Semiconductor Lasers

1
Semiconductor Lasers

• Aashwinder Lubana
• Brian Urbanczyk
• Harpaul Singh Kumar
• Kunal Chopra

2
Introduction

• Light Amplification by Stimulated Emission of
  Radiation.
• Laser light is monochromatic, coherent, and moves
  in the same direction.
• A semiconductor laser is a laser in which a
  semiconductor serves as a photon source.
• The most common semiconductor material that has
  been used in lasers is gallium arsenide.
• Einsteins Photoelectric theory states that light
  should be understood as discrete lumps of energy
  (photons) and it takes only a single photon with
  high enough energy to knock an electron loose
  from the atom it's bound to.
• Stimulated, organized photon emission occurs when
  two electrons with the same energy and phase
  meet. The two photons leave with the same
  frequency and direction.
• In 1916 Einstein devised an improved fundamental
  statistical theory of heat, embracing the quantum
  of energy. His theory predicted that as light
  passed through a substance it could stimulate the
  emission of more light. This effect is at the
  heart of the modern laser.

How Stuff Works http//science.howstuffworks.com/
laser6.htm
3
P- and N-type Semiconductors

• In the compound GaAs, each gallium atom has three
  electrons in its outermost shell of electrons and
  each arsenic atom has five. When a trace of an
  impurity element with two outer electrons, such
  as zinc, is added to the crystal. The result is
  the shortage of one electron from one of the
  pairs, causing an imbalance in which there is a
  hole for an electron but there is no electron
  available. This forms a p-type semiconductor.
• When a trace of an impurity element with six
  outer electrons, such as selenium, is added to a
  crystal of GaAs, it provides on additional
  electron which is not needed for the bonding.
  This electron can be free to move through the
  crystal. Thus, it provides a mechanism for
  electrical conductivity. This type is called an
  n-type semiconductor.
• 
  

4
Pictorial View
Under forward bias (the p-type side is made
positive) the majority carriers, electrons in the
n-side, holes in the p-side, are injected across
the depletion region in both directions to create
a population inversion in a narrow active
region.The light produced by radioactive
recombination across the band gap is confined in
this active region
5
Early Lasers

• The first laser diodes were developed in the
  early 1960s
• The device shown is an early example. It would
  require very high current flow to maintain a
  population inversion, and due to the heat
  generated by the steady-state current, the device
  would be destroyed quickly.

Laser Focus World http//lfw.pennnet.com/Articles/
Article_Display.cfm?SectionARCHISubsectionDispl
ayARTICLE_ID101065
6
Different types of Lasers are discussed
7
Vertical Cavity Surface-Emitting Lasers

• The VCSEL emits its coherent energy perpendicular
  to the boundaries between the layers. The
  vertical in VCSEL arises from the fact that laser
  diodes are typically diagrammed showing the
  boundaries as horizontal planes.
• The divergence of a laser beam is inversely
  proportional to the beam size at the sourcethe
  smaller the source, the larger the divergence.
• The cavity is along the vertical direction, with
  a very short length, typically 1-3 wavelengths of
  the emitted light.
• The reflectivity required for low threshold
  currents is greater than 99.9, Distributed Bragg
  Reflectors (DBRs) are needed for this
  reflectivity.
• DBRs are formed by laying down alternating layers
  of semiconductor or dielectric materials with a
  difference in refractive index.
• The DBR layers also carry the current in the
  device, therefore, more layers increase the
  resistance of the device. As a result,
  dissipation of heat and growth may become a
  problem if the device is poorly designed.
• Materials used include gallium arsenide (GaAs),
  aluminum gallium arsenide (AlGaAs),
• and indium gallium arsenide nitride (InGaAsN).

8
Examples of VCSELs
VCSELs have been constructed that emit energy at
850 and 1300 nanometers, which is in the
near infrared portion of the electromagnetic spect
rum.
Metallic Reflector VCSEL
Etched Well VCSEL
http//britneyspears.ac/physics/vcsels/vcsels.htm
Air Post VCSEL
Buried Regrowth VCSEL
9
Advantages of VCSEL vs. Edge Emitting Diode Lasers

• The VCSEL is cheaper to manufacture in quantity
• Easier to test on wafer
• More efficient
• The VCSEL requires less electrical current to
  produce a given coherent energy output.
• The VCSEL emits a narrow, more nearly circular
  beam than traditional edge emitters (used in
  optical fiber)
• Wavelength is tunable
• Efficiency and speed of data transfer is improved
  for fiber optic communications

10
Quantum Cascade Lasers

• When an electric current flows through a
  quantum-cascade laser, electrons cascade down an
  energy staircase emitting a photon at each step.
• It is composed of a sliver of semiconductor
  material. Inside, electrons are constrained
  within layers of gallium and aluminum compounds,
  called quantum wells, which are a few nanometers
  thick.
• The electrons jump from one energy level to
  another, and tunnel from one layer to the next
  going through energy barriers separating the
  wells. When the electrons jump, they emit photons
  of light.
• When the lower-energy electron leaves the first
  well, it enters a region of material where it is
  collected and sent to the next well.

11
Pictorial View
The invisible beam from a high-power quantum
cascade laser lights a match. It emits an
optical power in excess of 200 mW from each facet
at a wavelength of 8.0 µm.
12
Benefits of QC Lasers

• Typically 25 to 75 active wells are arranged in a
  QC laser, each at a slightly lower energy level
  than the one before -- thus producing the cascade
  effect, and allowing 25 to 75 photons to be
  created per electron journey.
• By simply changing the thickness of the
  semiconductor layers, the laser's wavelength can
  be changed as well.

The QCL can be regarded as an electronic
waterfall. When a proper bias is applied and an
electric current flows through the laser
structure, electrons cascade down an energy
staircase, and every time they fall down a step
they emit a photon
http//www.bell-labs.com/org/physicalsciences/proj
ects/qcl/qcl1.html
13
Quantum Dot Lasers

• Self-organized quantum dot lasers are grown by
  metal-organic vapor phase epitaxy (MOVPE),
  molecular beam epitaxy (MBE), and
  Stranski-Krastanow method
• Three dimensionally quantum-confined structures,
  quantum dots, provide atomic-like energy levels
  and a delta function density of states.
• Significant milestones in the development of the
  quantum dot lasers include demonstration of
• low threshold at room temperature
• large differential gain
• high output power
• wide spectral tunability
• better temperature insensitivity of the threshold
  current than quantum well lasers.

14
Quantum Dot Lasers

• Used in fields such as fiber-optic communications
  and pump sources
• The discrete energy levels in quantum dots
  provide for unique laser applications the lasing
  in self assembled quantum dot devices has been
  shown to exist for ground and excited state
  transitions, which allows for controlled
  wavelength switching.

15
Application of Lasers

• In telecommunications they send signals for
  thousands of kilometers along optical fibers.
• In consumer electronics, semiconductor lasers are
  used to read the data on compact disks and
  CD-ROMs.
• The power and tuning range properties of QC
  lasers makes it ideal for detection of gases and
  vapors in a smokestack.
• VCSEL has been proved to be an efficient emitter
  for fiber data communication in the speed range
  of 100Mbps to 1Gbps.
• Medical lasers are used because of their ability
  to produce thermal, physical, mechanical and
  welding effects when exposed to tissues. Some of
  the applications of lasers include stone removal
  (laser lithotripsy), activation of specific drugs
  or molecules and denaturizing of tissues and
  cells in body.
• Lasers are also used by law enforcement agencies
  to determine the speed and distance of the
  vehicles.
• Lasers are used for guidance purposes in
  missiles, aircrafts and satellites and make up
  for a potential replacement of ballistic
  missiles.

16
Problems of Nanostructured Lasers

• Good laser production above room temperature is a
  problem