R. Planel

Long-wavelength (≈15.5 μm) unipolar semiconductor laser in GaAs quantum wells

A unipolar semiconductor laser emitting in the mid-infrared spectral region is demonstrated. The laser scheme relies on a simple three-level system in GaAs/AlGaAs asymmetric coupled quantum wells. Population inversion between excited states is achieved by optical pumping of electrons from the ground state with a CO2 laser. Long-wavelength (≈15.5 μm) laser emission is demonstrated. The laser is operated in the pulsed regime up to a temperature of 110 K and with an output peak power ≈0.4 W at 77 K. Unipolar quantum well semiconductor lasers based on this principle are capable of covering the long wavelength mid-infrared spectral region above 12 μm.

Optical characterization of selectively intermixed GaAs/GaAlAs quantum wires by Ga+ masked implantation

Quantum wires were fabricated by selective intermixing of a GaAs/GaAlAs quantum well through masked Ga+ implantation and rapid thermal annealing. The evolution of the luminescence spectra of the wires with the width of the implantation masks, enabled us to characterize the lateral selectivity of our process as well as the degree of one‐dimensional confinement. The lateral extent of the intermixing was estimated at 20 nm giving rise to an important penetration of aluminum into the wires. From numerical simulations of the spatial distribution of implantation‐induced damage, it was concluded that some lateral diffusion of the defects occurred during annealing. However it has been possible to assess the confinement energies to be around 4 meV. The linewidth of the wires’ emission turned out to increase with decreasing mask size, indicating the presence of some fluctuations of the confining potential along the wires. The roughness of the lateral definition of the wires was evaluated at 20 nm, of the same order...

Giant Rabi splitting in a microcavity containing distributed quantum wells

We present a microcavity design where the overlap between quantum well excitons and the electromagnetic field is much larger than in previously reported structures: quantum wells are located not only in the central cavity layer but also in the Bragg mirrors. We obtain a Rabi splitting of 19 meV. Since the Rabi splitting is larger than the exciton binding energy, exciton excited states have to be taken into account in the reflectivity calculation to describe the spectral shape of the two polariton lines.

Investigation of mid-infrared intersubband stimulated gain under optical pumping in GaAs/AlGaAs quantum wells

We have investigated the mid-infrared intersubband stimulated emission under optical pumping in GaAs/AlGaAs coupled quantum wells. The quantum wells exhibit four levels bound in the conduction band. The energy between the ground and first excited subband is close to the optical phonon energy enabling population inversion. Intersubband stimulated gain between subbands E3 and E2 is observed around 14 μm wavelength by optically pumping the E1−E3 intersubband transition at 10 μm. The gain measurement is performed by time-resolved pump-probe experiments using a two-color picosecond free-electron laser. The dependence of the intersubband stimulated emission is analyzed as a function of the pump intensity, and the pump and probe wavelengths. We show that very large intersubband stimulated gain can be achieved at liquid nitrogen temperature in a 2 mm thick waveguide. The stimulated gain is resonant with the pump wavelength with a broadening ≈25 meV. The experimental results are explained with a nonperturbative ma...

Dynamical equilibrium between excitons and free carriers in quantum wells

Abstract We have investigated the exciton and free carrier populations dynamics in quantum wells by low temperature time-resolved photoluminescence spectroscopy. When the excitation energy is set above the free carrier quantum well gap, excitons are formed from random binding of electrons and holes. On the basis of this bimolecular formation process, we show that a 2D mass action law, describing the coexistence of free carriers and excitons, explains the time evolution of the exciton photoluminescence linewidth. This demonstrates the existence of a thermodynamic equilibrium between excitons and free carriers at each time delay following the excitation. The consequence is a very short exciton formation time (≲ 10 ps) in the density range investigated (10 9 – 10 10 cm −2 ), implying a bimolecular formation coefficient γ higher than 14 cm 2 s −1 .

Intersubband absorption of GaAs/AlGaAs quantum wells in MBE grown mid-infrared slab waveguides

A structure to enhance the absorbance due to intersubband transitions in GaAs/AlGaAs quantum wells is discussed. Mid-infrared slab waveguides including 30 quantum wells were grown using molecular-beam epitaxy (MBE). Photoluminescence experiments revealed an excellent uniformity of the samples. Absorption measurements over the whole 9-13.4- mu m spectral range were performed for the first time using the combination of CO/sub 2/ and NH/sub 3/ lasers. Effective absorbance due to intersubband transitions as high as 14 dB were measured for 3-mm-long waveguides. The waveguide structure is expected to be a good candidate for optoelectronic devices in the 10- mu m region.<<ETX>>

Prospects for further threshold reduction in bistable microresonators

We discuss recent results showing the possibility of reducing further the switching threshold of GaAs quantum well bistable microresonators, taking into account the materials optical properties, both in the nonlinear medium and in the Bragg reflectors, as well as their departure from ideality, due to fabrication constraints. Threshold minimization in the presence of nonlinear index saturation is discussed, and the implications of this optimization on the geometrical design of bistable cavities are described. We present a new topographic technique for the assessment of the cavity resonance at various sample points. This technique is found to be very valuable for the characterization of the optical quality of epitaxial wafers grown for vertical cavity devices. Finally, we present some new approaches for the restriction of the lateral size of the device, and discuss the ultimate threshold reductions that can be expected from these considerations.

Nonguiding half‐wave semiconductor microcavities displaying the exciton‐photon mode splitting

We present a monolithic epitaxially grown λ/2 semiconductor microcavity that contains two InGaAs quantum wells in its AlAs spacer. This particular design displays greatly reduced coupling of the quantum well emission to the in‐plane guided modes. The reflection and luminescence spectra of this structure at 77‐K display an exciton‐photon splitting of 4 meV with as few as two quantum wells, indicating the possibility of a strong coupling of the spontaneous emission of the excitons to the vertical cavity modes. This cavity design should thus be well suited for improving the coupling efficiency of the spontaneous emission to the lasing mode in vertically emitting structures.

Mixing of GaAs/(Ga,Al)As interfaces by Ga+ implantation

The intermixing of GaAs/(Ga,Al)As heterointerfaces by Ga+ implantation and annealing has been investigated. The damage accumulation in a GaAs/AlAs superlattice turned out to be less rapid than in a GaAs/GaAlAs quantum‐well structure. Low‐temperature photoluminescence (PL) spectroscopy of a GaAs/AlAs superlattice could be performed for doses as high as 1 × 1016 ions/cm2. The photoluminescence spectra exhibited several emission bands on the high energy side. The number and energy of these blue shifted peaks were found to depend on the implanted dose and as confirmed by secondary ion mass spectrometry, they could be interpreted as the emission of several quantum wells of the superlattice, disordered with different mixing rates. Two regimes were evidenced; while the depth extension of the disordering has been directly related to the post‐implantation defects distribution in the high dose regime, some diffusion of these defects during annealing has been pointed out in the low dose regime. Cross‐sectional trans...

Longitudinal electric field effects on GaAs‐AlAs type‐II superlattices

We have studied photoluminescence of direct‐gap and ‘‘indirect‐gap’’ GaAs‐AlAs superlattices under electric field perpendicular to the layers. In the former case, the quantum confined Stark effect is observed as expected. In the latter case, reverse effects are found, providing evidence that X‐like electron states are confined in the AlAs slabs.