Achieving control of light-material interactions for photonic device applications at nanoscale dimensions will require structures that guide electromagnetic energy with a lateral mode confinement below the diffraction limit of light. This cannot be achieved by using conventional waveguides or photonic crystals. It has been suggested that electromagnetic energy can be guided below the diffraction limit along chains of closely spaced metal nanoparticles that convert the optical mode into non-radiating surface plasmons. A variety of methods such as electron beam lithography and self-assembly have been used to construct metal nanoparticle plasmon waveguides. However, all investigations of the optical properties of these waveguides have so far been confined to collective excitations, and direct experimental evidence for energy transport along plasmon waveguides has proved elusive. Here we present observations of electromagnetic energy transport from a localized subwavelength source to a localized detector over distances of about 0.5 microm in plasmon waveguides consisting of closely spaced silver rods. The waveguides are excited by the tip of a near-field scanning optical microscope, and energy transport is probed by using fluorescent nanospheres.

Photosynthetic antenna complexes capture and concentrate solar radiation by transferring the excitation to the reaction center that stores energy from the photon in chemical bonds. This process occurs with near-perfect quantum efficiency. Recent experiments at cryogenic temperatures have revealed that coherent energy transfer-a wave-like transfer mechanism-occurs in many photosynthetic pigment-protein complexes. Using the Fenna-MatthewsOlson antenna complex (FMO) as a model system, theoretical studies incorporating both incoherent and coherent transfer as well as thermal dephasing predict that environmentally assisted quantum transfer efficiency peaks near physiological temperature; these studies also show that this mechanism simultaneously improves the robustness of the energy transfer process. This theory requires long-lived quantum coherence at room temperature, which never has been observed in FMO. Here we present evidence that quantum coherence survives in FMO at physiological temperature for at least 300 fs, long enough to impact biological energy transport. These data prove that the wave-like energy transfer process discovered at 77 K is directly relevant to biological function. Microscopically, we attribute this long coherence lifetime to correlated motions within the protein matrix encapsulating the chromophores, and we find that the degree of protection afforded by the protein appears constant between 77 K and 277 K. The protein shapes the energy landscape and mediates an efficient energy transfer despite thermal fluctuations.biophysics | photosynthesis | quantum beating | ultrafast spectroscopy | quantum biology E nergy transfer through photosynthetic pigment-protein complexes operates with exceptionally high quantum efficiency (1). Recent studies have demonstrated that energy moves through antennae using not only a classical hopping mechanism but also a manifestly quantum mechanical wave-like mechanism at cryogenic temperatures (2-5). Theoretical studies of this process within the Fenna-Matthews-Olson antenna complex (FMO) show that this quantum transport mechanism requires a balance between unitary (oscillatory) and dissipative (dephasing) dynamics; further, this balance appears to be optimized near room temperature and contributes to the robustness of the process (6-9). This theory demands that quantum coherence persist long enough to affect transport, but quantum beating has never been observed in FMO at physiological temperature.The FMO pigment-protein complex from Chlorobium tepidum serves as a model system for photosynthetic energy transfer processes (2, 10-13). This complex conducts energy from the larger light-harvesting chlorosome to the reaction center in green sulfur bacteria (14, 15). Each noninteracting FMO monomer contains seven coupled bacteriochlorophyll-a chromophores arranged asymmetrically, yielding seven nondegenerate, delocalized molecular excited states called excitons (11,16). The small number of distinct states makes this particular complex attractive for theoretical studies o...

Light-harvesting antenna complexes transfer energy from sunlight to photosynthetic reaction centers where charge separation drives cellular metabolism. The process through which pigments transfer excitation energy involves a complex choreography of coherent and incoherent processes mediated by the surrounding protein and solvent environment. The recent discovery of coherent dynamics in photosynthetic light-harvesting antennae has motivated many theoretical models exploring effects of interference in energy transfer phenomena. In this work, we provide experimental evidence of long-lived quantum coherence between the spectrally separated B800 and B850 rings of the light-harvesting complex 2 (LH2) of purple bacteria. Spectrally resolved maps of the detuning, dephasing, and the amplitude of electronic coupling between excitons reveal that different relaxation pathways act in concert for optimal transfer efficiency. Furthermore, maps of the phase of the signal suggest that quantum mechanical interference between different energy transfer pathways may be important even at ambient temperature. Such interference at a product state has already been shown to enhance the quantum efficiency of transfer in theoretical models of closed loop systems such as LH2.quantum biology | photosynthesis | ultrafast spectroscopy | biophysics | excitonic dynamics L ight-harvesting complex 2 (LH2) is the peripheral antenna pigment-protein complex of purple non-sulfur bacteria. LH2 contains two rings of BChl a pigments known as the B800 and B850 rings according to their respective room-temperature absorption bands in the infrared region of the spectrum (Fig. 1). These pigments are held in place by noncovalent interactions with pairs of low-molecular weight apoproteins. In most bacterial species, the LH2 complex consists of eight or nine of these protein heterodimers (αβ) organized in a highly symmetric ring (1). LH2 increases the effective cross-section for photon absorption from the solar spectrum in the membrane of purple bacteria. The energy absorbed by LH2 passes to another light-harvesting complex (LH1) tightly associated with the photosynthetic reaction center (2), wherein a stable charge separated state forms that ultimately drives the production of ATP.The energy transfer dynamics in LH2 has been studied for many years. Numerous time-resolved experiments have measured energy transfer from the B800 ring to the B850 ring in under a picosecond at room temperature (3-7). Förster resonance energy transfer (FRET) theory (8) estimates a slower transfer time by approximately a factor of five (9-13). Close examination of electronic coupling between pigments within each ring reveals, in part, the origin of this discrepancy. Studies on the excitation of the B850 ring (14-17) indicate the existence of Frenkel excitons (18,19), delocalized excitations that persist across several pigment molecules depending on the degree of structural symmetry present. In one limiting case, excitation is delocalized across the entire ring, invalidating a fundamen...

The Goldschmidt tolerance factor in halide perovskites limits the number of cations that can enter their cages without destabilizing their overall structure. Here we have explored the limits of this geometric factor and found that the ethylammonium (EA) cations which lie outside the tolerance factor range can still enter the cages of the 2D halide perovskites by stretching them. The new perovskites allow us to study how these large cations occupying the perovskite cages affect

Organic-inorganic hybrid halide perovskites are promising semiconductors with tailorable optical and electronic properties. The choice of A-site cation to support a three-dimensional (3D) perovskite structure AMX 3 (where M is a metal, and X is a halide) is limited by the geometric Goldschmidt tolerance factor. However, this geometric constraint can be relaxed in twodimensional (2D) perovskites, providing us an opportunity to understand how various the A-site cations modulate the structural properties and thereby the optoelectronic properties. Here, we report the synthesis and structures of single-crystals (BA) 2 (A)Pb 2 I 7 where BA = butylammonium, and A = methylammonium (MA), formamidinium (FA), dimethylammonium (DMA) or guanidinium (GA), a series of A-site cation varied in size. Single-crystal X-ray diffraction reveals that the MA, FA, and GA structures crystallize in the same Cmcm space group, while the DMA imposes the Ccmb space group. We observe that as the A-site cation becomes larger, the Pb−I bond continuously elongates, expanding the volume of the perovskite cage, equivalent to exerting "negative pressure" on the perovskite structures. Optical studies and DFT calculations show the Pb−I bond length elongation reduces overlap of the Pb s-and I p-orbitals and increases the optical bandgap, while Pb−I−Pb angles play a secondary role. Raman spectra show lattice softening with

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A time-of-flight imaging technique is introduced to visualize fluid flow and dispersion through porous media using NMR. As the fluid flows through a sample, the nuclear spin magnetization is modulated by RF pulses and magnetic field gradients to encode the spatial coordinates of the fluid. When the fluid leaves the sample, its magnetization is recorded by a second RF coil. This scheme not only facilitates a time-dependent imaging of fluid flow, it also allows a separate optimization of encoding and detection subsystems to enhance overall sensitivity. The technique is demonstrated by imaging gas flow through a porous rock.

Electronic structure and dynamics determine material properties and behavior. Important time scales for electronic dynamics range from attoseconds to milliseconds. Two-dimensional optical spectroscopy has proven an incisive tool to probe fast spatiotemporal electronic dynamics in complex multichromophoric systems. However, acquiring these spectra requires long point-by-point acquisitions that preclude observations on the millisecond and microsecond time scales. Here we demonstrate that imaging temporally encoded information within a homogeneous sample allows mapping of the evolution of the electronic Hamiltonian with femtosecond temporal resolution in a single-laser-shot, providing real-time maps of electronic coupling. This method, which we call GRadient-Assisted Photon Echo spectroscopy (GRAPE), eliminates phase errors deleterious to Fourier spectroscopies while reducing the acquisition time by orders of magnitude using only conventional optical components. In analogy to MRI in which magnetic field gradients are used to create spatial correlation maps, GRAPE spectroscopy takes advantage of a similar type of spatial encoding to construct electronic correlation maps. Unlike magnetic resonance, however, this spatial encoding of the nonlinear polarization along the excitation frequency axis of the two-dimensional spectrum results in no loss in signal while simultaneously reducing overall noise. Correlating the energy transfer events and electronic coupling occurring in tens of femtoseconds with slow dynamics on the subsecond time scale is fundamentally important in photobiology, solar energy research, nonlinear spectroscopy, and optoelectronic device characterization.MRI | nonlinear response | photon echo | ultrafast phenomena | spatial encoding U ltrafast optical spectroscopy can elucidate subpicosecond molecular dynamics, providing insight into vibrational and electronic structure and solute-solvent interactions in the IR and visible regions of the spectrum (1-6). Two-dimensional electronic spectroscopy directly probes correlations between electronic states providing detailed maps of energy transfer and coupling (7-10). However, as currently implemented two-dimensional spectroscopy provides no means by which to connect the initial dynamics in the first few hundred femtoseconds of electronic motion with slower dynamics associated with large structural changes. In a departure from the existing paradigm of pointby-point Fourier sampling, we map the multidimensional spectroscopic problem onto an imaging one by trading a temporal scan for a spatial dimension. This mapping permits the acquisition of the entire two-dimensional optical spectrum in a single-laser shot. Unlike other single-shot two-dimensional methods (11), which spatially encode the optical frequencies of the pulse and hence only provide static spectra, GRadient-Assisted Photon Echo (GRAPE) spectroscopy capitalizes on a temporal gradient imposed purely geometrically to encode time delays onto a spatial axis within the sample. The resultant spectrum provi...