Sharly Fleischer, Yan Zhou, Robert W. Field, Keith A. Nelson

FRISNO 11

Orientation and Alignment of Gas Phase Molecules by Single Cycle THz Pulses

Multiphoton Ionization Self focusing & Filamentation

0 2n n n I

High Harmonic GenerationUltrafast x-ray diffraction

Up – Down SymmetryConserved

DC field

+

-

Not Field Free !

Sakai and colleagues:

DC electric field + laser pulse with adiabatic turn-on and nonadiabatic turn-off.

Approaches for field – free orientation

A. Goban, S. Minemoto, and H. Sakai, “Laser-Field-Free Molecular Orientation,“ Phys. Rev. Lett. 101, 013001 (2008).

Vrakking and colleagues:

Hexapole state selector (Producing molecules in a single quantum state) + a combination of a DC and intense fs laser field.

Approaches for field – free orientation

O. Ghafur, A. Rouzée, A. Gijsbertsen, W. K. Siu, S. Stolte and M. J. J. Vrakking, “Impulsive orientation and alignment of quantum-state-selected NO molecules,” Nature Phys. 5, 289-293 (2009).

Approaches for field – free orientation

L. Holmegaard, J. L. Hansen, L. Kalhøj, S. L. Kragh, H. Stapelfeldt, F. Filsinger, J. Küpper, G. Meijer, D. Dimitrovski, M. Abu-samha, C. P. J. Martiny and L. B. Madsen, “Photoelectron angular distributions from strong-field ionization of oriented molecules,” Nature Phys. 6, 428-432 (2010).

Holmegaard and colleagues:

Electrostatic quantum state selector + deflector combination of a DC and femstosecond laser field.

Kling and colleagues:

Approaches for field – free orientation

two-color excitation scheme in which the fundamental laser frequency (800 nm) is mixed with its second harmonic with a specified relative phase.

S. De, I. Znakovskaya, D. Ray, F. Anis, Nora G. Johnson, I. A. Bocharova, M. Magrakvelidze, B. D. Esry, C. L. Cocke, I.V. Litvinyuk, and M. F. Kling “Field-Free Orientation of CO Molecules by Femtosecond Two-Color Laser Fields,” Phys. Rev. Lett. 103, 153002 (2009).

Optical Pulse vs. Half Cycle Pulse

),(2

ˆˆ

2

tVI

LH 2( (, c) os )V t

( )

( ) sin(2 )

dV

d

c( (, s) o )V t

( )

( ) sin( )

dV

d

1.3 fs

Ultrashort optical pulse

( ) sin(2 ) Half cycle pulse

( ) sin( )

Alignment2cos

Orientationcos

11

22

33

44

55

13

24

3531

42

53

11T

22T

33T

44T

55T

Population Transfer

2J J Coherences

2cosV

Alignment 2 2 2cos cos cospopulation coherence

Density matrix – non resonant

11

22

33

44

55

13

24

3531

42

53

11

22

33

44

55

Population Transfer

2J J Coherences

2cosV

Alignment 2 2 2cos cos cospopulation coherence

15

51

xx

xx

xx

xx

x

xx

x

Density matrix – non resonant

11T

22T

33T

44T

55T

11

22

33

44

55

Population Transfer

1J J Coherences

cosV

12

23

34

45

21

32

43

54

Orientation cos

Density matrix - resonant

11

22

33

44

55

Population Transfer

1J J Coherences

cosV

12

23

34

45

21

32

43

54

Orientation and Alignment

13

24

3531

42

53

14

25

41

52

15

51

Density matrix - resonant

ResonantNon resonant

| , | 2,J m J m | , | 1,J m J m

May induce all therotational cohernces

May induce only evenrotational coherences

No orientationOnly alignment

Both orientationand alignment

Short summary

Intense single cycle THz pulse

K. L. Yeh, M. C. Hoffmann, J. Hebling and K. A. Nelson, “Generation of 10 μJ ultrashort THz pulses by optical rectification”, Appl. Phys. Lett. 90, 171121 (2007);

LiNb

“But you don’t have a Half Cycle Pulse !!!”

Time (ps)

Am

plit

ude

(a.u

)

( ) 0E t dt

Time (ps)

Am

pli

tud

e (a

.u)

Frequency (THz)

Spec

tral

Am

plit

ude

(n

oram

lize

d)

Resonant Non - Resonant

Time(ps) Time(ps)J states (m=0) J states (m=0)

Am

plit

ud

e (a

.u)

Am

plit

ud

e (a

.u)

Population TransferJ,J+1 coherencesJ,J+2 coherences

39.8ps

Revival Revival

O=C=S

BS

G

LiNb

Laser

Pellicle BS

ZnTe W/ 4

PD

PD

Delay

EO sampling

1 Trev

2 Trev 3 Trev

Time (ps)

Am

pli

tud

e (a

.u)

EO sampling, OCS, 250torr

ReflectionsReflections

O=C=S

BS

G

LiNb

LaserDelay

P PD

Pellicle BS

BS

P 450 to THz polarization

Pellicle BS

ZnTe W/ 4

PD

PD

Delay

Detection of Molecular Alignment

Time (ps)

Inte

nsi

ty (

a.u

.)

12 revT

1 revT3

2 revT

Population Transferred To higher J’s

Alignment of OCS, 350torr, 300K

Each molecule is forced to rotate in a plane2cos 1/2

Non thermal rotational distributionTime independent alignment

17311.6 e

tps

populationI

Time (ps)

Inte

nsi

ty (

a.u

.)

Decay of population (T1)

Decay of coherence (T2)

5725.5 et

pscoherenceI

Time (ps)

Inte

nsi

ty (

a.u

.)

Rotational state distribution (J)

Inte

nsi

ty (

a.u

.)

sin( )I n L

I c

2

0

3( cos 1/ 3)

4

Nn

n

53.9 10

81.8 10

Experimental alignment factor

0 1/ 2 revT

0 1 revT

Max

Ori

enta

tion

@ 1

Tre

v (x

102 )

Max

Alig

nm

ent

@ 1

/2T

rev(

x104 )

Peak field amplitude (a.u.)

Max Alignment=1.207x10-4 x Peak2

Max Orientatio

n=1.441x10-2 x Peak

Time (ps)

Am

pli

tud

e (a

.u)

1% orientation5% orientation8% orientation

Summary

Table top single cycle THz pulses can inducesignificant field-free orientation under ambient conditions

Relatively high orientation is not necessarily associatedRequires with high degree of alignment.

With jet cold molecular samples, higher degree of orientationis expected.

Combining optical and THz pulses may enable two independent handles for manipulating molecular angular distributions in 3D.

Thank you