Scott Shaw conducted research investigating strong coupling of β-carotene in microcavities. He was successful in achieving the strong coupling regime, obtaining a Rabi splitting of ~2.1 eV, over twice as large as the next highest. Degradation experiments showed β-carotene films deteriorated quickly when exposed to light and water, but less so under nitrogen. While a large splitting was achieved, the system was likely not emissive due to aggregation-induced quenching. The research demonstrated strong coupling of β-carotene can be achieved under specified fabrication and storage conditions.

1. S.R. Shaw
Summer Research Project Report
Coupling Carrots: Strong Coupling Regime of
β-carotene with use of Microcavities
Scott R. Shaw1*
Abstract
Investigation into the possibility of achieving the strong coupling regime in β-carotene with the use of microcavities
was achieved with success and a associated Rabi splitting of ∼2.1 eV was obtained, a factor of two higher than
that of the next highest. A short study into the degeneration of β-carotene films with tetrahydrofuran as a solvent
was accomplished and a brief guide to fabrication conditions is offered.
Keywords
Strong Coupling — Carotenoid — Microcavity
1Department of Physics and Astronomy, University of Sheffield, Sheffield, United Kingdom
*Corresponding author: srshaw1@sheffield.ac.uk
Introduction
The interaction between light and matter is of fundamental im-
portance in a range of optoelectronic technologies. Quantum
electrodynamics (QED) describes the interaction of quantized
matter with a quantized electromagnetic field. A specific area
of research interest is cavity QED which concerns the interac-
tion within a resonant structure, for example a microcavity [1].
The microcavity is an efficient system for confining light to
study light-matter interactions [2]. It has a basic composition
of an active semiconductor layer sandwiched between two
highly reflective mirrors. The mirrors quantize the electro-
magnetic field within the cavity, meaning that only photons of
certain energy can be confined within the structure. A micro-
cavity can be thought of as an optical resonator analogous to
how a guitar string can be considered an acoustic resonator;
the separation of the two mirrors determines the allowed pho-
ton modes within the cavity much in the same way that the
length of the guitar string determines the pitch of sound that
can be supported. The wavelengths of light are thus described
as l = m/2n where l is the separation between the two mir-
rors, m is any positive integer, and n is the refractive index
of the active layer. This can give rise to the strong coupling
regime. The trapped cavity photons and the electronic states
of a material placed in the cavity undergo a mixing process.
This can yield a polariton state, a bosonic quasiparticle, result-
ing from the coupling of light and any electric or magnetic
dipole-carrying excitation. One method of determining this
process is achieved by measuring the white-light reflectivity
from the cavity.
Strong coupling using inorganic quantum well devices have
been exhaustively studied [3] while the study of organic
strongly coupled microcavities is rather young, having only
been developed since 1998 [4]. Organic materials possess
Frenkel excitons instead of Wannier ones with large binding
energy and oscillator strength [5]. This general difference
between inorganic and organic microcavities allows the or-
ganic microcavities to obtain larger values for Rabi splitting
and also offers the possibility of easily observing polaritons
at room temperature [6]. This means that the excitons are
better able to resist thermal disassociation facilitating the ob-
servation of polaritons at room temperature. The energy level
splitting between modes, also known as Rabi splitting which
is directly related to the square root of the molecule concentra-
tion, is a signature of formation of such hybrid states. It will
determine that it is in the strong couple regime if a discernible
experimental Rabi splitting is observed [7].
So far, investigation into organic strongly coupled material has
been carried out by using high fluorescent dyes with relatively
simple energy state diagrams [8]. One especially interest-
ing optically active molecule is β-carotene, since carotenoids
show a strong electron correlation, a rather complicated ex-
cited state structure and short excited state lifetimes [9]. Since
the transition to the first excited state is forbidden due to the
symmetry selection rules, the conventional absorption and flu-
orescence spectroscopy observes transitions involving the sec-
ond excited state of carotenoids, S2 [10]. By utilising strong
coupling with a sufficient Rabi splitting it may be possible to
achieve this electronic ‘forbidden’transition from changing
the symmetry of the system from AAB to ABA. This means
that the transition between the S2 and ground state would no
longer be a ‘forbidden’transition and thus emission from the
system may be able to be observed.
1. Experimental Methods
Fabrication of the thin films and microcavities was achieved
upon a glass slide with dimensions, 2cm by 1.5cm, which
were cleaned in a two stage process of sonication in hot and

2. Coupling Carrots: Strong Coupling Regime of β-carotene with use of Microcavities — 2/3
cold deionised water with added Hellmanex® III for 5 min-
utes each and then dried with a dry nitrogen flow. For the
microcavities, aluminium mirrors were placed on the glass
slides via evaporation in a vacuum of 10-6 mbar at a slow rate
to achieve a relatively smooth interface. The characteristic
metal thickness was 200 nm and 50nm and the sandwiched
active layer was varied via spin coating from 20 nm to 150
nm. The solution of β-carotene was made in a concentration
of 40 mg/ml with Tetrahydrofuran as a solvent. UV-Vis mea-
surements were made using the steady state FluoroMax with
xenon source and associated FluorEssenceTM computer soft-
ware. The white light reflectance spectroscopy was measured
with Deuterium-Halogen light source and Andor iDus 420
Series high speed spectroscopy CCD camera.
2. Results and Discussion
Degradation experiments of thin films of thickness 100 nm
were achieved through fluorescence spectroscopy, this data is
shown in Fig.1 and Fig.2. We can see in Fig.1 at the initial
time that the solution is slightly aggregated at 40 mg/ml as it
causes blue shift (around 400 nm) and red-shift (around 500-
520 nm). Fig.1 also shows the quick decay of the thin films
when left for 24 hours in ambient laboratory setting under
light and when placed in a dark box. It was discovered that
when left in light while under nitrogen flow the degeneration
is less pronounced. Fig.2 shows the degeneration of thin films
of thickness ∼100 nm when placed under nitrogen flow and
in the dark with being tested in intervals of 24 hours. We see
appreciable degeneration of the sample between 24 and 48
hours with the decay slowing in the next interval.
The white light reflectance spectroscopy experiment was con-
ducted upon a microcavity with an active layer of ∼100nm
and the data is shown in Fig. 3. There is clear evidence of
the strong coupling regime from observation of two distinct
branches above and below that of the exciton energy which
anticross around the resonant energy between that of the ex-
citon and the corresponding cavity photon mode. The Rabi
splitting was modelled using a classical two level model of
interacting oscillators and from this method a Rabi splitting
of ∼2.1 eV was obtained, a factor of two higher than that of
the next highest [11]. Although sufficient splitting may have
been achieved, the system may not be emissive for a num-
ber of other reasons, most likely due to aggregation-induced
quenching of fluorescence.
3. Conclusion
Degradation experiments have shown that sample fabrication
and storage is not a straight forward task. It is believed that
the degeneration of the samples arises mainly through inter-
action with the surrounding water molecules present in the
environment and a minor contribution through ambient light
absorption. Sample preparation should be conducted under
a nitrogen flow while storage of samples should not exceed
Figure 1. Degradation results via fluorescence spectroscopy of 40
mg/ml of β-carotene in THF for period of 24 hours in different
conditions. Initial: Results after fabrication, LN: light with nitrogen
flow, DA: dark in ambient laboratory setting, LA: light in ambient
laboratory setting
Figure 2. Degradation results via fluorescence spectroscopy of 40
mg/ml of β-carotene in THF for intervals of 24 hours while stored
under nitrogen
Figure 3. White light reflectance spectroscopy results clearing
showing two distinct branches above and below that of the exciton
energy.

3. Coupling Carrots: Strong Coupling Regime of β-carotene with use of Microcavities — 3/3
24 hours and should be placed under a vacuum in the dark.
As aggregation arises at 40 mg/ml and it is thought likely
that all aggregates either show efficient singlet fission (triplet
formation) or enhanced non-radiative decay from S2 to S0
(not passing S1) it is best to reduce the concentration of the
β-carotene solution. This research has shown that the strong
coupling regime with use of microcavities for β-carotene is
achievable under the specified conditions which yields a large
Rabi splitting.
Acknowledgments
The author would like to thank Richard T. Grant for continued
support throughout the project and The Rank Prize Fund for
the funding which made this research possible.
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