Tappe Fondamentali dello Sviluppo dei Laser in Italia Orazio Svelto Dipartimento di Fisica del Politecnico di Milano Accademia Nazionale dei Lincei
I Primi Lavori � O. Svelto Pumping Power Considerations on an Optical Maser Applied Optics 1 , 745 (April 1962) � M. Bertolotti, L. Muzii, D. Sette Considerazioni sulla Costruzione e sul Funzionamento di un Laser a Rubino Alta Frequenza , XXXI , 560 (Sett. 1962) � F. T. Arecchi, A. Sona He-Ne Optical Masers: Constructions and Measurements Alta Frequenza , XXXI , 718 (Nov. 1962) � G. Toraldo di Francia On the Theory of Optical Resonators Proc. Symp. on Optical Masers , Pol. Inst. Brooklyn (April 1963)
I Primi Laser (1962-1963) � Laser a Rubino (Fondazione Bordoni, Giugno 1962), M. Bertolotti e D. Sette � Laser a He-Ne (CISE, Ottobre 1962) F. T. Arecchi e A. Sona � Laser a Rubino (Centro Microonde, Politecnico di Milano, CISE)
La Impresa Maser-Laser del CNR (1963-1968) � Gruppo Promotore: Daniele Sette, Emilio Gatti e Giuliano Toraldo di Francia � Gruppi partecipanti CISE (F. T. Arecchi) Politecnico di Milano (O. Svelto) Centro Microonde (G. Toraldo di Francia) Fondazione Bordoni (M: Bertolotti)
La Seconda Ondata (1965-1970) 1965 Primo laser ad Ar + (CISE, A. Sona ) Primo Laser a CO 2 (CISE, A. Sona) 1966 Primo laser a Nd:YAG CW (Politecnico) 1967 Primo laser a ML, rubino (Politecnico) (5 ps nel 1968) 1969 Primo laser a He-Cd (CISE)
Le Ricerche sui Laser (1965-1970) � Gruppo di Roma (Bertolotti e Sette) Proprietà di coerenza di laser a più modi, confronto fra le proprietà di coerenza e proprietà statistiche (Bertolotti, Sette) � Gruppo di Firenze (Toraldo di Francia) Laser a molti elementi (Pratesi, Burlamacchi) Risonatori ottici (Checcacci, Scheggi) Teoria del laser multimodale (Bambini, Burlamacchi)
Le Ricerche sui Laser (1965-1970) � Gruppo CISE (Arecchi e Sona) Proprietà statistiche di laser a singolo modo e paragone con luce termica � Gruppo del Politecnico (Svelto, Sacchi) Laser a stato solido con singolo modo trasversale Teoria del Mode-Locking ed effetti dovuti alla dispersione V. Daneu, S. Riva Sanseverino, G. Soncini
La Ristrutturazione del CNR � Centro Ricerche sulle Microonde ⇒ Istituto di Ricerca sulle Onde Elettromagnetiche (FI, 1970) � Istituto di Elettronica Quantistica (FI, 1970) � Centro di Elettronica Quantistica e Strumentazione Elettronica (MI, 1975) � Gruppo Nazionale di Elettronica Quantistica e Plasmi
I Progetti Finalizzati del CNR � Laser di Potenza (A. Sona, 1978-1983) � Tecnologie Elettroottiche (A.M. Scheggi, 1989-1994) � Materiali e Dispositivi per l’Elettronica a Stato Solido, MADESS I (1987-1992) e II (1997-2002)
L’Inizio della Crisi del CNR � Scomparsa dei Gruppi Nazionali del CNR (metà anni ’90) � La creazione dell’Istituto Nazionale di Fisica della Materia (1994-2005) � La ristrutturazione del INFM nel CNR (2005- )
Ultrafast Laser Pulses: from Femtosecond to Attosecond Orazio Svelto Dipartimento di Fisica del Politecnico di Milano Accademia Nazionale dei Lincei
Ultrafast Optical Science � Generating faster and faster optical signals � Communicating by fast optical signals � Studying the dynamics of natural events
Microsecond Optical Pulses Harold Edgerton ( ≈ 1850) Electrical flashes of light ∼ 1 μ s Stroboscopy
Nanosecond Optical Pulses Abram and Lemoigne (1899) � Generation by a Spark � Measuremente by a Kerr Cell
Generation of Short Laser Laser Pulse Pulse Generation of Short -11 10 ps 10 Solid-State Laser Pulse duration (s) -12 10 1 ps Ti:sapphire -13 10 100 fs Dye Laser -14 10 fs 10 Compression -15 1 fs 10 1965 1970 1975 1980 1985 1990 1995 2000 Year ■ Dye lasers: 10 ps down to ■ Solid state lasers: 10 ps (Nd:glass ) down to ∼ 6 fs (Ti:Sapphire, 27 fs hundreds of pJ)
The “pump-probe” technique τ λ 1 λ 2 target A first pump pulse (at λ = λ 1 ) triggers a dynamical process. • A second, delayed, probe pulse (at λ = λ 2 ) , detects pump-induced • transmission, or fluorescence, changes in the target
Pump-probe Experimental Setup Beam splitter Pump Probe Chopper τ Sample Translation stage Slow detector Lock-in (photodiode) � Typical sensitivity: Δ T/T =10 -4 (for 1 kHz repetition rate) to 10 -6 (for 100 MHz repetition rate) � Temporal resolution: 10 to 100 fs
Impulsive Coherent Vibrational Spectroscopy � Eigenstate description: the short pulse excites, in phase, many vibrational eigenstates ⇒ a wavepacket is formed on the excited state potential energy surface
Femtosecond Molecular Dynamics ■ Ahmed H. Zewail, Nobel Prize for Chemistry 1999 (Femtochemistry) E 2 (R) ⇒ Na + + I - (ionic) E 3 (R) ⇒ Na( 2 P J ) + I Fluorescence [Na( 2 P J ) → Na( 2 S 1/2 )] NaI
From Femtosecond Femtosecond to to Attosecond Attosecond From -11 10 ps 10 Solid-State Laser □ Pulse compression : Pulse duration (s) -12 10 1 ps 6 fs (1987) nJ � Ti:sapphire -13 10 100 fs Hollow fiber Dye Laser 4.5 fs (1997) ∼ 1mJ -14 � 10 fs 10 Compression -15 1 fs 10 1965 1970 1975 1980 1985 1990 1995 2000 Year
Compression of Light Pulses Compression of Light Pulses � General scheme Phase Modulator Delay line φ (t) T( ω ) Phase Modulator: generation of extra-frequency band Delay line: re-phasing of the new frequency components
Self- -phase Modulation phase Modulation Self � Optical Kerr effect : n( r ,t) = n 0 + n 2 I( r ,t) ϕ = ω 0 − ( t ) t k n ( t ) l 0 Phase Modulator φ (t) ϕ d d I ( t ) ω = = ω 0 − ( t ) k n l 0 2 dt dt I, ω Intensity trailing edge t F requency linear chirp
Uniform Spectral Broadening Uniform Spectral Broadening I( r ,t) Δ ω = ω - ω 0 = -k 0 n 2 [ ∂ I( r ,t) / ∂ t] l Δω ( r ,t) non uniform SPM vs r Solution Solution � Kerr effect in a guiding nonlinear medium 1974, Ippen et al. : SPM in a multimode optical fiber filled � with liquid CS 2 1978, Stolen and Lin: SPM in single-mode silica core fibers �
High Energy Pulse Compression High Energy Pulse Compression � Requirements for uniform spectral broadening of high energy pulses ⇒ guiding medium of large transverse dimensions ⇒ single transverse mode ⇒ medium with fast and high χ 3 (electronic origin) ⇒ medium with high damage threshold and high critical power for self-focusing Solution Solution � SPM in hollow fiber filled with noble gases
SPM by Hollow- -Fiber Fiber SPM by Hollow Dielectric waveguide Noble gas � Advantages of hollow-fiber ⇒ large bore diameter (high energy) ⇒ losses caused by multiple reflections inside the fiber greatly discriminate against higher order modes � Advantages of noble gases ⇒ purely electronic third-order nonlinear susceptibility (instantaneous response) ⇒ control of nonlinearity strength by changing gas type and pressure
Pulse Compression by the Hollow Fiber Only way to produce powerful (sub TW) pulses in the sub-6-fs regime hollow waveguide � Guiding medium with a large 25 fs diameter mode, fast nonlinearity and high damage threshold � Ultrabroad-band dispersion control Argon p=0.5 bar by chirped-mirrors 8 Chirped-mirror τ = 5 fs 5 fs compressor SH Intensity (a.u.) 0.11 TW 6 4 2 M. Nisoli et al. , Appl. Phys. Lett. 68 , 2793 (1996) M. Nisoli et al. , Opt. Lett. 22 , 522 (1997) 0 -20 -10 0 10 20 Delay (fs)
Hollow Fiber Modulator Hollow Fiber Modulator
Hollow Fiber Output Beam � Fundamental mode with the lowest attenuation: EH 11 (hybrid mode) � radial intensity distribution ( a bore radius) ⎛ ⎞ r 2 ∝ ⎜ ⎟ I ( r ) J 2 . 405 0 a ⎝ ⎠ T runcated zero order Bessel function Measured beam profile
Chirped- -mirror Compressor mirror Compressor Chirped Wavelenght (nm) ⇒ Tailoring of dispersion compensation ⇒ ultra-broadband dispersion control with low losses ⇒ high intensity handling Thickness (nm)
Applications of Few-cycle Laser Pulses � Coherent dynamical vibrations in F-centers � Extreme Nonlinear Optics and attosecond pulse generation � Electron dynamics of electrons in molecules
Coherent Dynamics in KBr KBr F F- -c centers enters Coherent Dynamics in M. Nisoli et al., Phys. Rev. Lett. 77 , 3463 (1996).
Extreme Nonlinear Optics : Influence of Carrier-Envelop Phase E ( t )= A ( t )cos( ω t + ϕ ) ϕ = carrier-envelope offset (CEO) phase ϕ = 0 ϕ = π ϕ = π/2
Extreme Nonlinear Optics � Nonlinear optical effects which depend of the carrier- envelope phase � Examples of extreme nonlinear optics � High-harmonic and single attosecond pulse generation + molecule � Electron dynamics in D 2
High-order Harmonic Generation 1000 Intensity (arb. units) 100 Intensity (arb. units) Gas jet Harmonics 10 140 150 160 170 180 190 Photon energy (eV) 0 Red light (1.6 eV) 80 100 120 140 160 Photon energy (eV) Odd harmonics of the red light are generated up to � the soft X ray region
High Order Harmonic Generation Set-up gas jet laser z 0 � Grazing incidence toroidal mirror and spherical varied-line-spacing grating � Acquisition: micro-channel plate with output on phosphor screen, optically coupled to a CCD camera with single shot acquisition capability
Harmonic Generation Process ε (t) → ( ) = + 3 . 17 Photon Energy E U max IP p
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