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1、Ultrafast Optics: Introduction,The birth of ultrafast opticsUltrahigh intensityThe uncertainty principle and long vs. short pulsesGeneric ultrashort-pulse laserMode-locking and mode-locking techniquesGroup-velocity dispersion (GVD)Compensating GVD with a pulse compressorContinuum generationMeasuring
2、 ultrashort pulsesThe shortest event ever createdUltrafast spectroscopyMedical imaging,Prof. Rick TrebinoGeorgia Techwww.frog.gatech.edu,The “Galloping Horse” ControversyPalo Alto, CA 1872,The birth of ultrafast technology,Leland Stanford,Eadweard Muybridge,Bar bet: Do all four hooves of a galloping
3、 horse ever simultaneously leave the ground?,Time resolution:1/60th of a second,If you think you know fast, think again.,Ultrashort laser pulses are the shortest events ever created.,Ultrafast optics vs. electronics,1960,1970,1980,1990,2000,Time resolution (seconds),Year,Electronics,Optics,No one ex
4、pects electronics to ever catch up.,The metric system,Prefixes:,Milli (m)10-3,Micro ()10-6,Nano (n)10-9,Pico (p)10-12,Femto (f)10-15,Atto (a)10-18,Kilo (k)10+3,Mega (M)10+6,Giga (G)10+9,Tera (T)10+12,Peta (P)10+15,Small,Big,Well need to really know the metric system because the pulses are incredibly
5、 short and the powers and intensities can be incredibly high.,Exa (E)10+18,Timescales,Its routine to generate pulses 1 picosecond (10-12 s).Researchers generate pulses a few femtoseconds (10-15 s) long.,Such a pulse is to one minute as one minute is to the age of the universe.Such a pulse is to one
6、second as 5 cents is to the US national debt.,Harold EdgertonMIT, 1942,“How to Make Apple sauce at MIT” 1964,“Splash on a Glass”Junior High School student1996,Strobe photography,Time resolution: a few microseconds,Current record:60 attoseconds!,Ultrafast Ti:sapphire laser,Ultrafast lasers,The shorte
7、st pulses at different wavelengths,Short pulses at short wavelengths,90-degree relativistic Thompson scatteringLawrence Berkeley National Laboratory,Ultrafast set-ups can be very sophisticated.,The highest intensities imaginable,1 kHz “Chirped-Pulse Amplification (CPA)” system at the University of C
8、olorado (Murnane and Kapteyn),0.2 TW = 200,000,000,000 watts!,Even higher intensities!,National Ignition Facility (under construction),192 shaped pulses; 1.8 MJ total energy,Nova,Continuous vs. ultrashort pulses of light,A constant and a delta-function are a Fourier-Transform pair.,Continuous beam:U
9、ltrashort pulse:,Irradiance vs. time,Spectrum,time,time,frequency,frequency,Long vs. short pulses of light,The uncertainty principle says that the product of the temporaland spectral pulse widths is greater than 1.,Long pulse,Short pulse,Irradiance vs. time,Spectrum,time,time,frequency,frequency,Ult
10、rafast laser media,Solid-state laser media have broad bandwidths and are convenient.,Laser power,A generic ultrashort-pulse laser,A generic ultrafast laser has a broadband gain medium, a pulse-shortening device, and two or more mirrors:,Pulse-shortening devices include:Saturable absorbersPhase modul
11、atorsDispersion compensatorsOptical-Kerr media,One way to make short pulses: the saturable absorber,Like a sponge, an absorbing medium can only absorb so much. High-intensity spikes burn through; low-intensity light is absorbed.,Generating short pulses = Mode-locking,Locking vs. not locking the phas
12、es of the laser modes (frequencies),Intensity vs. time,Intensity vs. time,Group velocity dispersion (GVD) broadens ultrashort laser pulses,Different frequencies travel at different group velocities in materials, causing pulses to expand to highly chirped (frequency-swept) pulses.,Longer wavelengths
13、almost always travel faster than shorter ones.,Input ultrashort pulse,Anymedium,Chirped outputnot-so-ultrashort pulse,Pulse compressor,This device has negative group-velocity dispersion and hence can compensate for propagation through materials (i.e., for positive chirp).,Its routine to stretch and
14、then compress ultrashort pulses by factors of 1000.,The longer wavelengths traverse more glass.,Ultrafast optics is nonlinear optics.,At high intensities, nonlinear-optical effects occur.All mode-locking techniques are nonlinear-optical.Creating new colors of laser light requires nonlinear optics.,S
15、econd-harmonic-generation of infrared light yields this beautiful display of intense green light.,Continuum generation,Continuum Generation: focusing a femtosecond pulse into a clear medium turns the pulse white.,Recently developed techniques involving optical fibers, hollow fibers, and microstructu
16、re fibers produce very broadband continuum, over 500 THz (1000 nm) in spectral width!,The continuum from microstructure optical fiber is ultrabroadband.,The spectrum extends from 400 to 1500 nm and is relatively flat (when averaged over time).,This continuum was created using nJ ultrashort pulses.,C
17、ross section of the microstructure fiber.,The Dilemma,In order to measure an event in time, you need a shorter one.To study this event, you need a strobe light pulse thats shorter.But then, to measure the strobe light pulse, you need a detector whose response time is even shorter.And so on,So, now,
18、how do you measure the shortest event?,Using the pulse to measure itself: The Intensity Autocorrelator,Crossing beams in a nonlinear-optical crystal, varying the delay between them, and measuring the signal pulse energy vs. delay, yields the Intensity Autocorrelation, A(2)(t).,The Intensity Autocorr
19、elation:,Nonlinearcrystal,Pulse to be measured,Variable delay, t,Detector,Beamsplitter,E(t),E(tt),Esig(t,t),The signal field is E(t) E(t-t).So the signal intensity is I(t) I(t-t),Frequency-Resolved Optical Gating (FROG),“Polarization Gate” Geometry,Nonlinearmedium,Pulse to be measured,Variable delay
20、, t,Camera,Spec-trometer,Beamsplitter,E(t),E(t-t),Esig(t,t)= E(t) |E(t-t)|2,FROG involves gating the pulse with a variably delayed replica of itself in an instantaneous nonlinear-optical medium and then spectrally resolving the gated pulse vs. delay.,45 polarization rotation,Use any ultrafast nonlin
21、earity: Second-harmonic generation, etc.,FROG traces for linearly chirped pulses,The FROG trace visually displays the pulse frequency vs. time.,Frequency,Frequency,Time,Delay,Negatively chirped Unchirped Positively chirped,One of the shortest events ever created!,Baltuska, Pshenichnikov, and Weirsma
22、,J. Quant. Electron., 35, 459 (1999).,FROG traces,A 4.5 fs pulse!,FROG measurement of the ultrabroadband continuum,This pulse has a time-bandwidth product of 4000, and is the most complex ultrashort pulse ever measured.,Spatio-temporal distortions in pulses,Prism pairs and simple tilted windows caus
23、e “spatial chirp.”,Gratings and prisms cause both spatial chirp and “pulse-front tilt.”,Prism,Angularly dispersed pulse with spatial chirp and pulse-front tilt,Input pulse,Grating,Angularly dispersed pulse with spatial chirp and pulse-front tilt,Input pulse,We can shape ultrashort pulses.,Zewail use
24、d ultrafast-laser techniques to study how atoms in a molecule move during chemical reactions.,The 1999 Nobel Prize in Chemistry went to Professor Ahmed Zewail of Cal Tech for ultrafast spectroscopy.,The simplest ultrafast spectroscopy method is the Excite-Probe Technique.,This involves exciting the
25、sample with one pulse, probing it with another a variable delay later, and measuring the change in the transmitted probe pulse average power vs. delay:,The excite and probe pulses can be different colors.This technique is also called the “Pump-Probe” Technique.,Eex(tt),Excite pulse,Variable delay, t
26、,Epr(t),Samplemedium,Detector,Esig(t,t),Probe pulse,Beyond ultrafast spectroscopy: controlling chemical reactions with ultrashort pulses,You can excite a chemical bond with the right wavelength, but the energy redistributes all around the molecule rapidly (“IVR”).,But exciting with an intense, shape
27、d ultrashort pulse can control the molecules vibrations and produce the desired products.,Multi-photon imaging,Images due to Chris Schaffer, UCSD,In multi-photon imaging, we focus an ultrashort pulse tightly into an object and observe the multi-photon signal light.,F = Two-photon Fluorescence energy
28、,F I2,Two-photon fluorescence imaging,Pollen grain (Clivia Miniata),1.5 m axial resolution200 mW in 16 beamlets,46 sections separated by 0.5 min the axial dimension.2 seconds/image,Conventional image(using fluorescence),Real-time THG images,Anonymous microbes in Amsterdam canal water,Ultrashort puls
29、e lasers can precision machine many materials.,Metals,Ceramics,Diamond,Teeth,Polymers,High Explosives,Sub-micron sculpting,Photo-polymerization: light causes a polymer to solidify. Structural details of 120 nm (due to two-photon photo-polymerization).,Intralase performs vision-correction surgery usi
30、ng fs lasers.,This may be the most common application of ultrashort laser pulses.,Protection from lightning using amplified fs pulses,The pulse induces a conducting path, discharging the cloud charge before lightning can occur.,All that needs to be said about telecom,Actually, from the ruins, a new, more realistic telecom industry will emerge, and it will be ultrafast.,
