Large Nonlinear Refractive Index Research Articles

Versatile soliton emission from a [formula omitted] mode-locked fiber laser

Recently, few-layer tungsten disulfide (WS2), as a shining 2D material, has been discovered to possess both the saturable absorption ability and large nonlinear refractive index. Here, we demonstrate versatile soliton pulses in a passively mode-locked fiber laser with a WS2-deposited microfiber. The few-layer WS2 is prepared by the liquid-phase exfoliation method and transferred onto a microfiber by the optical deposition method. Study found, the WS2-deposited microfiber can operate simultaneously as a mode-locker and a high-nonlinear device. In experiment, by further inserting the WS2 device into the fiber laser, besides the dual-wavelength soliton, noise-like soliton pulse, conventional soliton and its harmonic form are obtained by properly adjusting the pump strength and the polarization states. For the dual-wavelength soliton pulses and noise-like pulse, the maximum output power of 14.2 mW and pulse energy of 4.74 nJ is obtained, respectively. In addition, we also achieve the maximum harmonic number (135) of conventional soliton, corresponding to a repetition rate of ∼497.5 MHz. Our study shows clearly that WS2-deposited microfiber can be as a high-nonlinear photonic device for studying a plenty of nonlinear soliton phenomena.

Synthesis of selenium nanoparticles and their nonlinear optical properties

Selenium nanoparticles were fabricated using a physical deposition method based on heterogeneous condensation. The Selenium nanoparticles were deposited on a quartz substrate and the ultrafast nonlinear optical responses were investigated by using the [Formula: see text]-scan technique. The nanosize films showed two-photon absorption and a large positive nonlinear refractive index at 800 nm excitation wavelength. The obtained real and imaginary parts of the intensity-dependent complex third-order nonlinear susceptibility of the Selenium nanoparticles ranged from [Formula: see text] to [Formula: see text] (esu) for different irradiance levels.

Optical nonlinearity of D-A-π-D and D-A-π-A type of new chalcones for potential applications in optical limiting and density functional theory studies

Two new chalcones namely, (2E)-1-(3-fluoro-4-methoxyphenyl)-3-(4-methoxyphenyl) prop-2-en-1-one and (2E)-3-(4-chlorophenyl)-1-(3-fluoro-4-methoxyphenyl)prop-2-en-1-one were synthesized and grown as single crystals by slow evaporation technique in methanol. The FTIR spectrum recorded confirms the presence of functional groups in these materials. The molecular conformation of the compounds was achieved by single crystal X-ray diffraction studies. The thermal stability of the crystals was determined from TGA/DSC curve. The third order optical nonlinearity of the chalcone compounds in DMF solution has been carried out using an Nd:YAG laser at 532 nm as the source of excitation. The nonlinear optical response was characterized by measuring the intensity dependent refractive index n2 of the medium using Z-scan technique. It is seen that the molecules exhibit a negative (defocusing) nonlinearity and large nonlinear refractive index of the order of −1.8 × 10−11 esu. The third-order nonlinearity of the studied chalcones is dominated by nonlinear refraction, which leads to strong optical limiting of laser. The result reveals that these two new chalcone molecules would be a promising material for optical limiting applications. In addition, the optimized molecular geometry, vibrational frequencies in gas, and the Molecular Electrostatic Potential (MEP) surface parameters of the two molecules were calculated using DFT/B3LYP method with 6–311++G(d,p) basis set in ground state. All the theoretical calculations were found in good agreement with experimental data.

Nonlinear Refraction Behaviour of Toluidine Blue O Dye-PVA Nanocomposite Films under CW Laser Light Excitation

Toluidine Blue O (TBO) dye-PVA nanocomposite films were prepared following a simple method based on solution-casting. Their morphology, linear as well as nonlinear optical properties were investigated. These nanocomposite films were characterized as nanoclustered aggregates grown within the PVA matrix. The nonlinear refraction behaviour of the nanocomposite films was investigated using Z-scan technique, under excitation with continuous wave (CW) laser light at 442nm for different peak incident intensities. The samples exhibited a negative nonlinearity and large nonlinear refractive index of the order of 10-6 cm2/W, which were better than some of the recently reported values for CW laser light excitation.

Kerr self-defocusing of multiple filaments in TW peak power UV laser beam

An effective suppression of multiple filamentation of the sub-TW peak power supercritical laser beam in xenon gas was demonstrated in direct amplification of subpicosecond UV pulses at Ti:sapphire/KrF laser facility GARPUN-MTW. A large negative nonlinear refractive index due to a two-photon resonance of KrF laser radiation with Xe state ensured Kerr self-defocusing of a few hundred filaments with a local peak intensity of ~0.2 TW cm−2, 200-fold higher than the average one over the beam cross section, and thus homogenized the laser beam. UV filaments in Xe produced a narrow-angle monochromatic coherent cone emission at 828 nm wavelength due to stimulated hyper-Raman scattering and amplified spontaneous emission at the transition .

Observation of Two-Photon Absorption and Nonlinear Refraction in AlN**Supported by the National Natural Science Foundation of China under Grant Nos 51372267, 51210105026 and 51172270, and the National Basic Research Program of China under Grant No 2013CB932901.

Two-photon absorption (TPA) and nonlinear refraction in AlN single crystals are revealed with the single beam Z-scan technique. A large TPA coefficient of 13±3cm/GW at 355nm and third-order nonlinear refractive indices of −1.91±0.38×10−13 cm2/W at 355 nm, 1.79±0.36×10−13 cm2/W at 532 nm and 1.61±0.32×10−12 cm2/W at 1064 nm are derived. It is inferred that the TPA-generated free carriers are responsible for the large negative nonlinear refractive index at 355 nm, while the oxygen impurity induced bandgap narrowing may account for the large third-order nonlinear refractive indices at 532 nm and 1064 nm. The large nonlinear optical responses of AlN promise wide applications in the deep ultraviolet nonlinear optical devices.

Measurement of large nonlinear refractive index of natural pigment extracted from Hibiscus rosa-sinensis leaves with a low power CW laser and by spatial self-phase modulation technique

We have reported here, for the first time, to the best of our knowledge, a high nonlinear refractive index (n2e) of a natural pigment extracted from Hibiscus rosa-sinensis leaves by using spatial self-phase modulation technique (SSPM) with a low power CW He-Ne laser radiation at 632.8nm. It is found by UV–Vis absorption spectroscopic analysis that chlrophyll-a, chlrophyll-b and carotenoid are present in the pigment extract with 56%, 25% and 19%, respectively. The photoluminescence (PL) emission characteristics of the extracted samples have also been measured at room temperature as well as in the temperature range of 283–333K to investigate the effect of temperature on luminescent properties of the sample. By analyzing the SSPM experimental data, the nonlinear refractive index value of pigment extract has been determined to be 3.5×10−5cm2/W. The large nonlinear refractive index has been assigned due to asymmetrical structure, molecular reorientation and thermally induced nonlinearity in the sample. The presented results might open new avenues for the green and economical technique of syntheses of organic dyes with such a large nonlinear optical property.

Third-order nonlinearity and saturable absorbed performance of Cr^4+:Gd_3Ga_5O_12 crystal

In this work, the third-order nonlinear optical properties and saturable absorbed performance of Cr4+-doped Gd3Ga5O12 (Cr4+:GGG) crystal were investigated. By using the Z-scan technique, the third-order nonlinear properties were analyzed systematically. Compared with Cr4+:YAG crystal, Cr4+:GGG crystal has a large third-order nonlinear refractive index, ground-state absorption, and excited-state absorption cross section. It has been successfully employed as a saturable absorber for passively Q-switched and Q-switching mode-locked lasers in this paper. In the Q-switched regime, the maximum average output power of 600 mW was obtained with the shortest pulse width of 4.76 ns and the repetition rate of 41.4 kHz, corresponding to single pulse energy and pulse peak power of 14.5 µJ and 3 kW, respectively. For the Q-switching mode-locking operation, the maximum output power of 468 mW was obtained with a repetition rate of 141.2 MHz. The results indicate that Cr4+:GGG crystal has the potential to be used for passive Q-switching and mode-locking laser generation.

Stimulated degenerate four-wave mixing in Si nanocrystal waveguides

Parametric frequency conversion via four-wave mixing (FWM) in silicon nanocrystal (Si NC) waveguides is observed at 1550 nm. To investigate the role of Si NC, different types of waveguides containing Si NC in a SiO2 matrix were fabricated. Owing to the increase of the dipole oscillator strength mediated by the quantum confinement effect, the non-linear refractive index () of Si NCs is found to be more than one order of magnitude larger than the one of bulk Si. Coupled differential equations for the degenerate FWM process taking into account the role of Si NC were numerically solved to model the experimental data. The modeling yields an effective for Si NCs in SiO2 waveguides which is similar to the one of Si waveguides. We also measured a large signal to idler conversion bandwidth of ∼22 nm. The large non-linear refractive index is joined with a large two photon absorption coefficient which makes the use of Si NC in non-linear optical devices mostly suitable for mid-infrared applications.

Wideband nonlinear spectral broadening in ultra-short ultra - silicon rich nitride waveguides.

CMOS-compatible nonlinear optics platforms with high Kerr nonlinearity facilitate the generation of broadband spectra based on self-phase modulation. Our ultra – silicon rich nitride (USRN) platform is designed to have a large nonlinear refractive index and low nonlinear losses at 1.55 μm for the facilitation of wideband spectral broadening. We investigate the ultrafast spectral characteristics of USRN waveguides with 1-mm-length, which have high nonlinear parameters (γ ∼ 550 W−1/m) and anomalous dispersion at 1.55 μm wavelength of input light. USRN add-drop ring resonators broaden output spectra by a factor of 2 compared with the bandwidth of input fs laser with the highest quality factors of 11000 and 15000. Two – fold self phase modulation induced spectral broadening is observed using waveguides only 430 μm in length, whereas a quadrupling of the output bandwidth is observed with USRN waveguides with a 1-mm-length. A broadening factor of around 3 per 1 mm length is achieved in the USRN waveguides, a value which is comparatively larger than many other CMOS-compatible platforms.

0.6-3.2 μm supercontinuum generation in a step-index germania-core fiber using a 4.4 kW peak-power pump laser.

An ultra-broadband supercontinuum was generated in a short piece of step-index germania-core fiber using a fiber laser with a peak power of 4.4 kW. The pure germania core made this fiber capable of propagating light towards the desirable mid-infrared region. The spectral broadening characteristics towards the mid-infrared region under different lengths of germania-core fiber were investigated using pump pulses of 4.4 kW and 1.1 ns at 1550 nm. The large nonlinear refractive index of germania and the small core size of germania-core fiber produced a nonlinear coefficient as high as 11.8 (W km)-1 at 1550 nm, which was beneficial for supercontinuum generation. The pump wavelength was located in the anomalous dispersion regime and close to the zero dispersion wavelength of this germania-core fiber, 1.426 μm. Eventually, an ultra-broadband supercontinuum source with a spectrum spanning from 0.6 to 3.2 μm was obtained and had a total output power of 350 mW at an optimized germania-core fiber length of 0.8 m. This work is the first demonstration, to the best of our knowledge, of a germania-core fiber-based ultra-broadband supercontinuum source that spans from the visible region to the mid-infrared region.

Quasi-three-level model applied to measured spectra of nonlinear absorption and refraction in organic molecules

Materials with a large nonlinear refractive index (n2) and relatively small linear and nonlinear absorption losses, namely, two-photon absorption (2PA, of coefficient α2), have long been sought after for applications such as all-optical switching (AOS). Here we experimentally determine the linear and 2PA properties of several organic molecules, which we approximate as centrosymmetric, and use a simplified essential-state model (quasi-three-level model) to predict the dispersion of n2. We then compare these predictions with experimental measurements of n2 and find good agreement. Here “quasi”-three-level means using a single one-photon allowed intermediate state and multiple (here two) two-photon allowed states. This also allows predictions of the figure-of-merit (FOM), defined as the ratio of nonlinear refractive phase shift to the 2PA fractional loss, that determines the viability for such molecules to be used in device applications. The model predicts that the optimized wavelength range for a large FOM lies near the short wavelength linear absorption edge for cyanine-like dyes where the magnitude of n2 is quite large. However, 2PA bands lying close to the linear absorption edge in certain classes of molecules can greatly reduce this FOM. We identify two molecules having a large FOM for AOS. We note that the FOM is often defined as the ratio of real to imaginary parts of the third-order susceptibility (χ(3)) with multiple processes leading to both components. As explained later in this paper, such definitions require care to only include the 2PA contribution to the imaginary part of χ(3) in regions of transparency.

Watt-level supercontinuum generation in As2Se3 fibers pumped by a 2-micron random fiber laser

Chalcogenide fibers are good candidates for generating supercontinuum (SC) radiation due to their large nonlinear refractive indices and high mid-infrared transmission, but their low damage thresholds hamper the SC power scaling, thereby limiting the ultimately achieved SC brightness. Here, we report an As2Se3 fiber SC system pumped by a novel random Q-switched 2 μm Tm3+ fiber laser. The maximum SC output power is 1.09 W with slope efficiency of 24%, and the SC spans from ~1980 to ~2500 nm with a spectral width of ~500 nm at the −20 dB points. The spectral power density is ~2 mW nm−1. To the best of our knowledge, this is the highest power and spectral density SC emission ever achieved in chalcogenide fibers.

Nonlinear Optical Response of Organic–Inorganic Halide Perovskites

Metal halide perovskites have exhibited excellent properties as absorbers in solar cells, but this may simply be the first of many applications for this intriguing class of materials. Here, we report the nonlinear optical response of triiodide (CH3NH3PbI3) and mixed halide (CH3NH3PbI3–xClx) perovskite absorbers. The results show that they have large nonlinear refractive index (NRI), 3 orders of magnitude larger than that of silicon. Particularly, the NRI of CH3NH3PbI3–xClx is more than two times larger compared to that of CH3NH3PbI3. Meanwhile, both of them have been proven to possess saturable absorption effects with small nonlinear absorption coefficients which indicate that they can maintain excellent absorption under high-intensity irradiation and are favorable to modulators toward large-energy pulsed laser. Taking into consideration the saturable absorption effect, we demonstrated a pulsed laser with the perovskite as a pulse modulator. These results above indicate the potential for perovskites to be...

Z-scan studies and optical limiting in a new organic-polymer composite film

Nonlinear optical (NLO) properties of a new poly[[(s)-1-(4-nitrophenyl)-2-pyrrolidinethyl]methacrylate] (PPM) in polymethylmethacrylate (PMMA) at different peak incident intensities ranging from 0.6 to 2 kW/cm2 are investigated using the open and closed aperture Z-scan technique under CW DPSS laser at 532 nm wavelength. The optical nonlinearity is intensity dependent. The experimentally results showed that the composite film exhibits self-defocusing nonlinearity with large nonlinear refractive index and good nonlinear absorption behavior. The observed nonlinear refraction and absorption are attributed to thermal in nature and reverse saturable absorption, respectively. The PPM/PMMA composite film also exhibits an interesting optical power limiting of the CW laser with good optical limiting threshold. These studies indicate that the new composite film may open new possibilities of using this material in all-optical limiting and switching devices.

SiO_2-Al_2O_3-La_2O_3 glass - a superior medium for optical Kerr gating at moderate pump intensity

Novel SiO2-Al2O3-La2O3 (SAL) glass shows a switching efficiency η ≤ 70% for dual wavelengths optical Kerr gating (OKG) at 0.7 TW/cm2 peak intensity IP of the gate pulse. It steadily increases with IP, is highest for the highest La2O3 content (24 mol%) and then superior to OKG media of large nonlinear refractive index n2 above 150 GW/cm2 (ZnS), 350 GW/cm2 (tellurite glass), and 450 GW/cm2 (N-SF56 glass). This superiority is attributed to negligible nonlinear counter processes in SAL glass which in the large n2 valued media make η(IP) decrease in the high IP region.

Large third-order nonlinear refractive index coefficient based on gold nanoparticle aggregate films

Au nanoparticles show large third-order nonlinear effect and ultra-fast response. Here a high nonlinear aggregate film based on self-assembled gold nanoparticles is reported and its third-order nonlinear refractive index coefficient is measured by Z-scan experiment. The third-order nonlinear refractive index coefficient of the Au nanoparticle aggregate film (γ1 = 9.2 × 10−9 cm2/W) is found to be larger than that of an 8-nm-thick sputtered Au film (γ2 = 6.5 × 10−9 cm2/W). This large nonlinear effect can be attributed to the strong field enhancement due to localized plasmon resonances between Au nanoparticles. The result shows that the self-assembled Au nanoparticle aggregate film could be a promising candidate as a third-order nonlinear optical material.

Prediction of an extremely large nonlinear refractive index for crystals at terahertz frequencies

We develop a simple analytical model for calculating the vibrational contribution to the nonlinear refractive index ${n}_{2}$ (Kerr coefficient) of a crystal in terms of known crystalline parameters such as the linear refractive index, the coefficient of thermal expansion, the atomic density, and the reduced mass and the natural oscillation frequency of the vibrational modes of the crystal lattice. We show that the value of this contribution in the terahertz spectral region can exceed the value of the nonlinear refractive index ${n}_{2}$ in the visible and near-IR spectral ranges (which is largely electronic in origin) by several orders of magnitude. For example, for crystal quartz the value of the Kerr coefficient in the low-frequency limit is ${n}_{2}=2.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}$ esu or, equivalently, $4.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}\phantom{\rule{0.28em}{0ex}}{\mathrm{m}}^{2}$/W, which is very much larger than its value of $3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}20}\phantom{\rule{0.28em}{0ex}}{\mathrm{m}}^{2}$/W in the visible range. Furthermore, we present an analysis of the dispersion of ${n}_{2}$ in the terahertz spectral range and show that even larger values of ${n}_{2}$ occur at frequencies close to the vibrational resonances.

Third-order nonlinear optical response of Ag–CdSe/PVA hybrid nanocomposite

Hybrid nanocomposites of II–VI semiconductor nanoparticles are gaining great interest in nonlinear optoelectronic devices. Present work includes the characterization of CdSe polymer nanocomposite prepared by chemical in situ technique. From X-ray diffraction, the hexagonal wurtzite structure of nanoparticles has been confirmed with spherical morphology from transmission electron microscopy. Ag–CdSe hybrid polymer nanocomposite has been prepared chemically at different Ag concentrations. The presence of Ag in hybrid nanocomposite has been confirmed with energy-dispersive X-ray spectroscopy. The effect of varying Ag concentration on the linear and nonlinear optical properties of the nanocomposites has been studied. In linear optical parameters, the linear absorption coefficient, refractive index, extinction coefficient and optical conductivity have been calculated. The third-order nonlinear optical properties have been observed with open- and closed-aperture Z-scan technique. The large nonlinear refractive index ~10−5 cm2/W with self-focusing behaviour is due to the combined effect of quantum confinement and thermo-optical effects. The enhanced nonlinearity with increasing Ag content is due to the surface plasmon resonance, which enhances the local electric field near the nanoparticle surface. Thus, Ag–CdSe hybrid polymer nanocomposite has favourable nonlinear optical properties for various optoelectronic applications.

Third-order nonlinearity and passive Q-switching of Cr⁴⁺:YGG garnet crystal.

We demonstrate the third-order nonlinear optical properties of Cr(4+):Y(3)Ga(5)O(12) (Cr(4+):YGG) and Q-switched lasers with Cr(4+):YGG as the saturable absorber for the first time to our knowledge. The third-order nonlinear properties, including the optical Kerr nonlinearity and saturable absorption, were systematically measured and analyzed in detail by using a Z-scan technique. The measured data show that Cr(4+):YGG has a large nonlinear refractive index, ground-state absorption cross section, and excited-state absorption cross section in contrast to Cr(4+):Y(3)Al(5)O(12) (Cr(4+):YAG). With a Nd:YGG crystal as the gain medium and a Cr(4+):YGG crystal as the saturable absorber, the passively Q-switched laser was performed. The shortest pulse width and largest pulse energy were achieved at the absorbed pump power of 8 W with the values of 9.1 ns and 26.1 μJ, respectively, corresponding to the average output power of 0.87 W and peak power of 2.9 kW. The results indicate that Cr(4+):YGG is an available and promising optical switcher for pulsed lasers.