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QCL Consider it an electronic waterfall: When an electric current flows through a quantum-cascade laser,

electrons cascade down an energy staircase emitting a photon at each step. The cascade of light

generated in this way makes the QC laser much more powerful than existing lasers -- exciting the

scientific community and convincing Bell Labs to draw up big plans for its tiny invention. The picture

above shows a bow-tie laser, which was first realized using a quantum cascade laser material. It can,

however, also be designed for telecom laser systems. -- Bell Labs

A quantum-cascade laser is a sliver of semiconductor material about the size of a tick. Inside,electrons are constrained within layers of gallium and aluminum compounds, called quantum wells

are nanometers thick -- much smaller than the thickness of a hair. In such a tight space, electrons take

on properties explained by quantum physics. Specifically, they jump from one energy level to

another, rather than moving smoothly between levels and tunnel from one layer to the next going

"through" rather than "over" energy barriers separating the wells. When the electrons jump, they emit

photons of light.

In existing lasers, a photon of light is emitted when a negative charge (an electron) jumps from asemiconductor's conduction band to a positive charge (or "hole") in the valence band. Once an

electron has been neutralized by a hole it can emit no more photons. The quantum-cascade laser

contains a series of electron "traps," or quantum wells. The semiconductor material in the laser is

arranged to sandwich an electron in two dimensions as it passes through, coaxing the electron into a

quantum well. As it exits, it emits a photon and loses energy. When the lower-energy electron leaves

the first well, it enters a region of material whereit is collected and sent to the next well. Typically 25

to 75 active wells are arranged in a QC laser, each at a slightly lower energy level than the one before

-- thus producing the cascade effect, and allowing 25 to 75 photons to be created per electron journey.

The QC laser is unique in that its entire structure is manufactured a layer of atoms at a time by a

crystal growth technique, Molecular Beam Epitaxy or MBE, invented at Bell Labs in the late sixties.

By simply changing the thickness of the semiconductor layers, the laser's wavelength can be changed

as well.

Bell Labs researchers Deborah Sivco, Alfred Cho, Albert Hutchinson, Claire Gmachl, AlessandroTredicucci, and Federico Capasso, demonstrated the first semiconductor laser that can simultaneously

emit light at multiple widely separated wavelengths.

From Bell Labs (http://www.bell-labs.com/org/physicalsciences/projects/qcl/qcl.html)

http://www.bell-labs.com/org/physicalsciences/projects/qcl/qcl.htmlhttp://www.bell-labs.com/org/physicalsciences/projects/qcl/qcl.html