Lasers and LEDs Lasers and LEDs or space. If you were to look from - PDF document

Lasers and LEDs 1 Lasers and LEDs 2 Introductory Question Introductory Question � In movies, laser beams are always shown as In movies, laser beams are always shown as bright pencils of light streaking through the air bright pencils of light streaking through the air Lasers and LEDs Lasers and LEDs or space. If you were to look from the side at a or space. If you were to look from the side at a beam from a powerful laser, would you be able beam from a powerful laser, would you be able to see that laser beam as it travels past you? to see that laser beam as it travels past you? A. Yes Yes A. B. No No B. C. Yes in air, No in space Yes in air, No in space C. Lasers and LEDs 3 Lasers and LEDs 4 Observations about Lasers Observations about Lasers 3 Questions about 3 Questions about and LEDs and LEDs Lasers and LEDs Lasers and LEDs � Lasers and LEDs often have pure colors Lasers and LEDs often have pure colors � How does laser light differ from regular light? How does laser light differ from regular light? � Lasers produce narrow beams of intense light Lasers produce narrow beams of intense light � How does a laser produce coherent light? How does a laser produce coherent light? � Lasers are dangerous to eyes Lasers are dangerous to eyes � How does an LED produce its light? How does an LED produce its light? � Reflected laser light has a funny speckled look Reflected laser light has a funny speckled look Lasers and LEDs 5 Lasers and LEDs 6 Question 1 Question 1 Light: Photons and Waves Light: Photons and Waves � How does laser light differ from regular light? How does laser light differ from regular light? � Electrons obey the Pauli exclusion principle Electrons obey the Pauli exclusion principle � � Each wave mode can have only one unique electron. Each wave mode can have only one unique electron. � That result gives structure to atoms and materials That result gives structure to atoms and materials � Photons don’t obey the Pauli exclusion principle Ph Photons don’t obey the Pauli exclusion principle Ph d d ’ ’ b b h P h P li li l l i i i i i l i l � Each wave mode can have many photons Each wave mode can have many photons � A radio wave has many photons in a single wave A radio wave has many photons in a single wave � Most light sources produce photons randomly Most light sources produce photons randomly � Each photon usually has its own wave mode Each photon usually has its own wave mode � But laser But laser light is an exception! light is an exception! 1

Lasers and LEDs 7 Lasers and LEDs 8 Spontaneous Emission Spontaneous Emission Clicker Question Clicker Question � Excited atoms normally emit light Excited atoms normally emit light spontaneously spontaneously � If you split the beam from a flashlight into two If you split the beam from a flashlight into two beams and overlap those beams on a white beams and overlap those beams on a white � These photons are uncorrelated and independent These photons are uncorrelated and independent screen, can you see interference effects? screen, can you see interference effects? � Each photons has its own wave mode Each photons has its own wave mode � These independent waves are incoherent light These independent waves are incoherent light A. Yes, because the beams contain light waves Yes, because the beams contain light waves A. B. No, because there are too many independent No, because there are too many independent B. light waves light waves C. No, because the beams don’t contain waves. No, because the beams don’t contain waves. C. Lasers and LEDs 9 Lasers and LEDs 10 Clicker Question Clicker Question Stimulated Emission Stimulated Emission � Atomic Atomic orbitals orbitals are electron standing waves and are electron standing waves and � Excited atoms can be Excited atoms can be stimulated stimulated into duplicating into duplicating don’t change with time. When a light wave don’t change with time. When a light wave passing light passing light passes an atom, its electrons passes an atom, its electrons � These photons are correlated and identical These photons are correlated and identical � The photons all have the same wave mode Th The photons all have the same wave mode Th h h ll h ll h h h d d A. remain in their atomic remain in their atomic orbitals orbitals and the atom is and the atom is A. � This single, giant wave is This single, giant wave is unchanged by the passing wave. unchanged by the passing wave. coherent light coherent light B. move into mixtures of move into mixtures of orbitals orbitals, allowing the , allowing the B. atom to polarize with the wave’s electric field. atom to polarize with the wave’s electric field. Lasers and LEDs 11 Lasers and LEDs 12 Clicker Question Clicker Question Question 2 Question 2 � If you split the beam from a laser pointer into If you split the beam from a laser pointer into � How does a laser produce coherent light? How does a laser produce coherent light? two beams and overlap those beams on a white two beams and overlap those beams on a white screen, can you see interference effects? screen, can you see interference effects? A. Yes, because the beams contain light waves Yes, because the beams contain light waves A. B. No, because there are too many independent No, because there are too many independent B. light waves light waves C. No, because the beams don’t contain waves. No, because the beams don’t contain waves. C. 2

Lasers and LEDs 13 Lasers and LEDs 14 Laser Amplification Laser Amplification Clicker Question Clicker Question � Light can be amplified using stimulated emission Light can be amplified using stimulated emission � If you place mirrors around a laser medium, If you place mirrors around a laser medium, � Excited atom Excited atom- -like systems can act as a laser medium like systems can act as a laser medium � That medium will duplicate any photons that have the That medium will duplicate any photons that have the A. nothing will happen because the mirrors will nothing will happen because the mirrors will A. right wavelength polarization and orientation right wavelength, polarization, and orientation right wavelength polarization and orientation right wavelength, polarization, and orientation prevent light from reaching the laser medium. prevent light from reaching the laser medium. li h f li h f hi hi h l h l di di � This duplication is perfect: the photons are true clones This duplication is perfect: the photons are true clones B. a photon emitted spontaneously by the laser a photon emitted spontaneously by the laser B. � This light amplification is the basis for lasers This light amplification is the basis for lasers medium will be duplicated endlessly. medium will be duplicated endlessly. Lasers and LEDs 15 Lasers and LEDs 16 Laser Oscillation Laser Oscillation Properties of Laser Light Properties of Laser Light � A laser medium can amplify its own light A laser medium can amplify its own light � Coherent Coherent – – identical photons identical photons � A laser medium in a resonator acts as an oscillator A laser medium in a resonator acts as an oscillator � Controllable wavelength/frequency Controllable wavelength/frequency – – colors colors � It duplicates its one of its own spontaneous photons It duplicates its one of its own spontaneous photons � Controllable spatial structure Controllable spatial structure – – narrow beams narrow beams � D pli � Duplicated photons leak from semitransparent mirror Duplicated photons leak from semitransparent mirror D pli t d ph t n l t d ph t n l k fr m k fr m mitr n p r nt mirr r mitr n p r nt mirr r � Controllable temporal structure Controllable temporal structure – – short pulses short pulses � The photons from this oscillator are identical The photons from this oscillator are identical � Energy storage and retrieval Energy storage and retrieval – – intense pulses intense pulses � Giant interference effects Giant interference effects � But apart from all this, laser light is still just light But apart from all this, laser light is still just light Lasers and LEDs 17 Lasers and LEDs 18 Introductory Question Introductory Question (revisited) Examples of Lasers Examples of Lasers (revisited) � In movies, laser beams are always shown as In movies, laser beams are always shown as � Gas lasers (powered by discharges) Gas lasers (powered by discharges) bright pencils of light streaking through the air bright pencils of light streaking through the air � Helium Helium- -neon lasers (red, green, yellow) neon lasers (red, green, yellow) or space. If you were to look from the side at a or space. If you were to look from the side at a � Carbon dioxide lasers (infrared) Carbon dioxide lasers (infrared) beam from a powerful laser, would you be able beam from a powerful laser, would you be able � Solid state lasers (powered by current or light) S lid Solid state lasers (powered by current or light) S lid l l ( ( d b d b li h ) li h ) to see that laser beam as it travels past you? to see that laser beam as it travels past you? � Diode lasers (red, blue, infrared) Diode lasers (red, blue, infrared) � Ruby lasers (red) Ruby lasers (red) A. Yes Yes A. � Nd:YAG Nd:YAG lasers (infrared) lasers (infrared) B. No No B. � Ti:Sapphire Ti:Sapphire lasers (infrared) lasers (infrared) C. Yes in air, No in space Yes in air, No in space C. 3