Asymmetric Line Shape Research Articles

Two-dimensional electron bound hole photoluminescence in GaAs in perpendicular magnetic fields

The photoluminescence of quasi two-dimensional (2D) electrons in modulation doped AlxGa1 − xAs-GaAs single heterostructure recombining with bound holes was studied at liquid helium temperatures in magnetic fields up to 9 T. Luminescence contributions from the ground and a weakly populated first excited 2D subband were observed as well as bulk-like contributions from the GaAs buffer. The 2D subband and the bulk signals are linked and oscillate with the magnetic field strength. It is not the filling factor that rules but the alignment of the two subband Landau-ladders paired with intersubband scattering that drives an oscillatory behavior of the subband populations. Asymmetric Landau-level transition line shapes indicate a perturbed 2D electron system. We performed a detailed analyses of Landau-level dependent line strengths, positions and half widths and compared the experiment with calculations of subband energies and transition matrix elements. The total electron density is not fixed in our experiment but decreases with magnetic field strength. This might indicate that the 2D system exchanges electrons with a reservoir and/or that the correlation between radiative and nonradiative recombination channels vary strongly with the magnetic field strength.

Ultrafast real-time observation of double Fano resonances in discrete excitons and single plasmon-continuum

Ultrafast observation of multiple asymmetric Fano line shapes in energetically overlapped single continuum and multiple discrete states is of great theoretical interest in different domains of physics, though their real-time experimental demonstrations are yet to be achieved. Here, we present an experimental observation on time-domain response of the double Fano asymmetries in a metal--two-dimensional semiconductor hybrid prototype at room temperature. The ultrafast interactions in two discrete spin-resolved excitons of ${\mathrm{MoS}}_{2}$ and a single metal plasmon-continuum allow us to observe the generation and evolution of the double Fano line profiles in real time. Apart from the subpicosecond development of double Fano line shapes, we calculate all the time-dependent nonlinear double Fano parameters. These results suggest a better understanding of ultrafast Fano physics in condensed-matter systems and have potential prospects in ultrafast photonic devices using double Fano resonances even at room temperature.

Plasmon resonances and tachyon ghost modes in highly conducting sheets

Plasmon-polariton modes in two-dimensional electron gases have a dual field-matter nature that endows them with unusual properties when electrical conductivity exceeds a certain threshold set by the speed of light. In this regime plasmons display an interesting relation with tachyons, the hypothetical faster-than-light particles. While not directly observable, tachyons directly impact properties of plasmon modes. Namely, in the ``tachyon'' regime, plasmon resonances remain sharp even when the carrier collision rate $\gamma$ exceeds plasmon resonance frequency. Resonances feature a recurrent behavior as $\gamma$ increases, first broadening and then narrowing and acquiring asymmetric non-Lorentzian lineshapes with power-law tails extending into the tachyon continuum $\omega>ck$. This unusual behavior can be linked to the properties of tachyon poles located beneath $\omega>ck$ branch cuts in the complex $\omega$ plane: as $\gamma$ grows, tachyon poles approach the light cone and hybridize with plasmons. Narrow resonances persisting for $\gamma>\omega$, along with the unusual density and temperature dependence of resonance frequencies, provide clear signatures of the tachyon regime.

Tunable Resonance and Phase Vortices in Kirigami Fano‐Resonant Metamaterials

AbstractEmerging internet‐of‐things technologies require widespread antennas and sensors. They should be cheap enough to be widely deployed, but still should be highly functional and tunable. Here, a thorough, in‐depth study of spectral shifts and phase vortices in kirigami Fano‐resonant metamaterials is presented. Microwave metamaterials are printed on paper using metal inks. Then, the printed metamaterials are cut line‐by‐line and folded, so that a step height between neighboring unit cells can be created. By varying step heights, significant spectral tuning as well as resonance switching are obtained. The large spectral shift of the highly asymmetric Fano lineshape also enables significant control of radiation direction. The observations are explained based on the interactions between neighboring unit cells. Moreover, phase singularities are found at the zero‐amplitude position of the Fano resonance spectrum, and a pair of phase vortices appear in parameter space. The kirigami metamaterials allow an easy parameter scan, and therefore significant control of the spectral phase becomes possible. Such drastic phase changes around singularity points could be very useful for various applications, including optical sensing and wavefront manipulation. The kirigami metamaterials enable easily fabricated but highly functional and tunable elements, which could be useful for various antennas and sensors.

Few-photon transport in Fano-resonance waveguide geometries

We present a theoretical study of Fano interference effects in few-photon transport. Under appropriate conditions, a local defect in an optical waveguide induces a highly asymmetric transmission lineshape, characteristic of Fano interference. For a two-level emitter placed adjacent to such a defect, here modeled as a partially transmitting element, we find an analytical expression for the full time evolution of single-photon wavepackets and the emitter excitation probability. We show how the partially transmitting element affects the emitter lifetime and shifts the spectral position of the effective system resonances. Using input-output formalism, we determine the single and two-photon $ S $-matrices for both a two-level emitter and a cavity-emitter system coupled to a waveguide with a partially transmitting element. We show how the Fano interference effect can be exploited for the implementation of a Hong-Ou-Mandel switch in analogy with a tunable linear or nonlinear beam splitter.

Lattice deformation in epitaxial Fe3O4 films on MgO substrates studied by polarized Raman spectroscopy**Project supported by the National Key Research and Development Program of China (Grant No. 2018YFB0703500) and the National Natural Science Foundation of China (Grant No. 11704401).

The lattice structures of epitaxial Fe3O4 films deposited on MgO were studied systematically using polarized Raman spectroscopy as a function of film thickness, where interesting phenomena were observed. Firstly, the spectral conflict to the Raman selection rules (RSRs) was observed under cross-sectional configuration, which can be attributed to the tetragonal deformation in the growth direction due to the lattice mismatch between Fe3O4 and MgO. Secondly, the blue shift and broadening of Raman peaks evidenced the decrease of the tensile strain in Fe3O4 films with decreasing thickness. Thirdly, distinct from the other Raman modes, the lowest T2g mode exhibited asymmetric lineshape, which can be interpreted using the spatial correlation model. The increased correlation length introduced in the model can well explain the enhanced peak asymmetry feature with decreasing thickness. These results provide useful information for understanding the lattice structure of epitaxial Fe3O4 film.

A Compact Laser Pumped 4He Magnetometer with Laser-Frequency Stabilization by Inhomogeneous Light Shifts

We propose a compact 4He magnetometer realizing magnetic field measurement and laser-frequency stabilization simultaneously in a single 4He atomic cell. The frequency stabilization scheme is based on the asymmetric line shape of magnetic resonance which is induced by spatially inhomogeneous light shifts. We investigate the asymmetric line shape of the magnetic resonance signal theoretically and experimentally in laser pumped 4He magnetometer with the magneto-optical double-resonance configuration. Notice that, due to the asymmetric line shape, the in-phase component of the magnetic resonance signal is shown to have a linear dependence with respect to the laser frequency detuning and is used to actively lock the laser frequency to the resonant point. The method reduces the complexity of the system and improves the stability of the magnetometer, making the laser-pumped 4He magnetometer more compact and portable.

Majorana fermions induced Fano resonance and fast-to-slow light in a hybrid semiconductor/superconductor ring device

We demonstrate theoretically the Fano resonance and the conversion from fast to slow light in a hybrid quantum dot (QD)–semiconductor/superconductor (S/S) ring device, where the QD is coupled to a pair of Majorana fermions (MFs) appearing in the hybrid QD–S/S ring device. The absorption spectra of the weak probe field can exhibit a series of asymmetric Fano line shapes, and their related optical propagation properties such as fast- and slow-light effects are investigated based on the hybrid system for suitable parametric regimes. The positions of the Fano resonances can be determined by the parameters, such as different detuning regimes and QD–MFs coupling strengths. Further, the transparency windows (i.e., the absorption dip approaches zero) in the probe absorption spectra are accompanied by the rapid steep dispersion of the Fano resonance profile, which promises the slow- or fast-light effect, and even tunable fast-to-slow light propagation (or vice versa) can be achieved by controlling different parameter regimes. Therefore, Fano resonance may indicate another means to investigate MFs and the investigation may open up promising applications in quantum information processing based on MFs in solid-state devices.

Fano Resonance in Artificial Photonic Molecules

AbstractThe spectral signatures of chemical molecules are dependent on the hybridization of electronic states. The artificial photonic molecules formed by structured optical microcavities, exhibiting Fano features with sharp asymmetric line shape and strong field enhancement, hold various potential applications in sensors, optical switches, lasing spasers, optical diodes, etc. To design high‐performance photonic devices, it is of great importance to gain the formation and modulation mechanisms of Fano resonance in various artificial photonic molecules. This review is focused on the Fano resonance in artificial photonic molecules. It starts by discussing the properties of Fano resonance, followed by a detailed discussion of the coupled oscillator model and coupled mode theory that could reveal the underlying mechanisms. Then, different types of photonic molecules for realizing Fano resonance are presented, and several representative exciting applications are introduced. Finally, a summary and a brief outlook on challenges are discussed for Fano resonance in artificial photonic molecules.

Mechanisms of Fano-resonant biosensing: Mechanical loading of plasmonic oscillators

Distinctively narrow and asymmetric line shape Fano resonances arise due to resonant interactions of sub-radiant and super-radiant modes in plasmonic nanostructures and metamaterials. A number of recent experimental studies have shown unique opportunities provided by highly dispersive Fano resonances in biosensing applications. However, there is limited understanding of Fano resonant optical response to biomolecular accumulation. Here, we introduce a phenomenological model that can precisely describe the intricate nature of the Fano resonances in plasmonic nanohole arrays and provide unambiguous physical insights into biosensing experiments. Using rigorous electromagnetic simulations and experimental measurements as benchmarking tools, we show that the non-trivial contribution of molecular accumulation to Fano resonant plasmonic response can be incorporated as a mechanical loading effect in a coupled-oscillator model. Quite remarkably, our phenomenological approach captures the complex spectral response of the Fano resonance profile and asymmetric linewidth broadening upon molecular accumulation. Furthermore, in strong agreement with our experimental measurements, we show that our parameterized​ model has predictive power in fine tuning the Fano resonant extraordinary light transmission lineshape using structural design parameters without resorting to electromagnetic simulations. Our phenomenological model provides a general analytical method that can be adapted to understand biomolecular detection measurements in different plasmonic and metamaterial systems.

Core level photoemission line shape selection: Atomic adsorbates on iron

Robust fitting of core level photoemission spectra is often central to reliable interpretation of X‐ray photoelectron spectroscopy (XPS) data. One key element is employment of the correct line shape function for each spectral component. In this study, we consider this topic, focusing on XPS data from atomic adsorbates, namely, O and S, on Fe(110). The potential of employing density functional theory (DFT) for generating adsorbate projected electronic density of states (PDOS) to support line shape selection is explored. O 1s core level XPS spectra, acquired from various ordered overlayers of chemisorbed O, all display an equivalent asymmetric line shape. Previous work suggests that this asymmetry is a result of finite O PDOS in the vicinity of the Fermi level, allowing O 1s photoexcitation to induce a weighted continuum of final states through electron‐hole pair excitation. This origin is corroborated by O DFT‐PDOS generated for an optimised five‐layer Fe(110)(2 × 2)‐O slab. Adsorbate DFT‐PDOS were also computed for Fe(110) ‐S. As, similar to adsorbed O, there is a significant continuous distribution of states about the Fermi level, it is proposed that the S 2p XPS core levels should also have asymmetric profiles. S 2p XPS data acquired from Fe(110) ‐S, and their subsequent fitting, verify this prediction, suggesting that DFT‐PDOS could aid line shape selection.

Photoluminescence and Raman spectroscopy of silicon thin films grown by laser ablation

Silicon thin films were prepared by Laser Ablation technique onto fused quartz substrates. The effect of thermal annealing on the silicon films and the pressure and temperature dependence show an unusual behavior of photoluminescence (PL) spectra of the silicon (Si) thin films. An intense PL energy band around 3.0 eV (400 nm) was observed at room temperature after thermal annealing. Raman spectra of the samples showed an asymmetric peak shifted to 517 cm−1, in comparison with the crystalline Si optical phonon at 520 cm−1; a broadening (7 cm−1) of the phonon line is also observed. These results agree quiet well with the relaxation of the q-vector selection rules for the Raman active optical phonons. The red-shifted asymmetric Raman line shape was fitted by using the phonon-confinement model, in order to estimate the size of the particles constituting the films.

Fano effect on neutron elastic scattering by open-shell nuclei

By focusing on the asymmetric shape of cross section, we analyze the pairing effect on the partial wave components of cross section for neutron elastic scattering off stable and unstable nuclei within the Hartree-Fock-Bogoliubov (HFB) framework. Explicit expressions for Fano parameters $q_{lj}$ and $\epsilon_{lj}$ have been derived and the pairing effects have been analyzed in term of these parameters, and the Fano effect was found on the neutron elastic scattering off the stable nucleus in terms of the pairing correlation. Fano effect was appeared as the asymmetric line-shape of the cross section caused by the small absolute value of $q_{lj}$ due to the small pairing effect on the deep-lying hole state of the stable nucleus. In the case of the unstable nuclei, the large $q_{lj}$ value is expected because of the small absolute value of the Fermi energy. The quasiparticle resonance with the large $q_{lj}$ forms the Breit-Wigner type shape in the elastic scattering cross section.

Fano resonance and amplification in a quadratically coupled optomechanical system with a Kerr medium

We discuss the Fano resonance and amplification in a quadratically coupled optomechanical system with a nonlinear Kerr medium. We find that effective cavity detuning and the strength of the Kerr nonlinearity lead to the appearance of the Fano resonance in the output probe field at room temperature, respectively. We show that the Kerr nonlinearity causes the asymmetric line shape to appear in the intensities of the output Stokes field and the second-order upper sideband generation. We also show that the Kerr nonlinearity can amplify the intensities of the output Stokes field and the second-order upper sideband generation, and the amplification factor is related to the strength of the Kerr nonlinearity. The findings of this study have potential applications in optical switching and frequency conversion.

An In-Fiber Coupler for Whispering-Gallery-Mode Excitation in a Microsphere Resonator

An integrated in-fiber coupler for excitation of whispering-gallery modes (WGMs) of a microsphere resonator is proposed. The device is fabricated by embedding a microsphere into an optical fiber inner air-cavity through an open micro-channel to excite various WGMs via evanescent field coupling. The asymmetric Fano resonance line shape and the symmetrical Lorentzian line shape with a slope of −13.88 dB/nm and a quality (Q)-factor of $1.81\times 10^{3}$ are observed in the transmission spectrum of the device. Such an in-fiber WGM resonator coupling structure has the features of compactness, low-cost, robust and alignment-free.

Multimode Fano resonances for low-power mode switching.

Intensive research in photonics has discovered many original and fascinating optical phenomena associated with the physics of Fano resonances. Here we extend the research of Fano resonance to on-chip multiple modes and high-order mode, which can be called multimode Fano resonance. By exploiting both wavelength and mode degrees of freedom, we demonstrate a compact structure to engineer wavelength- and shape-controlled multimode Fano resonances on an integrated photonic platform. Both simulated and experimental results indicate that the unambiguous asymmetric spectra line shapes for both transverse electric modes (${{\rm TE}_0}$TE0 and ${{\rm TE}_1}$TE1) can be further applied to low-power mode switching. The demonstrated on-chip multimode Fano resonances scheme may provide a new perspective for exploring the advantages and applications of multimode Fano resonances in integrated photonics.

Fano resonance induced fast to slow light in a hybrid semiconductor quantum dot and metal nanoparticle system

We theoretically demonstrate the Fano resonance and the conversion from fast to slow light in a hybrid semiconductor quantum dot (SQDs)-metal nanoparticle (MNPs) with cavity quantum electrodynamics treatment. The absorption spectra of the weak probe field exhibit a series of asymmetric Fano line shapes and their related optical propagation properties, such as fast and slow light effects, are investigated based on the hybrid system for suitable parametric regimes. Further, the transparency windows (i.e. the absorption dip approaches zero) in the probe absorption spectra are accompanied by the rapid steep dispersion of the Fano resonance profile, which promises the slow or fast light effect, and even tunable fast-to-slow light propagation (or vice versa) can be achieved by controlling different parameter regimes. Therefore the investigation may indicate promising applications in quantum information processing based on the hybrid SQD-MNP system.

Robust Fano resonance in a topological mechanical beam

The advances in topological condensed matter physics enable the manipulation of classic waves in different ways, such as unidirectional propagation featuring the suppression of backscattering and the robustness against impurities and disorder, making it possible to endow classical phenomena with topological properties. Fano resonance, a widely spread and basic kind of resonance, features an asymmetric line shape with an ultrahigh quality factor $Q$ that usually requires delicate designs and precise fabrication. In this work, we achieve a robust Fano mechanical resonance with topological protection by engineering band inversion of two different vibrating symmetries of a pillared beam that gives rise to dark and bright edge modes. The Fano resonance results from the constructive and destructive interferences between topological dark and bright modes. It is further demonstrated that the Fano asymmetric shape of the transmission peak and its frequency are robust against random perturbations in the pillars' position as long as the symmetry is conserved. If random perturbations break the symmetry and only band inversion is involved, the asymmetric line shape of the Fano resonance weakens until disappearing before the closure of the bulk band gap, since the excitation will couple all fundamental modes of the beam. The analysis of the robustness of Fano resonance originating from band inversion and symmetry protection reveals the nature of topological protection which can be applied to design topological high-$Q$ resonance in sensing application.

The vibronic absorption spectra and exciton dynamics of plasmon-exciton hybrid systems in the regimes ranged from Fano antiresonance to Rabi-like splitting.

The complex interplay between molecules and plasmonic metal nanoparticles (MNPs) presents a set of particular characteristics in absorption/scattering spectra such as excitonic splitting, asymmetric line shapes, plasmon-induced absorption enhancement and transparencies, etc. Although the MNP-molecule systems have been intensively investigated experimentally and theoretically, the construction of a theoretical framework which can produce all the disparate experimental observations and account for the electron-phonon (e-p) coupling is still in progress. Here, we present a theoretical approach which can account for both the plasmon-exciton coupling and the e-p interaction and produce all the spectral line shapes ranging from Fano antiresonance to Rabi splitting by simply tuning the coupling strength or plasmon damping rate. Additionally, we demonstrate the evolution of vibronic spectra and exciton dynamics with the coupling strength, plasmon damping rate, and detuning energy. It is found that the vibronic structures appearing in Rabi-like spectra are worse resolved, wider, and more largely shifted than those appearing in the Fano regime, attributed to the more significant deformation of the molecular vibrational wavepacket in the Rabi-like regime than that in the Fano regime as the molecular e-p interaction increases. The positive/negative value of detuning energy can induce different degrees of the vibrational wavepacket deformation and subsequently a different effect on the spectra in different coupling regimes.

Feshbach Resonances in p-Wave Three-Body Recombination within Fermi-Fermi Mixtures of Open-Shell 6Li and Closed-Shell 173Yb Atoms.

We report on the observation of magnetic Feshbach resonances in a Fermi-Fermi mixture of ultracold atoms with extreme mass imbalance and on their unique p-wave dominated three-body recombination processes. Our system consists of open-shell alkali-metal 6Li and closed-shell 173Yb atoms, both spin polarized and held at various temperatures between 1 and 20 μK. We confirm that Feshbach resonances in this system are solely the result of a weak separation-dependent hyperfine coupling between the electronic spin of 6Li and the nuclear spin of 173Yb. Our analysis also shows that three-body recombination rates are controlled by the identical fermion nature of the mixture, even in the presence of s-wave collisions between the two species and with recombination rate coefficients outside the Wigner threshold regime at our lowest temperature. Specifically, a comparison of experimental and theoretical line shapes of the recombination process indicates that the characteristic asymmetric line shape as a function of applied magnetic field and a maximum recombination rate coefficient that is independent of temperature can only be explained by triatomic collisions with nonzero, p-wave total orbital angular momentum. The resonances can be used to form ultracold doublet ground-state molecules and to simulate quantum superfluidity in mass-imbalanced mixtures.