Ultrafast Nonlinear Refraction Research Articles

Ultrafast nonlinear refraction measurements of infrared transmitting materials in the mid-wave infrared.

We utilize the conventional Z-scan technique to provide absolute measurements of third-order nonlinear refraction coefficients (n2) in the mid-wave infrared at 2 µm and 3.9 µm of common optical materials that have transparency windows spanning this regime. We study a variety of narrow band gap and wide band gap semiconductors, fluoride crystals (BaF2, CaF2, LiF, and MgF2) and optical glasses, and a series of chalcogenide glasses. The n2 is found to span on the order of ∼10-15 to ∼10-12 cm2/W for the semiconductors, ∼10-16 cm2/W for the fluoride crystals and glasses, and ∼10-14 to ∼10-13 cm2/W for the chalcogenides. The experimental results are compared to previous measurements of n2 conducted in the visible and near-infrared along with empirical and theoretical formulations.

Ultrafast nonlinear refraction measurements of infrared transmitting materials in the mid-wave infrared

Ultrafast nonlinear refraction measurements of infrared transmitting materials in the mid-wave infrared

Ultrafast nonlinear absorption and nonlinear refraction in few-layer oxidized black phosphorus

We experimentally investigated the nonlinear optical response in few-layer oxidized black phosphorus (OBP) by the femtosecond Z-scan measurement technique, and found that OBP not only possesses strong ultrafast saturable absorption but also a nonlinear self-defocusing effect that is absent in black phosphorus (BP). The saturable absorption property originates mainly from the direct band structure, which is still maintained in OBP. The emergence of self-defocusing might originate from the combined consequences of the oxygen-induced defects in BP. Our experimental findings might constitute the first experimental evidence on how to dynamically tune its nonlinear property, offering an inroad in tailoring its optical properties through chemical modification (oxidation, introducing defects, etc.). The versatile ultrafast nonlinear optical properties (saturable absorption and self-defocusing) imply a significant potential of the layered OBP in the development of unprecedented optoelectronic devices, such as mode lockers, optical switches, laser beam shapers, and wavelength converters.

Optical nonlinearities and ultrafast all-optical switching of m-plane GaN in the near-infrared

We reported a systematic investigation on the three-photon absorption (3PA) spectra and wavelength dispersion of Kerr refraction of bulk m-plane GaN crystal with both polarization E⊥c and E//c by femtosecond Z-scan technique in the near-infrared region from 760 to 1030 nm. Both 3PA spectra and Kerr refraction dispersion were in good agreement with two-band models. The calculated nonlinear figure of merit and measured ultrafast nonlinear refraction dynamics via femtosecond pump-probe with phase object method revealed that m-plane GaN would be a promising candidate for ultrafast all-optical switching and autocorrelation applications at telecommunication wavelengths.

Beam deflection measurement of time and polarization resolved ultrafast nonlinear refraction

We modify the well-known photothermal beam deflection technique to study ultrafast nonlinearities. Using phase-sensitive detection we directly measure the temporal and polarization dynamics of nonlinear refraction (NLR) with sensitivity to optically induced phase changes of approximately λ/20,000. We use the relative polarization dependence of excitation and probe to separate the isotropic and reorientational components of the NLR.

Ultrafast nonlinear optical response of photoconductive ZnO films with fluorine nanoparticles

The absorptive and refractive third order nonlinear optical properties exhibited by a ZnO thin solid film with fluorine nanoparticles were studied with picosecond and femtosecond pulses using different techniques. We were able to evaluate the photoconductivity of the material and the quenching of the induced birefringence observed in the presence of two-photon absorption. The samples were prepared by a chemical spray deposition technique. In order to investigate the different contributions of the third order nonlinearities of the film, we analyzed the vectorial self-diffraction effect and the optical Kerr transmittance observed in the sample. A dominantly absorptive nonlinearity was measured at a 532 nm wavelength with 50 ps pulses, while nonlinear refraction was found to be negligible in this regime. On the other side, a pure electronic refractive third order nonlinearity without the contribution of nonlinear absorption was detected at 830 nm with 80 fs pulse duration. A quasi-instantaneous optical response and a strong enhancement in the ultrafast nonlinear refraction with the inhibition of the picosecond two-photon absorption mechanism were measured for the case of the femtosecond excitation.

Comprehensive Finite-Difference Time-Dependent Beam Propagation Model of Counterpropagating Picosecond Pulses in a Semiconductor Optical Amplifier

In this paper, we present a numerical model to study counter pulse propagation in semiconductor optical amplifiers. An improved finite-difference beam propagation method for solving the modified nonlinear Schrodinger equation is applied for the first time in the counterpropagation regime. In our model, group velocity dispersion, two-photon absorption, ultrafast nonlinear refraction, and the change in the gain peak wavelength with carrier density are included, which have not been considered simultaneously in previous counterpropagation models. The model is applied to demonstrate how a subpicosecond and picosecond probe pulse shape and spectrum can be modified by a counterpropagating pump pulse. Based on the results obtained by this model, while subpicosecond probe pulses can be compressed by in this scheme, their time-bandwidth product are also improved significantly. Furthermore, the effects of several parameters are analyzed to obtain the proper probe spectral peak shift using counterpropagating probe pulses. The accuracy and computational efficiency of the new scheme are assessed through numerical examples and are shown to be superior to previously published approaches.

Nonlinear processes in strong ultrashort pulse pumped semiconductor optical amplifier

A theoretical model of semiconductor optical amplifier (SOA) is presented with carrier density pulsation (CDP)，free-carrier absorption (FCA)，stimulated emission (SE)，two-photon absorption (TPA)，spectral hole burning (SHB) and ultrafast nonlinear refraction (UNR) taken into account. The model is proved by comparing with the reported experimental results. The prevailing SOA model is revised. The influences of FCA and TPA processes on ultrashort strong optical pulses are analysed. When the strong optical pulse with several-picosecond pulsewidth is injected into the SOA operating under the transparency current，the intensity characteristics of the optical pulse are mainly influenced by TPA and FCA. As a result of taking the FCA effect into account，the intensity transmission characteristics of 200fs optical pulses obtained by the new model basically agree with the experimental results. It broadens the applicability of the model.

Wide spectral range third-order autocorrelator based on ultrafast nonresonant nonlinear refraction: erratum

We demonstrate a simple scheme for a wide spectral range, third-order autocorrelator based on ultrafast nonlinear Kerr-type refraction. The technique was successfully used to characterize high-energy ultrashort pulses at 1550 and 1300 nm, where the pulse’s shape and width are two of the most critical parameters. Because of its simplicity, this technique is also a powerful tool for the optimization of high-power chirped-pulse amplified laser systems, in which slight misalignment of the stretcher–compressor gratings can lead to spatiotemporal pulse distortions. In addition, it can be extended to low-power mode-locked oscillators.

Wide spectral range third-order autocorrelator based on ultrafast nonresonant nonlinear refraction.

We demonstrate a simple scheme for a wide spectral range, third-order autocorrelator based on ultrafast nonlinear Kerr-type refraction. The technique was successfully used to characterize high-energy ultrashort pulses at 1550 and 1300 nm, where the pulse's shape and width are two of the most critical parameters. Because of its simplicity, this technique is also a powerful tool for the optimization of high-power chirped-pulse amplified laser systems, in which slight misalignment of the stretcher-compressor gratings can lead to spatiotemporal pulse distortions. In addition, it can be extended to low-power mode-locked oscillators.

Amplification of picosecond optical pulses in midinfrared intersubband semiconductor optical amplifiers

A comprehensive theoretical model describing the amplification of picosecond optical pulses in midinfrared intersubband semiconductor optical amplifiers is developed, taking into account gain dispersion, short carrier relaxation time and ultrafast nonlinear refraction. It is shown that these factors are extremely important in determining the amplified optical pulse characteristics in both the time and frequency domains. The calculations also indicate that gain dispersion can be significantly enhanced by fast carrier relaxations, short pulse widths, and high pulse energies.

Characteristics of optical phase conjugation of picosecond pulses in semiconductor optical amplifiers

Nondegenerate four-wave mixing (FWM) of picosecond optical pulses in semiconductor optical amplifiers is investigated theoretically, taking into account various carrier processes, probe depletion, and cross-gain modulation effects. Analytical solutions for greatly simplified coupled-wave mixing equations are obtained from which conditions where probe depletion and cross-gain modulation effects cannot be neglected are found. On the other hand, the frequency detuning dependence of optimum input pump pulse energy and shaping of output conjugate pulses are observed from numerical simulations. In particular, calculations show that the frequency symmetry of conjugate and probe pulses with respect to the pump pulse does not hold for optical pulse FWM. Two-photon absorption and ultrafast nonlinear refraction effects are shown to decrease the FWM conversion efficiency.

Four-wave mixing of strong picosecond optical pulses in passive semiconductor waveguides

Four-wave mixing (FWM) of strong picosecond optical pulses in passive semiconductor waveguides is investigated numerically, taking into account two-photon absorption (TPA), ultrafast nonlinear refraction, probe depletion, and cross-gain modulation, as well as interband and intraband carrier processes. Simulations show good agreement with published experimental measurements, and indicate that interband and intraband nonlinear mixing and TPA-induced pulse reshaping play significant roles in determining conjugate pulse behaviors for strong pulse energy cases. In particular, for increased pulse energy, an increase in FWM conversion efficiency is predicted, and an optimum pulse width is identified.

Amplification of strong picosecond optical pulses in semiconductor optical amplifiers

Numerical simulations have been undertaken to investigate the amplification of strong picosecond optical pulses in semiconductor optical amplifiers (SOAs), taking into account carrier heating, spectral holeburning, two-photon absorption (TPA) and ultrafast nonlinear refraction (UNR) effects. It is shown that both the TPA and UNR play important roles in determining the output pulse properties if the pulse energy is larger than ~0.5 pJ, and that these effects can be significantly enhanced with increased pulse energy and decreased pulse width as well as with higher small signal gain of the SOA. Results also indicate that, for larger input pulse energy, the symmetry of the amplified optical pulse can be controlled simply by adjusting the injected current.

Strong picosecond optical pulse propagation in semiconductor optical amplifiers at transparency

The propagation of strong picosecond optical pulses in semiconductor optical amplifiers (SOAs) is investigated numerically by taking into account carrier heating, spectral hole-burning, and two-photon absorption, as well as ultrafast nonlinear refraction. Very good agreement with published experimental results are observed in both the time and frequency domains. It is shown that the effects of two-photon absorption and ultrafast nonlinear refraction are very important in determining the output pulse properties for pulse energy larger than 1 pJ.

Polarization Dependence of Ultrafast Nonlinear Refraction in Semiconductors at the Half-Bandgap

Polarization Dependence of Ultrafast Nonlinear Refraction in Semiconductors at the Half-Bandgap

Nonlinear-optical activity owing to anisotropy of ultrafast nonlinear refraction in cubic materials

The evolution of the polarization state in a cubic material with an anisotropic Kerr nonlinearity is examined. It is shown that in certain cases this provides a mechanism for nonlinear-optical activity, leaving the state of the polarization unchanged but causing a signif icant rotation in its major axis. The use of the anisotropic ultrafast nonlinear refraction that exists just beneath the half-gap in semiconductors to demonstrate these effects is discussed.

Polarization dependence of ultrafast nonlinear refraction in an AlGaAs waveguide at the half-band gap

We have obtained the polarization dependence of ultrafast nonlinear refraction by measuring the orientational dependence of self-phase modulation and cross-phase modulation in an Al0.18Ga0.82As waveguide at a frequency just beneath the two-photon absorption edge. It was found that nonlinear refraction exhibits a considerable anisotropy in accordance with theoretical calculations for GaAs. It was also found, as predicted, that perpendicular cross-phase modulation behaves differently from the isotropic Kleinmann result appropriate to silica fibers.

Polarization and field dependent two-photon absorption in GaAs/AlGaAs multiquantum well waveguides in the half-band gap spectral region

We report the observation of two photon absorption which is strongly dependent on the applied electric field and the optical polarization. At 1.55 μm wavelength, the two-photon absorption coefficient of the GaAs/AlGaAs multiquantum well (MQW) waveguides for transverse-magnetic light is about seven times lower than for transverse-electric polarized light and changes by a factor of approximately 4 for a change in applied direct-current electric field of ∼140 kV/cm. Ultrafast nonlinear refraction causing phase changes of over π radians without appreciable excess loss is observed. These measurements demonstrate that GaAs/AlGaAs MQW waveguides could be successfully used for subpicosecond all-optical switching near half-band gap, at wavelengths corresponding to the 1.55 μm optical communications band.