Electrically modulated diffraction gratings in organic chromophore thin films Y. Kutuvantavida,1,2 Grant V. M. Williams,2,* and M. D. H. Bhuiyan1 1

Callaghan Innovation, P.O. Box 31310, Lower Hutt 5040, New Zealand

2

MacDiarmid Institute, School of Chemical and Physics Sciences, Victoria University of Wellington, P.O. Box 600, Wellington 6012, New Zealand *Corresponding author: [email protected] Received 7 January 2014; revised 13 March 2014; accepted 20 March 2014; posted 21 March 2014 (Doc. ID 204238); published 18 April 2014

An electrically modulated diffraction grating has been demonstrated in poled polymer thin films containing the organic nonlinear optical chromophore, PYR-3 (2-{3-Cyano-4-[3-(1-decyl-1 H-pyridin-4-ylidene)propenyl]-5,5-dimethy l-5 H-furan-2-ylidene}-malononitrile), and amorphous polycarbonate. A dc electric field induced change in the diffraction efficiency of up to 9% was observed. The diffraction efficiency modulation was likely due to an electric field induced change in the film thickness via a piezoelectric effect rather than via an electronic linear electro-optic effect. © 2014 Optical Society of America OCIS codes: (250.4110) Modulators; (250.6715) Switching; (190.4710) Optical nonlinearities in organic materials. http://dx.doi.org/10.1364/AO.53.002687

1. Introduction

Organic nonlinear optical (NLO) chromophores are being researched for a number of applications that include optical modulators [1,2], computer interconnects [3], THz generation and detection [4–6], and electric field sensors [7]. They have a number of advantages over conventional solid state compounds (e.g., LiNbO3 ) that include the ability to easily create arbitrary shapes (e.g., optical fibers, thin films) when compared with LiNbO3 [1]. One disadvantage is that they are susceptible to photodegradation in the presence of oxygen [8–11]. However, the photostability can be significantly enhanced by reducing the oxygen content [8] or using an encapsulating barrier that reduces oxygen diffusion into chromophore/polymer films [12,13]. Chromophores in chromophore/polymer thin films can also be used to create phase diffraction gratings for narrow band optical filters by using two-beam interference and optical bleaching of the chromophores 1559-128X/14/122687-04$15.00/0 © 2014 Optical Society of America

that results in a periodic modulation of the refractive index [14]. Transmittance and reflection diffraction gratings have been reported in liquid crystal diffraction gratings [15–17]. An applied electric field can be used to orient the liquid crystals, and this can lead to electrically modulated diffraction gratings [16]. However, the modulation speed is limited by the liquid crystal reorientation time. It should be possible to create a fast electrically modulated diffraction grating by ensuring that the chromophores in a periodically bleached chromophore/polymer film have a hyperpolarizability and where the chromophores are oriented so that the chromophore/polymer film has a linear electro-optic coefficient, r13 . The applied electric field would change the refractive index [1] and lead to a change in the diffraction efficiency. It is also possible that the aligned chromophores lead to a change in the film thickness via the piezoelectric effect [18], which can also modulate the diffracted intensity. Thus, the diffraction efficiency can be modulated either by the linear electro-optic effect or by the piezoelectric effect. In this paper, we report the results from diffraction measurements on organic chromophore/amorphous 20 April 2014 / Vol. 53, No. 12 / APPLIED OPTICS

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polycarbonate (APC) thin films. The organic chromophore used was PYR-3 (2-{3-Cyano-4-[3-(1-decyl-1 H-pyridin-4-ylidene)-propenyl]-5,5-dimethy l-5 Hfuran-2-ylidene}-malononitrile). PYR-3 is a secondorder NLO compound with a dipole moment, μ, of 5.10 × 10−29 Cm and a hyperpolarizability measured by second harmonic generation, β, of 3.51 × 10−37 m4 V−1 at 800 nm [19,20]. We show that a diffraction grating can be made and the diffraction efficiency can be modulated by an applied electric field, where at low frequencies the diffraction efficiency modulation is likely to be predominantly due to a change in the film thickness via the piezoelectric effect. 2. Experimental Details

The synthesis of PYR-3 is described elsewhere [19,20]. The method used to make PYR-3/APC thin films is similar to that reported previously [10,11]. They were fabricated by spin-coating a solution containing PYR-3 and APC (glass temperature, T g  218°C) in 1,1,2-trichloroethane onto an indium tin oxide (ITO) coated glass substrate. They were dried overnight at room temperature followed by 80°C for 12 h, and then they were annealed at 150°C in a vacuum for 2 h. The films contained 5% PYR-3 by weight and the film thicknesses as well as refractive indices were measured using a Metricon 2010. The absorption spectra was collected using a Perkin-Elmer Lambda 1050 UV–Vis–NIR spectrophotometer. Poling of the films was achieved using a parallel plate configuration with a detachable gold coated top plate electrode where the ITO on the glass substrate formed the other electrode [21]. The films were heated to 175°C for 15 min in an inert atmosphere and with an applied electric field of ∼60 V∕μm. Diffraction gratings were made in the films in transmission mode and without the gold coated top plate electrode using a two-beam interference method and a 488 nm laser to periodically bleach the film in the charge transfer band that peaked at ∼614 nm [10,11]. The angle between one of the beams and the surface normal was 4.9°, which corresponds to a grating period of Λ  2850 nm. The diffracted light from one of the writing beams was used to monitor the diffraction efficiency. Periodic bleaching results in a modulation of the absorption coefficient as well as the refractive index, n. The experimental setup for probing the diffraction grating in reflection mode is shown in Fig. 1. A 760 nm laser diode was used where the incident light went through the glass substrate and the ITO coating and it was reflected at the gold coated block electrode. This wavelength was chosen because the absorption coefficient is negligible [10,11], and hence, only a phase grating was probed. The angle of incidence was 9° to allow for the zeroth-order beam to be measured. The incident laser light electric field vector was nearly parallel to the film surface and the grating wavevector. A high voltage power supply was used to generate the dc voltages and a function 2688

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Fig. 1. Experimental setup for probing the diffraction grating. Image shows the function generator (FG), high voltage power supply (HVPS), photodiodes (PDs), laser, attenuator, polarizer, and sample with a gold coated block electrode, chromophore/polymer film, and an ITO electrode on a glass substrate. The chopper was used for some measurements where it provided the reference frequency for the lock-in amplifier.

generator allowed ac voltages to be applied. The beam intensities were measured using Si photodiodes and a lock-in amplifier or a voltmeter. For some lock-in amplifier measurements, a chopper was used after the film and before the photodiode to measure the first-order diffracted beam intensity. For other measurements, the chopper was removed and the lock-in amplifier reference frequency was supplied by the function generator. 3. Results and Analysis

In the thin film limit with low diffraction efficiency and for a sinusoidal modulation of n, it can be shown that the first-order diffraction efficiency for a pure phase grating in transmission mode, ηT , can be written as [22,23]  ηT 

 πΔn∕2d 2 ; λ cosϑ

(1)

where d is the film thickness, λ is the wavelength of the incident light, ϑ is the angle, Δn0 is the peak-topeak modulation in n, and ηT is the ratio of the firstorder diffracted intensity, I 1 0, divided by the zeroth-order intensity, I 0 0. In our case, the diffraction efficiency was measured in reflection mode with a partly transmitting ITO top electrode and a reflecting Au coated bottom electrode. From consideration of the reflectance from both interfaces it can be shown that most of the diffraction occurs when the incident beam transverses the film from the ITO electrode to the gold coated bottom electrode and from the gold coated bottom electrode to the top ITO electrode. This means that Eq. (1) can be rewritten for the reflection mode as η≈

  πΔn∕22d 2 ; λ cosϑ

(2)

where η is the reflection mode diffraction efficiency. The first-order diffraction efficiency, η0, was measured for a 6.06 μm thick film by measuring I 1 0

with a lock-in amplifier with a chopper and measuring I 0 0 using the voltmeter. η0 was found to be 0.32%. Δn0 was estimated using Eq. (2) and found to be 2.2 × 10−3 . The normalized change in the first-order diffraction efficiency, ΔηE∕η0, was measured as a function of the applied electric field for a 6.06 μm thick film where ΔηE  ηE − η0 and ΔηE∕η0  I 1 E − I 1 0∕I 1 0. This was done by measuring I 1 E using the lock-in amplifier with a chopper. It can be seen in Fig. 2 that jΔηE∕η0j increased linearly with the applied electric field, and the highest value was 9%. This was significantly greater than the ∼0.54% reported in the only other known study [21]. If the electric field induced modulation of the diffraction efficiency seen in Fig. 2 arises from a change in n induced by the linear electro-optic effect, then it can be shown that [1] δnE  −n30 r13 E∕2;

(3)

where n0 is the refractive index for E  0 and r13 is the relevant electro-optic coefficient in our experimental configuration where the incident light is nearly perpendicular to the film surface. The effect of the applied electric field on the diffraction efficiency can be obtained by noting that ΔnE  Δn0  δnE, and hence, from Eqs. (2) and (3), ΔηE∕η0 is   ΔηE −n30 r13 E 1 n30 r13 E 2  ≈ ; (4) Δn0 η0 4 Δn0 where ΔηE∕η0 is linear with the applied electric field for small electric fields and when E ≪ 4Δn0∕n30 r13 . The data in Fig. 2 appear to be consistent with Eq. (4) and a linear electro-optic effect where jΔηE∕η0j is linearly dependent on the applied electric field. The resultant r13 estimated from Fig. 2 and Eq. (4) was r13  3 pm∕V, where we used the

Fig. 2. Plot of the magnitude of ΔηE∕η0 against the dc electric field for a 6.06 μm 5% PYR-3/APC film at 760 nm. The line is a linear fit to the data.

measured n0  1.609 and Δn0  2.2 × 10−3 . If the electric field induced modulation of the diffraction efficiency is due to a linear electro-optic effect, then the change in the zeroth-order beam intensity, ΔI 0 E  I 0 E − I 0 0, when divided by I 0 0 can be related to the change in the diffraction efficiency and approximated as ΔI 0 E∕I 0 0 ≈ −2ΔηE;

(5)

where we used the approximation, ΔI 0 E ≈ −2I 1 E. It is difficult to measure ΔI 0 E for an applied dc electric field, and hence, intensity and diffraction measurements were made with an applied ac electric field at 1 kHz. I 0 0 was first measured using the voltmeter, and then ΔI 0 E was measured using the lockin amplifier. ΔηE  ΔI 1 E∕I 0 0 was measured using the lock-in amplifier for the first-order diffracted beam and the voltmeter for the zeroth-order beam. The resultant jΔI 0 E∕I 0 0j and jΔηE∕η0j are plotted in Fig. 3 where it can be seen that jΔI 0 E∕I 0 0j  0.67% at 1.23 V∕μm. However, j2ΔηEj from Fig. 3(a) at the same electric field was 0.0028%, which is significantly smaller than the expected value. This means that the linear electrooptic effect was not the main reason for the electric field induced modulation of the diffraction efficiency. It is possible that there is a significant contribution to ΔηE∕η0 from a modulation in the film thickness by the applied electric field via a piezoelectric effect [18,24]. This is possible because the chromophores contain dipole moments and poling leads to partial dipole alignment. In this case, ΔI 0 E∕I 0 0  ΔI 1 E∕I 1 0, and hence, ΔI 0 E∕I 0 0  Δη∕η:

(6)

Fig. 3. (a) Plot of the magnitude of ΔηE∕η0 against the ac electric field for a 6.00 μm 5% PYR-3/APC film at 760 nm with f  1 kHz. The line is a linear fit to the data. (b) Plot of jΔI0 E∕I 0 0j for a 6.00 μm 5% PYR-3/APC film at 760 nm with f  1 kHz (open circles). Also shown is jΔηE∕η0j from (a) (filled circles). 20 April 2014 / Vol. 53, No. 12 / APPLIED OPTICS

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5.

6.

7.

8. Fig. 4. Plot of jΔηE∕η0j as a function of frequency for a 6.00 μm thick 5% PYR-3/APC film at 760 nm and 0.82 V∕μm.

It can be seen in Fig. 3(b) (open circles) that this provides a reasonable interpretation of the data for low electric fields, and there was only a small departure for higher electric fields. It is apparent in Fig. 4 that ΔηE∕η0 decreased with increasing frequency, f . Here, we plotted ΔηE∕η0 for the 6.00 μm thick film at 0.82 V∕μm. The decrease in ΔηE∕η0 with increasing frequency may be due to a film inertia effect where at high frequencies the film cannot respond to the driving frequency and the film thickness modulation decreases. The mid-frequency resonances may be due to mechanical coupling to the substrate and the Au coated block electrode.

9.

10.

11.

12.

13.

4. Conclusion

In conclusion, an electrically modulated diffraction grating has been made using an organic chromophore/APC thin film. The electric field induced change in the diffraction efficiency was as high as 9%. We found that at low frequencies the diffraction efficiency modulation is probably dominated by a change in the sample thickness by a piezoelectric effect rather than by a linear electro-optic effect. We acknowledge funding from MBIE (C08X0807, C08X01206) and the MacDiarmid Institute for Advanced Materials and Nanotechnology. We thank S. Raymond and J. Quilty for assistance with some of the measurements and Andy Kay for synthesizing the PYR-3 chromophore.

14.

15. 16.

17. 18. 19.

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Electrically modulated diffraction gratings in organic chromophore thin films.

An electrically modulated diffraction grating has been demonstrated in poled polymer thin films containing the organic nonlinear optical chromophore, ...
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