Coherent Transfer Research Articles

Dynamic nuclear polarization by two-pulse phase modulation.

The coherent transfer of electron spin polarization to nuclei by means of a microwave pulse sequence is a promising new approach to enhancing the sensitivity of solid-state nuclear magnetic resonance (NMR). The development of pulse sequences for dynamic nuclear polarization (DNP) of bulk nuclei is far from complete, as is the understanding of what makes a good DNP sequence. In this context, we introduce a new sequence, termed Two-Pulse Phase Modulation (TPPM) DNP. We provide a general theoretical description for electron-proton polarization transfer by periodic DNP pulse sequences and find it in excellent agreement with numerical simulations. In experiments at 1.2T, TPPM DNP generates a higher gain in sensitivity than existing sequences XiX (X-inverse-X) and TOP (Time-Optimized Pulsed) DNP but does so at relatively high nutation frequencies. In contrast, we find that the XiX sequence performs very well at nutation frequencies as low as 7MHz. A combination of theoretical analysis and experimental investigation makes clear that fast electron-proton polarization transfer, due to a well-preserved dipolar coupling in the effective Hamiltonian, correlates with a short build-up time of the dynamic nuclear polarization of the bulk. Experiments further show that the performances of XiX and TOP DNP are affected differently by the concentration of the polarizing agent. These results constitute important reference points for the development of new and better DNP sequences.

Cavity induced collective behavior in the polaritonic ground state

Cavity quantum electrodynamics provides an ideal platform to engineer and control light-matter interactions with polariton quasiparticles. In this work, we investigate collective phenomena in a system of many particles in a harmonic trap coupled to a homogeneous cavity vacuum field. The system couples collectively to the cavity field, through its center of mass, and collective polariton states emerge. The cavity field mediates pairwise long-range interactions and enhances the effective mass of the particles. This leads to an enhancement of localization in the matter ground state density, which features a maximum when light and matter are on resonance, and demonstrates a Dicke-like, collective behavior with the particle number. The light-matter interaction also modifies the photonic properties of the polariton system, as the ground state is populated with bunched photons. In addition, it is shown that the diamagnetic A^2A2 term is necessary for the stability of the system, as otherwise the superradiant ground state instability occurs. We demonstrate that coherent transfer of polaritonic population is possible with an external magnetic field and by monitoring the Landau-Zener transition probability.

An improved algorithm for design of broadband excitation, inversion, and mixing pulse sequences by iterative optimization of phases: TOPS-2

This paper presents an improved iterative algorithm (TOPS-2) for the design of broadband inversion, excitation and coherent transfer mixing sequence (TOCSY) pulses. The evolution of the Bloch vector is presented as a sequence of small constant flip angle pulses with varying phases and constant amplitude. This paper describes an improved algorithm for iterative optimization of piece-wise constant phases as we incorporate the quadratic terms in the propagators. In our iterative optimization we obtain a closed-form expression for each phase, and these phases are optimized sequentially using the new improved algorithm. This paper compares the simulation results of the TOPS vs TOPS-2 and shows that TOPS-2 perform better. Experimental validation of excitation and inversion TOPS-2 pulse sequence is performed with .5% H2O in 99.5% D2O, and experimental validation of TOPS-2 mixing (TOCSY) pulse sequence is done with 0.1% of Ethylbenzene (EB) in CDCl3 solvent.

MODE: Multiply modulated frame technique, excitation and coherence transfer

The study of modulated pulse (MODE pulse) approach for the application in NMR has been in the literature for over a decade. Although the method’s purpose was initially to decouple the spins, its application can be extended to broadband excitation, inversion and coherence transfer between spins (TOCSY). In this paper, the experimental validation of the TOCSY experiment with the help of MODE pulse and how the coupling constant varies over different frames are shown. We demonstrate that the TOCSY with a higher MODE pulse will result in less coherence transfer even with the same RF-power, and a lower MODE pulse will require a larger RF-amplitude to achieve TOCSY over the same bandwidth. We also present a quantitative analysis of the error due to fast oscillating terms that can be neglected, giving the required results.

Direct observation of coherence transfer and rotational-to-vibrational energy exchange in optically centrifuged CO2 super-rotors

Optical centrifuges are laser-based molecular traps that can rotationally accelerate molecules to energies rivalling or exceeding molecular bond energies. Here we report time and frequency-resolved ultrafast coherent Raman measurements of optically centrifuged CO2 at 380 Torr spun to energies beyond its bond dissociation energy of 5.5 eV (Jmax = 364, Erot = 6.14 eV, Erot/kB = 71, 200 K). The entire rotational ladder from J = 24 to J = 364 was resolved simultaneously which enabled a more accurate measurement of the centrifugal distortion constants for CO2. Remarkably, coherence transfer was directly observed, and time-resolved, during the field-free relaxation of the trap as rotational energy flowed into bending-mode vibrational excitation. Vibrationally excited CO2 (ν2 > 3) was observed in the time-resolved spectra to populate after 3 mean collision times as a result of rotational-to-vibrational (R-V) energy transfer. Trajectory simulations show an optimal range of J for R-V energy transfer. Dephasing rates for molecules rotating up to 5.5 times during one collision were quantified. Very slow decays of the vibrational hot band rotational coherences suggest that they are sustained by coherence transfer and line mixing.

Theoretical Description of Attosecond X-ray Absorption Spectroscopy of Frenkel Exciton Dynamics.

Frenkel excitons are responsible for the transport of light energy in many molecular systems. Coherent electron dynamics govern the initial stage of Frenkel-exciton transfer. Capability to follow coherent exciton dynamics in real time will help to reveal their actual contribution to the efficiency of light-harvesting. Attosecond X-ray pulses are the tool with the necessary temporal resolution to resolve pure electronic processes with atomic sensitivity. We describe how attosecond X-ray pulses can probe coherent electronic processes during Frenkel-exciton transport in molecular aggregates. We analyze time-resolved absorption cross section taking broad spectral bandwidth of an attosecond pulse into account. We demonstrate that attosecond X-ray absorption spectra can reveal delocalization degree of coherent exciton transfer dynamics.

Investigation of thermal performance of a ribbed solar air heater for sustainable built environment

Solar air heater plays a crucial role for development of sustainable built environment, which can fulfil the need of heating load of building by harnessing solar energy. A solar air heater (SAH) to warm a building has poor thermal performance due to the lack of a sufficiently coherent heat transfer rate. Utilizing a roughened surface is one of the most effective means for enhancing heat transfer of a SAH to the creation of a sustainable built environment. The impact of a discrete double-arc roughness was investigated in order to test the effects of current rib roughness on the characteristics on the thermohydraulic performance (THP) of rectangular SAH ducts. Reynolds number (Re) is varied from 3010 to 13935 with the relative roughness pitch was considered in the range of 6.67–8.33 while the angle of attack (α) varying from 30 degrees- 75 degrees. As a result of using discrete double arc ribs, Nusselt number (Nu) values increased as the Re increased, and friction factor (f) values decreased as Re increased. The Nu and f have been shown to increase by a maximum of 217% and 207% respectively. It was determined that THP reached the maximum level of 2.82. Correlations for Nu and f were also developed.

Coherent transfer of optical vortices via backward four-wave mixing in a double- Λ atomic system

We propose and analyze an efficient scheme for the coherent transfer of optical vortices in a cold atomic ensemble with four-level double-$\mathrm{\ensuremath{\Lambda}}$ configuration. The orbital angular-momentum (OAM) information can be transferred from the incident vortex fields to the generated backward signal field via resonant backward four-wave mixing (FWM). Considering the single vortex probe field initially acting on one transition of the atomic ensemble and using experimentally achievable parameters, we identify the conditions under which the resonant backward FWM allows us to greatly improve the conversion efficiency of optical vortices beyond what is achievable in the resonant forward OAM transfers. It is found that the complete energy conversion of optical vortices originates from the perfect competition between linear absorption and parametric gain. We also demonstrate that the phase mismatching would be detrimental to the high-efficiency transfer of optical vortices. Furthermore, we show that the generated backward signal field develops a new pure or mixing OAM state when the control field is also a vortex beam. Finally, we investigate the composite vortex beam generated by collinear superposition of the incident vortex probe and signal fields, which can controlled via adjusting the intensity of the control field. Our scheme may have potential applications in OAM-based optical communication and optical information processing.

Analytically Solvable Model for Qubit-Mediated Energy Transfer between Quantum Batteries.

The coherent energy transfer between two identical two-level systems is investigated. Here, the first quantum system plays the role of a charger, while the second can be seen as a quantum battery. Firstly, a direct energy transfer between the two objects is considered and then compared to a transfer mediated by an additional intermediate two-level system. In this latter case, it is possible to distinguish between a two-step process, where the energy is firstly transferred from the charger to the mediator and only after from the mediator to the battery, and a single-step in which the two transfers occurs simultaneously. The differences between these configurations are discussed in the framework of an analytically solvable model completing what recently discussed in literature.

Optimal control derived sensitivity-enhanced CA-CO mixing sequences for MAS solid-state NMR – Applications in sequential protein backbone assignments

We have recently introduced optimal-control derived pulse sequences for sensitivity-enhanced heteronuclear correlation NMR experiments of solid proteins. Preservation of equivalent coherence transfer pathways using transverse-mixing pulses (TROP) in multidimensional pulse schemes allows to increase the sensitivity of the experiments by more than a factor of 2 per each indirect dimension. In this article, we present homonuclear CA-CO transverse-mixing elements (homoTROP) that are based on dipolar interactions and achieve similar gains as the heteronuclear TROP pulses described previously. Both transfer elements were subsequently implemented in 3D se-hCAcoNH and se-hCOcaNH, that together with the previously introduced 3D se-hCANH and se-hCONH experiments yield a complete set of sensitivity-enhanced protein backbone assignment experiments. In contrast to the J-coupling based methods that are used at fast (60 kHz) and ultrafast MAS (>100 kHz), the homoTROP experiments employ about 10-times shorter mixing times making use of the larger magnitude of the dipolar coupling in comparison to the J couplings. The experiments are demonstrated using a microcrystalline, perdeuterated sample of the chicken alpha-spectrin SH3 domain in which all exchangeable sites are fully back-substituted with protons. We evaluated the gains in efficiency in all experiments site-specifically observing that the se-hCAcoNH and se-hCOcaNH experiments yield an increase in sensitivity by a factor of 1.36±0.09 and at least a factor of 1.8 with respect to the conventional hcoCAcoNH and hCOcaNH J-based experiments.

Experimental Demonstration of Velocimetry by Actively Stabilized Coherent Optical Transfer

We report on the development of a system called velocimetry by actively stabilized coherent optical transfer (VASCOT) that uses active optical phase tracking to measure the in-line velocity of retroreflecting targets with narrow-band photodetection. VASCOT is experimentally demonstrated over a 584-m atmospheric link to a corner-cube retroreflector (CCR) on an airborne drone. The in-line target-velocity measurement achieves residual uncertainties below $2\phantom{\rule{0.2em}{0ex}}{\text{nm s}}^{\ensuremath{-}1}$ within 5 s of averaging. Cycle-slip-free phase tracking over the full 3-min experiment allows the relative target range to be tracked with a one-standard-deviation uncertainty of 39.5 nm. VASCOT also provides an absolute target-range measurement with a statistical error of $\ifmmode\pm\else\textpm\fi{}13.7\phantom{\rule{0.2em}{0ex}}\mathrm{mm}$, which agrees with an independent Global-Positioning-System- (GPS) derived range measurement to within the GPS uncertainty.

High-precision Fourier ptychographic microscopy based on Gaussian apodization coherent transfer function constraints.

Fourier ptychographic microscopy (FPM) combines the concepts of phase retrieval algorithms and synthetic apertures and can solve the problem in which it is difficult to combine a large field of view with high resolution. However, the use of the coherent transfer function in conventional calculations to describe the linear transfer process of an imaging system can lead to ringing artifacts. In addition, the Gerchberg-Saxton iterative algorithm can cause the phase retrieval part of the FPM algorithm to fall into a local optimum. In this paper, Gaussian apodization coherent transfer function is proposed to describe the imaging process and is combined with an iterative method based on amplitude weighting and phase gradient descent to reduce the presence of ringing artifacts while ensuring the accuracy of the reconstructed results. In simulated experiments, the proposed algorithm is shown to give a smaller mean square error and higher structural similarity, both in the presence and absence of noise. Finally, the proposed algorithm is validated in terms of giving reconstruction results with high accuracy and high resolution, using images acquired with a new microscope system and open-source images.

Electronic Structure and Hole Transfer of All B-DNA Dimers and Homopolymers, via the Fishbone-Wire Model.

We employ the Tight Binding Fishbone-Wire Model to study the electronic structure and coherent transfer of a hole (the absence of an electron created by oxidation) in all possible ideal B-DNA dimers as well as in homopolymers (one base pair repeated along the whole sequence with purine on purine). The sites considered are the base pairs and the deoxyriboses, with no backbone disorder. For the time-independent problem, we calculate the eigenspectra and the density of states. For the time-dependent problem after oxidation (i.e., the creation of a hole either at a base pair or at a deoxyribose), we calculate the mean-over-time probabilities to find the hole at each site and establish the frequency content of coherent carrier transfer by computing the Weighted Mean Frequency at each site and the Total Weighted Mean Frequency of a dimer or polymer. We also evaluate the main oscillation frequencies of the dipole moment along the macromolecule axis and the relevant amplitudes. Finally, we focus on the mean transfer rates from an initial site to all others. We study the dependence of these quantities on the number of monomers that are used to construct the polymer. Since the value of the interaction integral between base pairs and deoxyriboses is not well-established, we treat it as a variable and examine its influence on the calculated quantities.

Partially coherent broadband 3D optical transfer functions with arbitrary temporal and angular power spectra.

Optical diffraction tomography is a powerful technique to produce 3D volumetric images of biological samples using contrast produced by variations in the index of refraction in an unlabeled specimen. While this is typically performed with coherent illumination from a variety of angles, interest has grown in partially coherent methods due to the simplicity of the illumination and the computation-free axial sectioning provided by the coherence window of the source. However, such methods rely on the symmetry or discretization of a source to facilitate quantitative analysis and are unable to efficiently handle arbitrary illumination that may vary asymmetrically in angle and continuously in the spectrum, such as diffusely scattered or thermal sources. A general broadband theory may expand the scope of illumination methods available for quantitative analysis, as partially coherent sources are commonly available and may benefit from the effects of spatial and temporal incoherence. In this work, we investigate partially coherent tomographic phase microscopy from arbitrary sources regardless of angular distribution and spectrum by unifying the effects of spatial and temporal coherence into a single formulation. This approach further yields a method for efficient computation of the overall systems' optical transfer function, which scales with O(n 3), down from O(mn 4) for existing convolutional methods, where n 3 is the number of spatial voxels in 3D space and m is the number of discrete wavelengths in the illumination spectrum. This work has important implications for enabling partially coherent 3D quantitative phase microscopy and refractive index tomography in virtually any transmission or epi-illumination microscope.

Coherent Phonon‐Induced Modulation of Charge Transfer in 2D Hybrid Perovskites

AbstractElectron–phonon interactions play an essential role in charge transport and transfer processes in semiconductors. For most structures, tailoring electron–phonon interactions for specific functionality remains elusive. Here, it is shown that, in hybrid perovskites, coherent phonon modes can be used to manipulate charge transfer. In the 2D double perovskite, (AE2T)2AgBiI8 (AE2T: 5,5“‐diylbis(amino‐ethyl)‐(2,2”‐(2)thiophene)), the valence band maximum derived from the [Ag0.5Bi0.5I4]2– framework lies in close proximity to the AE2T‐derived HOMO level, thereby forming a type‐II heterostructure. During transient absorption spectroscopy, pulsed excitation creates sustained coherent phonon modes, which periodically modulate the associated electronic levels. Thus, the energy offset at the organic–inorganic interface also oscillates periodically, providing a unique opportunity for modulation of interfacial charge transfer. Density‐functional theory corroborates the mechanism and identifies specific phonon modes as likely drivers of the coherent charge transfer. These observations are a striking example of how electron–phonon interactions can be used to manipulate fundamentally important charge and energy transfer processes in hybrid perovskites.

Modelling and design of ultra-high stable Fabry–Pérot cavity

Lasers having sub-Hz linewidth and long-term stability better than a few parts in 1016 is an unavoidable requisite to achieve unprecedented accuracy in the optical atomic clocks, intercomparing distributed atomic clocks and their networking, establishing coherent optical fibres, precision time & frequency transfer, production of optical qubits, and many more. For these wide ranges of applications, ultrastable Fabry–Pérot (FP) cavities, which act as the reference optical resonator, have got lots of attention. Designing such a FP cavity is a challenge due to the requirements of suppressing instabilities from all the thermo-mechanical noise sources at the highest degree. In this article, we report a comprehensive study to elucidate detailed design steps and associated numerical analysis procedures to build such a FP cavity. For this, we considered all sources of noises arising from: improper optical and opto-mechanical settings, ground vibrations transmitted to the cavity, deformations due to self-weight as it worsens the stability due to acoustic noise coupled to it, and temperature fluctuation. We present quantitative assessments of the instabilities for different shapes, geometries, and constituent materials of the cavities. In order to model the vibration-induced instability, we developed a novel forced-vibration based technique for accurate estimation of the instabilities, that reciprocates the experimental results reported in the literature, and the method is found to be superior compared to the other approaches, therefore estimating the overall cavity stability more accurately. We found in short term (¡ 10 s), the instability of the cavity is limited by temperature fluctuation, and in the long term (¿ 10 s), Brownian noise dominates the instability, therefore demands adequate precautions to reach the desired level of stability. Through systematic simulations, we have identified the best criteria for minimization of all these noises and obtain stabilities of 8.5×10−17 and 5.1×10−17 using dielectric and crystalline mirror coatings, respectively, which are similar to the best-reported results among its homologue cavities.

Molecular Vibrational Polariton Dynamics: What Can Polaritons Do?

ConspectusWhen molecular vibrational modes strongly couple to virtual states of photonic modes, new molecular vibrational polariton states are formed, along with a large population of dark reservoir modes. The polaritons are much like the bonding and antibonding molecular orbitals when atomic orbitals form molecular bonds, while the dark modes are like nonbonding orbitals. Because the polariton states are half-matter and half-light, whose energy is shifted from the parental states, polaritons are predicted to modify chemistry under thermally activated conditions, leading to an exciting and emerging field known as polariton chemistry that could potentially shift paradigms in chemistry. Despite several published results supporting this concept, the chemical physics and mechanism of polariton chemistry remain elusive. One reason for this challenge is that previous works cannot differentiate polaritons from dark modes. This limitation makes delineating the contributions to chemistry from polaritons and dark states difficult. However, this level of insight is critical for developing a solid mechanism for polariton chemistry to design and predict the outcome of strong coupling with any given reaction. My group addressed the challenge of differentiating the dynamics of polaritons and dark modes by ultrafast two-dimensional infrared (2D IR) spectroscopy. Specifically, (1) we found that polaritons can facilitate intra- and intermolecular vibrational energy transfer, opening a pathway to control vibrational energy flow in liquid-phase molecular systems, and (2) by studying a single-step isomerization event, we verified that indeed polaritons can modify chemical dynamics under strong coupling conditions, but in contrast, the dark modes behave like uncoupled molecules and do not change the dynamics. This finding confirmed the central concept of polariton chemistry: polaritons modify the potential energy landscape of reactions. The result also clarified the role of dark modes, which lays a critical foundation for designing cavities for future polariton chemistry. Aside from using 2D IR spectroscopy to study polariton chemistry, we also used the same technique to develop molecular polaritons into a potential quantum simulation platform. We demonstrated that polaritons have Rabi oscillations, and using a checkerboard cavity design, we showed that polaritons could have large nonlinearity across space. We further used the checkerboard polaritons to simulate coherence transfer and visualize it. A unidirectional coherence transfer was observed, indicating non-Hermitian dynamics. The highlighted efforts in this Account provide a solid understanding of the capability of polaritons for chemistry and quantum information science. I conclude this Account by discussing a few challenges for moving polariton chemistry toward being predictable and making the polariton quantum platform a complement to existing systems.

Particle conserving approach to ac-dc driven interacting quantum dots with superconducting leads

The combined action of a dc bias and a microwave drive on the transport characteristic of a superconductor-quantum dot-superconductor junction is investigated. To cope with time dependent nonequilibrium effects and interactions in the quantum dot, we develop a general formalism for the dynamics of the density operator based on a particle conserving approach to superconductivity. Without invoking a broken U(1) symmetry, we identify a dynamical phase connected to the coherent transfer of Cooper pairs across the junction. In the weak-coupling limit, we show that besides quasiparticle transport, proximity induced superconducting correlations manifest in anomalous pair tunneling involving the transfer of a Cooper pair. The resulting generalized master equation in presence of the microwave drive showcases the characteristic bichromatic response due to the combination of the ac Josephson effect and an ac voltage. Analytical expressions for all harmonics in the driving frequency of both the current and the reduced dot operator are given for arbitrary driving strength. For the net dc current, the resulting photon assisted processes give rise to rich current-voltage characteristics. In addition to photon assisted subgap transport, we find regions of total current inversion in the stability diagram. There, the junction acts as a pump with the net dc current flowing against the applied dc bias. The first harmonic of the current, being closely related to the nonlinear dynamic susceptibility of the junction, is discussed at finite applied dc bias.

On-site wave-wind observation and spectral investigation of dynamic behaviors for sea-crossing bridge during tropical cyclone

The dynamic behaviors of a sea-crossing bridge during a tropical cyclone event which passed through the bridge site in September 2015 are simulated using in-situ observation wave and wind data. The hourly observation data during the cyclone event at the bridge site including averaged wind speed, turbulence intensity, direction, significant wave height, mean period, spectral variations are recorded. The wave and wind spectra are analyzed from raw data and are fed to a frequency-domain framework for response modeling. The stochastic wave actions considering foundation geometry, diffraction effect, wave coherence and second-order quadratic wave force transfer functions are considered using boundary element methods and the wind actions including buffeting loads, aerodynamic stiffness and damping are modeled based on stationary buffeting theory. Results reveal that the maximal structural response occurs near the cyclone eye wall while the storm center is still 80–100 km away but experiences sudden drop when the cyclone eye reaches the bridge site. Deck responses are much more sensitive to wind, but wave dominates the tower and base force responses. Second-order wave load is found to be critical to responses because the sum-frequency wave force could undesirably coincide with the bridge eigenfrequencies.

Optical frequency transfer with below 10−21 uncertainty using a DFB–laser-based fiber Brillouin amplifier

Exploiting the outstanding performance of optical atomic clocks for improved timekeeping, relativistic geodesy, and fundamental physics beyond the standard model demands comparing distant state-of-the-art optical clocks. Interferometric optical fiber links have been demonstrated as an eminent method for such frequency comparisons over distances up to thousands of kilometers. However, for such distances, the optical fiber attenuation mandates signal amplification. Fiber Brillouin amplification (FBA) has been proven as an efficient amplification technique for coherent frequency transfer. Demonstrated FBA schemes have been designed based on costly narrow-linewidth pump lasers and analog pump-to-signal phase locking schemes. Furthermore, the high pump power requirement of these FBAs hinders the integration of FBA-based frequency dissemination on fiber connections for shared telecommunication signals in the C-band. In this paper, we propose and experimentally demonstrate a novel FBA module (FBAM) employing cost-effective distributed feedback (DFB) pump lasers assisted by a digital phase locking scheme based on a field programmable gated array. The new FBAM is compact, cost-effective, and directly applicable to different bands, which opens up new opportunities to establish a frequency metrology infrastructure within existing telecommunication fiber networks. Additionally, the small-footprint of the DFB-FBAM allows for frequent amplification stages with lower pump power to reach continental scale optical metrology links with an optimized signal-to-noise ratio. We characterized the DFB-FBAM’s frequency transfer uncertainty using a two-way layout over an in-lab 100 km long optical fiber link and reach a fractional frequency instability of 9.3 × 10−22 at a 10 ks integration time. The DFB-FBAM characterizations show uncertainty contributions of (−2.1 ± 3.3) × 10−22 and below for averaging times >100 ks. For the first time, we assess the temporal Brillouin frequency shift variations in an underground fiber link and implement a scheme to track these changes in a remote FBAM.