Synthesis and optical storage properties of a thiophene-based holographic recording medium

Article abstract of DOI:10.1039/b705648f

The results of the fabrication and optical data storage characteristics of a novel azothiophene polyester 9 for potential holographic storage are reported. The polyester is derived from an azothiophene diol 1 and diphenyl phthalate 8 via in vacuo melt transesterification. Inclusion of a 5-methoxy-2-thienyl moiety generates a trans π-π* transition centered close to 405 nm. An investigation of the optical data storage characteristics of a solution cast film of azopolyester with a thickness of 65 m is summarised. The optical anisotropy induced by a 532 nm frequency doubled YAG laser and probed at a wavelength of 633 nm outside the absorption band with a polarimeter reveals a very high induced anisotropy of 7 rad (laser intensity, 250 mW cm^-2). The calculated birefringence of the film is 0.02 per micron. Maximum anisotropy is reached after approximately 70 s of irradiation. The induced anisotropy disappears at approximately 90 °C. Room temperature stable gratings can be inscribed with high diffraction efficiency. The Royal Society of Chemistry 2007.

Full text of DOI:10.1039/b705648f

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Synthesis and optical storage properties of a thiophene-based holographic
recording medium
Avtar S. Matharu,*a Shehzad Jeeva,a Patrick R. Huddlestonb and P. S. Ramanujamc
Received 13th April 2007, Accepted 20th August 2007
First published as an Advance Article on the web 3rd September 2007
DOI: 10.1039/b705648f
The results of the fabrication and optical data storage characteristics of a novel azothiophene
polyester 9 for potential holographic storage are reported. The polyester is derived from an
azothiophene diol 1 and diphenyl phthalate 8 via in vacuo melt transesterification. Inclusion of a
5-methoxy-2-thienyl moiety generates a trans p–p* transition centered close to 405 nm. An
investigation of the optical data storage characteristics of a solution cast film of azopolyester with
a thickness of 65 mm is summarised. The optical anisotropy induced by a 532 nm frequency
doubled YAG laser and probed at a wavelength of 633 nm outside the absorption band with a
polarimeter reveals a very high induced anisotropy of 7 rad (laser intensity, 250 mW cm22). The
calculated birefringence of the film is 0.02 per micron. Maximum anisotropy is reached after
approximately 70 s of irradiation. The induced anisotropy disappears at approximately 90 uC.
Room temperature stable gratings can be inscribed with high diffraction efficiency.
irradiation either in the p–p* or n–p* region. The mechanism
Introduction
for azobenzene holographic data storage is postulated as a re-
Modern-day society lives in an electronic information-obsessed
orientation of the transition dipole moment (usually the
world. We are truly in the midst of the Information Age with a
molecular long axis in the case of the trans-isomer) of the azo-
need to handle increasing amounts of electronic information.
chromophore from an initial isotropic state to
a final
For example, satellite images, X-ray images, movies, music etc.
utilise mega to gigabytes of information requiring both regular
retrieval and archiving. However, we are faced with a huge
problem because classical storage media, such as magnetic, are
reaching their storage limits due to the superparamagnetic
effect, i.e., insufficient physical space on the surface of a disk to
store more bits of data without comprising resolution.1 New
physical formats, and possibly new technologies, are needed to
counteract this growing problem.2 One way of increasing
storage capacity is to use a shorter wavelength of light as
demonstrated by the new BluRay technology which operates
using a blue laser at 405 nm (hence the name BluRay) with a
numerical aperture of 0.85 offering storage capacity of 27 GB
(single-sided, single layer).3
anisotropic state. Incoming polarised light only interacts when
it is parallel with respect to the transition dipole moment.
Trans–cis–trans photoisomerisation is induced to such an extent
that the molecules reorganise whereby the transition dipole
moment is oriented perpendicular to the polarisation of the
incoming beam. In this instance there is no further interaction
and an aligned or anisotropic domain is created. Major
drawbacks of azobenzene polyesters are their inability to
fabricate thick films in the absence of a matrix or diluent of
sufficient optical quality to allow volumetric data storage and
their propensity to form liquid crystal microdomains which
cause excessive scattering.6 Thus, there is a need to design and
synthesise new materials for potential holographic data storage.
Herein we report a successful strategy (see Scheme 1) leading
to the synthesis of a monomer azothiophene diol 1 and its
subsequent polymerisation, in the presence of diphenyl
phthalate 8, to an amorphous photosensitive azothiophene
polyester 9 for potential use in holographic data. To the best of
our knowledge, although azothiophenes are reported in the
literature for solar cell and NLO applications7 none relate to
their potential as holographic data storage materials. The
chemical structure of 9 is designed to: (i) eliminate potential
liquid crystallinity via the inclusion of a thienyl moiety which
disrupts linear packing of molecules in order to produce
amorphous films of high optical clarity; (ii) contain a short
three carbon unsymmetrical diol tether to the photosensitive
group which also reduces liquid crystal formation; (iii) contain
an aromatic polyester backbone which is envisaged to give
materials with good film-forming properties; (iv) prevent the
p–p* transition from shifting too far towards longer wave-
lengths whilst maintaining a sufficient n–p* absorption in the
Holographic data storage may be a viable near- to mid-term
solution as exemplified by InPhase Technologies’ TapestryTM
media4 which offers in excess of a staggering 100 GB of storage
capacity. Unlike digital storage, holography allows data
storage and retrieval en masse with predicted storage volumes
in the region of 1 terabyte. Azobenzene-related polymers have
attracted considerable attention in the quest for novel
holographic storage media5 due to the ability of the azo-
linkage (–NLN–) to readily cis–trans-photoisomerise upon
aDepartment of Chemistry, University of York, Heslington, York,
EnglandYO10 5DD. E-mail: am537@york.ac.uk;Fax: +44 1904432516;
Tel: + 44 1904 434187
bDivision of Chemistry, Nottingham Trent University, Nottingham,
England. E-mail: Patrick.huddleston@ntu.ac.uk; Tel: +44 1159 8486656
cOptics and Plasma Research Department, Risoe National Laboratory,
Technical University of Denmark, PO Box 49, Frederiksborgvej 399,
DK-4000, Denmark. E-mail: p.s.ramanujam@risoe.dk;
Tel: +45 4677 4507
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followed by room temperature, atmospheric pressure, catalytic
hydrogenation (5% Pd/C) of nitro-compound 4 afforded the
desired suitably substituted aniline 5. Diazotization of 5
furnished the intermediate tetrafluoborate salt 6, which was
used immediately without purification for coupling with
2-methoxythiophene to produce the desired novel azothio-
phene diol 1 in moderate yield (40%). High temperature,
in vacuo, base-catalysed (K2CO3) melt polymerisation of
stoichiometric amounts of diol 1 and diphenyl phthalate 8
(derived from phthaloyl chloride following treatment of
phthalic acid 7 with an excess of oxalyl chloride) afforded
the crude azothiophene polyester. The polymerisation was
underataken using a commercial Kugelrohr apparatus com-
monly used for short-path distillations. The advantages of
such a set-up are: (i) ease of assembly as two glass bulbs are
used with the monomer mix placed in the first bulb; (ii) no
bespoke glassware needs to be fabricated; (iii) glass oven
allows instant visual inspection; and (iv) a central rotating
spindle ensures even mixing whilst heating. Diphenyl phthalate
8 was chosen because its melting point is close to that of the
diol 1 and during the course of the polymerisation phenol is
released which can be easily isolated in to second bulb. The
weight of phenol distillate can be easily determined as it forms
a solid in the second bulb. Repeated solubilisation in CHCl3
followed by preciptation in methanol afforded the desired
azopolyester 9.
Reagents and instrumentation
All reagents were purchased from Sigma Aldrich Chemical
Company. The structural integrity of the intermediates and final
products was evidenced by: 1H and 13C NMR spectroscopy
(JEOL EX270 spectrometer or Bruker AMX500 spectrometer)
with tetramethylsilane as the internal standard; Fourier-trans-
form infrared spectroscopy (Shimadzu IRPrestige-21 with
Specac GoldenGate platform). Percentage conversion of the
monomer to polymer is assumed to be 100% based on the
theoretical mass of phenol distillate collected. Glass transition
temperatures were determined using a Mettler Toledo DSC 822
instrument at heating and cooling rates of 10 uC min21. UV-vis
spectra were recorded on a Shimadzu UV 2401 PC instrument
using dilute solutions of the desired material dissolved in
spectroscopic grade THF. Mass spectra were determined using
a VG Autospec mass spectromter. Microanalyses were per-
formed by the service provided at the University of Newcastle.
Scheme 1 Reagents and conditions: (i) 2-bromomethyl oxirane,
K2CO3, KI (cat.), 2-butanone, reflux, 48 h; (ii) H2SO4 (2.0 M),
dioxane, rt, 12 h; (iii) H2, Pd/C, EtOH, rt, 12 h; (iv) fluoroboric acid
(8.0 M, 50% in H2O), isoamyl nitrite, EtOH, 0 uC to rt; (v)
2-methoxythiophene, glacial AcOH, NaOAc, rt, overnight. (vi) a.
(COCl)2. b. Phenol, pyridine, reflux. (vii) High temp, in vacuo, K2CO3.
green region such that addressing can take place at 532 nm
with read-out at 633 nm without bleaching.
Although there are many literature reports pertaining to the
synthesis of azothiophenes via diazonium coupling of suitably
substituted 2-aminothiophenes,8 our strategy involves the
moderate to good direct coupling of a benzenediazonium salt
to 2-methoxythiophene. Thus, we neither have to prepare
2-amino-5-methoxythiophene which we envisage to be rather
unstable nor employ the rather troublesome Gewald reaction.9
Our drive to further synthesise such materials was prompted
by the theoretical work of Astrand et al.10 who reported the
importance of five-membered rings as components in optical
data storage devices by calculating theoretical S1 (p–p*) and S2
(n–p*) excitation energies and absorptions for the trans-isomer
to be 3.00 eV (413 nm) and 2.24 eV (553 nm), respectively.
2-(4-Nitro-phenoxymethyl)oxirane 3
In an atmosphere of nitrogen, a mixture of 4-nitrophenol 2
(2.0 g, 14.37 mmol), 4-butanone (20 mL), anhydrous K2CO3
(5.95 g, 43.11 mmol), 2-bromomethyl oxirane (1.8 ml, 21.56
mmol) and KI (10 mg) was heated at reflux for 48 h. The
reaction mixture was filtered, the filtrate evaporated to dryness
and the resultant residue recrystallised (EtOH) to give 2-(4-nitro-
phenoxymethyl)oxirane 3 (2.61 g, 93%) as pale yellow crystals:
mp 67.3 uC (EtOH); umax/cm21 1330.8 (NLO assym.), 1249.8
(Ar–O); dH (270 MHz; CDCl3; Me4Si) 8.20 (d, Ar–H, J = 9.2),
7.02 (d, Ar–H, J = 9.2), 4.37 (dd, CH, J = 2.6, 11.1), 3.98 (q, J =
5.9, 11.1), 3.37 (m), 2.95 (q, J = 4.0), 2.77 (q, J = 2.6); dC (CDCl3,
75 MHz; CDCl3; Me4Si) 162.1 (Ar–O), 141.6 (Ar–NLO), 125.9
Experimental
Synthetic overview
As shown in Scheme 1 polyester 9 is derived from diol 1 and
diphenyl phthalate 8. Diol 1 is prepared from its intermediate
latent oxirane 3. Subsequent ring opening via acidic hydrolysis
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(Ar 6 2), 115.0 (Ar 6 2), 69.0 (–ArO–CH2–CH), 49.8 (CH2–
CH–CH2), 44.2 (O–CH2–CH); m/z (EI) 195 (M+, 55%); Found:
C, 55.17; H, 4.80; N, 7.18. Requires C, 55.39; H, 4.65; N, 7.18.
2), 145.7 (Ar 6 2), 132.0 (Ar–NLN), 121.8 (S–C–CH), 115.7
(S–C–NLN), 105.8 (OMe–C–CH), 70.0 (2u OH–CH), 69.9
(OAr–CH2), 62.8 (1u OH–CH2), 60.0 (OMe); m/z (EI) 308
(M+, 76%); Elemental analysis (%): C, 54.53; H, 5.23; N, 9.08.
Found: C, 54.07; H, 5.40; N, 8.84; UV-vis l p–p* (THF) 402 nm
(e/dm3 mol21 cm21 2.39 6 104).
3-(4-Nitro-phenoxy)propane-1,2-diol 4
A mixture of 2-(4-nitro-phenoxymethyl)oxirane 3 (2.5 g, 12.82
mmol), dioxane (70 mL) and aqueous H2SO4 (2.0 M, 50 mL)
was stirred at rt for 24 h. The organic content was extracted
with EtOAc, dried (MgSO4) and evaporated to furnish a
viscous yellow oil, (2.4 g, 88%) which was used without further
purification: umax/cm21 3529.7–3352.2 (–OH), 1338.6 (Ar–
NLO), 1257.5 (Ar–O); dH (270 MHz; CDCl3; Me4Si) 8.20 (d, 2
Ar–H, J = 9.1), 7.15 (d, 2 Ar–H, J = 9.1), 5.08 (s, 1 OH), 4.76
(s, 1 OH), 4.25–3.78 (m, 5H); dC (CDCl3, 75 MHz; CDCl3;
Me4Si) 162.0 (Ar–O), 141.2 (Ar–NLO), 126.0 (Ar 6 2), 115.4
(Ar 6 2), 71.9 (2u OH–CH), 70.3 (1u OH–CH2), 63.8 (OAr–
CH2); m/z (EI) 213 (M+, 73%).
Diphenyl phthalate 8
In an inert atmosphere of nitrogen, phthaloyl chloride (10.0 g,
49.26 mmol; freshly prepared from phthalic acid 7 and oxalyl
chloride catalysed with 1–2 drops of dimthylformamide)
dissolved in pyridine (20 mL) was added dropwise to a stirred
solution of phenol (13.8 g, 147.7 mmol) in pyridine (150 mL)
and allowed to stir overnight. The resultant mixture was
poured in to EtOAc (300 mL), washed with H2O (4 times) and
then with brine. The organic layer was dried (MgSO4), and
evaporated under reduced pressure to give pale a yellow solid,
which was recrystallised from toluene to give the desired
diphenyl phthalate (8) (12 g, 77%), as a white solid; mp
74.0 uC–74.2 uC; umax/cm21 1732.0 (CLO); dH (270 MHz;
CDCl3; Me4Si) 8.00 (Ar–H, m), 7.70 (Ar–H, m), 7.39 (Ar–H,
m), 7.24 (Ar–H, m); dC (75 MHz; CDCl3; Me4Si) 165.8
(Ar–CLO, 150.6 (Ar–C–CLO), 131.7 (Ar–C–O), 131.6 (Ar–C),
129.4 (Ar–C), 126.1 (Ar–C), 121.5 (Ar–C), 81.6 (Ar–C).
3-(4-Aminophenyloxy)propane-1,2-diol 5
Standard room temperature, atmospheric pressure catalytic
hydrogenation (5% Pd/C) of 3-(4-nitro-phenoxy)propane-1,2-
diol 4 (2.0 g, 9.38 mmol) dissolved in EtOH (200 mL) afforded
product as a light brown solid (1.1 g, 64%). The product was
used without further purification. mp 126–129 uC; umax/cm21
3344.5 (–NH2), 3275.1 (–OH), 1234.4 (Ar–O); dH (270 MHz;
DMSO; Me4Si) 6.61 (dd, Ar–H, J = 4.0, 8.9), 6.53 (d, Ar–H,
J = 8.9), 4.9–3.3 (m); dC (75 MHz; DMSO; Me4Si); 150.0,
150.0, 142.1, 142.0, 115.8, 115.1, 74.0, 70.0, 69.7; m/z (EI) 183
(M+, 50%); Found: C, 58.24; H, 7.10; N, 7.55. Requires C,
59.00; H, 7.15; N, 7.65.
Azothiophene polyester 9 (typical procedure)
A Kugelrohr bulb was charged with diphenyl phthalate 8
(213.1 mg, 0.67 mmol), diol 1 (206.4 mg, 0.67 mmol) and
K2CO3 (5 mol%) and heated in vacuo (10 mmHg) at 110 uC for
3 h. The resulting phenol distillate was continuously removed.
Thereafter, the bulb was heated to 140 uC at a reduced pressure
of (0.5 mm Hg) for 3 h until the theoretical amount of phenol
had been isolated (62.98 mg). The resultant crude polymer was
dissolved in chloroform and precipitated from methanol three
times to afford polyester 9 as a dark yellow powder.
4-(2,3-Dihydroxypropyloxy)benzenediazonium tetrafluoroborate
6
Isoamylnitrite (7 mL, mmol) was added to a cooled (215 uC)
solution of 3-(4-aminophenoxy)propane-1,2-diol 5 (1.0 g,
5.4 mmol) dissolved in EtOH (5 mL) and aqueous fluoroboric
acid (5 mL, 50% aqueous sol. 8.0 M). The reaction was allowed
to warm to room temperature overnight followed by solvent
removal. The resultant brown residue was washed several
times with hexane to leave a dark red semi-solid which was
used immediately without further purification.
Polyester 9 has been made several times with yield, post
purification, ranging from 200 mg (68%) to 230 mg (78%). The
absence of the OH stretching frequency in the infra-red
spectrum of the polyester 9 and the absence of proton NMR
signals characteristic of a mono-substituted benzene ring is a
good indication that the polyester is relatively free of any
unreacted monomer, either diol 1 or diester 8. dH (270 MHz;
CDCl3; Me4Si) 7.7–7.5 (4 H, Ar H), 7.45–7.25 (3 H, ArH + ThH
(Th = thiophene)), 6.9–6.7 (2 H, ArH) 6.3–6.1 (1 H, ThH), 5.60–
5.4 (1H, alkyl), 4.7–4.4 (2H, alkyl), 4.35–3.95 (2 H, alkyl), 3.9–
3.8 (3 H, OMe, br s); umax/cm21 1728.2 (CLO), 1597.0 (NLN);
DSC; Tg 91.7 uC (see Fig. 1); UV-vis lp–p* (THF) 405 nm; GPC:
varying the temperature and time the polymerisation reaction
was attempted several times. Of the polyesters synthesised the
3-[4-(5-Methoxythiophen-2-ylazo)phenoxy]-propane-1,2-diol 1
4-(2,3-Dihydroxypropyloxy)benzenediazonium
tetrafluoro-
borate 6 (1.5 g, 5.3 mmol) was added to 2-methoxythiophene
(606 mg, 5.3 mmol, 0.45 mL) dissolved in glacial acetic acid
(20 mL). To the above mixture, sodium acetate (600 mg) was
added and the whole mixture was stirred for 24 h at room
temperature. Extraction in to ethyl acetate followed by
evaporation and purification via column chromatography
afforded 3-[4-(5-methoxythiophen-2-ylazo)phenoxy]-propane-
1,2-diol (650 mg, 40%) as an orange solid; mp 92 uC; umax/cm21
3282.8 (–OH), 1597.0 (NLN), 1238.3 (Ar–O); dH (270 MHz;
DMSO; Me4Si) 7.66 (d, Ar–H, J = 9.28), 7.60 (d, J = 4.46),
7.06 (d, Ar–H, J = 9.28), 6.53 (d, J = 4.46), 5.01 (d, J = 5.20),
4.71 (t, J = 5.94), 4.1–3.7 (m), 3.96 (s, OMe); dc (75 MHz;
DMSO; Me4Si) 169.8 (S–C–OMe), 162.2 (O–Ar), 146.0 (Ar 6
weighted average and polydispersities ranged from: (a) Mw
=
18 900; Mw/Mn = 6.3 (see Fig. 2); (b) Mw = 53 200; Mw/Mn =
9.4; (c) Mw = 65 600; Mw/Mn = 11.3
The high polydispersity may be due to evaporative loss of
one of the monomers during in vacuo melt polymerisation
leading to a stoichiometric imbalance. Intractable tars were
observed on raising the temperature above 160 uC for extended
periods of time during polymerisation. The results reported in
the next section are based on the polymer that produced a
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Fig. 1 DSC thermogram of polyester 9
Fig. 3 Trans- to cis-photoisomerisation: UV steady-state data at
timed intervals for illumination at 410 nm for diol 1. Here, T represents
time in seconds.
good optical film. We appreciate that the optimal conditions
for polymerisation are still to be achieved. [GPC Condition:
column: PL gel MIXED-C 300 6 500 mm at 30 uC; mobile
phase: chloroform +5% hexafluoroisopropanol (HFIP); flow
rate: 1 mL min21; detection: differential refractive index at
30 uC; calibration: polystyrene 580–377 400 g mol21; injection
volume: 100 mL; sample concentration: approx 2 mg mL21.]
The spectrum shows clearly the absorption characteristics
of the trans-isomer; p–p*, lmax 406 nm (theoretical 413 nm);
n–p*, lmax 436 nm, thus, diol 1 is a blue–green sensitive
material which correlates well with the theoretical studies of
Astrand et al.10 To explore photoreversion further, the UV
spectrum was recorded initially at 5 s intervals on exposure to
UV light filtered at 410 nm and thereafter for longer periods.
Exposure at 410 nm probes the p–p* absorption of the trans-
isomer effecting trans- to cis-isomerisation. As shown in Fig. 3,
as the exposure time is increased the concentration of the
trans-isomer decreases (trans–p–p* transition lowers) whereas
the concentration of the cis-isomer increases (cis–p–p*, lmax
346 nm). A clear isobestic point at 357 nm is observed. A
steady-state is soon reached after approximately t = 55 s,
giving an approximate isomeric ratio of 25 (cis) : 75 (trans).
To monitor the photochemical back reaction from cis- to
trans- the filter was changed from 410 nm to 314 nm in order
to probe the p–p* absorption of the cis-isomer. The same
sample from the above study was removed from the UV
Results and discussion
Preliminary UV steady-state investigation of diol 1
Fig. 3 and 4 show the results of a UV-vis steady-state
investigation monitoring trans–cis–trans photoreversion of
diol 1. In order to maximise the concentration of the trans-
isomer, an appropriate strength solution of diol 1 in spectro-
scopic grade tetrahydrofuran was freshly prepared, stored at
220 uC in the dark overnight, and subsequently its UV
spectrum was recorded (Fig. 3, T = 0).
Fig. 4 Cis- to trans-photoisomerisation: UV steady-state data at
timed intervals for illumination at 314 nm for diol 1. Here, T represents
time in seconds.
Fig. 2 GPC molecular mass distribution of polyester 9
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Fig. 5 Set-up to measure optically induced anisotropy in the
azothiophene film. In the figure, ND filter is a neutral-density filter,
POL, polariser, HWP, half-wave plate and HeNe, He-Ne laser.
Fig. 7 Room temperature stability of the anisotropy.
instrument, allowed to equilibrate for two minutes at ambient
conditions and the spectrum recorded. A very rapid thermal
back relaxation from the cis- to the trans–state occurred during
this period (Fig. 4, T = 0). Thereafter, irradiation at 314 nm
(Fig. 4) reveals conversion of the cis-isomer back to the trans-
isomer via an isobestic point at 357 nm. Diol 1 readily undergoes
trans–cis–trans photoreversion with the trans-isomer being
predominantly more stable in ambient conditions.
Optical data storage characterisation of polyester 9
An investigation of the optical data storage characteristics (see
Fig. 5 for optical bench set-up) of a solution cast film of
azopolyester 9 (the best films were made from a polyester with
Mw 18 900 and Mw/Mn 6.3) with a thickness of 65 mm is
summarised in Fig. 6–8. Preliminary measurements are
presented for a wavelength of 532 nm, as the large film
thickness, together with maximum absorption at 410 nm,
precluded measurements at 405 nm. The optical anisotropy
induced by a 532 nm frequency doubled YAG laser and
probed at a wavelength of 633 nm outside the absorption band
Fig. 8 Thermal stability of the induced anisotropy.
with a polarimeter is shown in Fig. 6. The intensity of the laser
was 250 mW cm22. This induced anisotropy of 7 rad is very
high, mainly due to the increased film thickness. The
Fig. 9 Diffraction efficiency measurement. The grating was written
with two orthogonal circularly polarised beams, and read-out with one
circularly polarised beam.
Fig. 6 Anisotropy induced by the green laser at 532 nm as a function
of time, probed at 633 nm.
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Fig. 10 A polarisation holographic set-up to record diffraction gratings. In the figure, PBS is a polarisation beam-splitter, HWP, half-wave plate,
QWP, quarter-wave plate, POL, polariser and M, mirror.
calculated birefringence of the film is 0.02 mm21. Maximum
Conclusions
anisotropy is reached after approximately 70 s of irradiation.
We have demonstrated the fabrication and characterisation of
As can be seen from Fig. 7, the induced anisotropy decreases
a novel azothiophene polyester that can be cast into thick films
during the first 1000 s after irradiation, before levelling off due
providing a large induced anisotropy suitable for holographic
to the relaxation of the chromophores. For practical applica-
recording. Both the induced anisotropy induced with a single
tions, it is important that the temperature at which the induced
beam, and the diffraction efficiency recorded with two beams
anisotropy is erased be reasonably high. Fig. 8 shows the
have been proved to be stable in time. We acknowledge for
decrease in the anisotropy as the film is heated to higher
commercial application the recording time and stability need
temperatures. It is seen that the induced anisootropy
to be vastly improved via continual re-iteration of the polymer
disappears at approximately 90 uC. However, for practical
structure.
applications this is still low and will need further improvement.
Acknowledgements
Polarisation holographic recording
We thank the EPSRC (Grant award number EP/C542223/1)
The first order diffraction efficiency at a wavelength of 633 nm
for funding a Postdoctoral Research Fellow (S. J.). We also
when a polarisation holographic recording is made in the film,
thank Dr Andrew Penrose for GPC analyses.
with two orthogonally circularly polarised beams, is shown in
Fig. 9 measured using the optical set-up depicted in Fig. 10.
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Fig. 11 Atomic force microscopic scan of the diffraction grating.
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