Off-resonant Light Research Articles

Floquet engineering of interparticle correlations in electron-hole few-body system for strong radial confinement

We investigate the effects of off-resonant THz-frequency laser light coupling to bound few-body electron-hole system, i.e. the exciton and negatively charged trion confined in quantum wire. To solve this problem, we first conduct a unitary Hennerberger-Kramers transformation of the Hamiltonian and diagonalize its perturbative approximation to obtain the exciton and trion Floquet states. Within this framework, the light-matter coupling renormalizes an attractiveehinteraction, leaving the repulsiveeeunchanged, thus modifying corresponding two-particle correlation energies. Generally, the correlation energy ofehwould exceed theeeone for a semiconductor material with strongly localized heavy holes. However, as the former is weakened by increasing laser intensity, this relation can be reversed. Consequently, the trion may dissociate unconventionally, the hole gradually decouples from still strongly interacting electrons, and adequate energy and optical spectra changes accompany this process. The energy levels of the exciton and trion Floquet states are raised, while their optical brightness smoothly decreases for stronger laser intensities. We also show this process can be further modified by breaking the mirror symmetry of wire with a static electric field, and then the occurrence of the avoided crossings between the lowest energy levels of the trion depends on the laser intensity. These anticrossings shall be observed experimentally, confirming thus the usefulness of Floquet engineering for fast manipulations of the few-particle states in electron-hole systems on a subpicosecond time scale.

Photo- and exchange-field controlled spin and valley polarized transport in a normal/antiferromagnetic/normal (N/AF/N) junction based on transition metal dichalcogenides

Abstract We theoretically investigate spin- and valley-polarized transport within a normal/antiferromagnetic/normal (N/AF/N) junction based on transition metal dichalcogenides (TMDs), under the influence of off-resonance circularly polarized light and gate voltage. Antiferromagnetism modulates spin states and the effective gap, reducing the spin gap for one state while increasing it for the opposite, resulting in a broad spin polarization and a controlled gap. Off-resonance circularly polarized light adjusts the valley degree of freedom and the effective gap, providing a wide range of valley polarization. Harnessing the strong spin- orbit coupling in TMDs enables perfect spin-valley polarization in the proposed junction across a wide range of Fermi energies through AF and/or off-resonance light manipulation. AF manipulation effectively narrows the band gap of TMDs at lower light energies, enhancing potential applications of the proposed junction for spin-valley filtering.

Orbital angular momentum sensing of composite vortex light in a single-layer graphene system

This paper explores the impact of orbital angular momentum (OAM) in composite vortex light on the absorption and dispersion characteristics of a weak probe light interacting with a single-layer graphene system. Through systematic investigation, we demonstrate the exceptional control achievable over absorption and dispersion profiles by manipulating the OAM of light. Under resonance conditions for the probe light, transparent regions emerge in the spatial profile of probe absorption, and the number of these transparent regions can be precisely regulated by adjusting the OAM number of the composite vortex light. Conversely, in the case of off-resonance probe light, amplified regions surface in the absorption spectrum, with the number of these regions controllable by the OAM state of the composite vortex light. These findings hold significant implications for optical communication systems, offering a valuable tool for the detection and measurement of the OAM number of composite vortex light, and paving the way for advancements in tailored signal processing and communication technologies.

Sculpting ultrastrong light-matter coupling through spatial matter structuring.

The central theme of cavity quantum electrodynamics is the coupling of a single optical mode with a single matter excitation, leading to a doublet of cavity polaritons which govern the optical properties of the coupled structure. Especially in the ultrastrong coupling regime, where the ratio of the vacuum Rabi frequency and the quasi-resonant carrier frequency of light, ΩR/ω c, approaches unity, the polariton doublet bridges a large spectral bandwidth 2ΩR, and further interactions with off-resonant light and matter modes may occur. The resulting multi-mode coupling has recently attracted attention owing to the additional degrees of freedom for designing light-matter coupled resonances, despite added complexity. Here, we experimentally implement a novel strategy to sculpt ultrastrong multi-mode coupling by tailoring the spatial overlap of multiple modes of planar metallic THz resonators and the cyclotron resonances of Landau-quantized two-dimensional electrons, on subwavelength scales. We show that similarly to the selection rules of classical optics, this allows us to suppress or enhance certain coupling pathways and to control the number of light-matter coupled modes, their octave-spanning frequency spectra, and their response to magnetic tuning. This offers novel pathways for controlling dissipation, tailoring quantum light sources, nonlinearities, correlations as well as entanglement in quantum information processing.

Pseudospin collapse, multidirectional supercollimations, and all-electrons transmission and reflection in irradiated 8-Pmmn borophene

Propagation of ballistic electrons shows various optical-like phenomena. Here, we demonstrate a flexible method to modulate the band structure and manipulate the electron beams propagation in 8-Pmmn borophene by an off-resonant linearly polarized light. It is proposed to form fully tunable anisotropic dispersion by changing the polarization direction of the off-resonant light in an experimentally controllable way. Accompanied with it, the pseudospin symmetry of the electronic state in 8-Pmmn borophene collapses from a helical form into x or y direction, which undergoes a dramatic alteration. As a result of the wedge-shaped dispersions, the electron wave packet can be guided to propagate with undistorted shape along different directions, multidirectional electron supercollimations are exhibited in the system. Moreover, by constructing the optical sensing n–p and n–p–n junctions, interesting transport phenomena such as all-electrons Klein tunneling and omnidirectional reflection are realized by modulating the illumination parameters of the off-resonant light, both of them are independent of the incident energy and wave vector. It is expected that the peculiar transport properties in 8-Pmmn borophene modified by the off-resonant light field can offer more opportunities for device applications in valleytronics and electron-optics.

Quantum-Enhanced Magnetometry at Optimal Number Density.

We study the use of squeezed probe light and evasion of measurement backaction to enhance the sensitivity and measurement bandwidth of an optically pumped magnetometer (OPM) at sensitivity-optimal atom number density. By experimental observation, and in agreement with quantum noise modeling, a spin-exchange-limited OPM probed with off-resonance laser light is shown to have an optimal sensitivity determined by density-dependent quantum noise contributions. Application of squeezed probe light boosts the OPM sensitivity beyond this laser-light optimum, allowing the OPM to achieve sensitivities that it cannot reach with coherent-state probing at any density. The observed quantum sensitivity enhancement at optimal number density is enabled by measurement backaction evasion.

Toward improved quantum simulations and sensing with trapped two-dimensional ion crystals via parametric amplification

Improving coherence is a fundamental challenge in quantum simulation and sensing experiments with trapped ions. Here we discuss, experimentally demonstrate, and estimate the potential impacts of two different protocols that enhance, through motional parametric excitation, the coherent spin-motion coupling of ions obtained with a spin-dependent force. The experiments are performed on 2D crystal arrays of approximately one hundred $^9$Be$^+$ ions confined in a Penning trap. By modulating the trapping potential at close to twice the center-of-mass mode frequency, we squeeze the motional mode and enhance the spin-motion coupling while maintaining spin coherence. With a stroboscopic protocol, we measure $5.4 \pm 0.9$ dB of motional squeezing below the ground-state motion, from which theory predicts a $10$ dB enhancement in the sensitivity for measuring small displacements using a recently demonstrated protocol [Science $\textbf{373}$, 673 (2021)]. With a continuous squeezing protocol, we measure and accurately calibrate the parametric coupling strength. Theory suggests this protocol can be used to improve quantum spin squeezing, limited in our system by off-resonant light scatter. We illustrate numerically the trade-offs between strong parametric amplification and motional dephasing in the form of center-of-mass frequency fluctuations for improving quantum spin squeezing in our set-up.

Probing topological signatures in an optically driven α−T3 lattice

The $\alpha$-$T_3$ lattice, an interpolation model between the honeycomb lattice of graphene($\alpha=0$) and the dice lattice($\alpha=1$), undergoes a topological phase transition across $\alpha=1/\sqrt{2}$ when exposed to a circularly polarized off-resonant light. We study Berry phase mediated bulk magnetic and anomalous thermoelectric responses in order to capture the topological signatures of a driven $\alpha$-$T_3$ lattice. It is revealed that both the Berry curvature and the orbital magnetic moment associated with the flat band change their respective signs across $\alpha=1/\sqrt{2}$. The off-resonant light distorts the flat band near the Dirac points when $0<\alpha<1$ which eventually introduces two distinct well separated forbidden gaps of equal width in the quasienergy spectrum. The orbital magnetization varies linearly with the chemical potential, in the forbidden gaps. The slopes of the linear regions in the orbital magnetization are closely related to the respective Chern numbers on either side of $\alpha=1/\sqrt{2}$. We find that the slope for $\alpha>1/\sqrt{2}$ is approximately two times of that for $\alpha<1/\sqrt{2}$ which essentially indicates a topological phase transition across $\alpha=1/\sqrt{2}$. However, the anomalous Nernst coefficient vanishes when the chemical potential is tuned in the forbidden gaps. The anomalous Hall conductivity in the forbidden gap(s) approaches different quantized values on either side of $\alpha=1/\sqrt{2}$. All these topological signatures can be observed experimentally.

X-ray scattering exhibits time-resolved view of rhodopsin activation.

Visual rhodopsin is an excellent model system to understand the spatio-temporal dynamics of class-A G-protein-coupled receptors (GPCRs). We aim to understand the mechanistic details of how the changes in retinal cofactor propagate to become protein-wide conformational changes over large orders of time. Our hypothesis was that a volumetric expansion occurs in the light-activation process [1]. Here we show the first time-resolved X-ray scattering-based detection of the expansion of rhodopsin leading to the signaling state. We conducted time-resolved small-angle and wide-angle scattering (TR-SAXS and TR-WAXS, respectively) of rhodopsin solubilized in CHAPS detergent at the BioCARS synchrotron beamline (Advanced Photon Source). The photochemistry was initiated with a green (527-nm) laser and probed with X-rays at time delays between 10 ns to 128 ms to collect small and wide-angle difference scattering profiles. Our controls were the retinal-free apoprotein opsin pumped with either green (527-nm) or red (650-nm) light, and the rhodopsin holoprotein pumped with off-resonance red light (650 nm). Optical (pump) laser power titrations were conducted to understand the how visible light modulates the conformational changes following the activation. The absence of difference scattering signals in both controls confirmed the capture of structural information from excitation of the retinal chromophore. Our analysis shows time-dependent increases in the radius of gyration and the volume of the light-activated state versus the inactive dark state which are important for effector proteins to function in visual signaling. The solvent heating found in the high-q region (2-2.5Å−1) (TR-WAXS) was used to quantify time-resolved temperature changes in the protein. The analysis showed that multiple photons are absorbed by rhodopsin, which we attribute to sequential absorption by intermediates in the photocycle as the protein traverses its conformational landscape. [1] S.M.D.C. Perera et al. (2018) J.Phys.Chem.Lett. 9, 7064−7071.

Optomechanical Hot-Spots in Metallic Nanorod-Polymer Nanocomposites.

Plasmonic coupling between adjacent metallic nanoparticles can be exploited for acousto-plasmonics, single-molecule sensing, and photochemistry. Light absorption or electron probes can be used to study plasmons and their interactions, but their use is challenging for disordered systems and colloids dispersed in insulating matrices. Here, we investigate the effect of plasmonic coupling on optomechanics with Brillouin light spectroscopy (BLS) in a prototypical metal-polymer nanocomposite, gold nanorods (Au NRs) in polyvinyl alcohol. The intensity of the light inelastically scattered on thermal phonons captured by BLS is strongly affected by the wavelength of the probing light. When light is resonant with the transverse plasmons, BLS reveals mostly the normal vibrational modes of single NRs. For lower energy off-resonant light, BLS is dominated by coupled bending modes of NR dimers. The experimental results, supported by optomechanical calculations, document plasmonically enhanced BLS and reveal energy-dependent confinement of coupled plasmons close to the tips of NR dimers, generating BLS hot-spots. Our work establishes BLS as an optomechanical probe of plasmons and promotes nanorod-soft matter nanocomposites for acousto-plasmonic applications.

Topological Kerr effect in the graphene family materials.

Materials belonging to the graphene family are two-dimensional staggered monolayers that undergo topological phase transitions under the influence of an external electric field or off-resonant optical field. Inspired by the interplay between topological matter and the helicity of photons, we investigate various topological quantum phases of the graphene family materials (GFMs), when subject to an external electric field and irradiated by off-resonant light. Using the Kubo formalism, we derive analytic expressions of the valley and spin-resolved conductivities of silicene. We then show that the topological quantum phase transitions can be modulated by an external electric field or irradiating circularly polarized light on the surface. Based on a general beam propagation model, we theoretically investigate the transitional Kerr rotations in silicene in different phases. Our results identify topological phases where Kerr rotations and ellipticity can be maximized. We believe that our results are helpful for developing novel practical devices based on the Kerr effect of silicene.

Dynamics of optomechanical droplets in a Bose-Einstein condensate

We investigate numerically a one-dimensional Bose-Einstein condensate illuminated by off-resonant laser light which is retroreflected by a single feedback mirror. Studying the ground states of the system, we find density structures which are self-trapped via the optomechanical action of the diffracted light. We show that these structures are stable and exhibit Newtonian dynamics. We propose that these results allow continuous, nondestructive monitoring of condensate dynamics via the optical intensity and may offer opportunities for optical control and transport of coherent matter via gradients in optical phase alone.

Photo- and exchange-field controlled line-type resonant peaks and enhanced spin and valley polarizations in a magnetic WSe2 junction

Existing resonant tunneling modes in the shape of line-type resonances can improve the transport properties of the junction. Motivated by the unique structural properties of monolayer WSe2 e.g. significant spin–orbitcoupling and large direct band gap, the transport properties of a normal/ferromagnetic/normal WSe2 junction with large incident angles in the presence of exchange field (h), off-resonance light () and gate voltage (U) is studied. In a certain interval of U, the transmission shows a gap with optically controllable width, while outside it, the spin and valley resolved transmissions have an oscillatory behavior with respect to U. By applying (h), an optically (electrically) switchable perfect spin and valley polarizations at all angles of incidence have been found. For large incident angles, the transmission resonances change to spin-valley-dependent separated ideal line-type resonant peaks with respect to U, results in switchable perfect spin and valley polarizations, simultaneously. Furthermore, even in the absence of U, applying h or at large incident angles can give some spin-valley dependent ideal transmission peaks, making h or a transmission valve capable of giving a switchable fully spin-valley filtering effect. These findings suggest some alternate methods for providing high efficiency spin and valley filtering devices based on WSe2.

Spin-valley dependent Klein tunneling and perfect spin- and valley-polarized transport in a magnetic WSe2 superlattice

We study spin and valley transport through a magnetic ${\mathrm{WSe}}_{2}$ superlattice in the presence of off-resonance circularly polarized light and gate voltage. It is demonstrated by using appropriate values of off-resonance light on the ferromagnetic region that the junction becomes gapless with a linear dispersion spectrum for each spin-valley flavor. Therefore, for normal incidence, the junction is totally transparent ($T=1$), a manifestation of spin-valley dependent Klein tunneling. By increasing the number of barriers, we find optically and electrically tunable spin-valley dependent transmission gaps and transmission resonances, which are also confirmed using the Bloch theorem. Consequently, a perfect spin- and valley-polarized current can be obtained, which is tunable and switchable using an exchange field, off-resonance light, and gate voltage. Our findings show the potential applications of the proposed superlattices for the spin-valley polarization and the spin-valley filtering of ${\mathrm{WSe}}_{2}$-based devices.

Ultrafast polarization control in κ−(BEDT−TTF)2X

We present a theoretical investigation of the dynamics induced by higher-frequency off-resonant light pulse excitation in the metallic phase of $\ensuremath{\kappa}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{2}X$ [where BEDT-TTF is bis(ethylenedithio)-tetrathiafulvalene and $X$ represents a counteranion] by solving the time-dependent Schr\"odinger equation numerically in the quarter-filled extended Hubbard model for the material. If the magnitude of the applied light pulse exceeds a critical value, the transition to the charge-ordered (CO) state is then induced by the light pulse excitation. In the CO state, one site forming a dimer becomes charge rich, the other site becomes charge poor, and the intradimer electric dipole moments are aligned in a particular direction. The transition is driven by photoinduced reduction of the effective transfer integrals and can therefore be regarded as a dynamical localization-related phenomenon. There are two degenerate A phase and B phase CO states that have opposite polarization directions. The photogenerated CO state is given by the superposition of the A phase and B phase CO states with equal weights, and the polarization of the photogenerated CO state is effectively zero. Transitions to either of the two polarized CO states can be induced selectively, and the polarization can be generated by excitation using light and terahertz pulses. Furthermore, the polarization can be reversed on a timescale of several tens of femtoseconds by excitation using double light and terahertz pulses, which induce the following phase transitions: metal $\ensuremath{\rightarrow}$ polarized CO state of the A (B) phase $\ensuremath{\rightarrow}$ metal $\ensuremath{\rightarrow}$ polarized CO state of the B (A) phase $\ensuremath{\rightarrow}$ metal.

Light-modulated electron retroreflection and Klein tunneling in a graphene-based n–p–n junction

We investigate the electron retroreflection and the Klein tunneling across a graphene-based n–p–n junction irradiated by linearly polarized off-resonant light with the polarization along the x direction. The linearly polarized off-resonant light modifies the band structure of graphene, which leads to the anisotropy of band structure. By adjusting the linearly polarized light and the direction of n–p–n junction simultaneously, the electron retroreflection appears and the anomalous Klein tunneling, the perfect transmission at a nonzero incident angle regardless of the width and height of potential barrier, happens, which arises from the fact that the light-induced anisotropic band structure changes the relation of wavevector and velocity of electron. Our finding provides an alternative and flexible method to modulate electron retroreflection and Klein tunneling.

Topological metal phases in irradiated graphene sandwiched by asymmetric ferromagnets

The ill-defined Chern number and disappearing topological edge states make it difficult to characterize the topological properties of a metal. In this work, we investigate the abundant topological phases of monolayer graphene, which is sandwiched by asymmetric ferromagnets and irradiated by off-resonant light. In this system, there exist some rarely noticed metal phases, which are spin-valley polarized metal, topological spin metal (TSM), spin half metal, and topological spin half metal (TSHM). Particularly, for the TSM, the subband with spin up or spin down is topological, but the whole state becomes a metal. For the TSHM, a subband with one spin is topological, while the other subband with the opposite spin is insulated, and the whole state is a spin half metal. As a consequence, one topological protected spin current flows on the edge and the other spin propagates in the bulk. Further calculations indicate that the Berry curvatures for the metal phases are nonzero. We propose to probe the topological properties of the metal states with the anomalous Nernst effect. For these topological phases, spin and valley splitting and flip can be obtained by modulating the Fermi level. An electrically or magnetically controlled switch is designed by a two-terminal TSHM junction. It is expected that these topological metal phases can broaden the band engineering in monolayer graphene and support a promising platform for the studies of spin and valley caloritronics.

Light-modulated anisotropic Andreev reflection across an 8-Pmmn borophene-based superconducting junction

We investigate the Andreev reflection across an 8-Pmmn borophene-based superconducting junction irradiated by the linearly polarized off-resonant light with the polarization along x and y directions, respectively. In the regime of the interband conversion of electron–hole, Andreev retro-reflection happens in both cases of the polarization along x and y directions except perpendicular direction of junction. In the regime of the intraband conversion of electron–hole, specular Andreev reflection happens regardless of the directions of polarization except perpendicular direction of junction. In both regimes of the conversion of electron–hole, the values of differential conductance are different in the different directions of junction and different directions of polarization, i.e., an anisotropic conductance is generated. In addition, the nonzero incident angle of the perfect Andreev reflection can be tuned by adjusting the directions of junction and polarization. All these properties of Andreev reflection arise from the light-modulated anisotropic band structure of borophene.

Spin- and valley-polarized transport and magnetoresistance in asymmetric ferromagnetic WSe2 tunnel junctions

Transition metal dichalcogenides (TMDs) offer a new platform for theoretical study of two-dimensional materials and their applications, beyond graphene. Previous studies indicate that monolayers of TMDs become ferromagnetic when put in proximity to conventional ferromagnetic, leading to remarkable spin and valley transport properties when combined with the intrinsic spin-orbit coupling (SOC) inherent in these materials. In this work, we show that a magnetic tunnel junction setup consisting of an asymmetric ferromagnetic/ferromagnetic/normal ${\mathrm{WSe}}_{2}$ junction gives rise to a fully spin- and valley-polarized current for both parallel and antiparallel magnetization configurations in the presence of an off-resonance light. Due to the strong SOC of ${\mathrm{WSe}}_{2}$ together with off-resonance light, both the spin and valley polarizations are tunable and switchable. We also find that the tunneling magnetoresistance (TMR) could be tuned to 1 by off-resonance light. The relatively much stronger SOC in the conduction and valence bands of ${\mathrm{WSe}}_{2}$ compared to other compounds of the dichalcogenides family in collaboration with an off-resonance light leads to significant negative TMR at weak enough exchange fields. In appropriate parameter regimes, the TMR oscillations from negative to positive values can be tuned by the off-resonance light. Our results suggest that magnetic tunnel junctions involving ${\mathrm{WSe}}_{2}$ have very promising potential for applications in magnetic memory and spin and valleytronics devices.

Cavity-assisted preparation and detection of a unitary Fermi gas

We report on the fast production and weakly destructive detection of a Fermi gas with tunable interactions in a high finesse cavity. The cavity is used both with far off-resonant light to create a deep optical dipole trap, and with near-resonant light to reach the strong light–matter coupling regime. The cavity-based dipole trap allows for an efficient capture of laser-cooled atoms, and the use of a lattice-cancellation scheme makes it possible to perform efficient intra-cavity evaporative cooling. After transfer in a crossed optical dipole trap, we produce deeply degenerate unitary Fermi gases with up to 7 × 105 atoms inside the cavity, with an overall 2.85 s long sequence. The cavity is then probed with near-resonant light to perform five hundred-times repeated, dispersive measurements of the population of individual clouds, allowing for weakly destructive observations of slow atom-number variations over a single sample. This platform will make possible the real-time observation of transport and dynamics as well as the study of driven-dissipative, strongly correlated quantum matter.