Polarization Lifetime Research Articles

Enhancing the Optically Detected Magnetic Resonance Signal of Organic Molecular Qubits.

In quantum information science and sensing, electron spins are often purified into a specific polarization through an optical-spin interface, a process known as optically detected magnetic resonance (ODMR). Diamond-NV centers and transition metals are both excellent platforms for these so-called color centers, while metal-free molecular analogues are also gaining popularity for their extended polarization lifetimes, milder environmental impacts, and reduced costs. In our earlier attempt at designing such organic high-spin π-diradicals, we proposed to spin-polarize by shelving triplet M S = ±1 populations as singlets. This was recently verified by experiments albeit with low ODMR contrasts of <1% at temperatures above 5 K. In this work, we propose to improve the ODMR signal by moving singlet populations back into the triplet M S = 0 sublevel, designing a true carbon-based molecular analogue to the NV center. Our proposal is based upon transition-orbital and group-theoretical analyses of beyond-nearest-neighbor spin-orbit couplings, which are further confirmed by ab initio calculations of a realistic trityl-based radical dimer. Microkinetic analyses point toward high ODMR contrasts of around 30% under experimentally feasible conditions, a stark improvement from previous works. Finally, in our quest toward ground-state optically addressable molecular spin qubits, we exemplify how our symmetry-based design avoids Zeeman-induced singlet-triplet mixings, setting the scene for realizing electron spin qubit gates.

Room‐Temperature Single‐Molecule Infrared Imaging and Spectroscopy through Bond‐Selective Fluorescence

AbstractInfrared (IR) spectroscopy stands as a workhorse for exploring bond vibrations, offering a wealth of chemical insights across diverse frontiers. With increasing focus on the regime of single molecules, obtaining IR‐sensitive information from individual molecules at room temperature would provide essential information about unknown molecular properties. Here, we leverage bond‐selective fluorescence microscopy, facilitated by narrowband picosecond mid‐IR and near‐IR double‐resonance excitation, for high‐throughput mid‐IR structural probing of single molecules. We robustly capture single‐molecule images and analyze the combined polarization dependence, vibrational peaks, linewidths, and lifetimes of probe molecules with representative scaffolds. From bulk to single molecules, we find that vibrational lifetimes remain consistent, while linewidths are narrowed by approximately twofold and anisotropy becomes more pronounced. Additionally, unexpected peak shifts from single molecules were observed, attributed to the generation of new modes due to previously unexplored dimerization, supported by quantum chemistry calculations. These findings underscore the importance of infrared analysis on individual single molecules in ambient environments, offering molecular information crucial for functional imaging and the investigation of the fundamental properties and utilities of luminescent molecules.

Room-Temperature Single-Molecule Infrared Imaging and Spectroscopy through Bond-Selective Fluorescence.

Infrared (IR) spectroscopy stands as a workhorse for exploring bond vibrations, offering a wealth of chemical insights across diverse frontiers. With increasing focus on the regime of single molecules, obtaining IR-sensitive information from individual molecules at room temperature would provide essential information about unknown molecular properties. Here, we leverage bond-selective fluorescence microscopy, facilitated by narrowband picosecond mid-IR and near-IR double-resonance excitation, for high-throughput mid-IR structural probing of single molecules. We robustly capture single-molecule images and analyze the combined polarization dependence, vibrational peaks, linewidths, and lifetimes of probe molecules with representative scaffolds. From bulk to single molecules, we find that vibrational lifetimes remain consistent, while linewidths are narrowed by approximately twofold and anisotropy becomes more pronounced. Additionally, unexpected peak shifts from single molecules were observed, attributed to the generation of new modes due to previously unexplored dimerization, supported by quantum chemistry calculations. These findings underscore the importance of infrared analysis on individual single molecules in ambient environments, offering molecular information crucial for functional imaging and the investigation of the fundamental properties and utilities of luminescent molecules.

Growth and spectral properties of Er3+/Yb3+/Ce3+:YPO4 crystal

Er3+/Yb3+/Ce3+ tir-doped YPO4 crystals were successfully synthesized by high temperature solution method with Pb2P2O7 as flux. The growth, structures and spectral characterizations, including polarization absorption spectra, polarization fluorescence spectra and lifetime, were systematically discussed. It has broad absorption band around 973 nm with a full width at half maximum of 20.76 nm and 14.71 nm for π- and σ-polarizations, respectively, which is suitable for efficient pumping of InGaAs laser diodes. Based on J-O theory, spectral parameters such as oscillator strength, line strength, J-O intensity, fluorescence branching ratio, spontaneous transition rate and radiation lifetime were calculated systematically. In the polarization fluorescence spectra, four strong emission peaks were observed at 1505, 1537, 1561 and 1586 nm. The results show that Er3+/Yb3+/Ce3+:YPO4 crystal may be a promising laser crystal suitable for 1.5–1.6 μm.

A research program to measure the lifetime of spin polarized fuel

The use of spin polarized fuel could increase the deuterium-tritium (D-T) fusion cross section by a factor of 1.5 and, owing to alpha heating, increase the fusion power by an even larger factor. Issues associated with the use of polarized fuel in a reactor are identified. Theoretically, nuclei remain polarized in a hot fusion plasma. The similarity between the Lorentz force law and the Bloch equations suggests polarization can be preserved despite the rich electromagnetic spectrum present in a magnetic fusion device. The most important depolarization mechanisms can be tested in existing devices. The use of polarized deuterium and 3He in an experiment avoids the complexities of handling tritium, while encompassing the same nuclear reaction spin-physics, making it a useful proxy to study issues associated with full D-T implementation. 3He fuel with 65% polarization can be prepared by permeating optically-pumped 3He into a shell pellet. Dynamically polarized 7Li-D pellets can achieve 70% vector polarization for the deuterium. Cryogenically-frozen pellets can be injected into fusion facilities by special injectors that minimize depolarizing field gradients. Alternatively, polarized nuclei could be injected as a neutral beam. Once injected, the lifetime of the polarized fuel is monitored through measurements of escaping charged fusion products. Multiple experimental scenarios to measure the polarization lifetime in the DIII-D tokamak and other magnetic-confinement facilities are discussed, followed by outstanding issues that warrant further study.

Mechanistic Insights into Cellular and Molecular Targets of Zinc Oxide Quantum Dots (ZnO QDs) in Fungal Pathogen, Candida albicans: One Drug Multi-Targeted Therapeutic Approach.

Rationally designed multitargeted drugs, known as network therapeutics/multimodal drugs, have emerged as versatile therapeutic solutions to combat drug-resistant microbes. Here, we report novel mechanistic insights into cellular and molecular targets of ZnO quantum dots (QDs) against Candida albicans, a representative of fungal pathogens. Stable, monodispersed 4-6 nm ZnO QDs were synthesized using a wet chemical route, which exhibited dose-dependent inhibition on the growth dynamics of Candida. Treatment with 200 μg/mL ZnO QDs revealed an aberrant morphology and a disrupted cellular ultrastructure in electron microscopy and led to a 23% reduction in ergosterol content and a 53% increase in intracellular reactive oxygen species. Significant increase in steady-state fluorescence polarization and fluorescence lifetime decay of membrane probe 1,6-diphenyl-1,3,5-hexatriene (DPH) in treated cells, respectively, implied reduction in membrane fluidity and enhanced microviscosity. The observed reduction in passive diffusion of fluorescent Rhodamine 6G across the membrane validated the intricate relationship between ergosterol, membrane fluidity, and microviscosity. An inverse relationship existing between ergosterol biosynthetic genes, ERG11 and ERG3 in treated cells, related well with displayed higher susceptibilities. Furthermore, treated cells exhibited impaired functionality and downregulation of ABC drug efflux pumps. Multiple cellular targets of ZnO QDs in Candida were validated by in silico molecular docking. Thus, targeting ERG11, ERG3, and ABC drug efflux pumps might emerge as a versatile, nano-ZnO-based strategy in fungal therapeutics to address the challenges of drug resistance.

Spin-Polarized Charge Separation at Two-Dimensional Semiconductor/Molecule Interfaces.

Spin-polarized electrons can improve the efficiency and selectivity of photo- and electro-catalytic reactions, as demonstrated in the past with magnetic or magnetized catalysts. Here, we present a scheme in which spin-polarized charge separation occurs at the interfaces of nonmagnetic semiconductors and molecular films in the absence of a magnetic field. We take advantage of the spin-valley-locked band structure and valley-dependent optical selection rule in group VI transition metal dichalcogenide (TMDC) monolayers to generate spin-polarized electron-hole pairs. Photoinduced electron transfer from WS2 to fullerene (C60) and hole transfer from MoSe2 to phthalocyanine (H2Pc) are found to result in spin polarization lifetimes that are 1 order of magnitude longer than those in the TMDC monolayers alone. Our findings connect valleytronic properties of TMDC monolayers to spin-polarized interfacial charge transfer and suggest a viable route toward spin-selective photocatalysis.

Simultaneously Regulated Highly Polarized and Long-Lived Valley Excitons in WSe2/GaN Heterostructures.

Interlayer excitons, with prolonged lifetimes and tunability, hold potential for advanced optoelectronics. Previous research on the interlayer excitons has been dominated by two-dimensional heterostructures. Here, we construct WSe2/GaN composite heterostructures, in which the doping concentration of GaN and the twist angle of bilayer WSe2 are employed as two ingredients for the manipulation of exciton behaviors and polarizations. The exciton energies in monolayer WSe2/GaN can be regulated continuously by the doping levels of the GaN substrate, and a remarkable increase in the valley polarizations is achieved. Especially in a heterostructure with 4°-twisted bilayer WSe2, a maximum polarization of 38.9% with a long lifetime is achieved for the interlayer exciton. Theoretical calculations reveal that the large polarization and long lifetime are attributed to the high exciton binding energy and large spin flipping energy during depolarization in bilayer WSe2/GaN. This work introduces a distinctive member of the interlayer exciton with a high degree of polarization and a long lifetime.

Achieving highly stable sodium metal batteries with self-adapting and high-ionic-mobility ceramic fiber membranes

Tough issues like sodium (Na) dendrite growth and poor anode reversibility hinder the practical application of sodium metal batteries (SMBs) with moderate liquid electrolytes. To settle these problems, using a smart self-adapting Al2SiO5 ceramic fiber (CF) membrane is demonstrated to enable homogeneous Na depositions and inhibit the dendritic growth. This inorganic membrane itself has superb thermal stability, high ionic mobility (Na+ transference number: 0.65) and electrolyte wettability over traditional glass fiber (GF) or polymeric ones, guaranteeing the low voltage polarization (14 mV) and long-cyclic lifetime (over 600 h) in symmetric cells testing. Notably, aluminous components in CF membranes would interact with F-based molecules in the electrolyte phase, thereby releasing some Al3+ species that can be electrochemically deposited onto the anodic interface. The packed (+)Na3V2(PO4)3|CF|Na(−) full SMBs exhibit far superior cyclic stability (capacity retention over 78.7 % after 600 cycles at 1C) than other counterparts. The in-situ detection/postmortem analysis reveal that Al/F-based inorganics formed in as-built SEI layers play a vital role in Na metal anode protection. This work may provide a viable strategy to overcome the constraints of high-energy SMBs in practical applications.

A new charge transfer pathway in the MoSe2-WSe2 heterostructure under the conditions of B-excitons being resonantly pumped.

Most transition metal dichalcogenide (TMD) heterostructures (HSs) exhibit a type II band alignment, leading to a charge transfer process accompanied by the transfer of spin-valley polarization and spontaneous formation of interlayer excitons. This unique band structure facilitates achieving a longer exciton lifetime and extended spin-valley polarization lifetime. However, the mechanism of charge transfer in type II TMD HSs is not fully comprehended. Here, the ultrafast charge transfer process is studied in MoSe2-WSe2 HS via valley-solved broadband pump-probe spectroscopy. Under the conditions of B-excitons of WSe2 and MoSe2 being resonantly pumped, a new charge transfer pathway through the higher energy state associated with the B-exciton is found. Meanwhile, the holes (electrons) in the WSe2 (MoSe2) layer of MoSe2-WSe2 HS produce obvious spin-valley polarization even under the condition of B-exciton of WSe2 (MoSe2) being resonantly pumped, and the lifetime can reach tens of ps, which is in stark contrast to the absence of A-exciton spin-valley polarization in monolayer WSe2 (MoSe2) under the same pumping condition. The results deepen the insight into the charge transfer process in type II TMD HSs and show the great potential of TMD HSs in the application of spin-valley electronics devices.

Bright circularly polarized photoluminescence in chiral layered hybrid lead-halide perovskites.

Hybrid perovskite semiconductor materials are predicted to lock chirality into place and encode asymmetry into their electronic states, while softness of their crystal lattice accommodates lattice strain to maintain high crystal quality with low defect densities, necessary for high luminescence yields. We report photoluminescence quantum efficiencies as high as 39% and degrees of circularly polarized photoluminescence of up to 52%, at room temperature, in the chiral layered hybrid lead-halide perovskites (R/S/Rac)-3BrMBA2PbI4 [3BrMBA = 1-(3-bromphenyl)-ethylamine]. Using transient chiroptical spectroscopy, we explain the excellent photoluminescence yields from suppression of nonradiative loss channels and high rates of radiative recombination. We further find that photoexcitations show polarization lifetimes that exceed the time scales of radiative decays, which rationalize the high degrees of polarized luminescence. Our findings pave the way toward high-performance solution-processed photonic systems for chiroptical applications and chiral-spintronic logic at room temperature.

Over 20% Carbon-13 Polarization of Perdeuterated Pyruvate Using Reversible Exchange with Parahydrogen and Spin-Lock Induced Crossing at 50 μT.

Carbon-13 hyperpolarized pyruvate is about to become the next-generation contrast agent for molecular magnetic resonance imaging of cancer and other diseases. Here, efficient and rapid pyruvate hyperpolarization is achieved via signal amplification by reversible exchange (SABRE) with parahydrogen through synergistic use of substrate deuteration, alternating, and static microtesla magnetic fields. Up to 22 and 6% long-lasting 13C polarization (T1 = 3.7 ± 0.25 and 1.7 ± 0.1 min) is demonstrated for the C1 and C2 nuclear sites, respectively. The remarkable polarization levels become possible as a result of favorable relaxation dynamics at the microtesla fields. The ultralong polarization lifetimes will be conducive to yielding high polarization after purification, quality assurance, and injection of the hyperpolarized molecular imaging probes. These results pave the way to future in vivo translation of carbon-13 hyperpolarized molecular imaging probes prepared by this approach.

Persistent room-temperature valley polarization in graphite-filtered WS2 monolayer

Transition metal dichalcogenide (TMD) monolayers (1L) in the 2H-phase are two-dimensional semiconductors with two valleys in their band structure that can be selectively populated using circularly polarized light. The choice of the substrate for monolayer TMDs is an essential factor for the optoelectronic properties and for achieving a high degree of valley polarization at room temperature (RT). In this work, we investigate the RT valley polarization of monolayer WS2 on different substrates. A degree of polarization of photoluminescence (PL) in excess of 27% is found from neutral excitons in 1L-WS2 on graphite at RT, under resonant excitation. Using chemical doping through photochlorination we modulate the polarization of the neutral exciton emission from 27% to 38% for 1L-WS2/graphite. We show that the valley polarization strongly depends on the interplay between doping and the choice of the supporting layer of TMDs. Time-resolved PL measurements, corroborated by a rate equation model accounting for the bright exciton population in the presence of a dark exciton reservoir support our findings. These results suggest a pathway towards engineering valley polarization and exciton lifetimes in TMDs, by controlling the carrier density and/or the dielectric environment at ambient conditions.

Temperature dependence of Cr:CYA effective emission cross-section

A high-quality Cr:CYA (Cr:CaYAlO4) single crystal fiber was grown by μ-PD (Micro-Pulling-Down) method. The E//a and E//c polarized excitation spectra of Cr:CYA at room temperature was measured. The crystal field parameters were calculated and the energy level structure of Cr:CYA was analyzed. We also measured the polarization emission spectra and lifetimes at different temperatures and calculated the effective emission cross-section. Based on the above data and Boltzmann law, the thermal distribution between the excited energy levels 2E and 4A2 at different temperatures was analyzed. The present work may be useful for guiding laser research on Cr:CYA and other Cr3+-doped laser crystals above room temperature.

Enigmatic color centers in microdiamonds with bright, stable, and narrow-band fluorescence

Microdiamonds with color centers have unique photophysical properties. We discover extremely narrow spectral lines with full width at half maximum down to 0.5 nm (\ensuremath{\approx}390 GHz at wavelength 630.15 nm) at room temperature in photoluminescence of numerous high-pressure--high-temperature microdiamonds. Correlation spectroscopy proves that these lines originate from single photon emitters. Comprehensive experimental characterization of the emitters is performed by means of complementary scanning electron and fluorescence microscopy that allows morphological characterization and in time monitoring of the same labeled emitters. Study of photoluminescence and photoluminescence excitation spectra of single emitters showed negligible phonon sideband and narrowband excitation. Temperature sensitivity, stable photoluminescence, brightness, linear polarization, and fluorescence lifetime \ensuremath{\approx}1.5--2.5 ns are demonstrated. The results show great potential of the discovered ``enigmatic'' emitters for applications in quantum optics, fluorescence labeling, and high-resolution thermometry.

Spin Hyperpolarization in Modern Magnetic Resonance.

Magnetic resonance techniques are successfully utilized in a broad range of scientific disciplines and in various practical applications, with medical magnetic resonance imaging being the most widely known example. Currently, both fundamental and applied magnetic resonance are enjoying a major boost owing to the rapidly developing field of spin hyperpolarization. Hyperpolarization techniques are able to enhance signal intensities in magnetic resonance by several orders of magnitude, and thus to largely overcome its major disadvantage of relatively low sensitivity. This provides new impetus for existing applications of magnetic resonance and opens the gates to exciting new possibilities. In this review, we provide a unified picture of the many methods and techniques that fall under the umbrella term "hyperpolarization" but are currently seldom perceived as integral parts of the same field. Specifically, before delving into the individual techniques, we provide a detailed analysis of the underlying principles of spin hyperpolarization. We attempt to uncover and classify the origins of hyperpolarization, to establish its sources and the specific mechanisms that enable the flow of polarization from a source to the target spins. We then give a more detailed analysis of individual hyperpolarization techniques: the mechanisms by which they work, fundamental and technical requirements, characteristic applications, unresolved issues, and possible future directions. We are seeing a continuous growth of activity in the field of spin hyperpolarization, and we expect the field to flourish as new and improved hyperpolarization techniques are implemented. Some key areas for development are in prolonging polarization lifetimes, making hyperpolarization techniques more generally applicable to chemical/biological systems, reducing the technical and equipment requirements, and creating more efficient excitation and detection schemes. We hope this review will facilitate the sharing of knowledge between subfields within the broad topic of hyperpolarization, to help overcome existing challenges in magnetic resonance and enable novel applications.

Conceptual design of DIII-D experiments to diagnose the lifetime of spin polarized fuel

In magnetic fusion experiments, the cross sections for the D-T and D-3He fusion reactions are increased by as much as 50% if the fuel remains spin polarized parallel to the magnetic field. The goal of this study is to assess the feasibility of lifetime measurements of spin polarization, in magnetic fusion relevant conditions, on the DIII-D tokamak using relative changes in charged fusion product (CFP) loss measurements that depend upon the differential fusion cross section dσ/dΩ. Relative measurements that capture changes in the escaping CFP pitch, poloidal, and energy distributions are studied in two realistic TRANSP calculated plasma scenarios: (a) vector-polarized 3He and D pellets are injected into a hot hydrogen plasma to produce thermonuclear reactions, and (b) a tensor-polarized deuterium pellet is injected into an L-mode hydrogen background plasma that includes unpolarized 3He neutral beam injection. Ideal CFP signals in both scenarios show substantial pitch sensitivity to polarization for 14.7 MeV proton detection at a poloidal angle of (on the outer wall in the ion direction), and strong sensitivity to polarization for 3.6 MeV alpha flux detection by an array of poloidal detectors. Energy-resolved measurements of 14.7 MeV protons are also sensitive to the degree of polarization for the port in the beam-plasma scenario. A realistic assessment of CFP signals in the thermonuclear scenario show count rates in the range of cps for pitch-resolved proton detection and cps for alpha flux measurements. Reduced chi-squared calculations show polarization lifetime measurements are feasible for either proton or alpha detection of both enhanced or suppressed polarization for the thermonuclear scenario. Measurements of gamma rays produced in the weak branch complement the CFP measurements.

Redfield Propagation of Photoinduced Electron Transfer Reactions in Vacuum and Solution.

Dynamical simulations of ultrafast electron transfer reactions are of utmost interest. To allow for energy dissipation directly into an external surrounding environment, a solvent coupling model has been deduced, implemented, and utilized to describe the photoinduced electron transfer dynamics within a model triad system herein. The model is based on Redfield theory, and the environment is represented by harmonic oscillators filled with bosonic quanta. To imitate real solvents, the oscillators have been equipped with frequencies and polarization lifetimes characteristic of the corresponding solvent. The population was found to transfer through the energetically lowest electron transfer route regardless of the medium. The condensed population transfer dynamics were observed to be highly dependent on the solvent parameters. In particular, an increase in the solvent coupling entailed a detainment in the population transfer from the initially prepared diabatic state and a promotion in the population transfer through the other electron transfer route. Two explanations based on the diagonal and off-diagonal matrix elements of the Kohn-Sham Fock matrix, respectively, have been provided. The lifetime of the populated partially charge-separated state was prolonged with increasing solvent polarity, and it was explained in terms of attractive interactions between the solvent's dipole moments and the fragments' charges. The high-frequency vibrational fine-structure in the correlation function was demonstrated to be important for the transfer dynamics, and the importance of dephasing effects in polar solvents was verified and precised to concern the optical polarization of the solvents.

Prolonging valley polarization lifetime through gate-controlled exciton-to-trion conversion in monolayer molybdenum ditelluride

Monolayer 2D semiconductors provide an attractive option for valleytronics due to valley-addressability. But the short valley-polarization lifetimes for excitons have hindered potential valleytronic applications. In this paper, we demonstrate a strategy for prolonging the valley-polarization lifetime by converting excitons to trions through efficient gate control and exploiting the much longer valley-polarization lifetimes for trions than for excitons. At charge neutrality, the valley lifetime of monolayer MoTe2 increases by a factor of 1000 to the order of nanoseconds from excitons to trions. The exciton-to-trion conversion changes the dominant depolarization mechanism from the fast electron-hole exchange for excitons to the slow spin-flip process for trions. Moreover, the degree of valley polarization increases to 38% for excitons and 33% for trions through electrical manipulation. Our results reveal the depolarization dynamics and the interplay of various depolarization channels for excitons and trions, providing an effective strategy for prolonging the valley polarization.

Spin-Valley Depolarization in van der Waals Heterostructures.

The appearance of van der Waals heterostructures offers a new solution to valleytronics. Here, we observe the spin-valley depolarization process of electrons and holes in type-II MoS2-WSe2 heterostructures simultaneously for the first time by valley-resolved broad-band femtosecond pump-probe experiments. The different depolarization paths between electrons and holes make them have different spin-valley polarization lifetimes. The spin-valley depolarization pathway of holes is mainly dominated by a phonon-assisted intervalley scattering process, while intra- and intervalley coupling can trigger additional depolarization pathways for electrons. The hole polarization lifetime can be further prolonged to more than three times in trilayer heterostructure 2MoS2-WSe2. For MoS2-WS2 that has strong orbital hybridization of Mo and W atoms, both electrons and holes lose the spin-valley polarization extremely soon after charge separation, behaving similarly to intraexcitons in a monolayer. Our work advances the basic understanding of spin-valley depolarization of van der Waals heterostructures and facilitates the effort toward longer lifetime valleytronic devices for information transfer and storage applications.