4It took a long time….And in 1960 the LASER was invented: Soon to be described as a “solution looking for a problem”
5Where was I ?th grade student in Jerusalem, deciding to continue my studies in the US Berkeley, looking for a Thesis advisor Options: Shen, Townes, Hahn post doc position: Nico Bloembergen – Weizmann Institute (ever since)
6How does a laser work ?Monochromatic, Directional, Intense, Coherent
11Spontaneous Raman spectrum of CHCl3 Direct spontaneous Raman spectrum (from the catalogue)
12Conservation of Momentum Four Wave Mixing (FWM) and Coherent Anti Stokes Raman Scattering (CARS)Conservation of Momentum(phase matching)Energy conservationWRamanw1w2wAS2w1- w2- wAS = 0Dkk1k2kCARSDk = 2k1-k2-kAS= 0
13FWM Applications included: Molecular spectroscopyRotational and vibrational dynamicsSolid state fast relaxation phenomenaPhoton echoesCombustion diagnosticsSurface diagnosticsBiological applicationsMicroscopyRemote sensing…….
14Spectroscopy can be performed either in the frequency domain or in the time domain. In the frequency domain, we scan the frequency of excitation (absorption), or the frequency of observation (Spontaneous Raman spectroscopy), etc.Alternatively, we can capture the time response to impulse excitation, and then Fourier Transform this signal to obtain a frequency domain spectrum.We are always taught that the choice of one or the other is a matter of convenience, instrumentation, efficiency, signal to noise, etc. but that the derived physical information is the same, and therefore the measurements are equivalent.
15Time Resolved Four Wave Mixing A pair of pulses (Pump and Stokes) excites coherent vibrations in the ground stateA third (delayed) pulse probes the state of the system to produce signalThe delay is scanned and dynamics is retrieved
16However, practically ALL CARS and Four Wave Mixing experiments were/are performed in the frequency domain.i.e. one is not directly measuring the molecular polarization (wavefunction) which is oscillating at optical frequencies.
17Combined Time Frequency Detection of Four Wave Mixing With:Dr. Yuri Paskover (currently in Princeton)Andrey Shalit
18Outline Time Frequency Detection (TFD) : the best of both worlds Single Shot Degenerate Four Wave MixingTunable Single Shot Degenerate Four Wave MixingMultiplex Single Shot Degenerate Four Wave MixingTFD simplified analysisConclusions
27Spectral Distribution of the Observed Features 104 cm-1365 cm-1Observed frequency: cm-1Observed detuning : cm-1Observed frequency: cm-1Observed detuning : cm-1
281015 times faster, or in < 100 femtoseconds ! However, this is a long measurement, it takes approximately 10 minutes, or >> 100 seconds.In what follows I will show you how this same task can be performed much faster.1015 times faster, or in < 100 femtoseconds !
30Time Resolved Four Wave Mixing EaEbEcTime delayPhase matching~ femtosecond pulses ~ 0.1 mJ per pulse
31Spatial Crossing of two short pulses: Interaction regions k3k1k1 arrives firstk3 arrives first100 fsec = 30 microns5mmBeam diameter – 5 mmDifferent regions in the interaction zone correspond to different times delays
32Three pulses - Box-CARS geometry Time delays Spatial coordinates32
57ConclusionsTime Frequency combined measurements offer advantages over either domain separatelySpecific advantages in spectroscopy of unknown species, by the ability to identify the character of observed lines (fundamental or beat modes)Advantages in cleaning up undesirable pulse distortionsSingle mode FWM measurementsTunable single mode FWM measurementsMultiplex single mode FWM measurementsSignificant theoretical foundation (not discussed here)More work needed to improve resolution, bandwidth, accuracy, reproducibility, etc
58Thank you Acknowledgements Dr. Alexander Milner, Dr. Riccardo Castagna, Dr. Einat Tirosh, Sharly Fleischer, Andrey Shalit, Atalia Birman, Omer Korech, Dr. Mark Vilensky, Dr. Iddo PinkasThank you