Julien Nagle

GaAs/AlxGa1−xAs quantum cascade lasers

A unipolar injection quantum cascade (QC) laser grown in an AlGaAs/GaAs material system by molecular beam epitaxy, is reported. The active material is a 30 period sequence of injectors/active regions made from Al0.33Ga0.67As/GaAs-coupled quantum wells. For this device a special waveguide design, which complies with a GaAs heavily doped substrate and very short Al0.90Ga0.10As cladding layers, has been optimized. At a heat-sink temperature of 77 K, the laser emission wavelength is 9.4 μm with peak optical power exceeding 70 mW and the threshold current density is 7.3 kA/cm2. The maximum operating temperature is 140 K. This work experimentally demonstrates the general validity of QC laser principles by showing laser action in a heterostructure material different from the one used until now.

Low-loss Al-free waveguides for unipolar semiconductor lasers

A promising waveguide design for midinfrared (λ=5–20 μm) unipolar semiconductor lasers is proposed and demonstrated in (Al)GaAs quantum cascade structures. In the latter, the active region is embedded between two GaAs layers, with an appropriate doping profile which allows optical confinement, with low waveguide losses and optimal heat dissipation. Low internal cavity losses of 20 cm−1 have been measured using different techniques for lasers with emission wavelength at ∼9 μm. At 77 K, these devices have peak output power in excess of 550 mW and threshold current of 4.7 kA/cm2.

Quantum engineering of optical nonlinearities

Second-order optical nonlinearities in materials are of paramount importance for optical wavelength conversion techniques, which are the basis of new high-resolution spectroscopic tools. Semiconductor technology now makes it possible to design and fabricate artificially asymmetric quantum structures in which optical nonlinearities can be calculated and optimized from first principles. Extremely large second-order susceptibilities can be obtained in these asymmetric quantum wells. Moreover, properties such as double resonance enhancement or electric field control will open the way to new devices, such as fully solid-state optical parametric oscillators.

Observation of nonlinear optical rectification at 10.6 μm in compositionally asymmetrical AlGaAs multiquantum wells

We report the first experimental evidence of a nonlinear optical effect due to intersubband transitions in compositionally asymmetrical multiquantum wells. The effect is detected as an optical rectification signal appearing at the structure terminals when irradiated by a continuous 10.6 μm CO2 laser. The net electro‐optical coefficient of the structure is found to be 7.2 nm/V which is more than three orders of magnitude higher than for bulk GaAs. The results are in good agreement with theoretical predictions.

AlAs/GaAs quantum cascade lasers based on large direct conduction band discontinuity

The design and operation of quantum cascade (QC) lasers using AlAs/GaAs coupled quantum wells are reported. In this material system, the conduction band offset at the Γ point (∼1 eV) is much higher than in previously reported QC lasers. The use of high band discontinuity allows us to increase the energy separation among the subbands, thus suppressing thermally activated processes which limit device performance at high temperature. The measured thermal characteristics of these promising devices are strongly improved from previously reported GaAs-based QC lasers: The temperature dependence of the threshold current density is described by a very large T0 (320 K) and the laser slope efficiency does not vary for increasing heat sink temperatures. The maximum operating temperature is 230 K, limited by negative differential resistance effects that occur when the applied bias reaches 8 V.

Huge birefringence in selectively oxidized GaAs/AlAs optical waveguides

Selective wet oxidation of AlAs was used to obtain huge birefringence in GaAs/AlAs optical waveguides. A single polarization waveguide was obtained by oxidizing an AlAs layer buried in a GaAs guiding layer. The TM mode is below cutoff due to the high index contrast between the layers. Applications to phase matching in nonlinear optical conversion are envisaged.

Emission and capture of electrons in multiquantum-well structures

The mechanisms of unipolar emission and capture of electrons are studied in multiquantum-well structures in relation with the quantum-well infrared photoconductors (QWIPs). We clarify the roles played by the physical parameters which appear in the different QWIP photoresponse models, i.e., the photoconductive and the photoemissive ones. We then describe the experimental procedures which allow us to independently determine these different parameters: deep level optical spectroscopy for the electron emission probability, impedance spectroscopy for the quantum-well capture velocity and thermal emission time, as well as the infrared photoconductive gain for the unipolar electron capture time. The measured dependence of these parameters on photon energy and electric field sheds light on the microscopic physical phenomena which are involved in quantum-well infrared photodetection. Theoretical results on optical phonon mediated transitions in an applied electric field from barrier to well states show good agreement with experiment at low fields but less dependence on the field. Finally, this theoretical approach allows us to connect the different parameters and solve the apparent discrepancy between the QWIP photoresponse models. >

Improved temperature performance of Al0.33Ga0.67As/GaAs quantum-cascade lasers with emission wavelength at λ≈11 μm

The pulsed operation of a GaAs/AlGaAs quantum-cascade laser is reported up to 258 K. These devices emit at 11.3 μm and are based on a plasmon-confinement waveguide. To optimize the material gain, the active region is designed to diminish electron escape to continuum states. Gain and threshold measurement show evidence of better carrier confinement and improved thermal behavior compared to λ≈9 μm GaAs quantum-cascade lasers. The maximum peak-collected power at 77 K is 520 mW per facet and still 27 mW at 258 K. The temperature dependence of the threshold current density is characterized by a T0=128 K.

Design strategies for GaAs-based unipolar lasers: Optimum injector-active region coupling via resonant tunneling

Electron injection into the upper state of the laser transition in quantum cascade lasers is studied by investigating the electrical and optical characteristics of a set of electroluminescent devices. These devices exploit the active region of an (Al)GaAs laser based on a diagonal–anticrossed transition scheme with emission wavelength at 9.5 μm, and are identical except for the injection barrier thickness which varies from 3.9 up to 8.0 nm. We find that for thin barriers electron wave functions in the injector are directly coupled with those of the continuum. This leads to a parallel current path which strongly reduces the injection efficiency of electrons into the active region. The current leak is suppressed at low temperatures for samples with the thickest barriers, but it is still observable at high temperatures when electrons are thermally activated from the injector miniband into the continuum.

GaAs/Al/sub x/Ga/sub 1-x/As quantum cascade lasers

Summary form only given. By fabricating quantum cascade (QC) lasers in AlGaAs-GaAs we demonstrate that the fundamental concepts and design criteria can be truly extended in material systems, whilst still preserving the same basic characteristics in terms of threshold currents and output power.