NEUTRON DIFFRACTION FROM HOLOGRAPHIC NEUTRON DIFFRACTION FROM HOLOGRAPHIC POLYMER-DISPERSED LIQUID CRYSTALSPOLYMER-DISPERSED LIQUID CRYSTALS

NEUTRON DIFFRACTION FROM HOLOGRAPHIC NEUTRON DIFFRACTION FROM HOLOGRAPHIC POLYMER-DISPERSED LIQUID CRYSTALSPOLYMER-DISPERSED LIQUID CRYSTALS

I. Drevenšek-Olenika, M. A. Ellabbanb, M. Fallyb, K. P. Pranzasc J. Vollbrandtc

 

aFaculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, and J. Stefan Institute, Jamova 39, Ljubljana, SI 1001, Slovenia bNonlinear Physics Group, Faculty of Physics, University of Vienna, Boltzmanngasse 5, Vienna, A-1090, Austria cInstitute for Materials Research, GKSS-Research Center Geesthacht GmbH, PO Box 1160, Geesthacht, 21494, Germany

NEUTRON PHYSICS WITH NEUTRON PHYSICS WITH PHOTOREFRACTIVE MATERIALSPHOTOREFRACTIVE MATERIALS

NEUTRON PHYSICS WITH NEUTRON PHYSICS WITH PHOTOREFRACTIVE MATERIALSPHOTOREFRACTIVE MATERIALS

• 1990 - a beam of cold neutrons (0.5 nm < B < 5 nm) was for the first time diffracted from a periodic structure fabricated by optical holographic patterning (R. A. Rupp et al., Phy. Rev. Lett. 64, 301 (1990))

Photo-neutron-refractive (PNR) effect = optical illumination-induced modifications of the photosensitive material cause refractive index changes for neutrons.

PNR effect provides:• Structural characterization of holographic media on the nanoscale• Convenient way for fabrication of various neutron-optical devices for cold and ultracold neutrons (beam splitters, mirrors, lenses, interferometers, ...)

NEUTRON OPTICSNEUTRON OPTICSNEUTRON OPTICSNEUTRON OPTICSCoherent elastic scattering of cold neutrons with wavelengths of 0.5 nm < n < 5 nm,

can be described rigorously by a one-body, time-independent Schrödinger equation, which can be expressed as:

where the refractive index for neutrons is given as:

E

xV

E

xVxnn 2

)(1

)(1)(

Nuclear potential V(x) is usually replaced by a series of Fermi pseudopotentials:

)(2

)(2

)(22

xbm

hyxb

m

hxV

jjj

where bj is the bound coherent scattering length of a nucleus j located at the site yj,

and b(x) is the so-called coherent scattering length density. Consequently

)(

21)(

2

xbxn nn

By averaging over the “unit cell” of the medium one can take )]([)( xNbxb

,

.

.

,

.

nn hmEkkxn /2/2 ,0 )( 20

20

2

NEUTRON DIFFRACTIONNEUTRON DIFFRACTIONNEUTRON DIFFRACTIONNEUTRON DIFFRACTION

Medium with sinusoidal modification of b.

)cos()( 10 xKnnxn gnnn

Kg=| Kg |=2/ = grating vector

1

22

1 2)(

2bNbn nn

n

B

Bragg angle: Bn=sin-1(n/2)

ANGULAR DEPENDENCE OF ANGULAR DEPENDENCE OF DIFFRACTED INTENSITYDIFFRACTED INTENSITY

ANGULAR DEPENDENCE OF ANGULAR DEPENDENCE OF DIFFRACTED INTENSITYDIFFRACTED INTENSITY

Diffraction efficiency = Id/Iin : two wave coupling approximation (H. Kogelnik, 1969)

B

22

222

/1

sin),(

= n1nd/(cos)

grating strength = d/ deviation from Bragg angle

The measured () (rocking curve) is the convolution of (,) with angular and wavelength distributions of the incident neutron beam. (M. Fally, Appl. Phys. B 75, 405-426 (2002))

-20 -10 0 10 200.0

0.2

0.4

0.6

0.8

1.0

(arb. units)

Diff

ract

ion

effic

ienc

y

1.0 (*/2) 0.8 0.6 0.4 0.2

d

Iin

Id

OUR HPDLC SAMPLESOUR HPDLC SAMPLESOUR HPDLC SAMPLESOUR HPDLC SAMPLES

1D gratings fabricated from UV curable emulsion: • 55 wt% of the LC mixture (TL203, Merck), • 33 wt% of the prepolymer (PN393, Nematel),• 12 wt% of the 1,1,1,3,3,3,3-Hexafluoroisopropyl acrylate(HFIPA, Sigma-Aldrich). I. Drevenšek Olenik, M. E. Sousa, A. K. Fontecchio, G. P. Crawford, M. Čopič:Phys. Rev. E, 69, 051703-1-9 (2004).

• Standard glass cells with 50 or 100 m thick spacers, • Exposure: Ar laser, UV = 351 nm , 2 beams with j ~ 10 mW/cm2

• Unslanted transmission grating, = 0.43, 0.56, 1.0, 1.2 m,• Postcuring: Exposure to one beam for ~ 5 min.

Diffraction pattern observed for optical beam with O=543 nmat normal incidence.

SEM image of an “opened” sample

OPTICAL DIFFRACTION IN THE NEMATIC PHASEOPTICAL DIFFRACTION IN THE NEMATIC PHASEOPTICAL DIFFRACTION IN THE NEMATIC PHASEOPTICAL DIFFRACTION IN THE NEMATIC PHASE

I+1I-1

I+2I-2

Iin

Grating = 1.2 m:Angular dependence of the 1st order diffracted intensities.

• Strong difference between p- and s- polarized beams.• Overmodulated diffraction for p-polarized light.

I0

OPTICAL DIFFRACTION IN THE ISOTROPIC PHASEOPTICAL DIFFRACTION IN THE ISOTROPIC PHASEOPTICAL DIFFRACTION IN THE ISOTROPIC PHASEOPTICAL DIFFRACTION IN THE ISOTROPIC PHASE

Grating = 1.2 m

)cos()( 10 xKnnxn gopop

22

222

/1

sin),(

= n1opd/(cos), = d/

The fit gives:s =1.89 0.03 , p = 1.840.03

d = 30.50.4 m ,

n0op = 1.520.01 and n1op = 0.0110.001

Accordingly to the sample compositionmax possible n1,op=0.038.

I. Drevensek-Olenik, M. Fally, M. Ellaban, Phys. Rev. E 74, 021707 (2006).

NEUTRON DIFFRACTION EXPERIMENTSNEUTRON DIFFRACTION EXPERIMENTSNEUTRON DIFFRACTION EXPERIMENTSNEUTRON DIFFRACTION EXPERIMENTS• SANS-2 instrument at the Geesthacht Neutron Facility (GeNF)• Beam size: circular spot with 5 mm diameter • Central neutron wavelengths used: n = 1,16 nm; n = 1,96 nm

• Wavelength spread: ( n /n)= 10%• Full collimation distance of 40 m was used (angular spread ~0.5 mrad)• Maximum detector distance of 21 m was used.• 2D decetor with 256x256 pixels (2.2 x 2.2 mm2)

• All measurements were done at 22oC

NEUTRON DIFFRACTION IN THE NEMATIC PHASENEUTRON DIFFRACTION IN THE NEMATIC PHASENEUTRON DIFFRACTION IN THE NEMATIC PHASENEUTRON DIFFRACTION IN THE NEMATIC PHASE

• Diffraction pattern for n = 1,96 nm

measured at =0o.

• The diffraction efficiency of the 1st diffraction orders is around 10% !!!

• Diffraction peaks up to the 2nd order are visible.

(non-deuterated sample!!)

Grating = 1.2 m

ANGULAR DEPENDENCE of NEUTRON DIFFRACTIONANGULAR DEPENDENCE of NEUTRON DIFFRACTIONANGULAR DEPENDENCE of NEUTRON DIFFRACTIONANGULAR DEPENDENCE of NEUTRON DIFFRACTION

Sample 10f

Sample 10g

Red line = fit to 22

222

/1

sin),(

n = 1,16 nm

Bn=sin-1(n/2)=0.48 mrad From the fit it follows:n1n= (2.120.05)10-6, d=30 m

=> b1 = (9.890.26)1012 m-2

The corresponding modulation of the coherent scattering length density b1 is two

orders of magnitude larger than in the best PNR materials reported up to now!! b1

= (bN), due to phase separation of the constituent compounds

(b) can be large even if (N) is relatively small !!!

M. Fally, I. Drevensek-Olenik, M. Ellaban, K. P. Pranzas, J. Vollbrandt, Phys. Rev. Lett. 97, 167803 (2006).

WHAT ARE THE BENEFITS ?WHAT ARE THE BENEFITS ?WHAT ARE THE BENEFITS ?WHAT ARE THE BENEFITS ?From the info on chemical composition of our samples we calculated n0 ~ (1–7·10-5).While we detected n1n ~ 6.3 ·10-6 (for n = 1.96 nm).

This means that in our HPDLC gratings 10% variation of the V(x) for neutrons was induced by optical holographic patterning.

For observed n1n the value of = 100% can be reached by grating thickness d = 156 m !!!

Low thickness brings important advantages in construction of neutron optical devices:

• low level of incoherent scattering (no need for material deuteration = low price)

• alignment procedures for different neutron-optical elements become very simple!!

INCREASING GRATING THICKNESSINCREASING GRATING THICKNESSINCREASING GRATING THICKNESSINCREASING GRATING THICKNESSGrating = 1 m, spacers dS = 50 and 100 m

cell thickness dS n.d. efficiency grating thickness d ref. ind. modulation n1n (1.16 nm)

50 m 0.9 % 36 4 m 1.0 ·10-6

100 m 0.5 % 87 6 m 0.3 ·10-6

Increased sample thickness resulted in lower refractive index modulation for neutrons ?!?Besides this an anusual double Bragg peak was observed.

-0.06 -0.04 -0.02 0.00 0.02 0.04 0.060.000

0.001

0.002

0.003

0.004

0.005

0.006

+1st order -1st order

Diff

ract

ion

effic

ienc

y

Angle (rad)

According to two beam coupling theory:= at =/d=0.01 rad. So we actually see 2 separate Bragg peaks.

Possible explanation

DECREASING GRATING PITCHDECREASING GRATING PITCHDECREASING GRATING PITCHDECREASING GRATING PITCH

Grating = 0.56 m, spacers dS = 50 m (d = 30 5 m).

-0.06 -0.04 -0.02 0.00 0.02 0.04 0.06 0.08

0.0

0.2

0.4

0.6

0.8

1.0

-1st order

Diff

ract

ion

effic

ienc

y (%

)

Angle [rad]

Kogelnik

Gaussian

Lorentzian

Decreasing grating pitch resulted in smearing and broadening of the Bragg peak, which is attributed to structural inhomogeneities.

n1n ~ 1.1 ·10-6 (for n = 1.16 nm).

CONTRAST VERSUS GRATING PITCHCONTRAST VERSUS GRATING PITCHCONTRAST VERSUS GRATING PITCHCONTRAST VERSUS GRATING PITCH

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.40.0

5.0x10-7

1.0x10-6

1.5x10-6

2.0x10-6

2.5x10-6

n1

(at

n

= 1

.16

nm)

(micrometers)

Our acrylate-based HPDLC mixture is very convenient for fabricating structureswith > 1 m, but less suitable for submicron patterning.

AFM images of polymer matrix

)cos()( 10 xKnnxn gnnn

n

= 0.43 m 0.56 m 1.0 m

FUTURE PERSPECTIVESFUTURE PERSPECTIVESFUTURE PERSPECTIVESFUTURE PERSPECTIVES

HOLONS-setup at GeNF

• Probing of other sample compositions (different polymers, LCs, ...)• (Re)filling of the grating structures with high contrast materials (D2O, ...)

• Study of photopolymerization kinetics of H-PDLCs by neutron scattering in-situ• Synchronous probing of optical and neutron refractive properties• Investigations of sample morphology on the nanoscale• Investigations of structural modifications induced by external fields• Fabrication of 2D and 3D grating structures for neutrons• Assembling of neutron-optical elements

M. Fally, C. Pruner, R.A. Rupp, G. Krexner, Neutron Physics with Photorefractive Materials in Springer Series of Optical Sciences 115, eds. P. Gunter and J.-P. Huignard, Springer Verlag, 2007.

CONCLUSIONSCONCLUSIONSCONCLUSIONSCONCLUSIONS• Photopolymerization-induced phase separation of the constituent components in H-PDLCs causes a huge variation of refractive index for light as well as for neutrons.

• H-PDLC transmission gratings with the thickness of only few tens of micrometers act as extremely efficient gratings for neutrons.

• The light induced refractive index-modulation for neutrons in HPDLCs is two orders of magnitude larger than found in the best PNR materials probed up to now.

• These features make H-PDLCs very promising candidate for fabrication of neutron-optical devices

• Our results also demonstrate that neutron diffraction is a very convenient tool to investigate structural properties of the H-PDLCs.

C. Pruner et al., Nuclear Instruments and Methods in Physics Research Section A 560, 598 (2006).

PARTICIPATING RESEARCHERSPARTICIPATING RESEARCHERSPARTICIPATING RESEARCHERSPARTICIPATING RESEARCHERS

____________________________________________________________________________________________We acknowledge the financial support of ÖAD and Slovenian Research Agency (bilateral projects SI-A4/0708 and SI-AT/07-08-004), the Austrian Science fund FWF (project P-18988), and support of the GKSS–Research Center Geesthacht.

M. A. Ellabban H. Eckerlebe, M. Fally, M. Bichler

P. K. Pranzas J. Vollbrandt

I. Drevensek Olenik