OPTI510R: Photonics Khanh Kieu College of Optical Sciences, University of Arizona [email protected] Meinel building R.626 Important announcements Homework #1 assigned, due Jan 29 No class Wednesday Jan 24th TA office hour: 1-2 PM, Tuesdays (8th floor breakout area) Properties of Light Nature of light Velocity of light Frequency and wavelength Polarization Coherence Other light characteristics 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. 17th century known facts about light: • Light has different colors • Light can travel through vacuum • Light can be reflected and refracted i r = i r (Snell’s law) 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 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 (1629-1695) The Michelson–Morley experiment (1887) In 1845, Michael Faraday discovered Faraday rotation In 1873, Maxwell published his set of equations wikipedia.com 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 (1629-1695) In 1845, Michael Faraday discovered Reflection, refraction, diffraction, Faraday rotation Interference, and polarization explained In 1873, Maxwell published his set of equations Electromagnetic Waves (EM) 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 Infrared-waves Telecommunication bands Thermal imaging, remote sensing Measuring the temperatures of clouds Visible-waves Wavelength dimensions of 0.38 to 0.78 microns Human eye is most sensitive at 555mm 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. 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 (1629-1695) In 1845, Michael Faraday discovered Reflection, refraction, diffraction, Faraday rotation Interference, and polarization explained In 1873, Maxwell published his set of equations But… Photoelectric effect, black body radiation, Compton effect not explained 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 = 6.626068 x 10-34 m2kg/s The corpuscular theory of light came back? Nature of light Wave-particle duality: OK! It must be both wave and particle then! de Broglie formulated the de Broglie hypothesis in 1924- all matter has a wave- like structure (1929 Nobel prize): Confirmed by electron Interference (1937 Nobel prize) wordpress.com Book by de Broglie 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 1865. 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? Measure the velocity of light Rømer Observation: Delay in the time of eclipses of the moon as observed from earth Measure the velocity of light Fizeau apparatus Foucault apparatus (c = 315000 km/s) (c = 298000±500 km/s) Source: wikipedia.com Measure the velocity of light 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 1/299792458 of a second.“ • As a result of this definition, the value of the speed of light in vacuum is exactly 299 792 458 m/s Measure the velocity of light 1675 Rømer and Huygens, 220000 Astronomical measurements moons of Jupiter 1729 James Bradley, 301000 aberration of light Time of flight measurement 1849 Hippolyte Fizeau, 315000 toothed wheel 1862 Léon Foucault, 298000±500 Cavity resonance rotating mirror 1907 Rosa and Dorsey, EM 299710±30 constants Electromagnetic constants 1926 Albert Michelson, 299796±4 rotating mirror 1950 Essen and Gordon- 299792.5±3.0 Interferometry Smith, cavity resonator 1958 K.D. Froome, 299792.50±0.10 radio interferometry 1972 Evenson et al., 299792.4562±0.0011 laser interferometry 1983 17th CGPM, 299792.458 (exact) definition of the metre Velocity of light-Practical uses LiDAR 3D imaging Wikipedia.com GPS http://theexceptioncatcher.com/p/GizGPSBth#.Ut8x2vTn-IU osa-opn.org Frequency and wavelength of light 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 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) How to measure frequency and wavelength of light? Fourier-transform spectrometer A high-resolution spectrum of CO2 in the near-IR was obtained using a Fourier-transform spectrometer (laserfocusworld.org) How to measure frequency and wavelength of light? Fourier-transform spectrometer Fast acquisition speed, high resolution, low cost Momentum of light The momentum of light: Angular momentum of light? NASA Wikipedia.com Polarization Linear polarization Circular polarization Elliptical polarization Unpolarized light How to measure the polarization state of a light beam? Polarization Optical Isolators Imaging All-optical switching Magnetometer … Coherence Temporal coherence Spatial coherence Applications: Interferometry, coherent communication, imaging (OCT)… Other light characteristics Spectral bandwidth (OSA) Average power, peak power (power meter) Pulse energy, repetition rate (energy meter) Intensity Divergence, beam quality (M2 measurement) Properties of Light Nature of light Velocity of light Frequency and wavelength Polarization Coherence Other light characteristics 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? Good books to read Book by de Broglie (1937) by Albert Einstein and Leopold Infeld (1967).