And in 1960 the LASER was invented: Soon to be described as a solution looking for a problem It took a long time….
1960 - 7 th grade 1970 - student in Jerusalem, deciding to continue my studies in the US 1971 - Berkeley, looking for a Thesis advisor Options: Shen, Townes, Hahn 1977 - post doc position: Nico Bloembergen 1979 – Weizmann Institute (ever since) Where was I ?
How does a laser work ? Monochromatic, Directional, Intense, Coherent
So, what can we do with these Coherent sources?
Spontaneous Raman spectrum of CHCl 3 Direct spontaneous Raman spectrum (from the catalogue)
k k1k1 k1k1 k2k2 k CARS Energy conservation Conservation of Momentum (phase matching) Raman 1 1 2 AS 1 - 2 - AS = 0 k = 2k 1 -k 2 -k AS = 0 Four Wave Mixing (FWM) and Coherent Anti Stokes Raman Scattering (CARS)
FWM Applications included: Molecular spectroscopy Rotational and vibrational dynamics Solid state fast relaxation phenomena Photon echoes Combustion diagnostics Surface diagnostics Biological applications Microscopy Remote sensing …….
Spectroscopy 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.
Time Resolved Four Wave Mixing A pair of pulses (Pump and Stokes) excites coherent vibrations in the ground state A third (delayed) pulse probes the state of the system to produce signal The delay is scanned and dynamics is retrieved
However, 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.
Combined Time Frequency Detection of Four Wave Mixing With: Dr. Yuri Paskover (currently in Princeton) Andrey Shalit
Time Frequency Detection (TFD) : the best of both worlds Single Shot Degenerate Four Wave Mixing Tunable Single Shot Degenerate Four Wave Mixing Multiplex Single Shot Degenerate Four Wave Mixing TFD simplified analysis Conclusions Outline
Spectral Distribution of the Observed Features 104 cm -1 365 cm -1 Observed frequency: 104 cm -1 Observed detuning : 310 cm -1 Observed frequency: 365 cm -1 Observed detuning : 180 cm -1
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. 10 15 times faster, or in < 100 femtoseconds !
~ 50-100 femtosecond pulses ~ 0.1 mJ per pulse EaEa EbEb EcEc Time delay Phase matching Time Resolved Four Wave Mixing
Spatial Crossing of two short pulses: Interaction regions k3k3 k1k1 5mm Beam diameter – 5 mm 100 fsec = 30 microns Different regions in the interaction zone correspond to different times delays k 1 arrives first k 3 arrives first
Three pulses - Box-CARS geometry Time delays Spatial coordinates
Time Frequency combined measurements offer advantages over either domain separately Specific 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 distortions Single mode FWM measurements Tunable single mode FWM measurements Multiplex single mode FWM measurements Significant theoretical foundation (not discussed here) More work needed to improve resolution, bandwidth, accuracy, reproducibility, etc Conclusions
Acknowledgements Dr. Alexander Milner, Dr. Riccardo Castagna, Dr. Einat Tirosh, Sharly Fleischer, Andrey Shalit, Atalia Birman, Omer Korech, Dr. Mark Vilensky, Dr. Iddo Pinkas Thank you