Physics of Light and Optics: Lasers and LEDs

1
Physics of Light and OpticsLasers and LEDs

• Physics of Modern Devices
• March 11, 2009

2
Outline of whats next

• Physics of light and optics
• Lasers and LEDs (Today)
• Cameras
• Optical Recording and
• Communication

(After spring break)
3
LASERs

• Lasers show up in a large range of products and
  technologies.
• CD players
• Dental drills
• Metal cutting machines
• Clearing human arteries
• Land surveying
• Laser speed gun
• Laser light shows at concerts!
• They were invented in the late 1950s.

4
LASERS

• But what is a laser?
• And what makes a laser beam different from the
  beam of a flashlight?

5
Spontaneous Emission

• Excited atoms normally emit light spontaneously
• Photons are uncorrelated and independent
• Incoherent light
• light which consists mainly of electromagnetic
  waves of many different wavelengths

E2 - E1 h?
6
Stimulated Emission

• In stimulated emission, photon emission is
  organized
• Excited atoms can be stimulated into duplicating
  passing light
• Photons are correlated and identical
• Coherent light
• If an electron is already in an excited state,
  then an incoming photon can "stimulate" a
  transition to that lower level, producing a
  second photon of the same energy.

7
Population Inversion

• When a sizable population of electrons resides in
  upper levels, this condition is called a
  "population inversion", and it sets the stage for
  stimulated emission of multiple photons.
• This is the precondition for the light
  amplification which occurs in a laser, and since
  the emitted photons have a definite time and
  phase relation to each other, the light has a
  high degree of coherence.

8
Pumping

• A population inversion cannot be achieved with
  just two levels because the probability for
  absorption and for spontaneous emission is
  exactly the same.
• The lifetime of a typical excited state is about
  10-8 seconds, so the electrons drop back down by
  photon emission about as fast as you can pump
  them up to the upper level.
• Electronic pumping
• Currents of charged particles use their kinetic
  or electrostatic energies to excite atoms from
  ground state to excited state
• Optical pumping
• Shine intense light to cause the excitation
• Most common

9
Three-level Laser

• Level 1 ground state
• Level 2 intermediate metastable state with a
  long lifetime
• Level 3 high energy pump state
• In order to obtain population inversion, lifetime
  at level 2 must be greater than lifetime at level
  3.

10
Three-level Laser

• Atom starts in ground state.
• A collision or absorption of a photon shifts it
  to excited state (Level 3). Light amplified.
• Fast radiative transition to lower state (Level
  2) when suitable photon passes through atom and a
  duplicate photon emitted.
• Atom shifts to ground state by emitting photon or
  as the result of another collision.
• Cycle begins again.

11
Four-level laser

• Atom starts in ground state.
• A collision or absorption of a photon shifts it
  to excited state (Level 4). Light amplified.
• Atom emits a photon, shifts down to upper laser
  state (Level 3).
• Radiative transition to lower state (Level 2)
  when suitable photon passes through atom and a
  duplicate photon emitted.
• Atom shifts to ground state by emitting photon or
  as the result of another collision.
• Cycle begins again.

A demonstration .
12
Laser Amplification

• Producing coherent light requires amplification
• Stimulated emission can amplify light
• Laser medium contains excited atoms
• Photons must have appropriate wavelength,
  polarization, and orientation to be duplicated
• Duplication is perfect photons are clones

13
Laser Oscillation

• A laser oscillator is a device which uses the
  laser medium itself to provide the seed photon,
  which is duplicated many times
• Laser medium in a resonator produces oscillator
• A spontaneous photon is duplicated repeatedly
• Duplicated photons leak from semitransparent
  mirror
• Photons from oscillator are identical

Lets see this in action .
14
Properties of Laser Light

• Coherent identical photons
• Controllable wavelength/frequency colors
• Controllable spatial structure narrow beams
• Controllable temporal structure short pulses
• Energy storage and retrieval intense pulses
• Giant interference effects
• Apart from these issues, laser light is just light

15
Types of Lasers

• There are many different types of lasers.
• The laser medium can be a solid, gas, liquid or
  semiconductor.
• Gas (HeNe, CO2, Argon, Krypton)
• Powered by electricity
• have a primary output of visible red light.
• CO2 lasers emit energy in the far-infrared, and
  are used for cutting hard materials.
• Solid state (Ruby, NdYAG, TiSapphire)
• Powered by electricity or light
• The NdYAG (neodymiumyttrium-aluminum garnet)
  laser emits infrared light.
• Semiconductor lasers (diode lasers)
• generally very small and use low power.
• laser printers or CD players.
• Liquid (Dye, Jello)
• Powered by light
• use complex organic dyes in liquid solution
• broad range of wavelengths.

16
Wavelengths
Blue is hard!
17
Gas Laser Helium-Neon

• The most common and inexpensive gas laser, the
  helium-neon laser is usually constructed to
  operate in the red at 632.8 nm.
• It can also be constructed to produce laser
  action in the green at 543.5 nm and in the
  infrared at 1523 nm.
• One of the excited levels of helium at 20.61 eV
  is very close to a level in neon at 20.66 eV, so
  close in fact that upon collision of a helium and
  a neon atom, the energy can be transferred from
  the helium to the neon atom.
• Helium-neon lasers are common in the introductory
  physics laboratories, but they can still be
  dangerous!
• The helium gas in the laser tube provides the
  pumping medium to attain the necessary population
  inversion for laser action.

18
Solid State LaserNeodymium-YAG

• Uses a Neodymium-YAG crystal rod pumped by a
  flash lamp (rate of about 15 Hz).
• Neodymium-YAG lasers have become very important
  because they can be used to produce high powers.
• Such lasers have been constructed to produce over
  a kilowatt of continuous laser power at 1065 nm
  and can achieve extremely high powers in a pulsed
  mode.

19
Semiconductor LaserLaser Diodes

• Population inversion by concentrating current
  into a narrow p-n junction from heavily doped
  semiconductors
• Intense current injects large density of
  electrons in anodes conduction band and settle
  in lowest energy conduction levels (e.g. upper
  laser state level 3)
• Heavy doping empties most of anodes highest
  energy valence levels (e.g. lower laser state
  level 2)
• Many electrons in level 3 and few in level 2
  ?population inversion and amplification can occur
• Ends of anode are reflective enough to act as
  mirrors ? laser oscillator
• Wavelength and color depend on band gap of anode.
• The length of the junction must be precisely
  related to the wavelength of the light to be
  emitted.

20
Light-Emitting Diodes

• Light emitting diodes (LEDs) are found in all
  kinds of devices.
• form the numbers on digital clocks
• transmit information from remote controls
• light up watches
• tell you when your appliances are turned on
• can form images on a television screen
• illuminate a traffic light
• LEDs are just tiny light bulbs that fit easily
  into an electrical circuit
• They are illuminated by the movement of electrons
  in a semiconductor material.
• LEDs generate very little heat and are very
  efficient!

21
Light-Emitting Diodes

• LEDs are diodes
• They carry current only in one direction
• The charges release energy on crossing
  p-n-junction
• Sometimes that energy is emitted as light
• To emit higher-energy photons
• LEDs need larger band gaps
• LEDs need larger voltage drops to emit their light

22
What are LEDs made of?

• LEDs are usually constructed of gallium arsenide
  (GaAs), gallium arsenide phosphide (GaAsP), or
  gallium phosphide (GaP).
• Silicon and germanium are not suitable because
  those junctions produce heat and no appreciable
  IR or visible light.
• The junction in an LED is forward biased and when
  electrons cross the junction from the n- to the
  p-type material, the electron-hole recombination
  process produces some photons in the IR or
  visible in a process called electroluminescence.
• An exposed semiconductor surface can then emit
  light.

23
Electroluminescence

• When the applied forward voltage on the diode of
  the LED drives the electrons and holes into the
  active region between the n-type and p-type
  material, the energy can be converted into
  infrared or visible photons.
• The electron-hole pair drops into a more stable
  bound state, releasing energy on the order of
  electron volts by emission of a photon.
• The red extreme of the visible spectrum, 700 nm,
  requires an energy release of a 1.77 eV photon.
• At the other extreme, 400 nm in the violet, 3.1
  eV is required.

24
Some exercises (Chapter 14)

• 33. A CD player uses a beam of laser light to
  read the disc, focusing that light to a spot less
  than 1 µm (10-6 m) in diameter. Why cant the
  player use a cheap incandescent lightbulb for
  this task, rather than a more expensive laser?
• The lightbulbs photons are all different and
  wont focus to the same tiny spot.
• 34. Why cant a CD player use a light-emitting
  diode (LED) in place of its diode laser?
• The incoherent light from an LED wont focus as
  well as the coherent light of a diode laser. The
  CD player will not be able to illuminate the tiny
  features in the CD selectively enough to read the
  information.

25
Exercises

• 38. While some laser media quickly lose energy
  via the spontaneous emission of light, others can
  store energy for a long time. Why is a long
  storage time essential in lasers that produce
  extremely intense pulses of light?
• Long storage allows energy to be deposited into
  the laser medium over a longer period of time.
  That means that conventional light sources can be
  used to store the energy and huge amounts of
  total energy can be accumulated.
• 39. One of the first lasers used synthetic ruby
  as its laser medium. However, a ruby laser is a
  three-state laser its lower laser state is its
  ground state. Why does that arrangement make the
  ruby laser relatively inefficient?
• The ground state systems absorb much of the
  amplified light before it can leave the ruby.

26
Exercises

• 40. If most of the highest energy valence levels
  in a diode lasers p-type anode werent empty, it
  would become relatively inefficient and probably
  wouldnt emit laser light at all. Why not?
• The diode laser needs a substantial population
  inversion to operate well. Electrons in the
  highest valence levels can absorb the laser light
  while undergoing radiative transitions to the
  conduction levels. That absorption would trap the
  laser radiation and reduce the diodes ability to
  emit laser light.

27
Exercises

• 41. Why doesnt increasing the current passing
  through an LED affect the color of its light?
• The color of light emitted by an LED depends
  primarily on the band gap in the LEDs
  semiconductor.
• 42. Why does increasing the current passing
  through an LED affect the brightness of its
  light?
• With more current passing through the LED, the
  population of electrons in excited conduction
  states increases and the rate of radiative
  transition in the LED increases.

28
Summary

• Lasers are cool!
• LEDs are cool!
• http//phet.colorado.edu/simulations/sims.php?sim
  Lasers
• Next HW is due on Friday, March 13
• The one after will be due after spring break,
  March 23
• Both are now available on WebAssign
• Have a good spring break!