lasers - coas | drexel university
TRANSCRIPT
Nearsighted Eyesight
Near Point – closest point at which you can still focus on object – about 25 cm
Particle Mass Charge
Proton 1.67 X 10-27 kg 1.6 X 10-19 CNeutron 1.67 X 10-27 kg 0.0 CElectron 9.11 X 10-31 kg -1.6 X 10-19 C
Electric Potential is the potential energy per charge.
12 V
+ --
vo=0m / s
U=K
qV=12mv2
v= 2qVm= 2 1.6 X 10−19C 12V
9.11 X 10−31kg=2.05 X 106 m
s
PHOTOELECTRIC EFFECT
V
A
Anode Cathode
Evacuated Quartz Tube
+ --
Electron
If I give the electron enough energy, it could break away from the positive plate and jump the potential barrier.
PHOTOELECTRIC EFFECT
V
A
Anode Cathode
Evacuated Quartz Tube
+ --
Electron
If I give the electron enough energy, it could break away from the positive plate and jump the potential barrier.
PHOTOELECTRIC EFFECT
V
A
Anode Cathode
Evacuated Quartz Tube
+ -Electrons
- -
One way to give the electrons energy is to shine light on the anode.
PHOTOELECTRIC EFFECT
V
A
Anode Cathode
Evacuated Quartz Tube
+ -Electrons
- -
If the “retarding potential” is increased, fewer and fewer electrons make the jump.
PHOTOELECTRIC EFFECT
V
A
+ -
Energy distribution in the emitted electrons ( called photoelectrons) is independent of the intensity of the light.
More intense (stronger) light beam means more electrons, NOT higher energy electrons.
A strong beam yields more electrons than a weak beam of the same frequency, but the same average kinetic energy.
Photoelectron current is proportional to the light intensity for all retarding voltages. The extinction voltage V
O is the same
for all intensities of light of a given frequency, n.
PHOTOELECTRIC EFFECT
V
A
-
There is no time lag between the arrival of the light at the metal surface and the emission of the photoelectrons.
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PHOTOELECTRIC EFFECT
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A
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Consider sodium. A detectable photo electric current will result when 10-6 W/m2 of electromagnetic energy is absorbed by the surface.
Assume 1019 atoms in a layer of sodium 1 atom thick an 1 m2 , each atom will receive energy at 10-25 W. At this rate it would take about 2 weeks to accumulate the 1 eV of energy needed.
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Plot of maximum kinetic photoelectron energy, Kmax
,
versus frequency show that the maximum kinetic energy is proportional to the frequency.
K max=h −o=h−ho
h=6.626 X 10−34 Js Planck's Constant
PHOTOELECTRIC EFFECT - Summary
VA
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Dependence of Photoelectron kinetic energy on light Intensity:
Classical Prediction: Electrons absorb energy continuously from E-M Waves. A more intense light should transfer into metal faster and produce electrons with higher KE.
Experimental Results: Maximum Kinetic Energy is independent of light intensity.
PHOTOELECTRIC EFFECT - Summary
VA
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Time interval between incidence of light and ejection of photoelectrons:Classical Prediction: For a very weak light, a measurable time interval should pass between the incidence of the light and the ejection of an electron. This is the time required for an electron to absorb the incident radiation before it requires enough energy to escape metal.
Experimental Results: Electrons are emitted from the surface almost instantaneously, even at a very low light intensity.
PHOTOELECTRIC EFFECT - Summary
VA
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Dependence of ejection of electrons on light frequency:
Classical Prediction: Electrons should be ejected at any frequency of the incident light as long the intensity is strong enough.
Experimental Results: No electrons are emitted if the incident light frequency drops below a cut-off frequency, f
O , no matter how intense the
light is.
PHOTOELECTRIC EFFECT - Summary
VA
+ -
Dependence of photoelectron kinetic energy on light frequency:Classical Prediction: KE should not depend on frequency, only intensity.
Experimental Results: Maximum KE increases as frequency increases. Higher intensity means more electrons, not faster electrons.
peakT=2.88 X 10−3mK
Classical Theory
UltraViolet Catastrophe – Classical theory fits well at long wavelengths but does not do well at short wavelengths.
Max Planck: Imagine charged oscillators exist at surface.
●Energy of the oscillator is quantized.
En=nh f
h=6.626 X 10−34 Js
Max Planck: Imagine charged oscillators exist at surface.
●Energy of the oscillator is quantized. The number n is the quantum number.
En=nh f
h=6.626 X 10−34 Js
●Energy is absorbed or emitted in discrete units.
E=h f
Max Planck: Imagine charged oscillators exist at surface.
●Energy is absorbed or emitted in discrete units.
E=h
0
h2h3h4h5hE n
543210
Quantum effects become important and measurable only on the submicroscopic level of atoms and molecules.
A 2.0 kg block is attached to a massless spring of force constant of k = 25 N/m. The spring is stretched 0.40 m from its equilibrium position and released.
ETot=12k A2=0.525N /m0.40m2=2.0 J
f = 12 k
m= 1
2 25 N /m2.0 kg
=0.56 Hz
Quantum effects become important and measurable only on the submicroscopic level of atoms and molecules.
A 2.0 kg block is attached to a massless spring of force constant of k = 25 N/m. The spring is stretched 0.40 m from its equilibrium position and released.
ETot=2.0 Jf =0.56 Hz
En=nh f
n=En
hf= 2.0 J
6.63 X 10−34 Js 0.56Hz=5.4 X 1033
Quantum effects become important and measurable only on the submicroscopic level of atoms and molecules.
A 2.0 kg block is attached to a massless spring of force constant of k = 25 N/m. The spring is stretched 0.40 m from its equilibrium position and released.
ETot=2.0 Jf =0.56 Hz
En=nh f
n=5.4 X 1033
How must energy must be dissipated away for the oscillator to move to next energy level?
E=h f =6.63 X 10−34 Js0.56 Hz =3.7 X 10−34 J
Max Planck: Imagine charged oscillators exist at surface.
Using this theory of quantized energy levels, Max Planck was able to fit the experimental data.
PHOTOELECTRIC EFFECT - Photons
VA
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Albert Einstein used Planck's concepts to explain photoelectric effect.
h=K maxho
h=energy content of eachquatumof incident lightK max=maximum photoelectronenergy
ho=minimumenergy needed dislodge anelectron
PHOTOELECTRIC EFFECT - Summary
VA
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Albert Einstein used Planck's concepts to explain photoelectric effect.
h=K max
h=energy content of eachquatumof incident lightK max=maximum photoelectronenergy
=Work Function=min energy needed dislodge anelectron
Quantum Maximum work functionEnergy = electron energy + of surface.
PHOTOELECTRIC EFFECT - Summary
VA
+ - h=K maxQuantum Maximum work functionEnergy = electron energy + of surface.
Element Work Function(eV)Aluminum 4.08Beryllium 5.0Cadmium 4.07Calcium 2.9Carbon 4.81Cesium 2.1Cobalt 5.0Copper 4.7Gold 5.1Iron 4.5Lead 4.14Magnesium 3.68Mercury 4.5Nickel 5.01Niobium 4.3Potassium 2.3Platinum 6.35
PHOTOELECTRIC EFFECT – Photon Model
VA
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Dependence of photoelectron kinetic energy on light intensity.
h=K maxQuantum Maximum work functionEnergy = electron energy + of surface.
Photon Model: Max KE independent on intensity. Max KE depends on frequency of light and work function.
K max=h−Double the intensity, double the number of photoelectrons that leaves the surface.
PHOTOELECTRIC EFFECT – Photon Model
VA
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Time interval between incidence of light and ejection of photoelectrons.
h=K maxQuantum Maximum work functionEnergy = electron energy + of surface.
Photon Model: There is a one to one relationship between photoelectrons ejected and photons. Low intensity light means less photons but still eject electrons, just not as many.
PHOTOELECTRIC EFFECT – Photon Model
VA
+ -
Dependence of ejection of electrons on light frequency.
h=K maxQuantum Maximum work functionEnergy = electron energy + of surface.
Photon Model: Energy of photon depends on frequency of the light. Below the cutoff frequency the photons do not have enough energy to provide to the photoelectrons top overcome the work function and break free from the surface.
PHOTOELECTRIC EFFECT – Photon Model
VA
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Dependence of photoelectrons kinetic energy on frequency.
h=K maxQuantum Maximum work functionEnergy = electron energy + of surface.
Photon Model: Higher frequency of light means higher maximum kinetic energy.
K max=h−
Compton Effect:
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Incident Photon
Target Electron
Scattered Electron v
Scattered Photon
E=h
p=hc
E=moC2
p=0
E=h '
p=h 'c
E=mo2c4 p2c2
p= p
LASERSLASERS
Characteristics:
Coherent – Individual waves of light maintain a fixed phase relationship with one another resulting in no destructive interference.
Monochromatic – Very small range of wavelengths.
Small angle of Divergence – Beam spreads out very little, even over large distances.
Three kinds of transitions involving electromagnetic radiation can occur between two energy levels in an atom.
Ej
Ei
Induced Absorption
h
Spontaneous Emission
h
Induced Emission
h
hh
h=E j−E i
Three kinds of transitions involving electromagnetic radiation can occur between two energy levels in an atom.
Ej
Ei
Induced Emission
h
hh
h=E j−E i
Alternating Force
If the applied force is in phase with the oscillating pendulum, then the amplitude of the pendulum and the energy of the pendulum increases.
Three kinds of transitions involving electromagnetic radiation can occur between two energy levels in an atom.
Ej
Ei
Induced Emission
h
hh
h=E j−E i
Alternating Force
If the applied force is 180O out of phase with the oscillating pendulum, then the amplitude of the pendulum and the energy of the pendulum decreases (Induced Emission).
Lasers are used in many applications, Lasers are used in many applications, from CD Players to dental drills to high-from CD Players to dental drills to high-speed metal cutting machines to speed metal cutting machines to measuring systemsmeasuring systems..
The Optical Damage Threshold test station at NASA Langley Research Center has three lasers: a high-energy pulsed ND:Yag laser, a Ti:sapphire laser and an alignment HeNe laser.
●Atoms are constantly in motion. They continuously vibrate, move and rotate.
●Atoms can be in different states of excitation.
●They can have different energies.
●If we apply a lot of energy to an atom, it can leave what is called the ground-state energy level and go to an excited level.
●The level of excitation depends on the amount of energy that is applied to the atom via heat, light, or electricity.
Ej
Ei
Induced Absorption
h
h=E j−E i
Applying some energy (heat, light, etc.) to an atom, expect that some of the electrons in the lower-energy orbitals would transition to higher-energy orbitals farther away from the nucleus.
Ej
Ei
Induced Absorption
h
h=E j−E i
•In a laser, the lasing medium is “pumped” to get the atoms into an excited state. •Typically, very intense flashes of light or electrical discharges pump the lasing medium and create a large collection of excited-state atoms.
Ej
Ei
Induced Absorption
h
h=E j−E i
●It is necessary to have a large collection of atoms in the excited state for the laser to work efficiently. ●In general, the atoms are excited to a level that is two or three levels above the ground state.
Ej
Ei
Induced Absorption
h
h=E j−E i
●Population Inversion - the number of atoms in the excited state versus the number in ground state.●This increases the degree of population inversion.
Ej
Ei
Induced Absorption
h
h=E j−E i
●The excited electrons have energies greater than the more relaxed electrons. ●Just as the electron absorbed some amount of energy to reach this excited level, it can also release this energy. ●The electron can simply relax, and in turn rid itself of some energy.
Ej
Ei
Induced Absorption
h
h=E j−E i
●The emitted energy comes in the form of photons (light energy). ●The photon emitted will have very specific wavelength (color) that depends on the state of the electron's energy when the photon is released. ●Two identical atoms with electrons in identical states will release photons with identical wavelengths.
Ej
Ei
Induced Emission
h
hh
h=E j−E i
●Stimulated Emission●The photon that any atom releases has a certain wavelength that is dependent on the energy difference between the excited state and the ground state.
●If this photon should encounter another atom that has an electron in the same excited state, stimulated emission can occur.
● The first photon can stimulate or induce atomic emission such that the subsequent emitted photon (from the second atom) vibrates with the same frequency and direction as the incoming photon.
Ej
Ei
Induced Emission
h
hh
h=E j−E i
●Stimulated Emission●The photon that any atom releases has a certain wavelength that is dependent on the energy difference between the excited state and the ground state.
●If this photon should encounter another atom that has an electron in the same excited state, stimulated emission can occur.
● The first photon can stimulate or induce atomic emission such that the subsequent emitted photon (from the second atom) vibrates with the same frequency and direction as the incoming photon.
Ej
Ei
Induced Emission
h
hh
h=E j−E i
●Pair of mirrors, one at each end of the lasing medium.
●Photons, with a very specific wavelength and phase, reflect off the mirrors to travel back and forth through the lasing medium.
●In the process, they stimulate other electrons to make the downward energy jump and can cause the emission of more photons.
Ej
Ei
Induced Emission
h
hh
h=E j−E i
●A cascade effect occurs, and soon we have propagated many, many photons of the same wavelength and phase.
●The mirror at one end of the laser is "half-silvered"
● reflects some light and lets some light through.
● The light that makes it through is the laser light.
●Solid-state lasers ●Gas lasers ●Excimer lasers ●Dye lasers ●Semiconductor lasers
Types of Lasers:Types of Lasers:
Solid-state lasers
Have lasing material distributed in a solid matrix
(such as the ruby or neodymium:yttrium-aluminum garnet "Yag" lasers).
The neodymium-Yag laser emits infrared light at 1,064 nanometers (nm).
Gas lasers
(helium and helium-neon, HeNe, are the most common gas lasers) have a primary output of visible red light.
CO2 lasers emit energy in the far-infrared, and are used for cutting hard materials.
Excimer lasers
(the name is derived from the terms excited and dimers)
Use reactive gases, such as chlorine and fluorine, mixed with inert gases such as argon, krypton or xenon.
When electrically stimulated, a pseudo molecule (dimer) is produced. When lased, the dimer produces light in the ultraviolet range.
Dye lasers Use complex organic dyes, such as rhodamine 6G, in liquid solution or suspension as lasing media.
They are tunable over a broad range of wavelengths.
Semiconductor lasersSometimes called diode lasers, are not solid-state lasers.
These electronic devices are generally very small and use low power.
They may be built into larger arrays, such as the writing source in some laser printers or CD Players
First Ruby LaserFirst Ruby Laser
Ground State
2.25 eV
1.79 eV
OpticalPumping
=550nm
Radiationless Transition
Metastable State
LaserTransition
=694.3nm
Ruby – crystal of aluminum oxide, Al2O
3
- some Al3+ ions replaced with
chromium Cr3+
- Responsible for red color
- Cr3+ metastable level with a lifetime of about 0.003 s
- Xenon flash lamp excites Cr3+ to a higher energy
First Ruby LaserFirst Ruby Laser
Ground State
2.25 eV
1.79 eV
OpticalPumping
=550nm
Radiationless Transition
Metastable State
LaserTransition
=694.3nm
Ruby – crystal of aluminum oxide, Al2O
3
- Cr3+ fall to metastable level losing
energy to other ions.
- Photons from spontaneous decay are reflected back and forth between mirrored ends.
- These photons stimulate other excited Cr3+ ions to radiate.
- After a few microseconds, a large pulse of monochromatic, coherent red light.
- Rod length is an integral number of half wavelength, radiation trapped forms optical standing waves. Since induced emissions are stimulated by standing waves, their waves are all in step with it.
Helium-Neon Helium-Neon LaserLaser
Ground State
20.61 eV
Ground State
20.66 eV
18.70 eV
Radiationless Transition
Metastable State
LaserTransition =632.8nmCollision
Metastable State
Helium Atom
Neon Atom
Spontaneous Emission
●Mixture of about 7 parts helium with 1 part neon at low pressure ( approx. 1 mmHg ).●Glass tube has mirrors both ends, one partially transparent.●Spacing between mirrors is equal to integral number of half wavelengths.
●Electrical discharge is produced by electrodes outside the tube, connected to a high-frequency AC source.●Collisions with electrons from the discharge excite He and Nhe atoms to metastable state of 20.61 and 20.66 eV.●Some excited He atoms transfer their energy to ground state Ne atoms in collisions with the 0.05 eV of additional energy provided by KE of of atoms.
●He atoms help achieve population inversion in Ne atom population.●Laser transition is from the metastable state at 20.66 eV to excited state at 18.70 eV, with emission of 632.8 nm photon. ●Another photon is spontaneously emitted in a transition to a lower metastable state, yielding only incoherent light.
●Remaining excitation energy lost to collssions with tube walls.●Because electrons impacts that excite He and Ne atoms occur all the time, unlike pulsed excitation from Xenon lamp, He-Ne laser operates continuously.