Optical nonlinearity in Ar and N2 near the ionization threshold

We directly measure the nonlinear optical response in argon and nitrogen in a thin gas target to laser intensities near the ionization threshold. No instantaneous negative nonlinear refractive index is observed, nor is saturation, in contrast with a previous measurement [Opt. Express 17, 13429 (2009)] and calculations [Phys. Rev. Lett. 106, 183902 (2011)]. In addition, we are able to cleanly separate the instantaneous and rotational components of the nonlinear response in nitrogen. In both Ar and N2, the peak instantaneous index response scales linearly with the laser intensity until the point of ionization, whereupon the response turns abruptly negative and ～constant, consistent with plasma generation.     

Fifth-order phase-sensitive detection of ultrafast polarization beats in cascade four-level system

We study the colour-locked twin-noisy-field correlation effects in the fifth-order nonlinear susceptibility of ultrafast polarization beats in a cascade four-level system. More importantly, the fifth-order phase-sensitive heterodyne detection of ultrafast polarization beats has been exploited. The fifth-order nonlinear optical response can be controlled and modified through the colour-locked correlation of twin noisy fields. Thus, this method with the phase dispersion information is a good way to measure the real and imaginary parts of the fifth-order nonlinear susceptibility.     

Femtosecond Time-Resolved Third-Order Nonlinear Response of 4-(N, N-Diethylamino)-β-Nitrostyrene Solutions in the Nonresonant Region—Experimental Evidence of Group-Velocity Dispersion Effec—

We measure the femtosecond time-dependent, third-order nonlinear optical response for 4-(N, N-diethylamino)-β-nitrostyrene (DEANST) dissolved in N, N-dimethylformamide (DMF) from 0.1 to 1.4 M concentrations in the nonresonant region using time-resolved degenerate four-wave mixing (DFWM) spectroscopy. It is found that the DFWM signal profile with delayed responses depends on the concentration. This concentration dependence is determined to be mainly due to the group-velocity dispersion effect of the DEANST solution at the high concentration and the nonlinear response of the DMF solvent at the low concentration, but not due to the change in third-order nonlinear response of DEANST itself.     

Active microrheology: a proposed technique to measure normal stress coefficients of complex fluids

We propose a microrheological technique to measure normal stress coefficients (NSCs) of complex fluids, which would represent the first quantitatively accurate measurement of a nonlinear rheological property by microrheology. Specifically, the mechanical response of almost all complex fluids to "weakly nonlinear" deformations is described by the second-order fluid model. Two microrheological probes pulled with equal velocities through a second-order fluid experience a relative force that is linear in the first and second NSCs of the complex fluid. We compute the coupling matrix between NSCs and relative forces for probes translating parallel and perpendicular to their line of centers, which can be inverted to yield NSCs from measured relative forces. There exists an optimum probe separation for inversion of the coupling matrix and, hence, experimental recovery of NSCs.     

Sensitive signal detection using a feed-forward oscillator network

We present the results of an experimental investigation of a network of nonlinear coupled oscillators which are coupled in feed-forward mode. By exploiting the nonlinear response of each oscillator near its intrinsic Hopf bifurcation point, we have found remarkable amplification of small signals over a narrow bandwidth with a large dynamic range. The effect is exploited to extract a small amplitude periodic signal from an input time series which is dominated by noise. Specifically, we have used this relatively simple experimental system to measure responses with a bandwidth of approximately 1% of the central frequency, amplifications of approximately 60 dB, and a dynamic range of approximately 80 dB and can extract signals from a time series with a signal to noise ratio of approximately -50 dB.     

How to measure chi(3) of a nanoparticle

Most of the known methods to measure the nonlinear optical properties of materials deal with the bulk properties, but there are many demanding applications that require those measurements to be done on a single particle or a single molecule. We report a novel application of nonlinear optics to measure the third-order nonlinear optical susceptibility of nanoparticles in solutions. By measuring the power of the third harmonic generated in a diluted solution of nanoparticles, both the size and chi(3) can be extracted from a simple set of measurements.     

功率控制的强非局域空间光孤子短程相互作用

研究了功率控制的强非局域空间光孤子短程相互作用.由动量守恒,两个孤子短程相互作用时,孤子的质心轨迹因相位差而发生偏转.实验上证实了偏转幅度与功率比有关,当功率相等时,偏转最大.利用功率对相互作用的调控的特性,测量了液晶分子对光场的响应时间,发现比其对偏置电压的响应时间短很多.     

Generation of a forward-traveling phase-conjugate wave in germanium

In this communication, we report the generation of a picosecond forward-traveling phase-conjugate wave at 1.06 μm in thin germanium samples by degenerate four-wave mixing. At this wavelength, the conjugate wave is produced by diffraction of the pump from an absorption grating produced by direct absorption of pump and signal pulses. We measure and discuss the transient effects caused by the finite duration of the nonlinear material response.     

Nonlinear refractive index of vanadate crystals in the near IR region

We used the single-beam Z-scan method, employing picosecond laser pulses with λ = 1064 nm, to measure the nonlinear refractive index and nonlinear absorption in vanadate crystals, which are promising for design of laser sources. We have established that there is practically no nonlinear absorption in these crystals in the near IR range and that doping them with neodymium ions leads to a significant reduction in the nonlinear refraction. This effect can be used to partially compensate for the negative impact of nonlinear refraction on the lasing characteristics of lasers.     

Dynamical heterogeneity and nonlinear susceptibility in supercooled liquids with short-range attraction

Recent work has demonstrated the strong qualitative differences between the dynamics near a glass transition driven by short-ranged repulsion and one governed by short-ranged attraction. Here we study in detail the behavior of nonlinear, higher-order correlation functions that measure the growth of length scales associated with dynamical heterogeneity in both types of systems. We find that this measure is qualitatively different in the repulsive and attractive cases with regards to the wave vector dependence as well as the time dependence of the standard nonlinear four-point dynamical susceptibility. We discuss the implications of these results for the general understanding of dynamical heterogeneity in glass-forming liquids.     

Nonlinear refraction measurements in presence of nonlinear absorption using phase object in a 4f system

We report a technique to measure the value of the nonlinear refractive index of materials in presence of nonlinear absorption using a phase object at the entry of a 4f coherent imaging system. We show that it is possible to obtain a signal approximately due only to the induced nonlinear refraction in presence of two photon absorption. Experimental and simulated Z-scan transmittance profiles with and without phase object, as well as acquired and calculated images are presented here in order to validate our approach. We show also that the use of a reference material simplifies the measurement procedure avoiding computer fits.     

Pseudo-periodic surrogate test to sample time series in stochastic softening Duffing oscillator

Identification of typical noise-contaminated sample response is a hard task in a nonlinear system under stochastic background since irregularity of the sample response may come from measure noise, dynamical noise, or nonlinear effect, etc., and conventional dynamical methods are generally not useful. Here, the pseudo-periodic surrogate algorithm by Small is employed to test the sample time series in the softening Duffing oscillator under the Gaussian white noise excitation. The correlation dimensions of the noisy periodic and the noise-induced chaotic time series of the system are compared with those of their corresponding surrogate data respectively, the leading Lyapunov exponents by Rosenstein's algorithm are also presented for comparison.     

Engineering the nonlinear phase shift

We treat the nonlinear phase shift response in the weak perturbation limit as a linear digital filter that can be synthesized into the values of its poles and zeros and mapped onto an optical architecture. This procedure results in a significant enhancement in the nonlinear sensitivity with a response that is robust to frequency changes within the filter passband. A precompensation technique can be used to reduce distortions under strongly driven nonlinear operation to achieve a larger phase shift. We also show that nonlinear sensitivity improves with increasing filter group delay and can be increased within constant linear bandwidth by use of higher-order filters.     

Measurement of material nonlinearity using surface acoustic wave parametric interaction and laser ultrasonics

A dual frequency mixing technique has been developed for measuring velocity changes caused by material nonlinearity. The technique is based on the parametric interaction between two surface acoustic waves (SAWs): The low frequency pump SAW generated by a transducer and the high frequency probe SAW generated and detected using laser ultrasonics. The pump SAW stresses the material under the probe SAW. The stress (typically <5 MPa) is controlled by varying the timing between the pump and probe waves. The nonlinear interaction is measured as a phase modulation of the probe SAW and equated to a velocity change. The velocity-stress relationship is used as a measure of material nonlinearity. Experiments were conducted to observe the pump-probe interaction by changing the pump frequency and compare the nonlinear response of aluminum and fused silica. Experiments showed these two materials had opposite nonlinear responses, consistent with previously published data. The technique could be applied to life-time predictions of engineered components by measuring changes in nonlinear response caused by fatigue.     

Signal-to-noise ratio of a dynamical saturating system: Switching from stochastic resonator to signal processor

We consider an isolated dynamical saturating system for processing a noisy sinusoidal signal, and evaluate its performance with the measure of the signal-to-noise ratio. The considered system is linear for small inputs, but exhibits saturation in its response for large inputs. This nonlinearity displays the nonlinear phenomenon of stochastic resonance for a large biased sinusoid in appropriate system parameter regions. Without the stochastic resonance phenomenon, this dynamical saturating system can achieve a signal-to-noise ratio gain exceeding unity for a noisy unbiased sinusoid. These numerical results manifest the nonlinearities and the signal-processing ability of this system acting as a stochastic resonator or a signal processor.     

Nonlinear photonics with metallic nanostructures on top of dielectrics and waveguides

We review recent experimental and theoretical studies of the ultrafast and nonlinear optical response of metallic nanostructures on top of dielectric substrates and slab waveguides where plasmon hybridization is a key ingredient. In a first three-pulse all-optical control experiment a hybrid plasmonic mode is turned on or off only a few tens of femtoseconds after its excitation. A second experiment concentrates on the origin of the nonlinear response in a metallo-dielectric photonic crystal structure. We show that the shape of the nonlinear optical spectra provides unambiguous information about the nonlinear optical contribution of the metallic as well as the dielectric part of the structure. Furthermore, we discuss the influence of slow-light on the nonlinear response. All experimental results agree perfectly with numerical scattering matrix calculations as well as simulations based on a classical harmonic oscillator model.     

Nonlinear refractive indices of nonlinear liquids: wavelength dependence and influence of retarded response

We use liquid-filled capillary fibers with different core diameters to precisely characterize the nonlinear refractive index of the highly nonlinear liquids carbon disulfide, nitrobenzene, and toluene. We present measurements with two different femtosecond pump sources at wavelengths of 1032 and 1560 nm. The large nonlinearity of the liquids results from the retarded nonlinear optical response of the liquid molecules which includes a strong non-instantaneous contribution due to molecular reorientation. The nonlinear refractive index of the liquids is determined by fitting numerical simulations based on solving the generalized nonlinear Schrödinger equation including retarded response to the measured broadened output spectra. Our work is important for the novel field of near- and mid-IR supercontinuum generation in liquid-core optical fibers.     

Filament-length-controlled elasticity in 3D fiber networks

We present a model for disordered 3D fiber networks to study their linear and nonlinear elasticity. In contrast to previous 2D models, these 3D networks with binary crosslinks are underconstrained with respect to fiber stretching elasticity, suggesting that bending may dominate their response. We find that such networks exhibit a bending-dominated elastic regime controlled by fiber length, as well as a crossover to a stretch-dominated regime for long fibers. Finally, by extending the model to the nonlinear regime, we show that these networks become intrinsically nonlinear with a vanishing linear response regime in the limit of flexible or long filaments.