Download - Organic–Inorganic Hybrid Materials for Photonic Applications

IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 14, NO. 5, SEPTEMBER/OCTOBER 2008 1361

Organic–Inorganic Hybrid Materialsfor Photonic Applications

Shuichi Shibata, Tetsuji Yano, and Hiroyo Segawa

(Invited Paper)

Abstract—Organic–inorganic hybrid materials are derived fromthe chemical reaction of silane coupling reagents and metal alkox-ides, involving O–Si–C bonds in the matrix. They offer supe-rior characters of combined organic groups and inorganic link-ages for various optical applications. This paper reviews prepara-tion and characterization of optical devices and materials derivedfrom these hybrid materials. We mainly describe three topics formicrometer–nanometer scale optical components in our laborato-ries using organic–inorganic hybrid materials: 1) film formation;2) patterning by the photolithography techniques; and 3) laseremission from dye-doped hybrid spheres and coated glass spheres.

Index Terms—Coated glass sphere, dye-doped sphere, film,organic–inorganic hybrid material, patterning, spherical laser.

I. INTRODUCTION

N EW materials and fabrication technologies have been anessential for the development of various kinds of optical

devices, such as optical fibers, amplifiers, lasers, and detectors.Combination of different materials such as organics, inorganics,and metals in molecular level to achieve superior properties forphotonic applications is a dream of material researchers [1], [2].Moreover, integration of small-sized optical components havingvarious functions is essential for meeting the industrial require-ments [3]. In this paper, we describe three topics for micrometer–nanometer scale optical components in our laboratories usingorganic–inorganic hybrid materials: 1) film formation; 2) pat-terning by the photolithography techniques; and 3) laser emis-sion from dye-doped hybrid spheres and coated glass spheres.

In the past decade, sol–gel process has been actively inves-tigated as a tool for making all kinds of inorganics, ceramics,and glasses [4], [5]. The sol–gel process is a chemical techniquefor making solid materials through hydrolysis and condensationof precursors dissolved in solvents. A silicone alkoxide such astetraethoxysilane (TEOS) dissolved in alcoholic solution is hy-drolyzed and resultant ≡Si–OH reacts with adjacent hydrolyzedmolecule ≡Si–OH to form an Si–O–Si bond. Then, polymer-ization continues, and this leads to the formation of a solid gel.The gel is heated to evaporate solvents, resulting in shrinkageand lead to an oxide structure. Unfortunately, in these processes,weak bonds are easily broken and cracks are formed. Therefore,

Manuscript received December 29, 2007; revised January 6, 2008. Currentversion published October 3, 2008.

The authors are with the Department of Chemistry and Materials Sci-ence, Tokyo Institute of Technology, Tokyo 152-8550, Japan (e-mail:[email protected]; [email protected]; [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JSTQE.2008.922897

Fig. 1. Reagents for organic–inorganic materials.

by this technique, the thickness of crack-free films is limitedbelow 1 µm.

Chemically designed organic–inorganic hybrid materialswere proposed and fabricated in 1980s [1], [2] aiming atimproving the aforementioned problems on sol–gel process-ing. Typical starting materials for preparing the hybrid mate-rials are shown in Fig. 1: titanium tetra-n-butoxide (TTBu),diphenyldimethoxysilane (DPhDMS), phenyltriethoxysilane(PTES), 3-glycidoxypropyl-trimethoxysilane (GPTMS), etc.Sol for preparing hybrid materials is derived from the com-bination of silane coupling reagents in which organic groupsare covalently connected to Si and metal alkoxides. Silane cou-pling reagents containing organic groups, such as methacryloxy,phenyl, and epoxy, offer organic components linked to the silox-ane matrix, and incorporations of tetramethoxysilane (TMOS)and TTBu form Si–O–Si, Ti–O–Ti, Si–O–Ti networks as the in-organic components. Since hybrid materials involving organicgroups show superior mechanical flexibility, films in thicknessof 5–10 µm can be easily prepared by conventional dip- andspin-coating techniques.

For optical applications, glasses and polymers are well knownas highly transparent materials. Glasses are advantageous for

1077-260X/$25.00 © 2008 IEEE

1362 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 14, NO. 5, SEPTEMBER/OCTOBER 2008

their hardness and thermal and chemical stability. However,they are brittle and high temperature is needed in shaping andforming. On the other hand, polymers are fabricated at relativelylow temperatures and are mechanically flexible. However, theyare not scratch-resistant and their optical properties such as re-fractive indexes are limited as compared with inorganic glasses;for example, the attainable index for conventional polymers isabout nD = 1.7–1.8, while nD = 2.0, and higher index valuesfor glasses. Hybrid materials can be located between glasses andpolymers and wide variety of starting materials and processingtechniques enable us to achieve superior properties. Some of theproperties of hybrid materials and examples will be presentedin the following sections.

II. FORMATION OF ORGANIC–INORGANIC HYBRID FILM

Organic–inorganic hybrid materials have attained much at-tention for various applications, particularly, in film coating;hybrid materials are suitable to antireflective, protective, hardcoating, etc. A crack-free hybrid film of several micrometersin thickness can be easily obtained from these hybrid materi-als with good optical transparency and mechanical properties.Generally, conventional film preparation techniques, such asdip-, spin-, and spray-coatings, can be applied on a substrate oflarge area, with good controllability of refractive index at tem-peratures below 100 ◦C; they can thus be applicable to plasticsubstrate such as polymethylmethacrylate (PMMA). Wide vari-ety of hybrids is also favorable for the incorporation of organicdyes or nanometer-size inorganics, because they can offer bothof hydrophobic and hydrophilic sites surrounding the incorpo-rated materials.

A. Antireflective Film Coating

We have tried to make double-layered antireflective coatingon PMMA substrates at an early stage of investigation in 2000.Aiming at examining the controllability of refractive index andfilm thickness, the dip-coating experiment was carried out, andthe potential of the film for photonic applications was studied.In Fig. 2, reflectance of a PMMA substrate and that of calcu-lated antireflective two-layered film on the substrate is plottedas a function of incident wavelength. Decrease of reflectancedown to 0.5% at visible wavelength region is possible by form-ing the structure of [low-index hybrid film (nD = 1.42) of100 nm in thickness]/[high-index hybrid film (nD = 1.61) of80 nm in thickness]/[PMMA substrate (nD = 1.49)]. The mea-sured reflectance of this antireflective film/substrate is also plot-ted in the figure. At 550-nm wavelength, the measured value is0.7%. We can control the refractive index of the high-index filmby changing the titanium tetra-isopropoxide (TTIP) content inthe TTIP–DPhDMS–GPTMS hybrid system. In the dip-coatingtechnique, changing the pulling-up rate of the substrates cancontrol the film thickness. (To prevent the cellular pattern dur-ing film formation process, all the treatment was carried outunder dry N2 gas flow condition, as described in Section II-B.)A typical example of the low- and high-index films thicknessagainst pulling rate of substrates is shown in Fig. 3. The thick-ness was measured by interference technique. The arrows show

Fig. 2. Measured and calculated reflectance of two-layer antireflective film onPMMA substrate.

Fig. 3. Hybrid film thickness against pulling rate of substrate in dip-coatingprocess.

the thickness of antireflective structure designed by the calcula-tion. We can control the film thickness within 10 nm accuracywith good reproducibility. For the antireflective film, these val-ues are not surprising, while the accuracy and reproducibilityof essential parameters are high enough to try making opticaldevices at the next stage.

B. Cellular Patterns in Hybrid Film

In our previous works [6], a sol–gel derived hybrid film hasbeen prepared for making color coating on glass substrates. Thehybrid materials consisted of silane coupling reagents, Si- andTi-alkoxides, as the starting materials. The pigment incorporatedas a dopant in hybrid film was copper phthalocyanine of β-form (β-CuPc), and a good dispersion of β-CuPc was an urgentissue in the experiment. During the dip-coating trial in air, we

SHIBATA et al.: ORGANIC–INORGANIC HYBRID MATERIALS FOR PHOTONIC APPLICATIONS 1363

Fig. 4. Celluar patterns in CuPc-doped hybrid film.

encountered micrometer-sized patterns such as arrays of blueislands surrounded by colorless edges (see Fig. 4).

In the painting technology, it is well known that such patternsdue to the localized dispersion of pigments are formed in colorfilms [7]. The patterns, which were found in 1900, are nowknown as a “Benard cell” [8].

In the further experiments, however, we were surprised to no-tice that the similar patterns were also observed in the film with-out pigment. Moreover, the patterns on a silica glass substratestill remained after 1000 ◦C heating. Change in the patterns wasinvestigated as a function of preparation conditions such as filmthickness, film composition, and humidity. A 3-D image of thecells was observed by an atomic force microscopy (AFM), andthe AFM image of the cell patterns and their section profile areshown in Fig. 5. A cell pattern looks like valleys surrounded bychains of mountains; size of the cell was about 50 µm and thedepth from the top of the mountain to the bottom of the valleywas about 250 nm. The depth of the cells strongly depended onthe humidity in the atmosphere in the sol-preparation and thefilm-formation processes. The patterns in hybrid films were notobserved when they were prepared under dry gas condition, lessthan 20% relative humidity [6].

It is still difficult to exactly explain why atmospheric mois-ture dramatically determines the onset of cell formation, that is,the starting point of convective instability in hybrid film forma-tion. If the moisture-sensitive materials, such as precursors ofTiO2 nanoparticles originated from Ti-alkoxides, are localizedin the surface layer, dramatic change of surface tension causesMarangoni convection. In air, the convection starts immediatelyafter liquid film formation on the substrates in dip-coating pro-cess (also in spin-coating), and completes in less than 10–20 s,which can be observed even by naked eyes because the degreeof surface scattering changed clearly. Moreover, when high-

content Ti alkoxide is involved in the starting sol, the vigorousconvection occurred, and thus, the resultant depth reached up to1 µm in height. Careful attention should be paid to avoid cellformation in hybrid films; films should be formed always underlow-humidity condition.

C. High-Refractive-Index Film

The refractive index of inorganic TiO2 is known to be extraor-dinary high: anatase and rutile in TiO2 crystals show as highas nD = 2.493–2.554 and nD = 2.616–2.903, respectively [9].Therefore, the high TiO2 content system is favorable for high-refractive-index films, and we chose TTBu–DPhDMS–GPTMSsystem [10], [11]. Films of 0.1–1.0 µm in thickness in the systemwere prepared on glass substrates by the dip-coating technique.Nitrogen gas flowed into the coating chamber to avoid cellularpatterns. Refractive index of thin films was measured by an op-tical interference method. Refractive indexes of the thin filmsprepared at room temperature are shown in Fig. 6 in the hybridsystem TTBu–DPhDMS–GPTMS. The refractive index of thefilms increased with increasing TTBu and DPhDMS contents,and the highest index was nD = 1.72 in the composition of80TTBu–20DPhDMS.

For increasing the refractive index, the films were heatedup to 550 ◦C in an electric furnace under oxygen atmosphere.In Fig. 7, the refractive index change of the film of 80TTBu–20DPhDMS is shown against heating temperatures. The indexincreased dramatically above 200 ◦C, exceeded nD = 2.0 at450 ◦C and reached nD = 2.45 at 550 ◦C. Thermal analysisdifferential thermal analysis-thermal gravimetry (DTA-TG) ofthe films showed two exothermic peaks at around 250 ◦C and500 ◦C due to firing the organic groups in the hybrid matrix.Phenyl groups were also fired at around 400 ◦C and the totalvolume decreased about 50%, along with the weight loss ofabout 30%. Even after 550 ◦C heating, we observed no peakin the X-ray diffraction (XRD) pattern of the film, thus thosesamples remained in amorphous state. Unfortunately, severeshrinkage of the films occurred by heating, the thickness of thecrack-free film, nD = 2.45, was about 200–300 nm.

III. PATTERNING BY PHOTOLITHOGRAPHY TECHNIQUE

For achieving micrometer-sized optical devices and their in-tegration on the substrates, the patterning techniques are in-dispensable. Various lithography techniques, such as X-ray,electron-beam, laser, and UV-light lithography, have been de-veloped to make fine patterns of submicron to micrometer sizes.In this section, we describe the two investigation examples usingorganic–inorganic hybrid films as starting materials: 1) opticalwaveguides for special use to excite a microsphere of sphericalcavity structure and 2) top-gathering pillars for 2-D periodicarrays.

A. Optical Waveguide

The hybrid optical waveguide was fabricated on a silica glasssubstrate using sol–gel UV light lithography techniques [12]. 3-Methacryloxypropyl-trimethoxysilane (MOPS) was hydrolyzed


Fig. 5. AFM image and its section profile of cellular patterns.

Fig. 6. Refractive indexes of thin film prepared at room temperature (TTBu–DPhDMS–DPTMS three system).

Fig. 7. Refractive index change against heating temperature (80TTBu–20DPhDMS film).

in hydrochloric acid solution, then TMOS was added, andfinally, TTIP was titrated into the sol. Photoinitiator of IR-GACURE 184 (CIBA) was added into the resultant sol, andfilms of 10–15 µm in thickness were prepared by the dip-coatingtechnique under a dry N2 gas flow condition. Suitable conditions

Fig. 8. SEM phortograph of an optical waveguide of ridge structure on a silicaglass substrate.

should be chosen to carry out the precise photolithography; es-pecially prebaking time and temperature, UV light power, andcontent of photoinitiator are important parameters. The filmswere prebaked and exposed UV irradiation through a fused silicachromium photomask. Samples were then soaked in 2-propanolto remove the unexposed area, followed by the postbaking.

SEM photograph of an optical waveguide of ridge structureon a silica glass substrate is shown in Fig. 8; the width and theheight of the waveguide were 4 and 12 µm, respectively. Therefractive index of the waveguide was n = 1.490, and a typicaloptical loss, measured by the cut-back method, was 0.8 dB/cmat 633-nm wavelength.

A waveguide of half-buried structure (buried core/clad por-tion connected with ridge waveguide) was also prepared bycladding hybrid film of n = 1.483 (at 633-nm wavelength) onthe ridge-waveguide part, as shown in Fig. 9. The particularstructure of the half-buried waveguide was designed to intro-duce a pumping light into single-mode buried waveguide, andthen, couple through the bared-core portion into the spherical-cavity resonators.

In Fig. 10(a) and (b), the photographs of a rhodamine 6G(R6G)-doped hybrid sphere pumped through an optical waveg-uides are shown. Fig. 10(a) shows sphere placed with a waveg-uide and (b) shows sphere pumped by continuous wave (CW)


Fig. 9. SEM phortograph of an optical waveguide of half-buried structure.

Fig. 10. Dye-doped sphere pumped through ridge-waveguide (pumping lightwas cut by an edge filter).

Ar+ laser light (514.5-nm wavelength). Evanescent couplinglight pumped the sphere placed in contact with an optical waveg-uide (in the figure, the pumping light was cut by an edge filter).Ring-shaped emitted light was observed at the surface of thesphere. The emission spectrum showed the resonance peakscorresponding to whispering gallery modes (WGMs) [12].

B. Top-Gathering Pillars

2-D periodic arrays of dielectric materials are classified intoone of the photonic crystal structures and have potential uses ofvarious applications, such as diffraction gratings, backlighting,light-out coupling of displays [13], and biological separation, ina microfluid channel [14]. Using the similar photolithographytechniques as described earlier, 2-D periodic arrays of pillarsof hybrid materials were fabricated on glass substrates. Afterremoving the unirradiated area by soaking in suitable solvents,such as 1- or 2-propanol, the resultant pillar patters were rinsedagain with solvents: water, methanol, propanol, etc. In the nextdrying process, a peculiar phenomenon was observed. Capillaryforce between the pillars induced distortion and collapsed eachother, and finally, the unique structure “top-gathering periodicarray” was formed.

Typical examples of the top-gathering pillars of hybrid mate-rials prepared by the laser interference technique are shown in

Fig. 11. Top-gathering pillars of hybrid materials prepared by laser interfer-ence technique. (a-1) and (b-1) Top views. (a-2) and (b-2) Corresponding viewsfrom 20◦ angle with respect of the substrate.

Fig. 11 [15]. SEM photograph of (a-1) and (b-1) are top viewsand (a-2) and (b-2) are their corresponding views from an angleof 20◦ with respect to the substrate. In Fig. 11 (a-1) and (a-2),all pillars were vertical to the surface of a substrate and formeda 2-D periodic pattern. On the other hand, in Fig. 11 (b-1) and(b-2), a certain number of pillars gathered at the top and top-gathering unit of 2 × 2 pillars were formed. Different formationstyle of pillars, standing alone and top-gathering of 2 × 2, 3 ×3, . . . unit, can be controlled by choosing the parameters: heightand diameter of the pillars, distance of each other, and varioussolvent used in the rinse process.

Application of these unique “array of top-gathering pillars” isnow under investigation. As pointed out in [15], they can be usedas periodic arrays in the diffractive optics, light-out coupling unitfor light sources. Their unique structure, however, will open theway to nontraditional usages. One of the attractive researches isthe use of the pyramidal open space surrounded by top-gatheringunits, which is applicable as a template to construct a pyramidalstructure. We are trying to build pyramidal-shaped assembliesconsisted of fine particles such as SiO2 and ZnO. These particlessuspended in solvents were poured on the substrates of arraysof top-gathering pillars, and then, in drying process, they werecarried with liquid flow and were gathered into the pyramidal-shaped open space. The driving force for making pyramidal-shaped particle assembly seems to be the capillary force [16].

IV. MICROMETER-SIZED SPHERICAL RESONATOR

Encapsulation of light in the cavity structures has been anessential requirement for small-sized optical devices. Conven-tional optical fibers, fiber, and semiconductor lasers are success-ful examples of the light encapsulation in 1-D and 2-D cavities.Recently, there has been a considerable interest in a sphericalcavity of micrometer size: light encapsulation in 3-D cavity.Spherical particles are expected to have potential uses, such as


a light source of multiwavelengths [17], component of photoniccrystals [18], and a low-threshold laser [19].

The first spherical laser was reported in a Sm2+ in a CaF2sphere of millimeter size in 1961 [20]. It is well known thatthe Fabry–Perot-type conventional lasers have been developedrapidly and enormously. In spherical lasers, however, fundamen-tal research continued for a long time because of the difficul-ties in fabricating micrometer-size spheres and controlling thelaser actions. The previous investigations are classified into twogroups: 1) from liquid droplet to dye-doped spheres where thesizes of spheres progressively decreased for years and 2) high-quality factor (high-Q) solid spheres of relatively large sizes,along with the improvement in coupling efficiency of pumpinglaser light into spheres.

In the first research group, in 1984, lasing from dye dissolvedethanol droplets in 60-µm diameter was reported [21], whichis the revival of the spherical-laser research. After the report,many authors have demonstrated lasing from various dye-dopedspherical particles: plastic particles of ∼10–20 µm in diameters[22], [23] and organic–inorganic hybrid particles of 6 µm indiameter [24], [25].

In the second research group, much attention has been di-rected at investigating optical resonances (WGM) of silica glassmicrospheres [26]. Long confinement time (high-Q for high-purity silica glass spheres: Q =1010) achieves a strong in-teraction of light and materials with outstanding photostabil-ity [27], [28]. To excite high-Q microspheres, effective cou-pling of laser light into them is essential. Various techniquesfor achieving sufficient coupling have been tried using prismcouplers, fiber tapers, side-polished fibers, hybrid fiber-prismcouplers, slab waveguides, etc., [29], [30]. Recently, silica glassmicrospheres and microtroids pumped by fibers taper have beendemonstrated as superior resonators of high-quality factor andhigh optical performances [31].

Microcavity-based Raman lasers are highly attractive for ex-tending the wavelength range of the existing laser sources. Earlyworks showed that the stimulated Raman scattering is possiblein water droplets of 60 µm in diameter [32], [33]. Recently, a Ra-man laser was performed with ultrahigh-Q silica microspheres(40–70 µm in diameter) using fiber-taper couplers [34]. Thefiber taper is efficient for optical coupling, but it requires deli-cately drawn fiber of a few micrometers in diameter suspendedin air. Moreover, the refractive index of high-purity silica glassis as low as 1.458. A lower index than that for the cladding isnot readily available.

We recognize the urgent issues for high-Q microspheres as:1) high-refractive-index spheres; 2) coating of spheres; and 3)efficient coupling to spheres for meeting the requirement inpractical devices [35]. In this section, first, we describe the fab-rication and pumping of micrometer-sized spherical particles oforganic–inorganic hybrid materials to study the potential abilityas a spherical cavity laser [36]. Second, we present the prepa-ration of coated high-index glass spheres, and pumping themby laser coupling through the unique-shaped coated spheres,“terrace microspheres.” Stimulated Raman scattering was per-formed in these high-index glass spheres coated with hybridmaterials [37].

A. Microspheres Made by Vibrating Orifice Technique

The spherical particles were prepared by the vibrat-ing orifice technique [36]. Starting reagents PTES andDPhDMS were hydrolyzed and polymerized in hydrochlo-ric acid solution. For high-index particles, TTBu wasadded at 3 ◦C for controlling its high reactivity. Laserdyes, such as R6G and 4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran (DCM), were added to the start-ing sol.

In the fabrication of doped spheres, the good affinity betweenthe dopants and the matrix materials of the sphere is alwaysvery important. Organic groups connected covalently to Si insiloxane matrix are necessary to incorporate organic dyes in thesphere [38], because the hydrophobic–hydrophilic relation be-tween the matrix and dopant determines their affinity. Complexsalts surrounded by organic chelates, such as europium(III)-thenoyltrifuoroacetonate [Eu(TTFA)3] was also chosen as adoping reagent [39], which is easily dissolved in an ethanolsolvent and uniformly incorporated in the matrix.

The starting sol diluted with alcohol was supplied to theliquid droplet generator. A cylindrical liquid jet of diluted solpassing through an orifice (20 µm in diameter) breaks up intoequal-sized droplets by mechanical vibration. Then, the sol-vent was evaporated during flying with carrier gas, and sub-sequently, these droplets solidified into hybrid microspheres inaqueous ammonia water. By choosing the following parameters,we can control the sizes of the sphere precisely in the range of4–10 µm; solute concentration in the starting solution, feedingrate into the orifice, and the frequency of the orifice vibration.

B. Dye-Doped Microspheres for Laser Emission

The dye-doped spherical particles observed by an optical mi-croscopy are shown in Fig. 12. Narrow size distribution is re-markable, and the dye doping efficiency is nearly 100% in thecontent of ∼10−7–10−4 mol/g. Absorption and photolumines-cence of R6G-doped particles are shown in Fig. 13. Becausethe matrix of the spheres was chosen to have good affinity withorganic dyes, the incorporated dyes showed only a monomer-like absorption spectrum, which is similar to that dissolved inethanol. The dye-doped spheres showed a wide range of fluo-rescence in 520–680-nm wavelengths.

The wavelength region of laser emission was determinedspontaneously by incorporated dye species and its content, be-cause the overlap region of the absorption and the fluorescencebands did not show the laser emission.

Spherical particles were set on a glass plate under an opticalmicroscope in air, and one of them was pumped by a second har-monic pulse of a Q-switched Nd:YAG laser. Pulses were at 532-nm wavelength, 10-Hz repetition rate, and 5-ns duration time.A notch filter eliminated the pumped laser light, and then, theemission was measured by an image-intensified charge-coupleddevice (CCD) array with an electric gate. Photodegradation ofthe laser emission from various dye-doped sphere was measuredby pulse pumping up to 100 000 shot numbers [40].

A typical example of the emission spectrum from an R6G-doped sphere and the emission intensity as a function of the


Fig. 12. Dye-doped spheres prepared by the vibrating orifice technique.

Fig. 13. Absorption and photoluminescence spectra of R6G-doped hybridspheres.

pumping power are shown in Fig. 14. The wavelength region ofthe emission was ∼570–600 nm. Emission peaks correspondedto WGMs, and the number of modes depended upon the par-ticle diameter. Organic dyes usually lack photostability understrong laser pumping condition. With an increase of dye con-tent, a rapid decrease of emission intensity was observed. Onthe other hand, the threshold intensity decreased with an in-creasing dye content, which means that we can decrease thepumping power. Because the most stable lasing conditions areobtained by choosing the lower dye content and the lower pump-ing power, we should determine the cross-point conditions forachieving high-degradation-resistant dye-doped spheres fromthese contradictory results. The photostability of dye-dopedspheres are remarkable; by choosing the suitable dye-contentand pumping power, for example, in DCM-doped spheres, weknow that after 260 000 shot irradiation (above 7 h irradiation),

Fig. 14. Emission spectrum from R6G-doped spheres and the emission inten-sity as a function of the pumping power.

the emission consumes 50% of the initial intensity [40]. Theresults of dye-doped spheres are somewhat surprising becausethe lifetime (50% consumption of the initial intensity) of laseremission in dye-doped hybrid bulk materials is several thousandpulses [41], [42].

C. Coated Microspheres for Laser Emission

From the practical point of view, for preventing surface con-tamination and maintaining the optical performances, the micro-spheres should be coated with clad materials of lower refractiveindex than those of spheres. Therefore, to meet the requirementof the high relative refractive index nr (nr > 1.5) as a sphericalcavity, high-index spheres ncore > 2.0 are needed [11].

Commercially available high-index glass spheres of 30 µm indiameter made by the flame spray technique were used for the


Fig. 15. Raman emission spectra from terrace microsphere with various irra-diation points (A)–(E). SEM image of terrace microsphere is also shown.

coating experiments [37]. The composition of the glass sphereswas BaO–SiO2–TiO2 , and its refractive index is 1.93. In theflame spray technique, where small pieces of glass cullet weremelted in the flame, glass spheres with a smooth surface wereformed by the surface tension. The smooth surface by the melt-ing process is essential to the high-Q spherical cavity. Coatingmaterial was hybrid material, silicone oligomer GR100, havingmethyl and phenyl groups of a 2:1 molar ratio, with a refrac-tive index of 1.49 after curing. The glass spheres dispersed ina GR100-acetone solution were sprayed with air-spray equip-ment on a Teflon sheet substrate. Coated spheres were driedat room temperature followed by heating up to 130 ◦C. Be-low 20 mass% of GR-100 in acetone, glass spheres uniformlycoated glass spheres were obtained. Up to 40 mass%, however,the coated spheres having one flat side were obtained. Sincethe side attached to the Teflon sheet surface showed the flatportion, which resembled like a terrace, we named it “terracemicrosphere” from their impressive shapes.

Pumping experiments by Ar+ laser (514.5-nm wavelength)were carried out for terrace-shaped high-index glass spheres.Raman emission spectra of the “terrace microsphere” with vari-ous irradiation points are shown in Fig. 15. SEM photograph ofthe terrace microsphere is also involved in the figure. The cor-responding laser pumping spots [from (A) to (E)] are illustratedat the right side of the figure. When the terrace point (spot B)was pumped, the coated sphere showed the remarkable increaseof the emission intensity, as shown in the figure. On the otherhand, at the opposite point E (the uniformly coated area), wecannot observe the resonant emission.

Above the threshold input power at about 2 mW, the stimu-lated Raman emission from terrace microspheres showed las-ing action [43]. To achieve the resonant condition inside thesphere, the excitation at the tangent line is suitable, but un-der the perfect spherical condition, the light cannot penetrateinto the sphere and almost all of the energy will be reflected.The contradictory conditions, lowering the reflection at the sur-face and matching the angle for the resonant condition, seemto be satisfied by the terrace structure. Although commerciallyavailable glass spheres were used in the experiments, we now

developed a new fabrication technique to prepare supersphericalglass and confirmed the WGM resonant emission as an opticalresonator [44]–[46].

V. CONCLUSION

Organic–inorganic hybrid materials are designed to combineboth properties of organics and inorganics in molecular level,using various silane coupling reagents and metal alkoxides asstarting materials. The hybrid materials have performed remark-able results, which is higher than expected at the early stage ofthe investigation. Precise control of films coating on substrateswas achieved; flexible properties of organic groups can avoid thecracks, and good affinity between films and various substratesis obtained. Applicability of photolithography techniques haswidened the usefulness of the hybrid materials. High refractiveindex more than nD = 2.0, which is originated from inorganicssuch as TiO2 , was achieved as coating films and microspheres.It is noteworthy that coated spheres with unique structures canbe fabricated. The superior properties of hybrid materials nowexceed those of organics and inorganics, plastics, and glasses.

On the other hand, interesting but unfavorable phenomenawere induced, such as formation of cellular patterns, whichmeans that the hybrid materials are not so uniform as expectedin early stages. The sites in nanometer level seems to be formedin the host material; sometimes, the sites offer hydrophobicsurroundings of organic groups for organic dyes, and, in othercases, offer hydrophilic surroundings of –OH groups for metalor oxide nanoparticles. The different sites existed in the matrixand offer stable and affinitive effects on the various dopants.Furthermore, heating, UV-light irradiation, high-intensity laserirradiation, etc., influence and change the hybrids’ properties.We have already recognized the usefulness of the hybrid mate-rials, while still expect new chemical phenomana in the “O–Si–C” networks, and they will open new ways to various photonicapplications.

REFERENCES

[1] H. Schmidt, A. Kaiser, H. Patzelt, and H. Scholze, “Mechanical and phys-ical properties of amorphous solids based on (CH3 )2 -SiO-SiO2 gels,” J.Phys., Colloque C9, Suppl., vol. 12, pp. C9-275–C9-279, 1982.

[2] H. Schmidt, “New type non-crystalline solid between inorganic and or-ganic materials,” J. Non-Cryst. Solids, vol. 73, pp. 681–691, 1985.

[3] M. Kawachi, “Silica waveguides on silicon and their application tointegrated-optic components,” Opt. Quantum Elect., vol. 22, pp. 391–416, 1990.

[4] S. Sakka, “Sol–gel synthesis of glasses; present and future,” Amer. Ceram.Soc. Bull., vol. 64, pp. 1463–1466, 1985.

[5] J. D. Mackenzie, “Sol–gel optics,” J. Ceram. Soc. Jpn., vol. 101, pp. 1–10,1993.

[6] S. Shibata, K. Miyajima, H. Yoshikawa, T. Yano, and M. Yamane, “Cel-lular patterns in organic–inorganic hybrid film,” J. Sol-Gel Sci. Technol.,vol. 19, pp. 665–669, 2000.

[7] S. H. Bell, “Pigment flotation in paints: A dynamic phenomenon,” J. OilColour Chem. Assoc., vol. 35, pp. 373–395, 1952.

[8] H. Benard, “Les tourbillons cellulaires dans une nappe liquide,” Rev. Gen.Sci., Purea Appl., vol. 11, pp. 1261–1271, 1900.

[9] CRC Handbook of Chemistry and Physics, 71st ed. Cleveland, OH: CRCPress, 1991.

[10] Y. Arai, T. Yano, and S. Shibata, “High refractive-index thin film derivedfrom organic–inorganic hybrid materials,” J. Ceram. Soc. Jpn., vol. 112,pp. S248–S251, 2004.


[11] Y. Arai, T. Yano, and S. Shibata, “High refractive-index microspheres ofoptical cavity structure,” Appl. Phys. Lett., vol. 82, pp. 3173–3176, 2003.

[12] S. Shibata, “Hybrid microspheres for the optical applications,” presentedat the 20th Int. Congr. Glass, Kyoto, Japan, 2004, Paper I-II-020.

[13] Y. J. Lee, S. H. Kim, J. Huh, G. H. Kim, Y. H. Lee, S. H. Cho, Y. C. Kim,and Y. R. Do, “A high-extraction-efficiency nanopatterned organic light-emitting diode,” Appl. Phys. Lett., vol. 82, pp. 3779–3781, 2003.

[14] N. Kaji, Y. Tezuka, Y. Takamura, M. Ueda, T. Nishimoto, H. Nakanishi,Y. Horie, and Y. Baba, “Separation of long DNA molecules by quartznanopillar chips under direct current electric field,” Anal. Chem., vol. 76,pp. 15–22, 2004.

[15] H. Segawa, H. Misawa, T. Yano, and S. Shibata, “Fabrication of periodicarrays of top-gathering titania-organic hybrid pillars derived from multi-beam laser interference,” in Proc. SPIE, vol. 6106, pp. 6106N-1–6106N-9,2006.

[16] H. Segawa, Y. Yamazaki, T. Yano, and S. Shibata, “Pyramidal shapedassemblies of particles by micromoulding in top-gathering structures,”Presented at the PacRim, 2007, Shanghai, China, Nov. 11–14, Paper S9-6-0.

[17] T. Kippenberg, S. Spillane, D. Armani, B. Min, L. Yang, and K. Vahala,Optical Microcavities. Singapore: World Scientific, 2004.

[18] J. Joannopoulos, R. Meade, and J. Winn, Photonic Crystals. Princeton,NJ: Princeton Univ. Press, 1995.

[19] P. Barber and R. Chang, Optical Effects Associated With Small Particles.Singapore: World Scientific, 1988.

[20] C. G. B. Garrett, W. Kaiser, and W. L. Bond, “Stimulated emission intooptical whispering modes of spheres,” Phys. Rev., vol. 124, pp. 1807–1809, 1961.

[21] H. M. Tzeng, K. F. Wall, M. B. Long, and P. K. Chang, “Laser emissionfrom individual droplets at wavelength corresponding to morphology-dependent resonances,” Opt. Lett., vol. 9, pp. 499–501, 1984.

[22] M. Kuwata-Gonokami, K. Takeda, H. Yasuda, and K. Ema, “Laser emis-sion from dye-doped polystyrene microsphere,” Jpn. J. Appl. Phys.,vol. 31, pp. L99–L101, 1992.

[23] H. Misawa, R. Fujisawa, K. Sasaki, N. Kitamura, and H. Masuhara, “Si-multaneous manipulation and lasing of a polymer microparticle using aCW 1064 nm laser beam,” Jpn. J. Appl. Phys., vol. 32, pp. L788–L790,1993.

[24] S. Shibata, M. Yamane, K. Kamada, K. Ohta, K. Sasaki, and H. Ma-suhara, “Laser emission from dye-doped organic–inorganic particles ofmicrocavity structure,” J. Sol-Gel Sci. Technol., vol. 8, pp. 959–964,1997.

[25] S. Shibata, T. Yano, and M. Yamane, “Dye-doped spherical particles ofoptical cavity structure,” SPIE Sol-Gel Opt. IV, vol. 3136, pp. 68–76,1997.

[26] M. Cai, O. Painter, and K. J. Vahala, “Observation of critical couplingin a fiber taper to a silica-microsphere whispering-gallery mode system,”Phys. Rev. Lett., vol. 85, pp. 74–77, 2000.

[27] J. C. Knight, G. Cheung, F. Jacques, and T. A. Birks, “Phase-matchedexcitation of whispering-gallery-mode resonance by a fiber taper,” Opt.Lett., vol. 22, pp. 1129–1131, 1997.

[28] M. L. Gorodetsky, A. A. Savchenkov, and V. S. Ilchenko, “Ultimate Q ofoptical microsphere resonators,” Opt. Lett., vol. 21, pp. 453–455, 1996.

[29] M. L. Gorodetsky and V. S. Ilchenko, “Optical microsphere resonators:Optical coupling to high-Q whispering-gallery modes,” J. Opt. Soc. Amer.B, vol. 16, pp. 147–154, 1999.

[30] B. E. Little, J.-P. Laine, D. R. Lim, H. A. Haus, L. C. Kimerling, and S.T. Chu, “Pedestal antiresonant reflecting waveguides for robust coupling tomicrosphere resonator and for microphotonic circuits,” Opt. Lett., vol. 25,pp. 73–75, 2000.

[31] K. J. Vahala, “Optical microcavities,” Nature, vol. 424, pp. 839–846,2003.

[32] S. X. Qian and R. K. Chang, “Multiorder stokes emission frommicrometer-size droplets,” Phys. Rev. Lett., vol. 56, pp. 926–929, 1986.

[33] H. B. Lin, A. L. Huston, J. D. Eversole, and A. J. Campillo, “Double-resonance stimulated Raman scattering in micrometer-sized droplets,” J.Opt. Soc. Amer. B, vol. 7, pp. 2079–2089, 1990.

[34] S. M. Spillane, T. J. Kippenberg, and K. J. Vahala, “Ultralow-thresholdRaman laser using a spherical dielectric microcavity,” Nature, vol. 415,pp. 621–623, 2002.

[35] S. Shibata, T. Yano, and H. Segawa, “Sol–gel derived spheres for sphericalmicrocavity,” Acc. Chem. Res., vol. 40, pp. 913–920, 2007.

[36] S. Shibata, A. Tomizawa, H. Yoshikawa, T. Yano, and M. Yamane, “Prepa-ration of spherical particles by vibrating orifice technique,” SPIE Sol-GelOpt. V, vol. 3943, pp. 112–119, 2000.

[37] S. Shibata, S. Ashida, H. Segawa, and T. Yano, “Coated microsphere asspherical cavity Raman laser,” J. Sol-Gel Sci. Technol., vol. 40, pp. 379–384, 2006.

[38] S. Yagi, S. Shibata, T. Yano, A. Yasumori, M. Yamane, and B. Dunn,“Photostability of the laser dye DCM in various inorganic–organic hostmatrices,” J. Sol-Gel Sci. Technol., vol. 4, pp. 67–73, 1995.

[39] L. R. Matthews and E. T. Knobbe, “Luminescence behavior of europiumcomplexes in sol–gel derived host materials,” Chem. Mater., vol. 5,pp. 1697–1700, 1993.

[40] S. Shibata, A. Araya, T. Yano, and M. Yamane, “Photostability of thelaser emission from dye-doped spherical particles,” SPIE Sol-Gel Opt.VI, vol. 4804, pp. 44–51, 2002.

[41] J. C. Altman, R. E. Stone, F. Nishida, and B. Dunn, “Dye-activatedORMOSIL’s for lasers and optical amplifiers,” SPIE Sol-Gel Opt. II,vol. 1758, pp. 507–518, 1992.

[42] A. B. Wojcik, L. C. Klein, and S. Muto, “Rhodamine 6G-doped inor-ganic/organic gels for laser and sensor application,” SPIE Sol-Gel Opt.III, vol. 2288, pp. 392–399, 1994.

[43] S. Shibata, S. Ashida, H. Segawa, and T. Yano, “Low threshold spheri-cal Raman laser,” presented at the 14th Int. Sol–Gel Conf., Montpellier,France, Sep. 2–7, 2007.

[44] T. Kishi, S. Shibata, and T. Yano, “Preparation of micrometer-size super-spherical glasses for optical resonator,” presented at the 20th Int. Congr.Glass, Kyoto, Japan, 2004.

[45] T. Yano, S. Shibata, and T. Kishi, “Fabrication of micrometer-size glasssolid immersion lens,” Appl. Phys. B: Lasers Opt., vol. 83, pp. 167–170,2006.

[46] T. Kishi, S. Shibata, and T. Yano, “Fabrication of high-refractive-indexglass micron-sized solid immersion lenses by a surface-tension mold tech-nique,” J. Non-Cryst. Solids, vol. 354, pp. 558–563, 2008.

Shuichi Shibata was born in Hokkaido, Japan, in1948. He received the M.S. and Dr.Eng. degrees inoptical fibers from Hokkaido University, Sapporo,Japan, in 1973 and 1982, respectively.

From 1973 to 1991, he was with Nippon Telegraphand Telephone Corporation (NTT). Since 1991, hehas been a Professor in Tokyo Institute of Technology,Tokyo, Japan. His current research interests includeglasses and optical materials for the application.

Tetsuji Yano was born in 1963. He received the M.S.and Dr. Eng. degrees in material science from TokyoInstitute of Technology (TITech), Tokyo, Japan, in1989 and 1995, respectively.

From 1989 to 2003, he was an Assosciate Re-searcher at TITech, where he is currently an AssociateProfessor in the Department of Chemistry and Ma-terials Science. His current research interests includebasic science of glass materials to the fabrication ofoptical functional materials.

Hiroyo Segawa was born in Toyama, Japan, in 1972.She received the M.S. and Dr.Eng. degrees in materialscience from Tokyo Institute of Technology (TITech),Tokyo, Japan, in 1997 and 2000, respectively.

During 2000–2003, she was a Research Asso-ciate at Oita University. From 2002 to 2006, shewas a Researcher of Precursory Research for Em-bryonic Science and Technology (PRESTO) Pro-gram, Japan Science and Technology Agency (JST).Since 2004, she has been an Assistant Professor inthe Department of Chemistry and Materials Science,

TITech. Her current research interests include fabrication of optical materials byself-organization.

Download - Organic–Inorganic Hybrid Materials for Photonic Applications

Top Related