The existence of semiconductors, a material that behaves much like an insulator at low temperature but conducts electricity like metals at an elevated temperature, has been known since the early nineteenth century. It, however, took more than a century before the electrical properties of a semiconductor could be manipulated to build the p-n junction, which is now ubiquitous in electronic and optoelectronic devices. The p-n junction when suitably biased behaves like a diode which under certain conditions emits light spontaneously and is called light emitting diodes (LED). The invention of LED in early 1962 provided a major impetus to build a semiconductor diode laser, and by the end of that year, coherent light was reported to be emitted by a gallium arsenide diode. The inbuilt tiny size of the semiconductor lasers enabling its ready integration with other devices primarily led to an explosion of research activity across the world to perfect the diode laser technology by exploiting the unique properties that a semiconductor is known to offer. This has led to the emergence of diode lasers as the undisputed leader in the world of lasers. Today, they are present everywhere – CD players, DVD players, barcode readers, laser pointers, cellphones, HD TV, etc., and most importantly, they are central to the ever-expanding global communication network making possible the hassle-free long distance transpacific and transatlantic video/audio calls.

The underlying physics of semiconductors, beginning from the conceptualization of holes to the emission of light from a p-n junction diode, has been brought out in an engaging style in the beginning of Chap.12. The latter half of this chapter provides illuminating insight into the working of semiconductor lasers with a particular emphasis on heterojunction, both edge and surface emitting types, quantum well, and quantum cascade lasers.

1. 1.

   The resistivity of metals increases with temperature.

2. 2.

   In 2006, the American Institute of Physics announced five most important papers in the journals published by it since it was founded 75 years ago. One of these five papers happened to be theApplied Physics Letters paper that Holonyak coauthored with S. F. Bevacqua to report the creation of the first LED in 1962.

3. 3.

   The description of wave function is a little elaborative and is appended at the end of this chapter.

4. 4.

   1 eV of energy equals 1.6 × 10⁻¹⁹ joule.

5. 5.

   Readers may take another look at Sect. 2.3.1 of Chap.2 for refreshing their memory on Fresnel reflection.

6. 6.

   In 1952, Trevor Simpson Moss (1921– ), a British physicist, published a book (T.S. Moss, Photoconductivity in the Elements, Academic Press Inc., New York, 1952) during his affiliation to Cambridge University, connecting the electronic and optical properties of a semiconductor by a rule that is known as Moss rule and is given by Band Gap energy × refractive index = constant.

7. 7.

   To know the story of the invention of the quantum well lasers in the inventor’s own word, the readers may refer to the foreword that C. H. Henry wrote for the bookQuantum Well Lasers ed. by Peter S. Zory Jr.

8. 8.

   Molecular-beam epitaxy (MBE) is a process forthin film deposition ofsingle crystals. The ultrahigh vacuum and extremely low deposition rate adopted here allow exquisite control over material purity and layer thickness, respectively.

9. 9.

   In the operation of a laser, the oscillating mode usually does not have a complete overlap with the active volume of the lasing medium. The fraction of the gain volume that the mode utilizes for its growth is called the “mode filling factor.”

10. 10.

    The quantum tunneling effect is the ability of a particle to penetrate a potential barrier with an energy less than the height of the barrier. Exponential decay of the magnitude of the wave function of the particle allows its quantum mechanical penetration through the barrier.

11. 11.

    A Bragg reflector essentially is a mirror structure which consists of a stack of alternating high and low index layers of two different optical materials.

Authors and Affiliations

1. Laser and Plasma Technology Division, Bhabha Atomic Research Centre, Mumbai, India

   Dhruba J. Biswas

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