Waves, Light & Quanta Tim Freegarde Web Gallery of Art; National Gallery, London

2 Bandwidth theorem

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5 The LASER by Stimulated Emission of Radiation LIGHT AMPLIFICATION Theodore Maiman, 16 May 1960 beam splitter flash tube mirror ruby nm light amplifier optical resonator

6 Absorption and emission of photons ABSORPTION energy 0 n = 1 n = 3 n =  n = 2 absorptionemission EMISSION SPONTANEOUS ABSORPTION EMISSION STIMULATED

7 Absorption and emission of photons ABSORPTION EMISSION SPONTANEOUS ABSORPTION EMISSION STIMULATED EINSTEIN EQUATIONS spontaneous emission stimulated by vacuum field amplification of light if atomic population is inverted i.e. thermal equilibrium  blackbody spectrum Einstein A and B coefficients

8 The ruby LASER beam splitter flash tube mirror ruby nm light amplifier optical resonator energy absorptionemission Cr 3+ Cr 3+ ions in sapphire (Al 2 O 3 ) absorb blue and green from flash light internal transitions to metastable state metastable spontaneous emission is amplified by passage through ruby repeatedly reflected/amplified near-axial light builds up to form coherent laser beam

9 Laser beam characteristics beam splitter flash tube mirror ruby nm as initial source recedes down unfolded cavity, emission approaches that from distant point source long pulse  continuous wave (c.w.) narrow linewidth for long pulses ( ) noise from spontaneous emission gives lower limit to linewidth nonlinear processes have various effects in detail divergence determined by diffraction by limiting aperture Hecht section 13.1 focusable monochromatic constructive interference between reflections for certain wavelengths

10 The ruby LASER beam splitter flash tube mirror ruby nm light amplifier optical resonator ray optics colour diffraction interference quantum physics refraction, polarization, …