Applications of Photonics Technologies 2019 Introduction: what is light and basic properties Cristina Masoller [email protected] www.fisica.edu.uy/~cris MÀSTER UNIVERSITARI EN ENGINYERIA DE SISTEMES AUTOMÀTICS I ELECTRÒNICA INDUSTRIAL MÀSTER UNIVERSITARI EN ENGINYERIA AERONÀUTICA MÀSTER UNIVERSITARI EN ENGINYERIA INDUSTRIAL Introducing myself • Originally from Montevideo, Uruguay • PhD in physics (lasers, Bryn Mawr College, USA 1999) • Since 2004 @ Universitat Politecnica de Catalunya • Profesora Catedratica, Physics Department, research group on Dynamics, Nonlinear Optics and Lasers • Web page: http://www.fisica.edu.uy/~cris/ Introducing our research group Dynamics, Nonlinear Optics and Lasers Senior researchers / PhD students: 11/8 Introducing our research group . Research topics: Nonlinear phenomena (photonics, biophysics, complex systems) . Lab facilities in Gaia Building, UPC Terrassa: . Website: https://donll.upc.edu 4 Learning objectives and references . To understand the basic properties of light, which will allow us to understand photonic techniques and applications. There is no required text. The slides are based on previous courses by Prof. Ramon Vilaseca (UPC Prof. Emerito), the slides of Prof. Rick Trebino (Georgia Tech, USA) and freely available material in the Optical Society (OSA) web page. 5 what is light? Particles and waves Particles are localized in space and time (classical physics). Particles have well-defined trajectories. Waves are extended in space and time. Waves have poorly defined trajectories. waves bend around corners (diffraction) Two ways in which energy is transported Point-mass interaction, which transfers energy and momentum: particles. Extended regions wherein energy is transferred by vibrations and rotations (collective motions of particles): waves. Poll Is light constituted by particles or by waves? 9 The nature of light: Huygens Huygens promoted the wave theory. He felt that light propagates as a wave from the point of origin. Christiaan Huygens He realized that light slowed (1629-1695) down on entering dense media. He explained polarization, reflection, refraction, and double refraction. The nature of light: Newton Newton promoted the particle theory of light. Particles of light would travel in straight lines or rays. Explained (what were thought to be) sharp shadows. Explained reflection and refraction. Isaac Newton (1642-1727) "I procured me a triangular glass prism to try therewith the celebrated phenomena of colours." (Newton, 1665) Interference of light beams Thomas Young discovered interference of light—the ability of two light beams to either add or subtract. Thomas Young (1773-1829) His famous two-slit interference experiment proved convincingly that light is a wave. Young showed that light diffracted precisely as predicted by Fresnel’s wave theory. 13 Diffraction from one or two slits One slit Two slits In 1803, Thomas Young measured the two-slit pattern, which convincingly confirmed the wave nature of light (how could particles yield such a pattern?), ending centuries of debate as to whether light was a particle or a wave. In 19th-century Maxwell unified electricity and magnetism In free space: where E is the electric field, B James Clerk Maxwell 2 is the magnetic field, c = and c (1831-1879) is the velocity of light. Maxwell showed that the electromagnetic “field” is a wave that propagates at the speed of light. 22 The electric ( ) and magnetic ( ) ff1 0 fields obey the wave equation: xt2v 2 2 Electric field E Wavelength (l) Magnetic field B y x z E B Different wavelengths (distances between the peaks) or frequencies (the rate at which the peaks pass by) correspond to different colors, many of which we can’t see. Example of an harmonic Plane Wave that is a solution of Maxwell’s equations E(z,t) = i E0 cos(kz-wt+) B(z,t) = j B0 cos(kz-wt+) 푩 퐸0 푘(휔) (퐵0= ) Space 퓋 Wavelength: 휆 2휋 푛 휔 ·휔 Wavenumber: 푘 ≡ = 휆 푐 Velocity Wavevector: 풌 = 푘풌෡ “Phase” velocity: 푐 Time 퓋 = 휆 · 휈 = 푛(휔) 푬 Period: 푇 Frequency: 휈 = 1/푇 In vacuum: Angular frequency: 휔 = 2휋휈 퓋 = c = Initial phase: 휑 299792,458 km/s Vertical and Horizontal Polarizations Electric-field vector The simplest x polarizations y z Light can have Electric-field vector any sum of these two waves with any relative x phase. y z Linear polarization Adding two components in phase and with the same amplitudes yields 45º polarization. Electric-field vector The orientation of the field vector is 45º to the x and y x axes. y z Right circular polarization Adding two fields that have equal Ex (z , t ) E0 cos( kzw t ) amplitude and are 90° out of phase Ey (z , t ) E0 sin( kz w t ) gives a rotating field. Electric-field vector This polarization is also called positive helicity. x y z Left circular polarization and elliptical polarization Ex (z , t ) E0 cos( kzw t ) Electric-field vector Ey (z , t ) E0 sin( kzw t ) This polarization is also called negative helicity. x y z Unequal arbitrary-relative-phase E-field variation over time components yield elliptical y polarization Ex (z , t ) E0x cos( kz w t x ) x Ey (z , t ) E0 yy cos( kz w t ) The Intensity of a Wave How loud is a sound? How bright is a laser beam? We need a measure of the intensity of a wave. For a light wave, the intensity is given by: 1 2 where is the permittivity I 2 c E0 of the medium For other types of waves, the constants are different, but the intensity is always proportional to the square of the amplitude: IA 2 Real Waves vs. Plane Waves A plane wave has flat wave-fronts throughout all space. It also has infinite energy. It doesn’t exist. A real wave is localized. We can approximate a laser beam as a plane wave vs. z times a Gaussian in x and y: exp(x2/w2) xy22 E(,,,) x y z t E0 exp 2 exp[( i kzw t )] w w x z w y Localized wave-fronts x Laser beam spot on wall Pulsed Waves exp(t2/t2) Gaussian in time: If we can localize the t wave in space by t multiplying by a Gaussian E in x and y, we can also localize it in time by multiplying by a Gaussian in time (but it must be of z or t the form, z – vt or t z/v): (/t z v)2 xy22 E( x , y , z , t ) E0 exp 2 exp 2 exp[ i( kz wt)] t w This is the equation for a laser pulse about t long. A spherical wave (also solution of Maxwell's equations) E (r , t ) Re E0 / r exp[ i ( krw t )] where k is a scalar, and r is the radial coordinate. The directions of E and B must still be ┴ to the propagation direction, so the A spherical wave has polarization varies with angle. spherical wave-fronts. Unlike a plane wave, whose amplitude remains constant as it propagates, a spherical wave weakens: its intensity decreases as 1/r2. The spectrum indicates the frequencies present in a wave Plane waves are monochromatic (have only one frequency). Lightelectric field Time This wave has many frequencies. And the frequency increases in time (from red to blue). Superposition of waves with different frequencies 27 The Electromagnetic Spectrum Visible light: ~380 to ~780 nm. Spectra from common sources of visible light 29 The Sun spectrum Of the light at Earth’s surface, infrared comprises 49.4%, while visible light is 42.3%. Ultraviolet is the remaining 8%. 31 Another wave property of light: the Doppler effect Increased sound frequency if a source, such as a train (with whistle blowing), approaches a receiver and a decreased frequency as the source recedes. Christian Andreas Doppler (1803-1853) Waves from a source at rest Viewers at rest everywhere see the waves with their appropriate frequency and wavelength. The Doppler Effect . A receding source yields a red-shifted (longer-wavelength) wave, and an approaching source yields a blue- shifted (shorter- wavelength) wave. A source passing by emits blue- then red-shifted waves. The Doppler shift has a many uses Example: use it to sense rotation Red-shifted reflected light Incident light Blue-shifted reflected light Rotation Astronomers use it to measure recession velocities of distant galaxies and other astronomical objects. So light is a wave, no? By the mid-19th century, light was well-known to be a wave. But what exactly is “waving”? For a long time scientists searcher for the “luminiferous ether”, the hypothetical substance through which electromagnetic waves travel. Now is well-known that electromagnetic waves can propagate through empty space! 36 Thermal radiation: “Blackbody” Radiation When matter is heated, it not only absorbs light; it also emits it. The name “blackbody” comes from the assumption that the body absorbs at every frequency and hence would look black at low temperature. What does blackbody radiation look like? Depending on the temperature, look like this: Hot T (ºK) T (ºK) Light bulb Sun Almost everything is a blackbody! The sun, the earth, light bulbs, etc. The earth is cooler so emits in the IR. Images we see of the earth are of reflected light, which masks the IR. The same holds for humans. Comparison of light sources for lighting . A filament bulb has two kinds of losses: • losses due to the IR and UV radiation • heat-conduction losses in the filament, as a high intensity must be reached in order to reach a high temperature (2000-3000K). In contrast, discharge lamps and, mainly, LEDs, do not have so much heat conduction losses (smaller currents, as the radiation efficiency in the VIS is larger), and also they do not have IR and UV radiation losses. Therefore they are much more efficient.