A colour tunable microcavity by weak-to-strong coupling regime transition through a light-switchable material

Article abstract of DOI:10.1039/c3cc47691j

An organic based microcavity showing fully reversible colour tunability has been achieved for the first time. The emission output changes according to the modulation from pure photonic to polaritonic resonant modes through UV irradiation of the light-switchable matrix.

Full text of DOI:10.1039/c3cc47691j

ChemComm
View Article Online
View Journal | View Issue
COMMUNICATION
A colour tunable microcavity by weak-to-strong
coupling regime transition through a light-
switchable material†
Cite this: Chem. Commun., 2014,
0, 1122
5
Received 7th October 2013,
Accepted 21st November 2013
a
a
ab
a
Gianluca Accorsi,* Sonia Carallo, Marco Mazzeo, Armando Genco,
ac
abc
Salvatore Gambino and Giuseppe Gigli
DOI: 10.1039/c3cc47691j
www.rsc.org/chemcomm
An organic based microcavity showing fully reversible colour tunability
has been achieved for the first time. The emission output changes
according to the modulation from pure photonic to polaritonic reso-
nant modes through UV irradiation of the light-switchable matrix.
Scheme 1 Chemical structures of spiropyran (S, left) and merocyanine
The development of organic microcavities has attracted great
attention because of their advantages in terms of low cost, easy
(M, right). Interconversion between the two forms can be obtained by UV
l1) and Vis (l2) irradiation or heating (D), respectively.
(
1,2
fabrication, and mechanical flexibility. The cavities developed
3
so far usually work in a weak-coupling regime where the inter-
action rate between light and matter is slower than any energy loss greenish to deep red (through several colour shadings) in weak
pathway. Here, the resonance mode is mainly determined by the and strong coupling regimes, respectively, depending on external
optical length of the cavity, which can be varied by both UV irradiation time only. This approach leaves the thickness of the
the distance between the mirrors and the refractive index of microcavity unaltered leading to significant advantages in terms of
4
5
the medium. In 2008, Sch u¨ tte et al. have used a spatial wedge- time consumption, material saving and ease of fabrication process
8
shaped organic microcavity, reporting a tuning from 590 nm of the device. Moreover, the original colour output can be fully
(orange) up to 650 nm (red). Nevertheless, the strategy of recovered either by irradiating the cavity with visible light or by
introducing gradient thickness is limited by the use of thermal slight thermal treatment. These results provide an intriguing
6
deposition techniques. Recently, Schwartz et al. demonstrated alternative for the fabrication of a new kind of dynamical device
9
10–13
that soluble photoactive spiropyrans in poly(methylmethacrylate) to be employed in optical communication and lighting
fields.
Firstly, the photophysical properties of each component (Tb,
S and M, see Scheme 1) are varied by simply irradiating with S and M)‡ have been investigated in PMMA thin films in order
(
(
PMMA), in which the absorption profiles of the two forms
UV light, can change the degree of light-matter coupling in a to evaluate the suitable conditions for the fabrication of the
cavity. This leads to a modulation from weak-to-strong coupling microcavity. The choice of Tb has been dictated by the require-
regime, resulting in tuning of the cavity spectral response from ment of sharp emission bands centred on the blue-greenish
7
14
pure photonic to polaritonic states.
optical window and to avoid possible photo/electronic inter-
In this communication we demonstrated, for the first time, ferences with spiropyrans. In fact, the unique electronic proper-
that the introduction of a green dye (Tb, terbium(III) acetyl- ties of the Ln(III) complexes (internal f–f metal centred emission
acetonate hydrate),‡ suitably mixed with the red emission of shielded by external s and p orbitals) make them ideal candi-
1
5
the spiropyrans, leads to a colour tunability of the cavity output. dates for such an application.
In particular, we show that the cavity emission ranges from
In Fig. 1 (left) the absorption profiles of the spiropyrans at
different UV irradiation times, as well as the emission spectrum,
are shown. It can be noted that while S only absorbs in the UV
region (up to ca. 400 nm), M shows an additional broad band
a
Istituto di Nanoscienze – Consiglio Nazionale delle Ricerche,
NNL c/o distretto tecnologico Lecce, via Arnesano 16, 73100 Lecce, Italy.
6
E-mail: gianluca.accorsi@nano.cnr.it; Tel: +39 0832 1816216
centred at ca. 560 nm. The light-induced conversion between the
b
Department of Mathematics and Physics ‘‘Ennio De Giorgi’’, University of Salento,
two forms can also be easily appreciated by the naked eye because
of the colour change (from transparent S to dark violet M) of the
film. A total conversion from S to M is reached after B15 minutes
Campus Universitario, via Monteroni, 73100 Lecce, Italy
c
Center for Biomolecular Nanotechnology (CBN), Fondazione Istituto Italiano di
Tecnologia (IIT), Via Barsanti 1, Arnesano, 73010, Italy
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/c3cc47691j of UV irradiation (Fig. 1 left, inset). The absorption and emission
1
122 | Chem. Commun., 2014, 50, 1122--1124
This journal is ©The Royal Society of Chemistry 2014

View Article Online
Communication
ChemComm
Based on both the simulator results and the photophysical
data of the single components, a metallic microcavity has
been made by spin coating a CH Cl solution of S in PMMA
2
2
À1
(
PMMA–S 2 : 3 w/w, 26 mg ml ) on a glass substrate between
two thermally deposited semi-transparent Ag mirrors. The
resulting device consists of a PMMA doped layer (B130 nm
thick) between two Ag layers (each 35 nm thick), corresponding
to a cavity mode centred at B560 nm (suitably matched with
the absorption band of M). This is confirmed by the experi-
mental data provided in Fig. 2 (right inset, green line) showing
the cavity transmission spectrum at normal incidence. After UV
light irradiation, the rising absorption band of M changes the
degree of light-matter coupling within the cavity according to
the number of the switched photoactive molecules. This results
in a modulation of resonant modes from pure photonic to
polaritonic ones as clearly observable in the cavity transmission
Fig. 1 Left: electronic absorption (black; red lines refer to initial and final
spectra only) and emission (grey) spectra of spiropyrans in PMMA thin film.
The absorption profiles are taken at different UV light irradiation times and spectrum after total conversion where the two polariton peaks
the variation of the peak intensity at 560 nm (M) is provided in the inset.
Right: absorption (inset) and emission (main window) profiles of Tb in
PMMA thin film.
are centred at B470 and 640 nm, respectively (see Fig. 2, right
inset, red line).
Finally, we performed numerical electromagnetic simula-
tions using the Emissive Thin Film Optical Simulator (ETFOS)
profiles of Tb have also been obtained at room temperature in a that allowed the computing of the light-out-coupling (and thus
thin film and are depicted in Fig. 1 (right). Tb shows the typical the luminance and the CIE coordinates) of an emitting dipole
1
6
line-like emission bands in the B480–650 nm range and an distribution in a dielectric medium. To achieve such an analysis
absorption profile centred at B300 nm assigned to p–p transi- the experimental n and k values have been used as optical
1
7
tions of the coordinating ligands.
backgrounds of the cavity in the two limit cases (not and fully
Moreover, ellipsometry measurements have been performed converted). By using Tb and spiropyrans as emission dipoles,
on thin films in order to determine the optical constants, such we have obtained the simulated emission spectra of the cavity
as the real (n) and imaginary (k) parts of the refractive index. (Fig. 2, left inset). Therefore, thanks to this simulator response,
These are the key parameters required to obtain the thickness this feature has been successfully exploited by adding Tb
of the microcavity and hence, to properly tune its resonant (15%, w/w) into the cavity. The resulting light output at different
4
mode. These experiments were performed during the whole UV irradiation times has been recorded and it spans from greenish
UV photo-induced conversion process and the relative data are (t = 0 s) to red (t = 960 s), as depicted in Fig. 3. At t = 0 s the
shown in Fig. 2.
Fig. 2 Main window: dispersion of the real part of the refractive index (n)
through the studied wavelength range (dispersion of k is provided in
Fig. S1, ESI†). Left inset: simulated emission spectra of the cavity with Tb and
S as emission dipoles. Right inset: coupled and uncoupled cavity transmit-
tance. In all the three windows, the green and red lines represent the two limit
cases: before (t = 0 s) and after (t = 960 s) conversion, respectively.
Fig. 3 Emission spectra (lexc = 300 nm, normalized by area) of the
microcavity containing both S and Tb in PMMA. The initial (t = 0 s) and
final (t = 960 s) curves are represented by black solid lines and full circles,
respectively. The lines in between (grey) refer to irradiation times of 15, 30,
60, 120, 240, and 480 seconds, respectively. The restored emission profile
(1 h at 50 1C, red empty circles) is also shown.
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 1122--1124 | 1123

View Article Online
ChemComm
Communication
dye and a photo-switchable red emitting material, respectively,
have been investigated. Colour output tunability has been
achieved, for the first time, by inducing a strong coupling
interaction between the photo-switchable spiropyrans and the
cavity photonic mode. Colour outputs ranging from greenish to
red have been thus obtained through simple external UV irradia-
tion. Efforts toward obtaining white light emission based on the
same principle, via material replacements, are in progress.
This work was supported by an Italian project CNR-RADIUS
‘
‘Ricerca Avanzata su materiali e Dispositivi organici Innovativi
per l’Utilizzo nel Solare’’, EFOR—Energia da Fonti Rinnovabili
Iniziativa CNR per il Mezzogiorno L. 191/2009 art. 2 comma 44),
(
MAAT (MIUR – PON02_00563_3316357 – CUP B31C12001230005)
and progetto 5 per mille per la ricerca ‘‘Realizzazione di
dispositivi organici in regime di strong coupling’’ (CUP
F81J12000660001).
Notes and references
‡
Spectroscopic grade S, M, Tb and PMMA were purchased from Sigma-
Aldrich Chemical Company (St. Louis, MO, USA).
Absorption and emission spectra were obtained by a JASCO V-670
Fig. 4 Commission Internationale de l’Eclairage (CIE) coordinates corre-
sponding to the emission profiles are provided in Fig. 3. The related
irradiation times (from greenish down to red) are also indicated. CIE and
CRI values are provided in Table S1, ESI.†
§
spectrophotometer and a Varian Cary Eclipse spectrofluorometer,
respectively. Microcavity transmittance was measured by the J. A. Wollam
M-2000XI ellipsometer. Numerical simulations were performed using the
Emissive Thin Film Optical Simulator (ETFOS) Linux-version 1.5.
emission profile is characterized by three peaks at 488 nm,
1
2
3
M. Mazzeo, F. della Sala, F. Mariano, G. Melcarne, S. D. Agostino,
Y. Duan, R. Cingolani and G. Gigli, Adv. Mater., 2010, 22, 4696.
M. Mazzeo, F. Mariano, A. Genco, S. Carallo and G. Gigli, Org.
Electron., 2013, 14, 2840.
P. Zilio, D. Sammito, G. Zacco, M. Mazzeo, G. Gigli and F. Romanato,
Opt. Express, 2012, 20, A476.
K. J. Vahala, Nature, 2003, 424, 839.
B. Sch u¨ tte, H. Gothe, S. I. Hintschich, M. Sudzius, H. Frob,
V. G. Lyssenko and K. Leo, Appl. Phys. Lett., 2008, 92, 163309.
544 nm and B580 nm. The first two peaks are due to Tb emission,
while the latter is a contribution of both Tb and S. During the UV
conversion, the Tb greenish contribution decreases while the
red one from spiropyrans progressively enhances. Once the light
conversion between the spiropyran forms is complete, the emis-
sion from M definitely prevails.
4
5
It is worth noting that the original optical properties of the
microcavity can be fully recovered via soft thermal treatment
6 T. Schwartz, J. A. Hutchison, C. Genet and T. W. Ebbesen, Phys. Rev.
Lett., 2011, 106, 196405.
7
F. Li, L. Orosz, O. Kamoun, S. Bouchoule, C. Brimont, P. Disseix,
T. Guillet, X. Lafosse, M. Leroux, J. Leymarie, M. Mexis, M. Mihailovic,
G. Patriarche, F. Reveret, D. Solnyshkov, J. Zuniga-Perez and G. Malpuech,
Phys. Rev. Lett., 2013, 110, 196406.
(1 h at 50 1C), leading to a complete reversibility of the process
as provided in Fig. 3 (red empty circles).
A different way of representing the light output, typical of
display application, is by the use of the CIE coordinates. They
have been calculated from the emission spectra recorded in
Fig. 3 (CIEx, CIEy and CRI values are given in Table S1, ESI†)
and depicted in Fig. 4. Notably, the colour outputs, including
the intermediate ones, can be stably maintained (in the dark at
room temperature) by stopping UV irradiation, as signalled by the
superimposition of the emission spectra recorded over hours.
The next step will be to obtain white emission by improving the
blue spectral component. Such a target can be reached by a blue
shift of the microcavity mode through the replacement of the
photo-switchable material and/or by the use of a bluer emitter.
Experiments in this direction are in progress.§
8
9
D. Pisignano, M. Anni, G. Gigli, R. Cingolani, M. Zavelani-Rossi,
G. Lanzani, G. Barbarella and L. Favaretto, Appl. Phys. Lett., 2002,
81, 3534.
H. Li and K. Iga, Vertical-Cavity Surface-Emitting Laser Devices, Springer,
Berlin, 2003.
1
0 R. D. Costa, E. Orti, H. J. Bolink, F. Monti, G. Accorsi and
N. Armaroli, Angew. Chem., Int. Ed., 2012, 51, 8178.
1 B. W. D’andrade and S. R. Forrest, Adv. Mater., 2004, 16, 1585.
2 C. J. Humphreys, MRS Bull., 2008, 33, 459.
1
1
1
3 M. C. Gather, A. K o¨ hnen and K. Meerholz, Adv. Mater., 2011,
23, 233.
1
1
1
1
4 S. Quici, M. Cavazzini, G. Marzanni, G. Accorsi, N. Armaroli,
B. Ventura and F. Barigelletti, Inorg. Chem., 2005, 44, 529.
5 L. Armelao, S. Quici, F. Barigelletti, G. Accorsi, G. Bottaro, M. Cavazzini
and E. Tondello, Coord. Chem. Rev., 2010, 254, 487.
6 L. Armelao, G. Bottaro, S. Quici, C. Scalera, M. Cavazzini, G. Accorsi
and M. Bolognesi, ChemPhysChem, 2010, 11, 2499.
7 J. P. Leonard, C. B. Nolan, F. Stomeo and T. Gunnlaugsson,
Top. Curr. Chem., 2007, 281, 1.
In conclusion, the emission properties of a metallic micro-
cavity containing a Tb(III) complex and a spiropyran as a green
1
124 | Chem. Commun., 2014, 50, 1122--1124
This journal is ©The Royal Society of Chemistry 2014