Nonlinear Emission Research Articles

Tunable Bragg polaritons and nonlinear emission from a hybrid metal-unfolded ZnSe-based microcavity

Strong light-matter interaction in Bragg structures possesses several advantages over conventional microcavity system. These structures provide an opportunity to incorporate a large number of quantum wells without increasing the mode volume. Further, it is expected that the strong coupling could occur over the entire thickness of the Bragg structure, and the system offers an improved overlap between exciton wave function and light mode. However, advanced experiments in Bragg structures require a precise control and manipulation of quantum states of Bragg polaritons. Here, we propose and experimentally demonstrate novel methods for the modulation of Bragg polariton eigenstates. The modulation will be shown to even exceed 10 meV if the thickness of the top layer of the ZnSe-based Bragg structure is changed or if a thin silver layer is deposited on top of the structure. The Q value of the Bragg mode will be enhanced by a factor of 2.3 for a 30 nm silver layer. In addition, we report on the observation of nonlinear emission of the lower Bragg polariton mode in the hybrid structure being achieved when excitation dependent measurements are performed. Our results open the door to create a confined Bragg polariton system similar to conventional microcavities.

π-Conjugated Discrete Oligomers Containing Planar and Nonplanar Aromatic Motifs.

A new family of π-conjugated oligomers featuring a nonplanar polycyclic aromatic hydrocarbon, corannulene, and a planar aromatic unit, thiophene, is synthesized through an iterative metal-catalyzed coupling protocol. The two structural motifs are connected through an acetylene linkage. In the shorter oligomers, a thiophene unit is attached to one or two corannulenes. In the higher analogues, two, three, and four thiophene units are placed in an alternating fashion with three, four, and five corannulene units, respectively. Photophysical studies reveal extended π-effects that initially increase and then attenuate as a function of the oligomer length. Notably, longer oligomers are found to be highly active for nonlinear absorption and emission properties. The oligomer with three corannulene and two thiophene units exhibits a two-photon absorption cross section of 600 GM and two-photon-excited intense green luminescence. This work, therefore, introduces the concept of combining planar and nonplanar aromatic motifs in the design of π-conjugated discrete oligomers, establishes synthetic feasibility of such hybrid materials, reports on their photophysical properties that is anticipated to have significant implications for future research targets, and features the discovery that corannulene derivatives can exhibit excellent nonlinear optical activity when extended through π-bridges.

Vectorial control of nonlinear emission via chiral butterfly nanoantennas: generation of pure high order nonlinear vortex beams.

We report on a chiral gap-nanostructure, which we term a "butterfly nanoantenna," that offers full vectorial control over nonlinear emission. The field enhancement in its gap occurs for only one circular polarization but for every incident linear polarization. As the polarization, phase and amplitude of the linear field in the gap are highly controlled, the linear field can drive nonlinear emitters within the gap, which behave as an idealized Huygens source. A general framework is thereby proposed wherein the butterfly nanoantennas can be arranged in a metasurface, and the nonlinear Huygens sources exploited to produce a highly structured far-field optical beam. Nonlinearity allows us to shape the light at shorter wavelengths, not accessible by linear plasmonics, and resulting in high purity beams. The chirality of the butterfly allows us to create orbital angular momentum states using a linearly polarized excitation. A third harmonic Laguerre-Gauss beam carrying an optical orbital angular momentum of 41 is demonstrated as an example, through large-scale simulations on a high-performance computing platform of the full plasmonic metasurface with an area large enough to contain up to 3600 nanoantennas.

A Comprehensive Evaluation of Microwave Emissivity and Brightness Temperature Sensitivities to Soil Parameters Using Qualitative and Quantitative Sensitivity Analyses

Passive microwave remote sensing has experienced significant success for soil moisture (SM) inversion. However, quantifying the uncertainties caused by soil parameter sensitivities has not attracted sufficient attention. Although local sensitivity analysis (SA) has been used to describe parameter sensitivity in the past, it fails to quantify parameter sensitivities, especially interactions, for nonlinear microwave emission models. This paper presents a comprehensive evaluation that combines physically based emission models and various global SA algorithms to evaluate parameter sensitivity. All the algorithms exhibit highly consistent sensitivity measures, which means a reliable SA result is obtained. The results indicate that the sums of the main sensitivity indices of SM and surface roughness parameters—root-mean-square height (RMSH) and correlation length—are greater than 0.92 and 0.95 for emissivity and brightness temperature (TB), respectively. Furthermore, we find that: 1) the parameter probability distributions have little effect on the sensitivity measures; 2) the SM sensitivity decreases and the RMSH sensitivity increases as the frequency increases and the incidence angle decreases; and 3) the SM is more sensitive on V-polarized than on H-polarized emissivity and TB, while the RMSH is much more sensitive on the polarization index. The presented global SA quantitatively explains the optimal frequency, incidence angle, and polarization for SM inversion and extends the parameter SA for microwave emission models to a more general framework, as well as provides an implication for bare soil emission modeling and SM inversion.

Two-color pyrometry for low amplitude periodic heating.

Specimens subject to periodic heating must be probed for a calibrated temperature response if standard measurements of thermal diffusivity are to be extended to determine thermal conductivity. A variation on two-color pyrometry is developed to measure both the offset and harmonic amplitudes of temperature fluctuations caused by periodic heating. The requisite pyrometric formulae are derived for low amplitude heating using an expansion of the nonlinear thermal emission. Well-defined uncertainties in the temperature values are determined from experimental uncertainties in radiometric measurements. The accuracy demonstrated in this work is better than 2% for the temperature offset and 3%-8% for the fluctuating temperature amplitude.

Maxwell–Hydrodynamic Model for Simulating Nonlinear Terahertz Generation From Plasmonic Metasurfaces

The interaction between the electromagnetic field and plasmonic nanostructures leads to both the strong linear response and inherent nonlinear behavior. In this paper, a time-domain hydrodynamic model for describing the motion of electrons in plasmonic nanostructures is presented, in which both surface and bulk contributions of nonlinearity are considered. A coupled Maxwell-hydrodynamic system capturing full-wave physics and free electron dynamics is numerically solved with the parallel finite-difference time-domain (FDTD) method. The validation of the proposed method is presented to simulate linear and nonlinear responses from a plasmonic metasurface. The linear response is compared with the Drude dispersion model and the nonlinear terahertz emission from a difference-frequency generation process is validated with theoretical analyses. The proposed scheme is fundamentally important to design nonlinear plasmonic nanodevices, especially for efficient and broadband THz emitters.

Mixed Equilibria with Common Constraints on Transportation Networks

This study concerns about modeling the mixed equilibrium (ME) problem, including user equilibrium, system optimum, and Cournot-Nash players, with general common constraints (CCs) on transportation networks. The CCs capture the interactions of the decision variables of different players in ME, which could be internal interactions such as road link capacity constraints or external such as emission or congestion control policies. It is shown that ME with CCs can be modeled as a generalized Nash equilibrium problem (GNEP). The study proves that, under certain conditions, the GNEP-based ME is jointly convex, which can be reformulated as a variational inequality (VI). We then study the solution existence, uniqueness and solving method for the VI-based model, followed by a discussion on its potential applications. Numerical tests are conducted with common nonlinear link emission constraints as the CCs on a simple two-node, three-link network first, and then on the Nguyen Dupus network. The results show that modeling users’ route choice behavior with CCs is more general in evaluating system performance, planning link capacities, and making congestion or emission control related policies.

Metamorphic plasmonic nanoantennas for self-enhanced nonlinear light generation

Nanoplasmonic antennas are well established for focusing light from the far field into subwavelength-sized dimensions, while simultaneously increasing strongly the local electromagnetic field—an important ingredient for boosting nonlinear optical effects. Here, we study both the optical and structural properties of gold bowtie nanoantennas under illumination with femtosecond laser pulses and observe a pronounced metamorphism of the antennas, while the upconverted incoherent nonlinear emission increases simultaneously. Scanning electron microscopy images recorded before and after illumination show a clear modification of the antenna feedgap, accompanied by an 100× increase of the nonlinear signal. This is caused by laser-induced electromigration of gold nanoparticles, a process that is feedgap-size-dependent, self-limiting, and irreversible. Moreover, it is identified as the root cause for the strong enhancement of the nonlinear conversion efficiency by a factor of ≥4×106 as compared with an unpatterned gold film. By experimentally quantifying the electric field enhancement inside the nanoantenna feedgap to be >2000×, we demonstrate consistency with the observed enhancement of the nonlinear signal. Complete switching off of the nonlinear response of such metamorphic nanoantennas with a degree of polarization >99% indicates their potential for novel, nonlinear all-optical devices. Furthermore, we envision the controlled, laser-induced modification of plasmonic nanoantennas may provide a promising route to realize antennas with even higher field enhancements and, moreover, might enable deterministic quantum plasmonic experiments that require sub-nanometer-sized feedgaps.

Enhancement of Upconverted Fluorescence by Interference Layers.

Upconverting nanoparticles show potential applications in the field of photovoltaics and array-based detection devices. While fluorescence enhancement using interference of incident radiation is well known in Stokes-shift type systems such as fluorescent dyes; the effect of such interference geometry in nonlinear Anti-Stokes type emission, such as in upconversion rare earth photophysics is demonstrated for the first time. This work describes in detail the influence of the interference modulation on both the excitation (interion energy transfer) and radiative decay with nonradiative decay processes active between emissive levels. These effects are illustrated in the thickness dependence of the decay rate and rise time. Single particle upconverted spectra and time-resolved measurements show concurrent optimization of the infrared absorption and emission at 540 and 650 nm, with an average enhanced emission of 20 times at λ = 540 and 45 times at λ = 650 nm, dependent on the interference layer thickness and on the excitation intensity. The experimental results are correlated with finite element modeling. Both experiments and calculations show emission enhancement at an interference layer thickness of about 740 ± 20 nm, where such tolerance and the planar design, leads to ease in implementation in applications.

Optically Active Materials: Aggregation Induced Enhancement of Linear and Nonlinear Optical Emission from a Hexaphenylene Derivative (Adv. Funct. Mater. 48/2016)

Optically Active Materials: Aggregation Induced Enhancement of Linear and Nonlinear Optical Emission from a Hexaphenylene Derivative (Adv. Funct. Mater. 48/2016)

Mechanistic Understanding of Excitation-Correlated Nonlinear Optical Properties in MoS2 Nanosheets and Nanodots: The Role of Exciton Resonance

Low-dimensional molybdenum disulfide (MoS2) materials such as nanosheets and nanodots exhibit exotic optical, electronic, and catalytic properties, but so far the limited understanding of their nonlinear optical properties has restricted their potential use in nonlinear optoelectronics. In this work, we demonstrate that chemically prepared MoS2 nanosheets and nanodots have distinctive nonlinear emission properties: the former shows efficient second harmonic generation (SHG) with maximum intensity at its C-exciton resonance energy while the latter exhibits strong excitation-correlated two-photon luminescence (TPL). We combine two-photon photoluminescence excitation (TPPLE) and the Z-scan spectroscopies to study the second order response of the MoS2 nanodots and reveal, for the first time, that the most efficient and tunable TPL occurs through resonant two-photon absorption (TPA) induced transition from the 1Sh to 1Pe exciton state, followed by phonon-mediated exciton relaxation (1Pe → 1Se) and fast transit...

Tailorable second‐harmonic generation from an individual nanowire using spatially phase‐shaped beams

AbstractThe ability to control the optical field in the vicinity of an individual nano‐object is an obvious stepping‐stone in the tailoring of light‐matter interactions at the nanoscale. Earlier reports on tailoring light fields in the vicinity of a nano‐object have been restricted by their dependence on cumbersome optical or fabrication techniques, have relied mostly on in‐plane electric field polarizations, and have been demonstrated only for bulk materials and structures with strong in‐plane anisotropies. In addition, traditional methods for manipulating the longitudinal electric fields are significantly hindered by the lack of appropriate probes that can be used to unambiguously measure or calibrate the light coupling efficiency to nano‐objects. Here, we demonstrate such a possibility for the specific case of optical second‐harmonic generation (SHG). Our technique relies on spatial phase‐shaping of a high‐order laser beam to tailor the longitudinal fields at the beam focus and allows SHG from an individual and well‐defined vertically‐aligned GaAs nanowire to be manipulated on demand. Our technique is applicable to tailoring the efficiency of nonlinear emission on the nanoscale and to arbitrary polarization control at the beam focus in general. image

Plasmon-Enhanced Second Harmonic Generation: from Individual Antennas to Extended Arrays

We analyze the emission yield of the second harmonic generation (SHG) from dense ordered arrays of L-shaped Au nanoantennas within a well-defined collection angle and compare it to that of the isolated nanostructures designed with the same geometrical parameters. Thanks to the high antenna surface density, arrays display one order of magnitude higher SHG yield per unit surface with respect to isolated nanoantennas. The difference in the collected nonlinear signals becomes even more pronounced by reducing the collection angle, because of the efficient angular filtering that can be attained in dense arrays around the zero order. Albeit this key-enabling feature allows envisioning application of these platforms to nonlinear sensing, a normalization of the SHG yield to the number of excited antennas in the array reveals a reduced nonlinear emission from each individual antenna element. We explain this potential drawback in terms of resonance broadening, commonly observed in densely packed arrays, and angular filtering of the single antenna emission pattern provided by the array 0th order.

Nonlinear Generation of Vector Beams From AlGaAs Nanoantennas.

The quest for nanoscale light sources with designer radiation patterns and polarization has motivated the development of nanoantennas that interact strongly with the incoming light and are able to transform its frequency, radiation, and polarization patterns. Here, we demonstrate dielectric AlGaAs nanoantennas for efficient second harmonic generation, enabling the control of both directionality and polarization of nonlinear emission. This is enabled by specialized III-V semiconductor nanofabrication of high-quality AlGaAs nanostructures embedded in optically transparent low-index material, thus allowing for simultaneous forward and backward nonlinear emission. We show that the nanodisk AlGaAs antennas can emit second harmonic in preferential direction with a backward-to-forward ratio of up to five and can also generate complex vector polarization beams, including beams with radial polarization.

Modeling of Infrared-Visible Sum Frequency Generation Microscopy Images of a Giant Liposome.

The article explores the theory of infrared-visible sum frequency generation microscopy of phospholipid envelopes with dimensions larger than the wavelength of the nonlinear emission. The main part of the study concerns derivation and accounting for the contributions of effective nonlinear responses specific to sites on the surfaces of a bilayer envelope and their dependence on polarization condition and experimental geometry. The nonlinear responses of sites are mapped onto the image plane according to their emission directions and the numerical aperture of a sampling microscope objective. According to the simulation results, we discuss possible approaches to characterize the shape of the envelope, to extract molecular hyperpolarizabilities, and to anticipate possible heterogeneity in envelope composition and anisotropy of the environment proximal to the envelope. The modeling approach offers a promising analytic facility to assist connecting microscopy observations in engineered liposomes, cellular envelopes, and subcellular organelles of relatively large dimensions to molecular properties, and hence to chemistry and structure down to available spatial resolution.

Nonlinear Structured Illumination Using a Fluorescent Protein Activating at the Readout Wavelength.

Structured illumination microscopy (SIM) is a wide-field technique in fluorescence microscopy that provides fast data acquisition and two-fold resolution improvement beyond the Abbe limit. We observed a further resolution improvement using the nonlinear emission response of a fluorescent protein. We demonstrated a two-beam nonlinear structured illumination microscope by introducing only a minor change into the system used for linear SIM (LSIM). To achieve the required nonlinear dependence in nonlinear SIM (NL-SIM) we exploited the photoswitching of the recently introduced fluorophore Kohinoor. It is particularly suitable due to its positive contrast photoswitching characteristics. Contrary to other reversibly photoswitchable fluorescent proteins which only have high photostability in living cells, Kohinoor additionally showed little degradation in fixed cells over many switching cycles.

Plasmonic enhancement and polarization dependence of nonlinear upconversion emissions from single gold nanorod@SiO2@CaF2:Yb3+,Er3+ hybrid core\u2013shell\u2013satellite nanostructures

Lanthanide-doped upconversion nanocrystals (UCNCs) have recently become an attractive nonlinear fluorescence material for use in bioimaging because of their tunable spectral characteristics and exceptional photostability. Plasmonic materials are often introduced into the vicinity of UCNCs to increase their emission intensity by means of enlarging the absorption cross-section and accelerating the radiative decay rate. Moreover, plasmonic nanostructures (e.g., gold nanorods, GNRs) can also influence the polarization state of the UC fluorescence—an effect that is of fundamental importance for fluorescence polarization-based imaging methods yet has not been discussed previously. To study this effect, we synthesized GNR@SiO2@CaF2:Yb3+,Er3+ hybrid core–shell–satellite nanostructures with precise control over the thickness of the SiO2 shell. We evaluated the shell thickness-dependent plasmonic enhancement of the emission intensity in ensemble and studied the plasmonic modulation of the emission polarization at the single-particle level. The hybrid plasmonic UC nanostructures with an optimal shell thickness exhibit an improved bioimaging performance compared with bare UCNCs, and we observed a polarized nature of the light at both UC emission bands, which stems from the relationship between the excitation polarization and GNR orientation. We used electrodynamic simulations combined with Förster resonance energy transfer theory to fully explain the observed effect. Our results provide extensive insights into how the coherent interaction between the emission dipoles of UCNCs and the plasmonic dipoles of the GNR determines the emission polarization state in various situations and thus open the way to the accurate control of the UC emission anisotropy for a wide range of bioimaging and biosensing applications.

Changes in the Compressive Nonlinearity of the Cochlea During Early Aging: Estimates From Distortion OAE Input/Output Functions.

The level-dependent growth of distortion product otoacoustic emissions (DPOAEs) provides an indirect metric of cochlear compressive nonlinearity. Recent evidence suggests that aging reduces nonlinear distortion emissions more than those associated with linear reflection. Therefore, in this study, we generate input/output (I/O) functions from the isolated distortion component of the DPOAE to probe the effects of early aging on the compressive nonlinearity of the cochlea. Thirty adults whose ages ranged from 18 to 64 years participated in this study, forming a continuum of young to middle-age subjects. When necessary for analyses, subjects were divided into a young-adult group with a mean age of 21 years, and a middle-aged group with a mean age of 52 years. All young-adult subjects and 11 of the middle-aged subjects had normal hearing; 4 middle-aged ears had slight audiometric threshold elevation at mid-to-high frequencies. DPOAEs (2f1 - f2) were recorded using primary tones swept upward in frequency from 0.5 to 8 kHz, and varied from 25 to 80 dB sound pressure level. The nonlinear distortion component of the total DPOAE was separated and used to create I/O functions at one-half octave intervals from 1.3 to 7.4 kHz. Four features of OAE compression were extracted from a fit to these functions: compression threshold, range of compression, compression slope, and low-level growth. These values were compared between age groups and correlational analyses were conducted between OAE compression threshold and age with audiometric threshold controlled. Older ears had reduced DPOAE amplitude compared with young-adult ears. The OAE compression threshold was elevated at test frequencies above 2 kHz in the middle-aged subjects by 19 dB (35 versus 54 dB SPL), thereby reducing the compression range. In addition, middle-aged ears showed steeper amplitude growth beyond the compression threshold. Audiometric threshold was initially found to be a confound in establishing the relationship between compression and age; however, statistical analyses allowed us to control its variance. Correlations performed while controlling for age differences in high-frequency audiometric thresholds showed significant relationships between the DPOAE I/O compression threshold and age: Older subjects tended to have elevated compression thresholds compared with younger subjects and an extended range of monotonic growth. Cochlear manifestations of nonlinearity, such as the DPOAE, weaken during early aging, and DPOAE I/O functions become linearized. Commensurate changes in high-frequency audiometric thresholds are not sufficient to fully explain these changes. The results suggest that age-related changes in compressive nonlinearity could produce a reduced dynamic range of hearing, and contribute to perceptual difficulties in older listeners.

Simulations of high harmonic generation from plasmonic nanoparticles in the terahertz region

Metallic nanostructures are known for enhancing weak nonlinear processes when a resonant and coherent excitation takes place. With proper structural design, an additional boost of particular nonlinear processes is expected to be possible. Here, we present a numerical technique that is capable of simulating high harmonic generation from resonantly excited metallic nanoparticles in the terahertz frequency range. We demonstrate our method by investigating the nonlinear emission of arrays of plasmonic split-ring resonators at the range of ten terahertzs. Our multiscale, non-perturbative, and microscopic approach is based on a self-consistent combination of a hydrodynamic model for the nonlinear electronic material response and the discontinuous Garlerkin time-domain technique for the evaluation of the propagation of the electromagnetic field. It is predicted that the electronic nonlinearities of plasmonic nanoparticles give rise to several harmonics in the light emission when excited by intense terahertz radiation. Furthermore, our analysis predicts a non-perturbative scaling of higher harmonics at high excitation intensities.

Aggregation Induced Enhancement of Linear and Nonlinear Optical Emission from a Hexaphenylene Derivative

The discovery of the phenomenon known as aggregation‐induced emission (AIE) has opened the door to a variety of brilliant organic solid‐state light‐emitting materials. While AIE is well established in linear optics, the development of AIE luminogens (AIEgens) with highly efficient nonlinear optical (NLO) effects remains relatively unexplored. Particularly, second‐order NLO requires the AIEgens to be organized in a non‐centrosymmetric fashion, and such examples are rarely reported. Here, an AIEgen, 2,7‐di([1,1′‐biphenyl]‐4‐yl)‐fluorenone (4‐DBpFO), is designed and synthesized by introducing a carbonyl group onto the backbone of p‐hexaphenylene. The restricted rotation of the compound upon aggregation results in a dramatic enhancement of the linear optical emission when forming self‐assemblies. Furthermore, introducing the carbonyl group drives the formation of hydrogen bonded molecular chains, which are attached by the zigzag CH⋅⋅⋅π interactions in a non‐centrosymmetric way. As a result, the dipole of each individual molecule contributes accumulatively to a macroscopic dipole of the formed 4‐DBpFO microcrystals. This leads to a highly efficient second harmonic generation with very high laser damage treshold. This AIEgen, whose optical response is greatly enhanced in both linear and nonlinear optical regimes upon the formation of well‐defined self‐assemblies, has potential applications in next generation photonic circuits.