QC lasers produce high powers, dual wavelengths

A major benefit of quantum-cascade (QC) lasers is that the output wavelength can be chosen without being tied to the bandgaps of available materials. These lasers are made of "artificial materials," grown with multiple quantum wells designed to provide the wavelengths desired. In the four years since QC lasers were invented, they have been put to use in experimental mid-infrared (mid-IR) systems, replacing lead-salt diodes that had to be cryogenically cooled.

Now the group at Bell Labs (Murray Hills) that invented QC lasers is reporting two new design changes that make these lasers more useful. One change allows much higher output powers from the devices—up to 500 mW peak at room temperature from a device emitting at 7.6 µm. The other change designs three separate laser transitions into one device, which opens up a host of applications for differential measurements at IR wavelengths.

What is a cascade laser?

Quantum-cascade lasers are fundamentally different from semiconductor diode lasers—the emission wavelength depends on the thickness of the quantum well and barrier layers of the active region rather than the semiconductor bandgap. In addition, power can be high because one electron produces many photons—one per period, and most devices have 25 to 30 periods—and output wavelengths can be varied by altering the layer thicknesses of the active region during growth.

Two years ago, researchers demonstrated superlattice QC lasers, in which the active region consists of a periodic structure of many quantum wells separated by 1- or 2-nm barrier layers. This produces strong coupling between energy levels of the individual quantum wells, leading to the formation of broad electron-energy bands called minibands.

The lasing transition is between the bottom of the upper miniband and the top of the lower miniband, which arises from the overlap of the ground states of the quantum wells. Scaling the semiconductor superlattice by scaling the thickness of the aluminum indium arsenide (AlInAs) barriers and gallium indium arsenide (GaInAs) wells alters the separation between the minibands and therefore changes the output wavelength. Minibands can carry higher currents than the discrete energy levels of conventional QC lasers, thus allowing higher powers.

To achieve the high currents necessary to make QC lasers work at higher powers, researchers have previously added dopants to the active region. The dopants screen the electric field from the superlattice, where it would otherwise destroy the minibands.

The redesign by Federico Capasso’s group modifies the superlattice potential by varying the well thickness, which allows lower threshold currents.¹ In the absence of an applied field, the energy levels of the wells are misaligned, but when the appropriate voltage is applied, the minibands appear as desired and allow injection of a large current. The experimental chirped superlattice QC laser emits record-high 14-mW average-power and 500-mW peak-power pulses at room temperature, with the lowest threshold current densities (5 kA/cm²) yet reported for QC lasers. By comparison, last February the group reported making 8.2-µm lasers that emitted peak powers of about 180 mW at room temperature.

Tailoring three lines from one laser

Semiconductor lasers are more the exception than the rule for lasers in that they typically have one lasing transition. The Bell Labs group recently described a superlattice QC laser in which multiple transitions in the IR lase.² These devices are carefully designed to balance two constraints on the upper lasing levels—the difference between the upper levels must be large enough that the difference between the emitted photon energies is considerably larger than the broadening of the two transitions; it must also be less than the optical phonon energy to suppress fast electron relaxation between these levels, which could leach energy from the higher upper level. The laser emits a peak power of 100 mW at 6.6 and 8.0 µm. Higher currents generate a third line at 7.3 µm. "Three separate semiconductor lasers with a hundred times less power would normally be required to emit these widely different wavelengths," explains Capasso.

There are many applications that could use the ability of this new QC laser to emit at multiple wavelengths, especially differential sensing techniques, such as differential absorption lidar used for monitoring air pollution.

REFERENCES

1. A. Tredicucci et al., Appl. Phys. Lett. 73 (15), 12, 2101 (Oct. 1998).

2. A. Tredicucci et al., Nature 396, 350 (28 Nov. 1998).