Linear Optical Regime Research Articles

Quasicrystal Photonic Metasurfaces for Radiation Controlling of Second Harmonic Generation.

Photonic metasurfaces, a kind of 2D structured medium, represent a novel platform to manipulate the propagation of light at subwavelength scale. In linear optical regime, many interesting topics such as planar meta-lenses, metasurface optical holography, and so on have been widely investigated. Recently, metasurfaces have gone into the nonlinear optical regime. While it is recognized that the local symmetry of the meta-atoms plays a vital role in determining the polarization, phase, and intensity of the nonlinear waves, much less attention has been paid to the global symmetry of the nonlinear metasurfaces. According to the Penrose tiling and the newly proposed hexagonal quasicrystalline tiling, nonlinear optical quasicrystal metasurfaces are designed and fabricated based on the geometric-phase-controlled plasmonic meta-atoms with local rotational symmetry. It is found that the far-field radiation behavior of second harmonic generation waves are determined by both the tiling schemes of quasicrystal metasurfaces and the local symmetry of meta-atoms they consist of. The proposed concept may open new avenues for designing nonlinear optical sources with metasurface crystals.

Reconfiguring structured light beams using nonlinear metasurfaces.

Ultra-compact, low-loss, fast, and reconfigurable optical components, enabling manipulation of light by light, could open numerous opportunities for controlling light on the nanoscale. Nanostructured all-dielectric metasurfaces have been shown to enable extensive control of amplitude and phase of light in the linear optical regime. Among other functionalities, they offer unique opportunities for shaping the wave front of light to introduce the orbital angular momentum (OAM) to a beam. Such structured light beams bring a new degree of freedom for applications ranging from spectroscopy and micromanipulation to classical and quantum optical communications. To date, reconfigurability or tuning of the optical properties of all-dielectric metasurfaces have been achieved mechanically, thermally, electrically or optically, using phase-change or nonlinear optical materials. However, a majority of demonstrated tuning approaches are either slow or require high optical powers. Arsenic trisulfide (As2S3) chalcogenide glass offering ultra-fast and large χ(3)nonlinearity as well as a low two-photon absorption coefficient in the near and mid-wave infrared spectral range, could provide a new platform for the realization of fast and relatively low intensity reconfigurable metasurfaces. Here, we design and experimentally demonstrate an As2S3 chalcogenide glass based metasurface that enables reshaping of a conventional Hermite-Gaussian beam with no OAM into an OAM beam at low intensity levels, while preserves the original beam's amplitude and phase characteristics at high intensity levels. The proposed metasurface could find applications for a new generation of optical communication systems and optical signal processing.

All optical NOT and NOR gates using interference in the structures based on 2D linear photonic crystal ring resonator

All optical NOT and NOR logic gates using structures based on linear photonic crystal ring resonator (LPhCRR) are proposed in this paper. The switching function of the proposed gates can be obtained at very small power levels as it relies on the interference effect. Moreover, their functioning is possible with wide range of input powers and tolerant to input power fluctuations (as those work in linear optics regime). Optical behavior of the purposed structures is demonstrated by using Two Dimensional Finite Difference Time Domain (2DFDTD) method and dispersion diagram is extracted by Plane Wave Expansion (PWE) method. Performance of the proposed structures is found to be very good in terms of response time and contrast ratio.

Spin-dependent OAM flipping in multihelical optical fibres

We study the propagation of light beams through optical fibres that possess a multihelical distribution of the refractive index. Analytical expressions for the higher-order modes with the azimuthal numbers and the corresponding propagation constants of low-bi multihelical fibres are obtained. It is shown that the resonance modes are weighted superposition of optical vortices (OVs) with the same sign of circular polarization but opposite topological charges. We unveil the effect of spin-dependent flipping of topological charge (or, equivalently, orbital angular momentum) of an incoming OV. This paves the way to the implementation of the all-fibre stable logical element controlled-NOT gate, in which the circular polarization carries the control qubit and the topological charge carries the target. Such a gate should produce optical beams with entanglement between polarization and orbital degrees of freedom in the regime of linear optics.

Coherent Energy Transfer Dynamics and Photoluminescence of Coupled Quantum Dots: Fully Analytical Approach

We theoretically studied coherent excitation energy transfer between self-growth semiconductor quantum dots (QDs) by solving Heisenberg’s equations of motion for density matrix elements in second quantization regime. In a local excitation condition where only one QD electron is optically excited by the pump laser field, coherent excitation energy transfer to the other QD electron can be achieved through Coulomb (Forster) and electron-photon (radiation field) interactions. We calculated three diagonal and one off-diagonal Coulomb coupling constants, which are responsible for the biexcitonic frequency renormalization and the coherent energy transfer between QDs, respectively, and radiation field coupling coefficients by using electron and hole wave functions derived from eight-band kp-theorem, whose validity has already been tested by comparison with experiment. In linear optical regime where the occupation densities of electrons at higher energy level are negligibly small, we could successfully derive fully analytical behaviors of temporal dynamics of the interband polarizations and level occupation densities of both QDs by using Hartree-Fock approximation (HFA), in eventual, the stationary photoluminescence of the coupled QDs in an analytical form. Additionally, the validity of the HFA was examined by comparing the numerical results with those obtained from the exact correlation expansion model for different values of the pump field intensity.

Observation of Rotational Doppler Effect in Second Harmonic Generation in Reflection Mode

AbstractIn linear optical regime, one important application of Rotational Doppler Effect (RDE) is to measure the spinning speed of either molecules or macroscopic objects. The RDE can be further magnified using nonlinear harmonic generations. However, the previous measurement of nonlinear RDE in transmission mode, which relies on two beam interferometry, is very challenging due to the critical requirement of optical alignment in both spatial and time domains. Here, the observation of nonlinear RDE of second harmonic generation (SHG) waves from one spinning quartz crystal with silver coating in reflection mode is reported. The utilization of the post‐selection and beating frequency techniques greatly simplify the conventional nonlinear RDE measurement setup.

Imaging through Nonlinear Metalens Using Second Harmonic Generation.

The abrupt phase change of light at metasurfaces provides high flexibility in wave manipulation without the need for accumulation of propagating phase through dispersive materials. In the linear optical regime, one important application field of metasurfaces is imaging by planar metalenses, which enables device miniaturization and aberration correction compared to conventional optical microlens systems. With the incorporation of nonlinear responses into passive metasurfaces, optical functionalities of metalenses are anticipated to be further enriched, leading to completely new application areas. Here, imaging with nonlinear metalenses that combine the function of an ultrathin planar lens with simultaneous frequency conversion is demonstrated. With such nonlinear metalenses, imaging of objects with near infrared light while the image appears in the second harmonic signal of visible frequency range is experimentally demonstrated. Furthermore, the functionality of these nonlinear metalenses can be modified by switching the handedness of the circularly polarized fundamental wave, leading to either real or virtual nonlinear image formation. Nonlinear metalenses not only enable infrared light imaging through a visible detector but also have the ability to modulate nonlinear optical responses through an ultrathin metasurface device while the fundamental wave remains unaffected, which offers the capability of nonlinear information processing with novel optoelectronic devices.

Atomically precise clusters of gold and silver: A new class of nonlinear optical nanomaterials

Functional ligand-protected metal cluster nanomaterials with increased two-photon absorption and two-photon excited emission might result in new technologies within the fields of bio-imaging applications. During this article, I review the two-photon absorption/emission properties of atomically precise ligand-protected silver and gold nanoclusters, coined “ligand-core” NLO-phores. This includes quantum-chemical methodologies using time-dependent density theory. I will analyze physical phenomena and trends resulting in giant two-photon absorption/emission responses of a number of series of model nanoclusters. I will then focus my review on the effects of the relaxation pathways within the linear and nonlinear optical regime, similarly as innovative ways aiming at enhancing their two-photon emission responses.

Population of collective modes in light scattering by many atoms

The interaction of light with an atomic sample containing a large number of particles gives rise to many collective (or cooperative) effects, such as multiple scattering, superradiance and subradiance, even if the atomic density is low and the incident optical intensity weak (linear optics regime). Tracing over the degrees of freedom of the light field, the system can be well described by an effective atomic Hamiltonian, which contains the light-mediated dipole-dipole interaction between atoms. This long-range interaction is at the origin of the various collective effects, or of collective excitation modes of the system. Even though an analysis of the eigenvalues and eigenfunctions of these collective modes does allow distinguishing superradiant modes, for instance, from other collective modes, this is not sufficient to understand the dynamics of a driven system, as not all collective modes are significantly populated. Here, we study how the excitation parameters, i.e. the driving field, determines the population of the collective modes. We investigate in particular the role of the laser detuning from the atomic transition, and demonstrate a simple relation between the detuning and the steady-state population of the modes. This relation allows understanding several properties of cooperative scattering, such as why superradiance and subradiance become independent of the detuning at large enough detuning without vanishing, and why superradiance, but not subradiance, is suppressed near resonance.

Ultrathin Nonlinear Metasurface for Optical Image Encoding.

Security of optical information is of great importance in modern society. Many cryptography techniques based on classical and quantum optics have been widely explored in the linear optical regime. Nonlinear optical encryption in which encoding and decoding involve nonlinear frequency conversions represents a new strategy for securing optical information. Here, we demonstrate that an ultrathin nonlinear photonic metasurface, consisting of meta-atoms with 3-fold rotational symmetry, can be used to hide optical images under illumination with a fundamental wave. However, the hidden image can be read out from second harmonic generation (SHG) waves. This is achieved by controlling the destructive and constructive interferences of SHG waves from two neighboring meta-atoms. In addition, we apply this concept to obtain gray scale SHG imaging. Nonlinear metasurfaces based on space variant optical interference open new avenues for multilevel image encryption, anticounterfeiting, and background free image reconstruction.

Nonlinear photonic metasurfaces

In linear optical regime, many novel optical functions have been demonstrated by using ultrathin photonic metasurfaces. The main concept of metasurface is to appropriately assembly the spatially variant meta-atoms on a subwavelength scale, and realize the manipulations of polarization, phase and amplitude of light. Recently, the nonlinear optical properties of photonic metasurfaces have attracted a lot of attention. In this review, we discuss the design, material selection, symmetry consideration of the meta-atoms, as well as the applications such as nonlinear chiral optics, nonlinear geometric Berry phase and nonlinear wavefront engineering. Lastly, we point out the challenges and potentials of nonlinear photonic metasurfaces for manipulating the light-matter interactions.

Polarization rotation of hybridly polarized beams in a uniaxial crystal orthogonal to the optical axis: theory and experiment.

The polarization rotation of hybridly polarized beams passing through a uniaxial crystal whose optical axis is orthogonal to the beam axis is theoretically and experimentally investigated. Based on the existing theory, analytical expressions of hybridly polarized beams in uniaxial crystals orthogonal to the optical axis are derived. Simulation results show that the distributions of the intensity patterns, the states of polarization, and the spin angular momentum of hybridly polarized beams in uniaxial crystals depend strongly on the beam radius, the propagation distance in the crystal, and the ratio of the extraordinary refractive index to the ordinary refractive index. The experimental investigations observed the polarization rotation behavior of hybridly polarized beams in rutile crystal, which are in good agreement with the theoretical predictions. The presented work opens up novel approaches for manipulating the polarization rotation of the structured light field by anisotropic crystals in the linear optics regime, which has potential applications in material characterization, polarization-sensitive sensing, and all-optical switching.

Phenomenological modeling of nonlinear holograms based on metallic geometric metasurfaces.

Benefiting from efficient local phase and amplitude control at the subwavelength scale, metasurfaces offer a new platform for computer generated holography with high spatial resolution. Three-dimensional and high efficient holograms have been realized by metasurfaces constituted by subwavelength meta-atoms with spatially varying geometries or orientations. Metasurfaces have been recently extended to the nonlinear optical regime to generate holographic images in harmonic generation waves. Thus far, there has been no vector field simulation of nonlinear metasurface holograms because of the tremendous computational challenge in numerically calculating the collective nonlinear responses of the large number of different subwavelength meta-atoms in a hologram. Here, we propose a general phenomenological method to model nonlinear metasurface holograms based on the assumption that every meta-atom could be described by a localized nonlinear polarizability tensor. Applied to geometric nonlinear metasurfaces, we numerically model the holographic images formed by the second-harmonic waves of different spins. We show that, in contrast to the metasurface holograms operating in the linear optical regime, the wavelength of incident fundamental light should be slightly detuned from the fundamental resonant wavelength to optimize the efficiency and quality of nonlinear holographic images. The proposed modeling provides a general method to simulate nonlinear optical devices based on metallic metasurfaces.

Superradiance in a Large and Dilute Cloud of Cold Atoms in the Linear-Optics Regime.

Superradiance has been extensively studied in the 1970s and 1980s in the regime of superfluorescence, where a large number of atoms are initially excited. Cooperative scattering in the linear-optics regime, or "single-photon superradiance," has been investigated much more recently, and superradiant decay has also been predicted, even for a spherical sample of large extent and low density, where the distance between atoms is much larger than the wavelength. Here, we demonstrate this effect experimentally by directly measuring the decay rate of the off-axis fluorescence of a large and dilute cloud of cold rubidium atoms after the sudden switch off of a low-intensity laser driving the atomic transition. We show that, at large detuning, the decay rate increases with the on-resonance optical depth. In contrast to forward scattering, the superradiant decay of off-axis fluorescence is suppressed near resonance due to attenuation and multiple-scattering effects.

Chalcogenide optical microwires cladded with fluorine-based CYTOP.

We demonstrate optical transmission results of highly nonlinear As2Se3 optical microwires cladded with fluorine-based CYTOP, and compare them with microwires cladded with typical hydrogen-based polymers. In the linear optics regime, the CYTOP-cladded microwire transmits light in the spectral range from 1.3 µm up to >2.5 µm without trace of absorption peaks such as those observed using hydrogen-based polymer claddings. The microwire is also pumped in the nonlinear optics regime, showing multiple-orders of four-wave mixing and supercontinuum generation spanning from 1.0 µm to >4.3 µm. We conclude that with such a broadband transparency and high nonlinearity, the As2Se3-CYTOP microwire is an appealing solution for nonlinear optical processing in the mid-infrared.

Light interacting with atomic ensembles: collective, cooperative and mesoscopic effects

Cooperative scattering has been the subject of intense research in the last years. In this article, we discuss the concept of cooperative scattering from a broad perspective. We briefly review the various collective effects that occur when light interacts with an ensemble of atoms. We show that some effects that have been recently discussed in the context of ‘single-photon superradiance’, or cooperative scattering in the linear-optics regime, can also be explained by ‘standard optics’, i.e. using macroscopic quantities such as the susceptibility or the diffusion coefficient. We explain why some collective effects depend on the atomic density, and others on the optical depth. In particular, we show that, for a large and dilute atomic sample driven by a far-detuned laser, the decay of the fluorescence, which exhibits superradiant and subradiant dynamics, depends only on the on-resonance optical depth. We also discuss the link between concepts that are independently studied in the quantum-optics community and in the mesoscopic-physics community. We show that the coupled-dipole model predicts a departure from Ohm’s law for the diffuse light, that incoherent multiple scattering can induce a saturation of fluorescence and we also show the similarity between the weak-localization correction to the diffusion coefficient and the inaccuracy of Lorentz local field correction to the susceptibility.

Nonlinear optical mass sensor with an optomechanical microresonator

Mechanical oscillators have been widely used in mass sensing due to the direct relationship of the frequency-shift and mass-change. A conventional method for detecting the mass of deposited sample is to exploit the frequency variation of mechanical resonator in the linear optical regime. In the present letter, we demonstrate a nonlinear optical mass sensor with an optomechanical microresonator which will provide a route towards the use of cavity optomechanical system in biomedical sensors, deposition monitors, chemical reaction monitors and other nonlinear measurement fields. Based on the current experimental conditions, the nonlinear optical mass sensor proposed here can be achieved by experiments.

Fluidic laser beam shaper by using thermal lens effect

Laser measurement and laser processing techniques have been gaining strong attention from various applications. This research aims at the development of a fluidic laser beam shaper, and in order to fulfill the objective, thermal lens effect characteristics are studied. This phenomenon has the optical property of a divergent lens since the refractive index distribution on the optical axis is formed when a liquid is irradiated. In this research, effects of the pump power and the propagation distance to the probe beam profile are investigated experimentally and theoretically, with the purpose of developing a fluidic laser beam shaper. It is indicated that, by controlling some parameters in thermal lens system such as the pump power (in the regime of linear optics) and absorption coefficient, an input Gaussian beam can be converted into a flat-top beam profile. The relationship between the distance to obtain the flat-top beam, the pump power and absorption coefficient is investigated to show the flexibility of the fluidic laser beam shaper in many fields of laser application.

Sensing with Multipolar Second Harmonic Generation from Spherical Metallic Nanoparticles

We show that sensing in the nonlinear optical regime using multipolar surface plasmon resonances is more sensitive in comparison to sensing in the linear optical regime. Mie theory, and its extension to the second harmonic generation from a metallic nanosphere, is used to describe multipolar second harmonic generation from silver metallic nanoparticles. The standard figure of merit of a potential plasmonic sensor based on this principle is then calculated. We finally demonstrate that such a sensor is more sensitive to optical refraction index changes occurring in the vicinity of the metallic nanoparticle than its linear counterpart.

Asymmetric transmission and anomalous refraction in metal nanowires metasurface

Here we investigated the asymmetric transmission and the anomalous refraction introduced by a metasurface of bent gold nanowires. The refraction follows the generalized Snell’s law that takes into account the resonant behavior of metallic nanostructures located at the interface between two dielectrics. Measurements performed in the linear optical regime reveal a large sensitivity to the subwavelength features of the gold nanostructures.