OPTI510R: Photonics. Khanh Kieu College of Optical Sciences, University of Arizona Meinel building R.626

Transcription

1 OPTI510R: Photonics Khanh Kieu College of Optical Sciences, University of Arizona Meinel building R.626

2 Important announcements Homework #1 assigned, due Jan 24 No class Monday, Jan 21 th TA office hour: 3-4 PM, Wednesdays (8 th floor breakout area)

3 Properties of Light Nature of light Velocity of light Frequency and wavelength Polarization Coherence Other light characteristics

4 Nature of light The nature of light has been a fascinating topic for many generations of researchers. We all see and feel light but what it is exactly is still something that challenges our minds. 17 th century known facts about light: Light has different colors Light can travel through vacuum Light can be reflected and refracted i r i = r (Snel s law)

5 Nature of light So what is light? Corpuscular theory of light: light consists of corpuscles or very small particles flying at a finite velocity (Isaac Newton). Can be used to understand reflection, refraction and different colors of light But fails to explain diffraction, interference and polarization of light Isaac Newton

6 Nature of light The wave theory of light: At first, the nature of light is thought of as wave propagating in a medium called luminiferous aether. But all attempts to detect the aether have failed so far. So the theory based on the aether was more or less abandoned. The electromagnetic theory of light was then developed, culminating in the Maxwell s equations. In modern understanding, aether has no role in the theory of light. Christiaan Huygens ( ) In 1845, Michael Faraday discovered Faraday rotation The Michelson Morley experiment (1887) In 1873, Maxwell published his set of equations wikipedia.com

7 Nature of light The wave theory of light: At first, the nature of light is thought of as wave propagating in a medium called luminiferous aether. But all attempts to detect the aether have failed so far. So the theory based on the aether was more or less abandoned. The electromagnetic theory of light was then developed, culminating in the Maxwell s equations. In modern understanding, aether has no role in the theory of light. Christiaan Huygens ( ) In 1845, Michael Faraday discovered Faraday rotation Reflection, refraction, diffraction, Interference, and polarization explained In 1873, Maxwell published his set of equations

8 Electromagnetic Waves (EM)

9 Infrared-waves Wavelength dimensions of sub-mm to micron Infrared waves are thermal Infrared LED in remote control Image of a small dog in mid-infrared light Credit: Prof. Norwood

10 Infrared-waves Telecommunication bands Thermal imaging, remote sensing Measuring the temperatures of clouds Credit: Prof. Norwood

11 Visible-waves Wavelength dimensions of 0.38 to 0.78 microns Human eye is most sensitive at 555mm Credit: Prof. Norwood

12 Ultraviolet-waves Wavelength dimensions of 10 to 200 nm Sun emits UV radiation. Prolong exposure is bad for skin, eye and immune system. Exposure is needed for generation of Vitamin D. Credit: Prof. Norwood

13 Nature of light The wave theory of light: At first, the nature of light is thought of as wave propagating in a medium called luminiferous aether. But all attempts to detect the aether have failed so far. So the theory based on the aether was more or less abandoned. The electromagnetic theory of light was then developed, culminating in the Maxwell s equations. In modern understanding, aether has no role in the theory of light. Christiaan Huygens ( ) In 1845, Michael Faraday discovered Faraday rotation In 1873, Maxwell published his set of equations Reflection, refraction, diffraction, Interference, and polarization explained But Photoelectric effect, black body radiation, Compton effect not explained

14 Nature of light The classical physics prediction was completely wrong! (It said that an infinite amount of energy should be radiated by an object at finite temperature) The quantum theory of light: In an attempt to explain blackbody radiation, Planck postulated that electromagnetic energy could be emitted only in quantized form, in other words, the energy could only be a multiple of an elementary unit,, where h is Planck's constant. Different EM waves have diff. wavelengths E h hc / Planck constant: h = x m 2 kg/s The corpuscular theory of light came back?

15 Nature of light Wave-particle duality: OK! It must be both wave and particle then! de Broglie formulated the de Broglie hypothesis in all matter has a wavelike structure (1929 Nobel prize): Confirmed by electron Interference (1937 Nobel prize) Book by de Broglie

16 Velocity of light The velocity of light is a fundamental constant in physics. c = 299,792,458m/s Maxwell proposed that light travel with the speed of light Einstein postulated that the speed of light with respect to any inertial frame is independent of the motion of the light source. According to special relativity, c is the maximum speed at which all energy, matter, and information in the universe can travel. At the moment the meter is defined through the speed of light How to measure the velocity of light?

17 Measure the velocity of light Rømer Observation: Delay in the time of eclipses of the moon as observed from earth

18 Measure the velocity of light Fizeau apparatus (c = km/s) Foucault apparatus (c = ±500 km/s) Source: wikipedia.com

19 Measure the velocity of light

20 Measure the velocity of light In 1983: "The meter is the length of the path travelled by light in vacuum during a time interval of 1/ of a second. As a result of this definition, the value of the speed of light in vacuum is exactly m/s

21 Measure the velocity of light Astronomical measurements Time of flight measurement Cavity resonance Electromagnetic constants Interferometry 1675 Rømer and Huygens, moons of Jupiter 1729 James Bradley, aberration of light 1849 Hippolyte Fizeau, toothed wheel 1862 Léon Foucault, rotating mirror 1907 Rosa and Dorsey, EM constants 1926 Albert Michelson, rotating mirror 1950 Essen and Gordon- Smith, cavity resonator 1958 K.D. Froome, radio interferometry 1972 Evenson et al., laser interferometry th CGPM, definition of the metre ± ± ± ± ± ± (exact)

22 Velocity of light-practical uses LiDAR 3D imaging GPS Wikipedia.com osa-opn.org

23 Frequency and wavelength of light

24 Frequency and wavelength of light The wavelength of light changes in a medium. How about the frequency? The wavelength of light defines the smallest spot size that the laser beam can be focused down to. Diffraction limit

25 How to measure frequency and wavelength of light? The oscillation frequency of visible light is too fast to be measured with conventional electronics directly. OSA (Optical spectrum analyzer)

26 How to measure frequency and wavelength of light? Fourier-transform spectrometer A high-resolution spectrum of CO 2 in the near-ir was obtained using a Fourier-transform spectrometer (laserfocusworld.org)

27 How to measure frequency and wavelength of light? Fourier-transform spectrometer Fast acquisition speed, high resolution, low cost

28 Momentum of light The momentum of light: Angular momentum of light? NASA spin angular momentum orbital angular momentum Wikipedia.com

29 Polarization Linear polarization Circular polarization Elliptical polarization Unpolarized light How to measure the polarization state of a light beam?

30 Polarization Optical Isolators Imaging All-optical switching Magnetometer

31 Coherence Temporal coherence Spatial coherence Applications: Interferometry, coherent communication, imaging (OCT)

32 Other light characteristics Spectral bandwidth (OSA) Average power, peak power (power meter) Pulse energy, repetition rate (energy meter) Intensity Divergence, beam quality (M 2 measurement)

33 Properties of Light Nature of light Velocity of light Frequency and wavelength Polarization Coherence Other light characteristics

34 Question for thoughts Can you come up with your own theory of light? Why light travels at the speed that it travels? Can we use light to propel a satellite into its orbit? Can you come up with a better way to measure the speed of light? Can something move faster that the speed of light?

35 Good books to read Book by de Broglie (1937) by Albert Einstein and Leopold Infeld (1967) by Thomas Kuhn (4 th ed. 2012)