光学大三上高等一week

1、张利剑 Advanced Optics IWeek 1Spontaneous and stimulated emissions1Let there be lightAnd god said, let there be light; and there was light - Genesis 1:322015 International Year of LightApplications of light science and technology are vital for existing and future advances in medicine, energy, informati

2、on and communications, fiber-optics, astronomy, architecture, archaeology, entertainment and culture.3Nature, art, culture etc4Light is 5LifeHopeInformationLight is a collection of photon6Particle or wave?Particle AND wavePhysical properties of a photonA photon is massless, has no electric chargeA p

3、hoton has two possible independent polarisation state.The energy of a photonThe momentum of a photon7Key parametersVelocity of light in vacuum: c0 =3*108 m/sFrequency: Angular frequency: Wavelength: =c/Wave vector: Wavenumber (spatial frequency): 8Photons are the sameas long as they are in the same

4、mode (color, propagation direction, etc)independent of their generation schemes 9But light is different from light10even when they are in the same mode! This is due to the statistical behaviour of the photon ensemble,although our eyes may not be able to see the differentLaserLight Amplification by S

5、timulated Emission of Radiation LASERHigh monochromaticity, directionality, brightnessThe most coherent light in Nature, well, one of the most coherent light in Nature11A brief history of laser1917 the establishment of the theoretical foundation of laser and maser by Einstein1953 the first demonstra

6、tion of maser by Charles H. Townes1958 the design of laser using open resonator by Gordon Gould, Aleksandr Prokhorov, Charles H. Townes, Arthur L. Schawlow1959 the word LASER first published by Gordon Gould1960 the first demonstration of laser by Theodore H. Maiman12Various lasersGas lasers: HeNe, C

7、O2, Ar+Solid-state lasers: Ruby, Nd:YAG（掺钕钇铝石榴石), Ti:SapphireDye lasersSemiconductor lasersFiber lasersFree-electron lasers13Spectrum of EM field14Commercial laser lines15Source: WikiAppearance of lasers16Applications of lasersIndustrial and commercial: machining, display, lithography, communication

8、, laser pointers, CD/DVD, barcode scanner Military: weapon, targeting, navigation, LIDAR Medical: diagnosis, cosmetic surgery, eye surgery Scientific: spectroscopy, imaging, microscopy, nonlinear optics, quantum optics Entertainment18SynopsisFundamentalsSpontaneous and stimulated transitionsOptical

9、amplificationsAnalysis of optical systemsGaussian opticsOptical resonatorsOptical resonators with gain mediaLaser radiationsOperating principles of different laser systemsControl of laser oscillatorsTypes of lasers: solid-state laser, gas laser, tuneable laser, fiber laser, semiconductor laser20Syno

10、psisDetection of optical radiationsCoherence theoryLatest development of laser applicationsTextbook: 激光原理第6版（周炳琨，高以智，陈倜嵘著），激光和电光学（Lasers and Electro-Optics, Christopher C. Davis著，世界图书出版公司）References: Fundamentals of Photonics（B.E. Saleh and M.C. Teich）Useful link: (An Open Access Encyclopedia for Ph

11、otonics and Laser Technology) (where you can download the first 8 chapters of Lasers and Electro-Optics)21Quantum electronics22A subject that studies the generation, amplification and control of (coherent) electro-magnetic field with quantum effects, and related effects and applications. In short, q

12、uantum electronics equals laser scienceOscillator in conventional electronics23In conventional electronics (100 GHz), an oscillator requires an amplifierpositive feedbackfrequency selectionOscillator in quantum electronics24A laser is an oscillator that operates at very high frequencies (1011 to 101

13、5 Hz). amplifying medium provides gainresonant cavity provides feedback and frequency-selectionAmplifying medium25Electrons can take jumps in 3 waysTransition 1: spontaneous emission26An electron spontaneously decays from a higher energy level to a lower one and emits a photon (in random direction a

14、nd with arbitrary polarization)Einstein A coefficient27probability per second of a spontaneous jump (with unit s-1) If there are Ni particles per unit volume in level i, the rate at which jumps made between level i and j is For more than one lower levels: The total rate at which the population of le

15、vel i changes: Life time and metastable states28Natural lifetime of level i: i=1/AiAllowed transition: Aij is about 106-108 s-1, ij 1sForbidden transition: Aij 104 s-1, ij is 10-100 s, or even up to 1 hourMetastable level: levels that can only decay slowly, and usually only by forbidden transitions

16、Lineshape29The spontaneous emitted radiation is not of single frequency. The frequency spectrum of the radiation is described by the lineshape g() with normalisation conditiong() represents the probability density of the radiation at frequency g() is also frequently written as g(0, )Note: 激光原理uses g

17、(, 0). We will keep using the former one.Intensity and power30The amount of radiation emitted spontaneously by a collection of particles can be described by the radiant intensity Ie() The radiant power in a frequency interval , +d Transition 2: Stimulated emission31Jumps stimulated by external field

18、The photon generated by stimulated emission that same as the stimulating photon (frequency, direction, polarisation) Einstein B coeffient32The external energy density () (energy per unit volume per frequency interval, J m-3 Hz-1)probability per second of a stimulated jumpEnergy density and intensity

19、33c is the velocity of light in the medium, c = c0/n where c0 is the velocity of light in vacuum, n is the refractive indexWhite and monochromatic field34radiation field is whiteradiation field is monochromatic (single frequency)() = 21(-21)Two limits35Monochromatic external field() much broader tha

20、n g(0, )Transition 3: stimulated absorption3637Spontaneous emission A21Stimulated emission B21Stimulated absorption B12Black-body radiation38The radiation surrounding or within a body with thermodynamical equilibrium with it environment, or emitted by a black body held at constant, uniform temperatu

21、re.A good approximation to the thermal radiation by many objectsBlack-body radiation39The distribution of the radiation depends only on the equilibrium temperature, and is independent of the shape of the cavity. We choose a cubical cavity. For a plane waveOnly the field that satisfy the following co

22、nditions can survive in the cavity (why?)Black-body radiation40In k-space, one mode occupies, the possible modes form a lattice. The size of a unit cell iskx ky kz=(2/L)3The total number of modes with Remember k = 2/c, number of modes with frequency below is Black-body radiation41The mode density (n

23、umber of modes per unit volume per frequency interval Yet the energy of each mode is not the same: ultraviolet catastropheThe energy density is () = p()Black-body radiation: the Planck theory42Each oscillation mode (frequency ) can only take quantised energyMaxwell-Boltzmann distribution (thermal distribution): The average energy of mode Black-body radiation: the Planck theory43The energy density of the black-b