Page 175 - Photonics Essentials an introduction with experiments

Lasers

Lasers 169

many people believed that GaN was a hopeless material, and many
more were interested in another semiconductor, ZnSe, in which p-n
junctions could be made easily, and which emits blue-green light.
Akasaki’s message reached the ears of Shuji Nakamura, a student at
Tokushima University on Shikoku Island. Mr. Nakamura was a stu-
dent of Prof. Sakai, one of Akasaki’s students. Professor Sakai put
Nakamura in touch with Prof. Akasaki and thus he began to work on
GaN, too. Nakamura successfully made GaN blue LEDs for his mas-
ter’s thesis.
Nakamura got a job at Nichia Chemical Company. Nichia is a big
manufacturer of phosphors for color TV, and so they have a lot of ex-
perience in luminescent materials. There he successfully convinced
his management to invest significant resources in GaN materials
preparation and device fabrication to make optoelectronic diodes. In
1994, Nichia introduced a commercial blue LED based on GaN. Three
years later, Nakamura demonstrated a blue laser based on GaN oper-
ating continuously at room temperature with a threshold current den-
–2
sity of 3.6 kA-cm , very close to that for the GaAs-based laser men-
tioned above.
Akasaki, now in “retirement,” continues to develop and demon-
strate new laser devices, including an ultraviolet laser that has a
wavelength so short that it is invisible. The emission color can also be
bent in the other direction to make bright green emitters. These
achievements are regarded by many of my colleagues as among the
most important developments in laser device technology. Both Akasa-
ki and Nakamura have received numerous awards for their work, the
results of which can be seen on your local street corner in the form of
the green LED lamp in the stoplight.
In Fig. 7.12 we show the optical output spectrum of a GaN laser
made by Nakamura and his team. The evolution of the laser spectrum
with increasing current is shown in four stages starting at 1 mA,
which is well below threshold, to 53 mA, which is just above thresh-
old.
What can we learn from this wonderful story? Models are useful,
but they have limits. Sometimes, as in this case, the limits are not al-
ways easy to see. However, predictions of failure or impossibility are
often proved incorrect. As a postscript, I would like to mention that a
somewhat similar situation exists today in the field of polymer-based
optoelectronics. There is resistance to using optoelectronic polymers.
Devices to date are not very bright, and they tend to degrade during
operation. An obstacle to obtaining bright and long-lived polymer-
based optoelectronic devices is the poor purity of the starting materi-
als. Do you suppose you could solve this problem?

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