(苄基三乙基铵)双(1,3-二硫杂环戊烯-2-硫酮-4,5-二硫基)-金的三阶非线性光学性质

陈经纬 王新强 任 诠,* 李廷斌 杨洪亮 张敬楠 李国超

(1山东大学光学系,济南250100;2山东大学晶体材料国家重点实验室,济南250100; 3泰山学院材料与化学工程系,山东泰安271021)

1 Introduction

Third order nonlinear optical(NLO)properties have been the subject of numerous investigations by both theoreticians and experimentalists for long time.1Materials with large thirdorder NLO properties have attracted great interests because of their potential applications in telecommunication,optical computing,and optical signal processing.2,3In recent years,a variety of materials have been investigated for NLO properties in the form of films,in which organic materials are attractive due to their large variety,high nonlinearity,and the flexibility they offer to tune the optical properties through structural modification.4-7Among the organics,π-conjugated organic compounds and polymers have been investigated so far due to a great extent to the possibility of tailoring their nonlinear responses by manipulation of their chemical structures,and state of aggregation as well as the charge-transfer nature of the metal-ligand bonds,which can further enhance the nonlinearity.8-10

The synthesis and characterization of 2-thioxo-1,3-dithiole-4, 5-dithiolate(dmit)complexes and related selenium-and oxygen-substituted isologues have been reported in the past two decades.11,12In recent years,dmit and related ligands have been extensively investigated because they can be used in the assembly of highly electrically conducting radical anion salts and charge-transfer complexes as special π-electron delocalization in conjugated systems.They are used as important building blocks of organometallic complex.13,14

Additions of the dopant to the polymer matrixes modify the energy band gap of these materials depending on the type and magnitude of defect concentration of dopant.These modifications give information pertaining to optical,electronic and electrical conductivity behavior of polymer.15

Poly(methyl methacrylate)(PMMA)is a hard,rigid,and transparent polymer with a glass transition temperature of 125°C.It is a polar material and has large dielectric constant. Also its physical durability is far superior to that of other thermoplastics.16And it can be molten and molded into any shape we want,so we have selected PMMA as the polymer into which the NLO materials are doped.

Z-scan technique is an increasingly popular method for determination of the nonlinear refractive index(n2)and the nonlinear absorption coefficient(β)of the samples because it can indicates the sign and type of nonlinearity(refraction or absorption)easily.17,18

In this paper,we reported the synthesis of[C6H5CH2(C2H5)3N] [Au(dmit)2](BTEAADT)and preparation of BTEAADT-doped PMMA thin film.The NLO properties of the polymer thin film and BTEAADT acetonitrile solution were investigated using a picosecond laser by the Z-scan technique.

2 Experimental

The new dmit2-complex:BTEAADT,has been synthesized by the method as follows.It is a modification of that in references.23-25

4,5-Bis(benzoylthio)-1,3-dithiole-2-thione(1.247 g)was suspended in methanol(analytical reagent,15 mL).A sodium ethoxide solution obtained from Na(0.145 g)in methanol(15 mL) was added to the mixture to give a dark-red solution.Solutions of NaAuCl4·2H2O(0.597 g)dissolved in methanol(5 mL),and C13H22NBr(0.417 g)in methanol(5 mL)were added to the above-mentioned solution under stirring at room temperature. The reaction mixture was stirred for about 30 min.The product was collected by filtration and washed with methanol to afford dark-green precipitate of BTEAADT.

The BTEAADT-doped PMMA thin film with mass fraction 1%was prepared as follows:BTEAADT and PMMA were first separately dissolved in a quantity of acetonitrile(analytical reagent).Subsequently,the BTEAADT solution and PMMA solution were mixed completely and filtrated by a micro-sperture filter.Then the mixed solution was spread by spin-coating(1500 r·min-1,20 s)(KW-4A,Chinese Academy of Science,China)on clear quartz glass.Finally,the sample was dried at 60°C for 30 min.

The elemental analyses for C,H,N and S contents were performed on an Elementar Analysensystems(VarioEI III,Elementar GmbH,Germany)instrument.

The linear absorption spectra of the BTEAADT in 2.8×10-6mol·L-1solution of acetonitrile and BTEAADT-doped PMMA film were recorded using a UV-Vis-NIR scanning spectrophotometer(U-4100,Hitachi,Japan)in a wavelength region of 200-1400 nm at room temperature.The contributions of the quartz cell and acetonitrile were subtracted.

The linear refractive index and thickness of the thin film were measured by prism coupler(SPA-4000,Sairon Technology,Inc,Korea).The wavelengths of the laser beams were 632.8,1310,and 1550 nm,separatively.The refractive index of the prism used in the measurement was 1.965,the index resolution was±0.0005 and the index accuracy was less than 0.001.The polarization state of the laser light was transverse electric(TE)mode.

The transmission coefficient of the sample was measured by Z-scan technique using a mode-locked Nd:YAG laser(Leopard D-10,Continuum,America)producing 20 ps laser pulses (FWHM)at 1064 nm with a repetition rate of 10 Hz.In the experiment,the spatial profiles of the pulses were nearly Gauss-ian.The polymer thin film or the solution sample with concentration of 1.0×10-3mol·L-1was moved along the optical axis (the Z-direction)through the focus of the lens which has a focal length of 15 cm under the control of the stepper motor.The energy transmitted through an aperture in the far field is recorded as a function of the sample position.The radius of the beam waist was determined to be 59.6 μm for the polymer thin film and 57.64 μm for the solution.The Rayleigh lengths were calculated to be 10.49 mm for the polymer thin film and 9.80 mm for the solution,both are much longer than the thickness of the film(1.15 μm)or the quartz cell(1 mm).The reference beam and the on-axis transmitted beam energies through a closed-aperture(CA)or an open-aperture(OA)were measured by an energy ratio meter(EPM2000,Molectron,America)simultaneously.The CA detector is sensitive both to the nonlinear refraction and absorption,while the OA detector only to the later. The distance between the detector and the focus of the lens is far enough to satisfy the far-field approximation.As a reference,the Z-scan measurement for CS2was performed and used to estimate the on-axial peak irradiance I0at the focus.The value was measured to be 3.31×10-18m2·W-1,which is well accorded with(3.31±0.8)×10-18m2·W-1of the literature.17

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3 Results and discussion

The synthesized products of BTEAADT are transparent long-bar single crystals.The results of the elemental analyses are sunmmarized in Table 1,which are fairly identical with the theoretical values.It exhibits high solubility in acetone.Therefore,BTEAADT can be easily recrystallized and purified by acetone.

Fig.1 shows the linear absorption spectra of the BTEAADT acetonitrile solution and BTEAADT-doped PMMA film in a wavelength region of 200-1400 nm at room temperature.The position of the absorption peaks are at 290,353,and 465 nm for the solution.The 290 and 353 nm peaks represent the π-π*transition of the dmit ligand.The peak at 465 nm(Au←S charge-transfer transition26)represents the d-p interaction because the d orbital′s of the central metal ion Au3+interact with the p orbital′s on the two ethylene double bonds through the lone-pair electrons on the four sulfurat atoms,and the p orbital′s on the thiocarbonyl groups through the lone-pair electrons on the outer four sulfur atoms.27

The refractive-index curve of the BTEAADT-doped PMMA thin film is illustrated in Fig.2.We measured the refractive indices and thicknesses at 632.8,1310 and 1550 nm using a prism couple system.The variations of refractive index n with thewavelength λ were theoretical fitted by a Sell-meier dispersion formula,n=A+B/λ2+C/λ4,where A,B and C are Sellmeier coefficients.The linear refractive index(n)of the film at 1064 nm was calculated according to the fit line,in which the third-order NLO properties of the film were investigated.Its value was 1.4682.The thickness of the film(d)was 1.15 μm and the polymer thin film has good purity and uniform thickness.

Fig.1 UV-Vis-NIR absorption spectra of the BTEAADT acetonitrile solution using acetonitrile solvent as reference and BTEAADT-doped PMMAthin film using pure PMMAfilm as reference at room temperatureInset shows the molecular structure of BTEAADT.

The normalized Z-scan curves of the BTEAADT-doped PMMA thin film and BTEAADT acetonitrile solution were shown in Fig.3.The irradiance at focus of the laser beam(I0)was adopted as 2.18×1013W·m-2.Nonlinear absorption was negligible at 1064 nm in the experiment for both film and solution. The peak-to-valley configuration of the CA/OA Z-scan curves of the film and solution suggest that the refractive index change is negative,exhibiting a self-defocusing effect.And we also measured the Z-scan curve of the PMMA film coating on quartz substrate and the pure acetonitrile in the cell under the same measuring condition.Their effects were subtracted between the data processing in order to validate that the mea-sured NLO phenomena originate from BTEAADT only.

Fig.2 Linear refractive index curve of the BTEAADT doped PMMAthin filmInset shows the relationship between waveguide mode profile and laser incident angle.

Fig.3 Normalized Z-scan transmittance curves of BTEAADT-doped PMMAthin film OAand CA/OAZ-scan(a)and BTEAADT acetonitrile solution OAand CA/OAZ-scan(b)at 1064 nmThe solid lines are curves fitted to the experimental data.

According to the procedure given in reference,ΔΦ0,the onaxis nonlinear phase shift at the focus,was obtained through fitting the CA/OAcurve with the following equation.17

where x=Z/Z0,Z0is the Rayleigh length.T is the normalized transmittance for the CA/OAcurve.

Then one can easily get the nonlinear refraction coefficient n2(m2·W-1)through the equation

where k=2π/λ is the wave vector,Leff=[1-exp(-α0L)]/α0is the effective thickness of the sample(L denotes its thickness),α0is the linear absorption coefficient of the sample,and I0is the onaxis irradiance at focus.

Accordingly,the real and imaginary parts of the third-order nonlinear susceptibility χ(3)of the material can be calculated by the following equation28:

where n0is the linear refractive index of the sample,ω is the angular frequency of the light field,and c is the velocity of light in vacuum.

Forwards,the second-order hyperpolarizability γ of the sample molecule may be estimated through the equation29:

where Ncis the concentration of the solution and Lcis the local field correction factor which equals[(n20+2)/3]4.

From the above-mentioned equations and the numerical val-ue obtained through fitting the OA and CA/OA curve,we can calculate n2of the sample.The calculated results are listed in Table 2.

Table 2 Data of the results of the experiments

To evaluate the suitability of a material for all-optical integrated devices,two figures of merit are often used,1W=n2I/α0λ and T=βλ/n2.Materials used to manufacture AOS must satisfy figures of merit|W|＞＞1 and|T|＜＜1.The laser wavelength of 1064 nm is located on the off-resonant field of linear absorption of the BTEAADT-doped PMMA film material,while the nonlinear absorption of film material was negligible at 1064 nm.The values of W and T of the polymer thin film sample were calculated to be|W|=1.07 and|T|≈0.Both the W and T values satisfy the requirements for the application to all-optical integrated devices.

It is indicated that the values of the polymer thin film were about three orders of magnitude larger than that of organic solutions.It could be explained as follows:the number density of molecules in the polymer thin film was much larger than that of solution.Nonlinear absorption was negligible for the material reveals that it has potential applications in AOS field.The better results of NLO properties in the polymer thin film enhanced the application possibility of the material.

4 Conclusions

A new dmit2-complex:BTEAADT were synthesized.The BTEAADT-doped PMMA film with mass fraction 1%was prepared by using spin-coating method.Its linear properties were investigated by prism coupler.The NLO properties of the polymer thin film and solution were determined by Z-scan technique at the wavelength of 1064 nm with 20 ps laser duration. The phenomenon of nonlinear absorption was negligible in the experiment for both film and solution.They all show negative nonlinear refraction.The absolute value of nonlinear refraction index of the film is n2=3.978×10-15m2·W-1which is larger than that of the solution for about three orders.The refractive index of the film is 1.4682 and the thickness of the film is 1.15 μm. Overall,we can conclude that the material may be a promising candidate for application to nonlinear optical devices such as AOS manufacture at 1064 nm.

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