http://www.bell-labs.com/org/physicalsciences/projects/qcl/qcl1.html
http://www.bell-labs.com/org/physicalsciences/projects/qcl/qcl2.html
http://www.bell-labs.com/org/physicalsciences/projects/qcl/qcl3.html
http://www.bell-labs.com/org/physicalsciences/projects/qcl/qcl4.html