Orientation and Alignment of Gas Phase
Molecules by Single Cycle THz Pulses
Sharly Fleischer, Yan Zhou, Robert W. Field, Keith A. Nelson
FRISNO 11
Multiphoton Ionization
Self focusing &
Filamentation
n  n0  n2 I
Up – Down Symmetry
Conserved
High
Harmonic
Generation
Ultrafast x-ray
diffraction
DC field
Not Field Free !
+
-
Approaches for field – free orientation
Sakai and colleagues:
DC electric field +
laser pulse with adiabatic
turn-on and nonadiabatic
turn-off.
A. Goban, S. Minemoto, and H. Sakai, “Laser-Field-Free Molecular
Orientation,“ Phys. Rev. Lett. 101, 013001 (2008).
Approaches for field – free orientation
Vrakking and colleagues:
Hexapole state selector
(Producing molecules in
a single quantum state) +
a combination of a DC
and intense fs laser field.
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
Holmegaard and colleagues:
Electrostatic quantum
state selector + deflector
combination of a DC and
femstosecond laser field.
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 strongfield ionization of oriented molecules,” Nature Phys. 6, 428-432 (2010).
Approaches for field – free orientation
Kling and colleagues:
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


1.3 fs
 



ˆ2
L
Hˆ 
 V ( , t )
2I
V ( , t )   cos( )
V ( , t )   cos2 ( )
dV
 ( )  
d
 ( )   sin(2 )
dV
 ( )  
d
 ( )   sin( )
Ultrashort optical pulse
Half cycle pulse
 ( )   sin(2 )
 ( )   sin( )
Alignment
2
 cos  
Orientation
 cos  
Density matrix – non resonant
13

T
11

 24
T
22
31

35
T
33
 42

53
Population Transfer
T
44
V  cos 
J  J 2
2

T
55
Coherences
Alignment
 cos2     cos2   population   cos2  coherence
Density matrix – non resonant
11
x
x
13
x
15
x
x



x x

x x x

x x
 24
 22
31
42
51
35
33
44
53
55
Population Transfer
V  cos 
J  J 2
2
Coherences
Alignment
 cos2     cos2   population   cos2  coherence
Density matrix - resonant

T
11
12
 21 
T
22
 23
32 
T
33
34
 43 
T
44
 45
54 
T
55
Orientation
 cos  
Population Transfer
V  cos 
J  J 1
Coherences
Density matrix - resonant
11 12 13 14 15
 21  22  23  24  25
31 32 33 34 35
 41  42  43  44  45
Population Transfer
51 52 53 54 55
Orientation and Alignment
V  cos 
J  J 1
Coherences
Short summary
Non resonant
Resonant
| J , m | J  2, m 
| J , m | J  1, m 
May induce only even
rotational coherences
May induce all the
rotational cohernces
No orientation
Only alignment
Both orientation
and alignment
Intense single cycle THz pulse
LiNb
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);
“But you don’t have a Half Cycle Pulse !!!”

 E (t )dt  0
Amplitude (a.u)

Time (ps)
Spectral Amplitude
(noramlized)
Amplitude (a.u)
Time (ps)
Population
Transfer
J,J+1
J,J+2 coherences
Non - Resonant
Amplitude (a.u)
Amplitude (a.u)
Resonant
Frequency (THz)
J states (m=0)
Time(ps)
J states (m=0)
Time(ps)
  39.8ps
Revival
Revival
EO sampling
O=C=S
/4
Pellicle BS
ZnTe
W
PD
PD
G
LiNb
Delay
BS
Laser
EO sampling, OCS, 250torr
Amplitude (a.u)
1 Trev
2 Trev
Reflections
Reflections
Time (ps)
3 Trev
Detection of Molecular Alignment
Pellicle BS
O=C=S
/4
Pellicle BS
ZnTe
P
W PD
PD
PD
P 450 to THz
polarization
G
LiNb
Delay
BS
Delay
BS
Laser
Alignment of OCS, 350torr, 300K
Intensity (a.u.)
1 T
2 rev
1 Trev
3 T
2 rev
Population
Transferred
To higher J’s
Time (ps)
Non thermal rotational distribution
Time independent alignment

Each molecule is forced to rotate in a plane
 cos    1/2
2
Intensity (a.u.)
Decay of population (T1)
I population  11.6  e
Time (ps)
t
173 ps
Intensity (a.u.)
Decay of coherence (T2)
I coherence  25.5  e
Time (ps)
t
57 ps
Intensity (a.u.)
Rotational state distribution (J)
Experimental alignment factor
I
n L
 sin(
)
I
c
1.8 10
8
3N 
2
n 
( cos   1/ 3)
4n 0
3.9 10
5
0
0
1Trev
1/ 2Trev
Amplitude (a.u)
Max Orientation @ 1Trev (x102)
Max Alignment @ 1/2Trev(x104)
Time (ps)
8%
5% orientation
1%
Peak field amplitude (a.u.)
Summary
Table top single cycle THz pulses can induce
significant field-free orientation under ambient conditions
Relatively high orientation is not necessarily associated
Requires with high degree of alignment.
With jet cold molecular samples, higher degree of orientation
is expected.
Combining optical and THz pulses may enable two
independent handles for manipulating molecular angular
distributions in 3D.
Thank
you