Circularly Polarized Laser Research Articles

Circularly Polarized Lasing from a Microcavity Filled with Achiral Single-Crystalline Microribbons.

Organic circularly polarized (CP) lasers have received increasing attention due to their future photoelectric applications. Here, we demonstrate a CP laser from a pure organic crystal-filled microcavity without any chiral molecules or chiral structures. Benefited from the giant anisotropy and excellent laser gain of organic crystals, optical Rashba-Dresselhaus spin-orbit coupling effect can be induced and is conductive to the CP laser in such microcavities. The maximum dissymmetry factor of the CP lasing with opposite helicities reachs 1.2. Our strategy may provide a new idea for the design of CP lasers towards future 3D laser displays, information storage and other fields.

Timescales and contribution of heating and helicity effect in helicity-dependent all-optical switching

The heating and helicity effects induced by circularly polarized laser excitation are entangled in the helicity-dependent all-optical switching (HD-AOS), which hinders understanding the magnetization dynamics involved. Here, applying a dual-pump laser excitation, first with a linearly polarized (LP) laser pulse followed by a circularly polarized (CP) laser pulse, the timescales and contribution from heating and helicity effects in HD-AOS were identified with a Pt/Co/Pt triple-layer. When the LP laser pulses preheat the sample to a nearly fully demagnetized state, the CP laser pulses with a power reduced by 80% switch the sample’s magnetization. By varying the time delay between the two pump pulses, the results show that the helicity effect, which gives rise to the deterministic helicity-induced switching, arises almost instantly within 200 fs close to the pulse width upon laser excitation. The results reveal that the transient magnetization state upon which CP laser pulses impinge is the key factor for achieving HD-AOS, and importantly, the tunability between heating and helicity effects with the unique dual-pump laser excitation approach will enable HD-AOS in a wide range of magnetic material systems having wide-ranging implications for potential ultrafast spintronics applications.Graphical abstract

Generation of single circularly polarized attosecond pulses from near-critical density plasma irradiated by a two-color co-rotating circularly polarized laser.

In this paper, a new method is proposed to efficiently generate a single intense attosecond pulse with circular polarization (CP) through the interaction of an intense driving laser with a near-critical density plasma target. The driving laser is composed of two co-rotating CP lasers with similar frequencies but different pulse widths. When the matching condition is satisfied, the combined field is modulated to a short intense pulse followed by a weak tail. The resulting laser falling edge becomes steeper than the initial sub-pulses, which induces a quick one-time oscillation of the target surface. Meanwhile, the tail guarantees the energy to be compressed simultaneously in both polarization directions to the same extent, so that a single CP attosecond pulse can be produced efficiently and robustly via our method, which has been confirmed through extensive numerical simulations. In addition, our method makes it possible to generate a single CP attosecond pulse even for multi-cycle pulses that are already available for existing laser systems. This provides a novel way to advance the investigation of chiral-sensitive light-matter interactions in attosecond scales.

Wavelength‐Tunable Circularly Polarized Laser Arrays for Multidimensional Information Encryption

AbstractInformation security has attracted particular attention along with the exponential growth of information communications. Stimuli‐responsive microlasers show considerable potential for anti‐counterfeiting and encryption technologies due to their distinguishable signal readout, superior responsiveness, and distinct polarization states, yet their application is frequently restricted by the low security level due to the visualization of information recorded solely in emission wavelength. Here, a novel strategy is proposed to realize high‐security multidimensional information encryption based on wavelength‐tunable circularly polarized (CP) microlaser arrays by introducing additional covert polarization states as coding elements. CP laser emissions are obtained in cholesteric liquid crystal (CLC) arrays due to well‐organized periodic helical superstructures, and the lasing wavelength is thermally tailored across a broad range. Based on the synergistic use of wavelength and polarization state as cryptographic primitives, the multidimensional information encryption and decryption in thermo‐responsive CP microlaser arrays is realized. These results provide valuable enlightenment to the development of stimuli‐responsive CP laser for information security and anti‐counterfeiting applications.

Ionization of magnesium atoms in femtosecond 400-nm laser fields

Atomic magnesium (Mg) is reported to ionize more efficiently in 400-nm laser fields with circular polarization (CP) than with linear polarization. This experimental result is reproduced qualitatively by numerical solutions of a two-electron time-dependent Schr\"odinger equation. Theoretical analyses show that intermediate excited states play an important role during the ionization process. The $3s3p$ state and the $3s3d$ state are identified as the dominant intermediate states. The main ionization pathway is identified to be $3{s}^{2}\ensuremath{\rightarrow}3s3p\ensuremath{\rightarrow}3s3d\ensuremath{\rightarrow}$ ionized states. CP laser fields are shown to be able to pump more population from the ground state to the $3s3d$ state, leading to more efficient ionization of the Mg atoms.

Formation of periodic nanostructures induced by circularly-polarized femtosecond laser

We investigated the formation of periodic nanostructures on GaN induced by circularly-polarized femtosecond laser pulses. The structure shape changed from spiral to dots structures with increasing the pulse number. The structure change explained the previous inconsistent results, and we suggest a hypothesis for the formation dynamics. The period of the dots structures was approximately 150 nm which is almost 1/7 of the laser wavelength, and it kept crystalline comparable to the original substrate. The laser-induced periodic surface structures are expected to apply as a new fine processing technology.

Ultra-fast polarization of a thin electron layer in the rotational standing-wave field driven by double ultra-intense laser pulses

We explore radiative polarization of electrons in a standing-wave formed by two circularly-polarized laser pulses irradiating a thin layer. Here the electron radiative spin dynamics in external electromagnetic fields is described by the generalized Sokolov–Ternov model implemented in the particle-in-cell simulations. We find that significant polarization is established in roughly one laser period from the circular motion in the standing wave. However, such motion is unstable at the magnetic nodes such that electrons migrate to different phases. The beam polarization is then transferred to transverse directions following the T-BMT precession and splits into two groups with opposite signs. The induced polarization distribution allows for filtering out electron population of high polarization purity via certain emitting angles and energies, approaching maximum of 78% polarization at light intensities of the order ∼1023 W cm−2.

Efficient valley polarization of charged excitons and resident carriers in Molybdenum disulfide monolayers by optical pumping

The roadmap of future innovative device developments foresees the reduction of material dimensions down to nanometer scale and the incorporation of novel degrees of freedom. For instance, electrons and holes in 2D semiconductors like MoS2 monolayers exhibit a unique coupling between the spin and the crystal momentum, also referred to as the valley. A crucial requirement for future applications is therefore the possibility to initialise the spin/valley degree of freedom in these materials. Here we investigate the optical initialisation of the valley degree of freedom in charge-tunable MoS2 monolayers encapsulated with hexagonal boron nitride at cryogenic temperatures. We report in photoluminescence a large steady state valley polarization of the different excitonic complexes following circularly-polarized laser excitation. We reveal efficient valley initialisation of positively-charged excitons, which have so far proved to be elusive in non-encapsulated monolayers due to defect and laser-induced large electron doping. We find that negatively-charged excitons present a polarization of 70% which is unusually large for non-resonant excitation. We attribute this large valley polarization to the particular band structure of MoS2. In addition, we demonstrate that circular excitation induces a dynamical polarization of resident electrons and holes––as recently shown in tungsten-based monolayers.

Theoretical Study of the Efficient Ion Acceleration Driven by Petawatt-Class Lasers via Stable Radiation Pressure Acceleration

Laser-driven radiation pressure acceleration (RPA) is one of the most promising candidates to achieve quasi-monoenergetic ion beams. In particular, many petawatt systems are under construction or in the planning phase. Here, a stable radiation pressure acceleration (SRPA) scheme is investigated, in which a circularly-polarized (CP) laser pulse illuminates a CH2 thin foil followed by a large-scale near-critical-density (NCD) plasma. In the laser-foil interaction, a longitudinal charge-separated electric field is excited to accelerate ions together with the heating of electrons. The heating can be alleviated by the continuous replenishment of cold electrons of the NCD plasma as the laser pulse and the pre-accelerated ions enter into the NCD plasma. With the relativistically transparent propagation of the pulse in the NCD plasma, the accelerating field with large amplitude is persistent, and its propagating speed becomes relatively low, which further accelerates the pre-accelerated ions. Our particle-in-cell (PIC) simulation shows that the SRPA scheme works efficiently with the laser intensity ranging from 6.85×1021 W cm−2 to 4.38×1023 W cm−2, e.g., a well-collimated quasi-monoenergetic proton beam with peak energy ∼1.2 GeV can be generated by a 2.74 × 1022 W cm−2 pulse, and the energy conversion efficiency from the laser pulse to the proton beam is about 16%. The QED effects have slight influence on this SRPA scheme.

Negative Gilbert damping

Modern ferromagnetism theory is based on the Landau-Lifshitz-Gilbert equation that governs the dynamics of magnetic moment in a ferromagnet. As a deep-rooted notion, the Gilbert damping parameter must be positive because it describes the (energy and angular momentum) dissipation. In this paper, we report a negative Gilbert damping via the magneto-optical interaction of three orthogonal circularly-polarized laser beams with a submicron magnet placed in an optical cavity. We show that the off-resonant coupling between the driving laser and cavity photon in the far-blue detuning can induce a magnetic torque exactly of the Gilbert type with negative damping coefficient. A hyperbolic-tangent function ansatz is found to well describe the time-resolved spin switching as the intrinsic dissipation is overcome. Feasible experiments and materials are discussed to test our theory.

Relativistic Effects for a Hydrogen Rydberg Atom in a High-Frequency Laser Field: Analytical Results

Previously published analytical results for the effects of a high-frequency laser field on hydrogen Rydberg atoms demonstrated that the unperturbed elliptical orbit of the Rydberg electron, generally is engaged simultaneously in the precession of the orbital plane about the direction of the laser field and in the precession within the orbital plane. These results were obtained while disregarding relativistic effects. In the present paper, we analyze the relativistic effect for hydrogenic Rydberg atoms or ions in a high-frequency linearly- or circularly-polarized laser field, the effect being an additional precession of the electron orbit in its own plane. For the linearly-polarized laser field, the general case, where the electron orbit is not perpendicular to the direction of the laser field, we showed that the precession of the electron orbit within its plane can vanish at some critical polar angle θc of the orbital plane. We calculated analytically the dependence of the critical angle on the angular momentum of the electron and on the parameters of the laser field. Finally, for the particular situation, where the electron orbit is perpendicular to the direction of the laser field, we demonstrated that the relativistic precession and the precession due to the laser field occur in the opposite directions. As a result, the combined effect of these two kinds of the precession is smaller than the absolute value of each of them. We showed that by varying the ratio of the laser field strength F to the square of the laser field frequency ω, one can control the precession frequency of the electron orbit and even make the precession vanish, so that the elliptical orbit of the electron would become stationary. This is a counterintuitive result.

High Harmonic Generation in Monolayer and Bilayer of Transition Metal Dichalcogenide

In transition metal dichalcogenides (TMDCs), charge carriers have spin, pseudospin, and valley degrees of freedom associated with magnetic moments. The monolayers and bilayers of the TMDCs, in particular, MoS2, lead to strong couplings between the spin and pseudospin effects. This feature has drawn attention to TMDCs for their potential use in advanced tech devices. Meanwhile, high-order harmonic generation (HHG) has recently been applied to the characterization of the electronic structure of solids, such as energy dispersion, Berry-curvature, and topological properties. Here, we show theoretical results obtained with the ‘philosophy’ of using HHG to investigate the structural effects of the monolayer and bilayers of MoS2 on nonlinear optical emission. We use a simple model for MoS2 in the 3R AB staking. We find that the pseudospin and valley indexes (the Berry curvature and the dipole transition matrix element) in TMDC driven by a circularly polarized laser (CPL) can encode in the high-energy photon emissions. This theoretical investigation is expected to pave the way for the ultrafast manipulation of valleytronics and lead to new questions concerning the spin-obit-coupling (SOC) effects on TMDC materials, Weyl Semimetals, and topological phases and transitions in topological insulators.

Pulse envelope effects in nonlinear Breit-Wheeler pair creation

The effect of the pulse envelope on electron-positron pair creation in a circularly-polarised laser pulse is investigated. Interference on the length scale of the pulse envelope, and smoothness of the pulse edges are found to influence the pair spectrum. A toy model of a flat-top pulse is used to identify pulse envelope effects inaccessible to local approaches. Broadening of channel openings and a widening of the energy and transverse momentum distribution of the pair are found to receive contributions that are below the local harmonic threshold. By comparing pair yields in a flat-top, sine-squared and Gaussian pulse, a link between pulse shape and photon-polarised Breit-Wheeler is found. In the transverse momentum distribution, a signal of pulse envelope interference is found in an azimuthal asymmetry, which appears in intense fields and persists in long pulses.

Formula omitted]-boson production via the weak process [formula omitted] in the presence of a circularly polarized laser field

In the centre of mass frame, we have studied theoretically the Z-boson resonant production in the presence of an intense laser field via the weak process e+e−→μ+μ−. Dressing the incident particles by a Circularly Polarized laser field (CP-laser field), at the first step, shows that for a given laser field’s parameters, the Z-boson cross section decreases by several orders of magnitude. We have compared the Total Cross Section (TCS) obtained by using the scattering matrix method with that given by the Breit-Wigner approach in the presence of a CP-laser field and the results are found to be very consistent. This result indicates that Breit-Wigner formula is valid not only for the laser-free process but also in the presence of a CP-laser field. The dependence of the laser-assisted differential cross section on the Centre of Mass Energy (CME) for different scattering angles proves that it reaches its maximum for small and high scattering angles. At the next step and by dressing both incident and scattered particles, we have shown that the CP-laser field largely affects the TCS, especially when its strength reaches 109Vcm−1. This result is interpreted by the fact that heavy interacting particles require high laser field intensities to affect the collision’s cross section.

All-Optical Probe of Three-Dimensional Topological Insulators Based on High-Harmonic Generation by Circularly Polarized Laser Fields.

We report the observation of an anomalous nonlinear optical response of the prototypical three-dimensional topological insulator bismuth selenide through the process of high-order harmonic generation. We find that the generation efficiency increases as the laser polarization is changed from linear to elliptical, and it becomes maximum for circular polarization. With the aid of a microscopic theory and a detailed analysis of the measured spectra, we reveal that such anomalous enhancement encodes the characteristic topology of the band structure that originates from the interplay of strong spin-orbit coupling and time-reversal symmetry protection. The implications are in ultrafast probing of topological phase transitions, light-field driven dissipationless electronics, and quantum computation.

Effects of Linewidth Broadening Method on Recoil of Sodium Laser Guide Star

The linewidth broadening of the circular-polarized continuous wave laser mitigates the recoil effects of the sodium laser guide star very well. By choice of the optimal laser linewidth, the relations between the laser intensity and average spontaneous emission rates are obtained. The numerically simulated results show that the 1–100 MHz linewidth broadening effectively weakens recoil and enhances the average spontaneous emission rates. For laser powers from 10 W to 60 W, considering the intensity distribution with random at the mesospheric sodium layer, when the laser linewidth is broadened to be 1–100 MHz from 0 MHz, increments of the return photons go up to 110% from 50% and do not have an effect on the spot sizes of the sodium laser guide star. Several cases have proven that the linewidth broadening method is correct. Further calculations show that the linewidth broadening method similarly applies to the case of the multi-mode laser. Furthermore, the linewidth broadening of re-pumping should be taken into account.

Circular dichroism metamirror with diversified chiral molecules combinations.

In this paper, we propose a giant circular dichroism (CD) chiral metamirror that differentially absorbs circularly polarized (CP) waves with dual/multi bands. The structure is composed of two variable τresonators with counter split opening directions and different eigenfrequencies. Even more important, these two resonators are aligned vertically, and then integrated into one periodical unit cell, which results in the high-efficiency absorptive CD. The chiral metamirror has a narrow-band absorption of the left-handed circularly polarized (LCP) wave for lower resonance, and a broadband absorption of the right-handed circularly polarized (RCP) wave in the higher-frequency range. Optical resonator designs with diversified chiral molecules combinations are further studied in detail. Through the study of different periodic arrangements, parameters scanning optimization, and power loss distributions, it is proved that the overall permutation symmetry breakdown of total combined chiral molecules is the determining factor for the absorptive CD. Based on the principle in the microwave model, two mid-infrared chiral metamirrors are further realized, which can still show good spin-dependent absorption. This multi-polarization and multifunction response advance novel photonic devices for a variety of applications including CP laser, biomolecules detection, and energy harvester.

Laser-driven anisotropic and nonlinear Rashba spin splitting in GaAs monolayer

We investigate the electronic properties of the buckled honeycomb GaAs monolayer under a periodically circularly polarized laser (CPL) by first-principles calculations and Floquet theory. Under the irradiation of CPL with propagation direction in the $xy$ plane, a remarkable Rashba spin splitting (RSS) is induced in the valence and conduction bands at $\mathrm{\ensuremath{\Gamma}}$ point. The laser-driven RSS deviates from the classical linear Rashba model in a wide range of wave vectors, and can be interpreted by a nonlinear model involving the third-order terms. The phase diagram of Rashba effect with laser amplitude and propagation direction is worked out. The anisotropy of the nonlinear RSS is mainly determined by the laser propagation direction, providing the possibility of controlling the Rashba anisotropy characteristics. By carefully choosing the laser frequency and amplitude, giant anisotropic RSS can be obtained, which allows the generation of spin-polarized electrons via doping and facilitates the flexibility of spintronic devices.

Signature of Molecular Orbital Symmetry in High-Order Harmonic Generation by Bichromatic Circularly Polarized Laser Pulses.

Molecular orbital symmetry is shown to be an important factor in determining orders and helicities (polarizations) of high-order harmonic generation (HHG) by intense femtosecond counter-rotating bichromatic circularly polarized laser pulses. Numerical solutions of time-dependent Schrödinger equations (TDSE) for the one-electron molecular ions H2+ and H32+ for different initial electronic states show that harmonic orders and helicities are dependent on orbital symmetries and of the net incident pulse electric field. The numerical results and properties of the harmonics are described by dynamical symmetry theory and time profile analysis of the high-order harmonics, thus confirming that orbital and laser pulse symmetry dependence are generic in HHG of molecules.

3D Laser Displays Based on Circularly Polarized Lasing from Cholesteric Liquid Crystal Arrays.

3D laser displays play an important role in next-generation display technologies owing to the ultimate visual experience they provide. Circularly polarized (CP) laser emissions, featuring optical rotatory power and invariability under rotations, are attractive for 3D displays due to potential in enhancing contrast ratio and comfortability. However, the lack of pixelated self-emissive CP microlaser arrays as display panels hinders the implementation of 3D laser displays. Here, full-color 3D laser displays are demonstrated based on CP lasing with inkjet-printed cholesteric liquid crystal (CLC) arrays as display panels. Individual CP lasers are realized by embedding fluorescent dyes into CLCs with their left-/right-handed helical superstructures serving as distributed feedback microcavities, bringing in ultrahigh circular polarization degree values (gem = 1.6). These CP microlaser pixels exhibit excellent far-field color-rendering features and a relatively large color gamut for high-fidelity displays. With these printed CLC red-green-blue (RGB) microlaser arrays serving as display panels, proof-of-concept full-color 3D laser displays are demonstrated via delivering images with orthogonal CP laser emissions into one's left and right eyes. These results provide valuable enlightenment for the development of 3D laser displays.