UNIVERSIDADE DE SÃO PAULO Instituto de Física de São Carlos Coherent control of light matter interaction Prof. Dr. Cleber Renato Mendonça Photonic Group University of São Paulo (USP), Institute of Physics of São Carlos
ultrashort laser pulses Mode-locking E ( t) E n c n n 2L n exp i 2πν t n n c c 2L n 1, 2, 3,... f Laser System c/2l Modes of the free cavity N g c Laser Medium Femtosecond lasers : N 10 5 10 6 Δν g Laser Modes 1 pulse g
ultrashort laser pulses Intensity (arb. units) Laser Cavity Ti:Sapphire laser oscillator Espelho 100% refletor Prismas para compensação da dispersão da cavidade Espelho 100% refletor Espelho de Saída Pulso curto Espelho direcionador do bombeio Íris Lente convergente Espelho curvo Cristal de Ti:Safira Espelho curvo Laser de bombeio Ti:Sapphire Crystal Absorption Emission 1 pulse g Wavelenght (nm)
Signal (arb. units) ultrashort pulses Femtosecond pulses supplied for the Ti:Sapphire oscilador laser 0.8 ~11,6 ns 0.6 0.4 t Peak ~ 11,6 ns PML ~ 450 mw 0.2 0.0 f ~ 86 MHz E pulse ~ 5 nj -0.2-25 -20-15 -10-5 0 5 10 15 20 25 Time (ns)
Intensity (arb. units) ultrashort pulses Emission spectra of the Ti:Sapphire laser oscillator in CW and ML modes 2000 1500 CW ML 1000 FWHM p ~ 55 nm P CW ~ 250 300 mw 500 ~ 785 nm 0 P ML ~ 450 mw 0 700 750 800 850 900 Wavelength (nm) TL 2 0 0,441 c p TL ~16,5 fs Fourier Transform limited pulse
Dispersion of ultrashort pulse I(t) I(t) Dispersive medium 0 Initial pulse t 0 Final pulse t n( Bluer ) n( redder) Normal dispersive medium 0 Positively chirped pulse t 0 Fourier-transformed limit pulse t Anomaly dispersive medium n( Redder ) n( Bluer ) 0 Negatively chirped pulse t
Dispersion of ultrashort pulse - Chirp chirp E( t) Re I( t) exp{ i[ t ( t)]} 0 Positive chirp pulse inst ( t) 0 d dt smaller freq higher freq 1 ( t) 0 1t 2t 2 2 1 ( ) ( ) ( ) 2 2 0 1 0 2 0 t Grupo de Fotônica IFSC - USP
ultrashort pulses Short temporal duration wide spectral band High light intensity Allows nonlinear effects Control nonlinear process and photo-reactions Grupo de Fotônica IFSC - USP
Coherent control General idea: use the broad spectral band of ultrashort pulses to direct a given optical process ( t) O ( t) Target ( t) O ( t)
Coherent control Distinct combinations of photons of the same pulse can lead the system to a given final state through different pathways How does that actually work?
Nonlinear optics High intensities P E rad. E inter. Charges: Anharmonic Oscillator r P c ( 1 ) r.e c ( 2 ) : r r EE c ( 3 ) M r r r EEE... nonlinear effects are observaded with high intensities
Nonlinear optics 1961: Second Harmonic Generation (SHG), Franken et al. Ruby laser beam (694.2nm) Quartz Crystal (SiO 2 ) Laser light (347.1nm) h h 2h Two-photon Absorption (2PA), Kaiser and Garret Ruby laser beam (694.2nm) Eu 2 :CaF 2 + Crystal Fluorescent light (425 nm) h h X h, X2
Two-photon absorption (2PA) process Phenomenon does not described for the Classical Physics and does not observed until the development of the Laser. 1-photon absorption (Linear) 2-photon absorption (Nonlinear) Maria Göppert-Mayer was born June 28th 1906 in Kattowitz. In 1963 she received the Nobel Prize in Physics. Theoretical model: Maria Göppert-Mayer, 1931 Two photons from an intense laser light beam are simultaneously absorbed in the same quantum act, leading the molecule to some excited state with energy equivalent to the absorbed two photons.
Fluorescence Signal (arb. units) Two-photon absorption Absorbance 4.0 3.5 Materials are excitated by non-ressonant light (outside of the absorption band) 3.0 2.5 2.0 1.5 1.0 0.5 0.0 MEH-PPV Excitation (laser light) Fluorescence (2PAEF) dn dt 2 2 N1 2PAI 5 4 3 2 I I 2 2PAEF Excitation 400 500 600 700 800 Wavelength (nm) Spacial localization of the excitation F I 2 1 0 0 200 400 600 800 I 0 (GW/cm 2 ) Amos, W.B. & White, J.G. (2003)
Microscopy by 2PAEF Applications of 2PA 3D Imagems obtained via 2PAEF Cell 3D Microfabrication by 2PA (Polimerization) Human chromosome Nature 412, 697-698 (2001) Bull Mass-spring system
Applications of 2PA Given all the applications of 2PA, it seems to be interesting to be able to directly control it Just playing with the fs-pulse shape
Manipulating a two-photon process I 1 h i h j 0 I ll have to shape the phase profile of the pulse Grupo de Fotônica IFSC - USP
Shaping the pulses Transmission setup Reflection setup Grupo de Fotônica IFSC - USP
Shapping the pulses Deformable Mirror deformable mirror PC Curved mirrror Diffraction gratting Shaped pulse The deformable mirror acts in each spectral component of the pulse mirror Input pulse Grupo de Fotônica IFSC - USP
Ultrashort pulse shaping technique The MMDM is placed at the Fourier plane of a zero dispersion stretcher consisting of a 600 grove/mm ruled grating and a 25 cm focal-length mirror. V Deformable mirror Diffraction grating Computer Experimental setup for ultrashort pulse shaping technique using deformable mirror Experiment Curve dmirror Modulated pulse Micromachined deformable mirror (MMDM) Entrance pulse Shaping the ultrashort pulse in the phase domain. MMDM is a 600 nm gold-coated silicon nitride membrane (8 mm x 30 mm) suspended over an array of 19 actuator electrodes on a printed circuit board. Potential applied to the actuator creates an electrostatic attraction between the membrane and the electrode, deforming the mirror surface.
Shapping the pulses How to define which shape to use? Genetic Algorithm Phase Mask Grupo de Fotônica IFSC - USP
Genetic Algorithm Shaping the pulse + computer algorithm V Initial population V V Try Best Fathers crossing Mutation sons Evolution In this case, the process is optimized but the mechanism is not well understood Grupo de Fotônica IFSC - USP
Optimization of two-photon induced fluorescence Absorbance 4.0 3.5 3.0 2.5 2.0 1.5 H 3 CO CH 2 CH 3 O CH 2 CH (CH 2 ) 3 CH 3 CH CH N MEH-PPV Shaping pulse technique Computador 1.0 0.5 0.0 790 nm 400 500 600 700 800 Wavelength (nm) Lock-in Amplifier UV-Vis spectra of MEH-PPV in chloroform OUT IN I REFERENCE Ti:Saffire Converging lens Sample Chopper Optical Fiber Photodetector Chopper Control FREQUÊNCIA FREQUÊNCY OUT
Spectral Intensity (arb. units) Fitness Optimization of two-photon induced fluorescence 32000 Optimization process of the 2PAEF spectrum 30000 28000 26000 Best of Generation Best of Process 0 2 4 6 8 10 12 14 16 Generation 500 Initial 2PAEF spectrum Optimized 2PAEF spectrum 400 300 200 100 0 550 600 650 700 750 800 850 Wavelength (nm)
Optimization of two-photon induced fluorescence What is happening with the pulse during its evolution
Fitness Intensity (arb. units) Intensity (arb. units) Fitness Intensity (arb. units) Intensity (arb. units) Optimization of the two-photon induced fluorescence 5.2 5.0 Best of generation Best of process 4.8 4.4 P 50 fs 7.5 7.0 6.5 P 32 fs 4.8 4.0 6.0 4.6 3.6 5.5 4.4 4.2 3.2 2.8 5.0 4.5 4.0 4.0 2.4 3.5 3.8 0 5 10 15 20 25 30 Generation 2.0-150 -100-50 0 50 100 150 Delay (fs) 3.0-150 -100-50 0 50 100 150 Delay (fs) Evolution of the optimization process of the ultrashort pulse Autocorrelation obtained for non optimized pulse Autocorrelation obtained for optimized pulse 5.0 4.8 4.6 Best of generation Best of process 4.5 4.0 P 30 fs 5.6 5.2 4.8 P 20 fs 4.4 3.5 4.4 4.2 4.0 3.0 4.0 3.6 3.8 2.5 3.2 3.6 2.8 3.4 0 5 10 15 20 25 30 Generation 2.0-150 -100-50 0 50 100 150 Delay (fs) 2.4-150 -100-50 0 50 100 150 Delay (fs) Evolution of the optimization process of the ultrashort pulse Autocorrelation obtained for non optimized pulse Autocorrelation obtained for optimized pulse
Optimization of two-photon induced thermal lens Norm. Trans. R Intensity Lens Sample Lens Photodetector Z-Scan technique (Nonlinear absorption) 1.02 1.00 0.98 0.96 0.94 0.92 0.90-10 -5 0 5 10 Z(mm) Feedback Signal Nonlinear transmittance Shaping pulse technique Computer Samples Fluorescents and non-fluorescents Ti:Saffire Laser Chopper Converging lens? Sample Lock-in amplifier OUT Photodetector IN I REFERENCE Chopper Control FREQUÊNCIA OUT
Normalized Transmittance 2PAEF Intensity (arb. units) Optimization of two-photon induced thermal lens Fitness Normalized Transmittance 2PAEF Intensity (arb. units) 1,000 Sample: MEH-PPV 1,000 0,996 0,996 0,992 0,992 0,988 0,988 0,984 T ~ 1,2 % 0,074 0,984 T ~ 1,5 % -6-4 -2 0 2 4 6 Z (mm) 0,072-6 -4-2 0 2 4 6 Z (mm) 0,070 4,5 4,0 3,5 P ~ 29fs 0,068 0,066 0 5 10 15 20 25 30 Generation 5,5 5,0 4,5 P ~ 19 fs 4,0 3,0 3,5 2,5 3,0 2,0-100 -50 0 50 100 Delay (fs) 2,5-100 -50 0 50 100 Delay (fs)
Normalized Transmittance 2PAEF Intensity (arb. units) Optimization of two-photon induced thermal lens Fitness 2PAEF Intensity (arb. units) Normalized Transmittance 6,0 5,5 p ~ 50 fs Sample: MEH-PPV 3,6 P ~ 25 fs 5,0 3,2 4,5 4,0 2,8 3,5 2,4 3,0 0,16 2,0 2,5-100 -50 0 50 100 Delay (fs) 0,14 1,6-100 -50 0 50 100 Delay (fs) 0,12 1,000 0,996 0,10 0 5 10 15 20 25 30 Generation 1,000 0,996 0,992 0,992 0,988 0,988 0,984 T ~ 0,85 % 0,984 T ~ 1,6 % -6-4 -2 0 2 4 6 Z (mm) -6-4 -2 0 2 4 6 Z (mm)
Phase Mask In this case, we impose a known phase mask function to the ultra-short pulse V 0 deformable mirror Diffraction gratting x curved mirror Grupo de Fotônica IFSC - USP
Phase Mask Two-photon absorption transition is given by Anit-symmetric phase mask: S 2 is independent of the phase Symmetric phase mask: S 2 is minimized Grupo de Fotônica IFSC - USP
Controlling the MEH-PPV photo-degradation MEH-PPV: conductive and luminescent polymer with interesting properties for applications However, MEH-PPV photo-bleaches due to a photoxidation reaction, causing a decrease in its emission Grupo de Fotônica IFSC - USP
Controlling the MEH-PPV photo-degradation MEH-PPV sample Lens PC Optical fiber 0,12 210 Lock-in Amplifier Observe the two-photon excited emission as a function of the phase-mask Observe the photodegradation for distinct phase masks Grupo de Fotônica IFSC - USP
Controlling the MEH-PPV photo-degradation
Controlling the MEH-PPV photo-degradation =1 0.07 %/min = 0.5 0.1 %/min FTL 0.3 %/min Photobleaching rate is smaller for the phase-masked pulses
Coherent control of molecular orientation Use fs-laser (broad band) to induce two-photon absorption and, consequently, molecular orientation (optical storage) Coherently control the molecular orientation
Coherent control of molecular orientation
Coherent control of molecular orientation
Coherent control of molecular orientation
Conclusions Pulse shaping methods + coherent control of the nonlinear interaction seems to be an interesting method to further control nonlinear optical processes.
Thanks for your attention!!!