Introduction to Ultrafast

Spectroscopy(Pump-Probe,CARS)

Rapolu Mounika

Research scholar

ACRHEM

Under Supervison of

Dr.Soma venugopal Rao

Associate Professor

University of Hyderabad

Outline of talk

• Introduction.

• Basics of Ultrafast laser.

• Parameters that governs Ultrafast Experiments.

• Ultrafast Pump-Probe.

• Ultrafast CARS.

• Summary.

Introduction

• Ultrafast optics is nonlinear optics.

• Ultrafast lasers species of order of 500 femtoseconds or less.

• At high intensities, nonlinear-optical effects occur.

• All mode-locking techniques are nonlinear-optical.

• Creating new colors of laser light requires nonlinear optics.

.

Concepts of Mode Locking

Out of phase

RANDOM phase for all the laser modes

Irradiance vs. Time

Time Time

Out of phase Out of phaseIn phase

LOCKED phases for all the laser modes

Mode locking is a method to obtain ultrafast pulses from lasers, which are then

called mode-locked lasers mode

Basic principles of ultrafast lasers

Bandwidth vs Pulsewidth

narrow spectrum

continuous wave (CW)

broader spectrum

pulses (mode-locked)

broadest spectrum

shortest pulses

bandwidthDn

durationDt

DnDt = const.

Repetition Rate• No. of emitted pulses per second .

• In high repetition rate systems the material in

the focal region gains heat and is thermally

loaded by successive pulses, then permanent

structural changes take place

• It have a significant effect on the profile of

the index change at higher repetition rates

(tin the MHz region).

• This is shown to be due to the thermal

diffusion of the material being longer than

the pulse to pulse separation. This effect has

not been observed at lower repetition rates in

the kHz range.

Peak Intensity at focus • The key being that the peak power must

exceed the material threshold energy

• Peak Intensity at focus :

Ioo2=2*Energy/(pi*w0^2*pulsewidth)

Energy=Power/reprate

• 150fs,80MHz laser – Peak Power intensity in

MW.

• 2ps,1KHz laser – Peak Power intensity in

GW.

EKSPLA 30 ps Laser

• Nd:YAG Laser- 30ps

• :1064nm,532nm,355nm,266nm,216nm

• Beam Diameter: 12 mm

• Rep Rate: 10 Hz

• Pulse Energy at 1,064 nm: 100 mJ

MICRA OSCILLATOR+LEGEND AMPLIFIER

• O/P Micra Oscillator :λ≈ 800 nm,20 fs.

• Rep. rate:78 MHz.

• Power: 0.5 w.

• O/P of Ti:S regenerative amplifier : λ≈ 800 nm ,40 fs .

• Rep. rate :1 KHz.

• Power: 2.5 w.

Lasers at ACRHEM

INNOLAS 7ns Laser

• Nd:YAG Laser :7 ns

• :1064nm,532nm,355nm,266nm

• Beam Diameter: 8 mm

• Repetition Rate: 1 to 10 Hz

• Pulse Energy at 1064 nm: > 1.3 J

CHAMELEON 150fs Laser

• Ti:S Laser :150 fs

• :680-1080nm @800nm

• Beam Diameter: 3 mm

• Repetition Rate: 80 MHZ

• Pulse Energy at 1,064 nm: > 400 mJ

Lasers at ACRHEM

The 1999 Nobel Prize in Chemistry went to Professor Ahmed Zewail of

Cal Tech for ultrafast spectroscopy.

Zewail used ultrafast-laser techniques to study how atoms in a molecule

move during chemical reactions.

.

11

At a certain excitation energy:

1. Which reaction path is the most important for the excited-state relaxation?

2. How long does this relaxation take?

The Dynamics Problem

Ultrafast Laser Spectroscopy: Why?

• Most events that occur in atoms and molecules occur on fs and ps timescales.

• Collisions in room-temperature liquids occur on a few-fs time scale, so nearly

all processes in liquids are ultrafast.

• Investigating the dynamics of the system directly: energy transfer, the

movement of individual particles (electrons or atoms).

Ultrafast Pump-probe technique

• Pump probe spectroscopy is the simplest experimental technique used to study ultrafast electronic dynamics.

• An ultrashort laser pulse is split into two portions; a stronger beam (pump) is used to excite the sample, generating a non-equilibrium state, and a weaker beam (probe) is used to monitor the pump-induced changes in the optical constants (such as reflectivity or transmission) of the sample.

• Measuring the changes in the optical constants as a function of time delay between the arrival of pump and probe pulses yields information about the relaxation of electronic states in the sample.

Experimental Schematic

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.

• But exciting with an intense, shaped ultrashort pulse can control the molecule’s

vibrations and produce the desired products.

Super continuum

Generation :

Supercontinuum generation is the formation of broad continuous spectra by

propagation of high power pulses through nonlinear media

• Focusing a ultrafast laser pulse into

a clear medium turns the pulse

white.

• Generally, small-scale self-focusing

occurs, causing the beam to break

up into filaments.

• It is not a specific phenomenon but

rather a plethora of nonlinear

effects, which, in combination, lead

to extreme pulse broadening.

Coherent anti-Stokes Raman scattering (CARS)• Recent developments in nonlinear optics further provided powerful related spectroscopic

techniques such as stimulated Raman spectroscopy, coherent anti-Stokes Ramanspectroscopy, and higher order Raman effects.

• CARS involves three laser beams which interact with the sample and generate a newcoherent optical signal at the anti-Stokes frequency.

• The oscillation frequencies of molecular vibrations reflect the chemical structure and arewidely used as a spectroscopic fingerprint for chemical detection and identification. One ofthe most efficient optical techniques to acquire the vibrational spectrum is Coherent Anti-Stokes Raman Scattering (CARS) spectroscopy.

• CARS is a third order nonlinear optical process in which a pump field Ep and aStokes field Es interact with a sample to generate a signal field Eas at the anti-Stokesfrequency of ωas = 2ωp - ωs. When ωp - ωs is tuned to be resonant with a molecularvibration the CARS signal can be significantly enhanced, producing a vibrationalcontrast.

History of CARS• In 1965, a paper was published by two researchers of the Scientific Laboratory at

the Ford Motor Company. P. D. Maker and R. W. Terhune, in which the CARS phenomenon was reported for the first time.

• So, the its clear that acronym CARS have an inadvertent relation to automobiles. Maker and Terhune used a pulsed ruby laser to investigate the third order response of several materials.

• They first passed the ruby beam of frequency ω through a Raman shifter to create a second beam at ω-ωv, and then directed the two beams simultaneously onto the sample. When the pulses from both beams overlapped in space and time, the Ford researchers observed a signal at ω+ωv, which is the blue-shifted CARS signal.

• They also demonstrated that the signal increases significantly when the difference frequency ωv between the incident beams matches a Raman frequency of the sample. Maker and Terhune called their technique simply 'three wave mixing experiments'.

Fs-CARS experiment

55 O

t

Stokes

pump

signal

probe

Enhancement of CARS signal• The anti-Stokes radiation is unique to both the molecule and the vibrational mode

enabling high chemical specificity and thus aid in the identification of the scatteringmedium.

• The spectral bandwidth of the laser pulse is board enough to coherently andsimultaneously excite all the vibrational modes in the molecule of interest allowingfor amplification in the anti-Stokes Raman wave.

• The plot, CARS signal intensity versus wave number of Stokes, gives the line widthof emitted CARS radiation, from which we can calculate the dephasing timebroadening etc. can be determined.

• To get the CARS signal in liquids, the angle between pump and stokes at the sampleshould be 10-30 . In addition delay between both pump & Stokes pulse should beless than or equal to 1ns.

• For enhancement in CARS signal, SE-CARS can also used.

rotations

Incoherent processes are:

Radiative decay + Collisions

T. Lang , M. Motzkus, H.-M. Frey and P. Beaud, J. Chem. Phys., 115 (2001).

rotations &

vibrations

vibrations

Time-resolved CARS