Conformation of self-assembled porphyrin dimers in liposome vesicles by phase-modulation 2D fluorescence spectroscopy

By applying a phase-modulation fluorescence approach to 2D electronic spectroscopy, we studied the conformation-dependent exciton coupling of a porphyrin dimer embedded in a phospholipid bilayer membrane. Our measurements specify the relative angle and separation between interacting electronic transition dipole moments and thus provide a detailed characterization of dimer conformation. Phase-modulation 2D fluorescence spectroscopy (PM-2D FS) produces 2D spectra with distinct optical features, similar to those obtained using 2D photon-echo spectroscopy. Specifically, we studied magnesium meso tetraphenylporphyrin dimers, which form in the amphiphilic regions of 1,2-distearoyl-sn-glycero-3-phosphocholine liposomes. Comparison between experimental and simulated spectra show that although a wide range of dimer conformations can be inferred by either the linear absorption spectrum or the 2D spectrum alone, consideration of both types of spectra constrain the possible structures to a "T-shaped" geometry. These experiments establish the PM-2D FS method as an effective approach to elucidate chromophore dimer conformation.     

Nuclear Magnetic Resonance Studies of Lysine-Vasopressin: Structural Constraints

The 220-MHz proton NMR spectra of lysine-vasopressin and some related compounds are examined in deuterated dimethyl sulfoxide to obtain structural information that must be satisfied by any proposed conformation of the molecule. This structural information is in the form of dihedral angles (for rotation about the NH-C^αH bonds) from coupling constants, possible hydrogen bonding of the CONH₂ and backbone amide groups from the temperature-dependence of the chemical shift, and aromatic ring-aromatic ring interaction from the effect of the magnetically anisotropic groups on the chemical shift.     

Anomalously robust valley polarization and valley coherence in bilayer WS2

We report the observation of anomalously robust valley polarization and valley coherence in bilayer WS₂. The polarization of the photoluminescence from bilayer WS₂ follows that of the excitation source with both circular and linear polarization, and remains even at room temperature. The near-unity circular polarization of the luminescence reveals the coupling of spin, layer, and valley degree of freedom in bilayer system, and the linearly polarized photoluminescence manifests quantum coherence between the two inequivalent band extrema in momentum space, namely, the valley quantum coherence in atomically thin bilayer WS₂. This observation provides insight into quantum manipulation in atomically thin semiconductors.Tungsten sulfide WS₂, part of the family of group VI transition metal dichalcogenides (TMDCs), is a layered compound with buckled hexagonal lattice. As WS₂ thins to atomically thin layers, WS₂ films undergo a transition from indirect gap in bulk form to direct gap at monolayer level with the band edge located at energy-degenerate valleys (K, K′) at the corners of the Brillouin zone (1–3). Like the case of its sister compound, monolayer MoS₂, the valley degree of freedom of monolayer WS₂ could be presumably addressed through nonzero but contrasting Berry curvatures and orbital magnetic moments that arise from the lack of spatial inversion symmetry at monolayers (3, 4). The valley polarization could be realized by the control of the polarization of optical field through valley-selective interband optical selection rules at K and K′ valleys as illustrated in Fig. 1A (4–6). In monolayer WS₂, both the top of the valence bands and the bottom of the conduction bands are constructed primarily by the d orbits of tungsten atoms, which are remarkably shaped by spin–orbit coupling (SOC). The giant spin–orbit coupling splits the valence bands around the K (K′) valley by 0.4 eV, and the conduction band is nearly spin degenerated (7). As a result of time-reversal symmetry, the spin splitting has opposite signs at the K and K′ valleys. Namely, the Kramer’s doublet |K ↑ ? and |K′ ↓ ? is separated from the other doublet |K′ ↑ ? and |K ↓ ? by the SOC splitting of 0.4 eV. The spin and valley are strongly coupled at K (K′) valleys, and this coupling significantly suppresses spin and valley relaxations as both spin and valley indices have to be changed simultaneously.Open in a separate windowFig. 1.(A) Schematic of valley-dependent optical selection rules and the Zeeman-like spin splitting in the valence bands of monolayer WS₂. (B) Diagram of spin–layer–valley coupling in 2H stacked bilayer WS₂. Interlayer hopping is suppressed in bilayer WS₂ owing to the coupling of spin, valley, and layer degrees of freedom.In addition to the spin and valley degrees of freedom, in bilayer WS₂ there exists an extra index: layer polarization that indicates the carriers’ location, either up-layer or down-layer. Bilayer WS₂ follows the Bernal packing order and the spatial inversion symmetry is recovered: each layer is 180° in plane rotation of the other with the tungsten atoms of a given layer sitting exactly on top of the S atoms of the other layer. The layer rotation symmetry switches K and K′ valleys, but leaves the spin unchanged, which results in a sign change for the spin–valley coupling from layer to layer (Fig. 1B). From the simple spatial symmetry point of view, one might expect that the valley-dependent physics fades at bilayers owing to inversion symmetry, as the precedent of bilayer MoS₂ (8). Nevertheless, the inversion symmetry becomes subtle if the coupling of spin, valley, and layer indices is taken into account. Note that the spin–valley coupling strength in WS₂ is around 0.4 eV (the counterpart in MoS₂ ∼ 0.16 eV), which is significantly higher than the interlayer hopping energy (∼0.1 eV); the interlayer coupling at K and K′ valleys in WS₂ is greatly suppressed as indicated in Fig. 1B (7, 9). Consequently, bilayer WS₂ can be regarded as decoupled layers and it may inherit the valley physics demonstrated in monolayer TMDCs. In addition, the interplay of spin, valley, and layer degrees of freedom opens an unprecedented channel toward manipulations of quantum states.Here we report a systemic study of the polarization-resolved photoluminescence (PL) experiments on bilayer WS₂. The polarization of PL inherits that of excitations up to room temperature, no matter whether it is circularly or linearly polarized. The experiments demonstrate the valley polarization and valley coherence in bilayer WS₂ as a result of the coupling of spin, valley, and layer degrees of freedom. Surprisingly, the valley polarization and valley coherence in bilayer WS₂ are anomalously robust compared with monolayer WS₂.For comparison, we first perform polarization-resolved photoluminescence measurements on monolayer WS₂. Fig. 2A shows the photoluminescence spectrum from monolayer WS₂ at 10 K. The PL is dominated by the emission from band-edge excitons, so-called “A” exciton at K and K′ valleys. The excitons carry a clear circular dichroism under near-resonant excitation (2.088 eV) with circular polarization as a result of valley-selective optical selection rules, where the left-handed (right handed) polarization corresponds to the interband optical transition at K (K′) valley. The PL follows the helicity of the circularly polarized excitation optical field. To characterize the polarization of the luminescence spectra, we define a degree of circular polarization as P=I(σ+)−I(σ−)I(σ+)+I(σ−), where I(σ±) is the intensity of the right- (left-) handed circular-polarization component. The luminescence spectra display a contrasting polarization for excitation with opposite helicities: P = 0.4 under σ+ excitation and P = −0.4 under σ− excitation on the most representative monolayer. For simplicity, only the PL under σ+ excitation is shown. The degree of circular polarization P is insensitive to PL energy throughout the whole luminescence as shown in Fig. 2A, Inset. These behaviors are fully expected in the mechanism of valley-selective optical selection rules (3, 4). The degree of circular polarization decays with increasing temperature and drops to 10% at room temperature (Fig. 2B). It decreases as the excitation energy shifts from the near-resonance energy of 2.088 to 2.331 eV as illustrated in Fig. 2C. The peak position of A exciton emission at band edges shifts from 2.04 eV at 10 K to 1.98 eV at room temperature. The energy difference between the PL peak and the near-resonance excitation (2.088 eV) is around 100 meV at room temperature, which is much smaller than the value 290 meV for the low temperature off-resonance excitation at 2.331 eV. However, the observed polarization for off-resonance excitation at 10 K (P = 16%) is much higher than the near-resonance condition at room temperature (P = 10%). It clearly shows that the depolarization cannot be attributed to single process, namely the off-resonance excitation or band-edge phonon scattering only (10).Open in a separate windowFig. 2.Photoluminescence of monolayer WS₂ under circularly polarized excitation. (A) Polarization resolved luminescence spectra with σ+ detection (red) and σ− detection (black) under near-resonant σ+ excitation (2.088 eV) at 10 K. Peak A is the excitonic transition at band edges of K (K′) valleys. Opposite helicity of PL is observed under σ− excitation. Inset presents the degree of the circular polarization at the prominent PL peak. (B) The degree of the circular polarization as a function of temperature. The curve (red) is a fit following a Boltzmann distribution where the intervalley scattering by phonons is assumed. (C) Photoluminescence spectrum under off-resonant σ+ excitation (2.33 eV) at 10 K. The red (black) curve denotes the PL circular components of σ+ (σ−).Next we study the PL from bilayer WS₂. Fig. 3 shows the PL spectrum from bilayer WS₂. The peak labeled as “I” denotes the interband optical transition from the indirect band gap, and the peak A corresponds to the exciton emission from direct band transition at K and K′ valleys. Although bilayer WS₂ has an indirect gap, the direct interband optical transition at K and K′ valleys dominates the integrated PL intensity as the prerequisite of phonon/defect scattering is waived for direct band emission and the direct gap is just slightly larger than the indirect band gap in bilayers. Fig. 3A displays surprisingly robust PL circular dichroism of A exciton emission under circularly polarized excitations of 2.088 eV (resonance) and 2.331 eV (off resonance). The degree of circular polarization of A exciton emission under near-resonant σ± excitation is near unity (around 95%) at 10 K and preserves around 60% at room temperature. In contrast, the emission originating from indirect band gap is unpolarized in all experimental conditions.Open in a separate windowFig. 3.Photoluminescence of bilayer WS₂ under circularly polarized excitations. (A) Polarization-resolved luminescence spectra with components of σ+ (red) and σ− (black) under near-resonant σ+ excitation (2.088 eV) at 10 K. Peak A is recognized as the excitonic transition at band edge of direct gap. Peak I originates from the indirect band-gap emission, showing no polarization. Inset presents the circular polarization of the A excitonic transition around the PL peak. Opposite helicity of PL is observed under σ− excitation. (B) The degree of circular polarization as a function of temperature (black). The curve (red) is a fit following a Boltzman distribution where the intervalley scattering by phonons is assumed. (C) Photoluminescence spectrum of components of σ+ (red) and σ− (black) under off-resonance σ+ excitation (2.33 eV) at 10 K. A nonzero circular polarization P is only observed at emissions from A excitons.To exclude the potential cause of charge trapping or substrate charging effect, we study the polarization-resolved PL of bilayer WS₂ with an out-plane electric field. Fig. 4A shows the evolution of PL spectra in a field-effect-transistor-like device under circularly polarized excitations of 2.088 eV and an electric gate at 10 K. The PL spectra dominated by A exciton show negligible change under the gate bias in the range of −40 to 20 V. The electric-conductance measurements show that the bilayer WS₂ stays at the electrically intrinsic state under the above bias range. The PL spectra can be safely recognized as emissions from free excitons. As the gate bias switches further to the positive side (>20 V), the PL intensity decreases, and the emission from electron-bounded exciton “X⁻,” the so-called trion emerges and gradually raises its weight in the PL spectrum (11, 12). The electron–exciton binding energy is found to be 45 meV. Given only one trion peak in PL spectra, the interlayer trion (formed by exciton and electron/hole in different layers) and intralayer trion (exciton and electron/hole in the same layer) could not be distinguished due to the broad spectral width (13). Both the free exciton and trion show slight red shifts with negative bias, presumably as a result of quantum-confined stark effect (14). At all of the bias conditions, the degree of circular polarization of the free exciton and trion stays unchanged within the experiment sensitivity as shown in Fig. 4C.Open in a separate windowFig. 4.Electric-doping-dependent photoluminescence spectrum of bilayer WS₂ field-effect transistor. (A) Luminescence spectra of bilayer WS₂ at different gate voltage under near-resonant σ+ excitation (2.088 eV) at 10 K. X and X⁻ denote neutral exciton and trion, respectively. Green curve is a fitting consisting of two Lorentzian peak fits (peak I and X⁻) and one Gaussian peak fit (peak X). (B) Intensity of exciton and trion emissions versus gate. (Upper) The gate-dependent integral PL intensity consisting of exciton (X) and trion (X⁻). (Lower) The ratio of the integral PL intensity of exciton versus that of trion, as a function of the gate voltage. (C) Degree of circular polarization of exciton (X, red) and trion (X⁻, blue) versus the gate.It is also unlikely that the high polarization in bilayers results from the isolation of the top layer from the environments, as similar behaviors are observed in monolayer and bilayer WS₂ embedded in polymethyl methaccrylate (PMMA) matrix or capped with a 20-nm-thick SiO₂ deposition. The insensitivity of the circular-polarization degree on bias and environments rules out the possibility that the effects of Coulomb screening, charge traps, or charge transfers with substrates are the major causes for the robust circular dichroism in bilayers against monolayers.One potential cause may result from the shorter lifetime of excitons at K (K′) valley for bilayer system. The band gap shifts from K and K′ points of the Brillouin zone in monolayers to the indirect gap between the top of the valence band at Γ points and the bottom of the conduction band in the middle of K and Γ points in bilayers. Combining our time-resolved pump-probe reflectance experiments (Supporting Information) and the observed relative PL strength between monolayer and bilayer (10:1), we infer the exciton lifetime at K (K′) valleys around 10 ps, a fraction of that at monolayers. If we assume (i) the PL circular polarization P=P01+2ττk, where P₀ is the theoretical limit of PL polarization, and τ_k and τ denote the valley lifetime and exciton lifetime respectively; and (ii) the valley lifetime is the same for both monolayers and bilayers, the shorter exciton lifetime will lead to significantly higher PL polarization. However, the difference in exciton lifetime between bilayers and monolayers is not overwhelming enough to be the major cause of robust polarization observed in the time-integrated PL in bilayers.In monolayer WS₂ under circularly polarized resonant excitations, the depolarization mainly comes from the K ? K′ intervalley scattering. In bilayers, the depolarization could be either via K ? K′ intervalley scattering within the layer in a similar way as in monolayers, or via interlayer hopping, which also requires spin flip. As we discussed above, the interlayer hopping at K valley is suppressed in WS₂ as a result of strong SOC in WS₂ and spin–layer–valley coupling, which were experimentally proved by the circular dichroism in PL from bilayers. The robust polarization in bilayers implies that the intervalley scattering within a layer is diminished compared with that in monolayers. There are two prerequisites for intervalley scattering within layers: conservation of crystal momentum and spin flip of holes. The crystal momentum conservation could be satisfied with the involvement of phonons at K points in the Brillouin zone or atomic size defects, presumably sharing the similar strength in monolayers and bilayers. Spin-flip process could be realized by three different spin scattering mechanisms, namely D’yakonov–Perel (DP) mechanism (15), Elliot–Yaffet (EY) mechanism (16), and Bir–Aronov–Pikus (BAP) mechanism (17, 18). The DP mechanism acts through a Lamor precession driven by electron wavevector k dependent spin–orbit coupling. It is thought to be negligible for spin flip along out-plane direction as the mirror symmetry with respect to the plane of W atoms secures a zero out-plane crystal electric field. Another possible driving force behind the DP mechanism could be the asymmetry owing to the interface with the substrate. This can be excluded by the similar behaviors, where the monolayers and bilayers WS₂ are embedded in PMMA matrix or capped with a thin layer of SiO₂. The negligible effect of electric gating on polarization also implies that the DP mechanism is weak in monolayer and bilayer WS₂; the EY mechanism originates from scattering with phonons and defects. Its strength in bilayers and monolayers is likely to be at similar scale, and bilayers even have more low-frequency collective vibrational modes (19). Therefore, EY mechanism is unlikely to be the cause here; the BAP mechanism originates from the electron–hole exchange interaction. In monolayer and bilayer TMDCs, the optical features are dominated by the Wannier type, yet giant excitonic effect, and the exciton-binding energy in such intrinsic 2D semiconductors is estimated to be 0.6 ∼ 1 eV (20, 21). This giant exciton-binding energy indicates a mixture of electron and hole wavefunctions and, consequently, strong exchange interaction, which may contribute to the spin flip and intervalley scattering (5, 22). As the conduction band has a band mixing at K points, the spin flip of the electron would be a quick process. An analogous scenario is that the spin of holes relaxes in hundreds of femtoseconds or fewer in GaAs as a result of band mixing and spin–orbit coupling. The electron spin flip could lead to hole spin flip via strong exchange interaction accompanying intervalley scattering, which is realized by the virtual annihilation of a bright exciton in the K valley and then generation in the K′ valley or vice versa (22). This non-single-particle spin relaxation leads to valley depolarization instead of the decrease of luminescence intensity that results from coupling with dark excitons. Generally, the exciton-binding energy decreases with the relaxation of spatial confinement. However, first principle calculation shows that monolayer and bilayer WS₂ share the similar band dispersion and effective masses around K valley in their Brillouin zone as a result of spin–valley coupling (7). It implies that the binding energy of excitons around K valley in bilayer WS₂ is similar to or slightly less than that in monolayer WS₂. As the exchange interaction is roughly proportional to the square of exciton binding energy, the spin-flip rate and consequently intervalley scattering via exciton exchange interactions is presumably comparable or smaller to some extent in bilayer WS₂ (Supporting Information). Nevertheless, this is unlikely the major cause of the anomalously robust valley polarization in bilayer WS₂.Another possibility includes extra spin-conserving channels via intermediate intervalley-interlayer scatterings in bilayer WS₂, which are absent in monolayers (23). The extra spin-conserving channel may compete with the spin-flip process and reduce the relative weight of spin-flip intervalley scattering to some extent. However, the mechanism and the strength are unclear so far. Overall, the robust circular polarization in bilayers likely results from combined effects of the shorter exciton lifetime, smaller exciton-binding energy, extra spin-conserving channels, and the coupling of spin, layer, and valley degrees of freedom, indicating the relatively weak intervalley scattering in bilayer system. Further quantitative study is necessary to elaborate the mechanism.We also investigated the PL from bilayer WS₂ under a linearly polarized excitation. A linearly polarized light could be treated as a coherent superposition of two opposite-helicity circularly polarized lights with a certain phase difference. The phase difference determines the polarization direction. In semiconductors, a photon excites an electron–hole pair with the transfer of energy, momentum, and phase information. The hot carriers energetically relax to the band edge in a quick process around 10⁻¹ ∼ 10¹ ps through runs of inelastic and elastic scatterings, e.g., by acoustic phonons. During the quick relaxation process, generally the phase information randomizes and herein coherence fades. In monolayer TMDCs, the main channel for carrier relaxation is through intravalley scatterings including Coulomb interactions with electron (hole) and inelastic interactions with phonons, which are valley independent and preserve the relative phase between K and K′ valleys (24). In bilayer WS₂, the suppression of intervalley scattering consequently leads to the suppression of inhomogeneous broadening in carrier’s phase term. Subsequently, the valley coherence demonstrated in monolayer WSe₂ (24) is expected to be enhanced in bilayers (13). The valley coherence in monolayer and bilayer WS₂ could be monitored by the polarization of PL under linearly polarized excitations.Fig. 5A shows the linearly polarized components of PL under a linearly polarized excitation of 2.088 eV at 10 K. The emission from indirect band gap is unpolarized and A exciton displays a pronounced linear polarization following the excitation. The degree of linear polarization P=I(‖)−I(⊥)I(‖)+I(⊥) is around 80%, where I(∥)(I(⊥)) is the intensity of PL with parallel (perpendicular) polarization with respect to the excitation polarization. In contrast, the linear polarization is much weaker in monolayer samples (4% under the same experimental conditions, as shown in Fig. 5B). As presented in Fig. 5C, the polarization of A exciton is independent of crystal orientation and exactly follows the polarization of excitations. The degree of the linear polarization in bilayer WS₂ slightly decreases with the increased temperature and drops from 80% at 10 K to 50% at room temperature (Fig. 5D). This is the paradigm of the robust valley coherency in bilayer WS₂.Open in a separate windowFig. 5.Linearly polarized excitations on monolayer and bilayer WS₂. (A) Linear-polarization-resolved luminescence spectra of bilayer WS₂ under near-resonant linearly polarized excitation (2.088 eV) at 10 K. Red (black) presents the spectrum with parallel (cross) polarization with respect to the linear polarization of excitation source. A linear polarization of 80% is observed for exciton A, and the indirect gap transition (I) is unpolarized. (B) Linear-polarization-resolved luminescence spectra of monolayer WS₂ under near-resonant linearly polarized excitation (2.088 eV) at 10 K. Red (black) denotes the spectrum with the parallel (cross) polarization with respect to the linear polarization of excitation source. The linear polarization for exciton A in monolayer WS₂ is much weaker, with a maximum value of 4%. (C) Polar plot for intensity of the exciton A in bilayer WS₂ (black) as a function of the detection angle at 10 K. Red curve is a fit-following cos²(θ). (D) The degree of linear polarization of exciton A in bilayer WS₂ (black) as a function of temperature. The curve (red) is a fit following a Boltzmann distribution where the intervalley scattering by phonons is assumed. (E) Electric doping dependence of the linear polarization of exciton A in bilayer WS₂ at 10 K.The linear polarization of both exciton and trion in bilayer, contrasting to the circular polarization, which is insensitive to the electric field in the range, shows a weak electric gating dependence as shown in Fig. 5E. The PL linear polarization, presenting valley coherence, decreases as the Fermi level shifts to the conduction band. It does not directly affect intervalley scattering within individual layers and makes negligible change in circular dichroism. Nevertheless, the electric field between the layers induces a layer polarization and slightly shifts the band alignments between the layers by different amounts in conduction and valence bands (13, 25), although the shift is indistinguishable in the present PL spectra due to the broad spectral width. The layer polarization and the shift of band alignments may induce a relative phase difference between two layers and therefore affect the PL linear polarization via interference. Further study is needed to fully understand the mechanism.In summary, we demonstrated anomalously robust valley polarization and valley polarization coherence in bilayer WS₂. The valley polarization and valley coherence in bilayer WS₂ are the direct consequences of giant spin–orbit coupling and spin valley coupling in WS₂. The depolarization and decoherence processes are greatly suppressed in bilayer, although the mechanism is ambiguous. The robust valley polarization and valley coherence make bilayer WS₂ an intriguing platform for spin and valley physics.     

Mixed Conformations of Deoxyribonucleic Acid in Chromatin: A Preliminary Report

We have established empirical limits for the circular dichroism spectra appropriate for aqueous solutions of the B, C, and A forms of calf-thymus DNA and have analyzed DNA conformations in biological structures. The circular dichroism spectrum above 250 nm of purified calf-thymus chromatin can be satisfactorily accounted for as a linear combination of contributions of the B and C reference spectra without invoking higher-order structures such as supercoils. The amount of A contribution, if any, is below the limit of detection (相似文献   

Effect of Glass Composition on Luminescence and Structure of CsPbBr3 Quantum Dots in an Amorphous Matrix

Glass matrix embedding is an efficient way to improve the chemical and thermal stability of the halide perovskite QDs. However, CsPbX₃ QDs exhibit distinct optical properties in different glass matrixes, including photoluminescence (PL) peak position, PL peak width, and optical band gap. In this work, the temperature-dependent PL spectra, absorption spectra, high-energy X-ray structure factor S(Q), and pair distribution function (PDF) were integrated to analyze the structural evolution of CsPbBr₃ QDs in different glass matrixes. The results show that the lattice parameters and atomic spacing of CsPbBr₃ QDs are affected by the glass composition in which they are embedded. The most possibility can be attributed to the thermal expansion mismatch between CsPbBr₃ QDs and the glass matrix. The results may provide a new way to understand the effect of the glass composition on the optical properties of CsPbBr₃ QDs in a glass matrix.     

Comparative study of optical absorption and circular dichroism of bacteriochlorophyll oligomers in Triton X-100, the antenna pigment B850, and the primary donor P-860 of photosynthetic bacteria indicates that all are similar dimers of bacteriochlorophyll a

Dimers of bacteriochlorophyll a (Bchla) with optical absorption maximum at 853 nm and a nonconservative circular dichroism spectrum are formed in a solution of formamide/water that contains micelles of Triton X-100. The apparent equilibrium constant and the corresponding Gibbs energy change for the Bchl self-organization are 4.9 × 10⁶ M^-1 and -9.2 kcal/mol, respectively. The experimental absorption and circular dichroism spectra of the in vitro Bchl dimer (termed Bchl-853) are similar to the spectra of the bacterial light-harvesting complex B850 and the primary electron donor P-860 and probably point to a common structural motif. Indeed, simulation of the dimers' spectra (optical absorption and circular dichroism), achieved by using an extended version of the exciton theory, suggests the same geometry as recently elucidated for P-860 by x-ray diffraction crystallography. The proposed geometry is predicted to have the minimum energy in the gas phase. In conclusion, the spectral properties of the bathochromically shifted forms of Bchla are likely a result of strong dipolar interactions in self-organized structures of Bchls.     

Bacteriochlorophyll electronic transition moment directions in bacteriochlorophyll a-protein

The low-temperature 800-nm band absorption and circular dichroism spectra of the bacteriochlorophyll (Bchl) a-protein from Prosthecochloris aestuarii strain 2K are analyzed theoretically. These spectra show considerable structure that is attributed primarily to resonance (exciton) interactions among the lowest singlet transitions of the Bchl a molecules contained in each protein. We calculate these spectra from the known arrangement of the Bchl molecules in the protein. With the conventional assignment of the lowest singlet transition of Bchl a as Q_y (y-polarized), agreement of calculated spectra with experiment is poor. All of our attempts, based on this conventional assignment, to improve the theoretical fits to absorption and circular dichroism spectra simultaneously are unsuccessful. However, by making the simple but unconventional assumption that the lowest singlet transition in each of the Bchl a molecules in each protein is x-polarized rather than y-polarized, we find good agreement between calculated and observed spectra. If these results are not fortuitous, they indicate that there is a systematic error in the protein structural model, that the conventional assignments of Bchl a transitions are incorrect, or that the protein environment provides a sufficiently strong perturbation to rotate the lowest singlet transition moment direction by ~90°, presumably by changing the order of certain of the Bchl a orbitals.     

Lipid-protein interaction in the glycophorin-dipalmitoylphosphatidylcholine system: Raman spectroscopic investigation

Raman spectra have been recorded as a function of temperature for lipid-protein complexes of glycophorin isolated from erythrocyte membranes reconstituted with dipalmitoylphosphatidylcholine (DPPC) and its chain perdeuterated analogue ([²H₆₂]DPPC). The conformation of the phospholipid hydrocarbon chains in the vicinity of protein is drastically altered from that in pure lipid dispersions. Analysis of the chain C-²H stretching vibrations for complexes of [²H₆₂]-DPPC-glycophorin shows that at lipid:protein mole ratios of 125:1, a broad melting event occurs that is not observable by calorimetric techniques. The midpoint occurs at temperatures about 15°C below that of the gel/liquid crystal phase transition for [²H₆₂]DPPC in multilamellar dispersions. The same number of gauche rotamers form in the phospholipid hydrocarbon chains during the melting process as in the phase transition of the unperturbed molecule. Analysis of the C-H stretching region of the Raman spectrum in DPPC-glycophorin complexes indicates that lateral interactions between phospholipid chains in the complex are reduced so that interchain vibrational coupling is minimized. The observed differences between the Raman melting curves and the calorimetric endothermic transitions arise because different populations of phospholipid molecules are sampled in the two experiments. The advantages of Raman spectroscopy for the study of lipid-protein interaction are demonstrated in the current work. Implications for the structure of the lipid in the immediate vicinity of membrane protein are discussed.     

Oriented properties of the chlorophylls: Electronic absorption spectroscopy of orthorhombic pyrochlorophyllide a-apomyoglobin single crystals

The orientations of the transition dipole moments in chlorophyll (Chl) are among the most useful spectroscopic properties for determining macromolecular architecture in photosynthetic complexes; however, the relationships between these orientations and the Chl molecular geometry are unknown. In order to solve this problem, we have prepared single crystals of the synthetic 1:1 complex between pyrochlorophyllide a and apomyoglobin. The protein crystallizes readily in the orthorhombic (B) form, space group P2₁2₁2₁, and the unit cell dimensions are determined to be within 0.5% of those for native MetMb crystals of the same type. These green crystals are highly dichroic, and the strong absorption along the crystallographic a axis in the Q_y band is red-shifted by about 9 nm, relative to the corresponding feature in a solution of the protein. Although the crystal structure for native Mb in this space group has not been determined, the direction cosines of the heme normal relative to the crystal axes have been measured. By using these values, an appropriate trigonometric analysis, and the measured polarized single-crystal spectra, the orientation of the Chl transition dipole moment for the Q_y transition can be specified relative to the crystal axes. With the completion of the protein crystal structure, this result will lead directly to the orientations of the optical transition dipole moments relative to the molecular geometry. The effects of vibronic coupling and the protein environment on the absorption properties of Chl are discussed in detail.     

Exciton interactions in reaction centers of the photosynthetic bacterium Rhodopseudomonas viridis probed by optical triplet-minus-singlet polarization spectroscopy at 1.2 K monitored through absorbance-detected magnetic resonance

Linear dichroic triplet-minus-singlet [LD-(T - S)] spectra of isolated reaction centers of the photosynthetic bacterium Rhodopseudomonas viridis have been measured at 1.2 K with the linear dichroic absorbance-detected magnetic resonance (LD-ADMR) technique for two mutually perpendicular directions of the preferred axis. The LD-(T - S) spectra have been calibrated with respect to the corresponding (T - S) spectra as a function of applied microwave power and quantitatively interpreted using the formalism of photoselection. The transition moment of the optical transition at 1007 nm makes angles of 72° ± 5° and 15° ± 5° with the triplet x and y spin axes, respectively. The experimental spectra have been simulated employing exciton theory and using the atomic coordinates of the resolved crystal structure of the reaction center. The spectral interpretation yields the angles between the transition moments of the various absorption bands of the (T - S) spectra and the triplet axes, and between the moments themselves, with the triplet state of the primary donor ³P localized on the P-bacteriochlorophyll b in the “active” (L) chain.     

Crystal structure of Taq DNA polymerase in complex with an inhibitory Fab: The Fab is directed against an intermediate in the helix-coil dynamics of the enzyme

We report the crystal structure of Thermus aquaticus DNA polymerase I in complex with an inhibitory Fab, TP7, directed against the native enzyme. Some of the residues present in a helical conformation in the native enzyme have adopted a γ turn conformation in the complex. Taken together, structural information that describes alteration of helical structure and solution studies that demonstrate the ability of TP7 to inhibit 100% of the polymerase activity of the enzyme suggest that the change in conformation is probably caused by trapping of an intermediate in the helix-coil dynamics of this helix by the Fab. Antibodies directed against modified helices in proteins have long been anticipated. The present structure provides direct crystallographic evidence. The Fab binds within the DNA binding cleft of the polymerase domain, interacting with several residues that are used by the enzyme in binding the primer:template complex. This result unequivocally corroborates inferences drawn from binding experiments and modeling calculations that the inhibitory activity of this Fab is directly attributable to its interference with DNA binding by the polymerase domain of the enzyme. The combination of interactions made by the Fab residues in both the polymerase and the vestigial editing nuclease domain of the enzyme reveal the structural basis of its preference for binding to DNA polymerases of the Thermus species. The orientation of the structure-specific nuclease domain with respect to the polymerase domain is significantly different from that seen in other structures of this polymerase. This reorientation does not appear to be antibody-induced and implies remarkably high relative mobility between these two domains.     

Force-dependent polymorphism in type IV pili reveals hidden epitopes

Through evolution, nature has produced exquisite nanometric structures, with features unrealized in the most advanced man-made devices. Type IV pili (Tfp) represent such a structure: 6-nm-wide retractable filamentous appendages found in many bacteria, including human pathogens. Whereas the structure of Neisseria gonorrhoeae Tfp has been defined by conventional structural techniques, it remains difficult to explain the wide spectrum of functions associated with Tfp. Here we uncover a previously undescribed force-induced quaternary structure of the N. gonorrhoeae Tfp. By using a combination of optical and magnetic tweezers, atomic force microscopy, and molecular combing to apply forces on purified Tfp, we demonstrate that Tfp subjected to approximately 100 pN of force will transition into a new conformation. The new structure is roughly 3 times longer and 40% narrower than the original structure. Upon release of the force, the Tfp fiber regains its original form, indicating a reversible transition. Equally important, we show that the force-induced conformation exposes hidden epitopes previously buried in the Tfp fiber. We postulate that this transition provides a means for N. gonorrhoeae to maintain attachment to its host while withstanding intermittent forces encountered in the environment. Our findings demonstrate the need to reassess our understanding of Tfp dynamics and functions. They could also explain the structural diversity of other helical polymers while presenting a unique mechanism for polymer elongation and exemplifying the extreme structural plasticity of biological polymers.     

Gap-modulation infrared spectroscopy of high transition temperature superconductors

Conventional methods of determining the coupling factor α²(ω)F(ω) for the newly discovered high transition temperature (T_c) cuprate superconductors by using tunneling and infrared measurements have thus far failed to show the cause of the very high T_c of these compounds. This is due in part to difficulties in sample preparation for tunneling studies and to difficulties in obtaining good data at relatively high tunneling voltages. Also, in IR (infrared) measurements, small differences in absorptivity between the normal and superconducting state can be masked by changes in the phonon occupation at high and low temperatures. Here we propose a technique for determing the coupling constant, which should be less dependent on the surface quality of the sample than with tunneling and should allow measurements at higher energies with greater precision than do tunneling or simple IR observations. This should make possible a definitive determination of any possible exciton contribution to this coupling term, which would appear at energies well above the range where conventional IR or tunneling measurements are effective.     

Effects of Deposition and Annealing Temperature on the Structure and Optical Band Gap of MoS2 Films

In this study, molybdenum disulfide (MoS₂) film samples were prepared at different temperatures and annealed through magnetron sputtering technology. The surface morphology, crystal structure, bonding structure, and optical properties of the samples were characterized and analyzed. The surface of the MoS₂ films prepared by radio frequency magnetron sputtering is tightly coupled and well crystallized, the density of the films decreases, and their voids and grain size increase with the increase in deposition temperature. The higher the deposition temperature is, the more stable the MoS₂ films deposited will be, and the 200 °C deposition temperature is an inflection point of the film stability. Annealing temperature affects the structure of the films, which is mainly related to sulfur and the growth mechanism of the films. Further research shows that the optical band gaps of the films deposited at different temperatures range from 0.92 eV to 1.15 eV, showing semiconductor bandgap characteristics. The optical band gap of the films deposited at 200 °C is slightly reduced after annealing in the range of 0.71–0.91 eV. After annealing, the optical band gap of the films decreases because of the two exciton peaks generated by the K point in the Brillouin zone of MoS₂. The blue shift of the K point in the Brillouin zone causes a certain change in the optical band gap of the films.     

Investigations of Electron-Electron and Interlayer Electron-Phonon Coupling in van der Waals hBN/WSe2/hBN Heterostructures by Photoluminescence Excitation Experiments

Monolayers of transition metal dichalcogenides (TMDs) with their unique physical properties are very promising for future applications in novel electronic devices. In TMDs monolayers, strong and opposite spin splittings of the energy gaps at the K points allow for exciting carriers with various combinations of valley and spin indices using circularly polarized light, which can further be used in spintronics and valleytronics. The physical properties of van der Waals heterostructures composed of TMDs monolayers and hexagonal boron nitride (hBN) layers significantly depend on different kinds of interactions. Here, we report on observing both a strong increase in the emission intensity as well as a preservation of the helicity of the excitation light in the emission from hBN/WSe₂/hBN heterostructures related to interlayer electron-phonon coupling. In combined low-temperature (T = 7 K) reflectivity contrast and photoluminescence excitation experiments, we find that the increase in the emission intensity is attributed to a double resonance, where the laser excitation and the combined Raman mode A′₁ (WSe₂) + ZO (hBN) are in resonance with the excited (2s) and ground (1s) states of the A exciton in a WSe₂ monolayer. In reference to the 2s state, our interpretation is in contrast with previous reports, in which this state has been attributed to the hybrid exciton state existing only in the hBN-encapsulated WSe₂ monolayer. Moreover, we observe that the electron-phonon coupling also enhances the helicity preservation of the exciting light in the emission of all observed excitonic complexes. The highest helicity preservation of more than 60% is obtained in the emission of the neutral biexciton and negatively charged exciton (trion) in its triplet state. Additionally, to the best of our knowledge, the strongly intensified emission of the neutral biexciton XX⁰ at double resonance condition is observed for the first time.     

Structural and Optical Properties of La1?xSrxTiO3+δ

La_1−xSr_xTiO_3+δ has attracted much attention as an important perovskite oxide. However, there are rare reports on its optical properties, especially reflectivity. In this paper, its structural and optical properties were studied. The X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy and spectrophotometer were used to characterize the sample. The results show that with increasing Sr concentration, the number of TiO₆ octahedral layers in each “slab” increases and the crystal structure changes from layered to cubic structure. A proper Sr doping (x = 0.1) can increase the reflectivity, reaching 95% in the near infrared range, which is comparable with metal Al measured in the same condition. This indicates its potential applications as optical protective coatings or anti-radiation materials at high temperatures.     

Functional and quantitative measures of receptor-coupled G proteins in human mononuclear leukocytes: No change with age

Aging has been associated with alterations in signal transduction for a number of neurotransmitter receptors in human tissues. Heterotrimeric G proteins play a pivotal role in postreceptor information transduction, by coupling a variety of hormone and neurotransmitter receptors to several intracellular effector functions. Developmental and age-related changes in the abundance of specific Gα subunits have been shown in the human brain. In the present study, functional and quantitative measures of G proteins were conducted in human mononuclear leukocytes obtained from 19 healthy subjects of increasing age. Gs protein function, assessed through cholera toxin-sensitive β-adrenergic and dopaminergic agonists induced increases in ³H-Gpp(NH)p binding capacities to membranes of mononuclear leukocytes, and Gi protein function, assessed through pertussis toxin-sensitive muscarinic agonist induced increase in guanine nucleotide binding capacity, were found to be unaltered by increasing age. Immunobloting analyses with specific polyclonal antibodies against Gα_s, Gα_i, and Gα_q subunit proteins in mononuclear leukocyte membranes obtained from the same subjects showed that the quantities of these proteins in mononuclear leukocytes were as well independent of age. Insofar as age-related alterations in cellular information transduction mechanisms in peripheral tissues are important from the etiological, diagnostic, and pharmacological aspects of age-related disorders, it is important to know that both the coupling of receptors to G proteins, the function of these proteins, and their abundance in human peripheral mononuclear leukocytes stays unaltered by the aging process.     

Single quantum dot controls a plasmonic cavity’s scattering and anisotropy

Plasmonic cavities represent a promising platform for controlling light–matter interaction due to their exceptionally small mode volume and high density of photonic states. Using plasmonic cavities for enhancing light’s coupling to individual two-level systems, such as single semiconductor quantum dots (QD), is particularly desirable for exploring cavity quantum electrodynamic (QED) effects and using them in quantum information applications. The lack of experimental progress in this area is in part due to the difficulty of precisely placing a QD within nanometers of the plasmonic cavity. Here, we study the simplest plasmonic cavity in the form of a spherical metallic nanoparticle (MNP). By controllably positioning a semiconductor QD in the close proximity of the MNP cavity via atomic force microscope (AFM) manipulation, the scattering spectrum of the MNP is dramatically modified due to Fano interference between the classical plasmonic resonance of the MNP and the quantized exciton resonance in the QD. Moreover, our experiment demonstrates that a single two-level system can render a spherical MNP strongly anisotropic. These findings represent an important step toward realizing quantum plasmonic devices.Many quantum network and quantum information processing schemes build upon the enhanced light–matter interaction between a single quantum emitter and a cavity, enabling the effective conversion between photonic and matter-based quantum states (1–4). For example, if the absorption of a photon by a single atom placed inside a cavity can render it transparent to a second photon, then a variety of promising quantum information processing devices can be envisioned including quantum phase gates and repeaters (5). Such QED effects require a high atomic cooperativity c=g2γk, where the coupling strength g² ∝ 1/V is inversely proportional to the volume of the cavity mode V (6). γ and k are the linewidth of the atomic transition and the cavity mode, respectively. Typically, a high cavity quality factor Q (or low k) of conventional photonic cavities is required to compensate for relatively large (diffraction-limited) mode volumes and comes at a cost: The narrow linewidth of cavity modes places stringent requirements on their spectral alignment with the frequencies of quantum transitions. Plasmonic cavities, on the other hand, achieve high values of C while maintaining moderate Q values because of their ultrasmall modal volume (7–10). The relaxed spectral alignment requirements facilitate the experimental realization of various quantum phenomena, such as collective photon emission from a small ensemble of emitters (11) and single photon sources with tunable statistical properties (12).Prior experiments exploring cavity QED effects associated with single emitters coupled to plasmonic cavities or waveguides focused almost exclusively on the observations of reducing the emitter’s lifetime due to the enhanced radiative (proportional to F_P) and nonradiative energy transfers (13–15). The realms of quantum information science and plasmonics have also been bridged by demonstrating that photon emission statistics, such as antibunching behavior in the second-order correlation function for single photon sources, remain intact following the photon–plasmon–photon conversion process (16–19). The possibility of controlling the scattering of a plasmonic nanocavity by a single (and inherently quantum and nonlinear) two-level system has also been proposed (12, 20–22) but never experimentally observed.In this article we provide, to our knowledge, the first experimental demonstration that a single quantum dot (QD) dramatically modifies the scattering spectrum of a simple plasmonic cavity comprising a single metallic nanoparticle (MNP). The MNP–QD hybrid structure is assembled into a well-controlled geometry using the technique of atomic force microscope (AFM) nanomanipulation. The coupling between the MNP and QD is experimentally confirmed by measuring the exciton lifetime, which is reduced by more than an order of magnitude in the presence of the MNP. Analyzing the polarization and spectral properties of light scattered by the MNP-QD hybrid, we observe that the overall plasmonic cavity scattering is significantly modified over a broad spectral range. A Fano resonance spectrally aligned with the QD’s quantized exciton resonance is clearly identified when the polarization of the scattered photon is along the Fano axis (23) connecting the MNP’s center with the QD. The anisotropic scattering spectrum observed in our experiments suggests that a polarization-controlled, versatile quantum light source may be realized in this simple QD–MNP cavity system.The calculated polarization-resolved scattering spectra by the QD–MNP (diameters: 2r_QD = 6_.nm and 2r_MNP = 30 nm) hybrid are shown in Fig. 1A for three polarization angles ?_A of the analyzer placed in the collection path of the scattering signal to mimic the experimental setup (see Methods and Supporting Information for details). In the absence of the QD, all scattering spectra from a single MNP are independent of ?_A and possess a single broad peak at λ_MNP ≈ 520 nm corresponding to the plasmonic dipole resonance of the MNP. The introduction of a QD under the MNP, with the separation gap of g = 1?nm, modifies the scattering spectrum: A sharp Fano feature emerges at the exciton transition wavelength λ_MNP ≈ 520_.nm. The magnitude of the feature is a strong function of the analyzer orientation. If the projection of the Fano axis onto the analyzer plane is perpendicular to the analyzer direction (?_A = π/2 in Fig. 1A), then no Fano feature is predicted by our calculation. The strongest Fano feature is observed for ?_A = 0, and a weaker but finite Fano feature is observed for intermediate angles. Therefore, what is originally an isotropic scatterer (a spherical MNP) is transformed into a highly anisotropic one by the strong hybridization between the QD and the MNP.Open in a separate windowFig. 1.Calculation demonstrating how the near-field coupling modifies the far-field scattering spectra of a structure placed on a glass substrate. (A) The scattering spectra of an NP–QD hybrid excited by the unpolarized evanescent wave coming from the glass substrate side in all azimuthal angles. The angle φ_A indicates the orientation of the analyzer in the path of the scattered light. The Fano feature is the most (least) prominent when the orientation of the analyzer is parallel (perpendicular) with the in-plane component of the Fano axis, which connects the QD and MNP centers. (B and C) The field near the NP–QD hybrid at 500 and 552 nm, respectively, as indicated by the black arrows in the scattering spectrum at φ_A = 0 in A. The scattering signal at these two wavelengths is the same (indicated by the dotted line on the blue curve) whereas the MNP is excited much more strongly in C. This wavelength dependence proves that the presence of the QD indeed controls the MNP’s scattering and anisotropy resonantly.The ability of a single QD to modify the scattering spectrum of a much larger MNP possessing a scattering cross-section of (rMNP/rQD)6≈15,000-fold greater in magnitude than that of the QD seems surprising. Naively, one may expect the effect of a QD on the scattering spectrum of the hybrid system to be very small. As pointed out by a number of theoretical studies (12, 20, 21) and our own numerical calculation shown in Fig. 1, this expectation is not correct. Whereas the exciton dipole moment is too small to produce significant far-field scattering on its own, it is sufficient to depolarize the MNP in the near field, thereby dramatically modifying the electric polarizability of the combined hybrid system at the frequency of the exciton transition.To illustrate this point, the near-field distributions were calculated for λ₁ = 500 nm (Fig. 1B) and λ₂ = 552 nm (Fig. 1C), respectively. These two wavelengths were chosen because the scattering intensities are the same. The much higher (by almost a factor of 2) electric field induced on the MNP’s surface at λ = λ₂, is offset by strong near-field depolarization (light-color area near the QD) of the MNP by the exciton’s dipole. Because the electric field of a dipole rapidly decays with distance, such extreme depolarization (which can be alternatively interpreted as the excitation of high-order multipoles of the MNP by an exciton) can only occur if the QD is placed within nanometers from the MNP. Therefore, it is extremely crucial to precisely position the QD near the MNP as accomplished in our experiments.We assemble the hybrid structure using the technique of AFM nanomanipulation (24–26) (see Methods for details). Whereas other techniques including self-assembly (bottom-up approaches) and lithography (top-down approaches) can be used to create hybrid nanostructures, it is difficult to ensure that only one QD is present in the hybrid structure and to precisely control the distance between the QD and the MNP. AFM nanomanipulation allows us to carefully tailor the dimensions of an individual structure with <5-nm precision and to ensure the presence of only one QD.The assembly process begins by dispersing MNPs and QDs on a glass substrate randomly. We then simultaneously obtain an AFM topography image and a photoluminescence (PL) image by scanning the sample on a home-built integrated AFM–confocal microscope. We locate isolated MNPs and QDs in close proximity via the AFM topography image. We then manipulate a nearby MNP to approach the chosen QD as illustrated in Fig. 2 A–C. Because of the tip-convolution effect as well as the size difference between the MNP (∼30 nm in diameter) and the QD (∼5 nm in diameter), the lateral resolution of the AFM image is not sufficient to allow direct measurement of the distance between the MNP and QD in a hybrid structure. We push the MNP until the QD is no longer visible in the AFM image (Fig. 2C), and use nearby topography surfaces to estimate the position of the QD underneath with a precision of <5 nm. Ligand molecules on the surfaces of the MNP and QD prevent them from physically touching one another.Open in a separate windowFig. 2.Assembly and characterization of a strongly coupled MNP–QD hybrid structure. (A–C) Representative steps in a hybrid structure assembly. A single Au NP (A, Left) was placed in the close vicinity of a single CdSe/ZnS QD (A, Right). (D) Representative PL lifetime measurement of a single isolated QD (black) yielding ∼29 ns and lifetime of the assembled hybrid structure (red) with a fast component of ∼1.6 ns. Solid lines are fits to the data. We used a single exponential function to fit the bare QD lifetime and a double exponential function to fit the hybrid structure, yielding a fast decay of ∼1.6 ns and a slow decay of ∼13 ns.We then measure the lifetime to confirm that the MNP is indeed in the close proximity of the QD. Lifetime is, in fact, a rather accurate way to characterize the distance between the MNP and QD as demonstrated in our previous work (27). For the particular hybrid structure discussed in the rest of the paper, we measure a greatly reduced short lifetime (∼1.6 ns for the fast decay component, red curve in Fig. 2D). Compared with the representative bare QD lifetime of ∼29 ns (shown as the black curve in Fig. 2D), the QD in the hybrid structure shows a lifetime reduction of more than an order of magnitude, thus confirming the strong coupling between the QD and MNP.Measured ensemble absorption and PL emission spectra (taken in solution) of QDs are shown in Fig. 3B. The absorption spectrum features multiple discrete exciton resonances at lower energies and a continuous absorption spectrum at energies above the band gap of the crystal. Although all absorption resonances may influence the scattering spectrum of the hybrid structure, in the following analysis we focus on the lowest-energy exciton state featuring the strongest absorption resonance. This exciton transition is centered near λ_QD ≈ 615 nm with an ensemble-averaged spectral FWHM Δλ_1/2 ～ 15 nm.Open in a separate windowFig. 3.Scattering and anisotropy of a plasmonic cavity (i.e., an MNP) controlled by a single QD. (A) Experimental schematics for scattering measurements. (B) Experimentally measured absorption (black) and PL emission (red) spectra from an ensemble of colloidal CdSe/ZnS core–shell QDs. (C) Measured scattering spectra of a bare 30-nm-diameter MNP at different analyzer angles. The small changes near 550 nm or below are likely due to deviations from a perfectly spherical shape of the MNP. (D) Measured scattering spectra of the assembled QD–MNP hybrid structure at different analyzer angles. The Fano resonance indicated by the dotted vertical line spectrally aligns with the lowest exciton state measured in the absorption from an ensemble of QDs. The Fano resonance is most pronounced at ?_A = 30°.The dark-field scattering experiments are performed using a home-built optical system optimized for small MNP measurements (Fig. 3A). An unpolarized white-light source incident in a conical geometry generates evanescent fields and excites the hybrid structure from all directions. An analyzer is placed in the scattered light’s path to select the polarization of the collected scattering (see Methods for details of the optical setup and measurement). A series of such spectra S(λ, ?_A) is displayed in Fig. 3D as a function of the analyzer angle ?_A. A very sharp Fano feature, with ??S(λ, ?_A)/??λ actually reversing its sign at λ_QD ≈ 627?nm, can be clearly observed for ?_A = ?₁ = 30°. Weaker Fano features are observed for other polarizer orientations, all in qualitative agreement with our theoretical predictions in Fig. 1A. Remarkably, almost no Fano interference between the inherently quantum (single-exciton) and classical (plasmonic) transitions is observed for ?_A = ?₂ = 120°.Comparing the spectra which show the strongest and weakest Fano resonance at 30° and 120°, respectively, one observes modifications over a much broader spectral range than the lowest exciton resonance linewidth because other exciton states and absorption features in the QD also contribute to the resonantly enhanced depolarization of the MNP. We note that the numerical calculation presented in Fig. 1 is based on an MNP coupled to a single two-level system and only serves to illustrate the basic principle of QD controlled plasmonic cavity scattering and anisotropy. The calculation has incorporated a dipole moment of 50 Debye for the two-level system, which is an overestimate for a small colloidal QD used in the experiments. A quantitatively accurate model for calculating the scattering spectrum of this simplest MNP–QD hybrid structure remains a challenge. Higher-order multipole effects of the MNP and QD (28), the nanoparticle’s geometrical deviation from a perfect sphere, and all relevant electronic transitions in QDs need to be taken into account to quantitatively reproduce the experimental observations.To further confirm that the polarization dependence of the scattering spectra of the hybrid structure indeed originates from the coupling between the QD and MNP, we show the scattering spectra of a bare MNP in Fig. 3C. The spectra do not display any Fano features or analyzer angle dependence in the spectral proximity of the exciton resonance λ_QD. Therefore, it is indeed the coupling between a single QD and the MNP that turns an otherwise isotropic plasmonic cavity into a strongly anisotropic one. Because a single quantum absorber achieves this effect, one can envision the proposed hybrid system as an experimental platform for observing a plasmonic cavity anisotropy controlled by optical nonlinearity at the single-photon level. The small angular variations of S(λ, ?_A) at shorter wavelengths (around 550 nm or below) in Fig. 3 C and D most likely arise from a small intrinsic deviation of the MNP’s shape from an ideal sphere. Unlike the extrinsic anisotropy induced by the QD, it cannot be optically controlled and is not of interest for nonlinear quantum optics.The control of scattering and anisotropy of the MNP cavity by a single QD is not affected by the common photobleaching that puts severe limits on any fluorescence-based experiment. To confirm this property, we measure the PL of the hybrid structures again after the scattering experiments and find no measurable signal. This observation suggests that the QD has been severely photobleached by the combination of extended atmospheric exposure, strong near-field of the MNP cavity, and prolonged exposure to the halogen lamp used as the excitation source. Our experiments clearly demonstrate that even a photobleached QD may still be used to control the plasmonic cavity scattering by creating the Fano resonance due to its still intact absorption/scattering capability. Furthermore, the polarization-resolved scattering spectra presented here can be used to determine the location of a QD with respect to the nearby MNP based upon the recently developed plasmonic nanoprotractor concept (23). Thus, our work also suggests a hybrid sensor consisting of a nonfluorescent (photobleached) QD and an MNP.In summary, we have demonstrated that a single semiconductor QD coupled to an MNP cavity can effectively control the scattering spectrum of the latter, as well as render it highly anisotropic. The speculative implications of such extrinsic anisotropy are very intriguing. On the one hand, it serves as an orientation sensor to determine the relative locations of the QD and MNP. On the other hand, it should be possible to observe polarization-dependent photon statistics of light scattered from the QD-MNP nanohybrid. Furthermore, the photon statistics are wavelength tunable as previously proposed (12). By tuning the excitation wavelength to the spectral position corresponding to the destructive (constructive) interference side of the Fano resonance, strong bunching (antibunching) behavior should be observed in second-order correlation function g⁽²⁾ measurements. Furthermore, a single two-level system is intrinsically highly nonlinear. One may modulate the Fano resonance via such nonlinear effects associated with the single QD at higher incident light intensity. The studies presented here demonstrate the feasibility of QD–MNP hybrid nanostructures combined with polarization-sensitive detection schemes as a promising platform for new and exciting opportunities in plasmonic quantum technologies.