Polaritons in Living Systems: Modifying Energy Landscapes in Photosynthetic Organisms Using a Photonic Structure
David M Coles, Lucas C Flatten, Thomas Sydney, Emily Hounslow, Semion K Saikin, Alán Aspuru-Guzik, Vlatko Vedral, Joseph Kuo-Hsiang Tang, Robert A Taylor, Jason M Smith, David G Lidzey
PPolaritons in Living Systems: Modifying Energy Landscapes in Photosynthetic Organisms Using aPhotonic Structure
David M Coles, ∗ Lucas C Flatten, Thomas Sydney, Emily Hounslow, Semion K Saikin,
Al´an Aspuru-Guzik, Vlatko Vedral,
Joseph Kuo-Hsiang Tang, † Robert A Taylor, Jason M Smith, ‡ and David G Lidzey § Department of Physics & Astronomy, University of She ffi eld, She ffi eld S3 7RH, UK Department of Materials, University of Oxford, Oxford OX1 3PH, UK Department of Chemistry, University of She ffi eld, She ffi eld S3 7HF, UK Department of Chemical and Biological Engineering, University of She ffi eld, She ffi eld S1 3JD, UK Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138, USA Institute of Physics, Kazan Federal University Kazan, 420008, Russian Federation Department of Physics, University of Oxford, Oxford OX1 3PU, UK Centre for Quantum Technologies, National University of Singapore, Singapore 117543 Department of Chemistry and Biochemistry, Clark University, Worcester, MA 01610-1477, USA
Photosynthetic organisms rely on a series of self-assemblednanostructures with tuned electronic energy levels in order totransport energy from where it is collected by photon absorp-tion, to reaction centers where the energy is used to drive chem-ical reactions. In the photosynthetic bacteria
Chlorobaculumtepidum ( Cba. tepidum ), a member of the green sulphur bacte-ria (GSB) family, light is absorbed by large antenna complexescalled chlorosomes. The exciton generated is transferred to aprotein baseplate attached to the chlorosome, before travelingthrough the Fenna-Matthews-Olson (FMO) complex to the re-action center . The energy levels of these systems are generallydefined by their chemical structure. Here we show that by plac-ing bacteria within a photonic microcavity, we can access thestrong exciton-photon coupling regime between a confined cav-ity mode and exciton states of the chlorosome, whereby a coher-ent exchange of energy between the bacteria and cavity moderesults in the formation of polariton states. The polaritons havean energy distinct from that of the exciton and photon, and canbe tuned in situ via the microcavity length. This results in real-time, non-invasive control over the relative energy levels withinthe bacteria. This demonstrates the ability to strongly influenceliving biological systems with photonic structures such as micro-cavities. We believe that by creating polariton states, that are inthis case a superposition of a photon and excitons within a livingbacteria, we can modify energy transfer pathways and thereforestudy the importance of energy level alignment on the e ffi ciencyof photosynthetic systems. A photonic structure has the ability to modify the properties ofelectronic transitions due to changes in the local density of photonicstates. If both the resonator and exciton state display suitably lowlosses (i.e. narrow linewidths), the exciton has a strong interactionwith light (i.e. a large absorption coe ffi cient), and the resonator isdegenerate in energy with the exciton, the system may enter thestrong coupling regime. Here, energy is reversibly exchanged be-tween the resonator and exciton and two new eigenstates of the sys-tem are formed which are a coherent superposition of the photonicand excitonic states. Such states are called polaritons, and are quasi-particles that are delocalized throughout the resonator due to theirphotonic component, whilst retaining an interaction cross-section ∗ d.m.coles@she ffi eld.ac.uk † Currently at Materials and Manufacturing Directories, Air Force ResearchLaboratory, WPAFB, OH 45433, USA ‡ [email protected] § d.g.lidzey@she ffi eld.ac.uk N o r m a li z ed e x t i n c t i on ( a r b . un i t s ) Wavelength (nm) a bc d Trypan blue solutionAl mirror Al mirrorCba. tepidumPlinth
Figure 1 . Spectral properties of green sulphur bacteria and mi-crocavity configuration. (a)
TEM image of
Cba. tepidum . Scalebar is 1 µ m. Note that the size and shape of the bacteria is dependentupon the light conditions during growth . (b) Normalised extinctionspectra of 0.4% trypan blue (TB) aqueous solution (blue line) and
Cba. tepidum in water (green line). (c)
Optical microscope imageof
Cba. tepidum in a TB viability stain showing clusters of bacteriawith compromised cell membranes (stained blue, labeled ) and in-tact cell membranes (unstained, appear green, labeled ). The scalebar is 10 µ m. (d) Schematic of microcavity consisting of a bacterialsolution suspended between two semitransparent metallic mirrors,one of which is on a raised plinth.inherited from the exciton . These properties have led to strikingdisplays of phenomena such as polariton superfluids , inversionlesslasing and non-equilibrium Bose-Einstein condensates , the lattertwo being observed up to room temperature (Refs. 8–10). Strongexciton-photon coupling in optical microcavities was first observedalmost 25 years ago by Weisbuch et al. , when a series of semi-conductor quantum wells were embedded between two high qual-ity planar dielectric mirrors. Since then, a wide variety of materialshave been shown to be able to strongly couple to photonic modessuch as bulk semiconductors , organic molecules , polymers , 2Ddichalcogenides and proteins . Recently, single molecules wereeven shown to be able to strongly couple to plasmonic cavities .In this Letter, we show that living bacteria placed within an opti- a r X i v : . [ phy s i c s . b i o - ph ] F e b (cid:1) W = 1 0 3 m e V a Wavelength (nm)
O p t i c a l c a v i t y l e n g t h ( n m ) q = 2
L = 4 8 4 n m b Intensity (arb. units)
W a v e l e n g t h ( n m )
L = 6 0 2 n m (cid:1) W = 7 8 m e V c q = 3 Wavelength (nm)
O p t i c a l c a v i t y l e n g t h ( n m )
L = 7 6 2 n m fd Intensity (arb. units)
W a v e l e n g t h ( n m )
L = 8 7 7 n m (cid:1) W = 5 0 m e V e q = 4 Wavelength (nm)
O p t i c a l c a v i t y l e n g t h ( n m )
L = 1 0 5 7 n mL = 1 1 7 2 n m
Intensity (arb. units)
W a v e l e n g t h ( n m )
Figure 2 . Strong coupling of green sulphur bacteria to microcavity photonic modes. (a)
Transmission of the cavity at the point labeled in Fig. 3 as function of wavelength and cavity length while scanning the q = cavity mode through the chlorosome energy, showing theanticrossing of the polariton branches about the chlorosome energy. White horizontal line shows the chlorosome exciton energy and blacksquares show the unperturbed cavity mode energy. Red circles are the fitted polariton branch energies. (b) Individual transmission spectra,vertically o ff set, for given cavity lengths around exciton-photon resonance clearly showing the splitting of the cavity mode at exciton-photonresonance. Grey dashed line shows the chlorosome exciton energy. (c) and (d) , and (e) and (f) show the same for cavity modes q = q = ff ering an entirely new way for the GSBto collect or deliver energy.Figure 1(a) shows a TEM micrograph of a Cba. tepidum . Thebacteria were either grown following the procedure given in Ref. 1,or purchased as an active culture (Leibniz-Institut DSMZ), and werestored in anaerobic conditions prior to use. Each bacterial cell con-tains 200-250 light harvesting chlorosomes, which are large ovoidstructures (100-200 nm long, ∼
50 nm wide) consisting of tubular orplanar aggregates of bacteriochlorophyll c (BChl c ) molecules .The absorption spectrum of Cba. tepidum in aqueous solution (40mg biomass per ml) is shown in Fig. 1(b) (green line). The strongabsorption peak at 750 nm is due to aggregates of BChl c in thechlorosomes. The weak absorption shoulder at 676 nm is assigned toBChl c monomer and / or chlorophyll a that can be found in the reac-tion center (while the principle exciton energy of the reaction centeris 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 Soretband of BChl c and carotenoid molecules within the chlorosomes. Inorder to verify the bacteria are alive in the strong coupling regime,we use the cell viability stain trypan blue (TB) which is added to thebacterial solution. The dye is able to permeate the cell membrane ofdead cells and binds to intracellular proteins. Live cells with intactmembranes are unstained by the dye . The absorption spectrum ofTB is shown in Fig. 1(b) (blue line), and displays a strong absorptionpeak at 587 nm, with a shoulder at 630 nm. Figure 1(c) shows an optical microscope image of the Cba. tepidum solution stained withTB (0.4% in water) at a ratio of 1:1. Both dead and alive clusters ofbacteria are visible, labeled and 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 µ m × µ m onto which the mirror is grown. A 10 nm layer ofpoly(methyl methacrylate) (PMMA) is spincast onto each mirror.The two mirrors are mounted face-to-face to form the cavity within acustom built white-light transmission microscope that allows angularalignment of the mirrors. A piezoelectric actuator allows nanometriccontrol over the cavity length. The cavity is imaged onto the entranceslit of an imaging CCD spectrometer. The area to be spectrally im-aged is defined by the image position on the spectrometer slit and therow of pixels on the CCD, the former defining the horizontal coordi-nate and the latter the vertical coordinate. The stained Cba. tepidum solution is injected between the mirrors, before reducing the mirrorseparation to form a cavity with well-defined Fabry-Perot modes. Aschematic of the cavity geometry is shown in figure 1(d).The transmission spectra from a region of the cavity measuring 5.5 µ m × µ m as the cavity length is scanned from 450 nm to 725 nmis shown in figure 2(a) (see Supplementary Information for details on the calculation of the cavity length). The observed transmissionpeak corresponds to the q = q is the modeindex. As the cavity mode energy is scanned through the excitonenergy (solid white line), two peaks are observed that anticross aboutthat energy. These peaks are the upper and lower polariton branches(UPB and LPB, red circles) that reside at higher and lower energythan the exciton respectively. While a strongly coupled system maybe described using a fully quantum or semi-quantum formalism ,here the large number of exciton states within the cavity allow us touse a classically coupled oscillator model to fit the polariton stateenergies (see Supplementary Information).When the uncoupled photon and exciton energy are degenerate atthe point of anticrossing, the polariton state can be considered 50%photon and 50% exciton. At this point, the magnitude of the energysplitting between the UPB and LPB is the Rabi splitting energy ( (cid:126) Ω )which is dependent on the square root of the product of the transitionoscillator strength and number of states in the cavity mode volume.In the case of coupling with the q = is that (cid:126) Ω > ( γ x / + ( γ c /
2) where γ x and γ c are the full-width at half-maximum linewidthsof the uncoupled exciton and photon respectively. The chlorosomeexciton linewidth is 130 meV, and the q = q = ff set transmission spec-tra for decreasing cavity length (bottom to top). The two polaritonbranches and their anticrossing about the exciton energy (grey dashedline) is clearly visible. We note that the cavity could not be closedbeyond ∼
450 nm, likely due to the size of the bacteria within thecavity.Figure 2(c) and (d) show the microcavity transmission as the q = q = q = ∼
95 million if all chlorosomes are orientedin the plane of the cavity, and ∼
220 million if all dipoles are ran-domly oriented in the cavity. Assuming 200,000 BChl moleculesper chlorosome, the splitting corresponds to the coupling of excitonsfrom between 470 and 1100 chlorosomes, approximately the numberthat are in 2 to 6 bacteria.In order to ascertain whether the bacteria are alive during strongcoupling, we have performed micro-extinction spectroscopy on thebacteria involved in the coupling. A real-space CCD image of thecavity is shown in figure 3(a). The cavity was opened to approx-imately 100 µ m to allow a continuum of photonic states, and thenormalized extinction spectrum of the region marked ‘ ’ in Fig. 3(a)is shown in figure 3(b) (green line). We see that there is a strongabsorption peak at 750 nm due to the chlorosome absorption, but nosign of TB absorption in the 500-650 nm range, indicating that thecells had not been stained and remained viable. For comparison, themicro-extinction spectrum of an area containing compromised bac-teria is also shown (blue line) where TB absorption is the dominantfeature. Furthermore, there is no apparent dip in transmission in-tensity of the cavity modes when scanning through the 500-600 nmrange that would indicate the presence of TB (Supplementary Fig.S1). While the cavity acts to restrict the intensity of light reachingthe bacteria, they are known to survive in extremely low light en- a N o r m a li z ed e x t i n c t i on ( a r b . un i t s ) Wavelength (nm) b Figure 3 . Cba. tepidum within a microcavity and microabsorp-tion of
Cba. tepidum . ( a ) Real space optical image of the micro-cavity. White dashed lines mark the extent of the plinth in the ver-tical direction. White solid vertical line represents the position ofthe spectrometer slit when performing spectral imaging. White solidhorizontal line represents the position of the CCD track used for thespectra showing strong coupling shown in figure 2. The intersectionof the solid lines (marked ) is the position of the bacteria that areshown to undergo strong coupling to the cavity. The bacteria appearas pale spots on the image, however they are not clearly individuallyresolvable as they are smaller than the resolution of the microscope.( b ) Normalized absorption spectrum taken at position when thecavity was opened to allow a continuum of photonic states (greenline), and normalized absorption spectrum of stained bacteria (blueline). In both cases, the absorption spectra were taken from a spectralimage, with the reference taken from the same image but a separatetrack where no bacteria are present.vironments and display a low mortality rate even in the presence ofno light , making long-term experiments based on bacterial growthrates feasible. Indeed, the bacteria under investigation remained un-stained for the duration of the experiment, totaling several hours.We have previously suggested that the polariton branches mayprovide an alternative pathway for excitons to migrate through thephotosynthetic system, bypassing various states . The baseplate en-ergy is at 790 nm, while the FMO and reaction center are positionedat 810 nm and 840 nm respectively. Here, the lower polariton branchenergy can be widely tuned via the cavity length, and can be broughtinto resonance with each of these structures. For coupling to the q = L =
560 nm, the FMO complex at L =
580 nm and thereaction center at L =
605 nm. The excitonic percentage of polaritonstates at the BP, FMO and reaction center energy are 32%, 18% and8% respectively. The LPB may therefore act as a relaxation pathwayfor excitons from the chlorosome directly into lower energy states,including directly to the reaction center. This should modify the en-ergy transfer rates between the chlorosome and the other subunits and as a consequence a ff ect the growth rate of the bacteria.In conclusion, we have introduced living photosynthetic bacteriainto a photonic microcavity and shown that the system can enter thestrong coupling regime, thus creating exciton-photon superpositionstates within a living organism. It opens the opportunity to createhybrid-polariton systems in which the optical state that is coupled tothe chlorosome assembly is also coupled to a second semiconductormaterial placed within the optical cavity. This approach has previ-ously been used to hybridise a range of di ff erent semiconductor sys-tems, including di ff erent species of molecular dyes , and moleculardyes with semiconductor quantum wells . Such hybridisation hasbeen shown to facilitate rapid energy transfer between the excitonicstates by virtue of the intermediate hybrid-polariton , and could beused to either inject or extract energy from a chlorosome in a living bacteria. Furthermore, the optical cavity allows in situ control overthe relative energy levels within the bacteria, and by enhancing en-ergy transfer to the reaction center from the chlorosome, it may bepossible to direct the evolution of green-sulfur-bacteria towards or- ganisms that are more fit to live inside a microcavity than outside ofit, i.e. an organism tailored to live in a superposition state with aphoton.1. Overmann, J. The Prokaryotes , vol. 7 (Springer, New York, 2006),3rd edn.2. Weisbuch, C., Nishioka, M., Ishikawa, A. & Arakawa, Y. Ob-servation of the coupled exciton-photon mode splitting in a semi-conductor quantum microcavity.
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NatureMaterials , 712–719 (2014). Acknowledgements
D.M.C and D.G.L thank EPSRC grant number EP / M025330 / ffi ce of Scienceand O ffi ce of Basic Energy Sciences, under Award Number de-sc0001088.V.V thanks the Oxford Martin School, Wolfson College and the Universityof Oxford, the Leverhulme Trust (UK), the John Templeton Foundation, theEU Collaborative Project TherMiQ (Grant Agreement 618074), the COSTAction MP1209 and the EPSRC (UK). This research is also supported by theNational Research Foundation, Prime Ministers O ffi ce, Singapore, under itsCompetitive Research Programme (CRP Award No. NRF- CRP14-2014-02)and administered by Centre for Quantum Technologies, National Universityof Singapore. D.M.C further thanks EPSRC grant number EP / K032518 / Author Contributions
S.K.S, A.A.G and V.V conceived the experiment and provided theoreti-cal background. J.K.T cultured bacteria. Bacterial staining and optical mi-croscopy was performed by T.S and E.H. D.M.C and L.C.F fabricated themicrocavity samples. Experiments were performed by D.M.C and L.C.F un- der the supervision of R.A.T, J.M.S and D.G.L. All authors contributed to thepreparation of the manuscript.
SUPPLEMENTARY INFORMATIONCalibration of cavity length
The cavity mirrors are first aligned to be parallel by observing theinterference fringes in transmission as the cavity length is reduced,and adjusting the angle of one of the mirrors until only one fringe isvisible across the plinth. The cavity length is scanned by applying alinear voltage ramp to a piezoelectric actuator attached to one of thecavity mirrors, and the cavity transmission was spectrally imaged asa function of time. In order to calculate the cavity length for eachspectrum, an area of the spectral image is selected where no strongcoupling is observed (i.e. there is no bacteria or too few in a givenarea to couple). The cavity length ( L ) is set such that several Fabry-Perot modes are observed in transmission. The index ( q ) of each ofthe modes is calculated from the wavelengths of adjacent modes λ q and λ q − using q = λ q − λ q − − λ q (S1)Once the mode index of a transmission peak is known, the cavitylength can be calculated via L = λ q q n (S2)where n is the intracavity refractive index, which in this case is thatof water, 1.33. Polariton branch fitting
The energy of the lower and upper polariton branches is given bya coupled oscillator model S1 , equation S3. (cid:32) E c ( L ) (cid:126) Ω / (cid:126) Ω / E x (cid:33) (cid:32) α c ( L ) α x ( L ) (cid:33) = E p ( L ) (cid:32) α c ( L ) α x ( L ) (cid:33) , (S3)where E c ( L ) is the uncoupled cavity mode energy (which is foundfrom Eqn. S2), E x is the exciton energy, E p ( L ) is the polariton en-ergy and α c ( L ) and α x ( L ) are photon and exciton mixing coe ffi cientsrespectively. This can be solved to give E p ( L ) = E c ( L ) + E x ± (cid:113) ( E c ( L ) − E x ) + (cid:126) Ω (S4)This is fitted to the observed polariton branch energies with (cid:126) Ω and n as a fitting parameters. We note that refractive index of the bacte-ria varies slightly from that of the background index of water S2 , andfrom the fitting we find that n = . n = .
36 and n = .
35 for the q = q = q = n .The eigenvalues (mixing co-e ffi cients) α c ( L ) and α x ( L ) can also becalculated via α c , UPB ( L ) = E c ( L ) − E p,UPB ( L ) E x + E c ( L ) − E p,UPB ( L ) , α ( L ) = − α c , UPB ( L ) α c , LPB ( L ) = α ( L ) , α ( L ) = α c , UPB ( L )(S5) Transmission mode intensity
Figure S4 shows the transmission intensity for modes q = q =
6. We observe no obvious absorption feature at 587 nm thatwould correspond to the TB absorption band. The continuous fall intransmission intensity for increasing wavelength towards the excitonenergy is due to the polariton branch becoming more exciton like andless photon like in nature, while the opposite is true for increasingwavelengths away from the exciton energy. q = 7 q = 3 L P B q = 6 q = 3 U P B q = 5 q = 2 L P B q = 4 q = 2 U P B
Intensity (arb. units)
W a v e l e n g t h ( n m )
Figure S4 . Mode intensity of polariton branches.Calculation of the number of dipoles involved in strong coupling
The Rabi splitting energy ( (cid:126) Ω ) increases as the square root of thenumber of dipoles coupled to the field ( N ), with the splitting givenby S3 (cid:126) Ω = (cid:16) µ · ˆ E (cid:17) √ N (cid:32) π (cid:126) cn ff λ(cid:15) V (cid:33) (S6)where µ is the coupled dipole moment, V is the cavity mode volumeand ˆ E is a unit vector parallel to the polarization of the cavity elec-tric field. The dipole moment of BChl c is 5.48 D (Ref. S4), and thecavity mode volume S5 of the q = (cid:16) λ n (cid:17) .[S1] Skolnick, M. S., Fisher, T. A. & Whittaker, D. M. Strong cou-pling phenomena in quantum microcavity structures. Semicond.Sci. Technol. , 645 (1998).[S2] Liu, P. et al. Real-time measurement of single bacterium’s re-fractive index using optofluidic immersion refractometry.
Pro- cedia Engineering , 356 – 359 (2014).[S3] Fox, M. Quantum Optics - An Introduction (Oxford MasterSeries, 2006).[S4] Prokhorenko, V., Steensgaard, D. & Holzwarth, A. Excitondynamics in the chlorosomal antennae of the green bacteria chloroflexus aurantiacus and chlorobium tepidum.
BiophysicalJournal , 2105 – 2120 (2000).[S5] Ujihara, K. Spontaneous emission and the concept of e ff ectivearea in a very short optical cavity with plane-parallel dielectric mirrors. Japanese Journal of Applied Physics30