Nonlinear Refraction Research Articles

Intensity-Dependent Optical Response of 2D LTMDs Suspensions: From Thermal to Electronic Nonlinearities.

The nonlinear optical (NLO) response of photonic materials plays an important role in the understanding of light-matter interaction as well as pointing out a diversity of photonic and optoelectronic applications. Among the recently studied materials, 2D-LTMDs (bi-dimensional layered transition metal dichalcogenides) have appeared as a beyond-graphene nanomaterial with semiconducting and metallic optical properties. In this article, we review most of our work in studies of the NLO response of a series of 2D-LTMDs nanomaterials in suspension, using six different NLO techniques, namely hyper Rayleigh scattering, Z-scan, photoacoustic Z-scan, optical Kerr gate, and spatial self-phase modulation, besides the Fourier transform nonlinear optics technique, to infer the nonlinear optical response of semiconducting MoS2, MoSe2, MoTe2, WS2, semimetallic WTe2, ZrTe2, and metallic NbS2 and NbSe2. The nonlinear optical response from a thermal to non-thermal origin was studied, and the nonlinear refraction index and nonlinear absorption coefficient, where present, were measured. Theoretical support was given to explain the origin of the nonlinear responses, which is very dependent on the spectro-temporal regime of the optical source employed in the studies.

Interfacial modification of GO-FePt hybrid film leads to giant ultrafast optical nonlinearity

Excellent optical nonlinearity is highly promising for the development of multifunctional ultrafast photonics techniques and integrated optoelectronic devices. However, existing numerous optical materials commonly suffer from several challenges, such as bad functional integration, slow response speed, and weak optical nonlinearity thus severely hindering their extensive applications. Here, we first engineer new scalable graphene oxide (GO)-FePt films by chemistry induced self-assembly and then demonstrate the enhanced ultrafast optical nonlinearity by resorting to their interfacial modifications. It is found that the FePt magnetic nanoparticles (NPs) are easily assembled on the surface of the GO by exchanging the surface ligands of the former with evaporated solvent, leading to good water solubility and super-paramagnetism. The femtosecond Z-scan test results show that both the nonlinear saturation absorption and optical Kerr refraction of the pure GO are subjected to reversion once the FePt NPs are introduced. More importantly, we can greatly enhance the ultrafast nonlinear optics response of as-prepared GO-FePt hybrid film by promoting the concentration of magnetic NPs. In addition, the associated optical nonlinearity can be further boosted with femtosecond vectorial beams excitation instead of the linearly polarized one. In principle, the enhanced optical nonlinearity may arise from direct metal-to-semiconductor interfacial charge transfer transition (DICTT) related to the difference in work function and Fermi level together with complicated interaction between the vectorial light fields and the hybrid films. The GO-based magnetic hybrid films with giant ultrafast optical nonlinearity have potential wide-ranging applications in integrated optoelectronics, ultrafast nanophotonics, opto-magnetic storage, and beyond.

Nonlinear Refraction, Excited State Absorption, and Multiphoton Absorption of 1,3-Thiazolium-5-thiolate Mesoionic Compound.

We synthesized the mesoionic compound 2-(4-chlorophenyl)-3-methyl-4-(4-methylphenyl)-1,3-thiazole-5-thiolate and measured its refractive and absorptive nonlinear optical response in different temporal and spectral regimes. The experiments were performed by using the Z-scan technique with two pulsed light sources: the second harmonic (at 532 nm) of a mode-locked and Q-switched Nd-YAG laser (100 ps, 10 Hz) and a Ti: Sapphire laser system (100 fs, 1 kHz) operating at 800 nm. The observation and characterization of nonlinear refraction, two- and three-photon absorption, and excited state absorption of the mesoionic compound dissolved in dimethyl sulfoxide, in different concentrations, are presented and discussed with basis on the population redistribution in a three-energy-level model that allows the determination of the parameters which characterize the nonlinear response.

Third-order nonlinear optical properties of CsPbCl3 and CsPbBr3 perovskite quantum dots: Effects of particle size and surface traps

Particle size and surface trap effects on the third-order nonlinear optical (NLO) properties of CsPbCl3 and CsPbBr3 perovskite quantum dots (PQD) have been investigated using a femtosecond (fs) z-scan experiment. Firstly, optical, morphological, and structural characterizations of the synthesized PQDs were performed using linear absorption and photoluminescence (PL) spectra, XRD, and TEM methods. Next, NLO properties of CsPbCl3 and CsPbBr3 PQDs were studied at the operating wavelength of 800 nm and 3.564 × 1011 W/cm2 laser intensity. The nonlinear absorption coefficient (β), the nonlinear refraction coefficient (γ), and the third-order susceptibilities (χ(3)) values for both sample types were calculated to be on the order of 10−11 m/W, 10−17 m2/W, and 10−11 esu, respectively. The effects of particle size and surface trap sites on the nonlinear properties of the PQDs were studied thoroughly. γ, χ(3), and the unitless nonlinear figure of merits (FOM) parameters were realized to be proportional to the particle size for both types of materials. In addition, these parameters were improved by decreasing the surface trap sites in the PQDs. These results strongly indicate the significant potential of CsPbCl3 and CsPbBr3 PQDs for numerous nonlinear applications.

Formation of plasmon-exciton nanostructures based on quantum dots and metal nanoparticles with a nonlinear optical response

The establishment of the conditions for the formation of nanostructures with plasmon-exciton interaction based onquantum dots and plasmonic nanoparticles that provide unique nonlinear optical properties is an urgent task. The study demonstrates the formation of plasmon-exciton nanostructures based on hydrophilic colloidal Zn0.5Cd0.5S, Ag2S quantum dots and metal nanoparticles. Transmission electron microscopy and optical absorption and luminescence spectroscopy were used to substantiate the formation of plasmon-exciton hybrid nanostructures. The phase composition of the studied samples was determined by X-ray diffraction. The results obtained using ARLX’TRA diffractometer (Switzerland) indicated a cubic crystal structure (F43m) of synthesised Zn0.5Cd0.5S quantum dots and monoclinic (P21/C) crystal lattice of Ag2S. Transmission electron microscopy revealed that plasmonic nanoparticles are adsorption centres for quantum dots. The average sizes of the studied samples were determined: colloidal Ag2S quantum dots (2.6 nm), Zn0.5Cd0.5S(2.0 nm) and metal nanoparticles: silvernanospheres (10 nm) and gold nanorods (4x25 nm). The transformation of the extinction spectra of the light and the luminescence quenching of quantum dots have been established in mixtures of quantum dots and plasmonic nanoparticles. The nonlinear optical parameters of the studied samples were determined using the Z-scanning method at wavelengths of 355 and 532 nm in the field of nanosecond laser pulses. The conditions for the formation of hybrid nanostructures that provide an increase of the coefficient of nonlinear absorption of laser pulses (355 and 532 nm) up to 9 times with a duration of 10 ns due to the reverse saturable absorption occurring due to cascade two-quantum transitions in the intrinsic and local states of colloidal quantum dots and the suppression of nonlinear refraction, were determined. The observed changes were explained by the manifestation of the Purcell effect on the states of quantum dots in the presence of nanoresonators (gold nanorods and silver nanospheres). The results of these studies create new opportunities for the development of original systems for controlling the intensity of laser radiation, as well as quantum sensors of a new generation

Annealing and intensity dependent femtosecond nonlinear optical studies of sodium zinc borate oxide glasses incorporated with gold nanoparticles

Herein, we explored the annealing and intensity–dependent femtosecond (fs) nonlinear optical (NLO) studies on sodium zinc borate (NZB) glasses embedded with gold nanoparticles (NPs). The glasses were prepared by the melt–quench process and subjected for thermal treatment at 450 °C at different schedules in a normal atmosphere. It was identified that the glass annealed for 12 hr (i.e., the NZBAu12 glass) had the maximum saturation intensity and negative nonlinear refraction values. Therefore, the intensity–sensitive NLO investigations then conducted on that glass to understand the influence of intensity of laser radiation on NLO proeprties on gold NPs embedded glasses. The saturable absorption observed at lower intensity was switched to reverse saturable absorption (two–photon absorption) during the exposure with high fluence laser pulses. However, the nonlinear refraction sign was unchanged with increase of intensity of laser radiation. The strategy explored in this work might pave the way for achieving the strong nonlinear traits in borate–based glasses routed with gold NPs. The study outcomes endorse the benefits of NZBAu12 glass for optical switching functionalities under low–intensity pulse excitation, as well as for optical limiting applications under high–intensity pulse excitation.

Nonlinear refractive index enhancement of Nd3+ ions loaded borate glasses in the near–infrared region by silver nanoparticles

The study presents an evaluation and in–depth discussion of the efficacy of silver (Ag) nanoparticles (NPs) concentration on the third–order nonlinear optical (NLO) proeprties of borate glasses. The optical absorption data revealed Nd3+ ions and Ag NPs to be present in the studied glasses. Measurements carried out on the tested glass through transmission electron microscopy revealed spherical–shaped NPs with a median size of ∼16.53 nm (analyzed using the ImageJ software). Open and closed aperture Z–scan configurations were employed to ascertain the attributes of NLO absorption and refraction, respectively. Open–aperture Z–scan data revelaed two–photon absorption nonlinearity in the studied glasses, while the closed aperture Z–scan data revealed self–focusing nonlinear refraction. With the increase in silver NPs concentration, the nonlinear refraction coefficient was significantly increased compared to the nonlinear absorption parameter. The results instantiate large concentrations of Ag NPs embedded in borate glasses bearing Nd3+ ions are suitable for all–optical switching applications that excel in their functioning in near–infrared spectral range.

Thermo-optic coefficient of chemical vapor deposited graphene multilayers

Observation of strong inelastic light-matter interaction and large thermal conductivity of novel two-dimensional graphene has invigorated the present research. Herein, the thermo-optic coefficient of graphene multilayers is obtained from their nonlinear refraction coefficients calculated from Z-scan experiment in closed aperture geometry. The refractive optical nonlinearity was obtained employing a continuous wave He-Ne laser and occurs due to thermal lensing effect. The graphene multilayers behave as the diverging (concave) lens giving rise to self-defocusing effect leading to negative sign of refractive nonlinearity. Accordingly, a large negative thermo-optic coefficient (-2.43×10-3 K-1) inferred for the graphene multilayers suggests its utility for thermo-optic device applications.
 Keywords: Graphene, Optical Nonlinearity, Thermal lens, Self-defocusing, Thermo-optic coefficients

The rich nonlinear optical responses in a squaraine derivative: Nonlinear absorption and transient nonlinear refraction dynamics

The abundant nonlinear optical responses of a squaraine derivative (2, 4-bis[4-(diethylamino)-2-hydroxyphenyl]squaraine, SQ-640) are explored at visible and near-infrared (NIR) regions. The sign and magnitude of the third-order (intensity depend) nonlinear absorption and refraction for SQ-640 are determined under different pluses with Z-scan method. The results of this work show reverse saturable absorption (RSA) of SQ-640 at visible and NIR region and its potential for optical limiting (OL) application. Moreover, SQ-640 shows self-focusing at visible wavelength while shows self-defocusing at NIR wavelengths. The transient refraction dynamics indicate that positive excited-state refraction causes self-focusing at visible region while negative nonlinear refraction at NIR region mainly originates from bound electron response (Kerr effect) and negative excited state refraction. Besides, the nonlinear refraction index of SQ-640 under 532 nm ranges from ∼ 10-19 m2/W at femtosecond pulses to ∼ 10-16 m2/W at nanosecond pulses and the second order hyperpolarizability is about 4 × 10-28 esu at nanosecond case. The Kerr refraction coefficient is about −6.54 × 10-19 m2/W at 700 nm under femtosecond laser pulses. The obtained OL thresholds at 532 nm are ∼ 26.8 mJ/cm2 for picosecond pulses and ∼ 85.6 mJ/cm2 for nanosecond pulses. The enriching information extracted from NLR in SQ-640 may offer an optional perspective to understand the structure–property relationships in these compounds. The results of optical nonlinearities in SQ-640 gained from this study may be of assistance to its better applications in laser protection and optical switching.

Electrical, Thermal Lens and Optical Study of Fluorescein Film for Application As Organic Photovoltaic Devices.

This article is devoted to the study of various dielectric and optoelectrical parameters nonlinear optic behaviors, thermal lens and self-diffraction parameters of Fluorescein (FLs) doped polymethyl methacrylate (PMMA) films. The films were prepared with 60mM. These studies are based on the calculated values of refractive, absorption coefficient, energy gap, extinction coefficient and nonlinear Refraction index . The polymer films were prepared using the casting technique. All samples were previously investigated by UV-Vis-NIR spectrophotometric measurements and Optical microscopy SEM and ATM. Utilizing thermal lens spectrometry, an investigation of the thermo-optical characteristics as well as the nonlinear refractive index was carried out. In this method, a pump beam and a probe beam were brought into collinear alignment with one another. To determination the nonlinear Refraction index . High values of nonlinear refractive index predict a bright future for materials in optical applications. These results indicate that the new dye is a promising candidate for applications in nonlinear optical devices. Investigations were carried out on organic photovoltaic devices in addition to devices consisting of active layers with conducting polymer of PHPP:P3HT film and PHPP:P3HT/Fls. The methods of polymer and dyes synthesis are presented and their physical properties are given.

Linear and third-order nonlinear optical properties of spray pyrolysis deposited Mn doped ZnS nanostructured thin films for optoelectronic device applications

The undoped and 10 wt% of Mn-doped ZnS thin films have been deposited on a glass substrate at 350 °C using the spray pyrolysis technique. The structural, morphological, linear and third-order nonlinear (NLO) properties are reported. The XRD pattern showed that ZnS thin film has a cubic structure. The FESEM images show the uniformity of coating and improvement in the smoothness of the surface after adding the Mn dopant to the ZnS lattice. Using a UV–VIS spectrophotometer the optical energy band gap of ZnS film was found to be 3.71 eV and for 10 wt% of Mn-ZnS is 3.76 eV. By the method of Z-scan, the materials demonstrated good nonlinear absorption (NLA), nonlinear refraction (NLR) and third-order NLO susceptibilities. The magnitude of third-order NLO susceptibilities (χ)(3) 10-6 esu, NLA coefficient (β) 10-5 mW−1 and nonlinear refraction (n2) 10-8 cm2/W. The positive findings of NLO of Mn: ZnS thin film as optoelectronic devices and optical switching applications.

Enhanced electro-optical properties of nematic liquid crystal doped with Nitrogen doped graphene quantum dots

The first step of this study was to prepare mixtures that contained different concentrations of N doped graphene quantum dots (NGQDs) into nematic liquid crystals (NLCs). Then, through the Z-scan technology, the optical measurements in the nonlinear regime were conducted with a continuous wave (CW) at 632 nm wavelength and a low power laser beam. The nonlinear absorption (NLA) coefficient, nonlinear refractive (NLR) index, and third-order nonlinear susceptibility of NLCs doped with different concentrations of the NGQDs at distinctive laser powers and diverse applied voltages of external electric field was experimentally measured. The Z-scan results demonstrate that prepared cells have two photon absorption (2PA) and self-focusing effects. As the result, the concentration of the NGQDs in E7 and power of laser were optimum at 1% wt. and 11 mW, respectively. The effect of a DC electric field on optical nonlinearity was also investigated in the mixtures of NLCs and NGQDs at the concentration of 1% wt. Based on these findings, this sample can be used in both nonlinear optics and ultrafast broadband photonics.

Investigation of nonlinear absorption coefficient (β) and nonlinear refraction (n2) of L-threoninium P-toluenesulfonate monohydrate organic single crystal using two laser sources

Third order nonlinear optical properties of organic crystal L-threoninium P-toluenesulfonate monohydrate was investigated by ‘Z-scan’ technique employing two different laser sources under different intensities at 532 nm CW laser and 1064 nm pulsed laser respectively. Nonlinear absorption coefficient (αNL) and nonlinear refraction (n2) were evaluated at six different intensities using 532 nm CW laser source and with pulsed laser source (λ = 1064 nm) by changing the pulse rate keeping intensity same. It is found that this crystal possesses positive nonlinearity and exhibit saturation absorption.

Design of Single-Polarized Single-mode Photonic Crystal Fiber Without Air Holes in the Cladding

This study proposes a novel single-polarized single-mode (SPSM) photonic crystal fiber (PCF) without air holes in the cladding, and the silicon nitride holes and air holes in the core of the PCF are staggered. Based on the full vector finite element method, the birefringence characteristics with different hole sizes have been discussed in detail, and a SPSM-PCF is achieved through optimizing the core structure. Moreover, the extinction ratio, effective refractive index, dispersion, confinement loss and nonlinearity of the PCF are analyzed. Simulation results show that the PCF can obtain a high birefringence value of 2.142 × 10−3 , a large negative dispersion of −2750 ps/(nm · km), a low confinement loss of 1.57 × 10−7dB/m and a high nonlinear coefficient of 90.5W−1 · km−1 at the wavelength of 1.55 μm. In this paper, the proposed structure has the properties of high birefringence and high negative dispersion, which can be used in fiber laser, optical fiber sensing, dispersion compensation and other fields.

Nd3+ high order electrical susceptibility induced by CdS nanocrystals in a phosphate glass matrix

In this work the nonlinear optical properties of a phosphate glass matrix doped with Nd3+and codoped with CdS nanocrystals have been investigated by the time-resolved Z-scan technique. Analyzing the transient signal, it was possible to discriminate the thermal effect from the populational effects on the nonlinear refraction and absorption, allowing the determination of the parameters n2’, n2” and n4’. The dependence of these parameters on the relative concentration of Nd and CdS was also investigated, and a linear increase of these parameters with the CdS absorption was observed. Evidence of energy transfer between Nd and CdS nanocrystals was investigated by the determination of the non-radiative decay rate through lifetime measurements monitoring the emission at 1064 nm referring to the transition 4F3/2⟶ 4I11/2 of the Nd3+.

Single-Wall-Carbon-Nanotube– VSe2 Nanohybrid for Ultrafast Visible-To-Near-Infrared Third-Order Nonlinear Optical Limiters

An ideal third-order nonlinear optical material, which can efficiently absorb and refract light over a broad spectral range, is of both fundamental and technological significance as it enables many helpful functionalities in photonics. However, the optical nonlinearities are rather weak due to their perturbative nature and are limited to electronic resonances, thus restricting the response to a narrow spectral range. A charge-coupled donor-acceptor material pair can enhance the nonlinear optical response and circumvent the narrow spectral range limitation. However, such studies on potential material pairs remain largely unexplored. Here, we report the experimental observation of ultrafast third-order nonlinear optical response spanning the entire visible-to-near-infrared (400--900 nm) region in single-wall-carbon-nanotube (SWCNT)--${\mathrm{VSe}}_{2}$ nanohybrid in the strong coupling regime, enabled by efficient charge transfer. Compared to control systems, the measured nonlinear absorption and refraction of the nanohybrid show unprecedented or many-fold enhancements. Further, our density-functional theory and Bader-charge analysis show the strong electronic coupling of the nanohybrid in which the electrons are transferred from ${\mathrm{VSe}}_{2}$ to SWCNT, verified by steady-state and time-resolved photoluminescence measurements. The physics of the ultrafast nonlinear optical response is well captured by our five-level rate-equation model both qualitatively and quantitatively. Using the nanohybrid, we design a liquid cell-based optical limiter with an order of magnitude better device performance parameters, such as the optical limiting onset ($2.5--8.0\phantom{\rule{0.2em}{0ex}}\mathrm{mJ}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$) and the differential transmittance ($0.42--0.62$), compared to several other benchmark optical limiters in the femtosecond regime.

Nonlinear optical properties of gold nanoparticles containing borate glasses at spectral region of surface plasmon resonance

Herein, we subjected the sodium borate glass activated with gold nanoparticles (NPs) to thermal treatment at 500 °C for different holding time to improve their nonlinear optical (NLO) characteristics in visible spectral region. The linear optical density and microscopic measurements confirmed the presence gold NPs in the glass. The outcomes also divulged the growth of Au NPs with an annealing schedule. The NLO attributes were ascertained through the Z–scan method, in which the samples were pumped at 532 nm utilizing nanosecond laser pulses fired at a rate of 1 kHz. Open aperture Z–scan measurements disclosed the existence of two–photon absorption nonlinearity, whereas closed aperture Z–scan experiments revealed the presence of self–defocusing nonlinear refraction in the studied glasses. The measurements unveiled the enhance trend of two–photon absorption and nonlinear refraction coefficients of glass host with thermal holding time owing to the growth of NPs. It was identified that the titled glass embedded with gold NPs annealed for 7 h at 500 °C owes maximum NLO features. Therefore, NBAu–7 glass examined in the current sturdy is beneficial for bio–imaging, optical limiting and NLO nanophotonic applications.

Synthesis, band gap structure and third order non-linear optical properties of zinc tungsten oxide nanocomposite using a single CW laser beam.

In this study, a composite of zinc tungsten oxide nanoparticles (W-ZnO NPs) has been synthesized via mixing Na2WO4 and zinc acetate in water, followed by dropwise addition of NaOH. The synthesized W-ZnO NPs were characterized using measurement methods such as XRD, dynamic light scattering (DLS), Scanning electron microscopy (SEM) and UV-Vis. Also, the results were compared with the pure synthesized ZnO and WO3 NPs. Non-linear optical properties of the synthesized composite were measured using the Z-scan technique with a continuous wave Nd-Yag laser. The negative non-linear absorption coefficient of the components was obtained which indicates that the saturation absorption occurred in this composite. In comparison with pure ZnO NPs, non-linear absorption decreases which can be attributed to the negligible optical response of WO3 structures. Also, the negative value of the close aperture Z-scan curve shows that the thermal lensing effect is the main reason for the third-order non-linear refraction.

Fabrication, Characterization of Basic Blue 7 Dye-Doped PVA Films and Their Third-Order Nonlinear Optical Properties.

In this work, we report on the fabrication, characterization and the third-order nonlinear optical (NLO) properties of basic blue 7 (BB 7) dye‒doped polyvinyl alcohol (PVA) polymer thin films. The absorption spectra of BB 7 dye‒PVA films were measured by UV‒visible absorption studies. The functional groups of BB 7‒PVA films have been identified by FT‒IR spectroscopic analysis. The surface morphology was examined by AFM and SEM analysis which shows that the BB 7 dye‒doped PVA films have a homogenous and smooth surface. The nonlinear absorption and refraction of BB 7‒PVA films were respectively explored by open aperture and closed aperture Z‒scan technique using a 5 mW semiconductor diode laser of 635nm wavelengths. The BB 7‒PVA films exhibit the reverse saturable absorption (RSA) and self-defocusing effect and the measured third‒order nonlinear susceptibility χ(3) values of these films were found to be of the order of 10- 5 esu. The present experimental results show that BB 7‒PVA films may have potential applications in future photonic and NLO devices.

Measurements of Nonlinear Absorption and Refraction Coefficients of Pure and Nd Doped Calcium Lanthanum Borate Glasses

Measurements of Nonlinear Absorption and Refraction Coefficients of Pure and Nd Doped Calcium Lanthanum Borate Glasses