Kale J. Franz

High-power laterally coupled distributed-feedback GaSb-based diode lasers at 2 μm wavelength

We report on GaSb-based laterally coupled distributed-feedback (DFB) diode lasers designed to operate at wavelengths near 2.05 μm. Second-order Bragg gratings were etched alongside narrow ridge waveguides to enable single-mode DFB operation in 2-mm-long laser diodes. At a heat-sink temperature of 10 °C, the lasers emit more than 40 mW continuous-wave in a single longitudinal mode, while increasing the current beyond 300 mA results in multimode operation due to spectral shifting of the laser gain with respect to the peak grating reflectivity. At −10 °C, we observe DFB operation at higher current, with single-facet emission exceeding 80 mW.

Midinfrared semiconductor optical metamaterials

We report on a novel class of semiconductor metamaterials that employ a strongly anisotropic dielectric function to achieve negative refraction in the midinfrared region of the spectrum, ∼8.5–13 μm. We present two types of metamaterials, layered highly doped/undoped heterostructures and quantum well superlattices that are highly anisotropic. Contrary to other optical metamaterials these heterostructure systems are optically thick (up to 20 μm thick), planar, and require no additional fabrication steps beyond the initial growth. Using transmission and reflection measurements and modeling of the highly doped heterostructures, we demonstrate that these materials exhibit negative refraction. For the highly doped quantum well superlattices, we demonstrate anomalous reflection due to the strong anisotropy of the material but a determination of the sign of refraction is still difficult. This new class of semiconductor metamaterials has great potential for waveguiding and imaging applications in the long-wave inf...

ZnCdSe/ZnCdMgSe quantum cascade electroluminescence

This letter reports electroluminescence emission from a ZnCdSe∕ZnCdMgSe quantum cascade (QC) structure. With a two-well QC active region design, the II-VI heterostructure was grown lattice matched on an InP substrate by molecular beam epitaxy. Deep etched mesas were electrically pumped at current densities up to 10kA∕cm2, producing optical emission centered near 4.8μm, in good agreement with the structure design. The light is predominantly TM polarized, confirming its intersubband origin. Electroluminescence was observed from 78to300K.

Voltage Tunability of Quantum Cascade Lasers

The voltage tunability of three types of quantum cascade laser designs is investigated. The tuning coefficients and tuning ranges of electroluminescence and laser emission from all designs are measured and compared with the calculated results. A reduced tunability was observed in all lasers above threshold. This is attributed to the decrease of resistance across the laser active region (AR) as the photon density increases. A resumed tunability high above threshold occurs in all lasers with anticrossed injector ground and upper laser states. Lasers based on the anticrossed diagonal transition are tunable above threshold, with a tuning range of about 30 cm-1 (~3% of the laser emission wavenumber), i.e., a tuning rate of 750 cm-1 V-1middotperiod-1 of the AR and the injector.

Room temperature and narrow intersubband electroluminescence from ZnCdSe/ZnCdMgSe quantum cascade laser structures

We report ZnCdSe/ZnCdMgSe Quantum Cascade structures with “two-phonon” and “bound-to-continuum” active region designs. The electroluminescence shows more than 3 times higher luminescence efficiency and 40% narrower linewidth (<30 meV) than previous reports. The measured turn-on voltage matches closely the calculated value, indicating the improved electron transport characteristics in these structures. A waveguide design suitable for mode confinement in this material system is also presented, which resulted in a structure with a single narrow electroluminescence peak for all temperatures from 80 to 300 K.

Quantum cascade lasers with voltage defect of less than one longitudinal optical phonon energy

Efficient use of applied voltage in quantum cascade (QC) lasers is a critical factor in achieving high wall-plug efficiency and low compliance voltage. We demonstrate a QC laser emitting at 4.2 μm featuring a low voltage defect and short injector with only four quantum wells. Devices with a voltage defect of 20 meV, well below the energy of the longitudinal optical phonons, and a voltage efficiency of 91%, a record value for QC lasers, are reported for pulsed operation at 180 K. Voltage efficiencies of greater than 80% are exhibited at room temperature. Overall performance showed wall-plug efficiencies ranging from 21% at cryogenic temperatures to 5.3% at room temperature.

Evidence of cascaded emission in a dual-wavelength quantum cascade laser

This letter reports on a quantum cascade laser that exhibits simultaneous dual-wavelength emission from two consecutive optical transitions in each active region. These “cascaded” transitions—a second-excited state to first-excited state and a first-excited state to ground state—yield light at ∼9.5 and ∼8.2μm, respectively, in good agreement with simulations. The two lasing wavelengths have similar thresholds at the leading edge of a current pulse.

A widely voltage-tunable quantum cascade laser based on “two-step” coupling

A novel quantum cascade laser design with “two-step” coupling between the injector and the upper laser state is demonstrated to achieve a widely voltage-tunable laser spectrum. Electroluminescence from this design can be tuned over a range of more than 200 cm−1 with a tuning coefficient of ∼700 cm−1/V per stage. Lasers based on this design provide a tuning range of ∼100 cm−1 (8.3–9 μm) above threshold with a tuning coefficient of ∼900 cm−1/V per stage at a constant temperature of 295 K.

Short Injector Quantum Cascade Lasers

We report our study on the effects of shortened quantum cascade (QC) laser injector regions. While conventional short-wavelength QC lasers typically have around seven or more injector region quantum wells, we investigate QC structures with three and two injector wells. Improvements in threshold currents, output powers, and wall-plug efficiencies are expected for fundamental reasons. At heat sink temperatures near 80 K, we observe threshold current densities less than 0.5 kA/cm2, nearly 4 W peak output power, and wall-plug efficiencies in excess of 20%. At room temperature, we see threshold current densities around 2.3 kA/cm2, output powers in excess of 1 W, and wall-plug efficiencies around 7.6%. We also observe new effects in midinfrared QC lasers, such as a pronounced negative differential resistance, pulse instabilities, and multiple and varied turn-off mechanisms. These effects result from the greatly abbreviated injector regions with highly discrete states.

Low voltage-defect quantum cascade laser with heterogeneous injector regions.

We demonstrate an In0.635Al0.356As/In0.678Ga0.322As strain compensated quantum cascade laser that employs heterogeneous injector regions for low voltage defect operation. The active core consists of interdigitated undoped and doped injectors followed by nominally identical wavelength optical transitions. The undoped injector regions are designed with reduced voltage defect while the doped injectors are of a more conventional design. The measured average voltage defect is less than 79 meV. At 80 K, a 2.3 mm long, back facet high reflectance coated laser has an emission wavelength of 4.7 µm and outputs 2.3 W pulsed power with a peak wall-plug efficiency of 19%.