We report on the application of time-resolved inline digital holography in the study of the nonlinear optical properties of quantum dots deposited onto sample glass. The Fresnel diffraction patterns of the probe pulse due to noncollinear degenerate phase modulation induced by a femtosecond pump pulse were extracted from the set of inline digital holograms and analyzed. The absolute values of the nonlinear refractive index of both the sample glass substrate and the deposited layer of quantum dots were evaluated using the proposed technique. To characterize the inhomogeneous distribution of the samples’ nonlinear optical properties, we proposed plotting an optical nonlinearity map calculated as a local standard deviation of the diffraction pattern intensities induced by noncollinear degenerate phase modulation.

Synthesis of acrylate‐based nanocomposites (NCs) for photonic applications is performed, and the influence of gold and chalcogenide glass nanoparticles (NPs) on optical parameters of NC layers at high laser intensities is investigated, whereas the polymerization, recording of optical gratings or other elements, and variations of their functional parameters in As2S3 and Au‐containing NC layers are possible at low, 532 nm laser light intensities in the absorption edge range; optical stability under high intensity illumination and threshold laser damage intensities in NIR optical transmission range are important for nonlinear optical experiments, measurements of n2. Hence, these parameters are determined and show the increase in NCs’ stability due to the addition of Au NPs, as well as increase in n2 in gold‐ and chalcogenide‐containing NCs. Thus, the functionality of such NCs and optical elements on their basis can be extended to high‐intensity applications and nonlinear optics.

The nonlinear response of liquids in the terahertz regime has recently attracted significant interest, even though very few measurements have been reported. Here, we report on our measurements based on a zscan technique of the nonlinear refractive-index coefficient n2 at terahertz frequencies for several liquids with noncentrosymmetric molecules, specifically, water, ethanol, and ?-pinene. We describe how the value of n2 depends on the physical parameters of these molecules. The measured values of n2 of the liquids in the terahertz region are as much as 6 orders of magnitude larger than their corresponding values in the visible or near-IR. Through a simple theoretical model, we confirm that the predominant source of this large third-order nonlinearity is the second-order perturbative component of the vibrational response of these molecules, which have resonances in the mid-IR.

Experimental results and theoretical analysis of single-cycle terahertz pulse interaction with cubic nonlinear medium shows that the generation of radiation at triple frequencies disappears. It is replaced by the generation of radiation at quadruple frequencies.

The features of a liquid jet-based broadband terahertz (THz) generator that includes an optical part and system for a liquid jet formation are described. This system is based on the trapezium-shaped slit-type nozzle, the output of which is limited by two parallel blades. The applications of this generator for studying the optical-to-THz conversion efficiency for various liquids and THz energy temperature dependence are demonstrated. A trend for a decrease in the optical-to-THz conversion efficiency when using high-temperature water is revealed. This trend is interpreted through the increase in the THz wave absorption of the water jet. This system allows to achieve the optical-to-THz conversion efficiency up to 0.1% in the case of a-pinene double-pulse excitation. (C) 2021 Society of Photo-Optical Instrumentation Engineers (SPIE)

Single-pulse scheme and time-resolved experiments are implemented to explore the plasma formation in water and ethanol liquid jets. The measured dependences of the plasma reflection on the pump pulse energy and duration indicate the strong influence of intensity variation. Furthermore, ethanol demonstrates almost 2-fold higher reflectivity in comparison to water. Time-resolved experiments and corresponding analysis provide the insight into the electron density dynamics by measuring the reflection of laser pump and its third harmonic.

Laser-driven nonlinear phenomena can both reveal the structural features of materials and become the basis for the development of various translated technologies, including highly intense terahertz sources. Here we realize a modified single-color double-pulse excitation scheme for enhancing the terahertz wave generation in flat liquid jets, and we show that the pre-ionization effect is crucial for finding the optimal input conditions. The experimental results, being supported by numerical simulations, reveal the preference for longer pre-pulses to induce the effective ionization process and shorter signals for the strong laser-plasma interaction. In addition to the identified features of the terahertz wave energy enhancement with respect to the duration change for both pulses and their ratio variation, we state the possibility of achieving the optical-to-THz conversion efficiency value up to 0.1% in the case of double-pulse excitation of an α -pinene jet.

In this paper we present the results of both experimental and theoretical study of terahertz waves energy enhancement during liquid jets double-pulse excitation. The dependence of the terahertz radiation energy on mutual ratio of the pre-pulse and signal pump energies is investigated. Temperature dependence in the case of double-pulse irradiation reveals unexpected features displaying both exponential growth and decrease with a 18° optimal value.

Due to their high resonant third-order nonlinear response, lead sulfide quantum dots (QDs) are potential materials for applications in the field of nonlinear optics. In this paper, we implement the Z-scan method to study the resonant nonlinear response of lead sulfide QDs in colloidal solutions. We managed to measure the purely intrinsic resonant nonlinear response, free of thermal contribution. We report that the lead sulfide QD third-order nonlinear response per QD shows an unconventional increase. A figure of merit for QD nonlinearity grows as a power function with a factor of 2.9.

The work present investigates the dependence of the coherence time of an individual spectral supercontinuum on the intensity of femtosecond pumping during its generation in the water jet in the absence of filament. The results obtained show that the coherence time decreases along with pump intensity increase. Experimentally and by means of numerical simulation, it is demonstrated that this is caused by an increase in the linear frequency modulation coefficient in the temporal structure, which leads to the supercontinuum spectrum broadening. The relation can be used as an alternative way to assess the SC coherence.

The values of the nonlinear refractive index coefficient for various materials in the terahertz frequency range exceed the ones in both visible and NIR ranges by several orders of magnitude. This allows to create nonlinear switches, modulators, systems requiring lower control energies in the terahertz frequency range. We report the direct measurement of the nonlinear refractive index coefficient of liquid water by using the Z-scan method with broadband pulsed THz beam. Our experimental result shows that nonlinear refractive index coefficient in water is positive and can be as large as 7×10-10 cm2/W in the THz frequency range, which exceeds the values for the visible and NIR ranges by 6 orders of magnitude. To estimate n2, we use the theoretical model that takes into account ionic vibrational contribution to the third-order susceptibility. We show that the origins of the nonlinearity observed are the anharmonicity of molecular vibrations.

By irradiating a water jet with double pulses, we demonstrate 4-fold higher THz wave generation than for a single pump pulse. The dependence of the enhanced THz signal on the temporal delay between two collinear pulses reveals the optimal time for launching signal pulse is near 2-4 ps, which corresponds to the time needed to create the complete pre-ionization state when sufficient electron density is already induced, and there is no plasma reflection of the pump pulse radiation. The increase in THz waves generation efficiency corresponds to the case of water jet excitation by the pulses with an optimal duration for a certain jet thickness, which is determined by the spatial pulse size. Using a theoretical model of the interaction of a high-intensity sub-picosecond pulse with an isotropic medium, we held a numerical simulation, which well describes the experimental results when using 3 ps value of population relaxation time. Thus, in this work, double pump method allows not only to increase the energy of the generated THz waves, but also to determine the characteristic excited state lifetime of liquid water. The optical-to-terahertz conversion efficiency in case of double pulse excitation of water column is of the order of 0.5⋅10 -3, which exceeds the typical values for THz waves generation during two-color filamentation in air and comparable with the achievable values due to the optical rectification in some crystals.

Studying the nonlinearities in the resonant mode one may face thermo-optical effects due to sample overheating, occurring upon exposure to high-frequency repetition rate probing pulse. If present, thermooptical contribution to the refractive index are extremely hard to separate from the electronic contribution. In order to obtain purely electronic non-linear change in refractive index and to avoid thermal effects we modify classic Z-scan method to operate with low probing laser pulse repetition rate of 1 Hz. In our work, we present results of a Z-scan measured non-linear refraction coefficient of colloidal PbS QDs of different sizes free from thermal contribution. Our measured values of nonlinear refractive index is in order of 10(-16) cm(2)/W and independent on the QD concentration in the solution.

Here we report the direct measurement of the nonlinear refractive index coefficient of liquids by using the Z-scan method with broadband pulsed THz beam. The results fit with conventional Z-scan analytics well. We estimate the nonlinear refractive index coefficient n 2 of liquids through the use of the theoretical treatment that takes the ionic vibrational contribution into account. This estimation correlates with experimental data obtained.

Polar liquids are strong absorbers of electromagnetic waves in the terahertz range, therefore, historically such liquids have not been considered as good candidates for terahertz sources.However, flowing liquid medium has explicit advantages, such as a higher damage threshold compared to solid-state sources and more efficient ionization process compared to gases.Here we report systematic study of efficient generation of terahertz radiation in flat liquid jets under sub-picosecond single-color optical excitation.We demonstrate how medium parameters such as molecular density, ionization energy and linear absorption contribute to the terahertz emission from the flat liquid jets.Our simulation and experimental measurements reveal that the terahertz energy has quasi-quadratic dependence on the optical excitation pulse energy.Moreover, the optimal pump pulse duration, which depends on the thickness of the jet is theoretically predicted and experimentally confirmed.The obtained optical-to-terahertz energy conversion efficiency is more than 0.05%.It is comparable to the commonly used optical rectification in most of electro-optical crystals and two-color air filamentation.These results, significantly advancing prior research, can be successfully applied to create a new alternative source of terahertz radiation.

In modern optical fiber transmission systems, an important aspect is the temporal multiplexing of channels. Guided-wave optical technologies for creating communication lines with a terahertz repetition rate come to the fore. In this paper, the methods of numerical simulation have illustrated the possibility of forming a sequence of subpulses with any duration and with a terahertz repetition rate as well as to control it considering the discrepancy coefficient. This coefficient is related to the discrepancy between the central frequency of subpulses in the quasidiscrete temporal structure and the central frequency of the spectral lines in the quasidiscrete spectral structure. Its influence on the sequence of subpulses after encoding is shown. The results demonstrate the formation of a controlled sequence with a duration of more than 100 ps and a repetition rate of 0.4 THz, which is difficult to achieve by existing methods.

We show experimental results on the generation of terahertz radiation in at jets of modified water and various liquids. We compare the change in the efficiency of the THz waves generation with a change in the pH and kH values, as well as the salt concentration in water and the fat level in milk. Dependences of the terahertz radiation energy on the pump pulse energy are demonstrated.

Get PDF Email Share Share with Facebook Tweet This Post on reddit Share with LinkedIn Add to CiteULike Add to Mendeley Add to BibSonomy Get Citation Copy Citation Text A. N. Tcypkin, M. V. Melnik, M. O. Zhukova, I. O. Vorontsova, S. E. Putilin, S. A. Kozlov, and X. Zhang, "Direct Nonlinear Refractive Index Coefficient Measurement of Water in THz Frequency Range," in Nonlinear Optics (NLO), OSA Technical Digest (Optica Publishing Group, 2019), paper NTu2B.4. Export Citation BibTex Endnote (RIS) HTML Plain Text Citation alert Save article

Modern sources of THz radiation generate high-intensity pulses allowing to observe nonlinear effects in this spectral range. To describe many nonlinear effects theoretically, it is necessary to know the nonlinear refractive index coefficient of optical materials. The work studies the applicability of the Z-scan method to determine the nonlinear refractive index coefficient in the THz frequency range for few-cycle pulses. We have discussed the correctness of the known Z-scan method for calculating the nonlinear refractive index coefficient for broadband THz radiation regarding number of cycles pulses have. We have demonstrated that the error in determining the nonlinear refractive index coefficient is always greater than 70% for true single-cycle pulses. With the increase in the number of oscillations to the measurement error shows strong dependence on the sample thickness and can vary from 2% to 90% regarding the parameters chosen. The fact that such radiation dispersion length is commensurate with the nonlinear length or even less than the latter results in the discrepancy mentioned. It is demonstrated that the decrease in the sample thickness leads to the reduction of the nonlinear refractive index coefficient determination error, and this error is <2% when the ratio between the sample thickness and the pulse longitudinal spatial size is ≤1. This can relate to the fact that the nonlinear effects in such a thin sample occur faster than the dispersion ones.