Linear Optical Response Research Articles

Tight-binding model for opto-electronic properties of penta-graphene nanostructures

We present a tight-binding parametrization for penta-graphene that correctly describes its electronic band structure and linear optical response. The set of parameters is validated by comparing to ab-initio density functional theory calculations for single-layer penta-graphene, showing a very good global agreement. We apply this parameterization to penta-graphene nanoribbons, achieving an adequate description of quantum-size effects. Additionally, a symmetry-based analysis of the energy band structure and the optical transitions involved in the absorption spectra is introduced, allowing for the interpretation of the optoelectronic features of these systems.

Giant Self-Kerr Nonlinearity in the Metal Nanoparticles-Graphene Nanodisks-Quantum Dots Hybrid Systems Under Low-Intensity Light Irradiance.

Hybrid nanocomposites can provide a promising platform for integrated optics. Optical nonlinearity can significantly widen the range of applications of such structures. In the present paper, a theoretical investigation is carried out by solving the density matrix equations derived for a metal nanoparticles-graphene nanodisks-quantum dots hybrid system interacting with weak probe and strong control fields, in the steady state. We derive analytical expressions for linear and third-order nonlinear susceptibilities of the probe field. A giant self-Kerr nonlinear index of refraction is obtained in the optical region with relatively low light intensity. The optical absorption spectrum of the system demonstrates electromagnetically induced transparency and amplification without population inversion in the linear optical response arising from the negative real part of the polarizabilities for the plasmonic components at the energy of the localized surface plasmon resonance of the graphene nanodisks induced by the probe field. We find that the self-Kerr nonlinear optical properties of the system can be controlled by the geometrical features of the system, the size of metal nanoparticles and the strength of the control field. The controllable self-Kerr nonlinearities of hybrid nanocomposites can be employed in many interesting applications of modern integrated optics devices allowing for high nonlinearity with relatively low light intensity.

Application of the differential evolution for simulation of the linear optical response of photosynthetic pigments

For many applications the Differential Evolution (DE) algorithm had proved to be a very efficient tool for getting the best solution especially when the problem has hundreds of free parameters to optimize. This study aims to expend the list of DE successful scientific applications and to use the DE method for photosynthetic pigments optical response modeling. Chlorophyll (Chl) and bacteriochlorophyll (BChl) are the main pigment molecules of the photosynthetic light-harvesting complexes. These complexes are responsible for photon absorption and sequential transport of energy to the photosynthetic reaction center where the charge separation occurs. The DE routines were adopted to perform the fitting of Chl and Bchl Qy band absorption and fluorescence spectra measured in dimethyl ether. To simulate these spectra the stochastic Brownian oscillator model with 49 vibronic modes for Chl and 60 modes for BChl has been applied. By using the artificially calculated spectra, 10 strategies were tested with different settings of the scaling factor (F) and crossover parameter (Cr) before to fit the real experimental data. Once the proper strategy and settings, which give the fastest convergence, were found, the set of parameters representing the electron–phonon coupling (Huang–Rhys factors) and the lowest frequency modes were calculated. These parameters can be used for modeling of the light-harvesting complex nonlinear optical response.

Gauge invariance of excitonic linear and nonlinear optical response

We study the equivalence of four different approaches to calculate the excitonic linear and nonlinear optical response of multiband semiconductors. These four methods derive from two choices of gauge, i.e. length and velocity gauges, and two ways of computing the current density, i.e. direct evaluation and evaluation via the time-derivative of the polarization density. The linear and quadratic response functions are obtained for all methods by employing a perturbative density matrix approach within the mean-field approximation. The equivalence of all four methods is shown rigorously, when a correct interaction Hamiltonian is employed for the velocity gauge approaches. The correct interaction is written as a series of commutators containing the unperturbed Hamiltonian and position operators, which becomes equivalent to the conventional velocity gauge interaction in the limit of infinite Coulomb screening and infinitely many bands. As a case study, the theory is applied to hexagonal boron nitride monolayers, and the linear and nonlinear optical response found in different approaches are compared.

Broadband photocarrier dynamics and nonlinear absorption of PLD-grown WTe2 semimetal films

WTe2 is a unique material in the family of transition metal dichalcogenides and it has been proposed as a candidate for type-II Weyl semimetals. However, thus far, studies on the optical properties of this emerging material have been significantly hindered by the lack of large-area, high-quality WTe2 materials. Here, we grow a centimeter-scale, highly crystalline WTe2 ultrathin film (∼35 nm) by a pulsed laser deposition technique. Broadband pump-probe spectroscopy (1.2–2.5 μm) reveals a peculiar ultrafast optical response where an initial photo-bleaching signal (lasting ∼3 ps) is followed by a long-lived photoinduced absorption signature. Nonlinear absorption characterization using femtosecond pulses confirms the saturable absorption response of the WTe2 ultrathin films, and we further demonstrated a mode-locked Thulium fiber laser using a WTe2 absorber. Our work provides important insights into linear and nonlinear optical responses of WTe2 thin films.

Ordinary dielectric function of corundumlike α−Ga2O3 from 40 meV to 20 eV

The linear optical response of metastable $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ is investigated by spectroscopic ellipsometry. We determine the ordinary dielectric function from lattice vibrations up to the vacuum ultraviolet spectral range at room temperature for a sample with a $(0001)$ surface. Three out of four ${E}_{\mathrm{u}}$ infrared-active phonon modes are unambiguously determined, and their frequencies are in good agreement with density functional theory calculations. The dispersion of the refractive index in the visible and ultraviolet part of the spectrum is determined. High-energy interband transitions are characterized up to $20\phantom{\rule{0.222222em}{0ex}}\mathrm{eV}$. By comparison with the optical response of $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ and with theoretical results, a tentative assignment of interband transitions is proposed.

Optical Linearity Leading to Electromagnetically Induced Transparency in a Two Level System

In the present work, the possibility of observation of electromagnetically induced transparency in a two level system with the slow light propagation is investigated. The expression for the Rabi frequency is brought out within the rotating wave approximation and thereby it can be employed for the two level system in order to bring out the concepts of electromagnetically induced transparency in a two level system. The linear optical response of an atom with the resonant light in the medium is observed. The<div>linear optical susceptibilities with the controlled field is found. The real part of the optical linearity leads to the refractive index of the medium.</div><div><br></div>

Polarization Control with Plasmonic Antenna Tips: A Universal Approach to Optical Nanocrystallography and Vector-Field Imaging.

Controlling the propagation and polarization vectors in linear and nonlinear optical spectroscopy enables us to probe the anisotropy of optical responses providing structural symmetry selective contrast in optical imaging. Here, we present a novel tilted antenna-tip approach to control the optical vector-field by breaking the axial symmetry of the nanoprobe in tip-enhanced near-field microscopy. This gives rise to a localized plasmonic antenna effect with significantly enhanced optical field vectors with control of both in-plane and out-of-plane components. We use the resulting vector-field specificity in the symmetry selective nonlinear optical response of second-harmonic generation (SHG) for a generalized approach to optical nanocrystallography and imaging. In tip-enhanced SHG imaging of monolayer MoS2 films and single-crystalline ferroelectric YMnO3, we reveal nanocrystallographic details of domain boundaries and domain topology with enhanced sensitivity and nanoscale spatial resolution. The approach is applicable to any anisotropic linear and nonlinear optical response and enables the optical nanocrystallographic imaging of molecular or quantum materials.

A comparative study of media polarity effects on the linear and third-order nonlinear optical responses of thiazine dyes

In this paper, solvent effects on the linear and nonlinear optical responses of thiazine dyes were investigated. Spectroscopic and open aperture Z-scan techniques were used for studying linear and nonlinear responses of azure A and methylene blue dyes. The experimental results show that linear and nonlinear optical properties of used dyes depend highly on the molecular structure and their surrounding media characteristics. The calculated contribution of various media-induced interactions indicate that solvent hydrogen bond acceptor and solvent dipolarity and polarizability abilities have significant effects on the linear and nonlinear absorption characteristics of thiazine dyes. Moreover, by transition of the linear optics domain to nonlinear optics, similar resonance structures were obtained for thiazine dyes. So, the spectroscopic technique can be considered as a simple method for prediction of dominant resonance structures and effective solvent polarity parameters on the third-order nonlinear propertie...

Structural and optical properties of Ni atoms and $$\hbox {Ni}_{55}$$ Ni 55 cluster adsorbed on a rutile $$\hbox {TiO}_{2}$$ TiO 2 (110) surface

Adsorbed Ni in a clean rutile $$\hbox {TiO}_{2}$$ (110) surface has been investigated by computing the electronic band structure and the optical linear response properties within the density functional theory. We have determined the nucleation preferred sites by investigating the equilibrium geometries and their corresponding binding energies. The electronic properties of a closed-shell Mackay icosahedral $$\hbox {Ni}_{55}$$ cluster on $$\hbox {TiO}_2$$ show a strong trend to keep the free cluster shape with a small but significant charge transfer to the $$\hbox {TiO}_2$$ surface. The optical properties investigation of the $$\hbox {Ni}_{55}$$ cluster on rutile (110) indicates the development of an absorption band in the visible region.

Studies on spatial nonlocal optical absorption properties in AlGaAs/GaAs three-step quantum well

To investigate the nonlocal linear intersubband optical absorption properties in AlGaAs/GaAs three-step quantum well (3SQW) nanostructures for a p-polarized states, we deduced the expression of the optical absorption coefficient in the two-level 3SQW system by using Green’s function method and the nonlocal relation between electron and photoelectric field. For the given 3SQW nanostructure, we performed numerical calculations of the optical absorption spectra for different parameters such as angles of incidence, the well width, and the height of the step barrier potential. The numerical results show that spatial nonlocal linear optical response of electrons to light field can induce the radiation shift. It is also demonstrated that the radiation shift and optical absorbance can be controlled by the structure of the 3SQW. These results may bring about a helpful impact in designing nanomaterials with multifarious nonlocality and observing the spatial nonlocal effects in experiment.

Large Third-Order Optical Susceptibility with Good Nonlinear Figures of Merit Induced by Octupole Plasmon Resonance of Asymmetric Au–Ag Core–Shell Nanorods

The Au–Ag core–shell nanorods with asymmetric transverse cross section are synthesized, and their tunable linear and nonlinear optical responses are investigated. After the overgrowth of the asymmetric Ag shell on the Au nanorods, (i) a strong transverse octupole (TO) plasmon resonance at 390 nm is observed; (ii) the nonlinear refractive index γ at the TO resonance wavelength significantly increases from −0.15 × 10–4 to −0.85 × 10–4 cm2/GW, and the γ at the longitudinal dipole (LD) resonance wavelength slightly increases from −0.42 × 10–4 to −0.58 × 10–4 cm2/GW; (iii) but the nonlinear absorption coefficient β induced by the TO resonance is approximately 1 order of magnitude smaller than the one induced by the LD resonance. Consequently, the asymmetric Au–Ag core–shell nanorods at the TO resonance wavelength demonstrate a very large nonlinear refractive index as well as excellent nonlinear figures of merit (W = 4.4 > 1 and T = 0.14 < 1), which satisfy the demands for waveguide all-optical switching.

Simple model of saturable localised surface plasmon

Localised surface plasmons (LSPs) are now applied to various fields, such as bio-sensing, solar cell, molecular fluorescence enhancement and quantum-controlled devices at nanometre scale. Recent experiments show that LSPs are optically saturated by high-intensity light. Absorption saturation arises as a result of strong optical nonlinearity and cannot be explained by the conventional boson model of LSPs. Here, we propose a simple model of saturable LSPs using an effective dipole approximation. The strategy is to directly compare the classical linear optical response of an LSP with that obtained from a saturable quantum two-level system in the limit of weak excitation. The second quantization can then be performed by replacing a classical polarizability with a quantum dipole operator. Taking an ellipsoidal nanometal as an example, we analyse in detail the optical response of a single ellipsoidal nanometal to validate our model. Our numerical results show that the plasmon resonance frequency and spectral linewidth decrease as the aspect ratio of the ellipsoid increases, which is similar to the size dependence observed in early experiments.

Nonlinear optical response during the electron transition process originated from 3D spin-orbit splitting in NiO nanosheets.

NiO, a 3d transition-metal oxide with the strong electron correlation, has attracted great physical attention due to the spin-orbit splitting of 3d electrons. By taking advantage of electron transition process originated from 3d spin-orbit splitting, it may be applied to many photonics areas by linear or nonlinear optical response. To further broaden the photonics applications of NiO, we originally explore the nonlinear optical response, saturable absorption, during the electronic transition due to 3d spin-orbit splitting under a strong optical field and successfully applied in the ultrafast photonics as a mode-locker for the generation of visible laser pulses, which is the result of dynamic balancing process by the electron transition arising from ground state (3A2g) to excited state (1Eg) of spin-orbit splitting in the Ni2+ 3d configurations. With the NiO nanosheet film for saturable absorption, we experimentally realize a pulsed visible laser at a wavelength of 640.3 nm for the first time to our knowledge. These results indicate that the study of electron transition process generated by 3d spin-orbit splitting in 3d transition-metal oxides should be helpful for the development of ultrafast photonics and related devices design.

Comprehensive Study of Plasmonic Materials in the Visible and Near-Infrared: Linear, Refractory, and Nonlinear Optical Properties

Plasmonic nanostructures are used today for a variety of applications. Choosing the best suited plasmonic material for a specific application depends on several criteria, such as chemical and thermal stability, bulk plasma frequency, nonlinear response, and fabrication constraints. To provide a comprehensive summary, we compare these properties for eight different plasmonic materials, namely, Ag, Al, Au, Cu, Mg, Ni, Pd, and Pt. All these materials can be fabricated with electron beam lithography and subsequent evaporation of the desired material. First, we heated rod-antenna-type nanostructures made from these materials up to 1100 °C in air and investigated their linear optical response. Most structures lose their plasmonic properties at temperatures far below the melting point of the respective material. Gold, silver, and platinum structurally deform, whereas the other materials appear to chemically degrade. Second, to improve the thermal stability, structures with a 4 nm thin Al2O3 capping layer are fab...

Selective parallel G-quadruplex recognition by a NIR-to-NIR two-photon squaraine.

Fluorescence imaging probes for specific G-quadruplex (G4) conformations are of considerable interest in biomedical research. Herein, we present the synthesis and the binding properties of a new water-soluble near-infrared (NIR) amphiphilic squaraine dye (CAS-C1) which is capable of selective detection of parallel over non-parallel and non G4 topologies. The striking changes in its linear optical response upon binding to parallel G4s give rise to high fluorescence quantum yields (Φ f ≈ 0.7) and one-photon molecular brightness in the far-red-NIR region. The outstanding recognition process of CAS-C1 for parallel G4s via end-stacking provides binding constants in the nanomolar regime (K b = 107 to 108 M-1) awarding it as one of the most potent parallel G4 binders currently available. Moreover, the CAS-C1-parallel G4 system exhibits large two-photon absorption (TPA) cross-sections and molecular brightness in the second NIR biological transparency window (λ ≈ 1275 nm), making it an ideal candidate for NIR-to-NIR ultrasensitive two-photon procedures.

Generation of high-dimensional energy-time-entangled photon pairs

High-dimensional entangled photon pairs have many excellent properties compared to two-dimensional entangled two-photon states, such as greater information capacity, stronger nonlocality, and higher security. Traditionally, the degree of freedom that can produce high-dimensional entanglement mainly consists of angular momentum and energy time. In this paper, we propose a type of high-dimensional energy-time-entangled qudit, which is different from the traditional model with an extended propagation path. In addition, our method mainly focuses on the generation with multiple frequency modes, while two- and three-dimensional frequency-entangled qudits are examined as examples in detail through the linear or nonlinear optical response of the medium. The generation of high-dimensional energy-time-entangled states can be verified by coincidence counts in the damped Rabi oscillation regime, where the paired Stokes--anti-Stokes wave packet is determined by the structure of resonances in the third-order nonlinearity. Finally, we extend the dimension to $N$ in the sequential-cascade mode. Our results have potential applications in quantum communication and quantum computation.

Linear and nonlinear optical response of crystals using length and velocity gauges: Effect of basis truncation

We study the effects of a truncated band structure on the linear and nonlinear optical response of crystals using four methods. These are constructed by (i) choosing either length or velocity gauge for the perturbation and (ii) computing the current density either directly or via the time-derivative of the polarization density. In the infinite band limit, the results of all four methods are identical, but basis truncation breaks their equivalence. In particular, certain response functions vanish identically and unphysical low-frequency divergences are observed for few-band models in the velocity gauge. Using hexagonal boron nitride (hBN) monolayer as a case study, we analyze the problems associated with all methods and identify the optimal one. Our results show that the length gauge calculations provide the fastest convergence rates as well as the most accurate spectra for any basis size and, moreover, that low-frequency divergences are eliminated.

Optical conductivity of three and two dimensional topological nodal-line semimetals

The peculiar shape of the Fermi surface of topological nodal line semimetals at low carrier concentrations results in their unusual optical and transport properties. We analytically investigate the linear optical responses of three and two-dimensional nodal line semimetals using the Kubo formula. The optical conductivity of a three-dimensional nodal line semimetal is anisotropic. Along the axial direction (\textit{i.e.}, the direction perpendicular to the nodal-ring plane), the Drude weight has a linear dependence on the chemical potential at both low and high carrier dopings. For the radial direction (\textit{i.e.}, the direction parallel to the nodal-ring plane), this dependence changes from linear into quadratic in the transition from low into high carrier concentration. The interband contribution into optical conductivity is also anisotropic. In particular, at large frequencies, it saturates to a constant value for the axial direction and linearly increases with frequency along the radial direction. In two-dimensional nodal line semimetals, no interband optical transition could be induced and the only contribution to the optical conductivity arises from the intraband excitations. The corresponding Drude weight is independent of the carrier density at low carrier concentrations and linearly increases with chemical potential at high carrier doping.

Trion-Species-Resolved Quantum Beats in MoSe2.

Monolayer photonic materials offer a tremendous potential for on-chip optoelectronic devices. Their realization requires knowledge of optical coherence properties of excitons and trions that have so far been limited to nonlinear optical experiments carried out with strongly inhomogeneously broadened material. Here we employ h-BN-encapsulated and electrically gated MoSe2 to reveal coherence properties of trion species directly in the linear optical response. Autocorrelation measurements reveal long dephasing times up to T2 = 1.16 ± 0.05 ps for positively charged excitons. Gate-dependent measurements provide evidence that the positively charged trion forms via spatially localized hole states, making this trion less prone to dephasing in the presence of elevated hole carrier concentrations. Quantum beat signatures demonstrate coherent coupling between excitons and trions that have a dephasing time up to 0.6 ps, a 2-fold increase over those in previous reports. A key merit of the prolonged exciton/trion coherences is that they were achieved in a linear optical experiment and thus are directly relevant to applications in nanolasers, coherent control, and on-chip quantum information processing requiring long photon coherence.