (311)B GaAs substrate. Good agreement between theoretical results and the experimental data is obtained.

Polarization-dependent optical nonlinearities in fractional-layer superlattices

We measure the room‐temperature polarization‐dependent nonlinear absorption and refractive‐index spectra of a (Al0.5Ga0.5As)1/2(GaAs)1/2 fractional‐layer superlattice (FLS) structure grown by metalorganic chemical vapor deposition. The anisotropic nonlinear effects between the directions parallel and perpendicular to the superlattice give rise to a nonlinear optical birefringence in the plane of the growth surface. From our measurements using a femtosecond optical pulse, we derive the magnitude and spectral shape of the nonlinear optical birefringence. We describe the basis of an all‐optical polarization rotation switch using the FLS structure.

Excitonic optical properties in fractional‐layer‐superlattice wire structures

We report on the evolution of the excitonic optical features from two dimensions (2D) to one dimension (1D) in GaAs/AlAs fractional‐layer‐superlattice (FLS) wire structures. We demonstrate for the first time how the excitonic optical properties continuously evolve from 2D to 1D by introducing the FLS lateral compositional modulation in an AlGaAs quantum well. We also present a theory numerically analyzing the optical absorption properties in the FLS wires. Present theory accurately reproduces the experimental results using the AlAs/GaAs FLS wires and clarifies the main optical features associated with excitons in the intermediate quantum‐confinement regime between 1D and 2D.

RAPID RELAXATION OF HOT CARRIERS IN GAAS FRACTIONAL-LAYER SUPERLATTICE QUANTUM WIRES

We measure the hot carrier relaxation in two GaAs quantum wire arrays made from fractional‐layer superlattices (FLS). Using femtosecond optical pump–probe spectroscopy, we measure the differential transmission spectra for various pump–probe time delays from which we determine the carrier thermalization times. Although the two FLS structures have different degrees of one‐dimensional confinement at the band edge, we observe rapid (<2 ps) and efficient carrier relaxation in both cases with no sign of the inhibited relaxation predicted for ideal one‐dimensional structures. We believe that FLS quantum wire structures avoid relaxation bottlenecks because the shape of the FLS confinement potential produces high energy states which are two dimensional in character and which facilitate rapid energy relaxation.

Optical properties in fractional‐layer‐superlattice quantum wires calculated by multi‐band effective mass theory

We present the results of theoretical studies numerically analyzing the optical absorption and gain properties in AlGaAs fractional‐layer‐superlattice (FLS) quantum wires. With our calculation method we can treat any asymmetrical FLS wire with arbitrary dimensionality from 2D to 1D and to calculate not only absorption spectra but also gain properties. Our theory accurately reproduces the optical absorption anisotropy, experimentally evaluated for an AlGaAs FLS quantum wire. It is numerically shown how optical features evolve as the quantum confinement changes from 2D to 1D by varying the FLS lateral modulation. The small modulation of Al content in the AlGaAs FLS layer is found to change the optical properties markedly and improve the gain characteristics largely.