Bulk Nonlinearity Research Articles

Identification of weakly to strongly-turbulent three-wave processes in a micro-scale system

We find capillary wave turbulence (WT) spanning multiple dynamical regimes and geometries, all within a 40μL volume microfluidic system. This study is made viable with recent advances in ultra-high-speed digital holographic microscopy, providing 10μs time and 10nm spatial resolutions for images across the entire field at speeds sufficient to capture the salient wave phenomena. The observed WT types are: (i) discrete wave turbulence (DWT) dominated by finite domain effects, (ii) kinetic wave turbulence (KWT) that approximately satisfies weak wave turbulence (WWT) theory, and (iii) intermediate wave turbulence (IWT) that exhibits features from both DWT and KWT. We show that WT regime depends on input power and wavenumber, and we provide simple nondimensional parameters – derived from WWT theory – for intra-spectrum regime classification. Using the nondimensional parameters, a bulk nonlinearity metric is defined that employs bicoherence-based weighting. Analysis of experimental results reveals a correspondence between the theoretical regime classifiers and the observed phenomena. At sufficiently high input powers, the phenomena substantially depart from the WWT theory and reveal a regime of strongly nonlinear wave turbulence (SWT) defined by shallower spectral slopes that achieve a constant slope value over a range of input powers. This may suggest a corresponding power-law solution to the governing equations. This work augments current understanding of WT regimes and behaviors, and directly applies to many fields beyond fluid mechanics. For example, SWT appears upon the fluid interface at powers less than required for atomization, indicating that further study of SWT is needed to properly understand ultrasound-driven fuel spray atomization and drug and agricultural nebulization.

Few-photon transport via a multimode nonlinear cavity: Theory and applications

Few-photon transport via waveguide-coupled local quantum systems has attracted extensive theoretical and experimental studies. Most of the study has focused on atomic or atomic-like local quantum systems due to their strong light-matter interaction useful for quantum applications. Here, we study few-photon transport via a waveguide-coupled multimode optical cavity with second-order bulk nonlinearity. We develop a Feynman diagram approach and compute the scattering matrix of the one- and two-photon transport. Based on the calculated scattering matrix, we show highly nonclassical photonic effects, including photon blockade and $\pi-$conditional phase shift, are achievable in the waveguide-coupled multimode optical cavity system via quantum interference and linear response engineering. Our results might lead to significant applications of quantum photonic circuits in all-optical quantum information processing and quantum network protocols.

Design and synthesis of Phenylaminothiophene donor-based chromophore with enhanced electro-optic activity

Three novel chromophores B-D have been synthesized based on thiophene derivatived donors, including 5-(methyl(phenyl)amino)thiophene 5-(bis(4-methoxyphenyl)amino)thiophene 5-((4-methoxyphenyl)(methyl)amino)thiophene groups respectively, with the same isophorone-derived bridge and tricyanovinyldihydrofuran (TCF) acceptors. A new strategy was proposed to improve the electron donating ability of the chromophore, thienyl groups were explored as a replacement for the phenyl unit in the donor part of the chromophore. Density functional theory calculations suggested that the first hyperpolarizability of (arylamino) thiophene donor-containing chromophore B-D is about 20% higher than that of 4-(dialkylamino) phenyl counterpart chromophore A. At least 50% enhancement in bulk nonlinearity (r33) is realized as the donor subunits were changed from alkylaniline to (arylamino) thiophene donors. Polymeric thin films doped with 25 wt% chromophore B-D have been poled to afford large r33 values of 131, 143 and 138 p.m./V at 1.31 μm, respectively, which is much higher than the EO activity of chromophore A (88 pm/V).

Modeling of surface-induced second-harmonic generation from multilayer structures by the transfer matrix method.

We analytically and numerically investigate surface second-harmonic generation (SHG) from a stack of dielectric layers. We develop a theoretical formalism based on the transfer matrix method for the calculation of the surface-driven second-harmonic radiation from multilayer structures and elaborate it for the case of ultrathin dielectric layers using a power series expansion to derive the effective surface nonlinear tensor for the whole stack. We show that for deeply subwavelength thicknesses of the layers the surface responses from all interfaces can efficiently sum up, leading to largely enhanced efficiency of SHG. As a result, such surface-driven nonlinearity can become comparable to the bulk nonlinearity in noncentrosymmetric semiconductors and can yield high performance for nonlinear nanophotonic applications.

ENZ materials and anisotropy: enhancing nonlinear optical interactions at the nanoscale.

Epsilon-near-zero materials are exceptional candidates for studying electrodynamics and nonlinear optical processes at the nanoscale. We demonstrate that by alternating a metal and a highly doped conducting-oxide, the epsilon-near-zero regime may be accessed resulting in an anisotropic, composite nanostructure that significantly improves nonlinear interactions. The investigation of the multilayer nanostructure reveals the actual role of the anisotropy, showing that high degrees of anisotropy might be necessary to effectively boost nonlinear processes. Moreover, using a microscopic, hydrodynamic approach we shed light on the roles of two competing contributions that are for the most part overlooked but that can significantly modify linear and nonlinear responses of the structure: nonlocal effects, which blueshift the resulting resonance, and the hot electrons nonlinearity, which redshifts the plasma frequency as the effective mass of free electrons increases as a function of incident power density and enhances the nonlinear signal by several orders of magnitude. Finally, we show that, even in the absence of second order bulk nonlinearity, second order nonlinear processes are also significantly enhanced by the layered structure.

Electrically pumped efficient broadband CW frequency conversion in diode lasers using bulk χ2

Optical frequency conversion in chip-scale devices enables crucial applications in myriad fields, such as optical communication, spectroscopy, and sensing. With large nonlinearities and the possibility of monolithic pump laser integration coexisting within the same platform, compound semiconductor is a choice of material superior to many others. In this work, we demonstrate an intracavity difference frequency generation, electrically pumped diode lasers with external input signal beam. Thanks to exact phase matching and large bulk nonlinearity in the laser structures reported, there is a normalized internal conversion efficiency of 169% W−1 cm−2. We also demonstrate a broad tuning range between 1486 nm and 1686 nm, or 24 THz, which spans the S-C-L-U telecommunications bands.

Harmonic generation in the opaque region of GaAs: the role of the surface and magnetic nonlinearities.

Phase-locked second and third harmonic generation in the opaque region of a GaAs wafer is experimentally observed and analyzed both in transmission and reflection. These harmonic components, which are generated close to the surface, can propagate through an opaque material as long as the pump is tuned to a region of transparency or semitransparency and correspond to the inhomogeneous solutions of Maxwell's equations with nonlinear polarization sources. We show that measurement of the angular and polarization dependence of the observed harmonic components allows one to infer the different nonlinear mechanisms that trigger these processes, including not only the bulk nonlinearity but also the surface and magnetic Lorentz contributions, which usually are either hidden by the bulk contributions or assumed to be negligible. The experimental results are compared with a detailed numerical model that takes into account these different effects, including for the first time combined linear and nonlinear material dispersions in a nonlinear Lorentz oscillator model of the bulk nonlinearities. Our results suggest that the intensity of the second harmonic signal generated by the surface can be more intense than the signal generated by the bulk. These findings have significant repercussions and are consequential in nanoscale systems, which are usually investigated using only dispersionless bulk nonlinearities, with near-complete disregard of surface and magnetic contributions and their microscopic origins.

Enhancing second-harmonic generation using dipolar-parity modes in non-planar plasmonic nanocavities

There is an active demand to develop efficient nanoscale nonlinear sources for applications in photonic circuitry, quantum optics, and biosensing. To this end, plasmonic systems have been utilized to boost the nonlinear signal generation; however, high efficiencies of frequency conversion for realistic applications remain a challenge. Metal-insulator-metal (MIM) nanocavities are good candidates for strongly concentrating the fields at the nanoscale to enhance the optical nonlinearities; however, they typically suffer from the requirement to have a quadrupolar resonance at the emission wavelength. Here we introduce non-planar MIM nanocavities with a nonlinear spacer that can strongly enhance the second-harmonic generation (SHG), despite having fundamental and emission modes of the same parity. Our experimental and numerical results indicate that the enhancement is due to the non-planar design of the cavities and the bulk nonlinearity of the spacer layer.

Second harmonic generation and spontaneous parametric down-conversion in Mie nanoresonators

The recent active research in the field of nonlinear Mie nanoresonators has led to significant increase of harmonic generation efficiency from subwavelength structures. The progress in the second order nonlinear processes became evident after switching to nanoresonators made of materials that possess a bulk nonlinearity such as semiconductor and perovskite-type materials. Despite of that, the theory of second-harmonic generation from spherical Mie nanoparticles with a bulk nonlinearity has not been developed yet. In this work, we present a theoretical approach based on Mie theory for describing the second order nonlinear processes in all-dielectric nanoresonators. We focus on the second harmonic generation (SHG) process as well as on the inverse one, the process of spontaneous parametric down-conversion (SPDC). We demonstrate the SHG-SPDC correspondence in terms of Mie resonances and identify the selection rules governing these second order nonlinear processes. We show how one can control the spatial correlations of photons in SPDC from a single Mie-nanoparticle by properly choosing the parameters of the system, in particular, allowing for Kerker-type unidirectional emission of photons.

Nonlinear Goniometry by Second-Harmonic Generation in AlGaAs Nanoantennas

High-permittivity semiconductor nanoresonators have shown great potential for enhanced nonlinear light–matter interactions at the nanoscale due to the availability of a rich variety of resonances combined with low optical losses and a strong bulk nonlinearity. Second-harmonic generation in AlGaAs nanoantennas can be extremely efficient and exhibits a complex radiation pattern. However, the complexity of this far-field profile imposes severe constraints on practical applications and detection efficiency. In this work, we demonstrate, both experimentally and numerically, the control over the angular distribution of the second-harmonic radiation pattern from a monolithic AlGaAs-on-AlOx nanodisk by varying the polarization and the angle of incidence of the pump beam. By tuning the angle of incidence of a beam with s-polarized light from 0 to 45°, the detected second-harmonic signal is monotonically increased up to over an order of magnitude due to the strong dependence of the nonlinear radiation pattern on th...

Second-harmonic generation imaging of semiconductor nanowires with focused vector beams.

We use second-harmonic generation (SHG) with focused vector beams to investigate individual vertically aligned GaAs nanowires. Our results provide direct evidence that SHG from oriented nanowires is mainly driven by the longitudinal field along the nanowire growth axis. Consequently, focused radial polarization provides a superior tool to characterize such nanowires compared to linear polarization, also allowing this possibility in the native growth environment. We model our experiments by describing the SHG process for zinc-blende structure and dipolar bulk nonlinearity.

Basic Theory of Surface Sum-Frequency Generation

A detailed description on the basic theory of optical sum-frequency generation from an interfacial system is presented. Both the interface and the bulk generally contribute to the sum-frequency output. Two seemingly different approaches to specify bulk nonlinearity that includes electric-quadrupole and magnetic-dipole contributions are shown to yield the same sum-frequency output if surface nonlinearity is properly taken into account. The question of whether surface and bulk nonlinearities can be uniquely defined and separately measured is discussed. It is shown that the answer is affirmative. Truly bulk and truly surface nonlinear susceptibilities can be uniquely defined and separately deduced from measurements of transmitted and reflected sum-frequency generation.

Rational Enhancement of Second-Order Nonlinearity: Bis-(4-methoxyphenyl)hetero-aryl-amino Donor-Based Chromophores: Design, Synthesis, and Electrooptic Activity

Two new highly hyperpolarizable chromophores, based on N,N- bis-(4-methoxyphenyl) aryl-amino donors and phenyl-trifluoromethyl-tricyanofuran (CF3-Ph-TCF) acceptor linked together via pi-conjugation through 2,5-divinylenethienyl moieties as the bridge, have been designed and synthesized successfully for the first time. The aryl moieties on the donor side of the chromophore molecules were varied as to be thiophene and 1-n-hexylpyrrole. The linear and nonlinear optical (NLO) properties of all compounds were evaluated in addition to recording relevant thermal and electrochemical data. The properties of the two new molecules were comparatively studied. These results are critically analyzed along with two other compounds, reported earlier from our laboratories and our collaborator's, that contain (i) aliphatic chain-bearing aniline and (ii) dianisylaniline as donors, keeping the bridge (2,5-divinylenethienyl-), and the acceptor (CF3-Ph-TCF), constant. Trends in theoretically (density functional theory, DFT) predicted, zero-frequency gas-phase hyperpolarizability [beta(0;0,0)] values are shown to be consistent with the trends in beta HRS(-2omega;omega,omega), as measured by Hyper-Rayleigh Scattering (HRS), when corrected to zero-frequency using the two-level model (TLM) approximation. Similarly, trends in poling efficiency data (r33/E(p)) and wavelength dispersion measured by reflection ellipsometry (using a Teng-Man apparatus) and attenuated total reflection (ATR) are found to fit the TLM and DFT predictions. A 3-fold enhancement in bulk nonlinearity (r33) is realized as the donor subunits are changed from alkylaniline to dianisylaminopyrrole donors. The results of these studies provide insight into the complicated effects on molecular hyperpolarizability of substituting heteroaromatic subunits into the donor group structures. These studies also demonstrate that, when frequency dependence and electric-field-induced ordering behavior are correctly accounted for, ab initio DFT generated beta(0;0,0) is effective as a predictor of changes in r33 behavior based on chromophore structure modification. Thus DFT can provide valuable insight into the electronic structure origin of complex optical phenomena in organic media.

Experimental study of nonlinear mode mixing in dual-wavelength semiconductor lasers

New schemes for the generation of the sum, difference, and double frequencies in dual-wavelength injection lasers, based on an intracavity nonlinear mode mixing owing to a bulk nonlinearity, are experimentally investigated. Namely, the operation of the dual-wavelength laser diodes at the TE0 and TE1 modes, butt-joint laser diodes, and interband cascade lasers with tunnel junction are described.

Difference-frequency pulse generation in quantum well heterolasers

It is shown that mid-to far-infrared (IR) and terahertz (THz) pulse generation via difference-frequency mixing in quantum well (QW) dual-wavelength heterolasers can be rather efficient under the modelocking regime for one or both lasing fields even at room temperature. In such a device, the long-wavelength field is produced in the process of intracavity difference-frequency mixing of two optical fields: continuous wave (CW) and pulsed (or both pulsed), due to the resonant intersubband quantum coherence in QWs, as well as due to the nonresonant second-order semiconductor bulk nonlinearity. The mode-locking regime of the optical generation allows one to significantly enhance the pulsed driving fields in comparison with those under CW operation and, therefore, substantially increase the output difference-frequency power. Within a simple model, an explicit formula for the intensity and shape of the generated IR or THz pulse is derived. It is shown that this method is capable of producing picosecond pulses at a ∼ 1-GHz repetition rate with a peak power of the order of 1 W and ≲0.2 mW at 10 and 50 µm wavelengths, respectively.

Multipolar tensor analysis of second-order nonlinear optical response of surface and bulk of glass

We use two-beam second-harmonic generation to perform a quantitative tensor analysis of the effective dipolar surface nonlinearity and the separable multipolar bulk nonlinearity for BK7 glass. The most straightforward, self-consistent interpretation of the results is obtained when the effective surface response is assumed to have approximate Kleinman symmetry and the bulk contribution is dominated by magnetic, rather than quadrupole, effects.

Experimental investigation of Schottky barrier diodes as nonlinear elements in 800-nm-wavelength region

We report the results of experimental investigations of nonlinear properties of InP, GaAs and InGaAs Schottky barrier diodes (SBD) in the near-infrared range (λ≈800 nm). The results of our previous and present work show that SBD have a unique broadband spectral sensitivity (from the millimeter to the visible range). As video detectors the GaAs-SBD proved to be the most sensitive among the other diodes, but they concede to the InP-SBD as frequency mixer–multipliers in spite of their higher cut-off frequency. The higher efficiency of the InP-SBD in a mixer–multiplier mode is supposedly connected with the bulk nonlinearity properties of indium phosphide, lower noise and lower time constants, characterizing the charge-carrier energy growth in an electric field and its relaxation. We found that the optimal operation conditions for SBD suppose not only the optimal electric regime (applying a defined bias or a defined radiation source power) but also the optimal mutual orientation of a laser radiation polarization, a contacting wire (playing the role of an antenna) and a laser beam.

Second-order optical nonlinearity and change in refractive index in silica glasses by a combination of thermal poling and x-ray irradiation

Second-order optical nonlinearity was found to be generated in high-purity silica glasses when they were exposed to x-ray radiation and then thermally poled. Two kinds of second-order optical nonlinearity, near-surface nonlinearity localized in a thin layer near the sample surface and bulk nonlinearity spreading throughout the whole sample, were observed. The maximum χ33(2) values of near-surface and bulk nonlinearity were 0.20 and 0.43 pm/V, respectively. Further, a change in refractive index of Δn=4×10−3 was also observed in the sample. The χ33(2) value of the bulk nonlinearity and the change in refractive index increased with the x-ray intensity. On the other hand, the χ33(2) value of the near-surface nonlinearity had a tendency to saturate when the intensity was higher than 1.3 mW/cm2. Based on absorption spectra of the samples, both the bulk nonlinearity and the refractive index change were found to be associated with point defects such as the E′ center (≡Si⋅) and nonbridging oxygen ions (NBO−,≡Si–O−).

Solutal convection during growth of organic NLO crystals

Effect of solutal and thermal convection plays very important role when a large thermal gradient is applied during crystal growth. To address this problem, we have purified m.nitroaniline (m.NA) and m. dinitrobenzene (m.DNB) and studied crystal growth and effect of growth parameters on the optical quality. Crystals of pure and binary alloy of m.dinitrobenzene and m.nitroaniline were grown by vertical directional solidification method in a two zone transparent furnace. Effect of doping and temperature gradient on the solid-liquid interface morphology and quality of crystal was determined by studying the bulk transparency and nonlinearity.

Second‐order nonlinear optical response of thin films containing organic chromophores. Theoretical aspects

AbstractThis paper discusses some theoretical aspects associated with the unusual second‐order nonlinear optical response of chromophoric self‐assembled stilbalzolium superlattices, and of (N,N‐dimethylamino)‐4‐nitrostilbene (DANS)‐doped glass polymers poled in an “in‐plane” configuration. It is found that the bulk nonlinearity of such materials strongly depends upon specific chromophore interactions. While for the stilbalzolium superlattices the supramolecular hyperpola‐rizability is strongly affected by the ion‐pair configuration, the formation of (formally) centrosymmetric charged chromophore aggregates is likely responsible for the observed anomalous strong nonlinearity in DANS‐doped in‐plane poled polymers. The simple “chromophore gas” model is thus inadequate to describe the non‐linearity of such materials.