Nonlinear Optical Spectra Research Articles

Direct Comparison of Second Harmonic Generation and Two-Photon Photoluminescence from Single Connected Gold Nanodimers

In this article we compare the two-photon photoluminescence and second harmonic generation from single connected gold nanodimers. Analyzing the particle size-dependent nonlinear optical spectra and performing excitation polarization resolved measurements using an experimental setup combining a femtosecond laser source with a parabolic mirror, we show that second harmonic generation and two-photon photoluminescence have different behaviors despite the same expected fundamental intensity-dependence. For further understanding of the observed phenomena, the plasmon resonances of single nanodimers are investigated using dark-field optical microscopy, and calculations are performed with Green’s tensor method. Furthermore, the underlying mechanisms explaining the differences between these two optical processes are investigated using a surface integral equation method for the nonlinear computations. This study reveals that the different trends in the polarization-dependences of two-photon photoluminescence and se...

Effect of Spectral Density Shapes on the Excitonic Structure and Dynamics of the Fenna–Matthews–Olson Trimer from Chlorobaculum tepidum

The Fenna-Matthews-Olson (FMO) trimer (composed of identical subunits) from the green sulfur bacterium Chlorobaculum tepidum is an important protein model system to study exciton dynamics and excitation energy transfer (EET) in photosynthetic complexes. In addition, FMO is a popular model for excitonic calculations, with many theoretical parameter sets reported describing different linear and nonlinear optical spectra. Due to fast exciton relaxation within each subunit, intermonomer EET results predominantly from the lowest energy exciton states (contributed to by BChl a 3 and 4). Using experimentally determined shapes for the spectral densities, simulated optical spectra are obtained for the entire FMO trimer. Simultaneous fits of low-temperature absorption, fluorescence, and hole-burned spectra place constraints on the determined pigment site energies, providing a new Hamiltonian that should be further tested to improve modeling of 2D electronic spectroscopy data and our understanding of coherent and dissipation effects in this important protein complex.

Nonlinear Optical Microscopy Captures High-Resolution Images of Microstructures Within Three Types of Unlabeled Rat Cartilage

Cartilage is one of the most important tissues in mammalian growth and is classified into three types: hyaline, fibro, and elastic. To illustrate its potential as a clinical diagnostic tool, nonlinear optical microscopy (NLOM) was applied to simultaneously investigate images and spectra from three different types of rat cartilage. High-resolution images were obtained using the two-photon excited fluorescence and second-harmonic generation (SHG) of structures within fresh specimens. Since samples were imaged without hematoxylin and eosin (H&E) staining, detailed observations of tissue morphology and microstructural distribution were obtained without shrinkage or distortion. Nonlinear optical spectra confirmed that elastic cartilage is rich in elastic fibers, of which there are few in both hyaline and fibro cartilages. Structural dimensions were also measured with this technique, revealing the average area of the lacuna <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$(398.36 \pm 52.95\ \mu\text{m}^{2})$</tex-math></inline-formula> , or the average thickness of the territorial matrix <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$(0.83 \pm 0.26\ \mu\text{m})$</tex-math></inline-formula> , within elastic cartilage. Fourier analysis of SHG images was performed to further confirm that collagen in fibrocartilage showed significantly greater organization and directionality compared with collagen in hyaline cartilage <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$(p\ &lt;\ 0.01)$</tex-math></inline-formula> . Our results show that each type of cartilage has different structural features and proportion of matrix, which may significantly contribute to pathology when damaged. Our findings demonstrate that NLOM holds the potential as a clinically relevant diagnostic tool for imaging degenerative tissues or assessing wound repair following cartilage injury.

Nonlinear optical properties and supercontinuum spectrum of titania-modified carbon quantum dots

We have studied the nonlinear optical properties and supercontinuum spectrum of solutions of carbon quantum dots prepared by a hydrothermal process from chitin and then coated with titania. The titania coating has been shown to have an activating effect on the carbon quantum dots, enhancing supercontinuum generation in the blue-violet spectral region and enabling their nonlinear optical characteristics to be varied.

The role of permanent dipoles on the intensity-dependent nonlinear optical properties in asymmetric coupled quantum wells under a static electric field

We study theoretically the intensity-dependent nonlinear optical properties of an asymmetric coupled quantum well under a static electric field. Effects such as nonlinear absorption, optical rectification and refractive index are investigated theoretically by using the density matrix equations including the permanent dipole terms. Our results show that the effects of the permanent dipoles on the corresponding optical processes depend crucially on the direction and strength of the static electric field, especially for large optical intensities. Another interesting result of this work is that the nonlinear optical spectra studied here saturate with an increasing optical intensity and specifically the optical spectra in the case that the permanent dipoles are considered reach saturation for lower intensities than when the permanent dipoles are not considered.

Proposition of a Silica Nanoparticle-Enhanced Hybrid Spin-Microcantilever Sensor Using Nonlinear Optics for Detection of DNA in Liquid.

We theoretically propose a method based on the combination of a nonlinear optical mass sensor using a hybrid spin-microcantilever and the nanoparticle-enhanced technique, to detect and monitor DNA mutations. The technique theoretically allows the mass of external particles (ssDNA) landing on the surface of a hybrid spin-microcantilever to be detected directly and accurately at 300 K with a mass responsivity 0.137 Hz/ag in situ in liquid. Moreover, combined with the nanoparticle-enhanced technique, even only one base pair mutation in the target DNA sequence can be identified in real time accurately, and the DNA hybridization reactions can be monitored quantitatively. Furthermore, in situ detection in liquid and measurement of the proposed nonlinear optical spin resonance spectra will minimize the experimental errors.

Sensitivity of optical mass sensor enhanced by optomechanical coupling

Optical mass sensors based on cavity optomechanics employ radiation pressure force to drive mechanical resonator whose mechanical susceptibility can be described by nonlinear optical transmission spectrum. In this paper, we present an optical mass sensor based on a two-cavity optomechanical system where the mechanical damping rate can be decreased by adjusting a pump power so that the mass sensitivity which depends on the mechanical quality factor has been enhanced greatly. Compared with that of an optical mass sensor based on single-cavity optomechanics, the mass sensitivity of the optical mass sensor is improved by three orders of magnitude. This is an approach to enhance the mass sensitivity by means of optomechanical coupling, which is suitable for all mass sensor based on cavity optomechanics. Finally, we illustrate the accurate measurement for the mass of a few chromosomes, which can be achieved based on the current experimental conditions.

Nonlinear optical absorption via two-photon process in [formula omitted] quantum well

We study the combined effects of hydrostatic pressure, aluminum concentration, well width, and temperature on linear and nonlinear optical absorption spectrum in GaAs/Ga1−xAlxAs quantum well under the applied magnetic field via investigating the phonon-assisted cyclotron resonance (PACR) effect. Our result shows that the optical absorption behaviors are affected strongly by hydrostatic pressure, aluminum concentration, temperature and well width. Besides, the present work discovers a clear monotonic behavior of the resonant peaks and the half-width as functions of the pressure, concentration, temperature or well width in both one- and two-photon absorption processes.

Atomistic Modeling of Two-Dimensional Electronic Spectra and Excited-State Dynamics for a Light Harvesting 2 Complex

The Light Harvesting 2 (LH2) complex is a vital part of the photosystem of purple bacteria. It is responsible for the absorption of light and transport of the resulting excitations to the reaction center in a highly efficient manner. A general description of the chromophores and the interaction with their local environment is crucial to understand this highly efficient energy transport. Here we include this interaction in an atomistic way using mixed quantum-classical (molecular dynamics) simulations of spectra. In particular, we present the first atomistic simulation of nonlinear optical spectra for LH2 and use it to study the energy transport within the complex. We show that the frequency distributions of the pigments strongly depend on their positions with respect to the protein scaffold and dynamics of their local environment. Furthermore, we show that although the pigments are closely packed the transition frequencies of neighboring pigments are essentially uncorrelated. We present the simulated linear absorption spectra for the LH2 complex and provide a detailed explanation of the states responsible for the observed two-band structure. Finally, we discuss the energy transfer within the complex by analyzing population transfer calculations and 2D spectra for different waiting times. We conclude that the energy transfer from the B800 ring to the B850 ring is mediated by intermediate states that are delocalized over both rings, allowing for a stepwise downhill energy transport.

Nonlinear optical absorption via two-photon process in asymmetrical Gaussian potential quantum wells

In this paper, linear and nonlinear optical absorption spectrum in asymmetrical Gaussian potential quantum wells under the applied magnetic and electric fields are studied via investigating the phonon-assisted cyclotron resonance (PACR) effect. The results are calculated for GaAs and Ga1−xAlxAs materials. Our results show that the optical absorption behaviors and the half-width are significantly dependent on the height of the Gaussian potential, the well width, the magnetic field, and the temperature. It is also found that there is a clear monotonic behavior of the resonant peaks and the half-width as functions of the factors mentioned above in both one and two-photon absorption processes.

Nonlinear optical spectra having characteristics of Fano interferences in coherently coupled lowest exciton biexciton states in semiconductor quantum dots

Optical nonlinear effects are examined using a two-color micro-photoluminescence (micro-PL) method in a coherently coupled exciton-biexciton system in a single quantum dot (QD). PL and photoluminescence excitation spectroscopy (PLE) are employed to measure the absorption spectra of the exciton and biexciton states. PLE for Stokes and anti-Stokes PL enables us to clarify the nonlinear optical absorption properties in the lowest exciton and biexciton states. The nonlinear absorption spectra for excitons exhibit asymmetric shapes with peak and dip structures, and provide a distinct contrast to the symmetric dip structures of conventional nonlinear spectra. Theoretical analyses with a density matrix method indicate that the nonlinear spectra are caused not by a simple coherent interaction between the exciton and biexciton states but by coupling effects among exciton, biexciton and continuum states. These results indicate that Fano quantum interference effects appear in exciton-biexciton systems at QDs and offer important insights into their physics.

Complex third-order nonlinear optical susceptibility spectrum of a CdS film coat on a silver nanoparticle

We have fabricated a CdS-coated Ag nanoparticle using the reverse micelle method and observed the nonlinear optical response of the scattered light intensity caused by an individual particle. In addition, we succeeded in obtaining the complex third-order nonlinear susceptibility χ(3) spectrum near the energy band edge of the CdS film by comparing the experimental result and the result of the Mie calculation that considered the optical Kerr effect of the CdS. At ħωi = 2.43 eV, the χ(3) of the CdS was (1.1 + i0.6) × 10−17m2/V2. We conjecture that an increase in the carriers in the interband transition of the CdS originates from the increase in the χ(3) of the CdS at ħωi > 2.4eV.

Electric field effect on the third-order nonlinear optical susceptibility in inverted core–shell nanodots with dielectric confinement

Third-order nonlinear optical processes associated with the interlevel transitions in ZnS/CdSe core–shell quantum dots under electric fields are theoretically investigated. Taking into account the dielectric mismatch with the surrounding matrix, the electronic structure of the dots is obtained within the effective mass and parabolic band approximations. It is shown that large applied electric fields break the symmetry of the confinement potential and lead to a significant blue-shift of the peak positions in the nonlinear optical spectrum. The size effect is also discussed and it is proved that large nonlinear susceptibility can be obtained by increasing the thickness of the nanocrystal shell. Our results suggest that external factors such as the applied electric field and orientation of the incident light polarization can be used – in addition to spatial confinement – to improve the performances of the optical devices.

On the Shape of the Phonon Spectral Density in Photosynthetic Complexes

We provide a critical assessment of typical phonon spectral densities, J(ω), used to describe linear and nonlinear optical spectra in photosynthetic complexes. Evaluation is based on a more careful comparison to experiment than has been provided in the past. J(ω) describes the frequency-dependent coupling of the system to the bath and is an important component in calculations of excitation energy transfer times. On the basis of the shape of experimental J(ω) obtained for several photosynthetic complexes, we argue that the shape of J(ω) strongly depends on the pigment-protein complex. We show that many densities (especially the Drude-Lorentz/constant damping Brownian oscillator) display qualitatively wrong behavior when compared to experiment. Because of divergence of J(ω) at zero frequency, the Brownian oscillator cannot fit a single-site spectrum correctly. It is proposed that a log-normal distribution can be used to fit experimental data and exhibits desired attributes for a physically meaningful phonon J(ω), in contrast to several commonly used spectral densities which exhibit low-frequency behavior in qualitative disagreement with experiment. We anticipate that the log-normal J(ω) function proposed in this work will be further tested in theoretical modeling of both time- and frequency-domain data.

Nonlinear optical absorption and refraction in a strained anisotropic multi-level quantum dot system

Linear and nonlinear optical properties of disc shaped anisotropic multi-level quantum dot (QD) system has been theoretically investigated. The effect of dot size, shape anisotropy, strain and incident optical intensity on linear absorption, nonlinear absorption and nonlinear refractive index has been explored. The QD is modeled by in-plane anisotropic parabolic potential along x–y plane and by finite well potential along growth direction (z-axis). The contribution of strain is incorporated through various deformation potentials. The energy and wave function calculations are performed by multi-band envelope function approach based on k.p theory. The formulation is applied to the CdSe/CdS QD system. The numerical results show that, dot size, anisotropy and optical intensity have important effect on linear and nonlinear optical properties. The effect of strain is simultaneous red and blue shift of heavy hole (hh) and light hole (lh) transitions, respectively, which is clearly visible in terms of well resolved optical spectra. The theoretical results obtained are compared with the available experimental data and the results are in good agreement. Large blue shift and enhancement in magnitude of linear and nonlinear optical spectra of QD with size, anisotropy and strain make QD a promising candidate for application in tunable Nano-optoelectronic devices.

DFT Studies on the Electronic Spectrum and the Second-Order Nonlinear Optical Properties of Donor-Acceptor Helicenes

A series of chiral donor-acceptor helicenes have been designed using the molecular engineering of organic nonlinear optical materials. The geometries of donor-acceptor helicenes 1, 2, 3, 4, 5 are optimized using density functional theory (DFT-B3LYP) method at the 6-31g (d, p) basis set level. Based on the obtained stable molecular configuration, we adopt the TDHT/PM3 method and time-dependent density-functional theory (TD-DFT) to calculate the nonlinear optical (NLO) properties and electronic spectra of these molecules. Results show that molecule 4 has the largest static hyperpolarizability βμ of 40.4×10-30esu, which is enhanced by its two charge transfer orientations. In molecule 2, there exists two competitive charge transfers that weaken the second-order nonlinear optical effect of D-π-A structures greatly. The structural type of molecule 3, 4 and 5 are expected to be more efficient to achieve large β values.

Study on the Second-Order Nonlinear Optical Properties of [6]Helicenes with Chromophores

A series of chiral [6]helicenes have been designed using the molecular engineering of organic nonlinear optical materials. The geometries of [6]helicenes 1, 2, 3, 4,5 are optimized using density functional theory (DFT-B3LYP) method at the 6-31g (d, p) basis set level. Based on the obtained stable molecular configuration, we adopt the TDHT/PM3 method and time-dependent density-functional theory (TD-DFT) to calculate the nonlinear optical (NLO) properties and electronic spectra of these molecules. Results show that the static hyperpolarizability βµ alternates between positive value and negative value, whereas it remains positive for the molecues 2 and 3 which have medium magnitudes βµ, 3.4×10-30esu and 9.6×10-30esu respectively. In molecule 5, there exists two competitive charge transfers that reduce the hyperpolarizability β.

Two-photon absorption spectra of a near-infrared 2-azaazulene polymethine dye: solvation and ground-state symmetry breaking

Polymethine dyes (PDs) with absorption bands in the near-infrared region undergo symmetry breaking in polar solvents. To investigate how symmetry breaking affects nonlinear optical responses of PDs, an extensive and challenging experimental characterization of a cationic 2-azaazulene polymethine dye, including linear absorption, fluorescence, two-photon absorption and excited-state absorption, has been performed in two solvents with different polarity. Based on this extensive set of experimental data, a three-electronic-state model, accounting for the coupling of electronic degrees of freedom to molecular vibrations and polar solvation, has been reliably parameterized and validated for this dye, fully rationalizing optical spectra in terms of spectral position, intensities and bandshapes. In low-polarity solvents where the dye is mainly in its symmetric form, a nominally forbidden two-photon absorption band is observed, due to a vibronic activation mechanism. Inhomogeneous broadening plays a major role in polar solvents: absorption spectra represent the weighted sum of contributions from states with a variable amount of symmetry breaking, leading to a complex evolution of linear and nonlinear optical spectra with solvent polarity. In more polar solvents, the dominant role of the asymmetric form leads to the activation of two-photon absorption as a result of the symmetry lowering. The subtle interplay between the two mechanisms for two-photon absorption activation, vibronic coupling and polar solvation, can be fully accounted for within the proposed microscopic model allowing a detailed interpretation of the optical spectra of PDs.

Dissecting biexciton wave functions of self-assembled quantum dots by double-quantum-coherence optical spectroscopy

Biexcitons feature prominently in various scenarios for utilization of quantum dots (QDs) for enhancing the efficiencies of solar cells, and for the generation of entangled photon pairs in single QD sources. Two-dimensional double quantum coherence (2D-DQC) nonlinear optical spectra provide novel spectroscopic signatures of such states beyond global intensity and lifetime characteristics which are available by more conventional techniques. We report the simulation of a prototype 2D-DQC optical experiment of a self-assembled InAs/GaAs dot. The simulations consider the QD in different charged states and are based on a state-of-the-art atomistic many-body pseudopotential method for the calculation of the electronic structure and transition dipole matrix elements. Comparison of the spectra of negatively charged, neutral, and positively charged QD reveals optical signatures of their electronic excitations. This technique directly accesses the biexciton ($XX$) energies as well as the projections of their wave functions on the single-exciton manifold. These signals also provide a unique tool for probing the charged state of the QD and thus the occupation of the quantum state. Signatures of Pauli blockade of the creation of certain single and two excitons due to charges on the particles are observed. For all quantum states of the QD, the spectra reveal a strong multiconfiguration character of the biexciton wave functions. Peak intensities can be explained by interference of the contributing Liouville space pathways.

Optical and thermal characteristics of glasses based on TeO 2

Glass samples have been synthesized in quaternary system based on TeO 2–oxide within composition, 85TeO 2–5Nb 2O 5–5ZnO–5Ag 2O, 68TeO 2–5Nb 2O 5–20ZnO–7Na 2O and [(75−x)TeO 2–5Nb 2O 5–20ZnO–xPbO, x = 7, 18 mol%]. Structural characterization of the glasses was studied with respect to their thermal stability, refractive indices, third order nonlinear optical susceptibility, IR spectra and Vickers hardness. For four different prepared glasses, density in the range from 5·3744 to 6·0731 g· cm − 1, the glass transition temperature (T g) in the range from 326 to 350 °C and refractive indices, n, in the range from 2·1273 to 2·2123 at 435 nm and Vickers hardness, H v, in the range from 2·91 to 3·44 GPa were determined. The value of third order nonlinear optical susceptibilities \({\vert} {\chi}^{\text{(3)}}{\vert} {\approx} \) 17·9 ·10 − 13 esu of glass within composition, 68TeO 2–5Nb 2O 5–20ZnO–7PbO, was measured by using degenerate four-wave mixing (DFWM).