payne blue diode lasers

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Operation and Construction of Blue Diode Lasers

Ben Payne

Optics 325

April 21, 2006

Blue diode lasers are currently being experimentally produced. First a brief

lesson on how lasers operate is given, then how a blue diode laser works. Finally the

uses are talked about.


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Operation and Construction of Blue Diode Lasers

If one is interested in the properties of light, lasers offer a full spectrum of

possible investigations from classical optics to quantum mechanics. Lasers model

classical light rays very well for use with demonstrations of lenses but are produced by

quantum mechanical processes. The monochromatic coherent light is useful in many

applications, ranging from personal DVD and CD players to spectroscopy to more

experimental uses such as fusion (Hecht, 598). We will look at how lasers work, then the

operation of laser diodes, but not what happens to the beam after it leaves the apparatus.

Before one starts to look at the development of the laser, it is instructive to first

know what a laser is. When the device was first used in 1954 it was called Microwave

Amplification by Stimulated Emission of Radiation or MASER. After that it was

hypothesized that the same process could somehow produce visible light. The acronym

was then changed to LASER around 1965 after the development of just such a device.

Now the acronym has become so common that we call the device a laser, and if not in the

visible spectrum then it is a microwave laser or x-ray laser. What does this name actually

mean? The Light refers to the visible light that is emitted, as does the Radiation part.

The Amplification portion notates the process of increasing a signal. The Stimulated

Emission is the important, defining part of the process. The alternative is spontaneous

emission, which produces incoherent light. By using stimulated emission all the light is

in-phase both temporally and spatially, which is also known as coherence. The light is

also of the same wavelength, which is observed as monochromatic light. Next we will

look at how this is accomplished.


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The theory of operation for producing stimulated emission of electromagnetic

waves is challenging to the student of optics as it is based in quantum theory. In order to

understand where the light comes from, one must use idealized energy states and look

at the transitions between states. Given an atom in a high energy state, say E2, it decays

to a lower energy state, E1, and emits quantized energy (a photon). The amount of energy

is given the by first equation in Sveltos comprehensive Principles of Lasers:

2 1O

E Ev

h

=

One may recognize Planks constant, h. The equation signifies the energy transition

difference which emits some energy, h*O. The quantification of energy ties lasers to

quantum mechanics. The energy can either be an electromagnetic wave (a photon) or

non-radiative decay, which means thermodynamic (heat) dissipation. The atom is

initially in the higher energy state due to a process called pumping, which is discussed

later. The transition between states can occur one of two ways: spontaneous versus

stimulated emission. As the name suggests, a spontaneous transition emits energy that is

not related to external events. On the other hand, stimulated emission produces coherent

light because it responds to other light. Suppose there is an atom or molecule at an

elevated energy state and an incoming photon that has a frequency that matches the

energy transition. When the atom or molecule decays to the lower energy state the

photon it releases will be in phase with and the same wavelength of the photon that

prompted the change. This is the basic principle behind stimulated light emission.

Another fundamental concept in the field of lasers is the idea of a population. The

number of atoms or molecules in a given energy state is called the population, which is a

fraction of the total number of atoms or molecules. When more of the particles are in an


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elevated energy state than the lower energy state and then simultaneously decay (emitting

energy), a population inversion occurs. Instead of acting as an absorber, the material

emits energy. This population inversion occurs within a substance called the active

material. The substance is part of three idealized components, an active material, a

pumping scheme, and a resonator (Svelto, 1). The emission comes from the active

material. Once a signal is produced it has to be amplified. This is done using a

resonator, for example with a mirror or resonance cavity. Having looked at population

inversion in the active material and the produced light amplified in a resonator, we can

look at pumping. Pumping is how one gets the atom or molecule into the elevated energy

state. There must be some initial photons that stimulate the particles into emitting more

light. One could shine a light of the correct frequency on the material, but that would not

have any affect since absorption and stimulation cancel. In order to overcome this, the

stimulation has to increase the particles energy to a higher state than the elevated E2 in

order to pass through the transition from E2 to E1. Figure 1.4 in Svelto demonstrates the

two types of pumping which are termed three level or four-level lasers (page 7).

Figure 1: Production of laser light by energy decay leads to a burst of radiation


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There are various methods of pumping, but laser diodes use an electrical discharge in

order to pump the energy state up. Now that we have a basic understanding of the

physics behind lasers we can look at diode lasers.

Laser diodes are different from gas and liquid lasers as they use semiconductors

to produce light. Semiconductors are solid, mass producible, and not as fragile as the

alternatives. This makes them excellent candidates for commercialization. In order to

manufacture a diode laser, one simply layers different semiconductors. The hard part is

knowing what materials to layer in what thicknesses, how well one can keep the

substances pure and homogeneous, and the difficult techniques of putting layers down on

top of other materials. Semiconductors are used because they have band gaps: a gap in

energy levels between the next higher level for electrons and their current level. When

these electrons are pumped up and then fall back into the hole they left, light is emitted.

The motivation to use semiconductors comes from the fact that the diode is used for the

pumping process. Diodes prevent current from flowing in one direction but permit it in

the other. The semiconducting diodes used are either n-type, (negative from too many

electrons) or p-type, (positive from the absence of electrons) both are a result of adding

impurities, commonly called doping. Layering n-type and p-type semiconductors gives a

diode. The holes and excess electrons are excited by electricity and combine to produce

light. To figure out how thick the layers of semiconductor have to be, look at how deep

the electrons will permeate the p-type material to access available holes. The diffusion

equation is given by Svelto on page 397 as

*d D =


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where d = penetration depth of electron, D = diffusion coefficient, and = electron-hole

recombination time. As an example, Gallium Arsinide (GaAs) has a penetration depth d

of approximately 10-6 m. The reason to switch from GaAs to Gallium Nitride (GaN) is

because the the band gap is 3.4 eV, which corresponds to 365 nm (Johnson, page 31). By

adding other materials the wavelength changes slightly to blue or blue-green. The

elements used come from the third group and fifth group of the Periodic table and are

called III-V compounds. The specific materials used for blue lasers are Indium,

Aluminum, Gallium, and Nitrogen, which produce 480 to 520 nm light. (See figure 3, the

electromagnetic spectrum.) In order to use GaN as the active material one has to grow it

so that it is a thin film. To do this Silicon was initially used as a substrate. In

manufacturing one had to lay down thin layers of semiconductor at temperatures greater

than 1000C (steel melts at around 1370C) in a process called metal-organic chemical

vapor deposition (MOCVD). The GaN is grown on a sapphire surface. The two

materials are slightly different, as specified by their lattice constant. Since the

structures are different by 16%, cracks become present in the GaN. These cracks heat up

when energy is added in the pumping process, which destroys the material. This problem

was overcome using buffer layers of aluminum nitride (AlN) on the sapphire, as

developed by Isamu Akasaki in 1986 (Johnson, page 32). The primary developer of the

first blue nitride laser was Shuji Nakamura at Nichia Chemical Industries in Japan in

1995, now at the College of Engineering at the University of California at Santa Barbara.

At first he was able only to pulse the laser (due to the material instabilities), but by 1998

Nakamura was able to produce a continuous wave (CW) laser with a projected lifetime of

10,000 hours, or more than a year.


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The physical construction of diode lasers now uses multiple layers of

semiconductors to produce multiple quantum wells in devices called double

heterostructure lasers. These potential wells are in the active material and provide a place

for the electrons and holes to combine. The active material has dimensions such that the

device acts as a waveguide, and the edges are either rough or smooth to utilize the index

of refraction in such a way as to only allow stimulated light in one direction: out.

Figure 2: diagram of laser construction. Dimensions used by Nakamura are (4x450)*10-6

m

There are electrical inputs on the upper and lower surfaces, and the active material is

sandwiched between two semiconductors. When current is introduced it pumps electrons

up (gives them a higher energy state). When there are more electrons in the higher state

than the lower state, a population inversion occurs and energy is released. Some of the

energy is light, which is reflected until it is intense enough to leave the laser.


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Lasers have a relatively brief history, starting in the late 1950s. They have

evolved very quickly and have found many uses. Semiconductor lasers, including blue

diode lasers, are yet another example of the advancing progress. If a student of physics

continues on the path of optics, lasers will be a part of ones studies. More specifically

blue diode lasers will play in important role in technology, such as in DVDs, TV

displays, and printing. For optical storage like DVDs diffraction limits resolution, so a

lower wavelength allows higher data density. Currently Sony plans to offer a Blu-Ray

Disc recorder for about $4000. There are many companies attempting to harness the

capabilities of cheap, mass produced blue diode lasers. Current limitations are a lack of

understanding of the material physics, a low lifetime, and a low maximum voltage.

However, given the vast commercial interest in lasers, one can be sure that lasers will

remain a topic of much research. In his report on the status of blue laser diodes, Noble

Johnson says, The recent achievement of compact blue-emitting gallium nitride

semiconductor lasers is likely to have far-reaching technological and commercial

effects.


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Figure 3: Electro-magnetic spectrum


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Bibliography

Fasol, G. (1997). Longer Life for the Blue Laser. Science, 278(5345) p1902-1904.

Hecht, E. (2002). Optics. Reading, Massechusetts: Addison-Wesley

Johnson, N., Nurimkko, A., DenBaars, S. (2000). Blue Diode Lasers. Physics Today,53(10) p31-37.

Nakamura, S., Senoh, M., Nagahama, S., et. al (1997). High-Power, Long-LifetimeInGaN Multi-Quantum-Well-Structure Laser Diodes.Japanese Journal of AppliedPhysics 36 pL1059-L1061

Shimoda, K. (1983). Introduction to Laser Physics. Tokyo, Japan:Iwanami Shoten

Svelto, O., (1998). Principles of Lasers. New York, NY: Plenum Press

Whitten, J. (2001). Blue Diode Lasers: New Opportunities In Chemical Education.Journal of Chemical Education, 78(8) p1096-1101

Britney's Guide to Semiconductor PhysicsRetrieved from http://www.britneyspears.ac/lasers.htm on April 21, 2006

Laser, Diode LaserRetrieved from http://en.wikipedia.org/wiki/Laser andhttp://en.wikipedia.org/wiki/Diode_laser on April 21, 2006

Brian Kross of Jefferson LabRetrieved from http://education.jlab.org/qa/meltingpoint_01.html on April 21, 2006

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