Enhanced Transmission at the Zeroth-Order Mode of a Terahertz Fabry-Perot Cavity.

A planar Fabry-Perot cavity with intermirror spacing of d ≪ λ is explored for its "zero-order mode" terahertz transmission. The enhanced transmission observed as d → 0 indicates that such cavities satisfy the resonance conditions across a broad terahertz bandwidth. The experimental signatures from this elusive, "technically challenging" regime are evidenced using time-domain terahertz spectroscopy and are complemented by numerical calculations. The results raise intriguing possibilities for terahertz field modulation and pave new paths for strong coupling of multiple transition frequencies simultaneously.

Long Range Electronic Effects on the Host-Guest Complexation within the Oxygen Depleted 5,5'-Bicalixarene Cavities.

We report the synthesis of a series of the oxygen-depleted conjugated 5,5'-Bicalix[4]arene compounds bearing various substituents at the terminal positions of the conjugated chain and their fluorescence response to the presence of a cationic N-methylpyridinium guest. The complexation of this cation within the bicalixarene cavity results in the fluorescence quenching, with the host molecules bearing electron-donating groups demonstrating a stronger fluorescence response. These results show the importance of the electronic effects on the host-guest complexation within the hydrophobic calixarene scaffolds.

Intramolecular structural heterogeneity altered by long-range contacts in an intrinsically disordered protein.

Short-range interactions and long-range contacts drive the 3D folding of structured proteins. The proteins' structure has a direct impact on their biological function. However, nearly 40% of the eukaryotes proteome is composed of intrinsically disordered proteins (IDPs) and protein regions that fluctuate between ensembles of numerous conformations. Therefore, to understand their biological function, it is critical to depict how the structural ensemble statistics correlate to the IDPs' amino acid sequence. Here, using small-angle X-ray scattering and time-resolved Förster resonance energy transfer (trFRET), we study the intramolecular structural heterogeneity of the neurofilament low intrinsically disordered tail domain (NFLt). Using theoretical results of polymer physics, we find that the Flory scaling exponent of NFLt subsegments correlates linearly with their net charge, ranging from statistics of ideal to self-avoiding chains. Surprisingly, measuring the same segments in the context of the whole NFLt protein, we find that regardless of the peptide sequence, the segments' structural statistics are more expanded than when measured independently. Our findings show that while polymer physics can, to some level, relate the IDP's sequence to its ensemble conformations, long-range contacts between distant amino acids play a crucial role in determining intramolecular structures. This emphasizes the necessity of advanced polymer theories to fully describe IDPs ensembles with the hope that it will allow us to model their biological function.

From enhanced diffusion to ultrafast ballistic motion of hybrid light-matter excitations.

Transport of excitons and charge carriers in molecular systems can be enhanced by coherent coupling to photons, giving rise to the formation of hybrid excitations known as polaritons. Such enhancement has far-reaching technological implications; however, the enhancement mechanism and the transport nature of these hybrid excitations remain elusive. Here we map the ultrafast spatiotemporal dynamics of polaritons formed by mixing surface-bound optical waves with Frenkel excitons in a self-assembled molecular layer, resolving polariton dynamics in energy/momentum space. We find that the interplay between the molecular disorder and long-range correlations induced by coherent mixing with light leads to a mobility transition between diffusive and ballistic transport, which can be controlled by varying the light-matter composition of the polaritons. Furthermore, we show that coupling to light enhances the diffusion coefficient of molecular excitons by six orders of magnitude and even leads to ballistic flow at two-thirds the speed of light.

Collective Rayleigh Scattering from Molecular Ensembles under Strong Coupling.

Rayleigh scattering is usually considered to be the elastic scattering of photons from subwavelength physical objects, such as small particles or molecules. Here, we present a quantitative spectroscopic study of the scattering properties of molecules embedded in an optical cavity under strong coupling conditions, where the collective interaction between the molecules and the cavity gives rise to composite light-matter excitations known as cavity polaritons. We show that the polaritonic states exhibit strong resonant Rayleigh scattering, which depends on both the coupling strength and detuning and reaching ∼25% efficiency. Since the polaritonic wave functions in such systems are delocalized, our observations correspond to the collective scattering of each photon from a large ensemble of molecules.

Efficient Photosensitizing Capabilities and Ultrafast Carrier Dynamics of Doped Carbon Dots.

Carbon dots (C-Dots) are promising new materials for the development of biocompatible photosensitizers for solar-driven catalysis and hydrogen production in aqueous solution. Compared to common semiconducting quantum dots, C-Dots have good physicochemical, as well as photochemical stability, optical brightness, stability and nontoxicity, while their carbon based source results in tunable surface chemistry, chemical versatility, low cost, and biocompatibility. Herein we show that doping the C-Dots with phosphate or boron significantly influences their excited-state dynamics, which is observed by the formation of a unique long-lived photoproduct as a function of the different dopants. To probe the photosensitizing capabilities of the C-Dots, we followed the photoreduction of methyl viologen (MV2+), which acts as a molecular redox mediator (electron acceptor) to the C-Dots (the photosensitizer, i.e., electron donor) in aqueous solution, using steady-state and time-resolved fluorescence and absorption spectroscopic techniques as well as electrochemical measurements. We show that ultrafast electron transfer to MV2+ and slow charge recombination results in a high quantum yield of MV2+ photoreduction, while the doping drastically influences this quantum yield of MV2+ radical. Our findings contribute to the photophysical understanding of this intriguing and relatively new carbon-based nanoparticle and can improve the design and development of efficient photosensitizers over commonly used heterogeneous catalysts in photocatalytic systems by increasing the efficiency of radical generation.

Strong coupling of collective intermolecular vibrations in organic materials at terahertz frequencies.

Several years ago, strong coupling between electronic molecular transitions and photonic structures was shown to modify the electronic landscape of the molecules and affect their chemical behavior. Since then, this concept has evolved into a new field known as polaritonic chemistry. An important ingredient in the progress of this field was the demonstration of strong coupling with intra-molecular vibrations, which enabled the modification of processes occurring at the electronic ground-state. Here we demonstrate strong coupling with collective, inter-molecular vibrations occurring in organic materials in the low-terahertz region ([Formula: see text]1012 Hz). Using a cavity filled with α-lactose molecules, we measure the temporal evolution and observe coherent Rabi oscillations, corresponding to a splitting of 68 GHz. These results take strong coupling into a new class of materials and processes, including skeletal polymer motions, protein dynamics, metal organic frameworks and other materials, in which collective, spatially extended degrees of freedom participate in the dynamics.

Temporal Dynamics of Localized Exciton-Polaritons in Composite Organic-Plasmonic Metasurfaces.

We use femtosecond transient absorption spectroscopy to study the temporal dynamics of strongly coupled exciton-plasmon polaritons in metasurfaces of aluminum nanoantennas coated with J-aggregate molecules. Compared with the thermal nonlinearities of aluminum nanoantennas, the exciton-plasmon hybridization introduces strong ultrafast nonlinearities in the composite metasurfaces. Within femtoseconds after the pump excitation, the plasmonic resonance is broadened and shifted, showcasing its high sensitivity to excited-state modification of the molecular surroundings. In addition, we observe temporal oscillations due to the deep subangstrom acoustic breathing modes of the nanoantennas in both bare and hybrid metasurfaces. Finally, unlike the dynamics of hybrid states in optical microcavities, here, ground-state bleaching is observed with a significantly longer relaxation time at the upper polariton band.

Picosecond optomechanical oscillations in metal-polymer microcavities.

We experimentally study mechanical vibrations in planar Fabry-Perot microcavities made of metallic mirrors and a polymer spacer, using broadband pump-probe spectroscopy. These acoustic waves oscillate at a picosecond time-scale and result in spectral oscillations of the cavity transmission spectrum. We find that the oscillations are initiated at the metal mirrors and that their temporal dynamics match the elastic modes of the polymer layer, indicating that mechanical momentum is transferred within the structure. Such structures combine the strong optical absorption of metals with the elasticity and the processability of polymers, which open the road to a new class of optomechanical devices.

Supramolecular Translation of Enzymatically Triggered Disassembly of Micelles into Tunable Fluorescent Responses.

The need for advanced fluorescent imaging and delivery platforms has motivated the development of smart probes that change their fluorescence in response to external stimuli. Here a new molecular design of fluorescently labeled PEG-dendron hybrids that self-assemble into enzyme-responsive micelles with tunable fluorescent responses is reported. In the assembled state, the fluorescence of the dyes is quenched or shifted due to intermolecular interactions. Upon enzymatic cleavage of the hydrophobic end-groups, the labeled polymeric hybrids become hydrophilic, and the micelles disassemble. This supramolecular change is translated into a spectral response as the dye-dye interactions are eliminated and the intrinsic fluorescence is regained. We demonstrate the utilization of this molecular design to generate both Turn-On and spectral shift responses by adjusting the type of the labeling dye. This approach enables transformation of non-responsive labeling dyes into smart fluorescent probes.

Light-emitting self-assembled peptide nucleic acids exhibit both stacking interactions and Watson-Crick base pairing.

The two main branches of bionanotechnology involve the self-assembly of either peptides or DNA. Peptide scaffolds offer chemical versatility, architectural flexibility and structural complexity, but they lack the precise base pairing and molecular recognition available with nucleic acid assemblies. Here, inspired by the ability of aromatic dipeptides to form ordered nanostructures with unique physical properties, we explore the assembly of peptide nucleic acids (PNAs), which are short DNA mimics that have an amide backbone. All 16 combinations of the very short di-PNA building blocks were synthesized and assayed for their ability to self-associate. Only three guanine-containing di-PNAs-CG, GC and GG-could form ordered assemblies, as observed by electron microscopy, and these di-PNAs efficiently assembled into discrete architectures within a few minutes. The X-ray crystal structure of the GC di-PNA showed the occurrence of both stacking interactions and Watson-Crick base pairing. The assemblies were also found to exhibit optical properties including voltage-dependent electroluminescence and wide-range excitation-dependent fluorescence in the visible region.

Comprehensive study of ultrafast excited-state proton transfer in water and D2O providing the missing RO(-)···H(+) ion-pair fingerprint.

Steady-state and time-resolved optical techniques were employed to study the photoprotolytic mechanism of a general photoacid. Previously, a general scheme was suggested that includes an intermediate product that, up until now, had not been clearly observed experimentally. For our study, we used quinone cyanine 7 (QCy7) and QCy9, the strongest photoacids synthesized so far, to look for the missing intermediate product of an excited-state proton transfer to the solvent. Low-temperature steady-state emission spectra of both QCy7 and QCy9 clearly show an emission band at T < 165 K in H2O ice that could be assigned to ion-pair RO(-)*···H3O(+), the missing intermediate. Room-temperature femtosecond pump-probe spectroscopy transient spectra at short times (t < 4 ps) also shows the existence of transient absorption and emission bands that we assigned to the RO(-)*···H3O(+) ion pair. The intermediate dissociates on a time scale of 1 ps and about 1.5 ps in H2O and D2O samples, respectively.

Acid effect on photobase properties of curcumin.

Femtosecond UV-vis pump-probe spectroscopy was employed to study the acid effect on curcumin in the excited state. Curcumin in solutions of weak acids was found to be a photobase forming a protonated curcumin within a few tens of picoseconds from the time of excitation. The excited-state protonation reaction is also observed in the steady-state emission spectrum as a new red emission band with a maximum at 620 nm in the presence of weak acids. The transient pump-probe spectrum consists of four spectral bands, two emission bands, and two absorption bands. We assign a transient absorption band at ∼600 nm and an emission band at ∼540 nm to the neutral ROH form of curcumin. An absorption band at ∼500 nm and an emission band at 620 nm are assigned to the protonated ROH2(+) form of curcumin.

Thermodynamics of molecules strongly coupled to the vacuum field.

The thermodynamics of strong coupling between molecules and the vacuum field is analyzed and the Gibbs free energy, the enthalpy, and entropy of the coupling process are determined for the first time. The thermodynamic parameters are a function of the Rabi splitting and the microscopic solvation. The results provide a new framework for understanding light-molecule strong coupling.

Tuning the work-function via strong coupling.

The tuning of the molecular material work-function via strong coupling with vacuum electromagnetic fields is demonstrated. Kelvin probe microscopy extracts the surface potential (SP) changes of a photochromic molecular film on plasmonic hole arrays and inside Fabry-Perot cavities. Modulating the optical cavity resonance or the photochromic film effectively tunes the work-function, suggesting a new tool for tailoring material properties.

Polariton dynamics under strong light-molecule coupling.

We present a comprehensive experimental study of the photophysical properties of a molecule-cavity system under strong coupling conditions, using steady-state and femtosecond time-resolved emission and absorption techniques to selectively excite the lower and upper polaritons as well as the reservoir of uncoupled molecules. Our results demonstrate the complex decay routes in such hybrid systems and that, contrary to expectations, the lower polariton is intrinsically long-lived.

Disorder-enhanced transport in photonic quasicrystals.

Quasicrystals are aperiodic structures with rotational symmetries forbidden to conventional periodic crystals; examples of quasicrystals can be found in aluminum alloys, polymers, and even ancient Islamic art. Here, we present direct experimental observation of disorder-enhanced wave transport in quasicrystals, which contrasts directly with the characteristic suppression of transport by disorder. Our experiments are carried out in photonic quasicrystals, where we find that increasing disorder leads to enhanced expansion of the beam propagating through the medium. By further increasing the disorder, we observe that the beam progresses through a regime of diffusive-like transport until it finally transitions to Anderson localization and the suppression of transport. We study this fundamental phenomenon and elucidate its origins by relating it to the basic properties of quasicrystalline media in the presence of disorder.

Spontaneous pattern formation upon incoherent waves: from modulation-instability to steady-state.

We study the long-range propagation of incoherent light following the modulation instability (MI) process in non-instantaneous nonlinear Kerr-type media. We find that the system eventually reaches a steady-state characterized by a lower degree of coherence than in the initial state, with small fluctuations around a pronounced mean value. We find that the average values of the spatial correlation distance at steady-state and the fluctuations around it, which are obtained either through ensemble averaging, or by spatial averaging, or via temporal averaging, are all identical. This feature may be viewed as indication of ergodic behavior, which occurs in the long-time evolution following incoherent MI. Finally, we find that the steady-state properties of the system depend on the initial coherence but not on the nonlinearity strength, although the system evolves faster to steady-state as the strength of the nonlinearity is increased.