Nonlinear optical and electro-optic properties of hybrid sol–gels doped with organic chromophores

Optical Materials 28 (2006) 992–999 www.elsevier.com/locate/optmat

Nonlinear optical and electro-optic properties of hybrid sol–gels doped with organic chromophores Hongxi Zhang *, Dong Lu, Mahmoud Fallahi College of Optical Sciences, University of Arizona, 1630 East University Boulevard, Tucson, AZ 85721, United States Received 17 November 2004; accepted 6 May 2005 Available online 25 July 2005

Abstract This paper reports a systematic investigation on the nonlinear and electro-optic properties of hybrid sol–gel doped with azo-type chromophores. The host is based on photosensitive sol–gel prepared from 3-(methacryloxy)propyl trimethoxysilane-zirconium (or aluminum) oxide (MAPTMS-Zr(Al)) hybrid system. The chromophores are incorporated into the hybrid sol–gel as both guest and side-chain. Second harmonic generation experiment is conducted to optimize the poling parameters for the active sol–gel films. The stability of the second-order nonlinear optical coefficients is investigated by temporal decay test and heat-cool cycling experiment at elevated temperature. Electro-optic channel waveguides have been fabricated by direct ultra-violet exposure, direct blue laser writing, and reverse-mesa methods. Electro-optic effect of the waveguides is measured at 1550 nm and optical intensity modulation has been demonstrated. All the results show that the hybrid sol–gels are promising media for electro-optic devices for integrated optics in both performance and fabrication flexibility.  2005 Elsevier B.V. All rights reserved. PACS: 81.20.Fw; 42.65.k; 42.79.Gn; 78.20.Jq Keywords: Hybrid sol–gel; Second harmonic generation; Optical waveguide; Electro-optic effect

1. Introduction Electro-optic (EO) devices play key roles in optical communications, radio frequency sensing, and analog/ digital microwave links [1,2]. The increasing demand for high-speed EO devices has greatly stimulated the research and development of low dielectric constant materials including polymers and sol–gel silica that permit high-speed transmission of optical waves and microwaves with low mismatching. In the past decade, various organic chromophores and polymers have been synthesized and characterized as electro-optic media and a few devices with high EO coefficients have been demon-

*

Corresponding author. Tel.: +1 520 6211337; fax: +1 520 6214442. E-mail address: [email protected] (H. Zhang).

0925-3467/$ - see front matter  2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2005.05.016

strated [2–4]. In addition to large optical nonlinearity, the stability of EO performance has been a major concern. Some novel techniques such as lattice hardening have been developed to prevent the relaxation of the aligned chromophore and the degradation of the EO performance [5]. Compared with organic polymers, sol–gel silica materials have more rigid inorganic networks and smaller free volume for chromophore molecules, which can help freeze the chromophore molecules and, as a result, improve the stability of the electro-optic device [6]. Some reports on the nonlinear optical properties of azo-dye chromophores doped silica sol–gels have been presented recently [6–8]. However there are few reports on the fabrication and EO effect of the sol–gel waveguides. Among the sol–gel silica materials developed so far, the one based on the hybrid system of MAPTMS-Zr(Al)

H. Zhang et al. / Optical Materials 28 (2006) 992–999

is promising for its low loss and ease of processing. Its photopatternability provides fabrication flexibility and several passive devices such as waveguide Bragg gratings, multimode interferometric components, and diffractive optical elements have been recently demonstrated by one-step photopattern on conventional mask aligner [8,9]. There are two methods to incorporate nonlinear optical chromophores into sol–gel networks: guest–host and side-chain or main-chain [7]. In the guest–host method, the chromophores are physically doped into the sol–gel. The guest–host active sol–gels suffer from phase separation problem, which limits the chromophore loading density and results in poor thermal stability and high loss. While in the side-chain/main-chain method, the chromophores are covalently linked onto the silica backbone, which can provide higher temporal and thermal stability. In this paper, we report a systematic study on the azo-dye chromophores doped MAPTMS-Zr(Al) hybrid sol–gel films. In Section 2, we briefly describe the materials processing of MAPTMS-Zr(Al) active sol–gel doped with disperse red (DR) chromophores (DR1, DR13, and DR19). The third section will present the optical properties including refractive index and optical loss. Second-order optical nonlinearity of the active sol–gel films, including second-order optical coefficients and their thermal stability, will be given and discussed in Section 4. In Section 5, we present the fabrication and characterization of electro-optic waveguides. The evaluation of the EO effect of the waveguides is described in Section 6. The conclusion will discuss the flexibility and the promise of the active hybrid sol–gels for electro-optic devices.

2. Materials processing Material processing includes the preparations of passive sol–gel precursors, chromophore solutions, and the active sols. Passive MAPTMS-Zr(Al) sols are prepared following the procedure reported earlier [9]. In brief, MAPTMS is hydrolyzed with 0.01 M aqueous HCl. Zirconium propoxide (ZPO) is dissolved in methacrylic acid and then mixed with MAPTMS in a molar ratio of Si/Zr = 78:22 to produce MAPTMS-Zr sol. For MAPTMS-Al sol, di-s-butoxyaluminoxytriethoxysilane (BATES) is dissolved in 2-propanol and then mixed with prehydrolyzed MAPTMS in a molar ratio of MAPTMS:BATES = 84:16. The photosensitive MAPTMSZr(Al) sol is obtained by adding photoinitiator, CI-819, to the above sols in an amount of 0.5 wt.%. CI-819 has strong absorption below 425 nm, suitable for both direct patterning of passive devices and direct blue laser writing of EO waveguides. Azo-dye chromophores, DR1, DR13 and DR19, the molecular structures of which are shown in Fig. 1, are

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OH

OH

N

OH

HO N

N

N

N

N N

N

N

NO2

NO2

Cl

NO2

Fig. 1. Molecular structures of DR1 (left), DR13 (middle), and DR19 (right).

incorporated into the sols in both guest–host and sidechain. In guest–host method, the chromophores are dissolved in N,N-dimethylformamide (DMF) or pyridine and then mixed with MAPTMS-Zr(Al). In the sidechain method, DR chromophores are covalently linked to the silica backbone through the reaction between the hydroxyl in DRs and isocyanate in (3-triethoxysilyl propyl) isocyanate [(C2H5O)3Si(CH2)3NCO, TESPIC], as introduced by Choi et al. [11]. The reaction, as shown in Fig. 2 for DR1, is conducted in pyridine at 65 C for 1 day. DR-TESPIC is mixed with 0.01 M aqueous HCl in DMF or pyridine. Thus, triethoxysilyls in TESPIC are hydrolyzed, generating silanols that then are dehydrated with those in MAPTMS-Zr(Al) sol and densified at elevated temperature during poling and hard baking, resulting in rigid side-chain silica networks. To improve the thermal stability of the nonlinear and electro-optic properties, a thermal cross-linker (3-glycidoxypropyl)trimethoxysilane (GPTMS), which is also prehydrolyzed with 0.01 M HCl, is added to produce cross-linked sidechain active sol. The glass transition temperatures (Tg) of the active sol–gels are measured with a modulated differential scanning calorimeter (DSC). Fig. 3 shows the DSC scanning curve for side-chain active sol, DR1-TESPIC in MAPTMS-Zr, at a doping concentration of 10 mol%. OCH2CH3 O2N

CH2CH3

N N

+

N

OCN

(CH2)3 Si

TESPIC

DR1 O2N

N N

OCH2CH3

OCH2CH3

CH2CH2OH

CH2CH3 N

OCH2CH3

H

N (CH2)3 Si

CH2CH2O C

OCH2CH3

OCH2CH3

O DR1-TESPIC

Fig. 2. Synthesis process of side-chain chromophore monomer for DR1.

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H. Zhang et al. / Optical Materials 28 (2006) 992–999

(Rev Heat Flow (mW))

-1.8 -1.9 -2.0 -2.1 -2.2

160

180 200 220 240 Temperature (°C)

260

Fig. 3. DSC scanning curve for the measurement of Tg of the crosslinked side-chain active gel.

It reveals that the glass transition of the active sol–gel happens in a wide range from 160 C to 260 C. Thermogravimetric analysis is also conducted and no decomposition is observed at around 260 C.

3. Refractive index and optical loss Active sol–gel films are spin-coated onto indium tin oxide (ITO) precoated SiO2/Si substrates with 6 ± 0.5 lm SiO2. The ITO layer acts as the bottom electrode and the films are corona poled at 170 C for 1 h with a corona field of 6 kV and a tungsten needle–film distance of 1.5 cm. The TE mode refractive indices and optical loss of the films are measured on a prism coupler. The refractive index-wavelength dependencies of the films of DR1-TESPIC in MAPTMS-Zr (10.0 mol%) are shown in Fig. 4. For comparison, the refractive index of the undoped passive MAPTMS-Zr film hard baked at 170 C is also given. The doped active sol–gel film has an increased refractive index and the

poling also causes an increase of about 1.0% in the refractive index compared with the unpoled active film. The propagation loss of the films of side-chain DR1TESPIC and DR19-TESPIC in MAPTMS-Zr is measured to be about 0.9–1.0 dB/cm at 1550 nm, a little bit higher than that of the passive sol–gel film (0.7 dB/ cm) due to the contribution of the vibration overtone of –N–H bonds generated in the reaction between DR1 and TESPIC, as shown in Fig. 2. The recent experiment has shown that, by silylating the sol or passivating the sol–gel films with dielectric layer of SiO2, the loss caused by the retained and adsorbed water in the film can be reduced to around 0.3 dB/cm [12]. Correspondingly, by silylating the active sol or passivating the active sol–gel film, lower optical loss can be expected.

4. Nonlinear optical properties For second harmonic generation (SHG) measurement, the active sol–gel films are spin-coated onto ITO precoated glass substrates, soft baked at 100 C for 10 min and then corona poled at different temperatures from 120 C to 225 C. The absorption spectra of the poled films are shown in Fig. 5. Compared with the baked film, the maximum absorption wavelengths of poled films show blue shift due to the trans–cis transition of the chromophore molecules during poling. Poling at a temperature lower than the Tg results in less blue shift. But it is noted that the absorption tends to widen and has a shoulder in the shorter wavelength side (400 nm) for the films poled at 175 C and higher temperatures, indicating that chromophore decomposition occurs. The decomposition temperature of the films is evidently lower than that of the active sol–gel (>260 C) and chromophores (215 C for DRs). The reason is that the charged plasma and the current in

2.0

1.62

Baked @ 120 °C Active film, poled

1.58 1.56 1.54

Active film unpoled

1.52

MAPTMS-Zr

Absorption (A.U.)

Refractive index

1.60

1.5

Poled @ 120 °C 150 °C 165 °C 175 °C

1.0

0.5

200 °C 225 °C

1.50 600

800 1000 1200 1400 1600 Wavelength (nm)

Fig. 4. Refractive index of the passive MAPTMS-Zr, unpoled, and poled active films of side-chain DR1-TESPIC in MAPTMS-Zr. All the films are processed at 170 C.

0.0 400

600 800 Wavelength (nm)

1000

Fig. 5. Absorption spectra of the active sol–gel films poled at different temperatures.

H. Zhang et al. / Optical Materials 28 (2006) 992–999

d33 (pm/V)

40 30 20 10 0

120

140

160

180

200

Poling Temperature (°C) Fig. 6. Dependence of d33 on the poling temperatures of the films of DR1-TESPIC in MAPTMS-Zr.

VIS=6 kV VIS=0 V

SHG intensity (A. U.)

VIS=6 kV

VIS=0 V

Tpoling=150°C

Tpoling=170°C

Fig. 7. The influence of an in situ corona filed on the SHG intensity for the cross-linked side- chain films poled at 150 C and 175 C, respectively.

1.0

Crosslinked

0.8 Normalized d33

the poling process tend to bleach and destroy the chromophores [13]. SHG measurement is carried out on a setup similar with the one described in Ref. [14]. A Nd:YAG laser operating at 1064 nm with 1 kHz frequency and 90 ns pulse duration is used as the fundamental source. A p-polarized fundamental wave is illuminated on to the samples through a lens with a focal length of 50 cm. The samples are mounted on a rotation stage to adjust the incident angle of the fundamental. The second harmonic signal at 532 nm passes through a p-polarized analyzer and is detected by an optical multi-channel analyzer. The nonlinear optical coefficient d33 is derived by comparing the SHG intensity of the film with that of a Y-cut quartz crystal (d11 = 0.34 pm/V) [14–16]. A corona field is also applied during the SHG measurement to in situ evaluate the poling efficiency. Fig. 6 presents the dependence of d33 on the poling temperature for the films containing 10 mol% DR1TESPIC in MAPTMS-Zr. It is clear that the high d33 is obtained for the film poled at 165–175 C, and higher poling temperature results in decreased d33 due to the decomposition of the chromophore. Poling at lower temperature (<160 C) produces lower d33, which is attributed to the lower poling efficiency. Fig. 7 shows the influence of the in situ corona field (VIS = 6.0 kV, 1.5 cm far away from the film) on the SHG signals of the films poled at 150 C and 170 C, respectively. A dramatic enhancement in SHG signal is demonstrated for the film poled at 150 C, while the SHG signal increase is very small for the film poled at 170 C. Thermal stability of d33 of the films is investigated from 100 to 180 C, the common temperature range for device process and package. Three thermal experiments are conducted. First, we measure the temperature dependence of the SHG intensity from 24 C to 180 C and evaluate d33 values for the films. The results, as shown in Fig. 8, indicate that for the cross-linked sample, there is no apparent d33 decay below 110 C. At

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0.6

Non-crosslinked

0.4 0.2

Guest-host

0.0 40

80 120 160 Temperature (°C)

200

Fig. 8. Thermal stability of d33 for guest–host (DR13 doped MAPTMS-Zr), side-chain, and cross-linked side-chain (DR1-TESPIC doped MAPTMS-Zr) sol–gel films.

140 C, almost 90% of d33 is kept. Even at 180 C, over 70% of the d33 can be maintained. While the guest–host and noncross-linked films have poor thermal stability with obvious d33 decay from around 55 C and 70 C, respectively, and only 50% of d33 maintained at 110 C for the later. Second, a thermal cycling test is conducted for the cross-linked sample. The SHG intensity is monitored and d33 calculated when the sample is heated to 110 C at a rate of 3.0 C/min and then naturally cooled to room temperature. Three heating–cooling cycles are successively carried out. Fig. 9 shows the results of the third cycle experiment. It is clear that there is no evident d33 degradation observed during the thermal cycling test. Finally, temporal stability test is conducted for the cross-linked film samples. The results are also very encouraging. As shown in Fig. 10, after 24 h at 110 C the d33 decreases by only 4%. Even at 180 C, only 20% of the initial d33 degrades after 15 min.

H. Zhang et al. / Optical Materials 28 (2006) 992–999

Normalized d33

996 1.2 Heat-cool test

1.0

0.8 20

40

60 80 100 Temperature (°C)

120

Fig. 9. Thermal cycling test results of d33 of cross-linked side-chain (DR1-TESPIC doped MAPTMS-Zr) hybrid sol–gel film when heated (circles) and cooled (squares).

1.1

0

Time (hour) at 110 °C 5 10 15 20

25

Normalized d33

1.0 110 °C

0.9 0.8

180 °C

0.7 0.6 0

4 8 12 Time (min) at 180 °C

16

Fig. 10. The decay of d33 at 110 C and 180 C for the cross-linked side-chain film. d33 is normalized to its initial value at 110 C and 180 C, respectively.

The above results show that the cross-linked hybrid sol–gel can effectively freeze the aligned chromophores and has high thermal stability as a matrix for EO and nonlinear optical components. So the cross-linked side-chain active sols are used in the fabrication of EO waveguides.

tive index that is caused by either the reversible photoisomerization or irreversible photodecomposition of the chromophores during UV irradiation [17,18]. In experiment, cross-linked side-chain films are spin-coated on to ITO/SiO2/Si substrates, soft baked at 100 C for 10 min in an oven, and then corona poled at 170 C for 1 h. Then the films are irradiated through a mask on a mask aligner with I-line UV source at 365 nm. The unexposed strip (3 lm wide) acts as the waveguide and the exposed area as cladding. Fig. 11 shows the top-view of the waveguide of DR1-TESPIC in MAPTMS-Zr obtained by UV irradiation with a power density of 12 mW/cm2 and exposure time of 18 h. On the top, a cladding layer (3.0 lm thick) of passive MAPTMS-Al with lower refractive index (1.48 at 1550 nm) is spin-coated and then hard baked at 145 C for 1 h with a corona field being applied to avoid the relaxation of the already aligned chromophore molecules. A silver layer of 100 nm thick is coated with sputtering to form top electrode. This method is simple and of low cost. The drawback is that it is difficult to fabricate step-like waveguides because the absorption of the chromophores prevents the UV photons from penetrating deep into the films of a few microns thick [19], and the waveguide has poor confinement and heavy scattering, as shown in Fig. 11. 5.2. Blue laser writing Because the UV irradiation tends to induce degradation and decomposition of the chromophores, direct UV photopatterning with the exposed area as the waveguide is not an effective approach for the fabrication of EO waveguides. To overcome the limitations, a blue laser writer is developed to directly fabricate EO waveguides.

5. Waveguide fabrication The conventional technique to fabricate electro-optic devices is mainly based on the multi-step photolithography and wet/dry etching. For the present active sol–gel, the following easy and low-cost methods have been employed to fabricate EO channel waveguides. 5.1. Direct UV pattern Unlike the fabrication of passive sol–gel devices [9,10], direct UV photopattern of ridge waveguides of active sol–gel becomes difficult due to the absorption of the chromophores in UV spectral range. Here the direct UV pattern of the active sol–gel waveguides is based on that the UV irradiated materials have lower refrac-

Fig. 11. Top-view of the direct UV exposed waveguides of 3 lm wide from the cross-linked side-chain sol–gel film (upper) and the transmitted mode at 1550 nm (lower).

H. Zhang et al. / Optical Materials 28 (2006) 992–999

The high power of the blue laser can overwhelm the absorption of the chromophores and induce polymerization in photosensitive materials without destroying the chromophore molecules. The cross-linked side-chain active films of DR1-TESPIC in MAPTMS-Zr are spin-coated onto ITO/SiO2/Si substrates and soft baked at 100 C for 10 min in an oven. A continuous-wave semiconductor laser operating at 405 nm is used in the laser writing system. The output power of the laser (15 mW at maximum) and the moving speed of the translation stage are computer controlled. The blue laser beam is focused onto the film through objective lenses with different numerical apertures to control the width of the waveguide. During writing, polymerization of methacrylate occurs in the exposed area. After writing, the film is developed in acetone to remove the gel in the unexposed area. With this technique, a centimeter long waveguide can be written in a few minutes. Fig. 12 shows the photographs of the top-view and cross-section of the EO waveguide written with a laser

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power of 1.5 mW and a moving speed of 200 steps per second (30 nm per step). A defect-free and uniform waveguide is obtained with this method. The exposed channel is fully polymerized deep through the bottom, which is attributed to the high power density (2.0 · 103 W/cm2) of the blue laser and the strong absorption of the photoinitiator at 405 nm. It has to be pointed out that EO test (next section) shows that photodecomposition does not happen for the present active sol–gel film irradiated with 405 nm laser even at such a high power density of 2 · 103 W/cm2. We believe that the photodecomposition of the sidechain DR in sol–gel occurs at k < 400 nm. As a result, for direct patterning of electro-optic devices from UV sensitive chromophores doped sol–gels as well as polymers, the laser writing technique can be a practical method by using longer-wavelength laser sources and selecting proper photoinitiators in the corresponding spectral bands. The ridge waveguide is corona poled and hard baked at 170 C for 1 h with a voltage of 6 kV. An MAPTMSAl cladding layer is spin-coated onto the ridge and then the sample is baked at 145 C for 1 h with a 6 kV corona field being applied to maintain the alignment of the chromophore molecules. The sample is cooled down to room temperature and then coated with a silver layer of 100 nm thick as top electrode. 5.3. Reverse mesa method EO waveguides are also fabricated by a reverse mesa method, an all-wet chemical process. In brief, photosensitive MAPTMS-Al passive sol–gel film (with an index of 1.48 at 1550 nm) is spin-coated onto ITO coated SiO2/Si substrate and soft baked at 100 C for 10 min. The film is exposed through a photomask on a mask aligner and then developed in 2-proponal. Unexposed area is washed off and the reverse mesas are obtained after hard baking at 170 C for 2 h, as shown in Fig. 13 (top). Active sol is spin-coated onto the substrate with the reverse channels and soft baked at 100 C for 10 min. Then, the waveguide is corona poled and hard baked at 170 C for 1 h with a voltage of 6 kV. After that, an MAPTMS-Al top-cladding layer is spin-coated onto the waveguides and then the sample is baked at 145 C for 1 h. The sample is cooled down to room temperature and then coated with a silver layer of 100 nm thick as top electrode. The cross-section of the waveguide is shown in Fig. 13 (middle).

6. EO effect of the waveguides Fig. 12. Top-view of the direct blue laser written waveguide (upper), SEM micrograph of the cross-section of the waveguide (middle), and the transmitted optical mode at 1550 nm (lower).

The EO properties of the channel waveguides are tested with a transverse configuration [19]. A laser beam of k = 1550 nm from an extra-cavity diode laser,

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H. Zhang et al. / Optical Materials 28 (2006) 992–999

Figs. 11 (lower), 12 (bottom), and 13 (bottom), respectively. The later two have quite good modes at 1550 nm, while the UV irradiated one has some scattering. Driving field is applied to the waveguide through the bottom (ITO) and top (Ag) electrodes. In the transverse configuration, the output I from the waveguides can be described by the following equation:    h p. i 2p 3 V p I ¼ 2I 0 sin2 d þ 2 ¼ I 0 sin2 n reff L þ 2 ; 2 k d 2 where n is the refractive index of the waveguide, 2I0 is the peak-to-peak output intensity obtained by adjusting the compensator. L and d are the waveguide interaction length and distance between the electrodes. d is the phase shift caused by electric field V and reff the effective EO coefficient of the waveguide. Thus, by measuring the intensity variation at different voltage, the effective EO coefficient reff of the waveguide can be derived. Correspondingly, r33 of the waveguides can be obtained from the relations of r33 = 3r13 and reff = r33  r13 [20]. The EO coefficient r33 at 1550 nm is measured to be 9.0 pm/V for the waveguide of DR1-TESPIC in MAPTMS-Zr defined by direct blue laser and 12.4 pm/V for the waveguide of DR19-TESPIC in MAPTMS-Zr fabricated by reverse mesa method, respectively. The results are comparable with the best values obtained for the polymer counterparts doped with disperse red chromophores [21]. The corresponding VpL, the product of half wave modulation voltage Vp and the interaction length L, is estimated by VpL = kd/n3reffC to be 45 V cm and 32 V cm at 1550 nm for the above two waveguides, assuming the overlap factor C between the optical field and the electric field to be unit. The EO coefficient also exhibits good temporal stability. Fig. 14 shows the temporal stability of r33 of the waveguides fabricated by reverse mesa methods from side-chain DR19-TESPIC in MAPTMS-Al active sol–

Fig. 13. SEM photographs of the reverse mesa (3 lm · 3 lm, top) and the waveguide (middle). The bottom is the transmitted optical mode of the waveguide at 1550 nm.

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polarized at 45 in respect with the vertical direction, is coupled into a piece of polarization-maintaining single mode fiber and then end-fired into the EO waveguide. The output from the waveguide is collected with a 40· objective lens followed by a compensator and an analyzer that is put crossed with the input polarization direction. An infrared camera and an infrared detector are used to capture the transmitted optical modes and monitor the power variation of the output, respectively. The transmitted optical modes at 1550 nm from the waveguides fabricated by UV irradiation, direct blue laser writing, and reverse mesa methods are shown in

γ33 (pm/V)

12 9 6 3 0

0

50 100 150 200 250 Time after poling (day)

300

Fig. 14. Temporal stability of the EO coefficient of the waveguide fabricated by reverse mesa method from DR19-TESPIC in MAPTMSAl sol–gel.

H. Zhang et al. / Optical Materials 28 (2006) 992–999

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phore concentration, poling procedures, and fabrication processing. Another efficient approach is to incorporate high nonlinear hyperpolarizability chromophores into the hybrid sol–gel.

References

Fig. 15. EO intensity modulation response (bottom) of the waveguide fabricated by reverse mesa method from side-chain DR19-TESPIC in MAPTMS-Al sol–gel. The top trace is the applied drive voltage at 1 kHz.

gel. After experiencing a relaxation of around 12%, r33 keeps stable in the measurement period of 8 months. Intensity modulation has been conducted at 1550 nm with the above waveguides. Fig. 15 shows the intensity modulation response to the applied AC voltage of Vpp = 17 V for a 7 mm long waveguide fabricated from active MAPTMS-Al sol–gel doped with side-chain DR19-TESPIC.

7. Conclusions In summary, we have reported the materials processing, nonlinear optic effect, waveguide fabrication, and EO effect of hybrid MAPTMS-Zr(Al) sol–gel doped with azo-type nonlinear chromophores. High temporal and thermal stability has been obtained in cross-linked side-chain sol–gel films, showing that the hybrid sol–gels are promising host for EO devices. EO effect and modulation have been demonstrated on channel waveguides fabricated by low-cost and easy methods such as blue laser writing and reverse mesa methods, indicating that the active material systems permit high fabrication flexibility that is compatible with integrated circuits methodology. Even the current EO coefficients provide some advantages; we expect to improve the electro-optic coefficient of the active sol–gels by optimizing chromo-

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