I’m Beginning to See the Light: Properties and applications

of our friendly photons

Paul Kwiat

www.light2015.org

2015 = “Sesquicentennial” What happened 150 years ago??

A new old frontier: light

2015 is the UNESCO International Year of Light,

and light-based technologies

http://www.light2015.org

1865: James Clerk Maxwell publishes world-changing treatise: “A Dynamical Theory of the Electromagnetic Field”

electromagnetic waves, traveling at cradio, light, microwaves, x-rays

electric charges electric fields (attract/repel other charges)

oscillating electric charges oscillating electric fields

oscillating electric fields oscillating magnetic fields

oscillating magnetic fields oscillating electric fieldsx

z

y

Properties of light waves

Color, wavelength, frequency, energy

Brightness, power, intensity

Direction, beam shape

Polarization

Phase

Speed

Plan for the day: Properties of light0. Maxwell’s fabulous equations…1. The colors of light2. The speed of light3. The aether, NOT4. Photons and fluorescence5. Applications of Snell’s law6. Momentum (linear & orbital) of light(no devices )

“Things should be made as simple as possible, but not any simpler.”

No mathNo hard mathToday:

LOTS of demos

7

Kwiat’s Quantum Clan

Rebecca Holmes

Alex Hill JiaJun Wong

Erika Smith (Physics Demo Coordinator)

CourtneyKrafczyk

+ Kristina Meier

+ Sheldon Schlie

+ Austin Graf

Electromagnetic spectrum

What we don’t see…l We typically associate the term “light” only with electromagnetic

radiation that we can see.l Other animals can see other parts of the spectrum; e.g., birds

and bees can see UV light:

l We can use technology to extend our vision (e.g., digital cameras).

Laser

Measuring the speed of light

Laser

Change in Path Length = 2L= 2 * 6 inches = 1 foot= 0.3 m

L Change in time delay = 10-9s (= 1 nanosecond = 1 nsec)

Speed of Light = 0.3 m / 1 nsec= 3 x 108 m/s ≡ c(~ 1 foot/nsec)

Measuring the speed of light

As far as we know, this is the top speed in the universe.

Laser

How to change the speed of light…

What is light waving in??All other waves propagate in something!What about light?Many (including Maxwell) thought there must be an

“aether”.How to tell…

If we are moving through the aether (e.g., as the earth moves around the sun), the speed of light should depend on which way it is traveling -- like swimming up/downstream.

Optical Interferometersl Interference arises when there are two (or more) ways for

something to happen, e.g., two slits for the light to get from the source to the screen.

l An interferometer is a device using mirrors and “beamsplitters” (half light is transmitted, half is reflected) to give two separate paths from source to detector.

l Common type: Michelson : mirror

beam-splitter

l Works by varying the relative phase of the light waves for the two paths light can take

l One possibility is to vary the lengths L1 or L2l Makes possible very accurate measurements of displacements

Michelson-Morley Experiment (1897)• Experiment intended to detect the invisible “aether” that

19th-century scientists thought light must travel through. • Carefully observed the pattern of light and dark coming from

their interferometer. • Expected to see shift in the interference pattern as the Earth

traveled around the Sun, because the Earth’s motion relative to the aether would cause small changes in the speed of light.

• Saw no shift consistent with motion relative to the aether—instead, it seemed the speed of light stayed constant!

• 1905: Einstein built his Special Theory of Relativity on the idea that the speed of light is constant in all reference frames.

Application: Gravity Wave Detectionl Einstein predicted that when massive objects accelerate, they produce time-dependent gravitational fields – gravity waves –that propagate as “warpings” of spacetime at the speed of light. (~EM radiation from accelerating e)l The effect is very tiny: E.g., estimated ⊗L/L of ~10-21

for in-spiraling binary neutron stars. How to detect this???

Application: Gravity Wave DetectionLIGO:Laser InterferometricGravitational wave Observatory-World’s largest interferometers: 4-km -2 in Hanford, WA; 1 in Livingston, LO- >400 scientists-Projected sensitivity ~3 x 10-23 ⊗L ~ 10-19 m (10-9 Ang.) -Real searches now underway!

http://www.sr.bham.ac.uk/instrument/ligo_lho.jpghttp://www.sr.bham.ac.uk/instrument/ligo_lho.jpg

Light, Revisited1905: Einstein made a ‘quantum leap’ – proposed that light was really made of particles with tiny energy given by E = h f = h c/

frequency6.6 x 10-34J-s wavelength

1-mW red laser 3 x 1015 photons/sec =3,000,000,000,000,000/sec

This is an incredibly huge number – your eye basically cannot resolve this many individual photons (though the rods can detect single photons!).

And you MAY be able to see just one photon!! (Sign up TODAY:http://research.physics.illinois.edu/QI/Photonics/vision/ )

http://research.physics.illinois.edu/QI/Photonics/vision/

Come back on December 5Professor Virginia Lorenz

“How to Train your Photon”

(Followed by lab tours)

http://physics.illinois.edu/outreach/saturdayphysics/detail.asp?eventID=32895625&startDate=12/5/2015http://physics.illinois.edu/outreach/saturdayphysics/detail.asp?eventID=32895625&startDate=12/5/2015http://physics.illinois.edu/outreach/saturdayphysics/detail.asp?eventID=32895625&startDate=12/5/2015

Optical spectroscopy: a major window on the world

Quantum mechanics discrete energy levels,e.g., of electrons in atoms or molecules.

When an atom transitions between energy levels it emits light of a very particular frequency.

Every substance has its own signature of what colors it can emit.

By measuring the colors, we can determine the substance, as well as things about its surroundings (e.g., temperature, magnetic fields), whether its moving (via the Doppler effect), etc.

Optical spectroscopy is invaluable in materials research, engineering, chemistry, biology, medicine…

But how do we precisely measure wavelengths?

Spectroscopy DemonstrationWe have set up some discharge tubes with various gases. Notice that the colors of the various discharges are quite different. In fact the light emitted from the highly excited gases is composed of many “discrete wavelengths.” You can see this with the plastic “grating” we supplied.

Your eye

Hold grating less than 1 inch from your eye.

Light source

Your view through the grating:

Put light source at left side of grating.

View spectral lines by looking at about 45o. If you don’t see anything, rotate grating 90o.

Atomic SpectroscopyFigure showing examples of atomic spectra for H, Hg, and Ne.

You can keep the grating. See if you can see the atomiclines in Hg or Na street lights or neon signs.

Source for figure: www.physics.uc.edu/~sitko/CollegePhysicsIII/28-AtomicPhysics/AtomicPhysics.htm

The lines are explained by Quantum Mechanics.

Application: Red ShiftIt has been known since 1930’s that the universe is expanding.

How do we know?Distant galaxies are moving away from us

(and the farther away they are, the faster they appear to be moving!)

The speed of distant objects is determined by the “Doppler shift” of their atomic emission lines (cf. sirens of a passing firetruck)

FluorescenceA substance that absorbs high energy photons (e.g., UV) may lose some of the energy (e.g., to vibrations), and emit lower energy photons (e.g., green)Applications:

Application: “Brainbow”

Use fluorescent proteins to differentially label >100 individual neurons in the brain

Laser

Acrylic Block

How to change the speed of light…

Laser

Path Length in Acrylic Block= Refractive Index * Length= 1.48 * 4 inches * 2

Path Length in Air= Refractive Index * Length≈ 1.0 * 4 inches * 2

Change in Path Length= 0.48 * 4 inches * 2= 3.84 inches

Change in time Delay = 3.84 inches/(1 foot per nsec)= 0.32 nsec

How to change the speed of light…

Snell’s Law of Refraction

movie, demo

Snell’s Law Applications: • Imaging: telescopes, microscopes, movie projectors• Fiber optics: medical,

• Optical fingerprint scanner • ‘Invisibility’

communication

“Cloaking”Using interference effects and ‘structured materials’ with designer patterns of index of refraction to control the flow of light

Limitations: -Must stay rigid.-Only for limited range of wavelengths-Small features will not be cloaked

Momentum of lightBetween 1919 and 1923, A.H. Compton showed that x-ray photons collide elastically with electrons in the same way that two particles would elastically collide! “Compton Scattering”

Photons carry momentum!

Maxwell’s equations also predict that light waves have momentum.

Sun

Radiation pressurefrom sun

v

Comet’s tail

Photon in

Electron recoils

Application: Laser ‘tweezers’• The refraction of light can be used to

trap and hold small objects.• Used to study DNA strands, force on

individual muscle fibers, molecular motors, etc.

CONTEST TIME!!

Transmitter ReceiverOld school Old school

Any school: only get strong signal when antennas are aligned

New school New school

Polarization of lightPrepare and measure using a polarizer

Only transmits polarization aligned with axis.Perpendicular polarization is completely blocked.

Relative angle Transmissionθ = 0° 100%θ = 90° 0%θ = 45° 50%

Application: sunglasses for glare reduction

General rule: “Law of Malus”Itransmitted = Iincident x cos2(θ)

relative angle

The IMAX Solution

How to change polarization

Delay one component with respect to the other

How? Use “birefringent” material waves with H and V polarizations travel at different speeds.

What Causes Birefringence?Birefringence can occur in any material that possesses some asymmetry in its structure, so that the material is more “springy” in one direction than another.Examples: Crystals: quartz, calcite

Different atom spacings in and

Long stretched molecular chains: saran wrap, cellophane tape

Stress-induced birefringence:Applying a mechanical stress to a material will often produce an asymmetry

birefringence.This is commonly used to measure stress.

The possible ‘shapes’ of light• spherical waves (e.g., emitted from a single atom)• plane waves (also ‘plain’ mathematical idealization)• gaussian beam (e.g., from a laser pointer)• ‘vortex’ beam (orbital angular momentum)

gaussian beam ‘vortex’ beam(OAM = 0) (OAM = h/2π

per photon) Laser tweezers with vortex beam

Combine all properties of light

(10 colors) x (2 orbital angular momentum states) x (2 polarizations) 1.6 terabits/s (1012)

= 200 Gbytes/s (1 byte = 8 bits)~ 250 full length movies/s

“The most important thing is never to stop questioning.”

� I’m Beginning to See the Light: �Properties and applications �of our friendly photons��Paul Kwiat�Slide Number 2Slide Number 3Slide Number 4Slide Number 5Slide Number 6Slide Number 7Electromagnetic spectrumWhat we don’t see…Slide Number 11Slide Number 12Slide Number 13What is light waving in??Slide Number 15Slide Number 16Slide Number 17Slide Number 18Slide Number 19Slide Number 20Slide Number 21Spectroscopy DemonstrationAtomic SpectroscopyApplication: Red ShiftFluorescenceApplication: “Brainbow”Slide Number 27Slide Number 28Snell’s Law of RefractionSlide Number 30“Cloaking”�Using interference effects and ‘structured materials’ with designer patterns of index of refraction to control the flow of lightMomentum of lightApplication: Laser ‘tweezers’Slide Number 34Slide Number 35Slide Number 36Slide Number 37What Causes Birefringence?The possible ‘shapes’ of lightCombine all properties of lightSlide Number 41