Hideaki Page

300 K operation of a GaAs-based quantum-cascade laser at λ≈9 μm

The room-temperature (300 K), pulsed mode operation of a GaAs-based quantum-cascade laser is presented. This has been achieved by the use of a GaAs/Al0.45Ga0.55As heterostructure which offers the maximum Γ–Γ band offset (390 meV) for this material system without inducing the presence of indirect barrier states. Thus, better electron confinement is achieved, countering the loss of injection efficiency with temperature. These devices show ∼100 K increase in operating temperature with respect to equivalent designs using an GaAs/Al0.33Ga0.67As heterostructure. We also measure 600 mW peak power at 233 K a temperature readily accessible by Peltier coolers.

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.

GaAs-AlGaAs quantum cascade lasers: physics, technology, and prospects

In recent years, the performance of GaAs-AlGaAs-based quantum cascade (QC) lasers has improved markedly. These devices are capable of pulsed room temperature operation and can deliver respectable average powers (11 mW at /spl lambda//spl sim/9 /spl mu/m) operating on a Peltier cooler. This performance has been achieved by the suppression of thermally activated carrier leakage through increases in the heterobarrier band offset. We demonstrate that QC lasers, with wavelengths /spl lambda//spl ges/9 /spl mu/m, can operate using heterostructures encompassing the entire composition range of Al/sub x/Ga/sub 1-x/As, without encountering potential problems-of the satellite X-minima for x>45%. Furthermore, we present particular characteristics of these devices, such as a phonon-limited temperature dependence, electrical and optical self-oscillations, and novel design concepts that exploit this closely lattice matched material system. Finally, we discuss improvements in device fabrication to lower the operating current through a reduction of the area of current injection. Using this technology, devices can be designed to selectively pump the fundamental lateral mode. We, therefore, observe single spatial-mode operation over the entire current range of operation.

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.

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.

Improved CW operation of GaAs-based QC lasers: T/sub max/= 150 K

We present a substantial improvement in the CW performance of GaAs-based quantum cascade lasers with operation up to 150 K. This has been achieved through suitable changes in device processing of a well-characterized laser. The technology optimizes the current injection in the laser by reducing the size of the active stripe whilst maintaining a strong coupling of the optical mode to preserve low current densities. The reduction of total dissipated power is critical for these lasers to operate CW. At 77 K, the maximum CW optical power is 80 mW, threshold current is 470 mA, slope efficiency is 141 mW/A, and lasing wavelength /spl lambda//spl sim/10.3 /spl mu/m.

High-power room temperature emission quantum cascade lasers at /spl lambda/=9 /spl mu/m

We present two different techniques for processing InP-based /spl lambda/=9 /spl mu/m quantum cascade lasers which improve the thermal dissipation in the device. The first process is based on hydrogen implantation creating an insulating layer to inject current selectively in one part of the active region. The second process uses a thick electroplated gold layer on the laser ridge to efficiently remove the heat produced in the active region. Each process is designed to improve heat evacuation leading to higher performances of the lasers and will be compared to a standard ridge structure from the same wafer. We give evidence that the process of proton implantation, efficient in GaAs based structures, is not directly applicable to InP based devices and we present a detailed analysis of the thermal properties of devices with an electroplated gold thick layer. With these lasers, an average power of 174 mW at a duty cycle of 40% has been measured at 10/spl deg/C.

Gain measurements on GaAs-based quantum cascade lasers using a two-section cavity technique

A two-section cavity device has been used to measure gain spectra and waveguide losses of a GaAs-based quantum cascade laser. The device operates at 8.9 /spl mu/m and optical confinement is obtained by means of Al-free cladding layers. We investigated the gain characteristics in a spectral window of /spl sim/60 meV and up to 200 K. For current densities ranging from 1 to 8 kA/cm/sup 2/, we report a constant gain coefficient of 13 cm/kA at 4 K and 6 cm/kA at 200 K. At low temperatures and for current densities above 8 kA/cm/sup 2/, we observe gain saturation which we attribute to a reduced electron injection in the active region caused by space charge effects. We report a value of 22 cm/sup -1/ for the waveguide losses in good agreement with previous measurements.

High reflectivity metallic mirror coatings for mid-infrared (λ ≈ 9 μm) unipolar semiconductor lasers

We present a detailed study on the improvement of the performance of λ ~ 9 μm quantum cascade lasers by using a metallic high reflectivity facet coating. Higher operating temperatures, lower operating currents and higher peak powers are all seen as a consequence for this relatively straightforward technology. Furthermore, a dramatic approximately fourfold increase of average power is observed when operated on a Peltier cooler at −30 °C.

Effect of defect saturation on terahertz emission and detection properties of low temperature GaAs photoconductive switches

We present a study into the properties of terahertz (THz) emission and detection using low temperature grown GaAs photoconductive switches over a range of ex situ anneal temperatures. Our analysis focuses on the effect of defect saturation, which has been confirmed in many experiments. However its effect on the THz emission and detection has so far not been fully investigated. In this letter, we examine the dependence of the radiated THz pulse width (full width at half maximum) upon optical power, and show that the differences in the characteristics with annealing can be theoretically accounted for when defect saturation is taken into account. Defect saturation was found to substantially increase the trapping time of photoexcited electrons, which in turn can cause THz pulse broadening at high optical powers. This effect was found to increase with anneal temperature due to the decrease in defect density. The radiated peak THz amplitude from emitters increases monotonically with increasing optical power acr...