Combined Time Frequency Detection (TFD) by Single Shot Four Wave Mixing Yehiam Prior and Andrey Shalit Department of Chemical Physics Weizmann Institute of Science, Rehovot, Israel Coherent control, SafedSeptember 2012

Spectroscopy can be performed either in the frequency domain or in the time domain Scan the frequency to observe a spectrum OR capture the time response to impulse excitation, and Fourier Transform to obtain a spectrum We are taught that it is a matter of convenience, instrumentation, efficiency, signal to noise, etc. but otherwise the derived physical information is the same, and is all we can have.

Spontaneous Raman spectrum of chloroform (CHCl 3 ) Direct spontaneous Raman spectrum (Frequency domain - from the catalogue)

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

kk k1k1 k1k1 k2k2 k CARS Energy conservation Conservation of Momentum (phase matching)  Raman 11 11 22  AS  1 -  2 -  AS = 0  k = 2k 1 -k 2 -k AS = 0 Coherent Anti Stokes Raman Scattering (CARS)

~ femtosecond pulses ~ 0.1 mJ per pulse EaEa EbEb EcEc Time delay Phase matching Time Resolved Four Wave Mixing

F.T.

Time Domain vs. Frequency Domain In this TR-FWM the signal is proportional to a (polarization) 2 and therefore beats are possible

Experimental System (modified)

Time frequency Detection (CHCl 3 ) Summation over all frequencies (Δ)

F.T

Limited Band Detection Summation over 500cm -1 window

Open vs. Limited Detection Open band: Limited band: F.T

Time Frequency Detection CHCl 3

Spectral Distribution of the Observed Features 104 cm cm -1 Observed frequency: 104 cm -1 Observed detuning : 310 cm -1 Observed frequency: 365 cm -1 Observed detuning : 180 cm -1

Take home message 1: Working with spectrograms (combined time frequency domains) can be useful BUT

This is a long measurement, it takes approximately 10 minutes, or >> 100 seconds or >> 100,000 pulses. Do we care that this is a long measurement?

Chlorophyll derivatives for PDT Number of Pulses ( X100) Normalized Transmission 2x10 15 photons/cm 2 Pixels Pixels a b Transmission Avigdor Scherz Iddo Pinkas

Chlorophyll derivatives for PDT Avigdor Scherz Iddo Pinkas Transmission - Photo-bleaching Number of Pulses ( X100) Normalized Transmission Pixels Pixels a b

Take home message 2: Working with spectrograms (combined time frequency domains) can be useful Doing it rapidly is important

The measurement takes well over 100 seconds In what follows I will show you how this same task can be performed much faster times faster, or in < 100 femtoseconds !

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

+y-y k 1 firstk 3 first z k1k1 k2k2 k3k3 Pump-probe delay k1k1 k2k2 k3k3 Intersection Region: y-z slice

Single Pulse CARS Image CH 2 Cl 2

Time Resolved Signal and its Power Spectrum

CHBr 3 Several modes in the range

Time Resolved Signal and its Power Spectrum

Strict Phase Matching : Phase Matching Filtering x y z

Combined Time Frequency Measurements Scanning angle = Scanning frequency

Combined Time Frequency Measurements by Single Shot

+y-y k 1 firstk 3 first z Intersection Region: y-z slice

+y-y z Δ Intersection Region: y-z slice

Time Frequency Detection: Single Shot Image Focusing angle : δ = 3 mrad (CH 2 Br 2 )

Time Frequency Detection by Single Shot: Fourier Transformed (CH 2 Br 2 )

Scanned Measurements

Single pulse measurement Scanning method >10 5 pulses

Take home message 3: Working with spectrograms (combined time frequency domains) can be useful Doing it rapidly is important A full spectrum can be obtained within a single laser pulse

What can we do with this new capability?

Single Pulse Image (CH 2 Br 2 ) 180 cm cm -1 Ω= 180 cm -1 : Real vibrational frequency as a result of the inherent heterodyning due to the molecular anisotropy. Ω =360 cm -1 = 180cm -1 x2 : Homodyne beat due to the interference of two contributing spectroscopic pathways in non-resonant DFWM.

Magic Angle Polarization S total = S iso +S aniso k1k1 k1k1 k2k2 k2k2 k3k3 k3k3 360cm cm -1

|R zzzz | 2 |R zzmm | 2 Polarization Sensitive Single Shot

Time-Polarization Single Shot

Take home message 4: Working with spectrograms (combined time frequency domains) can be useful Doing it rapidly is important A full spectrum can be obtained within a single laser pulse Depolarization ratios can be easily measured

Two Dimensional Time Resolved Single Shot

Two Dimensional Image: IR 140

Photo-degradation of WST11

Pd Role of the Central Atom Without metal atom With Pd

Take home message 5 : Working with spectrograms (combined time frequency domains) can be useful Doing it rapidly is important A full spectrum can be obtained within a single laser pulse Depolarization ratios can be easily measured Photo-bleachable molecules are now more readily accessible for measurements

Summary Working with spectrograms (combined time frequency domains) can be useful Doing it rapidly is important A full spectrum can be obtained within a single laser pulse Depolarization ratios can be easily measured Photo-bleachable molecules are now more readily accessible for measurements [1] Y. Paskover, I. S. Averbukh, YP, Opt. Exp. 15, 1700 (2007) [2] A. Shalit, Y. Paskover, YP, Chem. Phys. Lett 450, 408, (2008) [3] Y. Paskover, A Shalit Y. Prior, Opt. Comm. 283, 1917 (2010) [4] A. Shalit and Yehiam Prior, PCCP, In press (2012)

+ 0 Detuning from a probe (k 3 ) carrier frequency k1k1 -k 2 k3k3 k1k1 k3k3 Time Frequency Detection

Detuning from a probe carrier frequency (Δ) 0 Spectral Distribution of the Signal Produced by a Fundamental Mode In TR-DFWM, we have shown that because of the quadratic dependence on the polarization, fundamental modes may be seen only after linearization of the signal, i.e. by heterodyne detection

Spectral Distribution of the Signal Produced by Intensity Beat Detuning from a carrier (Δ) 0

Identification of signals: Fundamental modes of frequency Ω 1 are spectrally peaked at Ω 1 /2 Intensity beats at frequency (Ω 1 ± Ω 2 ) spectrally peaked at [ (Ω 1 -Ω 2 )/2 ] Based on this result, it is now possible to directly and unambiguously identify the character of each peak

TFD analysis: CCl 4

Homodyne beat : (Ω 1 -Ω 2 )=99cm -1 Detuning : (Ω 1 +Ω 2 ) /2=260 cm -1 Ω 1 = 210 cm -1 ; Ω 2 = 309 Homodyne beat : (Ω 3 -Ω 4 ) = 246cm -1 Detuning : (Ω 3 +Ω 4 )/2 =337 cm -1 Ω 4 = 214 cm-1 ; Ω 3 = 460

Homodyne beat : (Ω 5 -Ω 6 ) = 147cm-1 Detuning : (Ω 5 +Ω 6 ) /2 = 385 cm -1 Ω 5 = 317 cm -1 ; Ω 6 = 464 cm DERIVED fundamental frequencies KNOWN CCl 4 Modes TFD analysis: CCl 4

Thank you