HANCOCK, N.H. Using photonic-crystal techniques, a research team at Lucent Technologies Bell Laboratories has created a vertically emitting quantum-cascade laser that is said to be more compact than previous QC lasers. Combined with vertical emission and tunability, the development could open new application areas in sensors, the researchers said.
Similar in geometry to vertical-cavity surface-emitting lasers, QC lasers can be integrated into arrays on semiconductor chips for sensing, imaging and spectroscopy applications. In contrast to the one-electron, one-photon operation of other lasers, QC lasers use multiple quantum-well structures to create a cascading effect in which many photons are generated by a single electron. The action creates a powerful pulsed operation at room temperature.
The key breakthrough in the device developed by Lucent (Murray Hill, N.J.) was the use of a totally reflecting microcavity built in a photonic crystal-a periodic pattern of etched holes that penetrate down into the waveguide region of the device. Photonic crystals are optical equivalents of semiconductors, creating similar forbidden bands at certain wavelengths. In this case, the two-dimensional etched pattern acts as a perfect reflector, since the optical radiation is in the forbidden band. The pattern also allowed researchers to build an optical grating that functions as a feedback loop to create laser action in the cavity.
Using the strong feedback effect, the group was able to shrink the device, since QC lasers typically require many lattice periods to develop a strong enough effect. In this case, laser action was achieved from a cavity length of only eight periods.
QC lasers are ideal for sensor applications because their output can be tuned to match the optical fingerprint of a given chemical. Researchers at Bell Labs have been developing the technology for remote sensing of environmental pollutants. The work may prove useful in medical diagnostics for the same reason.
Bell Labs workers Michael Sergent, Deborah Sivco and Alfred Cho were assisted by visiting researchers from a number of institutions, including the California Institute of Technology, Harvard University and the New Jersey Nanotechnology Consortium (Murray Hill).