Polaritons in Living Systems: Modifying Energy Landscapes in Photosynthetic Organisms Using a Photonic Structure

Photosynthetic organisms rely on a series of self-assembled nanostructures with tuned electronic energy levels in order to transport energy from where it is collected by photon absorption, to reaction centers where the energy is used to drive chemical reactions. In the photosynthetic bacteria Chlorobaculum tepidum (Cba. tepidum), a member of the green sulphur bacteria (GSB) family, light is absorbed by large antenna complexes called chlorosomes. The exciton generated is transferred to a protein baseplate attached to the chlorosome, before traveling through the Fenna-Matthews-Olson (FMO) complex to the reaction center. The energy levels of these systems are generally defined by their chemical structure. Here we show that by placing bacteria within a photonic microcavity, we can access the strong exciton-photon coupling regime between a confined cavity mode and exciton states of the chlorosome, whereby a coherent exchange of energy between the bacteria and cavity mode results in the formation of polariton states. The polaritons have an energy distinct from that of the exciton and photon, and can be tuned in situ via the microcavity length. This results in real-time, non-invasive control over the relative energy levels within the bacteria. This demonstrates the ability to strongly influence living biological systems with photonic structures such as microcavities. We believe that by creating polariton states, that are in this case a superposition of a photon and excitons within a living bacteria, we can modify energy transfer pathways and therefore study the importance of energy level alignment on the efficiency of photosynthetic systems.

Photosynthetic organisms rely on a series of self-assembled nanostructures with tuned electronic energy levels in order to transport energy from where it is collected by photon absorption, to reaction centers where the energy is used to drive chemical reactions.In the photosynthetic bacteria Chlorobaculum tepidum (Cba.tepidum), a member of the green sulphur bacteria (GSB) family, light is absorbed by large antenna complexes called chlorosomes.The exciton generated is transferred to a protein baseplate attached to the chlorosome, before traveling through the Fenna-Matthews-Olson (FMO) complex to the reaction center 1 .The energy levels of these systems are generally defined by their chemical structure.Here we show that by placing bacteria within a photonic microcavity, we can access the strong exciton-photon coupling regime 2 between a confined cavity mode and exciton states of the chlorosome, whereby a coherent exchange of energy between the bacteria and cavity mode results in the formation of polariton states.The polaritons have an energy distinct from that of the exciton and photon, and can be tuned in situ via the microcavity length.This results in realtime, non-invasive control over the relative energy levels within the bacteria.This demonstrates the ability to strongly influence living biological systems with photonic structures such as microcavities.We believe that by creating polariton states, that are in this case a superposition of a photon and excitons within a living bacteria, we can modify energy transfer pathways and therefore study the importance of energy level alignment on the efficiency of photosynthetic systems.
A photonic structure has the ability to modify the properties of electronic transitions due to changes in the local density of photonic states.If both the resonator and exciton state display suitably low losses (i.e.narrow linewidths), the exciton has a strong interaction with light (i.e. a large absorption coefficient), and the resonator is degenerate in energy with the exciton, the system may enter the strong coupling regime.Here, energy is reversibly exchanged between the resonator and exciton and two new eigenstates of the system are formed which are a coherent superposition of the photonic and excitonic states.Such states are called polaritons, and are quasiparticles that are delocalized throughout the resonator due to their photonic component, whilst retaining an interaction cross-section inherited from the exciton 4 .These properties have led to striking displays of phenomena such as polariton superfluids 5 , inversionless lasing 6 and non-equilibrium Bose-Einstein condensates 7 , the latter two being observed up to room temperature (Refs.8-10).Strong exciton-photon coupling in optical microcavities was first observed almost 25 years ago by Weisbuch et al. 2 , when a series of semiconductor quantum wells were embedded between two high quality planar dielectric mirrors.Since then, a wide variety of materials have been shown to be able to strongly couple to photonic modes such as bulk semiconductors 11 , organic molecules 12 , polymers 13 , 2D dichalcogenides 14,15 and proteins 16 .Recently, single molecules were even shown to be able to strongly couple to plasmonic cavities 17 .
In this Letter, we show that living bacteria placed within an opti- cal microcavity are able to strongly couple to the cavity field.Our work therefore demonstrates the formation of a 'living polariton'.This quasiparticle is part photon and part living organism in nature.This approach opens an opportunity to understand the interplay of the electronic states within a photosynthetic organism with its bio-logical function.Looking further ahead, we expect that our approach will permit optical hybridisation of the chlorosome states with other optoelectronic materials, offering an entirely new way for the GSB to collect or deliver energy.
Figure 1(a) shows a TEM micrograph of a Cba.tepidum.The bacteria were either grown following the procedure given in Ref. 1, or purchased as an active culture (Leibniz-Institut DSMZ), and were stored in anaerobic conditions prior to use.Each bacterial cell contains 200-250 light harvesting chlorosomes, which are large ovoid structures (100-200 nm long, ∼50 nm wide) consisting of tubular or planar aggregates of bacteriochlorophyll c (BChl c) molecules 18,19 .The absorption spectrum of Cba.tepidum in aqueous solution (40 mg biomass per ml) is shown in Fig. 1(b) (green line).The strong absorption peak at 750 nm is due to aggregates of BChl c in the chlorosomes.The weak absorption shoulder at 676 nm is assigned to BChl c monomer and/or chlorophyll a that can be found in the reaction center (while the principle exciton energy of the reaction center is at 840 nm).The shoulder at 810 nm is due to the FMO complex, while the absorption band in the 400-500 nm region is from the Soret band of BChl c and carotenoid molecules within the chlorosomes.In order to verify the bacteria are alive in the strong coupling regime, we use the cell viability stain trypan blue (TB) which is added to the bacterial solution.The dye is able to permeate the cell membrane of dead cells and binds to intracellular proteins.Live cells with intact membranes are unstained by the dye 20 .The absorption spectrum of TB is shown in Fig. 1(b) (blue line), and displays a strong absorption peak at 587 nm, with a shoulder at 630 nm. Figure 1(c) shows an optical microscope image of the Cba.tepidum solution stained with TB (0.4% in water) at a ratio of 1:1.Both dead and alive clusters of bacteria are visible, labeled 1 and 2 respectively.
An open microcavity structure is used as the photonic resonator.Two 15 nm thick semitransparent aluminium planar mirrors (80% reflectivity at 750 nm) were thermally deposited on silica substrates.One of the substrates has a raised 'plinth' of dimensions 100 µm × 100 µm onto which the mirror is grown.A 10 nm layer of poly(methyl methacrylate) (PMMA) is spincast onto each mirror.The two mirrors are mounted face-to-face to form the cavity within a custom built white-light transmission microscope that allows angular alignment of the mirrors.A piezoelectric actuator allows nanometric control over the cavity length.The cavity is imaged onto the entrance slit of an imaging CCD spectrometer.The area to be spectrally imaged is defined by the image position on the spectrometer slit and the row of pixels on the CCD, the former defining the horizontal coordinate and the latter the vertical coordinate.The stained Cba.tepidum solution is injected between the mirrors, before reducing the mirror separation to form a cavity with well-defined Fabry-Perot modes.A schematic of the cavity geometry is shown in figure 1(d).
The transmission spectra from a region of the cavity measuring 5.5 µm × 1.2 µm as the cavity length is scanned from 450 nm to 725 nm is shown in figure 2(a) (see Supplementary Information for details on the calculation of the cavity length).The observed transmission peak corresponds to the q = 2 cavity mode, where q is the mode index.As the cavity mode energy is scanned through the exciton energy (solid white line), two peaks are observed that anticross about that energy.These peaks are the upper and lower polariton branches (UPB and LPB, red circles) that reside at higher and lower energy than the exciton respectively.While a strongly coupled system may be described using a fully quantum or semi-quantum formalism 21 , here the large number of exciton states within the cavity allow us to use a classically coupled oscillator model 22 to fit the polariton state energies (see Supplementary Information).
When the uncoupled photon and exciton energy are degenerate at the point of anticrossing, the polariton state can be considered 50% photon and 50% exciton.At this point, the magnitude of the energy splitting between the UPB and LPB is the Rabi splitting energy ( Ω) which is dependent on the square root of the product of the transition oscillator strength and number of states in the cavity mode volume.
In the case of coupling with the q = 2 cavity mode, we find a splitting of 103 meV.The criteria for strong coupling 23 is that Ω > (γ x /2) + (γ c /2) where γ x and γ c are the full-width at half-maximum linewidths of the uncoupled exciton and photon respectively.The chlorosome exciton linewidth is 130 meV, and the q = 2 cavity mode linewidth away from the strong coupling region is 70 meV, therefore the strong coupling criteria is satisfied for coupling to the q = 2 mode.
Figure 2(b) shows a series of vertically offset transmission spectra for decreasing cavity length (bottom to top).The two polariton branches and their anticrossing about the exciton energy (grey dashed line) is clearly visible.We note that the cavity could not be closed beyond ∼ 450 nm, likely due to the size of the bacteria within the cavity.
Figure 2(c) and (d) show the microcavity transmission as the q = 3 cavity mode is scanned through the exciton energy.While an anticrossing is again visible, the mode splitting is reduced to 78 meV.This is because of a reduction in the interaction potential due to the weaker EM-field within the cavity.For the q = 4 mode (figure 2(e) and (f)), the splitting energy is reduced again to 50 meV, and the anticrossing is not clearly resolvable.
The magnitude of the Rabi splitting allows us to put bounds on the number of pigment molecules, and hence the number of bacteria involved in the coupling (see Supplementary Information for details).We find that the number of excitons simultaneously coupled to the q = 2 cavity mode is ∼95 million if all chlorosomes are oriented in the plane of the cavity, and ∼220 million if all dipoles are randomly oriented in the cavity.Assuming 200,000 BChl molecules per chlorosome, the splitting corresponds to the coupling of excitons from between 470 and 1100 chlorosomes, approximately the number that are in 2 to 6 bacteria.
In order to ascertain whether the bacteria are alive during strong coupling, we have performed micro-extinction spectroscopy on the bacteria involved in the coupling.A real-space CCD image of the cavity is shown in figure 3(a).The cavity was opened to approximately 100 µm to allow a continuum of photonic states, and the normalized extinction spectrum of the region marked '1' in Fig. 3(a) is shown in figure 3(b) (green line).We see that there is a strong absorption peak at 750 nm due to the chlorosome absorption, but no sign of TB absorption in the 500-650 nm range, indicating that the cells had not been stained and remained viable.For comparison, the micro-extinction spectrum of an area containing compromised bacteria is also shown (blue line) where TB absorption is the dominant feature.Furthermore, there is no apparent dip in transmission intensity of the cavity modes when scanning through the 500-600 nm range that would indicate the presence of TB (Supplementary Fig. S1).While the cavity acts to restrict the intensity of light reaching the bacteria, they are known to survive in extremely low light en- (b) Normalized absorption spectrum taken at position 1 when the cavity was opened to allow a continuum of photonic states (green line), and normalized absorption spectrum of stained bacteria (blue line).In both cases, the absorption spectra were taken from a spectral image, with the reference taken from the same image but a separate track where no bacteria are present.
vironments and display a low mortality rate even in the presence of no light 24 , making long-term experiments based on bacterial growth rates feasible.Indeed, the bacteria under investigation remained unstained for the duration of the experiment, totaling several hours.
We have previously suggested that the polariton branches may provide an alternative pathway for excitons to migrate through the photosynthetic system, bypassing various states 25 .The baseplate energy is at 790 nm, while the FMO and reaction center are positioned at 810 nm and 840 nm respectively.Here, the lower polariton branch energy can be widely tuned via the cavity length, and can be brought into resonance with each of these structures.For coupling to the q = 2 cavity mode, the LPB is resonant with the baseplate at a cavity length of L =560 nm, the FMO complex at L =580 nm and the reaction center at L =605 nm.The excitonic percentage of polariton states at the BP, FMO and reaction center energy are 32%, 18% and 8% respectively.The LPB may therefore act as a relaxation pathway for excitons from the chlorosome directly into lower energy states, including directly to the reaction center.This should modify the energy transfer rates between the chlorosome and the other subunits 26 and as a consequence affect the growth rate of the bacteria.
In conclusion, we have introduced living photosynthetic bacteria into a photonic microcavity and shown that the system can enter the strong coupling regime, thus creating exciton-photon superposition states within a living organism.It opens the opportunity to create hybrid-polariton systems in which the optical state that is coupled to the chlorosome assembly is also coupled to a second semiconductor material placed within the optical cavity.This approach has previously been used to hybridise a range of different semiconductor systems, including different species of molecular dyes 27 , and molecular dyes with semiconductor quantum wells 28,29 .Such hybridisation has been shown to facilitate rapid energy transfer between the excitonic states by virtue of the intermediate hybrid-polariton 30 , and could be used to either inject or extract energy from a chlorosome in a living bacteria.Furthermore, the optical cavity allows in situ control over the relative energy levels within the bacteria, and by enhancing energy transfer to the reaction center from the chlorosome, it may be possible to direct the evolution of green-sulfur-bacteria towards or-ganisms that are more fit to live inside a microcavity than outside of it, i.e. an organism tailored to live in a superposition state with a photon.
der the supervision of R.A.T, J.M.S and D.G.L.All authors contributed to the preparation of the manuscript.

Figure 1 .
Figure 1.Spectral properties of green sulphur bacteria and microcavity configuration.(a) TEM image of Cba.tepidum.Scale bar is 1 µm.Note that the size and shape of the bacteria is dependent upon the light conditions during growth 3 .(b) Normalised extinction spectra of 0.4% trypan blue (TB) aqueous solution (blue line) and Cba.tepidum in water (green line).(c) Optical microscope image of Cba.tepidum in a TB viability stain showing clusters of bacteria with compromised cell membranes (stained blue, labeled 1) and intact cell membranes (unstained, appear green, labeled 2).The scale bar is 10 µm.(d) Schematic of microcavity consisting of a bacterial solution suspended between two semitransparent metallic mirrors, one of which is on a raised plinth.

3 W 4 WFigure 2 .
Figure 2. Strong coupling of green sulphur bacteria to microcavity photonic modes.(a) Transmission of the cavity at the point labeled 1 in Fig. 3 as function of wavelength and cavity length while scanning the q=2 cavity mode through the chlorosome energy, showing the anticrossing of the polariton branches about the chlorosome energy.White horizontal line shows the chlorosome exciton energy and black squares show the unperturbed cavity mode energy.Red circles are the fitted polariton branch energies.(b) Individual transmission spectra, vertically offset, for given cavity lengths around exciton-photon resonance clearly showing the splitting of the cavity mode at exciton-photon resonance.Grey dashed line shows the chlorosome exciton energy.(c) and (d), and (e) and (f) show the same for cavity modes q = 3 and q = 4 respectively.

aFigure 3 .
Figure 3. Cba.tepidum within a microcavity and microabsorption of Cba.tepidum.(a) Real space optical image of the microcavity.White dashed lines mark the extent of the plinth in the vertical direction.White solid vertical line represents the position of the spectrometer slit when performing spectral imaging.White solid horizontal line represents the position of the CCD track used for the spectra showing strong coupling shown in figure 2. The intersection of the solid lines (marked 1) is the position of the bacteria that are shown to undergo strong coupling to the cavity.The bacteria appear as pale spots on the image, however they are not clearly individually resolvable as they are smaller than the resolution of the microscope.(b)Normalized absorption spectrum taken at position 1 when the cavity was opened to allow a continuum of photonic states (green line), and normalized absorption spectrum of stained bacteria (blue line).In both cases, the absorption spectra were taken from a spectral image, with the reference taken from the same image but a separate track where no bacteria are present.