Third-order nonlinear optical properties of graphene composites:A review

Graphene has excellent thirdorder nonlinear optical(NLO)properties due to its unique electronic band structure and wideband gap tunability.This paper focuses on the research progress of graphene and its composite materials in nonlinear optics in recent years.In this review,recent results on graphene(or graphene oxide)-metal nanoparticles(G-MNPs),graphene-metal-oxide nanoparticles(G-MONPs),graphene-metal sulfide nanoparticles(G-MSNPs),and graphene-organic molecular composites(G-OM)have been discussed.In addition,the enhancement mechanism of nonlinear absorption(NLA)and optical limiting(OL)have also been covered.

Study on the optical limiting properties of the mixed liquid of carbon black suspensions and green tea solution by multi-pulse laser

The optical limiting properties of the mixed liquid of carbon black suspensions （CBS） and green tea solution were studied by using an 8 ns laser pulse at 532 nm. The optical limiting effects of the CBS and mixed liquid have been compared between 5 and 10 Hz repetition frequencies with nanosecond laser pulse. The experimental results indicate that the optical limiting threshold of the sample with the incidence laser at 10 Hz repetition frequency is lower than at 5 Hz repetition frequency. The possible reasons for the influence of the repetition frequency on the samples are discussed. And by observing the optical radiant distributions when the laser pulse passing through different samples, a possible mechanism for the observed effects is suggested. At the same time, the result shows that the optical limiting of CBS is the dominant factor to optical limiting of the mixed liquid.

Measurement of stimulated Brillouin scattering(SBS) threshold based on waveform variation of SBS optical limiting

This paper proposes a method for measuring the stimulated Brillouin scattering （SBS） threshold based on waveform variation of SBS optical limiting. The output waveforms for different pump power densities are numerically simulated, and validated in the Nd：YAG seed-injected laser system. The results indicate that SBS does not take place in the case of a low pump power density and thus the output power scales up linearly with pump power. Once the pump power density exceeds the SBS threshold, SBS takes place and thereby the energies are transferred from pump to Stokes. As a result, a small shoulder appears in the trailing edge of the output waveform, which provides another method to determine the SBS threshold.

Tailoring optical properties of TiO_2 in silica glass for limiting applications

We present the linear and nonlinear optical studies on TiO2-SiO2 nanocomposites with varying compositions. Opti- cal band gap of the material is found to vary with the amount of SiO2 in the composite. The phenomenon of two-photon absorption （TPA） in TiO2/SiO2 nanocomposites has been studied using open aperture Z-scan technique. The nanocom- posites show better nonlinear optical properties than pure TiO2, which can be attributed to the surface states and weak dielectric confinement of TiO2 nanoparticles by SiO2 matrix. The nanocomposites are thermally treated and similar studies are performed. The anatase form of TiO2 in the nanocomposites shows superior properties relative to the amorphous and rutile counterpart. The involved mechanism is explained by rendering the dominant role played by the excitons in the TiO2 nanoparticles.

Two-photon Absorption and Optical Power Limiting Properties of Two Novel Dibenzothiophene-based Chromophores

Two novel V-shaped symmetric chromophores： E-2,8-bis（4-vinyl-4-carbazol-9-yl）diben- zothiophene （abbreviated as SK-G1） and E-2,8-bis（4-vinyl-4-triphenylamino） dibenzothiophene （abbreviated as ST-G1） have been synthesized and characterized. Their two photon absorption properties were measured by the open-aperture femtosecond Z-scan technique and the nanosecond nonlinear optical transmission （NLT）, respectively, when pumped by Ti： sapphire laser at 750 nm and 800 nm.

Optical power limiting of ultrashort hyper-Gaussian pulses in cascade three-level system

Propagation of strong femtosecond hyper-Gaussian pulses in a cascade three-level molecular system is studied by solving numerically the Maxwell–Bloch equations by the iterative predictor-corrector finite-difference time-domain method.Optical power limiting behavior induced by strong nonlinear two-photon absorption is observed for different orders of the femtosecond hyper-Gaussian pulses. Pulses of a higher order temporal profile are found to have a wider power range of optical limiting but a larger output saturation intensity. Both the output saturation value and the damage threshold of optical power limiting decrease with pulse duration increasing. The decrease of the pulse area along the pulse propagation is much slower than that obtained from the two-photon area theorem due to invalidity of the slowly varying amplitude approximation and the monochromatic field hypothesis.

Laser shaping and optical power limiting of pulsed Laguerre–Gaussian laser beams of high-order radial modes in fullerene C60

We study the strong nonlinear optical dynamics of nanosecond pulsed Laguerre–Gaussian laser beams of high-order radial modes with zero orbital angular momentum propagating in the fullerene C60molecular medium. It is found that the spatiotemporal profile of the incident pulsed Laguerre–Gaussian laser beam is strongly reshaped during its propagation in the C60molecular medium. The centrosymmetric temporal profile of the incident pulse gradually evolves into a noncentrosymmetric meniscus shape, and the on-axis pulse duration is clearly depressed. Furthermore, the field intensity is distinctly attenuated due to the field-intensity-dependent reverse saturable absorption, and clear optical power limiting behavior is observed for different orders of the input pulsed Laguerre–Gaussian laser beams before the takeover of the saturation effect;the lower the order of the Laguerre–Gaussian beam, the lower the energy transmittance.

Functional nonlinear optical nanoparticles synthesized by laser ablation

Nonlinear optics is an important research direction with various applications in laser manufacturing,fabrication of nano-structure,sensor design,optoelectronics,biophotonics,quantum optics,etc.Nonlinear optical materials are the funda-mental building blocks,which are critical for broad fields ranging from scientific research,industrial production,to military.Nanoparticles demonstrate great potential due to their flexibility to be engineered and their enhanced nonlinear optical properties superior to their bulk counterparts.Synthesis of nanoparticles by laser ablation proves to be a green,efficient,and universal physical approach,versatile for fast one-step synthesis and potential mass production.In this review,the development and latest progress of nonlinear optical nanoparticles synthesized by laser ablation are summarized,which demonstrates its capability for enhanced performance and multiple functions.The theory of optical nonlinear absorption,experimental process of laser ablation,applications,and outlooks are covered.Potential for nanoparticle systems is yet to be fully discovered,which offers opportunities to make various types of next-generation functional devices.

Capacity Scaling Limits and New Advancements in Optical Transmission Systems

Optical transmission technologies have gone through several generations of development.Spectral efficiency has significant ly improved,and industry has begun to search for an answer to a basic question：What are the fundamental linear and nonlin ear signal channel limitations of the Shannon theory when there is no compensation in an optical fiber transmission system？Next-generation technologies should exceed the 100G transmis sion capability of coherent systems in order to approach the Shannon limit.Spectral efficiency first needs to be improved be fore overall transmission capability can be improved.The means to improve spectral efficiency include more complex modulation formats and channel encoding/decoding algorithms,prefiltering with multisymbol detection,optical OFDM and Ny quist WDM multicarrier technologies,and nonlinearity compen sation.With further optimization,these technologies will most likely be incorporated into beyond-100G optical transport sys tems to meet bandwidth demand.

Optical Limiting Response in Er^(3+)-Doped TeO_2-Nb_2O_5-ZnO Glass

The nonlinear absorption properties of Er^3＋ doped telluride glass were investigated with picosecond laser pulses. The optical limiting response was measured with a transmission technique and reverse saturable absorption （RSA） with a Z-scan technique, which proved that the glass was a promising material for practical optical limiters. The experimental resulted showed that the excited absorption was responsible for the measured RSA, resulting in the optical limiting response. The measured data could be well simulated with a rate equation model to obtain the absorption cross sections of the excited state.

PREPARATION OF SOLUBLE SUBSTITUTED POLYACETYLENE OPTICAL LIMITING MATERIAL

Two novel polyacetylenes bearing nonlinear optical chromophoric group poly（3-（4-[4-（n-butyloxy） phenylazophenyl]carbonyl）oxy-1-propyne） （poly（1a）） and poly（3-（4-[4-（n-heptyloxy）phenylazophenyl]carbonyl）-oxy-l- propyne） （poly（lb）） were synthesized with [Rh（nbd）Cl]2-EtaN as catalysts. These polyacetylenes are soluble by using an alkyl spacer and an alkyloxyl group as a substituent. They were characterized by FTIR, NMR, GPC and UV-Vis and their optical limiting and nonlinear optical properties were investigated using 8 ns pulse at 532 nm wavenumber. The results show that these soluble functional polyacetylenes possess optical limiting properties and large nonlinear optical properties and poly（1b） possesses better optical limiting and nonlinear optical properties than poly（ 1 a）.

Spectroscopic Properties and Three-photon Absorption Induced Optical Limiting of Series of Novel Nonlinear Chromophores

Three novel nonlinear chromophores with symmetric D-π-D molecular structure and extended conjugated length were synthesized. Solvatochromism analysis shows great symmetric intramolecular charge transfer occurring in chromophores by the enhancement in the dipole moment between the ground and excited states. The properties of optical power limiting induced by three-photon absorption （3PA） are demonstrated. Large 3PA coefficients and the corresponding molecular cross sections as high as 10^-74 cm^6s^2 were obtained for nanosecond laser pulses at 1.06μm from nonlinear transmission measurements.

Investigation on the effect of beam divergence angle upon output waveform based on stimulated Brillouin scattering optical limiting

This paper investigates the effect of beam divergence angle on output waveform based on stimulated Brillouin scattering optical limiting.Output waveforms in the case of different pump divergence angles are numerically simulated,and validated in a Nd：YAG seed-injected laser system.The results indicate that a small pump divergence angle can lead to good interaction between pump and Stokes,and a platform can be easily realized in the transmitted waveform.In contrast,a peak followed by the platform appears when the divergence angle becomes large.

Theoretical Analysis on Optical Limiting Properties of Newly Synthesized Graphene Oxide-Porphyrin Composites

Optical limiting （OL） properties and two-photon absorption （TPA） of a series of covalently linked graphene oxide-porphyrin composite materials have been investigated by numerically solving the rate equations and field intensity equation with an iterative predictor-corrector finite-difference time-domain technique in nanosecond time domain. Our results show that graphene oxide-porphyrin composites exhibit enhanced OL behavior and possess larger TPA cross section compared with individual porphyrins. Interestingly~ unlike the previous result that porphyrin with heavier central metal shows better nonlinear abilities than that with- out metal substitute, graphene oxide-metal free porphyrin composite has stronger nonlinear absorption properties compared with graphene oxide-metal porphyrin composite. The com- putational results are in reasonable agreement with the experimental ones. Special attention has been paid to the influence of thickness of the medium and pulse width on TPA cross sections, which presents that larger TPA cross sections are obtained as the medium is thicker or the pulse duration is wider.

Variation of Two-photon Absorption Cross Sections and Optical Limiting of Compounds Induced by Static Electric Field

By numerically solving the Maxwell-Bloch equations using an iterative predictor-corrector finite-difference time-domain technique, we investigate propagating properties of a few-cycle laser pulse in a 4,4＇-bis（di-n-butylamino） stilbene （BDBAS） molecular medium when a static electric field exists. Dynamical two-photon absorption （TPA） cross sections are obtained and optical limiting （OL） behavior is displayed. The results show that when the static electric field intensity increases, the dynamical TPA cross section is enhanced and the OL behavior is improved. Moreover, both even- and odd-order harmonic spectral components are generated with existence of the static electric field because it breaks the inversion symmetry of the BDBAS molecule. This work provides a method to modulate the nonlinear optical properties of the BDBAS compounds.

Thiophene-fluorene derivatives with high three-photon absorption activities and their application to optical power limiting

The three-photon absorption （3PA） properties of two thiophene-fluorene derivatives （abbreviated as MOTFTBr and ATFTBr） have been determined by using a Q-switched Nd：YAG laser pumped with 38ps pulses at 1064nm in DMF. The measured 3PA cross-sections are 152×10^-78cm^6s^2 and 139× 10^-78cm^6s^2, respectively. The optimized structures were obtained by AM1 calculations and the results indicate that these two molecules show nonplanar structures, and attaching different donors has different effects on the molecular structure. The charge density distributions during the excitation were also systematically studied by using AM1 method. In addition, an obvious optical power limiting effect induced by 3PA has been demonstrated for both derivatives.

Numerical analyses on optical limiting performances of chloroindium phthalocyanines with different substituent positions

Optical limiting properties of two soluble chloroindium phthalocyanines with a- and β-alkoxyl substituents in nanosecond laser field have been studied by solving numerically the coupled singlet-triplet rate equation together with the paraxial wave field equation under the Crank-Nicholson scheme. Both transverse and longitudinal effects of the laser field on photophysical properties of the compounds are considered. Effective transfer time between the ground state and the lowest triplet state is defined in reformulated rate equations to characterize dynamics of singlet-triplet state population transfer. It is found that both phthalocyanines exhibit good nonlinear optical absorption abilities, while the compound with a-substituent shows enhanced optical limiting performance. Our ab-initio calculations reveal that the phthalocyanine with a-substituent has more obvious electron delocalization and lower frontier orbital transfer energies, which are responsible for its preferable photophysical properties.

Synthesis, Structure, and Optical Limiting Properties of an Axial Substituted Bis(8-oxide quinoline)zirconium Phthalocyanine

A novel bis(8-oxide quinoline)zironium phthalocyanine [(OQ)2 Zr Pc] with two 8-oxide quinolone anions at the same axial positions has been successfully synthesized and its chemical structure has been assigned by the 1 H NMR, MS and two-dimensional correlation infrared(2D-IR) spectroscopy as well as by single-crystal X-ray structural analysis. It possessed a moderately effective nonlinear absorption coefficient βeff of 6.69×10-14cm/GW at 532 nm in DMF solution, implying it is a promising candidate for nonlinear optical materials.

Synthesis and Three-photon Induced Nonlinear Properties of a Novel Organic Chromophore

A novel organic chromophore 4, 4＇-bis（9-carbazyl-trans-styryl）-biphenyl （BCSBP） has been synthesized and characterized by IHNMR and elemental analysis. Three-photon absorption（3PA） induced upconvention fluorescence was observed and large 3PA cross section as high as 10^-74 cm^6 s^2 was obtained for nanosecond laser pulses at 1064 nm from optical limiting measurements.

Effect of Water Vapor Absorption on Measurements of Atmospheric Nitrate Radical by LP-DOAS

During the measurement of atmospheric nitrate radical by long-path differential optical absorption spectroscopy, water vapor strong absorption could affect the measurement of nitrate radical and detection limits of system. Under the tropospheric condition, the optical density of water vapor absorption is non-linearly dependent on column density. An effective method was developed to eliminate the effect of water vapor absorption. Reference spectra of water vapor based on the daytime atmospheric absorption spectra, when fitted together with change of cross section with water vapor column densities, gave a more accurate fitting of water vapor absorptions, thus its effect on the measurements of nitrate radical could be restricted to a minimum and detection limits of system reached 3.6 ppt. The modified method was applied during an intensive field campaign in the Pearl River Delta, China. The NO3 concentration in polluted air masses varied from 3.6 ppt to 82.5 ppt with an average level of 23.6±1.8 ppt.