Single Cycle Optical Pulses Synchronized with Molecular Oscillators

Dr. Alexei Sokolov, Texas A&M (KITP Quantum Optics 7/26/02) 1

Single-Cycle Optical Pulses

Synchronized with Molecular Oscillations

Alexei SokolovTexas A&M University

Miaochan Zhi, Andrea Burzo,Igor Marienko, Roman Kolesov,Olga Kocharovskaya

Stanford University

David Walker, Deniz YavuzG. Y. Yin, and Steve Harris

University Electro-Communications,Chofu, Tokyo

Fam Le Kien, Kohzo Hakuta

- This work will show that coherent molecular motion can be used to modulate light,and as a result produce sub-femtosecond and sub-cyclepulses,

- show that this light sourcecan be applied for multiphotonionization, and the ionization,in turn, can be utilized tocharacterize the source,

- describe how this new lightsource can be used to studynew physics.

Single Cycle Optical Pulses Synchronized with Molecular Oscillators

Dr. Alexei Sokolov, Texas A&M (KITP Quantum Optics 7/26/02) 2

Ultrashort Pulses

Why?

- observation and control of molecular and atomic dynamics;

- time-resolved photoelectron andphotoionization spectroscopy;

- timed Coulomb explosion imaging;

- UV and X-ray amplification and lasing.

Techniques for attosecondpulse generation:

- HHG,

- Raman.

Single Cycle Optical Pulses Synchronized with Molecular Oscillators

Dr. Alexei Sokolov, Texas A&M (KITP Quantum Optics 7/26/02) 3

Atom in a strong laser field

V

x

High-order Harmonic Generation=

tunneling ionization + recollision

e-

1. Broadband collinear Raman generator:

- high coherence, low pressure regime;

- key idea: adiabatic preparation of a

single molecular superposition-state;

- two mechanisms of molecular modulation.

2. Multiphoton ionization with single-cycle pulses,

and waveform characterization by

pulse-shape dependent photoionization.

3. Sub-cycle pulse shaping.

4. Synchronization of the pulse train with respect

to the molecular motion.

Outline:

Single Cycle Optical Pulses Synchronized with Molecular Oscillators

Dr. Alexei Sokolov, Texas A&M (KITP Quantum Optics 7/26/02) 4

Strongly driven molecular systems

ΩΩ00 ΩΩ−−11δδωω

>> ||∆∆ωω||

ρab = ρaa = ρbb =1

2

ΩΩ00ΩΩ−−11||∆∆ωω||

δδωω

a

b

i

|b>

|a>

|i>

E 0 E 1 E 2 E 3E−2 E −1

1) Refractive index control in an EIT-like effect.

2) Broadband spectral generation.

3) Ultrashort pulse generation.

Sideband generation and propagation

∂Eq

∂z= − jη

ωq N aqρaa Eq + dqρbb Eq(+bq

*ρabEq−1 + cqρab* Eq +1)

Single Cycle Optical Pulses Synchronized with Molecular Oscillators

Dr. Alexei Sokolov, Texas A&M (KITP Quantum Optics 7/26/02) 5

Subfemtosecond pulse generation by molecular vibration in deuterium

0

0.3

0.6

0.9

0

8

16

24

0

0.3

0.6

0.9

0

8

16

24

Frequency (1,000 cm −1)

0

0.3

0.6

0.9

0 20 40 60 80

Time (fs)

0.2 fs

0

8

16

24

0 5 10 15 20

Nor

mal

ized

Ins

tant

aneo

us P

ower

Den

sity

|E(t

)|2

Nor

mal

ized

Spe

ctra

l Dis

trib

utio

n |E

( ωω)|

L = 0

L = 2 cm

L = 4 cm

|b>

|a>

E 0E −1

|i>

D2

P = 1 atm; L = 4 cmE −1

E 0

Generation and pulse compression occur in the same cell.

υab=2994 cm−1

Single Cycle Optical Pulses Synchronized with Molecular Oscillators

Dr. Alexei Sokolov, Texas A&M (KITP Quantum Optics 7/26/02) 6

Collinear generation of a wide vibrational Raman spectrum in low-density molecular deuterium (D2)

We observe:13 anti-Stokes + 2 Stokes + 2 pump frequencies.

narrow-linewidth

driving lasers

Nd:YAG

Ti:Slaser

P=71 torr, L=50 cm,T=77 K

D2 cell

screen

camera

Visible (attenuated by 100)IR UV

- 400MHz

100MHz

700MHz

∆ω =

Single Cycle Optical Pulses Synchronized with Molecular Oscillators

Dr. Alexei Sokolov, Texas A&M (KITP Quantum Optics 7/26/02) 7

Spectral modification

Nd:YAG

Ti:Slaser

D2 cell

testcell

- pulse shaping by spectral modification;

- selective ion production;

- number of photons, required for

ionization: 11 or 5 ?

- waveform characterization by

pulse-shape dependent photoionization.

Coherent Control of

Multiphoton Ionization on a

Few-Femtosecond Time Scale

Single Cycle Optical Pulses Synchronized with Molecular Oscillators

Dr. Alexei Sokolov, Texas A&M (KITP Quantum Optics 7/26/02) 8

Selective ion production

Mass spectrum produced by 5 sidebands phased to(a) maximize photoionization of Xenon, and(b) minimize photoionization of Xenon.

0

0.5

1

1.5

Ion

Sign

al, a

rb. u

.

(a)

0 0.4 0.8 1.2 1.6

C

Xe

0

0.5

1

Ion

Sign

al, a

rb. u

.

Time of flight, µs

(b)

0 0.4 0.8 1.2 1.6

Multiphoton ionization of Xenon.How many photons are required: 11 or 5 ?

Does that number depend on pulse shape?

0.01

0.1

1

50 60

all sidebands in phasevisible AM + infrared FMvisible FM + infrared AM

Ion

Sign

al, a

rb. u

.Total Laser Energy, mJ

6th-order response, regardless of relative sideband phases

Single Cycle Optical Pulses Synchronized with Molecular Oscillators

Dr. Alexei Sokolov, Texas A&M (KITP Quantum Optics 7/26/02) 9

Correlation of waveforms, synthesizedby subsets of Raman sidebands

Sin[w1t] + Sin[w2t] + Sin[w3t] + Sin[w4( t+t)] + Sin[w5( t+t)] 12

0

0.5

1

1.5

2

0

0.5

1

-15 -10 -5 0 5 10 15 20

Experiment Theory

Ion

Sign

al, a

rb. u

.

Time Delay, fs

Simple calculation, assuming equal sideband amplitudes and a 6th-order detector:

Sub-cycle phase selection

E(ω)

E(t)

E(t)

t

t

ω

2x

E(ω)

ω

2x

Single Cycle Optical Pulses Synchronized with Molecular Oscil lators

Dr. Alexei Sokolov, Texas A&M (KITP Quantum Optics 7/26/02) 10

Pulse shaping: “old” and “new”

“Traditional” pulse shaping:

Sub-cycle pulse shaping:

t

E(t)

t

E(t)

t

E(t)

t

t

t

E(t)

E(t)

E(t)

Atom (or molecule) in a strong

sub-cycle-shaped laser field.

V

x

The electron trajectory, the recollision time, and

the recolli sion energy can be controlled precisely.

e-

Single Cycle Optical Pulses Synchronized with Molecular Oscillators

Dr. Alexei Sokolov, Texas A&M (KITP Quantum Optics 7/26/02) 11

Molecular oscillations modulate light and produce trains of

pulses, which are (by the very nature of the modulation

process) perfectly synchronized with the molecular motion.

Ele

ctri

c F

ield

, a. u

.

0 10 20 30

T τ

Time, fs

H+

H+

2e

EIT-like control ofmultiphoton ionization

|a>|b>

1 2

e iϕ 2

Single Cycle Optical Pulses Synchronized with Molecular Oscillators

Dr. Alexei Sokolov, Texas A&M (KITP Quantum Optics 7/26/02) 12

EIT-like control ofmultiphoton ionization

with a possibility of amolecular wavepacket excitation

a

cd

ba

cd

b

Possible extensions of our technique range from studying

complicated multi-mode motion of complex molecules,

to probing ultrafast electronic dynamics in atoms.

t

4 1 2 3 4 1 2

Single Cycle Optical Pulses Synchronized with Molecular Oscillators

Dr. Alexei Sokolov, Texas A&M (KITP Quantum Optics 7/26/02) 13

0

0.5

1

1.5

2

0

0.5

1

-15 -10 -5 0 5 10 15 20

Experiment Theory

Ion

Sign

al, a

rb. u

.

Time Delay, fs

Extension of EIT ideas to molecular systems allowed us

to create a new light source: a collinear Raman generator.

This new light source allows us to study new physics:

We demonstrate photoionization with single-cycle pulses.

Theory: S. E. Harris and A. V. Sokolov, Phys. Rev. A 55, R4019 (1997);

S. E. Harris and A. V. Sokolov, Phys. Rev. Lett. 81, 2894 (1998);

A. V. Sokolov, Opt. Lett. 24, 1248 (1999).

Experiment: A. V. Sokolov, D. R. Walker, D. D. Yavuz, G. Y. Yin, and

S. E. Harris, Phys. Rev. Lett. 85, 562 (2000); A. V. Sokolov, D. R. Walker,

D. D. Yavuz, G.Y. Yin, and S. E. Harris, Phys. Rev. Lett. 87, 33402 (2001).

We have demonstrated that coherent molecular motion can modulate light, producing ultrashort pulses which are perfectly synchronized with the molecular oscillation.

This light source opens new horizons for sub-cycle pulse shaping, and for subfemto-second studies of atomic and molecular dynamics.

Conclusion: