Chiral Selection Rules Research Articles

Highly-Polarized Emission Provided by Giant Optical Orientation of Exciton Spins in Lead Halide Perovskite Crystals.

Quantum technologic and spintronic applications require reliable material platforms that enable significant and long-living spin polarization of excitations, the ability to manipulate it optically in external fields, and the possibility to implement quantum correlations between spins, i.e., entanglement. Here it is demonstrated that these conditions are met in bulk crystals of lead halide perovskites. A giant optical orientation of 85% of excitons, approaching the ultimate limit of unity, in FA0.9Cs0.1PbI2.8Br0.2 crystals is reported. The exciton spin orientation is maintained during the exciton lifetime of 55ps resulting in high circular polarization of the exciton emission. The optical orientation is robust to detuning of the excitation energy up to 0.3eV above the exciton resonance and remains larger than 20% up to detunings of 0.9eV. It evidences pure chiral selection rules and suppressed spin relaxation of electrons and holes, even with large kinetic energies. The exciton and electron-hole recombinations are distinguished by means of the spin dynamics detected via coherent spin quantum beats in magnetic field. Further, electron-hole spin correlations are demonstrated through linear polarization beats after circularly polarized excitation. These findings are supported by atomistic calculations. All-in-all, the results establish lead halide perovskite semiconductors as suitable platform for quantumtechnologies.

The Interaction of 2D Materials With Circularly Polarized Light.

2D materials (2DMs), due to spin-valley locking degree of freedom, exhibit strongly bound exciton and chiral optical selection rules and become promising material candidates for optoelectronic and spin/valleytronic devices. Over the last decade, the manifesting of 2D materials by circularly polarized lights expedites tremendous fascinating phenomena, such as valley/exciton Hall effect, Moiré exciton, optical Stark effect, circular dichroism, circularly polarized photoluminescence, and spintronic property. In this review, recent advance in the interaction of circularly polarized light with 2D materials covering from graphene, black phosphorous, transition metal dichalcogenides, van der Waals heterostructures as well as small proportion of quasi-2D perovskites and topological materials, is overviewed. The confronted challenges and theoretical and experimental opportunities are also discussed, attempting to accelerate the prosperity of chiral light-2DMs interactions.

Chirality logic gates.

The ever-growing demand for faster and more efficient data transfer and processing has brought optical computation strategies to the forefront of research in next-generation computing. Here, we report a universal computing approach with the chirality degree of freedom. By exploiting the crystal symmetry-enabled well-known chiral selection rules, we demonstrate the viability of the concept in bulk silica crystals and atomically thin semiconductors and create ultrafast (<100-fs) all-optical chirality logic gates (XNOR, NOR, AND, XOR, OR, and NAND) and a half adder. We also validate the unique advantages of chirality gates by realizing multiple gates with simultaneous operation in a single device and electrical control. Our first demonstrations of logic gates using chiral selection rules suggest that optical chirality could provide a powerful degree of freedom for future optical computing.

Unveiling Weyl-related optical responses in semiconducting tellurium by mid-infrared circular photogalvanic effect

Elemental tellurium, conventionally recognized as a narrow bandgap semiconductor, has recently aroused research interests for exploiting Weyl physics. Chirality is a unique feature of Weyl cones and can support helicity-dependent photocurrent generation, known as circular photogalvanic effect. Here, we report circular photogalvanic effect with opposite signs at two different mid-infrared wavelengths which provides evidence of Weyl-related optical responses. These two different wavelengths correspond to two critical transitions relating to the bands of different Weyl cones and the sign of circular photogalvanic effect is determined by the chirality selection rules within certain Weyl cone and between two different Weyl cones. Further experimental evidences confirm the observed response is an intrinsic second-order process. With flexibly tunable bandgap and Fermi level, tellurium is established as an ideal semiconducting material to manipulate and explore chirality-related Weyl physics in both conduction and valence bands. These results are also directly applicable to helicity-sensitive optoelectronics devices.

Exciton fine structure splitting and linearly polarized emission in strained transition-metal dichalcogenide monolayers

We study theoretically the effects of an anisotropic elastic strain on the exciton energy spectrum fine structure and optical selection rules in atomically thin crystals based on transition-metal dichalcogenides. The presence of strain breaks the chiral selection rules at the $\mathbit{K}$ points of the Brillouin zone and makes optical transitions linearly polarized. The orientation of the induced linear polarization is related to the main axes of the strain tensor. Elastic strain provides an additive contribution to the exciton fine structure splitting, in agreement with experimental evidence obtained from the uniaxially strained ${\mathrm{WSe}}_{2}$ monolayer. The applied strain also induces momentum-dependent Zeeman splitting. Depending on the strain orientation and magnitude, Dirac points with a linear dispersion can be formed in the exciton energy spectrum. We provide a symmetry analysis of the strain effects and develop a microscopic theory for all relevant strain-induced contributions to the exciton fine structure Hamiltonian.

Lifetime measurements in 80Br and a new region for observation of chiral electromagnetic selection rules

Level lifetimes for the candidate chiral doublet bands of 80Br were extracted by means of the Doppler-shift attenuation method. The absolute transition probabilities derived from the lifetimes agree well with the M1 and E2 chiral electromagnetic selection rules, and are well reproduced by the triaxial particle rotor model calculations. Such good agreements among the experimental data, selection rules of chiral doublet bands and theoretical calculations are rare and outstanding in researches of nuclear chirality. Besides odd-odd Cs isotopes, odd-odd Br isotopes in the A≈ 80 mass region represent another territory that exhibits the ideal selection rules expected for chiral doublet bands.

Harmonic Generation in Low‐Dimensional Materials

AbstractLow‐dimensional materials (LDMs) provide an unprecedented avenue having the potential to disruptively revolutionize the information and communication technologies. The rise of nonlinear photonics in LDMs began about in 2009 and has now become an important research direction. While harmonic generation, a widely studied nonlinear optical effect, can be a powerful probe to low‐dimensional physics, it may also find applications in bioimaging, optical signal processing, and novel coherent light sources. In this work, the state‐of‐the‐art advances in harmonic generation are reviewed in a range of emerging LDMs. The criteria of chiral selection rules for the second and third harmonic generations are also provided. In particular, different strategies to tune and enhance harmonic generation in LDMs are discussed, including excitonic effects, interlayer twisting angle, electric field, and cavity resonance among others. It is believed that harmonic generation in LDMs will continue to grow, thus lying the basis for practical applications.

Chiral selection rules for multi-photon processes in two-dimensional honeycomb materials.

We examine the chirality-dependent optical selection rules in two-dimensional monolayer materials with honeycomb lattices, and, based on symmetry argument, we generalize these rules to multi-photon transitions of arbitrary orders. We also present the phase relations between incident and outgoing photons in such processes. The results agree nicely with our experimental observations of second- and third-harmonic generation. In particular, we demonstrate that the phase relation of chiral second-harmonic generation can serve as a handy tool for imaging domains and domain boundaries of these monolayers. Our results can benefit future studies on chirality-related optical phenomena and opto-electronic applications of such materials.

Symmetries, Conserved Properties, Tensor Representations, and Irreducible Forms in Molecular Quantum Electrodynamics

In the wide realm of applications of quantum electrodynamics, a non-covariant formulation of theory is particularly well suited to describing the interactions of light with molecular matter. The robust framework upon which this formulation is built, fully accounting for the intrinsically quantum nature of both light and the molecular states, enables powerful symmetry principles to be applied. With their origins in the fundamental transformation properties of the electromagnetic field, the application of these principles can readily resolve issues concerning the validity of mechanisms, as well as facilitate the identification of conditions for widely ranging forms of linear and nonlinear optics. Considerations of temporal, structural, and tensorial symmetry offer significant additional advantages in correctly registering chiral forms of interaction. More generally, the implementation of symmetry principles can considerably simplify analysis by reducing the number of independent quantities necessary to relate to experimental results to a minimum. In this account, a variety of such principles are drawn out with reference to applications, including recent advances. Connections are established with parity, duality, angular momentum, continuity equations, conservation laws, chirality, and spectroscopic selection rules. Particular attention is paid to the optical interactions of molecules as they are commonly studied, in fluids and randomly organised media.

Control of Exciton Valley Coherence in Transition Metal Dichalcogenide Monolayers.

The direct gap interband transitions in transition metal dichalcogenide monolayers are governed by chiral optical selection rules. Determined by laser helicity, optical transitions in either the K^{+} or K^{-} valley in momentum space are induced. Linearly polarized laser excitation prepares a coherent superposition of valley states. Here, we demonstrate the control of the exciton valley coherence in monolayer WSe_{2} by tuning the applied magnetic field perpendicular to the monolayer plane. We show rotation of this coherent superposition of valley states by angles as large as 30° in applied fields up to 9T. This exciton valley coherence control on the ps time scale could be an important step towards complete control of qubits based on the valley degree of freedom.

Well separated trion and neutral excitons on superacid treated MoS2 monolayers

Developments in optoelectronics and spin-optronics based on transition metal dichalcogenide monolayers (MLs) need materials with efficient optical emission and well-defined transition energies. In as-exfoliated MoS2 MLs, the photoluminescence (PL) spectra even at low temperature consist typically of broad, overlapping contributions from neutral, charged excitons (trions) and localized states. Here, we show that in superacid treated MoS2 MLs, the PL intensity increases by up to 60 times at room temperature. The neutral and charged exciton transitions are spectrally well separated in PL and reflectivity at T = 4 K, with linewidth for the neutral exciton of 15 meV, but both transitions have similar intensities compared to the ones in as-exfoliated MLs at the same temperature. Time resolved experiments uncover picoseconds recombination dynamics analyzed separately for charged and neutral exciton emissions. Using the chiral interband selection rules, we demonstrate optically induced valley polarization for both complexes and valley coherence for only the neutral exciton.

Valley dynamics probed through charged and neutral exciton emission in monolayerWSe2

Optical interband transitions in monolayer transition metal dichalcogenides such as WSe2 and MoS2 are governed by chiral selection rules. This allows efficient optical initialization of an electron in a specific K-valley in momentum space. Here we probe the valley dynamics in monolayer WSe2 by monitoring the emission and polarization dynamics of the well separated neutral excitons (bound electron hole pairs) and charged excitons (trions) in photoluminescence. The neutral exciton photoluminescence intensity decay time is about 4ps, whereas the trion emission occurs over several tens of ps. The trion polarization dynamics shows a partial, fast initial decay within tens of ps before reaching a stable polarization of about 20%, for which a typical valley polarization decay time larger than 1ns can be inferred. This is a clear signature of stable, optically initialized valley polarization.

Carrier and Polarization Dynamics in MonolayerMoS2

In monolayer MoS2, optical transitions across the direct band gap are governed by chiral selection rules, allowing optical valley initialization. In time-resolved photoluminescence (PL) experiments, we find that both the polarization and emission dynamics do not change from 4 to 300K within our time resolution. We measure a high polarization and show that under pulsed excitation the emission polarization significantly decreases with increasing laser power. We find a fast exciton emission decay time on the order of 4ps. The absence of a clear PL polarization decay within our time resolution suggests that the initially injected polarization dominates the steady-state PL polarization. The observed decrease of the initial polarization with increasing pump photon energy hints at a possible ultrafast intervalley relaxation beyond the experimental ps time resolution. By compensating the temperature-induced change in band gap energy with the excitation laser energy, an emission polarization of 40% is recovered at 300K, close to the maximum emission polarization for this sample at 4K.

Baryon fields with UL(3)×UR(3)chiral symmetry. IV. Interactions with chiral(8,1)⊕(1,8)vector and axial-vector mesons and anomalous magnetic moments

We construct all SU_L(3)*SU_R(3) chirally invariant anomalous magnetic, i.e. involving a Pauli tensor and one-derivative, interactions of one chiral-[(8,1)+(1,8)] meson field with chiral-[(6,3)+(3,6)], [(3,\bar3)+(\bar3,3)], and [(8,1)+(1,8)] baryon fields and their "mirror" images. We find strong chiral selection rules: e.g. there is only one off-diagonal chirally symmetric anomalous magnetic interaction between J=1/2 fields belonging to the [(6,3)+(3,6)] and the [(3,\bar3)+(\bar3,3)] chiral multiplets. We also study the chiral selection rules for the anomalous magnetic interactions of the [(3,\bar3)+(\bar3,3)] and the [(8,1)+(1,8)] baryon fields. Again, no diagonal and only one off-diagonal chiral SU_L(3)*SU_R(3) interaction of this type is allowed, that turns out also to conserve the U_A(1) symmetry. We calculate the F/D ratios for the baryons' anomalous magnetic moments predicted by these interactions in the SU(3) symmetry limit and find that only the [(6,3)+(3,6)]-[(3,\bar3)+(\bar3,3)] one, reproduces F/D=1/3, in close proximity to the value extracted from experiment.

Partner bands of 126Cs – first observation of chiral electromagnetic selection rules

The lifetimes of the excited states belonging to the chiral partner bands built on the πh11/2⊗νh11/2−1 configuration in 126Cs have been measured using the DSA technique. For the first time the large set of the experimental transition probabilities is in qualitative agreement with all selection rules predicted for the strong chiral symmetry breaking limit.The selection rules originate from two general features of a chiral nucleus, namely, from the existence of well separated left- and right-handed systems built of three angular momentum vectors and extra symmetries appearing in addition to the chiral symmetry breaking.The B(M1) staggering resulting from these additional symmetries is sensitive to triaxiality of odd–odd nuclei as well as configuration of valence particles.

Quantum electrodynamics in two dimensions at a finite-temperature thermofield bosonization approach**The authors dedicate this work to the memory of Jorge André Swieca.

The Schwinger model at finite temperature is analyzed using the thermofield dynamics formalism. The operator solution due to Lowenstein and Swieca is generalized to the case of finite temperature within the thermofield bosonization approach. The general properties of the statistical–mechanical ensemble averages of observables in the Hilbert subspace of gauge invariant thermal states are discussed. The bare charge and chirality of the Fermi thermofields are screened, giving rise to an infinite number of mutually orthogonal thermal ground states. One consequence of the bare charge and chirality selection rule at finite temperature is that there are innumerably many thermal vacuum states with the same total charge and chirality of the doubled system. The fermion charge and chirality selection rules at finite temperature turn out to imply the existence of a family of thermal theta-vacua states parametrized with the same number of parameters as in the zero temperature case.

Determination of Photoswitching Dynamics through Chiral Mapping of Single Molecules Using a Scanning Tunneling Microscope

Single-molecule-resolved scanning tunneling microscopy of tetra-tert-butyl azobenzene (TTB-AB) molecules adsorbed onto Au(111) reveals chirality selection rules in their photoswitching behavior. This observation is enabled by the fact that trans-TTB-AB molecules self-assemble into homochiral domains. Cis-TTB-AB molecules produced via photoisomerization are found in two distinct conformations with final state chirality determined by the initial trans isomer chirality. Based on these observations and ab initio calculations, we propose a new inversion-based dynamical photoswitching mechanism for azobenzene molecules at a surface.

Non-perturbative renormalization of lattice QCD

In this talk I will discuss a number of approaches designed to deal with the problem of setting up a fully non-perturbative renormalization procedure in lattice QCD. Methods based on Ward-Takahashi identities on hadronic states, on imposing chiral selection rules on amplitudes with external quark/gluon legs, on the use of the Schrödinger functional and on “heating and cooling” Monte Carlo steps are reviewed. I conclude with some remarks on the possibility of defining next order terms (higher twists, “condensates”,...) in short distance expansions.

QCD selection rules in polarized hadron collisions

Plans are underway to measure spin asymmetries at large momentum transfer in hadron hadron collisions at RHIC and elsewhere. Proposals have focused on measuring quark transversity and quark and gluon helicity distributions in the nucleon. These experiments will also provide a strong and simple test of perturbative QCD, namely that A TT/ A LL ⪡ 1 in pp → 2 jets + X and several related processes, whereas A TT/ A LL ∼ 1 in Drell-Yan production of muon pairs. The prediction tests the helicity, twist and chirality selection rules of perturbative QCD that form the foundation for the analysis of spin dependent hard processes. We estimate the ratio A TT/ A LL numerically for polarized protons at RHIC.

Chirality selection rules and the U(1) problem

The conventional theory for the lack of U(1) chirality invariance in the presence of topological charge v fails to resolve the U(1) problem: it involves selection rules which forbid chirality changes ± 2 needed to generate the charged-pion decay constant F π . Possible remedies are considered; if they work. F 2 π is proportional to the average value of v 2 per unit Euclidean volume.