Electronic excitation of particles in fluorescent materials can now be controlled using laser-assisted energy-transfer techniques

Andrews, David

Bradshaw, David

English

Bradshaw, David S.

University of East Anglia digital repository

10.1117/2.1201006.003013Optical control of excited-statelifetimesDavid Andrews and David S. BradshawElectronic excitation of particles in fluorescent materials can now becontrolled using laser-assisted energy-transfer techniques.When light is absorbed by a fluorescent nanoparticle, the re-sulting excited state has a decay lifetime that is primarilydetermined by two intrinsic optical routes. One is decay throughemission of fluorescence, while the other is by resonance-energytransfer (RET), the transfer of the excitation to another site. Ithas recently become evident that the passage of a moderatelyintense, off-resonant laser beam may enable optical control ofexcited-state lifetimes in such a system. The general formulafor the excited-state lifetime,  , is very simply expressible as 1 D  1FL C  1RETCKI . The first two terms on the right are wellknown. They correspond to excited-state lifetimes associatedonly with fluorescence and RET, respectively. The additional,newly discovered term exhibits a proportionality to the intensityof an off-resonant laser beam, I . Detailed equations for the con-tributions to the proportionality constant,K, have been reportedin several recent publications. Using the theory of quantumelectrodynamics, we have shown thatK expresses a dependenceon laser-controlled fluorescence and laser-assisted RET.Suppose we have a single molecule or nanoparticle (such as aquantum dot) without any other in its immediate vicinity. Theinput of an off-resonant laser beam may modify the intensityof the molecule’s fluorescent emission.1, 2 The all-optical mech-anism involves no net absorption or stimulated light emission,since the throughput beam undergoes forward scattering andemerges unchanged (see Figure 1). Specifically, elastic for-ward scattering of the off-resonant light engages by nonlinearcoupling with the decay process. In essence, the probe laserconfers optical nonlinearity on the fluorescent emission (seeFigure 2). Unlike the well-known enhancement of emission bycouplingwith surface plasmons,3, 4 this mechanism signifies thatsuch modifications to spontaneous fluorescent emission may beproduced without the presence of any surface, through directinteraction with the oscillating electric field of the throughputelectromagnetic radiation.Figure 1. Representation of laser-controlled fluorescence. Blue line:Fluorescence. Orange: Off-resonant throughput laser beam.We have determined that fluorescence lifetimes, under spec-ified conditions, can be reduced by 10% for an input laser ir-radiance of 1011W/cm2 and proportionately more for higherintensities. Even at a level of 1010W/cm2, we anticipatethat the change in decay kinetics will be readily measur-able with modulation-based instrumentation. Time-correlatedphoton-detection techniques might represent a methodologyparticularly well-suited to such measurements when deployedin an adaptation of the setup for standard laser-fluorescencespectroscopy. With a tunable laser source, the capacity to controlthe wavelength of the throughput radiation introduces an ad-ditional experimental variable, offering a second dimension tosample interrogation. Establishing the variation with laser-pulseintensity will also validate the mechanism at work.We have also examined the ability of throughput light to exertcontrol over RET, a process that is favored when an electroni-cally excited component (acting as donor) relays its excitation toContinued on next page10.1117/2.1201006.003013 Page 2/3Figure 2. Energy-level diagram for laser-controlled fluorescence, athird-order nonlinear optical process. Electronic states (and their vi-brational manifolds) are represented by boxes, the wavy line is emittedfluorescence, the horizontal dashed arrow denotes the off-resonant laserbeam, and the green vertical arrow is an electronic transition.Figure 3. Energy-level scheme for laser-assisted resonance-energytransfer, a fourth-order process. As Figure 2, except that the wavy linenow denotes energy transfer. The energy levels for an initially excitedparticle are on the left, those for a nearby neighbor on the right.a neighboring particle with suitable acceptor levels. When REToccurs spontaneously, it usually represents the dominant processfor the decay of the species that holds the initial excitation.5On input of the off-resonant laser field, another, higher-order,all-optical mechanism may occur that again entails stimulatedforward scattering of the input beam (see Figure 3). Our calcu-lations show that laser-assisted RET may confer a several-folddecrease in the decay lifetime of a donor6 for moderately intenselaser irradiances of around 1012W/cm2.Figure 4. All-optical switching device comprising parallel 2D arrays,each composed of nanoparticles (spacer layer not shown).We recently proposed an all-optical switching device basedon this laser-assisted energy-transfer process. Working on theprinciple that conventional RET can be eliminated through asuitably designed geometric configuration (transition dipolesplaced mutually orthogonally to each other and to the donor-acceptor pair displacement), the transfer of energy will thenoccur only on application of the off-resonant laser. Switchingaction thus ensues, since donor decay by the transport of exci-tation energy is activated or deactivated when the throughputbeam is ‘on’ or ‘off,’ respectively.7 To exploit this switchingaction, one can envisage a sandwich structure comprising a pairof parallel 2D arrays (each composed of nanoparticle pixels) oneither side of an optically passive spacer layer (see Figure 4).Such a configuration provides a parallel signal-processingcapability.8Although the mechanisms for laser modifications to fluores-cence and energy transfer involve different orders of opticalresponse, they may both contribute to reducing excited-statelifetimes. Our attention now focuses on examination of theircombined effect, which could lead to unanticipated levels ofresponse. One reason for our interest in such a scenario is theContinued on next page10.1117/2.1201006.003013 Page 3/3possibility of an increased cycle frequency for excitation anddecay, increasing signal response and deferring the onset ofoptical saturation. Progress in theory is revealing more detailabout the characteristics of K and its determining factors. Forexample, we have already identified dispersion properties thatcan be exploited by tuning the throughput light in pre-resonanceregions. There are potentially associated spectroscopic applica-tions, because the influence of a tunable probe beam offers thescope to secure 2D spectra from complex materials, where thefluorescence is monitored as a function of not only the pumpbut also the probe wavelength. With pulsed sources, new tiers ofkinetic information should also become available. As always, thepath from new discoveries in optics to applications seems likelyto prove short and well-trodden.Research on energy transfer and fluorescence in the quantum-electrodynamics group at the University of East Anglia is supportedby the UK’s Leverhulme Trust.Author InformationDavid Andrews and David S. BradshawSchool of ChemistryUniversity of East AngliaNorwich, UKDavid Andrews leads research on fundamental molecularphotonics and energy transport, optomechanical forces, andnonlinear optics. He has published over 250 research papers and10 books. His current research focuses on optical nanomanipu-lation and switching, and light harvesting in nanostructures.David Bradshaw is a postdoctoral research associate whosecurrent interests include resonance-energy transfer, all-opticalswitching, and optical binding. He has co-authored over 35peer-reviewed papers, including an encyclopedia article onapplied spectroscopy.References1. D. S. Bradshaw and D. L. Andrews, Mechanism for optical enhancement andsuppression of fluorescence, J. Phys. Chem. A 113 (24), pp. 6537–6539, 2009.doi:10.1021/jp901830w2. D. S. Bradshaw and D. L. Andrews, All-optical control of molecular fluorescence,Phys. Rev. A 81 (1), p. 013424, 2010. doi:10.1103/PhysRevA.81.0134243. H. Rigneault, J. Capoulade, J. Dintinger, J. Wenger, N. Bonod, E. Popov,T. W. Ebbesen, and P.-F. Lenne, Enhancement of single-molecule fluorescence de-tection in subwavelength apertures, Phys. Rev. Lett. 95 (11), p. 117401, 2005.doi:10.1103/PhysRevLett.95.1174014. P. Anger, P. Bharadwaj, and L. Novotny, Enhancement and quenchingof single-molecule fluorescence, Phys. Rev. Lett. 96 (11), p. 113002, 2006.doi:10.1103/PhysRevLett.96.1130025. D. L. Andrews and D. S. Bradshaw, in D. L. Andrews ed., Resonance energytransfer, Encyclopedia of Applied Spectroscopy, pp. 533–554, Wiley-VCH, Wein-heim, 2009. ISBN: 978-3-527-40773-6.6. P. Allcock, R. D. Jenkins, and D. L. Andrews, Laser-assisted resonance-energy trans-fer, Phys. Rev. A 61 (2), p. 023812, 2000. doi:10.1103/PhysRevA.61.0238127. D. S. Bradshaw and D. L. Andrews, Optically controlled resonance energy trans-fer: mechanism and configuration for all-optical switching, J. Chem. Phys. 128 (14),p. 144506, 2008. doi:10.1063/1.28943198. D. S. Bradshaw and D. L. Andrews, All-optical switching between quan-tum dot nanoarrays, Superlatt. Microstruct. 47 (2), pp. 308–313, 2010.doi:10.1016/j.spmi.2009.10.017c 2010 SPIE

Optical control of excited-state lifetimes

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