基于染料掺杂Zr-ORMOSIL平面波导薄膜的ASE发射

ORIGINAL PAPERAmplified spontaneous emission from an infrared dye doped zirconia-organically modified silicate thin film waveguidesYu Yang ÆChao Ye ÆWeihai Ni ÆKing Young Wong ÆMinquan Wang ÆDennis Lo ÆGuodong QianReceived:19March 2007/Accepted:5June 2007/Published online:12July 2007ÓSpringer Science+Business Media,LLC 2007Abstract Near infrared dye,IR26,was doped into zir-conia-organically modified silicate (ORMOSILs)wave-guides by sol–gel technique.Amplified spontaneous emission (ASE)centered at 1.14l m was observed from the solid films and their spectroscopic characteristics were studied.The dependence of ASE on the preparation details was discussed.The threshold and full widths at half max-imum (FWHM)of ASE were 8.6mJ/cm 2and ~15nm,respectively.The output intensity dropped to 50%of its initial value after 1,200pulses at a repetition rate of 10Hz and pump intensity of higher than 50mJ/cm 2.These materials are promising for the distributed feedback (DFB)lasers tunable in infrared.Keywords Amplified spontaneous emission ÁNear infrared dye ÁZirconia-organically modified silicates ÁSol–gel1IntroductionTunable DFB lasers had extremely wide and prominent applications in many fields,especially in integrated optics and optics communication networks [1].In these areas,solid-state dye laser based on hybrid materials had obviousadvantages on other laser sources such as its low cost,ease of fabrication,non-toxic,high efficiency and wide spec-trum range and had generated extensive research interests in recent years [2–7].Up to now,the performances of so-lid-state dye lasers (SSDL)emitting in the visible region had been greatly improved,which paved the way for SSDL to be commercially available [8–11].In the near infrared region,a few research works had also been reported with frequency-doubled Nd:YAG laser as pump source.Thompson et al.observed ASE from IR140-doped in poly(methylmethacrylate)film in the near infrared region between 920and 970nm [12].Laser emission and DFB laser output in the 800–1,000nm region with laser lifetime of about several 1,000pulses were obtained with LDS dyes doped in thin film waveguides [13–15].However,the studies of SSDL emitting in the infrared region beyond 1.0l m,which corresponds to optical communication windows in actual applications,were scarce.This was largely due to the relatively inferior lasing performances of current very limited kinds of dyes emitting in the infrared region.For instance,the fluorescence quantum yields of several kinds of IR dyes were reported to be less than 0.02and the photostabilities of these IR dyes were low [16–18].Bezrodnyi et al.obtained lasing output from a polymethine dye thiopyrylotricarbocyanine doped in polymer bulk material with efficiency as high as 43%,tunable range of 63nm and laser lifetime of about 550pulses [19].However,concerning the actual use in inte-grated optics,organic-inorganic hybrid planar and channel thin film waveguides via sol–gel process were preferable to bulk materials due to versatility and compatibility to DFB configurations.Pizzoferrato et al.studied the absorption,emission and other spectroscopic parameter of several kinds of IR dyes and obtained light amplification from IR1051doped sol–gel solid thin film [16–18,20–22].ItY.Yang ÁM.Wang ÁG.Qian (&)Department of Materials Science &Engineering,State Key Laboratory of Silicon Materials,Zhejiang University,Hangzhou 310027,P.R.Chinae-mail:gdqian@C.Ye ÁW.Ni ÁK.Y.Wong ÁD.LoPhysics Department and the Center of Optical Science,The Chinese University of Hong Kong,Shatin,New Territories,Hong Kong SAR,P.R.ChinaJ Sol-Gel Sci Technol (2007)44:53–57DOI 10.1007/s10971-007-1593-zshould be noted that in order to obtain dye doped thinfilm waveguides emitting in the infrared region,precise control on preparation procedures of IR dyes doped solid materials was needed because not only the vulnerability of IR dyes to photo-,chemical-and thermal-degradation but also the waveguiding parameters of thinfilm should be considered.According to Eq.1,the condition for M order mode to be guided in an asymmetric waveguide wasD n¼n2Àn3!ð2Mþ1Þ32n2Âðk0dÞ2ð1Þwhere n2and n3were the refractive index of thefilm and substrate respectively,k0was the wavelength,d was the film thickness,M was the mode order[23].To obtain a thin film waveguide in the infrared region,the refractive index andfilm thickness should be increased,as compared with those of the thinfilm waveguides operating in the visible region.In this regard,zirconia waveguide was preferred because of its high refractive index,wide transparency and absence of catalytic photodegradation and organic modifier was chosen in order to obtainfilms with smooth andflat surfaces and good optical quality.The refractive index and thickness of zirconia-ORMOSILfilms could be easily ad-justed to be more than 1.52and 2.0l m,respectively, which allowed the mode to be guided in the infrared re-gion,as illustrated in Eq.1.So,it was suggested that DFB laser output tunable beyond1.1l m could be achieved from the IR dye doped zirconia-ORMOSIL thinfilm waveguides with Nd:YAG 1.064l m fundamental beam as pump source.In this letter we report the preparation of sol–gel derived zirconia-ORMOSIL hybridfilms doped with IR26dye in order to obtain a solid-state dye laser material,which is promising for future DFB operation.ASE from these dye dopedfilms was observed by the excitation of a1,064nm laser.The threshold and FWHM of ASE from IR26was about8.6mJ/cm2and15nm respectively,and the emis-sion lifetime was longer than1,200pulses.2Experimental detailsThe dye IR26was purchased from Exciton and used as received.The molecular structure of IR26can be found elsewhere[20].The solid host was prepared by hydrolysis and polycondensation of zirconium(IV)propoxide and c-glycidyloxypropyltrimethoxysilane(GLYMO)as organic modifier.The starting solution was obtained by mixing zirconium(IV)propoxide with acetic acid and dichlo-romethane(DCM)as solvent.After30min of vigorous stirring,the water needed for hydrolysis was mixed with acetic acid and2-methoxyethanol(MeOH)and introduced drop-wise into the solution followed by the addition of GLYMO.Molar percentage of GLYMO in thefilms was held at about50%(X GLY=[n GLY/(n GLY+n Zr)]·100)in order to obtainfilms with smooth andflat surfaces and good optical quality.After hydrolysis took place,IR26was added into the solution at an initial concentration of1·10–3mol/L. Then,the clear sol was kept in a refrigerator,aging for at least12h.Dye doped zirconia-ORMOSILfilms were ob-tained by spin-coating on glass substrates with thefinal solutions.Thefilms were then kept in a50°C oven,which is an essential step in the preparation procedure and will be discussed later.Depending on the viscosity of the sol,spin speed and the thermal treatment,films of thickness varying from 2.4to 2.8l m were obtained in this work,with refractive index up to1.52at633nm.The surface of the IR26doped zirconia-ORMOSIL thinfilm was examined by an atomic force microscope(AFM),as shown in Fig.1, which revealed that the surface of thefilm appeared to be amorphous with relatively small surface roughness.The waveguiding parameters such asfilm thickness and refractive index of IR26doped zirconia-ORMOSILfilms were characterized by a commercial prism coupler(Metr-icon model2010)at633and964nm.Two waveguide modes(for both TE and TM)at633nm and one waveguide mode at964nm were observed in all cases.A homemade scanning ellipsometerfitted with synchronously rotating polarizer and analyzer was also used to measure the refractive index n and the extinction coefficient k of the IR26doped zirconia-ORMOSILfilms from600to 1,500nm[24,25].Three types of detectors,viz.photo-multiplier tube(Hamamatsu R1104),photomultiplier tube (Hamamatsu R316)and InGaAs detector(Oriel70348), Fig.1AFM image of zirconia-ORMOSILfilmwere used in different spectral regions.The scanning el-lipsometer approach yields spectroscopic information for n and k,both critical parameters that define the propagation and loss of an optical wave in the waveguide[26].Silicon wafers were used as substrates.The measurements fol-lowed the standard procedures of ellipsometry.From the ratio of the intensities of the reflected polarized beams,the values of elliptical azimuth and phase angle were extracted. n and k were then determined at10nm interval by a numerical routine[24].The ellipsometry results for zirco-nia-ORMOSILfilm with an IR26concentration of1·10–3mol/L is illustrated in Fig.2.n and k were1.521and 2.8·10–3around1.10l m,respectively.Hence the IR26 doped zirconia-ORMOSILfilm behaved as an asymmetric waveguide with low propagation loss.Steady-state absorption spectra were recorded by a Hitachi spectrophotometer.The waveguides were trans-versely photoexcited by a Q-switched Nd:YAG laser at 1,064nm(7ns pulse duration,10Hz repetition rate).A CVI edgefilter was used tofilter the1,064nm pump wavelength,and the emission was coupled by afiber bundle to the entrance port of a0.3m focal length spec-trograph/near infrared InGaAs multichannel detector (SIENCETECH Lingas1-256)system.The wavelength accuracy of this detector system was about2.3nm.3Results and discussionThe absorption andfluorescence spectra of IR26doped in zirconia-ORMOSIL waveguide were presented in Fig.3a and b,respectively.Thefluorescence spectrum of IR26doped in zirconia-ORMOSIL waveguide was measured by the guided configuration at relatively low excitation intensity[18].In other words,the pump laser beam was focused on a strip(1.5mm long,0.25mm wide)on the waveguide by means of cylindrical lens and the emission was collected by thefiber bundle which was actually in the same plane of the dye doped waveguide.The absorption andfluorescence peaks were found to be about1.08and 1.11l m,respectively.The relatively small Stokes shift of ~30nm implies the existence of a large overlap between absorption and emission,resulting in reabsorption–ree-mission which might lead to a larger lasing threshold. Increasing the pumping density,spectral narrowing was observed,which centered at about1.14l m with FWHM of ~15nm,indicating the appearance of ASE,as shown in Fig.3b.The emission from IR26doped in zirconia-ORMOSIL thinfilms was further investigated by detecting the emis-sion at various angles against the planar waveguides.Re-sults presented in Fig.4demonstrated the strong dependence of emission on detecting angle.In case that the intersection angle between the dye doped waveguide and the detectingfiber bundle was0°,in other words,thefiber bundle was actually in the same plane of the dye doped waveguide,narrow band emission peaked at1.14l m could be achieved.While the detector tilted,the emission inten-sity detected decreased sharply and at least1-order of reduction could be observed.This observation confirmed the emission at around1.14l m to be ASE.The emission of IR26doped in solid thinfilms at various pump intensity were presented,as shown in Fig.5,by which the threshold of ASE could be ascertained.Excita-tion density above at8.6mJ/cm2gave rise to the spectrally narrowed emission with a FWHM of~15nm centered at ~1.14l m,while below that pump density,onlyfluores-cence could be observed.The main limitation to obtain the tunable narrow-line-width DFB laser output from solid-state dye materials in the infrared region was its stability.Without considering the difference between various dyes,the longer the output wavelength,the severer this problem was,due to the poorer thermal stability of dye molecules at the longer wave-length.In previous reports,the laser emission from IR dye doped solid bulk decreased to50%of its initial value after 550shots[19].However,the investigation on the photo-stabilities of IR dyes doped into thinfilms was scarce. Kobayashi et al.had reported that the ASE from LDS821 doped polymeric thinfilm at820nm ceased after the exposure of several thousands shots of pump pulses at a repetition rate of10Hz and pump intensity of165l J/cm2, which was just slightly above the lasing threshold[14].In this work,the photostability of IR26doped in zirconia-ORMOSILfilm was also investigated at a repetition rate of10Hz and pump intensity of higher than50mJ/cm2.With the increase of pump pulses,it was found that the ASE output intensity decreased to50%of its initial value after 1,200pump pulses.Considering the relatively higher pump intensity,it was suggested that the stability of IR26doped into zirconia-ORMOSIL thinfilms might be comparable or even higher than that reported previously[12,14].It was also proposed that by introducing photostable additives such as singlet state oxygen quencher,longer lasing lifetime might be expected.Thus,the IR26doped zirconia-ORMOSIL waveguides were promising materials for future DFB operations.It was well-known that the IR dyes were vulnerable to thermal-degradations[27].This was proved by checking the emission from the dye doped solidfilms and the sol which were aged in refrigerator for days.ASE was not detected in both cases even the sol and thefilms were preserved in an opaque environment for some time.So it seemed that the thermal-degradation might lead to the total bleaching of IR26dye molecules.However,it was also found that the thermal treatment in the preparation proce-dure was essential to obtain the ASE from IR26doped in zirconia-ORMOSIL waveguides.This phenomenon could be explained by the following mechanism.It has been re-ported that thefluorescence quantum yield of IR1051in-creased by a factor of3after solidification[20].During the thermal treatment,though parts of IR dye molecules might be thermally degraded,the evaporation and elimination of water and organic solvents from the solidfilms led to the reduction of the rotational and vibrational degrees of freedom in samples with respect to those without thermal treatment,decreasing the non-radiative decay.As a result of the increased quantum efficiency,emission could be detected from the thermally treated samples while it could not be detected from a freshfilm before thermal treatment due to insufficient gain.However,long period of thermal treatment would also lead to the total degradation of IR26 dye molecules.It should be noted that prolonged thermal treatment of the solidfilms would lead to the extinction of ASE at1.14l m,which we suggested the dye to be ther-mally degraded.4ConclusionsIn conclusion,ASE from IR26doped in zirconia-ORMO-SIL thinfilms prepared by sol–gel technique was observed.The spectroscopic properties of the dye dopedfilms were investigated.The dependence of ASE on the preparation process was discussed.The photostability of IR26doped in solidfilms were promising for future DFB operations. 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