Disk configuration creates compact quantum cascade lasers operating at low currents

Disk configuration creates compact quantum cascade lasers operating at low currents

Researchers at Lucent Technologies (Murray Hill, NJ) have fabricated quantum cascade (QC) lasers in disk form, obtaining tiny mid-infrared-emitting devices that operate at low threshold currents. In quantum cascade lasers, electrons tumble between upper and lower energy levels in a quantum well, emitting photons; well thickness determines the emission wavelength. The QC disk lasers consist of 20 periods of gallium indium arsenide/aluminum indium arsenide (GaInAs/AlInAs) layers, each containing three quantum-well active regions alternating with superlattice injectors. This stack is sandwiched between GaInAs/AlInAs waveguide layers, and the whole structure is grown on an n+-doped indium phosphide (InP) substrate.

After lithographically producing the quantum cascade structure, the grou¥etched the wafer, undercutting the quantum cascade region to produce 1.69-µm-thick disks on In¥pedestals. Operational devices with disk diameters ranging from 17 to 80 µm were fabricated and tested, the 80-µm disk producing approximately 1 µW of output at 5 µm. Threshold current increases as a function of disk area; currents ranged from 2.85 mA for a 17-µm disk to 48.4 mA for a 75-µm disk. The devices operated u¥to a temperature of 150 K. Light output increased linearly as a function of current until applied current reached twice threshold; the reduced slope that appears in the plot at that point may be caused by spatial hole burning.