Quantum well laser

=Origin of the concept of quantum wells=

In 1972, Charles H. Henry, a physicist and newly-appointed Head of the Semiconductor Electronics Research Department at
Bell Laboratories, had a keen interest in the subject of integrated optics, the fabrication of optical circuits in which the lighttravels in waveguides.

In late 1972, while pondering the problems associated with waveguides, he had a sudden insight, a realization that a
double heterostructure is a waveguide for electron waves, not just lightwaves. On further reflection, he saw that there is a completeanalogy between the confinement of light by a slab waveguide and the confinement of electrons by the potential well that is formed from the difference in bandgaps in a double heterostructure. Henry realized that there should be discrete modes (levels) in the potential well, and a simple estimate showed that if the active layer of the heterostructure is as thin as several tens of nanometres, the electron levels would be split apart by tens of milli-electron volts, which should be observable. This structure is now called a quantum well.

Henry then calculated how this quantization would alter the optical absorption edge of the semiconductor. His conclusion wasthat instead of the optical absorption increasing smoothly, the absorption edge of a thin heterostructure would appear as a seriesof steps.

Experimental verification of quantum wells

In early 1973, Henry proposed to R. Dingle, a physicist in his department, that he look for these predicted steps. The very thinheterostructures were made by W. Wiegmann using molecular beam epitaxy. The dramatic effect of the steps was observed in the ensuingexperiment, published in 1974 Dingle, Wiegmann, and Henry, Phys. Rev. Lett. 33, 827 (1974).] .

Invention of the quantum well laser

After this experiment showed the reality of the predicted quantum well energy levels, Henry tried to think of an application.He realized that the quantum well structure would alter the density of states of the semiconductor, and result in an improved
semiconductor laser requiring fewer electrons and electron holes to reach laser threshold. Also, he realized that the laser wavelengthcould be changed merely by changing the thickness of the thin quantum well layers, whereas in the conventional laser a change in wavelengthrequires a change in layer composition. Such a laser, he reasoned, would have superior performance characteristics compared to the standard double heterostructure lasers being made at that time.

Dingle and Henry received a patent on this new type of semiconductor laser comprising a pair of wide bandgap layers having an active region sandwiched between them, in which "the active layers are thin enough (e.g., about 1 to 50 nanometres) to separate the quantum levels of electrons confined therein. These lasers exhibit wavelength tunability by changing the thickness of the active layers. Also described is the possibility of threshold reductions resulting from modification of the density of electron states." The patent was issued on Sept. 21, 1976, entitled "Quantum Effects in Heterostructure Lasers," U.S. Patent No. 3,982,297 U.S. Patent #3,982,207, issued Sept. 21, 1976, InventorsR. Dingle and C. H. Henry ,"Quantum Effects in Heterostructure Lasers", filed March 7, 1975.] .

Quantum well lasers require fewer electrons and holes to reach threshold than conventional double heterostructurelasers. A well-designed quantum well laser can have an exceedingly low threshold current.

Moreover, since quantum efficiency (photons-out per electrons-in) is largely limited by optical absorption by theelectrons and holes, very high quantum efficiencies can be achieved with the quantum well laser.

To compensate for the reduction in active layer thickness,a small number of identical quantum wells are often used. Thisis called a multi-quantum well laser.

Early demonstrations

The term "quantum well laser" was coined in the late 1970's by Nick Holonyak and his students at the University of Illinois at Urbana Champaign. They demonstrated the first quantum well laser in 1977. W.T. Tsang at Bell Laboratories in the late 1970's and early 1980's showed thepromise of quantum well lasers by demonstrating that when the quantum well parameters are optimized, they have exceedingly low thresholdcurrent and very high efficiency in converting current-in to light-out, making them ideal for widespread use.

The story of the origin of the quantum well concept, its experimental verification, and the invention of the quantum welllaser is told by Henry in more detail in the Foreword to "Quantum WellLasers," ed. by Peter S. Zory, Jr. Foreword, http://www.books.google.com/books?isbn=0127818901"The Origin of Quantum Wells and the Quantum WellLaser," by Charles H. Henry, in "Quantum Well Lasers," ed.by Peter S. Zory, Jr., Academic Press, 1993, pp. 1-13. ]

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