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Advanced Laser and Photon Science (Kenichi ISHIKAWA) for internal use only (Univ. of Tokyo) Laser : the greatest invention of the 20th century レーザー 20世紀最大の発明 Industrial and medical application 産業 医療応 Industrial, daily life CPU Lithography リソグラフィー CD, DVD, Blu-ray, Copy machine Optical communication 光通信 Materials processing 材料加 Medical Hernia treatment, dental treatment, Laser scalpel, photodynamic therapy of cancer 腰痛の治療 科治療 術(レーザーメス) がん治療 LASIK レーシック birthmark removal あざ しみ治療 hair removal 脱 Baby gender selection 供の み分け 性別 4/5 No. 2
Laser-related Nobel laureates レーザー 関 連 のノーベル 賞 Townes, Basov, Prokhorov (1964-Physics):laser Gabor(1971-Physics) :invention and development of holography Bloembergen, Schawlow (1981-Physics):laser spectroscopy Kroto, Curl, Smalley (1996-Chemistry):fullerenes Chu, Cohen-Tannoudji, Phillips (1997-Physics):cool and trap atoms with laser light Zewail(1999-Chemistry):femtosecond chemistry Wieman, Ketterle, Cornell (2001-Physics) : Bose-Einstein condensation Tanaka, Fenn (2002-Chemistry):mass spectrometric analyses of biological macromolecules Glauber (2005-Physics):quantum theory of optical coherence Hall, Hänsch (2005-Physics):optical frequency comb Kao (2009-Physics): optical fiber Haroche, Wineland (2012-Physics) cavity QED Akasaki, Amano, Nakamura(2014-Physics) blue LED Betzig, Hell, Moerner (2014-Chemistry) super-resolved fluorescence microscopy Laser is omnipresent from basic science to our daily life. レーザーの 応 は 基 礎 研 究 から 常 活 までのすみずみにまで き 渡 っている 4/5 No. 3
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Advanced Laser and Photon Science (Kenichi ISHIKAWA) for internal use only (Univ. of Tokyo) Monochromaticity 単色性 Laser light has a single frequency or wavelength (pure color). 各種のレーザー光は それぞれある特定の波長のみ を含み その波長は時間的に一定である 4/5 No. 6
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Atom Energy level Bohr s condition hν light absorption spontaneous emission stimulated emission Emission of light (photon) upon transition to a lower level Spontaneous emission happens without an incident light E 2 E 1 ν hν = E 2 E 1 frequency h = 6.626 10 34 J s Planck constant Stimulated emission emits a photon induced by the incident light 4/5 No. 8
Before After spontaneous emission photon stimulated emission photon 2 photons (stimulated) absorption photon 4/5 No. 9
Light Amplification by Stimulated Emission of Radiation highly directional, high-intensity, very pure wavelength by spontaneous emission diverse direction and wavelength, low-intensity 4/5 No. 10
Unique properties of a laser Directionality & monochromacity Polarization E = E e ik x iωt+iφ 0 Frequency (wavelength) Phase Direction Laser is an ideal classical electromagnetic wave! 4/5 No. 11
Laser wavelength region 4/5 No. 12
1.2: Argon ion/ 488/514 nm CW/ Krypton ion/ 531/568/647 nm CW/ He-Ne/ 633 nm CW/ CO 2 10.6 µm CW or pulse/ Dye/ 450 nm 900 nm CW or pulse/ Diode/ 650 nm 900 nm CW or pulse/ Ruby/ 694 nm 1 250 µs Nd:YLF 1053 nm 100 ns 250 µs Nd:YAG 1064 nm 100 ns 250 µs Ho:YAG 2120 nm 100 ns 250 µs Ho:YSGG 2780 nm 100 ns 250 µs Er:YAG 2940 nm 100 ns 250 µs Alexandrite/ 720 nm 50 100 µs XeCl 308 nm 20 300 ns XeF Excimer 351 nm 10 20 ns KrF lasers 248 nm 10 20 ns ArF 193 nm 10 20 ns Nd:YLF 1053 nm 30 100 ps Nd:YAG 1064 nm 30 100 ps Ti:Sapphire/ 700 nm 1000 nm 5 fs 100 ps Short pulse laser Continuous wave laser (CW) Pulse laser Ultrashort pulse laser 4/5 No. 13
参 考 書 (Reference):W. T. Silfvast, Laser Fundamentals 4/5 No. 14
Temporal evolution of population density N 1 and N 2 N 1, N 2 Thermal equilibrium (T) T Boltzmann distribution ω spontaneous emission A absorption BW 12 stimulated emission E 2,N 2 Planck s law for cavity radiation BW 21 incident light E 1,N 1 W 4/5 No. 15
Advanced Laser and Photon Science (Kenichi ISHIKAWA) for internal use only (Univ. of Tokyo) Cavity (black body) radiation 4/5 No. 16
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dz I(z) I(z+dz) di dz = B(N 2 N 1 ) c I S Gain coefficient g = B(N 2 N 1 ) c Extended Lambert-Beer law I(z) =I 0 e gz = I 0 e (N 2 N 1 )z = B c 4/5 No. 18
Stimulated emission cross section for a variety of lasers Laser λ(nm) σ(m -2 ) He-Ne 632.8 3.0 10-17 Argon 488.0 2.5 10-16 He-Cd 441.6 9.0 10-18 Copper (CVL) 510.5 8.6 10-18 CO 2 10,600.0 3.0 10-22 Excimer 248.0 2.6 10-20 Dye (Rh6G) 577.0 2.5 10-20 Semiconductor 800.0 1.0 10-22 Nd:YAG 1064.1 6.5 10-23 Nd:Glass 1062.3 3.0 10-24 Ti:Sapphire 800.0 3.4 10-23 Cr:LiSrAlF 850.0 4.8 10-24 4/5 No. 19
I(z) >I 0 for z>0 N 2 >N 1 a necessary condition Stimulated emission > absorption At thermal equilibrium N 2 = N 1 exp ω /k B T [ ] << N 1 Energy E 2 thermal equilibrium Energy e E kt e E kt E 1 N 2 N 1 E 1 E 2 N 2 N 1 population inversion Population density Population density 4/5 No. 20
Solid, liquid, gas Plasma Free electron R 1 R 2 Pumping energy source is necessary. Flash lump LED Gas discharge Electric current Chemical reaction Another laser, Oscillator (resonator) Gain medium Pump Laser light 4/5 No. 21
pump Γ spontaneous emission N 2 A = N 2 stimulated emission N 2 N 2 B I c N 1 steady state dn 2 dt = N 2 1 + BI c =0 N 2 = 1 + BI c 4/5 No. 22
I sat = c B = = B c sufficient condition saturation length gl sat = (N 2 N 1 )L sat = 12 ± 5 e gl sat 10 5 4/5 No. 23
gl sat = (N 2 N 1 )L sat = 12 ± 5 g =0.15 m 1 L sat 80 m! one path L =0.2m e gl = 1.03 amplification by one path is small in general L sat L 400 paths is necessary 4/5 No. 24
R 1 R 2 The gain medium is put in a cavity (resonator) with two flat mirrors for lasing. Oscillator (resonator) Gain medium Pumping Laser light Amplifier Feedback amplifier βi o I i I o = AI i I o I i A I o = 1 Aβ I i I o ( 1 β)i o Aβ <1 4/5 No. 25
R 1 I i Oscillator (resonator) Feedback amplifier Gain medium R 2 Pumping βi o I o 1 ( β)i o Laser light I o = Aβ =1 A 1 Aβ I i Infinite amplication Lasing condition exp[2(g a)l]r 1 R 2 =1 A Necessary population inversion β g =(N 2 N 1 ) N 2 N 1 = a ln R 1R 2 2L 4/5 No. 26
Laser g (m -1 ) L (m) m He-Ne 0.15 0.2 400 Argon 0.5 1.0 24 He-Cd 0.3 0.5 80 Copper (CVL) 5 1.0 2.4 CO 2 0.9 1.0 13 Excimer 2.6 1.0 4.6 Dye (Rh6G) 500 0.02 1.2 GaAs 100,000 0.0001 1.2 Nd:YAG 10 0.1 12 Nd:glass 3 0.1 40 4/5 No. 27
R 1 R 2 Gain medium Laser light Oscillator (resonator) Pumping energy source 4/5 No. 28